Physostigmine increases the dose of propofol required to induce anaesthesia

Purpose: This prospective, randomized, double-blind study was performed to determine the effect of administration of physostigmine on the dose of propofol required to produce loss of consciousness. Methods: Forty female unpremedicated patients were assigned in a random blind design to receive either 2 mg physostigmine or equal volume of normal saline iv, five minutes before induction of anaesthesia with propofol. All patients received general anaesthesia for breast surgery. Propofol was infused at a constant rate of 200 ml · hr^-1 while patients were breathing oxygen 100% via a face mask. In each patient the dose of propofol required to produce loss of the ability to grasp a 20 ml synnge was recorded as the end-point of loss of consciousness. At this point the protocol was terminated and, after intubation of the trachea, anaesthesia was maintained with a nitrous oxide-isoflurane or sevoflurane mixture in oxygen, increments of an opioid and a muscle relaxant. Doses of anaesthetic drugs and duration of anaesthesia vaned and depended on the type of breast surgery, determined by frozen section. Results: The mean ± SD dose of propofol required to produce loss of consciousness was 2.4 ± 0.6 mg · kg^-1 and 2.0 ± 0.4 mg · kg^-1 in the physostigmine and in the normal saline groups respectively (P = 0.014). Conclusion: Physostigmine pretreatment increases the dose of propofol required to produce loss of consciousness.