Myocardial Tension and Oxygen Uptake

Abstract

Data obtained in 94 tests in nine anesthetized, thoracotomized dogs were re-examined to determine relationships among oxygen uptake per minute (qO₂), cardiac frequency (F), mean aortic pressure (P), stroke volume (V), and heart weight (G). Statistical analysis supports the concept that during each systole qO₂/G is empirically related most closely to the peak mechanical tension (T) developed by a spherical heart, in accord with the relationship qO₂/G-b/F = aT, in which a and b are derived constants for each heart. Peak tension is half the product of the mean pressure and the radius (estimated from the stroke volume). Myocardial oxygen uptake is viewed as developing through a mechanical positive feedback mechanism which triggers the shortening of contractile elements. The number of such elements triggered in each beat determines the peak tension, the oxygen requirement of that beat, the resulting energy release, and the work. The oxygen costs of tension increase with the size of the heart since tension must be applied throughout the entire surface of each shell of contractile elements. Mechanical efficiency varies with the square of the stroke radius; it is not directly affected by mean aortic pressure or cardiac frequency. Factors affecting the all-or-none phenomenon, mobilization of tension, myocardial efficiency, the law of the heart, myocardial oxygen uptake, and the coronary flow are discussed in light of these findings.