A randomized prospective study of BIS guided low-flow sevoflurane anesthesia; is air safer than nitrous oxide?

Abstract

Objective: This prospective randomized BIS controlled study was conducted to compare low-flow anesthesia (LFA) techniques with or without nitrous oxide (N₂O) using remifentanil and sevoflurane, with respect to ventilation parameters and sevoflurane consumption.

Methodology: Forty-five, ASA I/II women younger than 65-year-old, scheduled for gynecological surgery lasting nearly two hour under general anesthesia were enrolled. Electrocardiogram (ECG), pulse oximetry, non-invasive arterial pressure, train-of-four (TOF) and bispectral index (BIS) were monitored. Anesthesia was induced by inj propofol 2 mg/kg with increments of 10 mg until BIS was under 60 and rocuronium 0.6 mg/kg. Patients were randomized to one of three groups, 15 patients in each, to receive either N₂O (Group N) or N₂O-free anesthesia (Groups R I and R II). All groups received bolus remifentanil 0.5 µg/kg and then infusions @ 0.2 µg/kg/min (Group R I), or 0.05 µg/kg/min (Group R II) as maintenance. Anesthesia was maintained with sevoflurane in O_{2 +} N₂O or air. Signs indicating adequate depth of anesthesia during maintenance phase of anesthesia were HR, arterial blood pressure and BIS. The goal was to obtain a BIS value between 40 and 60 and hemodynamic parameters within 20% of baseline values. Opioid infusions were constant as sevoflurane vaporizer dial setting was adjusted in ± 0.5% volumes to maintain this goal. Systolic, diastolic and mean arterial pressures, HR, SpO₂, the inspired and expired gas partial pressure measurements of O₂, sevoflurane, N₂O, and CO₂, BIS values sevoflurane vaporizer dial settings, and recovery times were recorded. Measuring points were at every 5 min during surgery. A minimum inspired oxygen concentration (FiO₂) of 0.3 was maintained. Consumption and costs for sevoflurane were calculated.

Results: Demographic data, duration of surgery and anesthesia were similar between the groups. A significant decrease was observed in FiO₂ by time in all groups. For all recording times FiO₂ was statistically greater in Group N. The difference between delivered O₂ and FiO₂ was the lowest in Group N. The difference between inspired and expired fractions of sevoflurane (Fi_sevo and Fet_sevo) reduced by time during the low flow period. It was lower in Group N than in remifentanil groups. Total sevoflurane consumption was significantly greater in Group R II than Group N but there was no significant difference in sevoflurane consumption and costs per patient per minute between groups. Recovery times were comparable between the groups.

Conclusions: We concluded that risk of hypoxia and volatile anesthetic consumption did not differ with or without N₂O in remifentanil-sevoflurane, low flow anesthesia. Monitoring FiO₂ is essential in both air/O₂ and N₂O/O₂ mixtures. Both are safe to administer unless FiO₂ is lower than 30%. BIS-guided sevoflurane with its low solubility feature successfully adapts quickly to variable anesthetic depth levels during low-flow anesthesia.

Key words: Anesthesia; Closed Circuit, Anesthesia; Rebreathing; Nitrous oxide; Consciousness Monitors; Bispectral Index Monitor

Citation: Inan G, Celebi H. A randomized prospective study of BIS guided low-flow sevoflurane anesthesia; is air safer than nitrous oxide? Anaesth Pain & Intensive Care 2016;20(3):266-272

Received: 31 January 2016; Reviewed: 27 March 2016; Corrected: August 2016; Accepted; 9 September 2016