of Cardiac Conduction
In an adult with a healthy
heart, the heart rate is usually about 72 beats per minute.
The excitatory and electrical
conduction system of the heart is responsible for the contraction and relaxation
of the heart muscle. The sinoatrial node (SA node) is the pacemaker where the
electrical impulse is generated. This node is located along the posterior wall
of the right atrium right beneath the opening of the superior vena cava. It
is crescent shaped and about 3 mm wide and 1 cm long.
The impulse travels from
the SA node through the internodal pathways to the atrioventricular node (AV
node). The AV node is responsible for conduction of the impulse from the atria
to the ventricles. The impulse is delayed slightly at this point to allow complete
emptying of the atria before the ventricles contract.The impulse continues through
the AV bundle and down the left and right bundle branches of the Purkinje fibers.
The Purkinje fibers conduct the impulse to all parts of the ventricles, causing
contraction (Guyton, 1982).
Abnormal heart rhythms occur
for several reasons.
- The vagal stimulation
of the parasympathetic nervous system can cause a decrease in the rate at
the SA node and can also decrease the excitability of the AV junction fibers.
This causes a slowing of the heart rate, and in severe cases a complete blockage
of the impulse through the AV junction.
- Sympathetic stimulation
also effects cardiac rhythm and conduction. It increases the rate at the SA
node and increases the rate of conduction and excitability throughout the
heart. It also increases the force of myocardial contraction. Subsequently,
the overall workload on the heart is increased.
- A small area of the heart
can become more excitable than normal, which causes abnormal heart beats called
ectopy. Ectopic foci are usually caused by an irritable area in the heart.
This irritability can be caused by ischemia, stimulants such as nicotine and
caffeine, lack of sleep or anxiety (Guyton, 1982).
- Aberrant conduction
pathways can cause arrhythmias by providing an alternate route for the
wave of depolarization. Normally, atrial and ventricular tissues are isolated
by nonconductive fat and connective tissue. Aberrant pathways form a short
circuit bridge across the nonconductive tissue. Hence, tissue can be depolarized
prematurely or out of sync.
- Inherited or acquired
channelopathy can predispose an individual to dangerous
de pointes. Faulty ion channels
may result in prolonged repolarization, lengthening the QT interval. The
heart is most vulnerable during repolarization. Channelopathies can be
due to "leaky" or slow closing sodium channels, ineffective potassium
channels or drugs and toxins which can affect the transport of ions across
stimulation of the heart can...
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