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Correlates of Myocardial Oxygen Consumption when Afterload Changes during Halothane Anesthesia in Dogs

Philip L. Wilkinson, MB, BS^*, John V. Tyberg, MD, PhD, John R. Moyers, MD^*, and Anne E. White, MS

*Assistant Professor, Anesthesia. Departments of Anesthesia, Medicine, and the Cardiovascular Research Institute, University of California Medical Center, San Francisco, California 94143. Assistant Professor in Residence, Physiology, Medicine and Cardiovascular Research Institute. Departments of Anesthesia, Medicine, and the Cardiovascular Research Institute, University of California Medical Center, San Francisco, California 94143. Staff Research Associate, Anesthesia. Departments of Anesthesia, Medicine, and the Cardiovascular Research Institute, University of California Medical Center, San Francisco, California 94143.

Abstract

We have examined whether several clinically measurable correlates of myocardial oxygen consumption remain valid correlates when afterload changes during morphine or halothane plus morphine anesthesia. The correlates measured were heart rate, systolic blood pressure, left ventricular end diastolic pressure (LVEDP), and rate-pressure product (RPP). In six dogs we measured myocardial oxygen consumption (mV_O2) and hemodynamic variables during morphine (4 mg/kg) and during morphine-halothane (1.5% end tidal concentration) anesthesia. We changed ventricular afterload by infusion of nitroprusside or phenylephrine and measured mV_O2 as the product of left anterior descending coronary artery flow and arterial-coronary sinus oxygen content difference. RPP was the best correlate of mV_O2 during both morphine-halothane (R = 0.84) and morphine anesthesia (R = 0.71). Systolic pressure and LVEDP also significantly correlated with mV_O2 during both anesthetic states. The slope of the line relating RPP and mV_O2 was significantly (p < 0.05) depressed during added halothane anesthesia. This slope depression (0.016 vs 0.021) is of little practical consequence within the physiologic range of RPP. Regression analysis showed that systolic pressure and heart rate alone account for the major part of changes in mV_O2 during morphine-halothane anesthesia under the conditions of this experiment. After allowing for changes in LVEDP associated with systemic pressure, LVEDP still had significant effects on mV_O2 during morphine anesthesia, but not during morphine-halothane anesthesia. We conclude that RPP is the best correlate of mV_O2 under morphine-halothane anesthesia over a wide range of ventricular afterload states.

Key Words: ANESTHETICS • Volatile: halothane • ANESTHETICS • Intravenous: morphine • HEART: oxygen consumption

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