THERMOREGULATORY VASOCONSTRICTION DURING PROPOFOL NITROUS-OXIDE ANESTHESIA IN HUMANS - THRESHOLD AND OXYHEMOGLOBIN SATURATION

To determine the thermoregulatory effects of propofol and nitrous oxid e, we measured the threshold for peripheral vasoconstriction in seven volunteers over a total of 13 study days. We also evaluated the effect of vasoconstriction on oxyhemoglobin saturation (Spo2). Anesthesia wa s induced with an intravenous bolus dose of propofol (2 mg/kg), follow ed by an infusion of 180 mug.kg-1.min-1 for 15 min, and maintained wit h 60% nitrous oxide and propofol (80-160 mug.kg-1.min-1). Central and skin surface temperatures and Spo2 (using two different pulse oximeter s) were measured continuously; plasma propofol concentrations and arte rial Po2 were measured at 15-min intervals. Volunteers were cooled wit h a circulating water blanket until definitive peripheral vasoconstric tion was detected. The tympanic membrane temperature triggering vasoco nstriction was considered the thermoregulatory threshold. Vasoconstric tion developed on seven study days during propofol/nitrous oxide anest hesia at a central temperature of 33.3 +/- 1.0-degrees-C (mean +/- SD) and plasma propofol concentration of 3.9 +/- 1.1 mug/mL. The threshol ds during anesthesia were significantly lower than those during the co ntrol period (36.7 +/- 0.3-degrees-C), but the correlation between pla sma propofol concentrations and vasoconstriction thresholds was poor. On the remaining six study days, vasoconstriction did not develop desp ite central temperatures ranging from 32.1 to 32.7-degrees-C. Correspo nding propofol concentrations were 4.1-10.9 mug/mL. These data suggest that anesthesia with propofol, in typical clinical concentrations, an d 60% nitrous oxide substantially inhibits thermoregulatory vasoconstr iction. Vasoconstriction increased Spo2 by approximately 2% without a significant concomitant change in Po2. The observed increase in Spo2 p robably reflects decreased transmission of arterial pulsations to veno us blood in the finger.