Heart
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Hypertrophic cardiomyopathy is characterized by myocardial wall
thickening, particularly a disproportionate thickening of the interven-
tricular septum in comparison with the posterior wall. Echocardiogra-
phy accurately assesses the degree of thickening and its effect on systolic
function, such as dynamic left ventricular outflow obstruction, systolic
anterior motion of the aortic mitral valve leaflet and mid-systolic
closure of the aortic valve. There may also be a degree of diastolic dys-
function. Serial short-axis gradient echo MRI allows accurate measure-
ment of wall thickness and is particularly useful in assessing apically
confined hypertrophy. A number of histological changes are observed,
including cardiomyocytic disarray with replacement fibrosis and col-
lagenous component expansion. Treatment is usually medical, except
for refractory cases and those in whom the left ventricular outflow tract
obstruction has a gradient of greater than 50 mmHg. Ventricular septal
myotomy and myectomy are performed in such cases. Catheter alcohol
septal ablation has been introduced as a non-surgical alternative. A
number of patients may also require implantation of cardiac defibrilla-
tors to prevent sudden cardiac death.
An athlete’s heart may physiologically hypertrophy but in a
uniform fashion; the left ventricle cavity is usually less than 55 mm in
size, and thickness decreases on deconditioning. In contrast, hyper-
trophic cardiomyopathy reveals asymmetric patterns of left ventricular
hypertrophy, often with sharp segmental transitions, left atrial enlarge-
ment and bizarre electrocardiographic patterns. Furthermore, there is
an autosomal dominant inheritance pattern of abnormalities in genes
coding for myocardial proteins associated with hypertrophic cardio-
myopathy. Individuals with mutations of the β-MHC (major histo-
compatibility complex) gene usually develop the classic form of
hypertrophy, whereas those with cardiac troponin T gene mutations
generally have only mild or clinically undetectable hypertrophy. Rare
forms of hypertrophy include localized left ventricular apical hypertro-
phy as a result of cardiac troponin I mutations, and isolated mid-
cavity hypertrophy caused by cardiac actin and MLC (myosin light
chain) gene mutations.


HEART
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7
NOITCES
Aorta
Pulmonary trunk
Superior vena cava
Anterior interventricular
artery
Right atrium
Left atrial appendage Fossa
ovalis
Right upper
Venous mponent pulmonary vein
ule
Body co
Right lower
b
Circumflex coronary artery esti pulmonary vein
V
Mitral valve Left atrium
Fig. 57.26 The posterior wall of the left atrium close to the posterior interatrial groove in a cadaveric heart. The smooth-walled venous component of the
left atrium is the most extensive component. The septal aspect of the left atrium shows the crescentic line of the free edge of the flap valve against the
rim of the fossa ovalis. The orifices of the right superior and inferior pulmonary veins are adjacent to the plane of the septal aspect of the left atrium.
(Specimen courtesy of M Loukas MD, PhD.)
Anterior interventricular vein Serous pericardium, visceral layer (epicardium)
Myocardium
Left coronary artery,
Endocardium
anterior interventricular branch
Right ventricle
Anterior interventricular sulcus
Septomarginal trabecula
(moderator band)
Superoposterior Right coronary artery,
papillary muscle right marginal branch
Left coronary artery, Myocardium
left marginal branch
Superolateral papillary
muscle
Trabeculae
Left ventricle carneae
Interventricular septum,
muscular part
Inferoseptal papillary
muscles Tricuspid valve, septal leaflet
Chordae tendineae
Inferior interventricular sulcus
Middle cardiac vein Right coronary artery, inferior interventricular branch
Fig. 57.27 Left and right ventricles: cross-section perpendicular to the axis of the heart, superior aspect. (With permission from Waschke J, Paulsen F
(eds), Sobotta Atlas of Human Anatomy, 15th ed, Elsevier, Urban & Fischer. Copyright 2013.)
the anulus increases linearly with body surface area in children and commissure. These anteromedial (inferoseptal) and posterolateral
young adults (Poutanen et al 2006). The anulus is strongest at the (superoposterior) extremities may be regarded as two independent


internal aspects of the left and right fibrous trigones. Extending from commissures, each positionally named as indicated in brackets.
these structures, the anterior and posterior coronary prongs (tapering, Although simple, the official names for these leaflets – anterior and
fibrous, subendocardial tendons) partly encircle the orifice at the atrio- posterior, respectively – are somewhat misleading because of the obliq-
ventricular junction (see Fig. 57.23). Between the prong tips, the atrial uity of the valve.
and ventricular myocardial masses are separated by a more tenuous When the valve is laid open, the anterior leaflet (aortic, septal,
sheet of deformable fibroelastic connective tissue. Spanning anteriorly ‘greater’ or anteromedial) is seen to guard one-third of the circumfer-
between the trigones, the fibrous core of the central part of the aortic ence of the orifice and to be semicircular or triangular, with few or no
leaflet of the mitral valve is a continuation of the fibrous subaortic marginal indentations. Its fibrous core (lamina fibrosa) is continuous
curtain that descends from the adjacent halves of the left and adjacent on the outflow aspect, beyond the margins of the fibrous subaortic
(non-coronary) valve leaflets (see Fig. 57.29). curtain, with the right and left fibrous trigones (see Figs 57.17, 57.21,
57.24C). Between the trigones, it is continuous with the fibrous curtain
Mitral valve leaflets itself and, beyond the trigones, with the roots of the anular fibrous
The mitral valvular leaflets have long been described as paired struc- prongs (see Fig. 57.23). The leaflet has a deep crescentic rough zone
tures. (The name ‘bicuspid valve’ is explicit but erroneous because the that receives various chordae tendineae. The ridge limiting the outer
leaflets are not cuspid, or ‘peaked’, in form). Small accessory leaflets are margin of the rough zone indicates the maximal extent of surface
almost always found between the two major leaflets and so the mitral contact with the mural leaflet in full closure. A clear zone is seen


valve should be described as a continuous veil that is attached around between the rough zone and the valvular anulus; it is devoid of attach-
the entire circumference of the mitral orifice. Its free edge bears several ments of chordae, although its fibrous core carries extensions from
indentations, of which two are sufficiently deep and regular to be chordae attached in the rough zone. The anterior leaflet has no basal
nominated as the ends of a solitary and oblique zone of apposition or zone and continues into the valvular curtain. Hinging on its anular


Heart
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Superior vena cava
Right upper
pulmonary vein
Aorta
comV pe on no eu ns
t
pulmR oig nh at
r
ylo vw ee inr
Fossa
Pulmonary trunk ovalis
Body
Left atrium
Septal perforator
A arn tete ryrior interventricular Vestibule
Circumflex
coronary artery
Superolateral Mitral valve
papillary muscle
Inferoseptal
papillary muscle
Fig. 57.28 A dissection of the left ventricle in a cadaveric heart, exposing the papillary muscles of the ventricle. Notice the thick wall of the left ventricle
and the chordae tendineae of the mitral valve attaching to the papillary muscles. (Specimen courtesy of M Loukas MD, PhD.)
Left coronary leaflet Left coronary artery
Aortic bulb Left coronary artery,
anterior interventricular branch
Non-adjacent leaflet
Interventricular septum, membranous part
Aortic sinus
Myocardium
Pulmonary trunk Chordae tendineae
Posterolateral
papillary muscle
Left coronary leaflet
Right coronary artery
Nodule of left coronary
leaflet
Left atrioventricular orifice
Right coronary leaflet
Mitral valve, aortic leaflet
Non-adjacent leaflet
Inferoseptal papillary muscle
Fig. 57.29 The aortic orifice opened from the front to show the leaflets of the aortic valves, their nodules, lunules, commissures and the triple-scalloped
line of their anular attachment. Also shown are the continuity of the subaortic curtain with the mitral aortic leaflet (i.e. ‘aortic baffle’) and the coronary
ostia, and the spatial relationship of the aortic orifice to the pulmonary orifice and to the left ventricle. (With permission from Waschke J, Paulsen F (eds),
Sobotta Atlas of Human Anatomy, 15th ed, Elsevier, Urban & Fischer. Copyright 2013.)


HEART
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NOITCES
attachment, and continuous with the subaortic curtain, it is critically the atrial cavity, and the atrial aspects of the rough zones come into
placed between the inlet and the outlet of the ventricle. During passive maximal contact. Precise papillary contraction, and increasing tension
ventricular filling and atrial systole, its smooth atrial surface is impor- in the chordae, continue to prevent valvular eversion and maintain
tant in directing a smooth flow of blood towards the body and apex of valvular competence. The orifices and the leaflets of both atrioventricu-
the ventricle. After the onset of ventricular systole and closure of the lar valves undergo considerable changes in position, form and area
mitral valve, the ventricular aspect of its clear zone merges into the during a cardiac cycle. Both valves move anteriorly and to the left during
smooth surface of the subaortic curtain, which, with the remaining systole, and reverse their motion in diastole. The mitral valve reduces
fibrous walls of the subvalvular aortic vestibule, forms the smooth its orificial (anular) area by as much as 40% in systole; its shape changes
boundaries of the ventricular outlet. from circular to crescentic at the height of systole, the anular attachment
The posterior leaflet (mural, ventricular, ‘smaller’ or posterolateral) of its aortic leaflet being the concavity of the crescent. The attachment
usually has two or more minor indentations. Lack of definition of the of its mural leaflet, although remaining convex, contracts towards the
major intervalvular commissures has led to disagreement and confu- anterior cardiac wall. The smooth left ventricular outflow tract (aortic
sion concerning the territorial extent of this leaflet and the possible vestibule) terminates at the aortic valve leaflets. Although stronger in
existence of accessory scallops. Examination of the valve in the closed construction, the aortic valve resembles the pulmonary valve (see Figs
position reveals that the posterior leaflet may conveniently be regarded 57.21, 57.24, 57.29) in possessing three semilunar leaflets, supported


as comprising all the valvular tissue posterior to the anterolateral within the three aortic sinuses of Valsalva. Although the aortic valve,
(inferoseptal) and posteromedial (superoposterior) ends of the major like the pulmonary valve, is often described as possessing an anulus in
zone of apposition with the aortic leaflet. Thus defined, it has a wider continuity with the fibrous skeleton, there is no complete collagenous
attachment to the anulus than does the anterior leaflet, guarding two- ring that supports the attachments of the leaflets. As with the pulmo-
thirds of the circumferential attachments. Further indentations usually nary valve, the anatomy of the aortic valve is dominated by the fibrous
divide the mural leaflet into a relatively large middle scallop and semilunar leaflet attachment (see Fig. 57.24C).
smaller lateral and septal commissural scallops. Each scallop has a
crescentic opaque rough zone, receiving on its ventricular aspect the Mitral chordae tendineae
attachments of the chordae that define the area of valvular apposition Mitral chordae tendineae resemble those supporting the tricuspid valve.
in full closure. From the rough zone to within 2–3 mm of its anular False chordae are also irregularly distributed as in the right ventricle.
attachment, there is a membranous clear zone devoid of chordae. They are single or multiple, thin, fibrous or fibromuscular structures
The basal 2–3 mm is thick and vascular, and receives basal chordae. that traverse the cavity of the left ventricle and have no connection with
The ratio of rough to clear zone in the anterior leaflet is 0.6 and in the the valvular leaflets (Fig. 57.30). They occur commonly in human left
middle scallop of the posterior leaflet is 1.4. Much more of the mural ventricles and often cross the subaortic outflow. Histologically, false
leaflet is in apposition with the aortic leaflet during closure of the chordae sometimes contain extensions from the ventricular conducting
mitral valve. tissues; these left ventricular bands are often identified on echocardiog-
raphy. It has been suggested that false chordae produce premature
Opening of the mitral valve ventricular contractions and be the possible cause of functional heart


