Effect of enflurane on the baroreflex control of heart rate in decerebrate rats

Volatile anesthetics alter the arterial baroreflex (BRX) but its mechanisms are poorly understood. This study was designed to determine the effect of 1 and 2 minimal alveolar concentrations (MAC) of enflurane on the BRX parameters in unanesthetized brain stem-intact and decerebrate rats. Under enflurane anesthesia, the femoral artery and both femoral vein were catheterized for pressor (phenylephrine) and depressor (nitroprusside) drug delivery and continuous blood pressure measurements. Decerebration was performed at midcollicular level. BRX tests were performed in 3 time periods; before enflurane (conscious brain-intact), during 1 or 2 MAC enflurane exposure 1 hour after a sham operation or a decerebration operation, and 2 hours after the termination of enflurane (zero enflurane). Mean arterial pressure (MAP) and heart rate (HR) were fitted to a sigmoid logistic equation, the Boltzman equation. The curve of best fit was obtained with a computer program. 1 MAC and 2 MAC of enflurane shifted MAP-HR baroreflex curves to the left in the all groups and significantly attenuated the baroreflex range. The slope of conscious intact period and zero enflurane period of each group did not change significantly, but during the enflurane period the slope was significantly lowered. Enflurane depressed the baroreflex sensitivity (slope) and the HR range in a similar dose-dependent manner in both brain stem-intact and decerebrate rats. Such results draw into question whether the suprapontine sites contribute to enflurane's actions on cardiovascular autonomic regulation.