Chapter 11: THE RESTING ELECTROCARDIOGRAM
NORMAL ACTIVATION OF THE HEART: VENTRICULAR DEPOLARIZATION
After emerging from the sinus node, the cardiac impulse propagates throughout the atria in its journey toward the atrioventricular (AV) node. The normal P wave (resulting from activation of the myocardium of both atria) is a consequence of, but does not directly represent, sinus node activity. During sinus rhythm, the right atrium is activated before the left atrium.6 This explains why high-fidelity recordings of the P waves of some normal persons show a small notch at the top. The latter simply reflects the normal asynchrony existing between the atria.6 Because of the anatomic position of the sinus node, the sequence of atrial depolarization occurs in an inferior, leftward, and somewhat posterior direction. The normal P waves are always positive in leads I, II, aVF, and V3 to V6 and negative in lead aVR. According to the anatomic position of the heart, the P wave may be diphasic in V1 and aVL or negative in the latter lead. Atrial repolarization, also called Ta, is directly opposite in polarity to the P wave.6!! It is usually not seen because it coincides with the PR segment (not to be confused with the PR interval) and QRS complex. The PR interval (used to estimate AV conduction time) includes conduction through the "true" AV structures (AV node, His bundle, bundle branches, and main divisions of the left bundle branch), as well as through those parts of the atria located between sinus and AV nodes.8 The onset of ventricular depolarization (given by the beginning of the normal q wave) reflects activation of the left side of the interventricular septum. This has been attributed to the fact that the left bundle system is shorter than the right bundle branch.8,!5 In addition, the large fanlike distribution of the ramifications of the fascicles of the left bundle branch on the left septal surface produces activation of a greater number of ordinary muscle cells per unit of time.6,8,15 For this reason, the normal initial depolarization is oriented from left to right, therefore explaining the small q wave in lead V6 and the small r wave in lead V1. After the cardiac impulse descending through the right bundle branch reaches the right septal surface, the interventricular septum is activated in both directions. Septal activation is thereafter encompassed within or neutralized by free-wall activation. The most distal ramifications of both bundle branches (Purkinje fibers) form networks within the subendocardial regions of both ventricular walls. The latter are activated as soon as the multiple ramifications emerge from the Purkinje fibers.6,15 The greater mass of the left ventricular (LV) free wall explains why LV free-wall events overpower those of the interventricular septum and right ventricular free wall.
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