Persisting concentrating and second gas effects on oxygenation during N 2 O anaesthesia

Summary Theoretical modelling predicts that the concentrating effect of nitrous oxide (N 2 O) 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 ( P a o 2 ) was measured after a minimum of 30 min of relaxant general anaesthesia with an inspired oxygen ( F I o 2 ) of 30%. Patients were randomly allocated to two groups. The intervention group had N 2 O introduced following baseline blood gas measurements, and the control group continued breathing an identical F I o 2 in nitrogen (N 2 ). The primary outcome variable was change in P a o 2 . Mean (SD) in P a o 2 was increased by 1.80 (1.80) kPa after receiving a mean of 47.5 min of N 2 O 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.