At the onset of diastole, opening is passive but rapid, the leaflets parting murmurs or innocent murmurs in children and young adults.
and projecting into the ventricle as left atrial pressure exceeds left ven- True mitral valve chordae may be divided into four types: inter-
tricular diastolic pressure. Passive ventricular filling proceeds as atrial leaflet (commissural), rough zone (including the special strut chordae),
blood pours to the apex, directed by the pendant aortic valvular leaflet. ‘cleft’ and basal chordae. Most true chordae divide into branches from
The leaflets begin to float passively together, hinging on their anular a single stem soon after their origin from the apical third of a papillary
attachments and partially occluding the ventricular inlet. Atrial systole muscle, or proceed as single chordae that divide into several branches
now occurs, jetting blood apically and causing re-opening of the leaf- near their attachment. Basal chordae, in contrast, are solitary structures
lets. As maximal filling is achieved, the leaflets again float rapidly passing from the ventricular wall to the mural leaflet. There is such
together. Closure is followed by ventricular systole, which starts in the marked variation between the arrangement of the chordae that any
papillary muscles and continues rapidly as a general contraction of the detailed classification loses much of its clinical significance. Suffice it
walls and septum. Coordinated contraction of the papillary muscles to say that, in the majority of hearts, the chordae support the entire free
increases the tension in the chordae and promotes joining of the cor- edges of the valvular leaflets, together with varying degrees of their
responding points on opposing leaflets, preventing their eversion. With ventricular aspects and bases, and there is some evidence to suggest that
general mural and septal excitation and contraction, left ventricular those valves with unsupported free edges become prone to prolapse in
pressure increases rapidly (see Fig. 57.20). The leaflets ‘balloon’ towards later life.
Fig. 57.30 The aorta has been
transected at the level of the
Aorta Right coronary orifice
sinutubular junction in this
cadaveric heart in order to
Left atrium reveal the closed leaflets of the
aortic valve. Note how the
supporting sinuses may be


Interleaflet triangle described as left coronary, right
Pulmonary trunk
coronary and non-adjacent,
according to the origin of the
Membranous septum coronary arteries. However, this
Aortic-mitral
Circumflex continuity terminology could be
coronary artery ambiguous if the coronary
arteries arise in variable
locations, whereas the
Anterior interventricular
terminology of right, left and
artery
Superolateral non-facing aortic sinus,
Left bundle papillary muscle respectively, is not ambiguous.
branch Note the inter-leaflet triangles,
membranous septum, and
fibrous continuity between the
aorta and mitral valve.
Inferoseptal
papillary muscle (Specimen courtesy of M
Loukas MD, PhD.)
Left ventricle
Fine trabeculations


Heart
1011
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RETPAHC
Papillary muscles except at its midpoint, where there is an aggregation of fibrous tissue,
The two muscles supporting the leaflets of the mitral valve vary in length the valvular nodule of the semilunar leaflet. The fibrous core that flanks
and breadth, and may be bifid. The anterolateral (superolateral) muscle each nodule is tenuous, forming the lunules of translucent and occa-
arises from the sternocostal mural myocardium, and the posteromedial sionally fenestrated valvular tissue; fenestrations are of no functional
(inferoseptal) from the diaphragmatic region (see Fig. 57.27). Chordae significance. The aortic surface of each leaflet is rougher than its ven-
tendineae arise mostly from the tip and apical third of each muscle but tricular aspect.
sometimes take origin near their base. The chordae from each papillary Confusingly, three sets of names are used to describe the aortic
muscle diverge and are attached to corresponding areas of closure on leaflets. Posterior, right and left refer to their fetal positions before
both valvular leaflets. full cardiac rotation has occurred (Ch. 52). Corresponding terms based
on their approximate positions in maturity are anterior, left and right
posterior. Widespread clinical terminology, which links both leaflets
Aortic root
and sinuses to the origins of the coronary arteries, has replaced anterior,
left and right leaflets with right and left coronary and non-adjacent
The aortic root, the anatomical bridge between the left ventricle and the (and, usually, non-coronary) leaflets, respectively; these clinical terms
ascending aorta, consists of the aortic valvular leaflets (supported by are preferable in the normal heart because they are simple and
the aortic sinuses of Valsalva) and the inter-leaflet triangles interposed unambiguous.
between their basal attachments (see Fig. 57.24). As such, it possesses
significant length, but because of the semilunar attachment of the leaf- Aortic sinuses (of Valsalva)
lets, it has no discrete proximal border. It is limited distally by the The aortic sinuses are more prominent than those in the pulmonary


sinutubular junction (see Fig. 57.30). The essential feature is the semi- trunk. The upper limit of each sinus reaches considerably beyond the
lunar attachments of the valve leaflets; their hinge lines cross the ana- level of the free border of the leaflet and forms a well-defined circum-
tomical ventriculoarterial junction, marking a transition from the ferential sinutubular ridge on the aortic inner surface, just above the
myocardium of the left ventricle to the fibroelastic tissue of the valve aortic valvular leaflets (see Fig. 57.24C). Coronary arteries usually open
sinuses (Fig. 57.31). The muscular portion of the aortic root is roughly near this ridge within the upper part of the sinus but are markedly vari-
two-thirds of its widest circumference. Descriptions of the aortic root able in their origin. The walls of the sinuses are largely collagenous near
over the years have been bedevilled by accounts of a valve anulus. the attachment of the leaflets but the amount of lamellated elastic tissue
Although echocardiographers used to describe this proximal border in increases with distance from the zone of attachment. At the midlevel
terms of an ‘anulus’, examination of cross-sections of the left ventricular of each sinus, its wall is about half the thickness of the supravalvular
outflow tract has never found any circular anatomical boundary or a aortic wall and less than one-quarter of the thickness of the sinutubular
distinct boundary of any kind (Loukas et al 2014a). There are at least ridge. At this level, the mean luminal diameter at the commencement
two rings within the root; neither serves to support the valve leaflets, of the aortic root is much larger than that of the ascending aorta; these
which are attached in semilunar fashion from the sinutubular junction details are functionally significant in the mechanism of valvular motion.
to a basal ventricular attachment. Two leaflets are supported by muscle, A linear relationship between the diameter of the aortic sinus and the
and the third has an exclusively fibrous attachment. The root acts as a square root of body surface area has been described in children (Kaiser
bridging structure not only anatomically, separating the myocardial and et al 2008).
arterial components of the left ventricular pathway, but also function-


ally because its proximal and distal components can withstand consid-
Opening of the aortic valve
erable changes in ventricular and arterial pressures.
During diastole, the closed aortic valve supports an aortic column of
blood at high but slowly diminishing pressure (see Fig. 57.20). Each
Aortic valve leaflets
sinus and its leaflet form a hemispherical chamber. The three nodules
The aortic valve leaflets are attached in part to the aortic wall and in
are apposed and the margins and lunular parts of adjacent leaflets are
part to the supporting ventricular structures. The situation is more
tightly apposed on their ventricular aspects. From the aortic aspect, the
complicated than in the pulmonary valve because parts of the leaflets
closed valve is triradiate, three pairs of closely compressed lunules
also take origin from the fibrous subaortic curtain, and are continuous
radiating from their nodules to their peripheral commissural attach-
with the aortic leaflet of the mitral valve (see Fig. 57.29). This area of
ments at the sinutubular junction (see Fig. 57.21). As ventricular systolic
continuity is thickened at its two ends to form the right and left fibrous
pressure increases, it exceeds aortic pressure and the valve is passively
trigones (see Fig. 57.21). As with the pulmonary valve, the semilunar
opened.
attachments incorporate segments of ventricular tissue within the bases
The fibrous wall of the sinuses nearest the aortic vestibule is almost
of two of the aortic sinuses. The sinuses and leaflets are conveniently
inextensible but, more superiorly, the wall is fibroelastic. Under left
named as right, left and non-coronary, according to the origins of the
ventricular ejection pressure, the radius here increases 16% in systole,
coronary arteries (see Fig. 57.24C). However, the so-called non-coronary
as the commissures move apart to form a fully open triangular orifice.
leaflet is better termed the non-adjacent leaflet because it rarely gives
The free margins of the leaflets then become almost straight lines
rise to a coronary artery.
between peripheral attachments. However, they do not flatten against
The semilunar attachments incorporate three triangular areas (trigo-


the sinus walls, even at maximal systolic pressure, which is probably an
nes) of aortic wall within the apex of the left ventricular outflow tract.
important factor in their subsequent closure. During ejection, most
They are interposed between the bulbous aortic sinuses and separate
blood enters the ascending aorta but some enters the sinuses, forming
the cavity of the left ventricle from the pericardial space. Removal of the
vortices that help to maintain the triangular ‘mid position’ of the leaflet
trigones in an otherwise intact heart is instructive in demonstrating the
during ventricular systole and also probably initiate their approxima-
relationships of the aortic valve, which, justifiably, may be considered
tion at the end of systole. Tight and full closure ensues with the rapid
as the keystone of the heart. The first triangle, between the non-coronary
decrease in ventricular pressure in diastole. Commissures narrow,
and left coronary leaflets, has a base continuous inferiorly with the
nodules aggregate and the valve reassumes its triradiate form. Experi-
fibrous aortic–mitral curtain. The second triangle, between right and
ments indicate that 4% of ejected blood regurgitates through a valve
non-coronary leaflets, has the membranous components of the inter-
with normal sinuses, whereas 23% regurgitates through a valve without
ventricular septum as its base and thus ‘faces’ the right ventricle, whereas
them. The normal structure of the aortic sinuses also promotes non-
its apex ‘points’ towards the transverse pericardial space behind the
turbulent flow into the coronary arteries.
origin of the right coronary artery. The third triangle, between the two
coronary leaflets, has its base on the muscular interventricular septum
Echocardiography
and its apex ‘points’ to the plane of space found between the aortic wall
and the free-standing sleeve of right ventricular infundibular muscula-
ture that supports the leaflets of the pulmonary valve. Although the Echocardiography allows a detailed assessment of the functional
basal attachments of each aortic valvular leaflet are thickened and col- anatomy of the heart. The gross anatomy of the heart can be evaluated


lagenous at their ventricular origins, the leaflets lack a continuous col- by two-dimensional echocardiography in the parasternal, apical,
lagenous circular skeletal support; valvular function depends primarily suprasternal and subcostal positions (Fig. 57.32). The standardized
upon the semilunar attachments of the leaflets. planes used are long-axis, short-axis and four-chamber. The long-axis
The leaflets are endocardial folds with a central fibrous core. With view is obtained by placing the ultrasound transducer in the left apico-
the valve half-open, each equals slightly more than a quarter of a sphere, sternal position and provides detailed images of the left ventricle, aorta,
an approximate hemisphere being completed by the corresponding left atrium, and mitral and aortic valves (Fig. 57.32C). Angling the
sinus. Each leaflet has a thick basal border, deeply concave on its aortic beam towards the right also allows assessment of the right atrium, right
aspect, and a horizontal free margin that is only slightly thickened, ventricle and tricuspid valves. Rotating the transducer by 90° in the


