Effect of Severity of Pulmonary Disease on Nitrous Oxide Washin and Washout Characteristics

The influence of chronic obstructive pulmonary disease (COPD) on the nitrous oxide (N₂O) washin and washout characteristics was evaluated in 90 (ASA II-III) males undergoing elective peripheral surgery under general anaesthesia with controlled ventilation. Patients were classified by preoperative bedside pulmonary function testing into three groups. Group I (n=30), patients without COPD (FEV₁/FVC > 80% predicted values; control group); Group II (n=30), patients with mild COPD (FEV₁/FVC = 65-79% of predicted values); and Group III (n=30), patients with moderate COPD (FEV₁/FVC = 50-64% of predicted values). The anaesthetic technique was standardized for all patients. The Datex Capnomac Ultima™ monitor was used to measure the inspired and expired concentrations of nitrous oxide (N₂O), carbon dioxide (CO₂), and isoflurane. The duration of both N₂O washin (time from start of N₂O administration to equilibrium of inspired and expired N₂O concentrations) and 5 per cent washout (time from discontinuation of N₂O to an expired N₂O concentration of 5 per cent of the equilibrium value) were recorded. The duration of N,O washin and washout were significantly prolonged in Groups II and III (P < 0.001) as compared to the control group (Group I). The end-tidal CO₂ concentration decreased significantly during N₂O washout without causing oxygen desaturation (SpO₂ < 90%). We conclude that the duration of N₂O washin and washout were significantly prolonged in anaesthetized patients with COPD which may delay the induction and recovery from N₂O anaesthesia.