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

Abstract

SummaryOne hundred ASA grade I 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. Normocapneic ventilation was maintained and analgesia was provided with 0.5 mg.kg-' 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 ( S D 4.04) (awake) and 7.35 (SD 3.15) (after 30 rnin), the partial pressure of oxygen 13.17 ( S D 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-', whilst pyruvate levels were 0.1 (SD 0.02) (awake) and 0.12 (SD 0.21) (after 30 min) mmol.1-I. Patients' recovery and postoperative course were uneventful. Key wordsOxygen; blood level. Measurement techniques; pulse oximetry.It is common practice to provide at least 3 170 oxygen with nitrous oxide during anaesthesia. This concept has been well accepted by anaesthetists in past decades when proper monitoring facilities were not available [l-31. At that time anaesthetists had to depend on calculated alveolar oxygen tension (PAo,) and intermittent sampling of arterial blood to assess oxygenation. Such guesswork has been eliminated by the availability of in-line inspired oxygen (Ro,) monitors (0, analyser) and continuous noninvasive arterial oxygen saturation monitors (pulse oximeter). However, even after the recognition of these monitors as minimum standard monitoring devices, anaesthetists are still providing 31 ' YO oxygen during anaesthesia.It is known that cardiac output [4,5] does not decrease during thiopentone and nitrous oxide anaesthesia with spontaneous ventilation, and that oxygen consumption falls to approximately 15% below basal level during controlled ventilation [6]. In spite of these findings, no study has used the pre-operative arterial oxygen saturation (Spo2) value as a guide for adjustment of the intraoperative Ro2 below 31%.The main objective of this study was to determine whether the conventional 3 1 ?LO inspired oxygen fraction is necessary to maintain a constant (pre-operative) Spo, during balanced anaesthesia using nitrous oxide. MethodsOne hundred adult patients of ASA grade 1 or 2 who had no cardiac or respiratory disease or diabetes mellitus were recruited into the study. Patients were of either sex and they acted as their own control. Ethics committee approval and inf...