as einthoven's triangle. lead i is the voltage between the (positive) left arm (la) electrode and right arm (ra) electrode: i = l a − r a lead ii is the voltage between the (positive) left leg (ll) electrode and the right arm (ra) electrode: i i = l l − r a lead iii is the voltage between the (positive) left leg (ll) electrode and the left arm (la) electrode: i i i = l l − l a augmented limb leads leads avr, avl, and avf are the augmented limb leads. they are derived from the same three electrodes as leads i, ii, and iii, but they use goldberger's central terminal as their negative pole. goldberger's central terminal is a combination of inputs from two limb electrodes, with a different combination for each augmented lead. it is referred to immediately below as "the negative pole". lead augmented vector right (avr) has the positive electrode on the right arm. the negative pole is a combination of the left arm electrode and the left leg electrode: a v r = r a − 1 2 ( l a + l l ) = 3 2 ( r a − v w ) (la+ll)=(ra-v_)} lead augmented vector left (avl) has the positive electrode on the left arm. the negative pole is a combination of the right arm electrode and the left leg electrode: a v l = l a − 1 2 ( r a + l l ) = 3 2 ( l a − v w ) (ra+ll)=(la-v_)} lead augmented vector foot (avf) has the positive electrode on the left leg. the negative pole is a combination of the right arm electrode and the left arm electrode: a v f = l l − 1 2 ( r a + l a ) = 3 2 ( l l − v w ) (ra+la)=(ll-v_)} together with leads i, ii, and iii, augmented limb leads avr, avl, and avf form the basis of the hexaxial reference system, which is used to calculate the heart's electrical axis in the frontal plane. older versions of the nodes (vr, vl, vf) use wilson's central terminal as the negative pole, but the amplitude is too small for the thick lines of old ecg machines. the goldberger terminals scale up (augments) the wilson results by 50%, at the cost of sacrificing physical correctness by not having the same negative pole for all three. precordial leads the precordial leads lie in the transverse (horizontal) plane, perpendicular to the other six leads. the six precordial electrodes act as the positive poles for the six corresponding precordial leads: (v1, v2, v3, v4, v5, and v6). wilson's central terminal is used as the negative pole. recently, unipolar precordial leads have been used to create bipolar precordial leads that explore the right to left axis in the horizontal plane. specialized leads additional electrodes may rarely be placed to generate other leads for specific diagnostic purposes. right-sided precordial leads may be used to better study pathology of the right ventricle or for dextrocardia (and are denoted with an r (e.g., v5r). posterior leads (v7 to v9) may be used to demonstrate the presence of a posterior myocardial infarction. the lewis lead or s5-lead (requiring an electrode at the right sternal border in the second intercostal space) can be used to better detect atrial activity in relation to that of the ventricles. an esophogeal lead can be inserted to a part of the esophagus where the distance to the posterior wall of the left atrium is only approximately 5–6 mm (remaining constant in people of different age and weight). an esophageal lead avails for a more accurate differentiation between certain cardiac arrhythmias, particularly atrial flutter, av nodal reentrant tachycardia and orthodromic atrioventricular reentrant tachycardia. it can also evaluate the risk in people with wolff-parkinson-white syndrome, as well as terminate supraventricular tachycardia caused by re-entry. an intracardiac electrogram (iceg) is essentially an ecg with some added intracardiac leads (that is, inside the heart). the standard ecg leads (external leads) are i, ii, iii, avl, v1, and v6. two to four intracardiac leads are added via cardiac catheterization. the word "electrogram" (egm) without further specification usually means an intracardiac electrogram. lead locations on an ecg report a standard 12-lead ecg report (an electrocardiograph) shows a 2.5 second tracing of each of the twelve leads. the tracings are most commonly arranged in a grid of four columns and three rows. the first column is the limb leads (i, ii, and iii), the second column is the augmented limb leads (avr, avl, and avf), and the last two columns are the precordial leads (v1 to v6). additionally, a rhythm strip may be included as a fourth or fifth row. the timing across the page is continuous and notes tracings of the 12 leads for the same time period. in other words, if the output were traced by needles on paper, each row would switch which leads as the paper is pulled under the needle. for example, the top row would first trace lead i, then switch to lead avr, then switch to v1, and then switch to v4, and so none of these four tracings of the leads are from the same time period as they are traced in sequence through time. contiguity of leads diagram showing the contiguous leads in the same color in the standard 12-lead layout each of the 12 ecg leads records the electrical activity of the heart from a different angle, and therefore align with different anatomical areas of the heart. two leads that look at neighboring anatomical areas are said to be contiguous. category leads activity inferior leads leads ii, iii and avf look at electrical activity from the vantage point of the inferior surface (diaphragmatic surface of heart) lateral leads i, avl, v5 and v6 look at the electrical activity from the vantage point of the lateral wall of left ventricle septal leads v1 and v2 look at electrical activity from the vantage point of the septal surface of the heart (interventricular septum) anterior leads v3 and v4 look at electrical activity from the vantage point of the anterior wall of the right and left ventricles (sternocostal surface of heart) in addition, any two precordial leads next to one another are considered to be contiguous. for example, though v4 is an anterior lead and