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.

The AAI index, the BIS index and end-tidal concentration during wash in and wash out of sevoflurane.

The bispectral index (BIS), auditory evoked potential index (AAI) and the end-tidal sevoflurane concentration were studied during induction and emergence in 10 ASA I-II patients. Both during 'wash-in' and 'wash-out' of sevoflurane, the AAI and BIS indices show huge variability and an overlap of indices between awake and not responding to command. This was the most pronounced during induction and the range of values was larger for the AAI index as compared with the BIS index. Mean (range) BIS was 85 (73-98) and 48 (10-83) awake and unconscious, respectively, and mean AAI index was 71 (43-99) and 21 (4-85), respectively. This study demonstrates the difficulties of using processed EEG variables in real time in a clinical situation of non-steady state pharmacodynamics.

Sevoflurane requirements during ambulatory surgery: a clinical study with and without AEP-index guidance.

BACKGROUND: Several monitors have been developed to measure anesthetic depth. The auditory evoked response uses an auditory signal to actively test the level of brain activity. The aim of the present study was to determine whether sevoflurane titration with A-line auditory guidance from the evoked potential monitor would reduce gas consumption and improve recovery times. METHODS: Patients (n=60, aged 18-65 years) undergoing elective knee arthroscopy were randomized to titrate the main anesthetic sevoflurane with O2:N2O (1:2), either clinically (30 patients) or in combination with a target auditory evoked potential index of 30+/-5 (30 patients) using the A-line monitor (version 1.4, Danmeter A/S; Odense, Denmark). Induction was supplemented with fentanyl, and randomized to 0.05, 0.10 and 0.15 mg immediately before propofol (10 in each group). Sevoflurane consumption and emergence times were the primary and secondary study end-points. RESULTS: Guidance from the A-line monitor did not reduce the sevoflurane consumption time or the emergence, regardless of the fentanyl dose. However, it did reduce the time from the recovery room to discharge eligibility (P<0.05). Sevoflurane consumption decreased inversely with the fentanyl dose (P<0.01), with no impact on emergence times. CONCLUSION: The auditory evoked potential index provided by the A-line monitor does not decrease sevoflurane consumption or emergence times for ambulatory knee arthroscopy.