Relaxation time constant of isolated rabbit left ventricle

Experiments were performed on isolated rabbit left ventricles. Controlled ejections during otherwise isovolumic contractions were studied. The time constant of relaxation was defined as the slope of the linear approximation of the ln(P)-t relation over a 40-ms period starting 20 ms after the minimum of the first time derivative of left ventricular pressure (dP/dt) of the isovolumic contraction. Variations in time of ejection, its amplitude, and velocity are applied independently. No direct effect of the variations in time and velocity of the ejection on the time constant of relaxation was found. This is in conflict with the findings of Hori et al. (Circ. Res. 55: 31-38, 1984). The difference is due to the influence of the recovery of pressure directly after the end of ejection in their study. This effect is present especially when ejection was timed to take place late in the contraction phase. The effect of the variation of the amplitude of the ejection on the time constant was similar to the effect of the end-diastolic pressure on the end-diastolic volume. It is concluded that the time constant of relaxation depends linearly on the same processes that are responsible for the height of the end-diastolic pressure.