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TO DETERMINE cardiac performance, a comparison between the whole heart and isolated (skeletal) muscle often has been made. Otto Frank (1895) designed his beautiful experiments on the frog heart following "the same pattern which Fick and Von Kries used in their studies of skeletal muscle. . . ." A similar comparison between heart and muscle can be found in Starling's formulation of, his "law of the heart" (Patterson et al., 1914): "The law of the heart is therefore the same as that of skeletal muscle that the mechanical energy set free on passage from the resting to the contracted state depends . . . on the length of the muscle fibers." Presently, the same line of thinking prevails, since variables derived from studies on isolated muscle are determined for the whole heart to establish its performance (Mason et al., 1970). The task of the heart is to transport arterial blood to the various organs and venous blood to the lungs. It does this by ejecting a sufficient amount of blood at a given pressure against the resistance of the arterial system. Although this pump function of the heart has been fully recognized since Harvey wrote his "de motu cordis" (1628), it is still not clear how to quantify it. Cardiac output, stroke volume, left ventricular function curves, peak systolic pressure, stroke work, peak wall stress, rate of rise of left ventricular pressure, maximum velocity of shortening, and many other variables all have been put forward as a measure of the function or performance of the heart (Abel, 1976; Ahmed et al., 1977; Baylen et al., 1977; Frank et al., 1965; Kreulen et al., 1975; McDonald et al., 1972; Raphael et al., 1977; Sonnenblick and Downing, 1963). However, a

How to quantify pump function of the heart. The value of variables derived from measurements on isolated muscle.