Determinants of left ventricular preload-adjusted maximal power

Maximal left ventricular (LV) hydraulic power output (PWRmax), corrected for preload as PWRmax/(V-ed)(beta) (where V-ed is the end-diastolic volume and beta is a constant coefficient), is an index of LV contractility. Whereas preload-adjusted maximal power (PAMP) is usually calculated with beta = 2, there is uncertainty about the optimal value of beta (beta = 1 for the normal LV and 2 for the dilated LV). The aim of this work is to study the determining factors of beta. The data set consisted of 245 recordings (steady state and vena cava occlusion) in 10 animals in an ischemic heart pig model. The occlusion data yielded the slope (E-es; 2.01 +/- 0.77 mmHg/ml, range 0.71-4.16 mmHg/ml) and intercept (V-0; -11.9 +/- 22.6 ml; range -76 to 39 ml) of the end-systolic pressure-volume relation, and the optimal beta-factor (assessed by fitting an exponential curve through the V-ed-PWRmax relation) was 1.94 +/- 0.88 (range 0.29-4.73). The relation of beta with V-ed was weak [beta = 0.60 + 0.02(V-ed); r(2) = 0.20]. In contrast, we found an excellent exponential relation between V-0 and beta [beta = 2.16e(0.0189(V0)), r(2) = 0.70]. PAMP, calculated from the steady-state data, was 0.64 +/- 0.40 mW/ml(2) (range 0.14-2.83 mW/ml(2)) with a poor correlation with E-es (r = 0.30, P < 0.001). An alternative formulation of PAMP as PWRmax/(V-ed - V-0)(2), incorporating V-0, yielded 0.47 +/- 0.26 mW/ml(2) (range 0.09-1.42 mW/ml(2)) and was highly correlated with E-es (r = 0.89, P < 0.001). In conclusion, correct preload adjustment of maximal LV power requires incorporation of V-0 and thus of data measured under altered loading conditions.