FACTORS INFLUENCING THE ARTERIAL OXYGEN-TENSION DURING ANESTHESIA WITH ARTIFICIAL-VENTILATION

Factors influencing oxygenation of the arterial blood were studied in 26 patients, during routine anaesthesia with artificial ventilation. T he mean minute volume was 10 l./min and the arterial Pco(2) 27 mm Hg. The mean oxygen consumption was 215 ml/min (STPD) and the physiologica l deadspace 36 per cent of tidal volume (no allowance being made for t he volume of the air passages bypassed by the endotracheal tube). At h igh levels of inspired oxygen concentration, the mean alveolar-arteria l Po-2 gradient was 145 mm Hg, corresponding to a calculated shunt of 11 per cent. At an alveolar Po-2 of 155 mm Hg the gradient was 52 mm H g, 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 fa lls and their mean change was significant. Attempts were made to reduc e the venous admixture by hyperinflation of the lungs. This was succes sful in some patients, when a pressure of 40 cm H2O was maintained for 40 seconds. Lower pressures, and the use of a sustained expiratory pr essure of 5 cm H2O were not effective. Indices of disordered respirato ry function found in this study were not significantly different from those in a comparable study of anaesthetized patients breathing sponta neously.