Background and objective: The mechanism underlying stable cuff pressure in the Brandt rediffusion endotracheal tube during anaesthesia with nitrous oxide is not fully understood. The present study assessed changes in gas concentrations in the cuff during and after nitrous oxide anaesthesia.
Methods: The pressure of an air-filled Brandt endotracheal tube cuff was measured; anaesthesia was maintained with nitrous oxide 67% and oxygen 33% for 6 h in the Continuous Group; oxygen was substituted for nitrous oxide after 3 h of nitrous oxide anaesthesia in the Discontinuous Group (n = 8 for each). In some other patients, the study was terminated at 90, 180, 270 and 360 min (n = 8 for each). Gas concentrations in the cuff were measured at the end of the study.
Results: Cuff pressures increased slightly during the first 180 min of anaesthesia (P < 0.001). Thereafter, the cuff pressure did not change significantly in the Control Group but decreased in the Discontinuous Group (P < 0.001); there was a significant difference between the two groups (P < 0.0001). Cuff pressure never exceeded 22 mmHg and there were no air leaks in either group during the 6 h anaesthesia. Nitrous oxide concentrations in the cuff were 11.3 ± 2.6% and 2.6 ± 0.8% in the Control Group and Discontinuous Groups, respectively; however, changes in carbon dioxide and oxygen concentrations in the cuff were within approximately 1%.
Conclusions: Small changes in the nitrous oxide concentration in the cuff contribute to a stable cuff pressure in the Brandt rediffusion system, but changes in carbon dioxide or oxygen concentrations have little effect.