Effect of heart rate and preload on index of myocardial performance in the normal and abnormal left ventricle

Background: 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 the effect of heart rate and preload alteration on IMP has not been performed with normal or reduced left ventricular (LV) function. Methods: We studied 14 mongrel dogs at baseline, after the induction of acute ischemic LV dysfunction, and with chronic LV dysfunction. Heart rate was altered by atrial pacing 10 and 20 beats above baseline, and volume loading was accomplished with 10 mL/kg of saline at a paced rate. Hemodynamics, and transmitral and transaortic pulsed Doppler, were obtained. Results: With normal LV function, there were no changes in IMP with pacing. With acute LV dysfunction, IMP was also unchanged with pacing, although both LV ejection time (ET) (192 +/- 23 vs 208 +/- 25 milliseconds, P <.05) and isovolumic contraction time (58 +/- 25 vs 72 +/- 31 milliseconds, P <.05) declined. With chronic LV dysfunction, IMP was unchanged although LV ET declined (188 +/- 15 vs 204 +/- 18 milliseconds, P <.01). Volume loading did not alter the IMP with normal LV function although LV ET increased (208 +/- 25 vs 220 +/- 20 milliseconds, P <.001). With acute LV dysfunction, IMP decreased (0.66 +/- 0.11 vs 0.82 +/- 0.20, P <.05) because of a decrease in isovolumic relaxation time (63 33 vs 76 +/- 38 milliseconds, P <.05). With chronic LV dysfunction, IMP also declined with volume loading (0.59 +/- 0.29 vs 0.73 +/- 0.28, P <.01) because of an increase in LV ET (224 +/- 30 vs 198 +/- 22 milliseconds, P <.0001). Conclusion: Heart rate incrementation does not change IMP. However, volume loading reduces IMP primarily as a result of LV ET lengthening with chronic LV dysfunction. Further systematic evaluation of IMP is needed if this index is to be useful as a prognostic indicator.