Heart
1011.e1
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RETPAHC
Sinus wall
Ventriculo-
arterial
junction Leaflet
Transitional
zone
Hinge
Ventricular
myocardium
Fig. 57.31 A histological section through one of the coronary aortic
sinuses to demonstrate the way in which ventricular muscle supports the
transition from the fibroelastic wall of the sinus to the tissues of the
leaflet. The transitional area is anchored to the ventricular muscle. The
hinge of the leaflet is well below the level of the ventriculoarterial junction.
Importantly, there is no ring-like structure in the form of an ‘anulus’
supporting the hinge of the leaflet within the ventricle. The muscular
tissue of the ventricle is stained pink; fibrous tissue, which would show
up as a characteristic light colour with this stain, is absent. Haematoxylin
and eosin stain. (Courtesy of Diane E Spicer.)


HEART
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7
NOITCES
RV RV
PV
R LV
TV
AV
RV NA
LV L
RA MV
LA
LA
RA
LA
A B C
RV
RV
RV
AV
RA LV PV
RA AV
LV
LA
LA MV
LA
D E F
Fig. 57.32 Cardiac anatomy shown by transthoracic echocardiography and CT. A, Four-chamber view. B, Short-axis view at aortic valve level. Note
three aortic leaflets – right, left and non-adjacent – and the central position of the aorta. C, Parasternal long-axis view. D–F, Corresponding images by
CT. Note the different orientation compared with transthoracic echocardiography (looked at from below, foot to head). Abbreviations: A, anterior; AV,
aortic valve; F, foot; H, head; L, left; LA, left atrium; LV, left ventricle; MV; mitral valve; NC, non-adjacent; P, posterior; PV, pulmonary valve; R, right; RA,
right atrium; RV, right ventricle; TV, tricuspid valve. (Images courtesy of Dr Konstantinos Dimopoulos, Royal Brompton and Chelsea and Westminster
Hospitals, London.)
clockwise direction produces the short-axis view, which allows assess- connective tissue matrix itself is interconnected laterally to form
ment of the left ventricle, papillary muscles, chordae tendineae and bundles, strands or sheets of macroscopic proportions showing a
mitral valves (Fig. 57.32B). The four-chamber view demonstrates the complex geometric pattern. Surrounding and attaching to larger myo-
ventricles, atria, and mitral and tricuspid valves (Fig. 57.32A). Rotation cardial bundles are stronger perimysial condensations. The overall
of the transducer allows two-chamber views of the heart and more pattern is described in terms of struts and weaves. Despite its impor-
detailed assessment of the aorta and aortic valves. Cardiac MRI and CT tance, the myocardial matrix cannot be grossly dissected.
provide similar information on cardiac structure and function (see Figs Running at the ventricular base is a complex framework of dense


57.2, 57.35A,B), together with complementary information on great collagen with membranous, tendinous and fibroareolar extensions,
vessels (Fig. 57.32D–F) and other extracardiac intrathoracic structures. intimately related to atrioventricular valves and the aortic orifice. The
whole is sufficiently distinct to be termed the fibrous skeleton of the
heart, but although it is often stated that all four valves are contained
CONNECTIVE TISSUE AND FIBROUS SKELETON OF
within this skeleton, this is not the case. The leaflets of the pulmonary
THE HEART valve are supported on a free-standing sleeve of right ventricular
infundibulum that can easily be removed from the heart without dis-
From epicardium to endocardium and from the orifices of the great turbing either the fibrous skeleton or the left ventricle. Another point
veins to the roots of the arterial trunks, the intercellular spaces between of confusion is the idea that the ‘skeleton’ provides the support for the
contractile and conduction elements are permeated by connective cardiac valves. In reality, it is the overall structure of the atrioventricular
tissue; the amount, arrangement and texture vary greatly with location. junctions that supports the mitral and tricuspid valves, whereas the
Over much of the heart, a fine layer of areolar tissue is found beneath arterial valves are hinged within the valvular sinuses. The fibrous skel-
the mesothelium of the serous (visceral) epicardium that accumulates eton is strongest at the junction of the aortic, mitral and tricuspid valves,
subepicardial fat, concentrated along the acute margin, the atrioven- the so-called central fibrous body (see below) (see Figs 57.21, 57.23).
tricular and interventricular grooves, and their side channels. The coro- Two pairs of curved, tapering, collagenous prongs, fila coronaria, extend
nary vessels and their main branches are embedded in this fat; the from the central fibrous body. They are stronger on the left, where they
amount increases with age. The endocardium also lies on a fine areolar pass partially around the mitral and tricuspid orifices. These orifices are


tissue rich in elastic fibres. Fibrocellular components of these subepi- almost co-planar and incline to face the cardiac apex. In contrast, the
cardial and subendocardial layers blend on their mural aspects with the aortic valve faces superiorly, lying anterosuperior and to the right of the
endomysial and perimysial connective tissue on the myocardium. Each mitral orifice. Two of the leaflets of the aortic valve are in fibrous con-
cardiac myocyte is invested by a delicate endomysium composed of fine tinuity with the aortic leaflet of the mitral valve; this subaortic curtain
reticular, collagen and elastin fibres embedded in ground substance. is also an integral part of the fibrous skeleton (see Figs 57.21, 57.24C).
This matrix is lacking only at desmosomal and gap junctional contacts The two ends of the curtain are strengthened as the right and left fibrous
of intercalated discs. Similar arrangements apply to ventricular conduc- trigones, which are the strongest parts of the skeleton. The right trigone,
tion myocytes and their extensive working myocardial contacts. The together with the membranous septum, constitutes the central fibrous


Heart
1013
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RETPAHC
body, which is penetrated by the atrioventricular bundle of His (see
below). The membranous septum is crossed on its right aspect by the
attachment of the tricuspid valve, dividing the septum into atrioven-
tricular and interventricular components.
The aortic root is central within the fibrous skeleton and is often
described in terms of an ‘anulus’ integrated within the fibrous skeleton
(Anderson et al 2013b). However, as with the pulmonary valve, the
structure of the aortic root corresponds to the triple fibrous semilunar
attachments of its leaflets. Within this complex circumferential zone
are three crucially important triangular areas that separate, on the ven-
tricular aspect, the aortic bulbous sinuses that house the valvular leaf-
lets. As a whole, three triangles, known as the subaortic spans, can be
conceptualized in terms of a three-pointed coronet; their triangular
apices correspond to the tips of the valvular commissures and their
walls, significantly thinner than those of the sinuses, consist variously
of collagen or admixed muscle strands and fibroelastic tissue. They
form the subvalvular extensions of the aortic vestibule. The first span is
the interval between the non-coronary and left coronary sinuses that is
filled with the deformable subaortic curtain. The second span between
the non-coronary and right coronary sinuses is continuous with the
anterior surface of the membranous septum. The third span, between
the two coronary aortic sinuses, is filled with loose fibroelastic tissue
and separates the extension of the subaortic root from the wall of the
free-standing subpulmonary infundibulum. Previously, this was
Fig. 57.33 A reconstruction of a porcine heart made following diffusion
thought to be the location of the tendon of the infundibulum (conal tensor MRI of an autopsied heart. The tracks are created from the first
(conus) ligament) (Loukas et al 2007); research using mouse hearts eigenvectors and represent the alignment of the aggregated myocytes.
has shown that the ligament is found at the level of the sinutubular This technique enables the identification of reproducible tracks, or


junctions at the site of initial fusion of the distal outflow cushions. The pathways, through the myocardium that connect remote end and
line of proximal fusion is the longitudinal raphe sometimes seen in epicardial regions by means of simultaneous changes in helical and
the muscular subpulmonary infundibulum. intrusion angles. (With permission from Anderson R, Smerup M, Sanchez-
Similar fibrous triangles are found separating the sinuses of the Quintana D, Loukas M, Lunkenheimer P. 2009. The three-dimensional
pulmonary trunk but these are significantly less robust. Use of the terms arrangement of the myocytes in the ventricular walls. Clin Anat 22:64–76.)
mitral and tricuspid ‘anuli’ implies that the atrioventricular valves are
supported by discrete circular fibrous rings, when, in reality, they are
D-shaped structures integrally attached to the fibrous cardiac skeleton.
estimated at 8 per 1000 live births but they are found in up to 2% of
It is therefore inaccurate to consider these valvular orifices as circular.
stillbirths. Only a small proportion of the anomalies are directly attrib-
Rather, the straight edge of the ‘D’-shaped mitral valve represents a
utable to genetic or environmental factors and the majority are the
fibrous continuity between the anterior leaflet and the aortic root, and
result of multifactorial events.
the remainder supports the mural leaflet. The straight edge of the tri-
cuspid valve represents the attachment of the septal leaflet, marking Abnormalities of cardiac position
the inferior border of the triangle of Koch, and the remainder supports
the anterosuperior and inferior leaflets. The valvular attachments of the
Available with the Gray’s Anatomy e-book
atrioventricular valves are not simple, rigid collagenous structures but
dynamic, deformable lines that vary greatly at different peripheral
points and change considerably with each phase of the cardiac cycle Acyanotic cardiac defects
and with increasing age. The tricuspid attachments are even less robust
than those of the mitral valve. At several sites, only fibroareolar tissue Available with the Gray’s Anatomy e-book
separates the atrial and ventricular muscular masses.
The fibrous skeleton ensures electrophysiological discontinuity
Cyanotic cardiac defects
between the atrial and ventricular myocardial masses (except at the site


