[Limitations of the clinical assessment of myocardial contractility]

The mechanical performance of the myocardium depends upon factors intrinsic to the myofibrils ("contractility") and upon the physicochemical conditions surrounding the cells ("inotropic state"). A precise description of cardiac muscle performance requires a knowledge of 4 variables: length, force, velocity and time. In vivo the problem is further complicated by the presence in the ventricle of spatio-temporal nonuniformities (activation sequence, fiber orientation) and by the fact that major determinants of the inotropic state, such as sympathetic tone, are hard to verify. The indices expected to reflect the tension-velocity-length relation of the myofibrils, either during isovolumic contraction (peak(+)dP/dt) or during ejection (end-systolic relations between pressure or stress and volume, relations between ejection fraction and stress) can generally detect acute changes in inotropic state. Up to now, however, none of these indices has been shown to be very sensitive in detecting basal alterations in contractility. Considering the practical and theoretical limitations on study of an intact ventricle, it would appear indispensable, if we want greater precision in detecting functional alterations of the myocardium, to supplement study of ventricular mechanics by biochemical and morphological analysis of the myocardium.