The effects of posture and isoproterenol on the velocity of left ventricular contraction in man. The reciprocal relationship between left ventricular volume and myocardial wall force during ejection on mean rate of circumferential shortening.

A study was performed in five normal men in whom left ventricular volume was measured by thermodilution in the supine and 60 degrees head-up postures, in the control state, and then during steady-state response to isoproterenol. The mean rate of circumferential shortening of the left ventricle was calculated for each of the postures in both inotropic states and was found to remain constant in the control state at 12.5 +/-0.6 cm/sec in the supine posture and 13.3 +/-0.5 cm/sec in the tilted posture. Similarly, mean rate of circumferential shortening remained constant in response to the positive inotropic effect of isoproterenol at 20.9 +/-0.5 cm/sec in the supine position and 20.7 +/-0.5 cm/sec in the tilted posture. It is concluded that the constancy of mean rate of circumferential shortening over the relatively broad physiologic range of left ventricular end-diastolic volume and mean force of ejection during a given state of myocardial contractility represents the coupled reciprocal influences of ventricular wall tension and myocardial fiber length on the velocity of ventricular wall shortening. Unlike stroke work, stroke power, and mean rate of left ventricular ejection, which are volume-dependent parameters of myocardial performance, the mean rate of circumferential shortening appears to be a reasonable index of left ventricular contractility, which in steady-state conditions is independent of left ventricular end-diastolic volume and mean ventricular wall force of ejection. In this study, changes in mean rate of circumferential shortening associated with changes of heart rate were small and variable.