Hexamethonium and midazolam terminate dysrhythmias and hypertension caused by intracerebroventricular bupivacaine in rabbits

Abstract

Previous studies have demonstrated that bupivacaine administered directly into the central nervous system (CNS) is capable of producing signs of bupivacaine cardiovascular toxicity. To investigate the mechanisms by which bupivacaine may act within the CNS to produce cardiovascular toxicity, we studied four groups of halothane-anesthetized rabbits in which infusion of intracerebroventricular (icv) bupivacaine or intravenous (iv) phenylephrine resulted in dysrhythmias and hypertension. In group 1 (n = 5), icv bupivacaine (500 ± 79 μg [mean ± SEM]) produced dysrhythmias lasting 73 ± 13 min, whereas icv saline caused no dysrhythmias or hypertension. In group 2 (n = 9), icv bupivacaine-induced hypertension and dysrhythmias were abolished by icv midazolam in 4.4 ± 0.6 min, and when dysrhythmias and hypertension recurred (22 ± 0.9 min), hexamethonium (10 mg/kg iv) promptly terminated dysrhythmias and hypertension (14 ± 1 s). In group 3 (n = 10), icv bupivacaine-induced dysrhythmias and hypertension were not affected by increasing the inspired halothane concentration from 0.8 to 1.6%. In group 4 (n = 6), iv phenylephrine-induced dysrhythmias and hypertension were not affected by icv midazolam. These results suggest that icv bupivacaine produces dysrhythmias and hypertension by increasing autonomic nervous system (ANS) outflow from the brain stem. The finding that peripheral autonomic blockade by hexamethonium rapidly terminated dysrhythmias and hypertension supports this mechanism. We speculate that icv bupivacaine produces an increase in autonomic outflow by blockade of the inhibitory γ-aminobutyric acid (GABA) neurons that are known to be the principal tonic inhibitors of the ANS. This hypothesis is supported by the finding that CNS GABA potentiation by icv midazolam, but not generalized CNS depression by halothane, terminated dysrhythmias and hypertension.