Background Maximal left ventricular power (PWR_max) can index contractile function and reserve; however, its marked preload dependence mandates load adjustment to yield a more cardiac-specific measurement. Prior studies have used varying methods, but supporting data have generally been lacking. We hypothesized that the optimal approach for preload adjustment varies with ventricular volume (particularly end-systolic volume) and is significantly different for dilated hearts with reduced left ventricular function compared with small to normal-sized hearts with normal systolic function.

Methods Left ventricular pressure-volume relations were measured by the conductance catheter method in 36 patients, with preload altered by inferior vena cava obstruction. Patients with normal ventricles (n = 16), hypertrophy or mitral stenosis (n = 12), and dilated cardiomyopathy (n = 8) were divided into three groups based on resting end-diastolic volume: group 1, 66.3 ± 12; group 2, 118.1 ± 20; and group 3, 218.2 ± 48 ml. PWR_max was the maximal product of simultaneous left ventricular pressure and rate of volume change. PWR_max end-diastolic volume (EDV) data were fit to a power function, PWR_max = ⍺EDV^β (where ⍺ is a scaling factor and β is the power coefficient), and the preload sensitivity of β and PWR_max/EDV^β ratios (β = 1, 2, or best fit) were compared.

Results β Varied directly with chamber size: β = 0.004 · (EDV + 0.56), r = 0.65, p < 0.0001. However, it was equally well approximated by 1.0 in groups 1 and 2 (ESV <75 ml, EF >40%), whereas β = 2 was more appropriate in group 3.

Conclusion PWR_max/EDV provides adequate preload independence in all but dilated hearts with reduced LV function, whereas PWR_max/EDV² is required in the latter. These data should help clinical application of a noninvasive PWR_max index for assessing chamber contractility and contractile reserve in human beings. (Am Heart J 1998;136:281-88.)