of penetration of the conduction tissue). It also provides direct attach-
ment for the myocardium and for fibrous tissue throughout the Available with the Gray’s Anatomy e-book
heart as a support matrix for a three-dimensional meshwork of
cardiomyocytes.
CONDUCTION TISSUES
Arrangement of cardiomyocytes The microstructure of cardiac
muscle is described in detail in Chapter 6. Ventricular myocytes form a The cells of cardiac muscle differ from those of skeletal muscle in having
three-dimensional mesh in a supporting fibrous matrix. One popula- the inherent ability to contract and relax spontaneously. This myogenic
tion is aligned so that the long axis of the aggregated cells is tangential rhythm is shown by small pieces of cardiac tissue, and even isolated
to the epicardial and endocardial borders, albeit with marked variation myocytes. Ventricular cells contract and relax at a lower frequency than
in the angulation relative to the ventricular equator. Correlation with atrial cells, but in the intact heart, both are synchronized to a more
measurements taken using force probes shows that these myocytes rapid rhythm, generated by pacemaker tissue in the sinus and atrioven-
produce the major unloading of the blood during ventricular systole. A tricular nodes and conveyed by a system of specialist conduction fibres
second population is aligned at angles of up to 40° from the epicar- in the atrioventricular conduction axis and the ventricular conducting
dium towards the endocardium and produces auxotonic forces during pathways (Purkinje cells). For an account of excitation–contraction
the cardiac cycle. The three-dimensional arrangement of the mesh as coupling in cardiac muscle, see page 138.
demonstrated in MRI of the porcine heart also mediates the realign- The anatomical arrangement of these tissues is described in the
ment of cardiomyocytes that must take place during ventricular con- context of the heart. Here, consideration is restricted to the cells that
traction and accounts for the extent of systolic mural thickening make up the impulse-generating and conduction system. All are modi-
(Fig. 57.33). fied cardiac cells. Three types within a continuum of morphology may
The finding that the number of cardiomyocytes in the left ventricle be distinguished from normal working cells, namely: nodal cells, tran-
increases between the first and the twentieth year of life suggests that sitional cells and Purkinje cardiomyocytes.


children may be able to regenerate myocardium (Mollova et al 2013).
Overview of the conduction system
CONGENITAL CARDIAC MALFORMATIONS
Of all the cells in the heart, those of the sinu-atrial node generate the
Congenital malformations of the heart are common and amount to most rapid rhythm, and therefore function as the cardiac pacemaker.
about one-quarter of all developmental anomalies. Their incidence is The impulse, believed to be generated in the nodal cells, is transmitted


Heart
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The most severe abnormality of position is an extrathoracic heart Most congenital heart abnormalities can be detected during antena-
(ectopia cordis), where the heart usually projects to the surface through tal ultrasound screening (Fig. 57.34). Neonates with severe congenital
the lower thoracic and upper abdominal wall, remaining covered, in heart disease may present with tachypnoea, difficulty in feeding, cyano-
most instances, by the fibrous pericardium, and usually accompanied sis and/or cardiovascular collapse, and there may be audible murmurs
by herniation of the abdominal contents. A mirror image, i.e. reversal on auscultation. The vast majority of abnormalities may be detected by
in cardiac shape and position, occurs predominantly in the right postnatal echocardiography; in a very few cases, cardiac catheterization
hemithorax (dextroposition). The apex of the heart may be directed to and direct measurements of pressure, oxygen saturations and angiogra-
the right instead of the left (dextrocardia). This arrangement may phy may be required.
be part of ‘situs inversus totalis’, where the heart, great vessels and
abdominal organs all occupy mirror-imaged positions. The heart may Acyanotic cardiac defects are the result of either left-to-right cardiac
also be right-sided in Kartagener’s syndrome (a subgroup of ciliary shunting through heart defects (intra- or extracardiac) or of obstruction.
dyskinesias). Left-to-right shunting leads to an increased workload and stress on the
More usually, an abnormal location of the heart occurs in cases of heart and the lungs as a consequence of increased pulmonary blood
isomerism, in which both sides of the thorax, including the main flow and increased pulmonary venous return. Depending on the loca-
bronchi and the atrial appendages, retain features of either morphologi- tion of the shunt and the magnitude of left-to-right shunting, patients
cal right- or left-sidedness. Isomerism is also commonly associated with are at risk of developing pulmonary arterial hypertension unless
the anomalous arrangement of the abdominal organs: right isomerism timely heart surgery is undertaken. Examples of defects leading to left-


with absence of the spleen (asplenia) and left isomerism with multiple to-right shunting are simple septal defects such as atrial or ventricular
spleens (polysplenia). Intracardiac anatomy in cases of isomerism is septal defects or patent ductus arteriosus, or more complex atrioven-
almost universally abnormal, and there is a range of heart defects tricular septal defects, and/or a combination of any of these defects with
(usually more simple in left and more complex in right isomerism). abnormal atrioventricular or ventriculoarterial connections (e.g. a
These intrathoracic abnormal arrangements, with or without abdom- double-outlet right ventricle, where more than 50% of both the aorta
inal abnormalities, often first manifest themselves incidentally follow- and the pulmonary artery originate from the right ventricle) (Ch. 52).
ing chest radiography. Reversal of normal anatomy has obvious clinical Obstructive lesions may involve the atrioventricular or arterial valves
implications, a typical example being a patient with mirror-image (mitral, aortic or pulmonary valve stenosis), or narrow vessels (e.g.
arrangements and appendicitis, who presents with an acute left lower coarctation of the aorta or interrupted aortic arch). Depending on the
quadrant pain. level and severity of the obstruction, patients may have a range of
RV
LV MV
VSD RA LA LV RV LV RV TV
RA
A B C
VSD
RV
LV
RV RV Ao Ao
PA
RA LV
D E F
Fig. 57.34 A, Tricuspid atresia at 21 weeks’ gestation. A four-chamber view showing a large left ventricle and a hypoplastic right ventricle. The mitral
valve is seen; no tricuspid valve is identified. There is an associated ventricular septal defect. B, An atrioventricular septal defect at 20 weeks’ gestation.
A common atrioventricular valve (arrow) is associated with a defect in the atrioventricular septum. C, Hypoplastic left heart syndrome at 32 weeks’
gestation. This four-chamber view shows a large right atrium, right ventricle and tricuspid valve. There is a hypoplastic left ventricle; no mitral valve is


identified. D, Hypoplastic left heart syndrome at 32 weeks’ gestation. This four-chamber view shows colour flow from right atrium to right ventricle
across the tricuspid valve; no mitral valve is identified and there is no flow in the left side of the heart. E, Tetralogy of Fallot at 33 weeks’ gestation. The
aorta arises astride a ventricular septal defect. F, Transposition of the great arteries at 21 weeks’ gestation. The aorta and pulmonary artery arise in
parallel orientation. The aorta arises from the right ventricle and the pulmonary artery from the left ventricle. Abbreviations: Ao, aorta; LA, left atrium; LV,
left ventricle; MV, mitral valve; PA, pulmonary artery; RA, right atrium; RV, right ventricle; TV, tricuspid valve; VSD, ventricular septal defect. (Courtesy of
Gurleen Sharland, Evelina London Children’s Hospital.)


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A B
Fig. 57.35 A, Cardiac magnetic resonance angiography in a patient with a normal heart. Note the criss-cross relationship of the great arteries; the aorta
arises from the left ventricle and runs to the right and posteriorly to the pulmonary trunk (at its origin) and towards the head. The pulmonary artery arises
from the right and anterior right ventricle, crosses over and then runs to the left of the aorta and towards the back (where it bifurcates into the right and
left pulmonary arteries at the underside of the aortic arch). B, A cardiac magnetic resonance angiogram from a patient with transposition of the great
arteries. Note the parallel or side-by-side relationship of the great vessels and loss of their normal criss-cross relationship. The aorta runs anteriorly and
comes off the right ventricle. The hypertrophied right ventricle supports the systemic circulation, and the ventricular septum bows from right to left.
Abbreviations: Ao, aorta; LV, left ventricle; PT, pulmonary trunk; RV, right ventricle. (Courtesy of Dr Philip Kilner, Royal Brompton Hospital, London.)
clinical presentations from an asymptomatic heart murmur (common) position of the great arteries (Fig. 57.35; p. 924) and tricuspid and
to cardiovascular collapse (uncommon). Treatment in all cases is pulmonary atresia (often in the setting of hypoplastic right ventricle
directed towards normalizing heart workload, systemic and pulmonary and functionally univentricular physiology). Neonates with severe cyan-
blood flow, and cardiac output, and may be surgical and/or catheter- otic congenital heart defects may be dependent on the patency of the
based. ductus arteriosus; early diagnosis, treatment with intravenous prosta-
glandin infusion and timely transfer to a tertiary centre for more defini-
Cyanotic cardiac defects may be attributable to a right-to-left intra- or tive therapy is key to survival and good long-term outcomes.
extracardiac shunt or to a severe reduction in pulmonary blood flow. Major advances in the diagnosis and management of infants with
They may be caused by simple lesions such as severe pulmonary steno- congenital cardiac malformations in recent years have resulted in an


sis with an atrial septal defect and right-to-left shunting; moderate increased number of adolescents and adults with palliated or repaired,
lesions, including Fallot’s tetralogy, in which there is a ventricular septal but not cured, congenital heart disease. This is an area that poses mul-
defect, right ventricular outflow obstruction, right ventricular hypertro- tiple challenges to both cardiovascular and other medical disciplines.
phy and an overriding aorta; or more complex lesions, including trans-


HEART
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over preferentially conducting pathways to right and left atria, and to parts of the right atrium (see Figs 57.13A, 57.37). In hearts over 65 years
the atrioventricular node. At the atrioventricular node, the impulse is of age, a layer of connective or fatty tissue between the subendocardium
delayed by 40 ms, allowing the atria to eject their contents fully before and the body of the node may sometimes render it visible to the naked
commencement of ventricular contraction, and also placing an upper eye. Extending on the right from the crest of the right appendage, the
limit on the frequency of signals that can be transmitted to the ventri- node typically courses posteroinferiorly into the upper part of the ter-
cles. The transitional cells interpose between the atrial cardiomyocytes minal groove; in about 1 in 10 hearts it extends in horseshoe fashion
and the nodal cells. There is no transition in humans between the across the crest of the appendage. Nodal tissue does not occupy the full
node and the penetrating bundle, and the cells are virtually identical. thickness of the right atrial wall from epicardium to endocardium but
Here, they become continuous with the more distinctively appearing sits as a wedge of specialized subepicardial tissue within the terminal
Purkinje cells. Conduction of the impulse is rapid in the bundle and groove. Its location is consistently marked by a large central artery,
its branches (2–3 m/sec, as opposed to 0.6 m/sec in normal myocar- which originates from either the proximal part of the right coronary or
dium). The cardiac impulse therefore arrives at the apex of the heart circumflex arteries in equal proportion. Nodal cells are grouped circum-
before spreading through the ventricular walls, producing a properly ferentially around the sinu-atrial nodal artery, packed within a dense
coordinated ventricular ejection. The human heart beats ceaselessly at matrix of connective tissue as interlacing strands of myocytes. They are
70 cycles every minute for many decades, maintaining perfusion of smaller, paler and more empty-looking than working atrial myocardial
pulmonary and systemic tissues. The rate and stroke volume fluctuate fibres, being 5–10 µm in their greatest diameter, with a large central


