Effect of alterations of the left ventricular activation sequence on left ventricular performance

Abstract

It is difficult to determine the effect of ventricular pacing on left ventricular (LV) performance using isovolumic or ejection phase indices, because ventricular pacing also produces changes in loading conditions. Accordingly, we investigated the effect of pacing from the atrium and various ventricular sites on the LV end-systolic pressure-volume relation, a relatively load-independent measure of LV performance. Studies were performed following autonomic blockade in dogs chronically instrumented to measure left ventricular pressure and determine left ventricular volume from three ultrasonic endocardial dimensions. During ventricular pacing, left ventricular end-diastolic volume, stroke volume, and end-systolic pressure were decreased, while the end-systolic volume was relatively unchanged. LV end-systolic pressure-volume relations were generated by vena caval occlusions during pacing at a constant rate from atrial and ventricular sites. Compared to atrial pacing, the LV end-systolic pressure-volume relations were shifted to the right during pacing from ventricular sites. The volume intercept of the LV end-systolic pressure-volume relation increased during ventricular pacing, while the slope of the LV end-systolic pressure-volume relation changed only slightly. We concluded that alterations of the normal activation sequence produced by ventricular pacing depress LV pumping function, independent of loading conditions, as indicated by a rightward shift of the LV end-systolic pressure-volume relation. The decreased stroke volume during ventricular pacing is due both to a decreased end-diastolic volume (decreased preload) and the rightward shift of the end-systolic pressure-volume relation (decreased pump function).