Anesthetic induction agents, sympathetic nerve activity and baroreflex sensitivity: A study in rabbits comparing thiopental, propofol and etomidate

The mechanisms of arterial hypotension following intravenous anesthetic ind uction agents are multifactorial. The purpose of this study was to evaluate and compare the effects of thiopental, propofol and etomidate on hemodynam ics, sympathetic outflow and arterial baroreflex sensitivity using not only neuraxis-intact but also totally baro-denervated rabbits. A total of 60 ra bbits was anesthetized with urethane, tracheotomized, and mechanically vent ilated with oxygen in nitrogen (FiO(2) 0.5). The left renal sympathetic ner ve was isolated and placed on a bipolar electrode to record renal sympathet ic nerve activity (RSNA). Thirty animals underwent a surgical preparation o f total baroreceptor denervation. Bolus injections of an anesthesia inducti on dose of thiopental 4 mg/kg and twice the induction dose of propofol 4 mg /kg significantly decreased RSNA to the same extent (19.4 +/- 6.7 and 19.7 +/- 5.2% reduction, mean +/- SEM) and mean arterial pressure (MAP) also to the same extent (19.5 +/- 4.6 and 22.1 +/- 3.1% reduction) in the neuraxis- intact animals. RSNA was increased (34.5 +/- 6%) without reduction of MAP b y an induction dose of etomidate, 0.3 mg//kg. Sympathetic barosensitivity w as attenuated even 10 min after thiopental at 4 mg/kg or propofol at 4 mg/k g (68% and 54% of control, respectively). Propofol at 2 mg/kg (induction do se) and etomidate at 0.6 mg/kg decreased RSNA and MAP only in the baro-dene rvated animals. It was found from the barosensitivity study that patients c an be hemodynamically unstable even though blood pressure has returned to n ormal after thiopental and propofol administration. Data suggest that etomi date can even stimulate the sympathetic nervous system and increase sympath etic outflow. It was also clearly found from the baro-denervated animal stu dy that thiopental was stronger than propofol in directly suppressing sympa thetic outflow at the induction dose.