Cardiac cycle length modulates cardiovascular regulation that is dependent on previous beat contraction history.

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Abstract
Previous studies of the beat-to-beat regulation of left ventricular performance in the intact cardiovascular system have shown that the performance of the left ventricle on any one beat is influenced by the mechanical events of the previous beat, so-called previous beat contraction history. The general hypothesis investigated in this study is that previous beat contraction history occurs because of the perturbation of one or more biochemical processes with time courses that are long relative to one cardiac cycle. If this is true, then previous beat contraction history should depend on the interval between beats and, therefore, should extend beyond the previous beat to two, or even three, previous beats as heart rate is increased. Hemodynamic responses to random preload and afterload changes were measured in 11 anesthetized open-chest dogs on right heart bypass at three paced heart rates. Multiple linear regression was used to analyze these hemodynamic sequences and identify variables from the previous one, two, or three beats that were important in the mechanical history dependence of left ventricular function. The results of this analysis showed that under baseline conditions, all 11 hearts showed one beat of history dependence, with only two of 11 hearts showing a dependence on the previous two or three beats. At the highest heart rate, all 11 hearts still showed one beat of history dependence, but 10 of 11 hearts showed two beats of history dependence, and four of 11 showed three beats of history dependence (p less than 0.05). A general framework for the mechanism underlying these findings, which relates previous beat contraction history to the interval-dependent mechanical restitution phenomenon, is proposed and discussed.