Calculation of inspired oxygen fraction during anaesthesia based on awake oxygen saturation

One hundred ASA grade 1 or 2 patients were studied to assess the use of awake oxygen saturation to determine the necessary inspired oxygen fraction during anaesthesia. The inspired oxygen fraction was adjusted gradually to the lowest possible value to maintain the pre-operative oxygen saturation at a constant level. Normocapnic ventilation was maintained and analgesia was provided with 0.5mg.kg-1 of pethidine given intravenously. In addition to routine monitoring and measurement (electrocardiograph, oxygen saturation, expired carbon dioxide concentration and noninvasive blood pressure), arterial samples for blood gas analysis and venous samples for lactate and pyruvate levels were taken when awake and 30 min after achieving a steady state with a minimum inspired oxygen fraction. The mean inspired oxygen fraction required in these patients was 20.91 (SD 1.92%) ranging from 18-25%. The arterial pH were 7.34 (SD 4.04) (awake) and 7.35 (SD 3.15) (after 30 min), the partial pressure of oxygen 13.17 (SD 1.97) (awake) and 13.83 (SD 1.6 kPa) (after 30 min) and the partial pressure of carbon dioxide and base excess were normal. Serum lactate levels were 1.04 (SD 0.25) (awake) and 1.15 (SD 0.48) (after 30 min) mmol.l-1, whilst pyruvate levels were 0.1 (SD 0.02) (awake) and 0.12 (SD 0.21) (after 30 min) mmol.l-1. Patients' recovery and postoperative course were uneventful.