Chronic obstructive pulmonary disease and anaesthesia: formation of atelectasis and gas exchange impairment

Gas exchange impairment and the development of atelectasis during enflurane anaesthesia were studied in 10 patients (mean age 70 yrs) with chronic obstructive pulmonary disease (COPD). Awake, no patient displayed atelectasis as assessed by computed X-ray tomography. The ventilation/perfusion distribution (VA/Q), studied by the multiple inert gas elimination technique, displayed an increased dispersion of VA/Q ratios (the logarithmic standard deviation of the perfusion distribution, mean log Q SD 0.99; upper 95% confidence limit of normal subject: 0.60), and increased perfusion of regions with low VA/Q ratios (0.005 less than VA/Q less than 0.1: 5.4% of cardiac output). Shunt was negligible (mean 0.6%). Computed chest tomography showed significantly larger cross-sectional thoracic areas than previously seen in subjects with healthy lungs (p less than 0.01). No atelectasis was seen in any patient. During anaesthesia there was a further worsening of the VA/Q mismatch with significantly increased log Q SD (1.29, p less than 0.05) but no increase in shunt (mean 1%). Minor atelectatic areas were noted in three patients, the others displayed no atelectasis at all. Chest dimensions were reduced by no more than 3% during anaesthesia, suggesting an unchanged or only minimally affected functional residual capacity. These findings contrast with those seen in patients with healthy lungs in whom atelectasis and shunt regularly develop during anaesthesia.