PROPOFOL AND AUTONOMIC REFLEX FUNCTION IN HUMANS

The effects of continuous infusions of propofol on baroreceptor reflex regulation of cardiac rate and peripheral sympathetic nerve activity were evaluated in seven healthy, normotensive, young (19-26 yr), male volunteers. Heart rate, radial artery pressure, and continuous recordi ngs of efferent sympathetic vasoconstrictor outflow (from the peroneal nerve) were monitored. Baroreceptor perturbations were produced by bo lus intravenous injections of nitroprusside (100 mu g) followed 60 s l ater by phenylephrine (150 mu g) These stimuli were delivered to subje cts while conscious and during propofol anesthesia (200 mu g kg(-1) mi n(-1)) at least 25 min after subjects were paralyzed (vecuronium), had tracheas intubated, and were ventilated (30% O-2:70% N-2) to maintain normocarbia. Additional data were collected during hypercarbic condit ions and during a lower infusion rate of propofol (100 mu g kg(-1) min (-1)) combined with 70% nitrous oxide. Propofol infusions significantl y lowered sympathetic nerve activity (SNA) and blood pressure (BP) and increased heart rate (HR). Cardiac baroreceptor sensitivity determine d during nitroprusside was reduced 60% during propofol infusions and w as only subtly improved during simultaneous N2O administration. In con trast, reflex sensitivity during phenylephrine was not changed from aw ake values during each of the three experimental conditions. Reflex re gulation of SNA was nearly abolished during normocarbic conditions und er propofol anesthesia but restored to conscious levels during hyperca rbia and during N2O administration. These data indicate that propofol markedly attenuates reflex responses to hypotension, but that reflex s ympathetic responses are better maintained in hypercarbic conditions a nd when lower doses of propofol are used in conjunction with N2O. In c ontrast, reflex responses to a hypertensive stimulus seem to be well p reserved during propofol infusions regardless of the prevailing Pace, or the presence of N2O.