Depressed inotropic state and reduced myocardial oxygen consumption in the human heart

The relation between myocardial oxygen uptake (MVO₂) and the mechanical properties of left ventricular contraction were studied in 14 patients with and without left ventricular dysfunction. Coronary blood flow was estimated by helium washout, and left ventricular inotropic state was characterized from the mean left ventricular circumferential fiber shortening velocity in the minor equator (mean V_CF) and the value at peak tension (V_CF at max T). Ten patients with left ventricular dysfunction whose mean V_CF values were less than 1 circumference/sec were compared with patients whose mean V_CF values fell within the normal range. Patients with depressed shortening velocity had lower than normal average coronary blood tlow values (53 ± 3 vs. 70 ± 3 ml/100 g per min, respectively, P < 0.005) and reduced MVO₂ (75 ± 5 vs. 120 ± 6 μl/100 g per beat, P < 0.001). Coronary venous washout curves were well fitted by mono-exponential functions in normal patients but deviated appreciably from a single exponential in patients with left ventricular dysfunction, thereby indicating nonuniform distribution of coronary blood flow in the latter group. Peak and mean developed tension at the minor circumference, tension-time index, stroke work and fiber-shortening work per myocardial volume correlated poorly with MVO₂. However, mean V_CF and V_CF at max T correlated significantly with MVO₂ (r = 0.63, P < 0.02). These findings are in agreement with previous experimental findings demonstrating the importance of the contractile state as a determinant of myocardial oxygen consumption.