The influence of nitrous oxide on propofol dosage and recovery after total intravenous anaesthesia for day-case surgery.

We studied the influence of nitrous oxide on the maintenance dose of propofol and recovery characteristics in 42 patients, aged 18-62 years, ASA 1 or 2, scheduled for day case inguinal herniotomy. Using a double-blind, randomised design, patients received anaesthesia with propofol-alfentanil-vecuronium-oxygen and either nitrous oxide or room air (FIO2 = 0.30). The rate of propofol infusion was adjusted depending on anaesthetic depth as judged using standard clinical criteria; alfentanil was administered on a weight basis. Patients' lungs were manually ventilated after tracheal intubation and muscle relaxation was reversed at the end of surgery with neostigmine and atropine. A series of psychomotor tests was performed pre-operatively and 30 and 120 min postoperatively. The mean maintenance doses of propofol were 0.084 mg.kg-1.min-1 in the N2O group and 0.088 mg.kg-1.min-1 in the air group (p = 0.97). In the nitrous oxide group the mean (SD) interval to spontaneous eye opening was 13.1 (7.3) min compared to 8.1 (4.9) min in the air group (p = 0.01). Similarly, the interval until obtaining a standardised response was 13.5 (5.3) min and 9.8 min (5.4) in the nitrous oxide and air groups, respectively (p = 0.04). The addition of nitrous oxide to propofol-alfentanil-vecuronium anaesthesia does not reduce propofol requirements and prolongs early recovery compared to air.