Evaluation of contractile state by maximal ventricular power divided by the square of end-diastolic volume.

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Abstract
BACKGROUND Maximal ventricular power (PWRmax) reflects contractile state and has the potential to be noninvasively determined. However, its sensitivities to preload, afterload resistance, and inotropic state are incompletely defined. The present study determines these dependencies and proposes a novel power-based contractile index that is little altered by load. METHODS AND RESULTS Seven open-chest, autonomically blocked dogs were instrumented with a proximal aortic flow probe, central aortic and ventricular micromanometers, and a conductance catheter for ventricular chamber volume. Preload was transiently reduced by left atrial hemorrhage, and afterload was increased by intra-aortic balloon inflation. Inotropic state was pharmacologically altered by lidocaine, dobutamine, propranolol, or verapamil. PWRmax was highly preload sensitive, altering 1.7 +/- 0.1-fold a given percent change in end-diastolic volume (EDV). This preload dependence was reduced by dividing PWRmax by EDV but was virtually eliminated when PWRmax was divided by EDV2. This latter index also displayed little change in response to as much as 60% increases in afterload resistance. PWRmax/EDV2 varied directly with inotropic state, correlating to both the slope (Ees) of the end-systolic pressure-volume relation (PWRmax x 1,000/EDV2 = 0.31 x Ees - 0.04, r = 0.82, p less than 0.001) and the slope (A) of the dP/dtmax-EDV relation (PWRmax x 1,000/EDV2 = 0.025 x A + 0.02, r = 0.86, p less than 0.001). PWRmax values determined from the product of ventricular pressure and flow versus central aortic pressure and flow were nearly identical over a broad loading range, indicating that PWRmax may be noninvasively assessed (i.e., without requiring left ventricular chamber pressure). CONCLUSIONS PWRmax divided by EDV2 provides a measure of contractile function that is little influenced by loading conditions and has potential for noninvasive clinical use.