Time-dependent middle ear pressure changes under general anaesthesia in children: N2O-O2 mixture versus air-oxygen mixture.

OBJECTIVES: The aim of this study was to investigate the effects of N2O-O2 mixture (Inspired O2 30%) on middle ear pressure (MEP) in children compared with the effects of an air-oxygen mixture (Inspired O2 50%). METHOD: The study included thirty child patients who underwent general anaesthesia for different reasons, with the exception of ENT problems and ear interventions. They were randomly divided into two groups. Group 1 (15 children: 10 male and 5 female) received a N2O-O2 mixture (Inspired O2 30%); and group 2 (15 children: 10 male and 5 female) were given an air-oxygen mixture (Inspired O2 50%). MEP was measured using a portable impedance analyser before the operation (PreO),10 minutes after intubation (10AEn), 30 minutes after intubation (30AEn), 10 minutes before extubation (10BEx), 15 minutes after the operation (PO15), 30 minutes after the operation (PO30), 1 hour after the operation (PO1h) and 6 hours after the operation (PO6h). RESULTS: The pressure and compliance values were the same in groups 1 and 2. The pressure-time graphs for the two groups were different: in Group 2, MEP rose quickly at 10AEn and positive pressure values were seen in the middle ear. MEP then fell rapidly until the end of the surgery and lower and negative pressures (Mean -50 daPa) were observed at PO6h. In Group 1, MEP was elevated at 10AEn and positive pressure was found (but not as high as in Group 2). MEP then fell more slowly. In other words, positive pressure in the middle ear persisted longer and the middle ear was subjected to positive pressure and nitrogen over a longer period. Separate analyses were made in Groups 1 and 2 of pressure differences and of compliance values at eight measurement points using the Friedman test. Differences in pressure values were found to be statistically significant in both Group 1 (p = 0.000) and Group 2 (p = 0.000). In Group 1, all the 10AEn and 30AEn values were significantly higher than the PreO, PO30, PO1h and PO6h values. The 10BEx value was significantly higher than the PreO and PO1h values. The PO15 value was significantly higher than the PreO value. In Group 2, the PO6h value was significantly lower than the 10BEx, 10AEn and 30AEn values. The PO1h value was significantly lower than the 30AEn values. The MEP values increased in Group 1 in younger and taller children and in children receiving anaesthesia for shorter periods. MEP values increased in Group 2 in younger and taller children, and in heavier children. MEP values fell with the length of anaesthesia. CONCLUSION: In brief anaesthesia, nitrogen was not removed from the middle ear quickly in Group 1: middle ear pressure values were higher. The nitrous oxide remained in the middle ear longer and so the possibility of ear toxicity may increase. In Group 2, 50% O2 was rapidly absorbed and removed from the middle ear and so middle ear pressure was not as high. It may be concluded that air-oxygen mixture (Inspired O2 50%) anaesthesia should be recommended as being more reliable in tympanoplasties and other middle ear interventions than a N2O-O2 mixture (Inspired O2 30%).