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. In nonejecting beats (single-beat aortic occlusion plus volume step), early steps (during the first third of systole) caused an increase in the duration of systole (245 ± 18 to 257 + 19 msec; p < .05); when the volume step was placed later (near the time of peak pressure), the duration of systole fell (245 ± 18 to 231 + 17 msec; p < .05). These results indicate the presence of load-dependent relaxation in the intact canine heart. Although these data were obtained during abrupt load changes in single cardiac cycles, it is possible that alterations in the mechanisms underlying the phenomenon of load-dependent relaxation can be responsible for disordered relaxation in clinical and experimental studies of the diastolic properties of the left ventricle. Circulation 75, No. 6, 1287-1294, 1987 RELAXATION, which refers to the process by which myocardium returns to its initial length and tension, is influenced by the interaction of deactivation (the decay of active force-generating capacity) and loading conditions (forces affecting myocardial fiber length). These forces or loads may be categorized as those that are applied early in the cardiac cycle and those that are abruptly applied late in the cycle. The application of an early systolic load (when activating calcium is avail- Vol. 75, No. 6, June 1987 able to the contractile proteins) results in the recruitment of more cross-bridges and a prolonged systole. By contrast, late ...
Peak systolic elastance (Emax) was measured in the intact canine circulation by means of a new experimental technique. In this technique the heart is isolated from the circulation during a single systole and subjected to controlled ventricular loads. An electropneumatic aortic occluder is used to isolate the ventricle, and a servo-controlled syringe pump is used to control the ventricular load. Because the experimental load is applied for a single heartbeat only, ventricular function can be measured without the interference of regulatory feedback mechanisms. In eight dogs, weighing 17-42 kg, the relationship between changes in endsystolic pressure and volume was determined from the single-beat application of purely compliant loads. The end-systolic relations were linear, and their slope, Emax, was inversely related to weight. The observed relation between Emax and body weight allows comparisons to be made between different preparations in which Emax has been determined. Values of Emax obtained from the single-beat preparation were found to be 27-74% above those reported in isolated heart preparations and nearly identical to those reported for in vivo or denervated in situ preparations.
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