The step response of left ventricular pressure to ejection flow: a system oriented approach.

Left ventricular pressure is dependent on both ventricular volume and ventricular ejection flow. These dependencies are usually expressed by ventricular elastance, and resistance, respectively. Resistance is a one-valued effect only, when ejection flow either is constant or increases. Decreasing ejection flow elicits a third effect: a decrease of elastance. The effects of elastance, resistance and elastance depression were modeled in a three-compartment model consisting of a dead-volume compartment, an elastance compartment, and a second series-elastance compartment connected to the elastance compartment by a resistance. This model was identified with the pressure response determined experimentally by imposing pumped constant-flow ejection epochs on isolated rabbit hearts. The experimental flow epochs consisted of two phases of constant flow separated by an increasing or decreasing flow step. It was found that elastance is not changed after the flow step if this is positive or zero. Negative flow steps induced a deactivation of elastance that is linearly dependent on the difference between isovolumic pressure that would be developed at the volume existing at the time of measurement and actual pressure. The parameters found from the identification procedure are ventricular active volume, nondepressed elastance, series-elastance, resistance, and the elastance deactivation factor. The first four parameter values were found in agreement with other results reported in literature. The elastance depression factor is a new parameter that could be of physiological or clinical significance since it may be related to the inability of the force generators in the heart muscle to be restored to their full number, after being inactivated or decoupled by filament sliding associated with ejection. On the basis of the results, an alinear state-model of the ventricle, for arbitrary, including physiological flow patterns is proposed.