PREDICTION OF MOVEMENT DURING PROPOFOL NITROUS-OXIDE ANESTHESIA - PERFORMANCE OF CONCENTRATION, ELECTROENCEPHALOGRAPHIC, PUPILLARY, AND HEMODYNAMIC INDICATORS

Background: Movement in response to painful stimulation is the end poi nt classically used to assess the potency of anesthetic agents. In thi s study, the ability of modeled propofol effect-site concentration to predict movement in volunteers during propofol/nitrous oxide anesthesi a was tested, then it was compared with the predictive abilities of th e Bispectral Index and 95% spectral edge frequency of the electroencep halogram, pupillary reflex amplitude, and systolic arterial blood pres sure. In addition, the relationships between simple end points of loss and recovery of consciousness, and pupillary, hemodynamic, and propof ol concentration indicators were studied. Methods: Ten healthy volunte ers were anesthetized with an infusion of propofol, which was increase d in three equal steps to 21 mg . kg lean body mass(-1). h(-1). After loss of the ability to hold a syringe and of the eyelash reflex, 60% n itrous oxide was introduced and the trachea was intubated without the use of muscle relaxants. The propofol infusion rate then was decreased to 15.4 mg kg lean body mass(-1). h(-1). Ten minutes later, tetanic e lectrical stimulation was administered to the thigh via needle electro des: if movement was observed within 1 min, the propofol infusion rate was increased by 1.75 mg . kg lean body mass(-1). h(-1) 5 min after t he stimulus; if not, it was similarly decreased. This 15-min sequence was repeated until volunteers ''crossed over'' from movement to no mov ement (or vice versa) four times. The propofol infusion rate then was increased to 21 mg . kg lean body mass(-1). h(-1), nitrous oxide was d iscontinued, the trachea was extubated, and the infusion rate was decr eased in five equal steps over 50 min. The times at which the eyelash reflex returned and the birth date was recalled were recorded. The ele ctroencephalogram was monitored continuously (FP1, FP2, ref: nasion, g round: mastoid). Measurements of the pupillary response, arterial bloo d pressure, and heart rate were recorded during induction and awakenin g, just before and for 5 min after each stimulation. Arterial blood sa mples were obtained for propofol assay, and propofol effect-site conce ntrations were calculated at each time. The predictive value of indica tors was compared using a new statistic, the prediction probability (P -K). Results: Loss and return of the eyelash reflex occurred at greate r propofol effect-site concentrations than either dropping the syringe or recall of the birthday. The propofol effect-site concentration (in the presence of 60% nitrous oxide) predicted to prevent movement afte r a supramaximal stimulus in 50% of volunteers was 1.80 mu g/ml (95% c onfidence limits: 1.40-2.34 mu g/ml). The Bispectral Index (P-K = 0.86 ), 95% spectral edge frequency (P-K = 0.81), pupillary reflex amplitud e (P-K = 0.74), and systolic arterial blood pressure (P-K = 0.78) did not differ significantly from modeled propofol effect-site concentrati on (P-K = 0.76) in their ability to predict movement. Conclusions: Ind icators of pharmacodynamic effect, such as the electroencephalogram, p upillary light reflex, and systolic arterial blood pressure, predict m ovement as well as effectsite concentration during propofol/nitrous ox ide anesthesia. Loss and return of the eyelash reflex correspond to a deeper level of anesthesia than syringe-dropping or recall of the birt h date.