Late-systolic pumping properties of the left ventricle. Deviation from elastance-resistance behavior.

Elastance-resistance [E(t)-R] representations of the left ventricle (LV) were evaluated for their ability to reproduce instantaneous pressure [P(t)] and outflow [Q(t)]. Experiments were performed in open-chest rats. P(t) and Q(t) were measured during steady-state ejecting beats and during a beat in which the aorta was suddenly clamped. The degree of clamping varied from partial to total occlusion. The total occlusion beat was considered an isovolumic beat that generated an isovolumic pressure [Piso(t)] with a characteristic time to maximal Piso(t) [Tpisomax]. In ejecting beats, 34% of stroke volume was delivered after Tpisomax. P(t) and Q(t) from the steady-state ejecting beats and Piso(t) from the clamped beat were then used to estimate parameters of an E(t)-R model. Components of P(t) and Q(t) not accounted for by E(t)-R were identified and termed extra-pressure [Pext(t)] and extra-outflow [Qext(t)]. Pext(t) and Qext(t) were near-zero valued until Tpisomax; then they became systematically positive and finally negative valued after end ejection. During partial aortic occlusion, P(t) was elevated and Q(t) was reduced. However, the time of ejection was extended, and the fraction of stroke volume delivered after Tpisomax increased as P(t) was made higher. Partial occlusion also prolonged the positive phase of Pext(t) and Qext(t). Elements possessing "active" and "deactive" properties were added to the E(t)-R model in an attempt to account for Pext(t) and Qext(t) during partial occlusion. Optional forms of these elements were considered. These expanded E(t)-R models were fitted to basal ejecting data and then asked to predict data from a partial occlusion beat. All expanded models failed to adequately predict the partial occlusion pressure and/or outflow. It was concluded that 1) late ejection was quantitatively important to LV pumping, 2) behavior during late ejection was inconsistent with E(t)-R, and 3) ad hoc modification of E(t)-R models was not likely to yield LV pumping models that could satisfactorily reproduce instantaneous P(t) and Q(t) behavior over the entire ejection period.