Influence of pacing site on canine left ventricular contraction.

We investigated the influence of pacing site on several aspects of left ventricular (LV) performance to test the hypothesis that "effective ventricular muscle mass" is reduced with direct ventricular pacing. All studies were performed on isolated supported canine hearts that were constrained to contract isovolumically. To determine the influence of pacing site on magnitude and time course of isovolumic LV pressure (P) generation, LVP waves were recorded in eight isolated hearts paced at 130 beats/min. Pacing was epicardially from atrium, LV apex, LV free wall, right ventricular free wall (RVF), and endocardially from right ventricular endocardium. In a given heart, peak LVP was greatest with atrial pacing and smallest with RVF pacing, the difference being on average 26 +/- 10% (mean +/- SD) of the former pressure. The other pacing sites produced intermediate peak LVPs. When instantaneous LVP waves, obtained while pacing from each of the five sites, were normalized by their respective amplitudes, they were virtually superimposable up to the time of peak pressure and only slightly different during the remainder of the cardiac cycle. With changes in pacing site there was a linear negative correlation (r = 0.971) between changes in peak pressure and changes in duration of the QRS complex of a bipolar epicardial electrogram with an average slope of -0.51 mmHg/ms. Compared with atrial pacing, the slope of the end-systolic pressure-volume relation, Ees, was decreased with ventricular pacing, but Vo, the volume axis intercept, was relatively constant.(ABSTRACT TRUNCATED AT 250 WORDS)