Postoperative pulmonary function

Abstract

Absorption collapse of alveoli may develop if partial or complete occlusion of the airways occurs during the inhalation of 100% oxygen. 1--3 In addition, it has been suggested, on theoretical grounds, that alveoli with 'critical' v/Q ratios may collapse with only modest increases in the inspired oxygen c~ncentration.~ It is known that a reduction in functional residual capacity (FRC) occurs during anaesthesias* and it has beer, postulated' that this is associated with an increase in the number of closed airways, resulting in an increase in maldistribution of the inspired gas in relation to the pulmonary capillary blood flow.Nitrous oxide is a more soluble gas than nitrogen and there may be a greater tendency for absorption collapse to occur when oxygen1 nitrous oxide mixtures, as compared with oxygenlnitrogen mixtures, are inspired. It is known that the absorption collapse occurring as a result of breathing 100% oxygen at residual volume does not resolve for at least 14 hours' so it is likely that if collapse occurs in association with nitrous oxide anaesthesia, its effects may continue at least until the day after operation.This study compares the relative influence of nitrogenloxygen and nitrous oxideloxygen mixtures, inspired during anaesthesia, on Pao2, FRC and other subdivisions of lung volume in the immediate postoperative period. Patients and methods48 patients aged between 20 and 65 years, undergoing either vagotomy and pyloroplasty or cholecystectomy, were allocated randomly to two groups. One group was ventilated during anaesthesia with a mixture of 30% oxygen in nitrogen (N2 group), the other received nitrous oxide in place of nitrogen (NzO group).Patients of both sexes in the age range 21-65 years and within 10% of the normal weight range were admitted to the study. Apart from the reasons for surgery, the patients were healthy. Informed consent was obtained from each patient before the study commenced. Lung function tests were performed on the day before operation and on the lst, 2nd and 5th days after operation. At all times during the measurements, the patients lay in a standard semirecumbent position in the laboratory.Arterial blood was withdrawn from the radial artery into heparinised plastic syringes. Pao2, Pacoz and pH were measured immediately using appropriate Radiometer electrodes. A blood-gas factor (1,065) was applied to the measured Po2 of blood to allow for the difference between the response of the electrode to air and blood. Using the Severinghaus slide rule," blood-gas values were corrected for the difference in temperatures between the electrodes, which were maintained at 37WC, and the patient's core temperature. The alveolar-arterial oxygen partial pressure difference (PAo~ -Paoz) was calculated as:(Pro, -Pacoz) R -Pa0 P A O~-P~~Z =