The effects of fresh gas flow on the amount of sevoflurane vaporized during 1 minimum alveolar concentration anaesthesia for day surgery: a clinical study

BACKGROUND: Even small costs per case can become economically significant in high volume day surgical units. While general anaesthesia with higher fresh gas flow rates has technical advantages, they result in higher costs. The aim of the present study was to evaluate drug consumption and direct costs related to variations in the fresh gas flow and use of nitrous oxide at a 1 minimum alveolar concentration (MAC) sevoflurane end-tidal anaesthesia for day surgery. METHODS: Thirty-two ASA I-II patients undergoing elective day surgery under general anaesthesia 14 (10-21) min] were studied. Induction was with propofol and fentanyl 100 microg. After laryngeal mask airway placement, patients were randomized to one of four different fresh gas flows: 1 or 2 l/min oxygen in air (50% oxygen), 3 l/min (33% oxygen), or 3 l/min oxygen in nitrous oxide (33% oxygen). Anaesthesia was maintained at 1 MAC. The vaporizer was weighed before and after each procedure. The primary study variable was the sevoflurane utilization per minute. RESULTS: Sevoflurane utilization increased with increasing fresh gas flow for oxygen in air (r2 = 0.89). The nitrous oxide in oxygen group had the lowest sevoflurane utilization, even compared with the lowest oxygen in air group (0.36 vs. 0.48 g/min). CONCLUSION: Sevoflurane utilization during 1 MAC anaesthesia increases linearly with fresh gas flow and is still higher than when nitrous oxide is used even with very low fresh gas flow rates. Direct inhaled anaesthesia-related costs are consequently 20% higher than when nitrous oxide is used, even for the lowest oxygen in air fresh gas flows.