Sympathetic Responses to Induction of Anesthesia in Humans with Propofol or Etomidate

Anesthetic induction with propofol commonly results in hypotension. This study explored potential mechanisms contributing to hypotension by recording cardiovascular responses including sympathetic neural activity from patients during induction of anesthesia with propofol (2.5 mg.kg-1 plus 200 micrograms.kg-1.min-1) or, for comparison, etomidate (0.3 mg.kg-1 plus 15 micrograms.kg-1.min-1). Twenty-five consenting, nonpremedicated, ASA physical status 1 and 2, surgical patients were evaluated. Measurements of R-R intervals (ECG), blood pressure (radial artery), forearm vascular resistance (plethysmography), and efferent muscle sympathetic nerve activity (MSNA microneurography: peroneal nerve) were obtained at rest and during induction of anesthesia. In addition, a sequential bolus of nitroprusside (100 micrograms) followed by phenylephrine (150 micrograms) was used to obtain data to quantitate the baroreflex regulation of cardiac function (R-R interval) and sympathetic outflow (MSNA) in the awake and anesthetized states. Etomidate induction preserved MSNA, forearm vascular resistance, and blood pressure, whereas propofol reduced MSNA by 76 +/- 5% (mean +/- SEM), leading to a reduction in forearm vascular resistance and a significant hypotension. Both cardiac and sympathetic baroslopes were maintained with etomidate but were significantly reduced with propofol, especially in response to hypotension. These findings suggest that propofol-induced hypotension is mediated by an inhibition of the sympathetic nervous system and impairment of baroreflex regulatory mechanisms. Etomidate, conversely, maintains hemodynamic stability through preservation of both sympathetic outflow and autonomic reflexes.