VENTRICULAR CONTRACTION MODELLED AS A BALANCE BETWEEN POSITIVE AND NEGATIVE INFLUENCES OF EJECTION.

Previously, cardiac ejection was thought to exert only negative effects upon end-systolic left ventricular pressure (LVP). However, recent experimental evidence has shown that (1) end-systolic LVP of an ejecting beat often exceeds the peak LVP of an isovolumic contraction at the end-systolic volume, and (2) the duration of systole is 50% longer in ejecting versus isovolumic beats. A model based on mechanisms of cardiac muscle contraction that reproduces both of these positive effects of ejection is described. The model suggests that the positive and negative effects of ejection could be, individually, quite powerful. Thus, the near load-independence of end-systolic pressure could result from the counterbalance between several strong but opposing influences of ejection.