in response to prevailing physiological demands. The principal events nucleus. Their pale appearance is attributable to the sparsity of
in a cardiac cycle are summarized in Figure 57.20, including the elec- organelles; myofibrils are few and irregularly arranged, and there is no
trical events recorded in the electrocardiogram; the mechanical proper sarcotubular system with little glycogen.
sequences of diastole, atrial systole, volumetric contraction, ejection The sinu-atrial node is described as having a head, body and tail,
and isovolumetric relaxation in ventricular systole; the acoustic phe- from which 5–8 short digitations of nodal tissue radiate towards the
nomena recorded in the phonocardiogram; the pressure profiles of superior vena cava, subepicardium and crista terminalis, penetrating the
right and left hearts and arterial trunks; and the sequences of valvular working atrial myocardium (Figs 57.38–57.39). At the nodal periphery,
events. nodal cells intermingle with slender, fusiform, transitional ‘linking’
Cardiac efficiency depends on precise timing of the operation in cells that are part of a heterogeneous cellular group intermediate in
interdependent structures. Passive diastolic filling of the atria and ven- appearance between nodal cells and normal working atrial myocytes.
tricles is followed by atrial systole, stimulated by discharge from the (For further reading on the use of molecular markers to map the extent
sinu-atrial node, which completes ventricular filling. Excitation and of nodal tissue, see Monfredi et al (2010).) There are no autonomic
contraction of the atria must be synchronous and finish before ventricu- ganglion cells within the node, although many border it anteriorly and
lar contraction; this is effected by a delay in the conduction of excitation posteriorly. Nerve fibres are present but do not appear to contact the
from atria to ventricles. Thereafter, ventricular contraction proceeds in nodal myocytes.
a precise manner. The atrioventricular valves are closed by ventricular
Paranodal area
systole, which must occur prior to closure; ventricular activation must
therefore precede valvular closure, which spreads from the ventricular A paranodal area within the terminal crest, between the cells of the


apices towards the outflow tracts and orifices. sinu-atrial node and the working atrial cardiomyocytes, and possessing
Cardiac contraction originates unequivocally in specialized cardio- properties of both nodal and atrial tissues, has been identified (Molenaar
myocytes, but neural influences are important in adapting the intrinsic et al 2011). Its precise function remains to be determined, but computer
cardiac rhythm to functional demands from the entire body. All cardio- simulations suggest that it is an integral part of normal atrial activation
myocytes are excitable. They display autonomous rhythmic depolariza- and may play a role in pacemaking.
tion and repolarization of their cell membranes, conduction of waves
Internodal atrial myocardium
of excitation via gap junctions to adjacent cardiomyocytes, and
excitation–contraction coupling to their actomyosin complexes. These Specialized pathways have been alleged to connect the sinus and atrio-
properties are developed to different degrees in different sites and in ventricular nodes, but there is no evidence that tracts insulated from
different types of cardiomyocyte (Ch. 6). The rate of depolarization and the working myocardium, as occurs in the ventricular conduction path-
repolarization is slowest in the ventricular myocardium, intermediate ways, exist within the atrial walls. The impulse generated by the sinu-
in the atrial muscle, and fastest in the myocytes of the sinu-atrial node. atrial node is conducted more rapidly via the long axis of the atrial
The latter override those generating slower rhythms and, in the normal muscle bundles than it is transversely. The main pathways for conduc-
heart, are the locus for the rhythmic initiation of cardiac cycles. Con- tion towards the atrioventricular node are the terminal crest and the
versely, conduction velocity is slow in nodal myocytes, intermediate in margins of the oval fossa. Conduction to the left atrium is preferentially
general ‘working’ cardiac myocytes, and fastest in the myocytes of the through Bachmann’s bundle; additional pathways occur through the
ventricular conduction system. margins of the oval fossa and through the muscular connections
The nodes and networks of the so-called specialized myocardial cells between the walls of the coronary sinus and left atrium. During devel-


constitute the cardiac conduction system (Figs 57.36–57.37). The com- opment, the walls of the systemic venous sinus are exclusively com-
ponents of this system are the sinus and atrioventricular nodes, the posed of primary myocardium. Although pathways within these walls
atrioventricular bundle with its left and right bundle branches, and the may be identified in prenatal stages, primary myocardium is slowly
subendocardial plexus of ventricular conduction cells (Purkinje cells). conducting, which means that these pathways do not represent areas of
The main pacemaker rhythm of the heart (sinus rhythm) is generated preferentially rapid conduction.
within the system but is influenced by nerves. It is transmitted specifi-
Atrioventricular node
cally from atria to ventricles via the atrioventricular node and bundle.
The spread of excitation is very rapid but not instantaneous. Different The atrioventricular node is an oblique, half-oval atrial structure, located
parts of the ventricles are excited at slightly different times, with impor- within the atrial component of the muscular atrioventricular septum
tant functional consequences. Failure of the conduction system will not and separated from the ventricular musculature by the insulating tissues
block cardiac contraction but the system will become poorly coordi- of the atrioventricular groove (see Fig. 57.37). Its anatomical landmarks
nated. The rhythm will be slower because it originates from a spontane- are the boundaries of the triangle of Koch, i.e. the attachment of the
ous (myogenic) activity in the working cardiac myocytes or in a septal leaflet of the tricuspid valve inferiorly, the ostium of the coronary
subsidiary pacemaker in a more distal part of the diseased or disrupted sinus basally and the tendon of Todaro superiorly (see Fig. 57.13). The
conduction system. There are no specialized internodal and interatrial atrial aspect of the node is convex and overlain by atrial myocardium.
conduction pathways; the excitation emanating from the sinu-atrial Its left margin is concave and abuts on to the superior aspect of the
node spreads to the atrial musculature and to the atrioventricular node central fibrous body. The basal end projects into the atrial muscle and


through ordinary atrial working myocardium. The geometric arrange- the anteroinferior end enters the central fibrous body to become the
ment of fibres along well-organized atrial muscle bundles, e.g. the ter- penetrating atrioventricular bundle.
minal crest, the rims of the fossa ovalis and Bachmann’s bundle, ensures The node consists of two zones, compact and transitional, pervaded
that conduction is marginally more rapid than elsewhere within the by an irregular collagenous reticulum that enmeshes the myocytes (Fig.
atrium. 57.40). The compact zone consists of frequently stratified, interlocking
nodal cells. Cells from the right and left atrial walls and the atrial
Sinu-atrial node
septum feed directly into the compact zone. The transitional zone
In the majority of hearts, the sinu-atrial node is a crescent-shaped struc- envelops the compact portion of the node and consists of elongated
ture, between 8 and 25 mm long, located at the embryonic junction ‘transitional cells’, intermediate in morphology and function between
between the venous part (sinus venosus) and the atrium proper derived compact nodal cells and working atrial cardiomyocytes.


Heart
1014.e1
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RETPAHC
Sinu-atrial node artery Terminal crest
Endocardium
A
F
Width Terminal crest
A B C G
D E F
Detail of panel B
5 mm
Fig. 57.38 Measurements of the width and height of the sinu-atrial node and the distances of nodal tissue to epicardium (a) and endocardium (b),
measured in cadaveric tissue. (Top) A gross cadaveric specimen together with an example of a histological section and a diagram to indicate where
measurements were made on each histological section. (Bottom) Histological sections (Masson trichrome stain) taken through levels A–F as indicated on
the gross specimen. (G) The red dotted line delineates the nodal boundaries. Note the irregular contour of the node and the extensions towards the
neighbouring myocardium (arrows). Abbreviations: SVC, superior vena cava. (With permission from Sánchez-Quintana D, Cabrera JA, Farré J, Climent V,
Anderson RH, Ho SY. 2005. Sinu-atrial node revisited in the era of electroanatomical mapping and catheter ablation. 2005 91:189–94.)
muidracipE
Terminal groove SVC
Height
b
a
Histological sections
Head
Body Body
Body Tail Tail
f
f
f
f
f
A B C
Fig. 57.39 Histological sections of the nodal body. A, Fibro-fatty tissue (f) between the caudal aspect of the nodal body and the subendocardium.
B–C, Fragmentation of the nodal tail into clusters. Masson trichrome stain. (With permission from Sánchez-Quintana D, Cabrera JA, Farré J, Climent V,
Anderson RH, Ho SY. 2005. Sinu-atrial node revisited in the era of electroanatomical mapping and catheter ablation. 2005 91:189–94.)


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Atrial Transitional Compact Atrial Penetrating Fig. 57.40 Histological sections showing the
overlay cells cells AV node myocardium AV bundle features of (A) the atrioventricular (AV) node and
(B) the penetrating atrioventricular bundle.
Trichrome method, fibrous tissue stained green.
(With permission from Anderson RH, Boyett MR,
Dobrzynski H, Moorman AF. 2013. The anatomy
of the conduction system: implications for the
clinical cardiologist. J Cardiovasc Transl Res.
6:187–196.)
A B
Atrial Insulating Ventricular Insulating Ventricular
septum tissues septum tissues myocardium


Heart
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RETPAHC
A
Aorta
Right pulmonary artery Right appendage
Pulmonary valve
Superior vena cava
Sinu-atrial node
Aortic mound
Fossa ovalis Membranous part of
interventricular septum
Tendon of Todaro Radiation of left branch
Valve of inferior vena cava Right bundle branch
Coronary sinus
Septal leaflet of tricuspid valve
Inferior vena cava
Atrioventricular node
Septomarginal trabecula
(moderator band)
Papillary muscles
B
Aorta
Left appendage
Left pulmonary artery
Cut chordae tendineae
of mitral valve
Right pulmonary veins
Superoposterior papillary muscle
Aortic valve
Radiation of left branch
Inferior vena cava
Fig. 57.36 The conduction tissue of the heart. A, Right aspect. B, Left aspect. The elements of the conduction system are shown in purple. Conduction
tissue accompanies fine trabeculae carneae and false chordae. In reality, the radiation of the left bundle branch is directly related to the leaflets of the
aortic valve.
Extensions of nodal cardiomyocytes run in the direction of the coro-
Atrioventricular bundle
nary sinus along the tricuspid anulus (the putative ‘slow pathway’); in
the anterior portion of the triangle of Koch near the compact portion of The atrioventricular bundle (of His) is the direct continuation of the
the atrioventricular node (the putative ‘fast pathway’); and in the direc- atrioventricular node. It becomes oval, quadrangular or triangular in
tion of the mitral anulus (the left atrial extension) (Mani and Pavri transverse sectional profile as it enters the central fibrous body. It
2014). In both sinus and atrioventricular nodes, the intercellular con- traverses the fibrous body and branches on the crest of the muscular
tacts between nodal cells, and between nodal and transitional cells, are interventricular septum; the branching tract is sandwiched between the
much less specialized than the intercalated discs between normal cardiac muscular and the membranous components of the septum.


