The effect of controlled changes in volume on the active state of the rabbit isolated left ventricle

The contractile properties of isovolumically contracting isolated rabbit left ventricles are studied under the influence of controlled rapid volume changes during systole and diastole. The time integral of the pressure curve (TTI), representing the active state, is used to quantify the energy consumption of the ventricle. Steady state conditions resulting from an introduced volume change show a TTI/EDV relation which represents the Starling curve. However, immediately after a quick volume increase (decrease) introduced in diastole, the TTI/EDV ratio has a higher (lower) value than indicated by the Starling relation. This shows a volume dependent activation (deactivation), related to changes in the inotropic state of the heart muscle cells within the ventricular wall. A volume increase at a later moment (in systole) always produces a lower rate of activation. Indeed, if the rapid volume change is introduced at moments later than 70% of time to peak pressure, TTI is less than observed from the Starling mechanism, indicating a deactivation. When comparing the decreasing effect on the active state introduced by volume decrease during systole, it is shown that this effect is not only a function of the amplitude of the decrease itself but is highly dependent upon the way EDV is reached.