Optimal preload adjustment of maximal ventricular power index varies with cardiac chamber size

Maximal left ventricular power (PWRmax) 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.

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. PWRmax was the maximal product of simultaneous left ventricular pressure and rate of volume change. PWRmax end-diastolic volume (EDV) data were fit to a power function, PWRmax = alphaEDVbeta (where alpha is a scaling factor and beta is the power coefficient), and the preload sensitivity of beta and PWRmax/EDVbeta ratios (beta = 1, 2, or best fit) were compared.

Beta varied directly with chamber size: 0 = 0.004 x (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 beta = 2 was more appropriate in group 3.

PWRmax/EDV provides adequate preload independence in all but dilated hearts with reduced LV function, whereas PWRmax/EDV2 is required in the latter. These data should help clinical application of a noninvasive PWRmax index for assessing chamber contractility and contractile reserve in human beings.