chapter  114
Coronary Artery Physiology and Intracoronary Ultrasonography
Pages 14

Myocardial Oxygen Consumption The principal function of the coronary arteries is to provide oxygen and nutrients to the myocardium. Myocardial oxygen consumption (MV•O2) is equal to the product of coronary blood flow and the arteriovenous oxygen gradient across the coronary vascular bed, that is, arterial oxygen content minus coronary sinus oxygen content. In the resting state, myocardial oxygen extraction is near maximum and coronary sinus oxygen saturations are typically 30% or less (or PO2 < 20 mm Hg). Because myocardial oxygen extraction is already near maximum, MV•O2 can increase only by increasing coronary blood flow. MV•O2 is dependent on coronary blood flow, and changes in MV• O2 closely parallel changes in coronary blood flow. Important determinants of MV•O2 are heart rate, inotropic state (contractility), and intramyocardial wall stress. MV• O2 can be approximated clinically by the product of systolic blood pressure and heart rate (called the “rate-pressure product”). The rate-pressure product is an estimate of MV• O2 (and, thus, coronary blood flow) and is frequently used during exercise testing.