Comparison of measures of left ventricular contractile performance derived from pressure-volume loops in conscious dogs

Abstract

Three measures of left ventricular (LV) performance derived from pressure (P)-volume (V) loops have been proposed: the end-systolic P-V (P(ES)-V(ES)) relation, the stroke work-end-diastolic V (SW-V(ED)) relation, and maximum dP/dt-V(ED) (dP/dt(max)-V(ED)) relation. We evaluated the variability of repeated determinations, and inotropic and load sensitivity of these relations in conscious dogs. LVV was determined from three orthogonal LV diameters measured by sonomicrometry. Three to six sets of variably loaded P-V loops were generated by transient caval occlusions before and again after increasing inotropic state by infusing dobutamine (6 ± 1 μg/kg/min, mean ± SD) and after increasing P(ES) by 49 ± 17 mm Hg with phenylephrine following autonomic blockade. The slope (M(SW)) of the SW-V(ED) relation was the least variable at constant inotropic state (coefficient of variation, 4 ± 3%) compared with the slope (E(ES)) of the P(ES)-V(ES) relation (8 ± 3%) or the slope (dE/dt(max)) of the dP/dt(max)-V(ED) relation (11 ± 6%, p < 0.05). The extrapolated volume-axis intercept of the SW-V(ED) relation was much less variable than the intercepts of the P(ES)-V(ES) or dP/dt(max)-V(ED) relations. M(SW), E(ES), and dE/dt(max) all increased (p < 0.05) in response to dobutamine. The extrapolated volume-axis intercepts of the SW-V(ED) relation was unchanged. M(SW) had the smallest increase in response to dobutamine (124 ± 22% of control) compared to E(ES) (178 ± 67% of control) and dE/dt(max) (211 ± 68% of control, p < 0.05). The position of the P(ES)-V(ES) relation, quantified as the V(ES) at P(ES) = 100 (V100), showed less variability (2 ± 1%) than the slope of the P(ES)-V(ES) relation (8 ± 3%, p < 0.05). V100 decreased from 30.8 ± 17.4 to 26.7 ± 13.7 ml during dobutamine (p < 0.05). After phenylephrine, E(ES), M(SW), and dE/dt(max) decreased by less than 10% (p = NS). The P(ES)-V(ES) relation shifted to the left with this increased afterload and V100 decreased by 3.2 ± 1.5 ml (p < 0.05), whereas the position of the SW-V(ED) relation that integrates data from the entire cardiac cycles is the most stable but also the least sensitive to changes in inotropic state, whereas the dP/dt(max)-V(ED) relation is the most sensitive but also most variable measure of the contractile state. In contrast to the P(ES)-V(ES) and dP/dt(max)-V(ED) relations, the volume-axis intercept of the SW-V(ED) relation can be reproducibly determined in conscious animals and is not altered by enhanced contractile state.