Load-dependent relaxation with late systolic volume steps: servo-pump studies in the intact canine heart.

In isolated heart muscle preparations an abrupt increase in load during the latter portion of contraction (at a time when there is little if any potential to develop additional force) causes a premature and more rapid relaxation; this load-dependent relaxation characterizes relaxation in myocardium with normal sarcoplasmic reticulum. The purpose of our study was to assess whether or not the phenomenon of load-dependent relaxation is present in the intact heart and to describe the left ventricular response to abrupt load increments (volume steps) throughout the cardiac cycle. Using a microcomputer-controlled servo-pump attached to the apex of an intact canine heart, we studied the effects of 6 ml steps on left ventricular pressure transients during relaxation. Each volume step was carried out in a single beat with 20 stabilization beats separating the intervention beats; thus, at a heart rate of 120 beats/min, a sequence of 10 intervention beats could be carried out in less than 2 min. By performing the experiments on a single-beat basis (control vs intervention beat), we were able to minimize reflex and other physiologic feedback mechanisms that might alter the results. Studies were performed in five anesthetized dogs. In ejecting beats, an early step (immediately after aortic valve opening) caused an increase (3%) in the duration of systole (the time from the onset of pressure rise to the instant at which left ventricular pressure had declined to one-half its maximal value); in contrast, a late step (just before aortic value closure) caused a decrease (7%) in the duration of systole.(ABSTRACT TRUNCATED AT 250 WORDS)