v5 is a lateral lead, they are contiguous because they are next to one another. electrophysiology main article: cardiac electrophysiology the study of the conduction system of the heart is called cardiac electrophysiology (ep). an ep study is performed via a right-sided cardiac catheterization: a wire with an electrode at its tip is inserted into the right heart chambers from a peripheral vein, and placed in various positions in close proximity to the conduction system so that the electrical activity of that system can be recorded. standard catheter positions for an ep study include "high right atrium" or hra near the sinus node, a "his" across the septal wall of the tricuspid valve to measure bundle of his, a "coronary sinus" into the coronary sinus, and a "right ventricle" in the apex of the right ventricle. interpretation interpretation of the ecg is fundamentally about understanding the electrical conduction system of the heart. normal conduction starts and propagates in a predictable pattern, and deviation from this pattern can be a normal variation or be pathological. an ecg does not equate with mechanical pumping activity of the heart, for example, pulseless electrical activity produces an ecg that should pump blood but no pulses are felt (and constitutes a medical emergency and cpr should be performed). ventricular fibrillation produces an ecg but is too dysfunctional to produce a life-sustaining cardiac output. certain rhythms are known to have good cardiac output and some are known to have bad cardiac output. ultimately, an echocardiogram or other anatomical imaging modality is useful in assessing the mechanical function of the heart. like all medical tests, what constitutes "normal" is based on population studies. the heartrate range of between 60 and 100 beats per minute (bpm) is considered normal since data shows this to be the usual resting heart rate. theory qrs is upright in a lead when its axis is aligned with that lead's vector schematic representation of a normal ecg interpretation of the ecg is ultimately that of pattern recognition. in order to understand the patterns found, it is helpful to understand the theory of what ecgs represent. the theory is rooted in electromagnetics and boils down to the four following points: depolarization of the heart toward the positive electrode produces a positive deflection depolarization of the heart away from the positive electrode produces a negative deflection repolarization of the heart toward the positive electrode produces a negative deflection repolarization of the heart away from the positive electrode produces a positive deflection thus, the overall direction of depolarization and repolarization produces positive or negative deflection on each lead's trace. for example, depolarizing from right to left would produce a positive deflection in lead i because the two vectors point in the same direction. in contrast, that same depolarization would produce minimal deflection in v1 and v2 because the vectors are perpendicular, and this phenomenon is called isoelectric. normal rhythm produces four entities – a p wave, a qrs complex, a t wave, and a u wave – that each have a fairly unique pattern. the p wave represents atrial depolarization. the qrs complex represents ventricular depolarization. the t wave represents ventricular repolarization. the u wave represents papillary muscle repolarization. changes in the structure of the heart and its surroundings (including blood composition) change the patterns of these four entities. the u wave is not typically seen and its absence is generally ignored. atrial repolarisation is typically hidden in the much more prominent qrs complex and normally cannot be seen without additional, specialised electrodes. background grid ecgs are normally printed on a grid. the horizontal axis represents time and the vertical axis represents voltage. the standard values on this grid are shown in the adjacent image at 25mm/sec: a small box is 1 mm × 1 mm and represents 0.1 mv × 0.04 seconds. a large box is 5 mm × 5 mm and represents 0.5 mv × 0.20 seconds. the "large" box is represented by a heavier line weight than the small boxes. measuring time and voltage with ecg graph paper the standard printing speed in the united states is 25 mm per sec (5 big boxes per second), but in other countries it can be 50 mm per sec. faster speeds such as 100 and 200 mm per sec are used during electrophysiology studies. not all aspects of an ecg rely on precise recordings or having a known scaling of amplitude or time. for example, determining if the tracing is a sinus rhythm only requires feature recognition and matching, and not measurement of amplitudes or times (i.e., the scale of the grids are irrelevant). an example to the contrary, the voltage requirements of left ventricular hypertrophy require knowing the grid scale. rate and rhythm in a normal heart, the heart rate is the rate at which the sinoatrial node depolarizes since it is the source of depolarization of the heart. heart rate, like other vital signs such as blood pressure and respiratory rate, change with age. in adults, a normal heart rate is between 60 and 100 bpm (normocardic), whereas it is higher in children. a heart rate below normal is called "bradycardia" (<60 in adults) and above normal is called "tachycardia" (>100 in adults). a complication of this is when the atria and ventricles are not in synchrony and the "heart rate" must be specified as atrial or ventricular (e.g., the ventricular rate in ventricular fibrillation is 300–600 bpm, whereas the atrial rate can be normal or faster ). in normal resting hearts, the physiologic rhythm of the heart is normal sinus rhythm (nsr). normal sinus rhythm produces the prototypical pattern of p wave, qrs complex, and t wave. generally, deviation from normal sinus rhythm is considered a cardiac arrhythmia. thus, the first question in interpreting an ecg is whether or not there is a sinus rhythm. a criterion for sinus rhythm is that p waves and