Effect of Cardiac Cycle Length upon Magnitude of Ventricular Gradient

The ventricular gradient of Wilson 1 may be defined as the net electrical effect of differences in time course of repolarization of different parts of the ventricular muscle as this effect is projected onto the frontal plane of the body. In the electrocardiogram it is measured by the mean manifest net area of the QRS-T group of deflections. In previous papers^2,3 it has been pointed out that the magnitude of the gradient varies inversely as the heart rate or directly as the average cardiac cycle length. The present report seeks to explain this relationship.

Three main procedures were used. In one, the string galvanometer recorded the action current derived from the apex and base of the excised turtle ventricle, in air. Apical negativity yielded an upward deflection. Records were obtained when the ventricle was driven at various rates, as shown by the cycle lengths in Table I. In a second set of experiments the apex was cooled, though, as the irregularity of the results shows, no attempt was made to keep the temperature constant. Table I also shows the Q-T interval for each cycle length, and the net area of QRS-T in units of about 30 microvolt-seconds. The method of measuring the areas has been published elsewhere. 4 Since the initial deflections were small, the larger part of the net QRS-T area is within the T wave, except when the cycles were short.

It may be observed that the net areas are negative, that is, below the base line of the electrogram, when the apex was at the room temperature of 24°C. This finding indicates that in this heart the repolarization of the apex was more rapid than that of the base.