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

Abstract

Elastance-resistance [E(t)-R] representations of the left ventricle (LV)were evaluated for their ability to reproduce instantaneous pressure [P(t)J and outflow [Q(t)J. 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 [Pis,(t)J with a characteristic time to maximal P1,0(t) [Tpisomaxl. In ejecting beats, 34% of stroke volume was delivered after Tpisomax. P(t) and Q(t) from the steady-state ejecting beats and P,so(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)I and extra-outflow [Qe,x(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 PeX,(t) and Qe,d(t). Elements possessing "active" and "deactive" properties were added to the E(t)-R model in an attempt to account for Pe,xt(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. (Circulation Research 1990;66:218-233) It has become popular to relate instantaneous left ventricular pressure [P(t)], volume [V(t)], and outflow [Q(t)] by using mathematical models based on time-varying elastance [E(t)] and resistance (R).1-8 Such representations are appealing because, if valid, they allow identification of instantaneous left ventricular (LV) pump properties from data recorded from only one or two heart beats. In previous study,3 however, we found serious deficiencies in the performance of E(t)-R models: 1) Errors in the prediction of P(t) increased progressively as end ejection was approached. 2) E(t)-R models failed to predict P(t) and Q(t) data other than that to which these models were fitted. 3) The estimated parameter values of the E(t)-R model were unrealistic estimates of physical entities the parameters supposedly represented. 4) Interdependencies between parameters were found and resulted in nonuniq...

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