Physiological Basis of the Electrocardiogram

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8 The Physiological Basis of the Electrocardiogram atria, teleologically, finish contraction before the ventricles begin), and accelerated conduction of the depolarization wave via specialized conducting fibers (so that disparate parts of the heart are depolarized in a more synchronized fashion). Nine different temporal states are shown. The dotted line below each illustrated state summarizes the preceding trajectory of heart vectors. First, atrial depolarization is illustrated. As the wave of depolarization descends throughout both atria, the summation vector is largely pointing down (to the subject’s toes), to the subject’s left, and slightly anterior. Next there is the delay at the AV node, discussed above, during which time there is no measurable electrical activity at the body surface unless special averaging techniques are used. After activity emerges from the AV node it depolarizes the His 10 bundle, followed by the bundle branches. Next, there is the septal depolarization. The septum is the wall between the ventricles, and a major bundle of conducting fibers runs along the left side of the septum. As the action potential wave enters the septal myocardium it tends to propagate left to right, and so the resultant heart vector points to the subject’s right. Next there is apical depolarization, and the wave of depolarization moving left is balanced by the wave moving right. The resultant vector points towards the apex of the heart, which is largely pointing down, to the subject’s left, and slightly anterior. In left ventricular depolarization and late left ventricular depolarization, there is also electrical activity in the right ventricle, but since the left ventricle is much more massive its activity dominates. After the various portions of myocardium depolarize, they contract via the process of excitation-contraction coupling described above (not illustrated). There is a plateau period during which the myocardium has depolarized (ventricles depolarized) where no action potential propagates, and hence there is no measurable cardiac vector. Finally, the individual cells begin to repolarize and another wave of charge passes through the heart, this time originating from the dipoles generated at the interface of depolarized and repolarizing tissue (i.e., ventricular repolarization). The heart then returns to its resting state (such that the ventricles are repolarized), awaiting another electrical stimulus that starts the cycle anew. Note that both the polarity and the direction of propagation of the repolarizing phase are reversed from those of depolarization. As a result, repolarization waves on the ECG are generally of the same polarity as depolarization waves. To complete the review of the basis of the surface ECG, a description of how the trajectory of the cardiac vector (detailed in Figure 1.4) results in the pattern of a normal scalar ECG is now described. The cardiac vector, which expands, contracts, and rotates in three-dimensional space, is projected onto 12 different lines of welldefined orientation (for instance, lead I is oriented directly to the patient’s left). Each lead reveals the magnitude of the cardiac vector in the direction of that lead at each instant of time. The six precordial leads report activity in the horizontal plane. In practice, this requires that six electrodes are placed around the torso (Figure 1.5), and the ECG represents the difference between each of these electrodes (V1–6) and the central terminal [as in (1.3)]. 10. The His bundle is a collection of heart muscle cells specialized for electrical conduction that transmits electrical impulses from the AV node, between the atria and the ventricles to the Purkinje fibers, which innervate the ventricles.
1.2 The Physical Basis of Electrocardiography 9 Figure 1.5 The six standard chest leads. (From: [2]. c○ 2004 MIT OCW. Reprinted with permission.) Figure 1.6 Frontal plane limb leads. (From: [2]. c○ 2004 MIT OCW. Reprinted with permission.)
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Physiological Basis of the Electrocardiogram

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