Cardiac muscle mechanics and ventricular performance: force and time parameters

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Abstract
Studies were designed to determine parameters of force and velocity derived from isolated cardiac muscle mechanics which would permit comparison with related performance characteristics in the intact ventricle. In the papillary muscle, for a wide range of resting muscle lengths, tension developed isometrically (Po) was found a linear function of initial muscle length and proportional to the extent of isotonic shortening (δL). Length-tension curves obtained isotonically were virtually identical to those obtained isometrically. In the intact heart stroke volume is a function of δL. Ventricular end-diastolic pressure is a function of resting muscle length. As increments of resting muscle length increased δL, increasing ventricular end-diastolic pressure augmented stroke volume. Increasing the load carried by the muscle (afterload) at a given resting muscle length increased work performed by the papillary muscle. Similarly, increasing aortic pressure increased stroke work for a given ventricular end-diastolic pressure in the intact heart. In the papillary muscle time to maximal δL was an inverse function of maximal velocity of shortening (Vmax), but independent of both Lr and afterload. The inverse relation of time to max δL and Vmax were shown for norepinephrine and frequency. In the intact heart time to peak ventricular pressure, was found to be largely independent of ventricular end-diastolic and aortic pressures and inversely related to the inotropic state of the myocardium. Contractility of the papillary muscle strictly defined by Vmax and Po, was thus characterized by time to maximal δL, as well as δL. Contractility of the ventricle at a given ventricular end-diastolic pressure could then be defined by stroke volume and peak pressure time, which are indices of force and velocity.

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