cells (p. 137). A sparsity of gap junctions is consistent with the absence The right branch of the bundle (crus dextrum) is a narrow, discrete,
from these areas of connexin-43 (a major protein component of mam- rounded group of fascicles that courses at first within the myocardium
malian gap junctions), and probably accounts for the observed difficulty and then subendocardially towards the ventricular apex, entering the
in exciting these cells from adjacent cells. The atrioventricular delay may septomarginal trabecula to reach the anterior papillary muscle. Its
owe much to this relative non-excitability, which appears to disturb the branches to the ventricular walls are few in its septal course. At the
spread of potential, delaying propagation; the narrow diameter of the origin of the anterior papillary muscle, it divides profusely into fine
transitional cells may also contribute to conduction delay. subendocardial fascicles that diverge, first embracing the papillary
The arterial supply to the atrioventricular node is from a character- muscle, then recurving subendocardially for distribution to the remain-
istic vessel that originates from the dominant coronary artery at the ing ventricular walls.
cardiac crux. Autonomic ganglia are present between the node and the The left branch (crus sinistrum) arises as numerous fine, intermin-
coronary sinus. gling fascicles that leave the left margin of the branching bundle through


HEART
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NOITCES
oblique inverted crown within the atrioventricular groove. They also
R
form a variable and often insignificant anastomosis (in the non-
pathological state) via marginal and interventricular (descending)
loops that intersect at the cardiac apex. The main arteries and major
P Q S T
branches are usually subepicardial, but those in the atrioventricular and
interventricular grooves are often deeply sited, and occasionally hidden
by overlapping myocardium or embedded in it (myocardial bridging).
The term ‘dominant’ is used to refer to the coronary artery giving off
the posterior interventricular (posterior descending or inferior interven-
Fibro-fatty atrioventricular
groove (separation of atrial tricular) branch, which supplies the posterior (inferior) part of the
SVC and ventricular myocardium) ventricular septum and often part of the posterolateral (inferolateral)
wall of the left ventricle. The right artery is the dominant artery in 60%
LA
of hearts. Anastomoses between right and left coronary arteries are
Sinu-atrial node
(pacemaker) abundant in the fetus but are much reduced by the end of the first year
RA of life. Anastomoses providing collateral circulation may become prom-
Atrioventricular inent in conditions of chronic hypoxia and in coronary artery disease.
node (delay)
An additional collateral circulation is provided by small branches from
Atrioventricular mediastinal, pericardial and bronchial vessels.
bundle and branches
(insulated) Coronary arterial original calibre, based on arterial casts or angio-
gram measurements, ranges between 1.5 and 5.5 mm. The calibre of
the left origin exceeds the right in 60% of hearts, the right being larger
in 17%, and both vessels being of approximately equal calibre in 23%.
The external diameter of the left coronary artery increases from 2.1 mm
at the age of 1 year to 3.3 mm at the age of 15 years (Pesonen et al 1991)
Purkinje fibres and the diameters of the coronary arteries may increase up to the 30th
(activation) year. A significant association has been found between the diameters of


the right and left coronary arteries at birth and at 1 and 6 months of
Fig. 57.37 The basic structure of the conduction system and its
age and birth weight, height and body surface area (Karagol et al 2012).
relationship with the electrocardiogram. Note the foramen ovale allowing
for communication between the right atrium (RA) and the left atrium (LA).
Right coronary artery
Other abbreviations: SCV, superior vena cava. (Redrawn by courtesy of
Professor RH Anderson, Institute of Child Health, University College, The right coronary artery arises from the anterior (‘right coronary’)
London.) aortic sinus; its ostium is usually below the sinutubular junction. The
artery is usually single but as many as four right ostia have been
observed, reflecting the independent origins of the conal, sinu-atrial
node and ventricular arteries (Figs 57.42–57.43). The right coronary
most of its course along the crest of the muscular ventricular septum artery passes at first anteriorly and slightly to the right between the right
(see Fig. 57.36). These fascicles form a flattened sheet down the smooth atrial appendage and pulmonary trunk, where the sinus usually bulges.
left ventricular septal surface that diverges apically and subendocar- On reaching the atrioventricular groove, it descends almost vertically to
dially across the left aspect of the ventricular septum, trifurcating into the right (acute) cardiac border, curving around it into the posterior
anterior, septal and posterior divisions. Fine branches leave the sheets, (inferior) part of the groove, where the latter approaches its junction
forming subendocardial networks, which first surround the papillary with both interatrial and interventricular grooves, the appropriately
muscles and then curve back subendocardially to be distributed to all termed cardiac crux. The artery ends a little to the left of the crux, often
parts of the ventricle. The principal branches of the bundle are insulated by anastomosing with the circumflex branch of the left coronary artery.
from the surrounding myocardium by sheaths of connective tissue. The right coronary artery may end near the right cardiac border, between
Functional contacts between ventricular conduction and working this and the crux, or more often it reaches the left border, replacing the


myocytes become numerous only in the subendocardial terminal rami- more distal part of the circumflex artery. Its branches supply the right
fications. Hence, papillary muscles contract first, followed by a wave of and variable parts of the left chambers and atrioventricular septum.
excitation and ensuing contraction that travels from the apex of the Usually, the first branch is the right conal artery (conus artery, artery of
ventricle to the arterial outflow tract. Because the Purkinje network is the conus, arteria coni arteriosi; Loukas et al 2014b). (This vessel arises
subendocardial, muscular excitation proceeds from endocardium to independently from the anterior aortic sinus in approximately one-
epicardium. third of hearts and is therefore sometimes termed the ‘third coronary
In the developing heart, the bundle responsible for atrioventricular artery’; a similarly named vessel arises from the left coronary circulation
conduction is a much more extensive structure. Immunohistochemical and so this title is inappropriate.) The right conal artery ramifies antero-
analysis has revealed that the precursor of the system is a ring of cells inferiorly over the pulmonary conus and over the superior aspect of the
that surrounds the inlet and outlet components of the developing ven- right ventricle, sometimes anastomosing with a similar branch from the
tricular loop (Ch. 52). left interventricular (anterior descending) artery to form the anulus of
Vieussens, a tenuous anastomosis around the right ventricular outflow
Cardiac pacing
tract (Figs 57.44–57.45) (Loukas et al 2009).
The first segment of the right coronary artery (between its origin and
Available with the Gray’s Anatomy e-book the right cardiac margin) gives off anterior atrial and ventricular
branches that diverge widely, approaching a right angle in the case of
Cardiac conduction studies the ventricular arteries, an arrangement that is in marked contrast to
the more acute origins of the left coronary ventricular branches. The
Available with the Gray’s Anatomy e-book anterior ventricular branches, usually two or three, ramify towards the
cardiac apex, which they rarely reach (unless the right marginal artery
Congenital conduction abnormalities is included within this group of branches, as it is by some authors). The
right marginal artery is greater in calibre than the other anterior ven-


tricular arteries and long enough to reach the apex in most hearts. When
Available with the Gray’s Anatomy e-book
it is very large, there may be only one other remaining anterior ven-
tricular branch, or even no other branches. Up to three small posterior
(inferior) ventricular branches, most often two, arise from the second
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE
segment of the right coronary artery between the right border and crux
to supply the right ventricular diaphragmatic aspect. Their size is
Coronary arteries
inversely proportional to that of the right marginal artery, which usually
extends to the cardiac diaphragmatic surface. On approaching the crux,
The right coronary artery arises from the anterior (‘right coronary’) sinus the right coronary artery normally produces up to three posterior (infe-
and the left coronary artery from the left posterior (‘left coronary’) sinus rior) interventricular branches (occasionally none). The posterior (infe-
of the ascending aorta (see Figs 57.21–57.22; Fig. 57.41); ostia levels rior) interventricular artery itself, lying within the interventricular
vary. The two arteries, as indicated by their generic name, form an groove, may therefore be flanked, either to the right or to the left, or on


Heart
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This ring is in continuity with the atrioventricular bundle, itself part
of an interventricular ring of specialized cardiomyocytes. The segment
of the interventricular ring that encircles the developing left ventricular
outflow tract subsequently breaks down. The segment that persists
within the ventricles extends along the septum beyond the branching
atrioventricular bundle and forms a dead-end tract. The dorsal compo-
nent of the interventricular ring is part of the definitive atrioventricular
node, continuous with the atrioventricular bundle. The rightward com-
ponent of the atrioventricular ring system, emerging inferiorly from the
atrioventricular node, is part of the slow nodal pathway. The cardio-
myocytes that formed the ring initially possessed a primary phenotype,
i.e. they would have conducted slowly.
Wolff–Parkinson–White syndrome is caused by abnormal small
strands of otherwise unremarkable ventricular myocardium that run
through the fibroareolar tissue of the atrioventricular groove, connect-
ing the atrial and ventricular myocardial masses at some point around
the atrioventricular junctions.
Temporary pacing wires are usually inserted by cannulation of either
the internal jugular or subclavian veins; the latter approach carries a
slightly greater risk of a pneumothorax because of the proximity of the
pleural cavity. Other potential risks are brachial plexus injury if the entry
Conal artery Right coronary artery
site is too posterior, and thoracic duct injury if the left subclavian vein
Fig. 57.43 The right coronary sinus, revealing a coronary orifice for the
is cannulated. The most common location for permanent pacemaker
right coronary artery and a coronary orifice for the conal artery. (With
devices is subcutaneously on the anterior chest wall. Access to the
permission from Loukas M, Sharma A, Blaak C, Sorenson E, Mian A.
chambers and endocardium of the right heart is gained via the cephalic
2013. The clinical anatomy of the coronary arteries. J Cardiovasc Transl
vein within the deltopectoral groove.
Res. 6:197–207.)
Intracardiac electrocardiography and electrophysiology are used to
assess cardiac conduction and rhythm abnormalities. A catheter is


