Effects of halothane, enflurane, and isoflurane on coronary blood flow autoregulation and coronary vascular reserve in the canine heart

Abstract

To investigate the effects of volatile anesthetics on coronary blood flow (CBF) autoregulation and coronary vascular reserve, studies were performed on chronically instrumented dogs, awake and during the administration of 1.0 MAC halothane, enflurane, and isoflurane. Coronary pressure-flow plots were generated by measuring left anterior descending coronary artery blood flow while varying coronary inflow pressure with a hydraulic occluder. Autoregulation was quantitated by two measures: the slope of the horizontal 'autoregulated' portion of the pressure-flow relationship and the autoregulation index (ArI) or Norris. Slope values (ml·min-1·mmHg-1 ± SD) were: awake, 0.243 ± 0.043; halothane, 0.414 ± 0.044; enflurane, 0.587 ± 0.187; and isoflurane, 0.795 ± 0.246. The increase in slope was statistically significant only for halothane and isoflurane (P < .05). The ArI approaches 1.0 when autoregulation is perfect, and approaches zero or is a negative number when autoregulation is absent. The authors found ArI values of: awake, 0.55; halothane, -0.08; enflurane, -0.01; isoflurane, -0.02. These values indicate good autoregulation while awake, but impaired autoregulation with all three anesthetics (P < .05). Coronary vascular reserve was calculated, at a diastolic coronary pressure of 40 mmHg, as the difference between resting flow and flow during maximal coronary vasodilation induced by intracoronary adenosine. Coronary vascular reserve, maximal coronary conductance and coronary zero-flow pressure were not significantly altered by these anesthetics. The authors conclude that 1.0 MAC enflurane, halothane, and isoflurane mildly disrupt CBF autoregulation, increasing CBF out of proportion to myocardial demands. Under the conditions of this study, these anesthetics do not affect maximal CBF or coronary vascular reserve.