INFLUENCE OF AGE ON ATELECTASIS FORMATION AND GAS-EXCHANGE IMPAIRMENT DURING GENERAL-ANESTHESIA

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 (Pa(O2)) decreased with increasing age (P < 0.001) and the alveolar-arterial oxygen partial pressure difference (PA(O2) - Pa(O2)) 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 Pa(O2) was perfusion of regions of low VA/Q ratios, which increased with age. During anaesthesia shunt influenced Pa(O2) 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.