FACTORS INFLUENCING THE ARTERIAL OXYGEN TENSION DURING ANAESTHESIA WITH ARTIFICIAL VENTILATION

Abstract

Factors influencing oxygenation of the arterial blood were studied in 26 patients, during routine anaesthesia with artificial ventilation. The mean minute volume was 10 1./min and the arterial Pco2 27 mm Hg. The mean oxygen consumption was 215 ml/min (STPD) and the physiological deadspace 36 per cent of tidal volume (no allowance being made for the volume of the air passages bypassed by the endotracheal tube). At high levels of inspired oxygen concentration, the mean alveolar-arterial Po2 gradient was 145 mm Hg, corresponding to a calculated shunt of 11 per cent. At an alveolar Po2, of 155 mm Hg the gradient was 52 mm Hg, corresponding to a calculated venous admixture of 9 per cent. There was no progressive increase in venous admixture during anaesthesia in patients under the age of 43 but most of the older patients showed falls and their mean change was significant. Attempts were made to reduce the venous admixture by hyperinflation of the lungs. This was successful in some patients, when a pressure of 40 cm H2O was maintained for 40 seconds. Lower pressures, and the use of a sustained expiratory pressure of 5 cm H2O were not effective. Indices of disordered respiratory function found in this study were not significantly different from those in a comparable study of anaesthetized patients breathing spontaneously.