Left ventricular activation time in normal men.

IN DIRECT LEADS obtained through a small electrode attached to the epicardium, there is a nearly perpendicular return of the R wave from its peak to the base line. This intrinsic deflection marks the extinction of the electric potential of the small core of ventricle to which the electrode is applied. In precordial leads, where a relatively large electrode is at a considerable distance from the heart, there is a more gradually sloping downstroke, or intrinsicoid deflection, since electric activity does not disappear simultaneously in all parts of the wide area subtended by the electrode. In clinical electrocardiography, the time from the onset of the initial deflection to the peak of the R wave in precordial leads over the left ventricle is widely used for timing and measuring left ventricular activation. An electrode placed at V5 or V6 registers an R wave because of the predominating positive potentials derived from the anterior and lateral wall of the left ventricle. Negative potentials derived from the activation of the posterior wall of the left ventricle influence the duration of the positive phase in these leads and cause slurring or notching on the descending limb of the R wave. Thus these leads reflect counteracting positive and negative potentials. A reversed situation occurs at Vg