Chapter 493. Electrocardiography

George F. Van Hare

Topics Discussed: cardiac conduction system; cardiology; conduction, cardiac; electrocardiogram; electrocardiographic axis; electrodes, electrocardiographic; qrs axis; scalar value; vectorcardiography; ventricular depolarization.

Sections: Approach, P Wave, Q Wave, QRS and T Waves, Ventricular Hypertrophy, ST Segment and T Waves, References.

Excerpt:"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..."

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