Effects of diazepam and midazolam on baroreflex control of heart rate and on sympathetic activity in humans.

The effects of induction of anesthesia with diazepam and midazolam on baroreflex control of heart rate and on plasma levels of catecholamines were investigated in this study. Group 1 subjects (n = 10) received diazepam, 0.4 mg/kg. Group 2 subjects (n = 10) received midazolam, 0.3 mg/kg. Baroreflex function was assessed using a pressor test (phenylephrine). In addition, samples for subsequent determination of plasma norepinephrine and epinephrine levels and plasma diazepam or midazolam concentrations were collected before and 5, 10, and 15 min after intravenous drug administration. The pressor baroreflex slope declined significantly after diazepam or midazolam administration with the maximal changes (-45 and -43%, respectively) observed when plasma diazepam or midazolam concentrations were the highest. Norepinephrine plasma concentrations decreased at each measurement with both drugs. In contrast, epinephrine concentration decreased only after midazolam. The authors conclude that diazepam or midazolam used for induction of anesthesia results in a transient depression of baroreflex function and a sustained decrease of sympathetic tone. This study also indicates that the depression of arterial baroreflex heart rate responses under diazepam or midazolam anesthesia are less pronounced than the depression of baroreflex responses reported by other investigators with potent inhalational anesthetics. However, this disruptive effect of diazepam and midazolam on sympathetic control of circulation might induce a limited ability to compensate for hemodynamic alterations related to hypovolemia.