The apparent source resistance of heart and muscle

Abstract

The pumping ability of the left ventricle may be characterized by a graph relating mean left ventricular pressure with mean left ventricular output. This graph is obtained by making the heart eject against different arterial loads, and measuring the resulting left ventricular pressure and aortic flow. For isolated heart muscle an equivalent relationship can be found. The isolated muscle is made to contract in such a fashion that it seems part of the myocardium, i.e., force relaxation precedes lengthening, just as isometric relaxation of pressure precedes left ventricular filling. Mean force should now be related with mean velocity of shortening. This relationship is a straight line and its slope has the dimension of resistance. We cannot understand such a relationship in terms of resistance in a preparation that changes its characteristics with time during contraction. A attempt is made to elucidate this relationship. The contracting muscle is represented by a time varying compliance. Calculations show that this model exhibits an almost linear relationship between mean force and mean velocity of shortening just as the muscle does. The intercept with the mean force axis (mean isometric force,$\bar F_{ISO}$) is:$\bar F_{ISO}$, whereF _a is preload,C _max is compliance of the passive muscle,T is repetition period, andC (t) is the instantaneous muscle compliance. The intercept, with the mean velocity axis$(\bar v_{max} )$ is:$\bar v_{max}$ withC _min the minimum value of compliance during the contraction. It is concluded that the straight line relationship found in an isolated heart muscle between mean force and mean velocity of shortening can be explained in terms of a time varying compliance.