thaler chapter 1

The Normal 12-Lead EKG

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The Rest of the Ventricular Myocardium Depolarizes The remainder of the ventricles, the vast bulk of the myocardium, depolarizes next. Because the left ventricle is so much more massive than the right ventricle, it dominates the remainder of the QRS complex, and the average vector of current flow swings leftward. Normally, this vector points anywhere from 0° to +90°. In the frontal plane, therefore, large positive deflections (R waves) may be seen in many of the left lateral and inferior leads. Lead aVR, lying rightward, records a deep negative deflection (S wave).

− 90°

lead aVR

0°

180°

lead I

+90°

lead II

Ventricular depolarization as seen in leads I, II, and aVR. Lead I records a small Q wave due to septal depolarization and a tall R wave. Lead II also records a tall R wave and, less often, a small Q wave. The QRS complex in lead aVR is also deeply negative.