Low-dose halothane anesthesia does not affect the hemodynamic estimation of myocardial oxygen consumption in dogs

The accuracy of the product of systolic blood pressure and heart rate as a correlate of myocardial oxygen consumption has been questioned in anesthetized patients, suggesting that anesthesia alters the relationship between myocardial oxygen consumption and the hemodynamics. This relationship was reexamined in the awake state and during 0.95% end-tidal halothane anesthesia in seven chronically instrumented dogs. Measurements of myocardial oxygen consumption were compared with two indices, the systolic pressure rate product and the pressure work index. The latter is a weighted combination of the systolic pressure rate product and external cardiac work. A wide range in myocardial oxygen consumption (3.0-38.2 ml O2.min-1.100g-1) and hemodynamic states (range in pressure rate 4,100-40,700 mmHg/min and range in pressure work 3.7-37.0 ml O2.min-1.100g-1) was achieved with infusions of norepinephrine, nitroglycerin, neosynephrine, nitroprusside, and isoproterenol. Both the pressure rate product and the pressure work index correlated closely with myocardial oxygen consumption not only when the animals were awake but also during anesthesia. For each animal best-fit regression analysis for the awake and the anesthetized data did not reveal any significant change in the slope or y-intercept. Pooled data analysis that only permitted a change in slope with the addition of anesthesia (intercept unchanged) revealed a 17% decrease (P less than 0.05 on t test) in the slope constant when the pressure rate product was applied and no change in slope when the pressure work index was used to estimate myocardial oxygen consumption. It was concluded that myocardial oxygen consumption can be estimated during 0.95% halothane anesthesia by either index but that the pressure work index performs better than the systolic pressure rate product.