Indirect assessment of myocardial oxygen consumption in the evaluation of mechanisms and therapy of angina pectoris

Exercise testing, applied in a systematic, standardized manner, is a more reliable means of assessing angina pectoris than clinical evaluation. In utilizing exercise testing to determine functional impairment In angina, it is essential to distinguish between external stress, or the load on the skeletal muscles, and internal or cardiac stress. This distinction is critical to meaningful evaluation of angina since the performance of the heart is of primary concern in this syndrome. Evaluation of external exercise capacity alone is limited as a measure of cardiac performance since the relationship between external function and cardiac performance may not be direct. A readily applicable approach to determination of cardiac functional capacity in angina is afforded by indirect assessment of myocardial oxygen consumption (MVO₂). This is achieved through derived indexes of heart rate and blood pressure, two major determinants of MVO₂, usually in the form of the product of heart rate × systolic blood pressure × systolic ejection period (triple product) or the product of the first two variables (double product). Although these indexes do not encompass all the major determinants of MVO₂, they demonstrate a constancy at the point of angina in each individual that is consistent with the occurrence of this syndrome at a peak, critical threshold of MVO₂ above which regional myocardial perfusion cannot rise. Utilization of indirect indexes of MVO₂ allows evaluation of the mechanisms of action of a variety of conditions and therapeutic interventions in angina. Thus, enhanced perfusion of ischemic myocardium is suggested by augmentation of the triple product when angina occurs during exercise, as observed after successful aortocoronary bypass graft surgery. Medical therapy of angina acts through a fundamentally different mechanism demonstrated by reduction in the triple product at given levels of exercise with no increase at the point of angina, thus indicating a major effect on the circulatory response to the physical stress. Indirect indexes of MVO₂ are thus useful in interpreting mechanisms and evaluating therapy of angina but they must be applied with appropriate caution because of their inherent limitations.