Alveolar-arterial PO2 differences in anaesthesia. 1966

During routine anesthesia, normal arterial oxygenation cannot be ensured by observation of the patient or by any practicable measurement technique. It is therefore essential to have an understanding of the factors which influence the arterial PO2. The factors influencing the alveolar PO2 are simple and the actual level may be predicted with considerable accuracy. However, the arterial PO2 cannot easily be predicted because of the large and variable alveolar-arterial PO2 differences which occur during anaesthesia. Measurement of alveolar-arterial PO2 difference during anaesthesia requires the calculation of the ideal alveolar PO2 by an equation which allows for the fact that inert gas is seldom in equilibrium under these circumstances. Analytical techniques must be selected for their freedom from interference by anaesthetic gases. Alveolar-arterial PO2 differences encountered in routine surgical patients correspond to a calculated venous admixture within the range 5-25 per cent of total pulmonary blood flow. Somewhat higher values are encountered when the alveolar PO2 falls below 120 mm.Hg. It is not possible to foresee those patients who develop extensive venous admixture. It is probable that the principal component of the venous admixture is blood flow through atelectatic lung, although its recognition by radiography is difficult. It is often possible to reduce the alveolar-arterial PO2 difference by hyperinflation of the lung.