Chapter 493

Normal electrical activation of the heart begins pacemaker activity in the sinoatrial node, and the wave of activation spreads through the right and left atria (Fig. 493-1). In the right atrium the wave of depolarization passes inferiorly, and the left atrium is activated via Bachmann bundle, which also triggers an inferiorly directed activation front. These activation fronts generate a potential that is detected on the body surface as the P wave. Any force that has magnitude and direction is termed a vector and can be represented by an arrow with direction and magnitude proportional to the force. The impulse is delayed at the atrioventricular (AV) node, producing the PR interval. This allows ventricular filling to be completed before ventricular contraction begins. Beyond the AV node, the impulse moves rapidly down the bundle of His into the right and left bundle branches. As the impulses pass down the septum, they activate septal muscle predominantly from the left side, so that the initial ventricular vector passes from left to right, anteriorly and superiorly (Fig. 493-2), and begins the Q wave in lead V6 or the first part of the R wave in lead V1.1 After reaching the apex, the impulse activates the ventricular free walls from endocardium to epicardium and from apex toward the base, thus inscribing the R and S waves; the last part of the heart to be activated is the posterior ventricular muscle just under the AV ring. In adults and older children, there is more left than right ventricular muscle, so the major cardiac vectors point to the left and posteriorly and produce a tall R wave in V6 and a deep S wave in V1. In a normal newborn infant with a thick right ventricle, the major cardiac vectors pass to the right and anteriorly and produce a dominant R wave in V1 and a large S wave in V6. After depolarization has occurred, there is slower repolarization that produces the T wave.

###### Figure 493-1.

Diagram of conduction system. AV, atrioventricular node; His, bundle of His, dividing into right and left bundles; LA, left atrium; RV and LV, right and left ventricles; SA, sinoatrial node; SVC and IVC, superior and inferior venae cavae.

###### Figure 493-2.

Normal ventricular depolarization, starting with 1 and ending with 5. A: Section through ventricles with thicker-walled left ventricle and thinner-walled right ventricle. Arrows are vectors, indicating direction and magnitude of electrical forces at each time. B: Vectors are superimposed on common center, and their tips are joined by a dashed line. C: The dashed line remains to give a vector loop. The vector at any moment would be a line joining the central point to the corresponding part of the loop. The arrows...

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