Influence of age on atelectasis formation and gas exchange impairment during general anaesthesia

Abstract

We have studied the effects of anaesthesia on atelectasis formation and gas exchange in 45 patients of both sexes, smokers and non-smokers, aged 23-69 yr. None of the patients showed clinical signs of pulmonary disease, and preoperative spirometry was normal. In the awake patient, partial pressure of arterial oxygen (PaO2) decreased with increasing age (P < 0.001) and the alveolar-arterial oxygen partial pressure difference (PAO2 - PaO2) increased with age (P < 0.001). Shunt, assessed by the multiple inert gas elimination technique, was small (mean 0.5%) and uninfluenced by age. However, there was an increasing dispersion (log SD Q) of ventilation/perfusion ratios (VA/Q) and increasing perfusion of regions of low VA/Q (VA/Q <0.1) with increasing age (P < 0.001 and P < 0.05, respectively). No patient displayed any atelectasis as assessed by computed x-ray tomography of the chest. During inhalation anaesthesia (halothane or enflurane) with mechanical ventilation, 39 of 45 patients developed atelectasis and shunt. There was a strong correlation between the atelectatic area and the magnitude of shunt (r = 0.81, P < 0.001). Atelectasis and shunt did not increase significantly with age, whereas log SD Q and perfusion of regions with low VA/Q ratios did (r = 0.55, P < 0.001 and r = 0.35, P < 0.05, respectively). Awake, the major determinant of PaO2 was perfusion of regions of low V;A/Q ratios, which increased with age, During anaesthesia shunt influenced PaO2 most low Va/Q being a secondary factor which, however, was increasingly important with increasing age, thus explaining the well-known age-dependent deterioration of arterial oxygenation during anaesthesia. © 1991 Copyright: 1991 British Journal of Anaesthesia.