Index of myocardial performance is afterload dependent in the normal and abnormal left ventricle

The index of myocardial performance (IMP) has been used as a prognostic systolic-diastolic index for patients with dilated cardiomyopathy and postmyocardial infarction. To date, systematic evaluation of afterload alteration (arterial pressure) on IMP has not been performed with normal or reduced left ventricular (LV) function. We studied 15 mongrel dogs at baseline, after the induction of acute ischemic LV dysfunction, and with chronic LV dysfunction. Each dog was atrially paced, and the arterial pressure was reduced with nitroprusside (NTP) (>10 mm Hg) and increased with methoxamine (Methox) (>30 mm Hg) in random order. Hemodynamics and transmitral and transaortic pulsed Doppler were obtained. With normal LV function, there were no changes in IMP with NTP. Methox reduced IMP (0.51 ± 0.12-0.45 ± 0.12, P < .05) as a result of a shortened isovolumic contraction time (ICT). With acute LV dysfunction, IMP declined with NTP (0.74 ± 0.19–0.65 ± 0.17, P < .01) because of a shortened ICT and isovolumic relaxation time. Methox prolonged IMP (0.73 ± 0.16–0.83 ± 0.21, P < .05). With chronic LV dysfunction, NTP resulted in a reduced IMP (0.75 ± 0.27–0.57 ± 0.27, P < .01) as a result of a reduced ICT and isovolumic relaxation time and a prolonged LV ejection time associated with an increased LV ejection fraction. Methox increased IMP (0.72 ± 0.26–1.31 ± 0.43, P < .001) because of an increased ICT and isovolumic relaxation time and a reduced LV ejection time associated with a reduced LV ejection fraction. Forward stepwise regression indicated that both LV systolic pressure ( P = .0006) and LV ejection fraction ( P = .0222) were independent predictors of IMP. IMP is afterload dependent in the normal LV. IMP is afterload dependent with acute and chronic LV dysfunction by influencing the isovolumic indices and LV ejection time in opposite directions. Further systematic evaluation of IMP is needed if this index is to be useful as a prognostic indicator.