New technique for analysis of cardiac energetics using a modified Fenn equation

In 12 dogs being supported by cardiopulmonary bypass, the relationship among myocardial oxygen consumption and four energy-consuming factors (basal metabolism, heart rate, tension development, and external work) was studied. Tension (internal work) in the left ventricular wall was evaluated by myocardial tissue pressure with a Mikro-Tip pressure transducer. In an empty beating heart with constant perfusion pressure, both systolic tissue pressure and developed tissue pressure represented the same characteristics as developed tension measured by other methods. As the heart rate was increased, the systolic tissue pressure and developed tissue pressure continued to increase stepwise (Bowditch effect) up to some stimulation rate, at which, however, a decrease began despite a further increase in heart rate (Woodworth effect). Significant regression was established between myocardial oxygen consumption and heart rate, tension (developed tissue pressure x heart rate), and external work (minute work): myocardial oxygen consumption = (9.05 x 10(-3) heart rate) + (1.95 x 10(-4) developed tissue pressure) x heart rate + (1.63 x 10(-3) minute work) + 1.42 (r = 0.7999), where activation energy = 9.05 x 10(-3) ml/100 gm per beat, tension-related energy = 1.95 x 10(-4) ml/100 gm per unit of internal work, energy for work = 1.63 ml/100 gm per unit of external work, and basal metabolism = 1.42 ml/min/100 gm. We concluded that myocardial tissue pressure is a good substitute for tension and that multiple regression with heart rate, tension, and external work (as by modified Fenn's equation) seems indispensable to predict myocardial oxygen tension in the whole heart.