Effects of fentanyl-diazepam-nitrous oxide anaesthesia on arterial baroreflex control of heart rate in man.

The effects of fentanyl 7.5 micrograms kg-1 (group I), 10.0 micrograms kg-1 (group II) and 12.5 micrograms kg-1 (group III) with diazepam 0.25 mg kg-1 and 70% nitrous oxide on baroreflex control of heart rate in humans were investigated. Phenylephrine (the pressor test), sodium nitroprusside (the depressor test) and graded neck suction provoked baroreflex responses. In group I the pressor, depressor and neck suction baroreflex slopes decreased during anaesthesia. In groups II and III the depressor test slopes were also decreased during anaesthesia. However, the slopes derived from the pressor and neck suction tests did not decrease. These data suggest that baroreflex control of heart rate is attenuated during low doses of fentanyl (7.5 micrograms kg-1). Baroreflex mediated tachycardia is decreased by higher doses of fentanyl (10.0 and 12.5 micrograms kg-1). However, baroreflex-mediated bradycardia is maintained during the higher doses of fentanyl. We suggest this effect is the result of enhanced vagal efferent activity mediated by fentanyl.