Shortening of the Intraventricular Conduction Time in Premature Ventricular Beats During Ventricular Extrastimulation: Possible Role of Dimensional Shortening of the Conducting Distance During Ventricular Systole

Supernormal Intraventricular Conduction. Introduction: Conduction time (CT) is given by the formula: conducting distance divided by conduction velocity. Based on this formula, we hypothesized that CT shortening (i.e., supernormal conduction) may result from dimensional shortening of the distance of impulse propagation, which naturally occurs during ventricular systole. Methods and Results: To test the above, two separate groups of patients were studied, group A (14 patients) for electrophysiologic study and group B (12 patients) for echocardiographic study. In group A patients, CT from the stimulus artifact to the basal lateral wall of the left ventricle (LV) (S-LV interval) was measured using right ventricular (RV) apical extrastimulus testing. S-LV interval shortening in premature RV beats was demonstrated in all 14 patients. The maximum shortening was 20 ± 9 msec (range 10 to 40), and the maximum% shortening was 16%± 6% (7% to 27%). In group B patients with implanted pacemakers, the major (long) and minor (short) axis dimensions of the LV were measured with echocardiography. The major axis dimension was used as an approximate measure of the linear length from the RV apex to the basal lateral wall of LV. The maximum% shortening of the major axis dimensions was 15%± 4%, 16%± 2%, and 11%± 4% during VVI pacing, respectively, at paced cycle lengths of 1,000 (11 patients), 800 (5 patients), and 600 msec (12 patients). The maximum% shortening of the S-LV intervals was comparable in magnitude with that of the major axis dimensions: 20% versus 15%± 4%, 15%± 7% versus 16%± 2% and 16%± 6% versus 11%± 4%, respectively, at paced cycle lengths of 1,000, 800, and 600 msec. There was also a good temporal correlation between the electrophysiologic (CT shortening) versus echocardiographic (dimensional shortening) parameters. Thus, the intraventricular CT and the major axis dimension of the LV were shortened in a similar magnitude and also at a similar timing in the cardiac cycle. Conclusion: These findings suggest the possibility that supernormal conduction may result, at least in part, from dimensional shortening of the pathway length of impulse propagation from the stimulating to recording electrodes, which naturally occurs during ventricular systole.