Exercise induced augmentation of myocardial oxygen extraction in spite of normal coronary dilatory capacity in dogs.

Myocardial O2-extraction rate was studied during exercise induced augmentation of cardiac work in dogs. The O2-extraction rate at rest was 75% of arterial content. Progressive levels of exercise increased the animals' O2-consumption from 7 ml/min-kg up to 91 ml/min-kg. Cardiac output rose from 108 ml/min-kg at rest to 484 ml/min-kg at the highest exercise level. The increase in myocardial O2-consumption from 9 ml/min-100 g at rest up to 57 ml/min-100g at the highest exercise level was met by an increase in coronary flow from 59 to 256 ml/min-100 g and a rise of myocardial AVDO2 from 15 to 22 Vol%. Thus the latter contributed 40% to the augmented myocardial O2-requirements. Coronary venous O2-saturation decreased to 9% saturation during highest levels of exercise. This low value was not the result of a limited coronary dilatory capacity, of inadequate state of exercise training, or of a relative underperfusion of the inner layers of the left ventricle. Thus, augmentation of myocardial O2-extraction rate seems to be a mechanism of physiological relevance during exercise induced elevation of myocardial O2-requirements in dogs and may be explained by capillary recruitment in the myocardium.