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 N2O 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.