FACTORS INFLUENCING THE ARTERIAL OXYGEN TENSION DURING HALOTHANE ANAESTHESIA WITH SPONTANEOUS RESPIRATION

Abstract

Factors influencing oxygenation of the arterial blood were studied, during routine anaesthesia, in thirty-six patients anaesthetized with halothane and allowed to breathe spontaneously, with a mean minute volume of 5 l./min. There was evidence of an unsteady respiratory state during the first hour of anaesthesia. Oxygen consumption was 87 per cent of basal. Physiological deadspace amounted to 33 per cent of the expired tidal volume (all patients intubated). At high levels of inspired oxygen concentration, the mean alveolar-arterial Po, gradient was 184 mm Hg, corresponding to a shunt of 14 per cent of pulmonary bloodflow. At lower levels of alveolar Po₂, the alveolar-arterial Po, gradient diminished but was above the value which would be caused by a shunt of 14 per cent. This was probably due to uneven ventilation perfusion ratios (maldistribution) corresponding to a calculated venous admixture rising as high as 30 per cent. It is concluded that, to ensure the maintenance of a normal arterial Poa in the majority of patients, the alveolar Po, is required to be as high as 200 mm Hg and this needs an inspired oxygen concentration of 35 per cent under the conditions investigated in this study.