Arterial oxygen saturation during induction of anaesthesia

Three groups of 10 ASA 1 patients were studied to determine the incidence of hypoxaemia (oxygen saturation less than or equal to 90%) using pulse oximetry during induction of 'mask' anaesthesia, and whether simple oxygenation techniques could prevent its occurrence. We also surveyed all anaesthetists in three major hospitals to ascertain their techniques for this method of anaesthesia. Anaesthesia was induced in all patients with thiopentone and maintained with nitrous oxide and isoflurane. The first group received 33% oxygen in nitrous oxide as carrier gases, a second group a few normal breaths of 100% oxygen during thiopentone administration followed by 33% oxygen in nitrous oxide, while a third group received 100% oxygen after loss of eyelash reflex until spontaneous breathing was established. No patient received positive pressure ventilation before spontaneous breathing was established. Six of the 10 patients in the first group became hypoxaemic compared to none in the second group, and three patients became hypoxaemic in the third group. Thirty-seven percent of anaesthetists who responded to the survey either did not apply positive pressure ventilation before establishment of spontaneous breathing, or only did so if apnoea was prolonged. Only one anaesthetist fully pre-oxygenated patients lungs. We conclude that to avoid the likely occurrence of hypoxaemia during induction of mask anaesthesia, a minimum of a few breaths pre-oxygenation is necessary.