LOADING SEQUENCE PLAYS AN IMPORTANT ROLE IN ENHANCED LOAD SENSITIVITYOF LEFT-VENTRICULAR RELAXATION IN CONSCIOUS DOGS WITH TACHYCARDIA-INDUCED CARDIOMYOPATHY

Background Left ventricular relaxation rate in the failing heart depen ds more on the systolic load than in the normal heart. To elucidate th e mechanisms for the enhanced load sensitivity of left ventricular rel axation in heart failure, we examined the relative contributions of ch anges in end-systolic volume and loading sequence to the left ventricu lar relaxation rate. Methods and Results In seven conscious dogs, the time constant (T-d) of left ventricular pressure decay, end-systolic v olume, systolic circumferential force, and time to peak force during c aval occlusion were compared before and after development of tachycard ia-induced heart failure. Rapid ventricular pacing decreased the slope of the end-systolic pressure-volume relation from 4.5 to 2.8 mm Hg/mL (P<.01) and prolonged T-d from 33 to 49 ms (P<.01). In normal conditi ons, caval occlusion reduced end-systolic force (-580 g, P<.01) and en d-systolic volume (-7 mL, P<.01) but did not change T-d or time to pea k force. In heart failure, however, caval occlusion shortened T-d (-11 ms, P<.01), with a concomitant decrease in the time to peak force (-3 0 ms, P<.01), while end-systolic volume and force declined slightly. C onsequently, for a comparable reduction in end-systolic force, T-d dec reased more in heart failure than in normal hearts, suggesting enhance d load sensitivity. Moreover, changes in T-d correlated well with thos e in the time to peak force (r=.79, P<.01) but not with those in end-s ystolic volume. Conclusions Loading sequence rather than elastic recoi l seems to play the predominant role in the enhanced load sensitivity of left ventricular relaxation in heart failure.