The Doppler-derived myocardial performance index (MPI), defined as the sum of isovolumetric contraction and relaxation durations divided by ejection time and reflecting both systolic and diastolic myocardial function, has been found to be related to morbidity and mortality in cardiac diseases. The MPI is easily obtained, reproducible, and has a narrow range in healthy subjects. The goal of this study was to study the influence of heart rate changes on the MPI. Thirty patients with sick sinus syndrome treated with a pacemaker with a right atrial lead were studied. The patients were paced at increasing rates from 50 to 100 bpm. The MPI increased on average 0.02 +/- 0.03 per 10-bpm increase in rate. The correlation between MPI and heart rate was weak (rho = 0.31, P <.01).