Persisting concentrating and second gas effects on oxygenation during N2O anaesthesia

Theoretical modelling predicts that the concentrating effect of nitrous oxide (N2O) uptake on alveolar oxygenation is a persisting phenomenon at typical levels of ventilation - perfusion (V/Q) inhomogeneity under anaesthesia. We sought clinical confirmation of this in 20 anaesthetised patients. Arterial oxygen pressure (PaO2) was measured after a minimum of 30 min of relaxant general anaesthesia with an inspired oxygen (FIO2) of 30%. Patients were randomly allocated to two groups. The intervention group had N2O introduced following baseline blood gas measurements, and the control group continued breathing an identical FIO2 in nitrogen (N-2). The primary outcome variable was change in PaO2. Mean (SD) in PaO2 was increased by 1.80 (1.80) kPa after receiving a mean of 47.5 min of N2O compared with baseline conditions breathing O-2/N-2 (p = 0.01). This change was significantly greater (p = 0.03) than that in the control group: + 0.09 (1.37) kPa, p = 0.83 and confirms the presence of significant persisting concentrating and second gas effects.