Loading and performance of the heart as muscle and pump

Evaluation of ventricular muscle-pump function of the failing heart still constitutes a major problem in cardiology. More than a decade ago it was proposed that the mechanical performance of the heart should be approached as a muscle rather than as a pump. The purpose was to find properties of cardiac muscle which would help in distinguishing alterations of ventricular performance due to changes in loading conditions (Frank-Starling adaptation; heterometric autoregulation) from either positive (circulating catecholamines, changes in heart rate) or negative (myocardial or muscle failure) inotropic alterations (homeometric autoregulation; changes in contractility). Obviously, the pragmatic aim was to distinguish a good from a bad heart more accurately. Several contractile properties of isolated mammalian cardiac muscle have since been well characterised and recent studies have permitted a coherent and unifying description of cardiac muscle performance. The basis of it is that mechanical activity of myocardial muscle can be defined in terms of the interrelationship between force development, instantaneous length during shortening, and velocity of shortening, independently of the time course of activation. This analysis provides a model independent description of what muscle really does and how it behaves when its loading conditions are altered (Brutsaert, 1974). Yet, the application of these basic muscle concepts to the intact heart still remains controversial for several reasons: insufficient understanding of basic cardiac muscle mechanics, failure to take into consideration some conceptual limitations, the many oversimplified assumptions in modelling the ventricle, the complex physiological loading of a combined muscle-pump system, etc. Pragmatic usefulness as the only justification for applying muscle concepts to the intact ventricle is no longer tenable as a sufficient argument. The ventricle is primarily a combined muscle-pump system and neither a papillary muscle nor even less a skeletal muscle. The nature of loading during muscle shortening in the ventricular wall is quite different from that in the isolated muscle since the ventricle is not called on to lift a weight but to eject a viscous fluid into a viscoelastic vascular system (Abbott and Gordon, 1975; Milnor, 1975). Muscle concepts as directly derived from skeletal muscle physiology should therefore no longer be forcibly applied to the intact heart for ventricular function evaluation. Instead, emphasis should be placed on the integration of muscle and pump function and on properties of heart muscle that help to define its behaviour as an integral part of the wall of a pumping system (Paulus et al.,1976).