inserted via the femoral, subclavian or internal jugular veins using a
guidewire technique. Fluoroscopy, echocardiography and, more
recently, cardiac magnetic resonance are used to guide accurate place- Aorta
ment of the catheters to the appropriate position. The sites of study are
Right
the high right atrium (for assessing the atrioventricular bundle and right coronary
bundle branch) and the coronary sinus (for evaluating atrioventricular artery
junctional arrhythmias and accessory pathways). The multipolar elec- Conal artery
trodes provide detailed electro-anatomical mapping of the sequence of
excitation from the atria, atrioventricular junction and ventricles. The
*
origin of supraventricular arrhythmias, ventricular tachycardias, acces-
sory conduction pathways and re-entrant pathways can be identified Anterior
and used to guide radiofrequency ablation. interventricular
artery
In the past, there was an anatomical basis for the majority of presenting
congenital conduction abnormalities: they were the product of either Ventricular
accessory pathways or conduction tissue dysgenesis at any point from branches
the atrioventricular node to the atrioventricular bundle. Today, conduc-
tion abnormalities are increasingly likely to relate to long-standing
haemodynamic problems and/or the effects of previous surgery for
**
patients with congenital heart defects. This reflects the fact that, although
surgery for most of these defects has been available for the past four
decades, surgery itself has not been curative; more often than not,
patients develop conduction abnormalities and arrhythmia from surgi- ***
cal scars that cause haemodynamic problems such as chamber dilation
and/or hypertrophy. Occasionally, conduction abnormalities are caused
by tumours such as multifocal Purkinje cell tumours, or benign con-
genital polycystic tumours of the atrioventricular node.
Congenital abnormalities of the coronary arteries are found rarely
in children. The two most common abnormalities are coronary arterio-
venous fistula and anomalous left coronary artery arising from the
Fig. 57.44 A variation of the anulus of Vieussens (*). This corrosion cast
pulmonary artery (Uysal et al 2014). Other congenital abnormalities
specimen shows the right conal artery, a branch of the right coronary


include ectopic origin of left circumflex artery from right coronary
artery, anastomosing with a proximal ventricular branch of the anterior
artery, single coronary artery arising from right sinus of Valsalva, and interventricular artery. A second anastomosis (**) occurs between the
ectopic right coronary artery arising from the left sinus of Valsalva ventricular branches of the right coronary and anterior interventricular
(Clemente et al 2010). arteries. Close to the apex, a third anastomosis (***) appears between the
distal part of the anterior interventricular artery and branches of the acute
marginal artery. (With permission from Loukas M, Bilinsky S, Bilinsky E,
Matusz P, Anderson RH. 2009. The clinical anatomy of the coronary
collateral circulation. Clin Anat. 22:146–160.)


Heart
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A B
Arch of aorta
Pulmonary trunk
Left coronary artery (main stem)
Superior vena cava
Anterior interventricular
(descending) artery
Sinu-atrial nodal
artery Circumflex artery
Left atrial auricle
Right coronary artery Left atrial rami
Outlines of: Left conal artery
Anterior aortic sinus Circumflex artery Sinu-atrial artery
Right posterior aortic sinus Left (obtuse)
Left posterior aortic sinus marginal artery C
Right conal artery Diagonal artery
Right anterior
ventricular arteries
Atrioventricular
Interventricular
nodal artery
anterior septal branches
Inferior interventricular
(descending) arteries
Right (acute)
marginal artery
D E
Ascending aorta
Right posterior
Pulmonary trunk Superior vena cava atrial arteries
Left pulmonary veins
Stem of posterior Right pulmonary veins
atrial arteries
Circumflex artery Region of
crux of heart
Inferior vena cava
Right coronary artery
Anterior interventricular
(descending) artery (termination) Inferior interventricular
(descending) arteries
Fig. 57.41 Anterior views of the coronary arterial system, with the principal variations. The right coronary arterial tree is shown in purple, the left in red.
In both cases, posterior distribution is shown in a paler shade. A, The most common arrangement. B, A common variation in the origin of the sinu-atrial
nodal artery. C, An example of left ‘dominance’ by the left coronary artery, also showing an uncommon origin of the sinu-atrial nodal artery.
Posteroinferior views of the coronary arterial system. The right coronary arterial tree is shown in purple, the left in red. D, An example of the more normal
distribution in right ‘dominance’. E, A less common form of left ‘dominance’. In these ‘posterior’ views, the diaphragmatic (inferior) surface of the
ventricular part of the heart has been artificially displaced and foreshortening ignored, to clarify the details of the so-called posterior (inferior) distribution
of the coronary arteries.


HEART
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Aorta Fig. 57.42 A dissection of a cadaveric
heart in which both coronary arteries
have been exposed to reveal their
branches to the right and left ventricles,
respectively. Notice how the great
cardiac vein is intertwined around the
Left atrium
anterior interventricular artery.
Left coronary (Specimen courtesy of M Loukas MD,
Arterial branch artery PhD.)
Right coronary Sinu-atrial nodal
artery artery
Acute marginal Circumflex
artery coronary artery
Coronary sinus
Diagonal arteries
Ventricular branches
Anterior
Infundibular branch interventricular
artery
Great cardiac vein
Conal artery Anterior lateral branches – occasionally double, and very rarely triple – mainly
interventricular artery supply the right atrium. The posterior branch is usually single and sup-
plies both atria.
The artery of the sinu-atrial node is an atrial branch, distributed
largely to right atrial myocardium. Its origin is variable: most commonly,
it arises from the anterior atrial branch of the right coronary artery, less
*
often from its right lateral part, and least often from its posterior atriov-
entricular part. This ‘nodal’ artery thus usually passes posteriorly in the
groove between the right atrial appendage and aorta. It may originate
from the circumflex branch of the left coronary artery. Whatever its
** origin, it usually branches around the base of the superior vena cava,
typically as an arterial loop from which small branches supply the right
atrium. A large branch, the ramus cristae terminalis, traverses the sinu-
atrial node; it would seem more appropriate to name this branch the
‘nodal artery’, on the grounds that the vessel that currently bears this
name actually supplies the atria and serves as the ‘main atrial branch’.
Septal perforating branches of the right coronary artery are relatively
*** short, and leave the posterior (inferior) interventricular branch to
supply the posterior interventricular septum. They are numerous but
do not usually reach the septal apex. The largest posterior septal perfo-
rating artery, usually the first, commonly arises from the inverted loop
Right coronary Ventricular
artery branches said to characterize the right coronary artery at the crux; it almost always


supplies the atrioventricular node. Small, recurrent atrioventricular
Fig. 57.45 A coronary angiogram demonstrating three collateral arterial
branches originate from the ventricular branches of the right coronary
anastomoses. At the area of the subpulmonary infundibulum, the right
artery as they cross the atrioventricular groove, and supply adjacent
conal artery, a branch of the right coronary artery, anastomoses with the
atrial myocardium.
left conal artery, a branch of the anterior interventricular artery, to form
the anulus (arterial circle or vascular ring) of Vieussens (*). Ventricular
Left coronary artery
branches of the right coronary artery anastomose with the proximal (**)
and distal portions (***) of the anterior interventricular artery, forming two The left coronary artery is usually larger in calibre than the right. It
collateral pathways. As a result, there is contrast medium in the anterior supplies a greater volume of myocardium, including almost all of the
interventricular artery. (With permission from Loukas M, Bilinsky S, left ventricle and atrium, and most of the interventricular septum (see
Bilinsky E, Matusz P, Anderson RH. 2009. The clinical anatomy of the Fig. 57.42; Figs 57.49–57.51). In hearts with ‘right dominance’, the
coronary collateral circulation. Clin Anat. 22:146–160.) right coronary artery supplies a variable posterior region of the left
ventricle (see Fig. 57.41A–C).
The left coronary artery arises from the left posterior (left coronary)
aortic sinus; the ostium sometimes lies inferior to the margin of the
both sides, by these parallel branches. When these flanking vessels exist, leaflets and may be double, leading into major initial branches, usually
branches of the posterior (inferior) interventricular artery are small and the circumflex and anterior interventricular (descending) arteries. Its
sparse. The posterior (inferior) interventricular artery is occasionally initial portion, between its ostium and its first branches, varies in length
replaced by a left coronary branch. Although the atrial branches of the from a few millimetres to a few centimetres. The artery lies between the


right coronary artery are sometimes described as part of anterior, lateral pulmonary trunk and the left atrial appendage, emerging into the atrio-
(right or marginal) and posterior groups, they are usually small single ventricular groove, where it turns left. This part is loosely embedded in
vessels, 1 mm in diameter (Figs 57.46–57.48). The right anterior and subepicardial fat and usually has no branches, but may give off a small


Heart
1018.e1
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RETPAHC
A PA
Fig. 57.47 Normal ECG-gated multidetector row CT anatomy of the right
coronary artery and its branches. This lateral oblique volume-rendered
image shows the caudal course of the proximal right coronary artery (long
Fig. 57.46 Normal ECG-gated CT anatomy of the right coronary artery
arrow), which gives off an acute marginal branch (short arrows) to the
and its branches. This oblique volume-rendered image of the superior
right ventricle. (With permission from Kim SY, Seo JB, Do KH, et al. 2006.
aspect of the heart shows the right coronary artery (arrow) arising from
Coronary artery anomalies: classification and ECG-gated multi-detector
the right sinus of Valsalva and coursing in the right atrioventricular groove
row CT findings with angiographic correlation. Radiographics. 26:317–33.)
towards the posterior interventricular septum. Abbreviations: A, aorta; PA,
pulmonary artery. The conal artery and the sinu-atrial nodal artery were
too small to be seen in this case. (With permission from Kim SY, Seo JB,
Do KH, et al 2006. Coronary artery anomalies: classification and
ECG-gated multi-detector row CT findings with angiographic correlation.
Radiographics. 26:317–33.)
PA
A
Fig. 57.49 Normal ECG-gated multidetector row CT anatomy of the left
coronary artery and its branches. This oblique volume-rendered image of
Fig. 57.48 Normal ECG-gated multidetector row CT anatomy of the right the top of the heart shows the left coronary artery (long white arrow)
coronary artery and its branches. This posterior oblique volume-rendered arising from the left sinus of Valsalva and trifurcating into the left anterior
image shows that the distal right coronary artery divides into the posterior interventricular artery (thick black arrow), the left circumflex artery (thin
(inferior) interventricular artery (long black arrow) and posterior left black arrow), and the ramus intermedius (short white arrow), which takes


ventricular branches (short black arrows). The posterior (inferior) a course similar to that of the usual first diagonal branch. The left anterior
interventricular artery courses in the posterior (inferior) interventricular interventricular artery then gives rise to diagonal branches (short black
groove, parallel to the middle cardiac vein (white arrow). (With permission arrows) to the anterior free wall of the left ventricle. Abbreviations: A,
from Kim SY, Seo JB, Do KH, et al. 2006. Coronary artery anomalies: aorta; PA, pulmonary artery. (With permission from Kim SY, Seo JB, Do
classification and ECG-gated multi-detector row CT findings with KH, et al. 2006. Coronary artery anomalies: classification and ECG-gated
angiographic correlation. Radiographics. 26:317–33.) multi-detector row CT findings with angiographic correlation.
Radiographics. 26:317–33.)


