FORCE-VELOCITY RELATION IN CARDIAC-MUSCLE - ANALYSIS IN THE TIME DOMAIN - APPLICATION TO THE DETERMINATION OF QUANTITATIVE AND TIME-DEPENDENT MECHANISMS

This paper questions the convention that force-velocity relation in cardiac muscle determines the fundamental character of contraction. It is proposed that force-velocity relation is the consequence of force and velocity behaviour as functions of time. To demonstrate this, a simplified model was constructed. The model assumes each active site to be a load-dependent force-generator. It is proposed that the rate of appearance of calcium in the myofilament area is a preset function of time and determines that the contraction variables - tension, shortening and velocity of shortening - are also preset functions of time. Hence, force-velocity, length-velocity and other inter-relations between these contraction variables are simply obtained by plotting the appropriate variables one vs the other. The model's ascending protion of isometric tension and the force-velocity relation plotted due to variations in initial muscle length, heart rate and cardio-active agents, are in good agreement with the curves produced by real cardiac muscle. The model's length-velocity relation depends on instantaneous length, regardless of initial muscle length and time, as observed in real muscle. According to the model, Vmax and dp dtmax depend on calcium and preload to the same extent as P0, suggesting a clue to the interpretation of similar real muscle response. Analysis of the contractile variables in the time-domain rather than in the force-velocity plane enables the determination of practical parameters of cardiac performance. The model predicts two independent clinical indices: (1) t- dp dt, the time interval from the onset of contraction to dp dtmax for the estimation of the 'time-dependent factors' of contraction, and (2) Ap, the product of t- dp dt and dp dtmax for the estimation of the 'quantitative factors' of contraction. Experimental data support this prediction. © 1979.