Contractile Element Work: A Major Determinant of Myocardial Oxygen Consumption*

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Abstract
Cardiac contraction in the intact dog heart has been analyzed in terms of Hill''s series elastic model for muscle, and the total work performed by the contractile element was calculated. Contractile element work was derived as the sum of the work done in stretching the series elastic component to peak force, plus the work performed by the contractile element in shortening of the muscle fiber. The latter quantity was calculated as the total fiber shortening work minus the amount performed by the recoiling series elastic component. Over a wide range of altered hemodynamics, an excellent correlation between contractile element work and left ventricular oxygen consumption was observed (r = 0.91). This relationship was significantly better (p< 0.05) than that found between oxygen consumption and pressure-time per minute(r = .74), force-time per minute(r = .74), or fiber shortening work(r = .74). An index of contractile element work was derived using only heart rate, left ventricular systolic mean pressure, and mean left ventricular volume and was shown to correlate well with the calculated contractile element work(r = .995). From an analysis of the contractile element work-oxygen consumption relationship, the oxygen consumption of the beating, empty dog heart has been estimated to be 4.3 ml/100 g/minute. Multiple regression of oxygen consumption as a linear function of both contractile element work and heart rate was also performed in an effort to examine activation energy per beat as a possible determinant of myocardial oxygen consumption. In this manner the energy requirement of the non-beating heart was estimated to be 1.9 ml/100 g/minute, and the energy of activation estimated to be 0.02 ml/beat/100 g. A method is described by which the Fenn effect may be studied in the intact dog heart. Using this approach, the Fenn effect could not be demonstrated in these studies.