HEART
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NOITCES
A
PA
Fig. 57.50 An anterior oblique volume-rendered image showing the left
anterior ventricular artery (thin black arrows) coursing along the anterior
interventricular groove, and the left circumflex artery (thick black arrow)
coursing in the left atrioventricular groove. Obtuse marginal branches
(short white arrow) and diagonal branches (short black arrows) are also Fig. 57.51 A posterior oblique volume-rendered image showing the left
shown. Abbreviations: A, aorta; PA, pulmonary artery. (With permission anterior ventricular artery (long white arrows) coursing along the anterior
from Kim SY, Seo JB, Do KH, et al. 2006. Coronary artery anomalies: interventricular groove, and the left circumflex artery (thick black arrow)
classification and ECG-gated multi-detector row CT findings with coursing in the left atrioventricular groove. Obtuse marginal branches
angiographic correlation. Radiographics. 26:317–33.) (short white arrows) and a diagonal branch (short black arrow) are also
shown. (With permission from Kim SY, Seo JB, Do KH, et al 2006.
Coronary artery anomalies: classification and ECG-gated multi-detector
row CT findings with angiographic correlation. Radiographics. 26:317–33.)


Heart
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RETPAHC
Fig. 57.52 A dissection of a cadaveric
heart revealing the course of the left
Aorta
coronary artery and its branches. (With
Superior
vena cava permission from Loukas M, Sharma A,
Blaak C, Sorenson E, Mian A. 2013. The
clinical anatomy of the coronary arteries.
J Cardiovasc Transl Res. 6:197–207.)
Conal artery
Left atrium
Circumflex
Right atrium artery
Diagonal artery
Right coronary
artery
Anterior
interventricular
artery
Acute marginal
artery
atrial ramus and, rarely, the sinu-atrial nodal artery. Reaching the atrio- posterior branches of the circumflex artery also supply the left ventricle.
ventricular groove, the left coronary artery divides into its main branches: Anterior ventricular branches (from one to five, commonly two or
namely, the circumflex and anterior interventricular arteries. three) course parallel to the diagonal artery when it is present, and
The anterior interventricular artery is commonly described as the replace it when it is absent. Posterior ventricular branches are smaller
continuation of the left coronary artery. It descends obliquely forwards and fewer because the left ventricle is partly supplied by the posterior
and to the left in the interventricular groove (Fig. 57.52), sometimes (inferior) interventricular artery. When this artery is small or absent, it
deeply embedded in or crossed by bridges of myocardial tissue (myo- is accompanied or replaced by often doubled or tripled interventricular
cardial bridges), and by the great cardiac vein and its tributaries. continuations of the circumflex artery.
Almost invariably, the anterior interventricular artery reaches the The artery to the sinu-atrial node is often derived from the anterior
apex, where it terminates in one-third of hearts. More frequently, it circumflex segment (less often from the circum-marginal segment). It
turns round the apex into the posterior interventricular groove and passes over and supplies the left atrium, encircles the superior vena cava
passes one-third to one-half of the way along its length, meeting the (like a right coronary nodal branch) and sends a large branch through


terminal twigs of the posterior (inferior) interventricular branches of the node; despite its name, it is predominantly atrial in distribution.
the right coronary artery (see above). The artery to the atrioventricular node, sometimes the terminal branch
The anterior interventricular artery gives off right and left anterior of the circumflex artery (20%), arises near the crux. When this occurs,
ventricular and anterior septal branches, and a variable number of cor- the circumflex artery usually gives off the posterior interventricular
responding posterior branches. Anterior right ventricular branches are artery, an example of so-called ‘left dominance’.
small and rarely number more than one or two; the right ventricle is
Coronary arterial distribution
supplied almost entirely by the right coronary artery. Up to nine large
left anterior ventricular arteries branch at acute angles from the anterior Details of coronary arterial distribution require integration into a
interventricular artery, crossing the anterior aspect of the left ventricle concept of total cardiac supply. Most commonly, the right coronary
diagonally, with the largest reaching the rounded (obtuse) left cardiac artery supplies all of the right ventricle (except a small region to the
border. One often dominates, sometimes arising separately from the right of the anterior interventricular groove); a variable part of the dia-
left coronary trunk, which then ends by trifurcation. This left diagonal phragmatic aspect of the left ventricle; the posteroinferior third of the
artery, reported to exist in at least 33–50% of hearts, may be doubled interventricular septum; the right atrium and part of the left atrium; and
(20%). A small left conal artery frequently leaves the anterior interven- the conduction system as far as the proximal parts of the right and left
tricular artery near its origin, and anastomoses on the conus with its crura. Left coronary distribution is reciprocal and includes most of the
counterpart from the right coronary artery and with the vasa vasorum left ventricle; a narrow strip of right ventricle; the anterior two-thirds of
of the pulmonary artery and aorta. The anterior septal perforating the interventricular septum; and most of the left atrium. As noted previ-


branches leave the anterior interventricular artery almost perpendicu- ously (see Fig. 57.41), variations in the coronary arterial system mainly
larly, and pass posteroinferiorly within the septum, usually supplying affect the diaphragmatic aspect of the ventricles and reflect the relative
its ventral two-thirds. The first septal perforator artery usually supplies ‘dominance’ of coronary arterial supply. The term is misleading because
the atrioventricular bundle at the point of its division. Small posterior the left artery almost always supplies a greater volume of tissue than
septal branches from the same source supply the posterior third of the the right. In ‘right dominance’, the posterior (inferior) interventricular
septum for a variable distance from the cardiac apex (Loukas et al artery is derived from the right coronary artery; in ‘left dominance’, it
2009). is derived from the left coronary artery. In the so-called ‘balanced’
The circumflex artery, comparable to the anterior interventricular pattern, branches of both arteries run in or near the posterior (inferior)
artery in calibre, curves left in the atrioventricular groove, and continues interventricular groove.
round the left cardiac border into the posterior part of the groove, ter- Less is known about variations in atrial supply because the small
minating left of the crux in most hearts, although sometimes continu- vessels involved are not easily preserved in the corrosion casts that are
ing as the posterior (inferior) interventricular artery. Proximally, the left used for analysis. In more than 50% of individuals, the right atrium is
atrial appendage usually overlaps it. A large ventricular branch, the left supplied only by the right coronary artery, and in the remainder the
marginal artery, normally arises perpendicularly from the circumflex supply is dual. More than 62% of left atria are supplied mainly by the
artery and ramifies over the rounded ‘obtuse’ margin, supplying much left coronary artery, 27% by the right coronary artery (in each group a
of the adjacent left ventricle, typically to its apex. Smaller anterior and small accessory supply from the other coronary artery exists), and 11%


Heart
1019.e1
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RETPAHC
Aorta
Pulmonary
vein
Pulmonary Left auricle
trunk
Conal artery Circumflex
artery
Right coronary Diagonal artery
artery
Myocardial
bridge
Anterior
interventricular
artery
Fig. 57.53 A dissection of a cadaveric heart in which a large part of the anterior interventricular artery is covered by a myocardial bridge.
Myocardial bridges are reported to have a frequency varying from
0.5 to 40% when identified clinically and from 15 to 85% when found
at autopsy (Fig. 57.53). The wide variation in frequency indicates that
many bridges may be asymptomatic during life. The major clinical
conditions produced by a myocardial bridge are cardiac ischaemia,
atherosclerosis and sudden cardiac death. The incidence of atheroscle-
rosis is increased when the right coronary artery is bridged. Although a
relationship between myocardial bridges and sudden cardiac death has
not been established, autopsy series have shown histological evidence
of otherwise unexplained ischaemia in individuals with myocardial
bridges; many died during exercise and had no other risk factors for
coronary arterial disease.


HEART
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7
NOITCES
are supplied almost equally by both arteries. Arterial supply to the sinu- develop later in life. Congenital fistulae are more common and account
atrial and atrioventricular nodes also varies: the sinu-atrial node is for 50% of paediatric coronary vascular aberrations; they are believed
supplied more often by the right coronary artery; fewer than 10% of to be derived from Thebesian vessels. Acquired coronary artery fistulae
sinu-atrial nodes receive a bilateral supply. The atrioventricular node is are most commonly iatrogenic in aetiology but may also occur after
usually supplied by the right coronary artery. traumatic injury; these most commonly are of the coronary cameral
type, from the right coronary artery into the right side of the heart.
Coronary anastomoses
The cardiac collateral circulation represents a native system for coronary Cardiac veins
arterial bypass. The first few centimetres of the arterial main stems are
devoid of anastomotic branches, but further distally, collateral channels
The cardiac venous system is divided into two major parts. The greater
are abundant, exhibit variable calibres and occupy numerous locations,
system consists of large vessels that lie within the subepicardial myo-
allowing for bidirectional flow between most native arteries. Approxi-
cardium and drain most of the outer myocardium. These veins extend
mately 30 different sites of collateral extramural vessels have been
over the myocardial surface and do not adhere to topographical borders,
described, the most frequent being at the apex; the anterior aspect of
although the larger collecting vessels lie within the interventricular
the right ventricle; the posterior aspect of the left ventricle; the crux;
grooves before draining into the coronary sinus. The intercommunicat-
the interatrial and interventricular grooves; and between the sinu-atrial
ing parts of this system are the coronary sinus and its tributaries, the
nodal and other atrial vessels (see Figs 57.44, 57.45). Anastomoses
anterior cardiac venous system, and the ventricular septal and atrial
between branches of the coronary arteries, both subepicardial and myo-
veins. The coronary sinus and its tributaries return blood to the right


cardial, and between these arteries and extracardiac vessels, are of prime
atrium from the entire heart (including its septa), except for the anterior
medical importance. Clinical studies suggest that anastomoses cannot
region of the right ventricle and small, variable parts of both atria and
rapidly provide collateral routes sufficient to circumvent sudden coro-
the left ventricle. The anterior cardiac veins drain an anterior region of
nary obstruction (see Figs 57.44, 57.45, 57.54). Nevertheless, it has long
the right ventricle, expanding to include a region around the right
been established that anastomoses do occur, particularly between fine
cardiac border when the right marginal vein joins this group.
subepicardial branches, and they may increase during individual life by
The smaller system functions primarily to return venous blood from
mechanisms of angiogenesis and arteriogenesis. The anulus of Vieus-
the inner myocardial walls into the right atrium and ventricle and, to
sens is a collateral vessel that crosses the subpulmonary infundibulum,
a lesser extent, into the left atrium and sometimes the left ventricle. It
providing an anastomosis between the conal branch of the right coro-
contains the smallest cardiac veins (Thebesian veins) that drain the
nary artery and the anterior interventricular artery. The artery to the
subendocardial myocardium either directly, via connecting intramural
sinu-atrial node commonly provides a communication between the
arteries and veins, or indirectly, via subendocardial sinusoidal spaces.
proximal parts of the coronary arteries. The apical collateral artery joins
the interventricular arteries. The frequent enlargement of the first septal
Variation in cardiac veins Attempts to categorize variations in
branch of the anterior inter ventricular artery, Kugel’s anastomotic artery
cardiac venous circulation into ‘types’ have not produced any accepted
(arteria anastomotica auricularis magna), has been described as running
pattern. The coronary sinus may receive all the cardiac veins other than
between the proximal parts of the coronary arteries along the antero-
the Thebesian veins, including the anterior cardiac veins, which may be
superior margin of the oval fossa to anastomose with the distal part of