MEASURING RECOVERY FROM GENERAL-ANESTHESIA USING - CRITICAL FLICKER FREQUENCY - A COMPARISON OF 2 METHODS

Critical flicker frequency (CFF) is the frequency at which a flickering light appears steady. It is a sensitive measure for assessing recovery from anaesthesia. The CFF is almost always determined with the method of limits by which the flickering frequency is progressively decreased (or increased) until the patient reports a change from fusion to flicker (or flicker to fusion). This method has two disadvantages: it is influenced by the response bias (i.e., the subjective criterion used by the subject to decide that flicker is present or absent) and by the response delay (i.e., the interval between the perceptual change and the response). To avoid these problems, the method of forced choice is recommended. For each trial, the subject observes the light during two short successive periods. The light flickers during only one period, according to chance. The patient must indicate the period during which flickers occur. If uncertain, the patient has to make a guess. The aim of this study was to compare the two methods for assessing recovery from general anaesthesia. Two induction agents were used to obtain different recovery profiles. Twenty patients undergoing uncomplicated surgery lasting less than two hours were tested. They received either thiopentone or midazolam for induction, according to a randomized design. Vecuronium was used to facilitate tracheal intubation and anaesthesia was maintained with fentanyl, isoflurane and nitrous oxide. The CFF was measured before induction and at 60, 120 and 180 minutes after arrival in the recovery room. The person measuring CFF was unaware of the induction agent used. The CFF determined by forced choice was higher than that by the method of limits (53.3 vs 49.3 Hz across induction agents, periods and patients, P < 0.01). Both methods yielded a similar profile of recovery, indicating that maximal CFF depression occurred at 60 min and that it was more pronounced for midazolam than for thiopentone. The forced choice method was easier to use than that of limits. We conclude that the method of forced choice and that of limits yield similar results. However, the method of forced choice is to be preferred because it eliminates response bias and response delay.