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 i.v. 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 syringe 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 varied 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.