[Middle ear pressure variations during anesthesia with nitrous oxide].

Some authors have reported that significant pressure changes occured in middle ear clefts when nitrous oxide (N2O) is inhaled during general anesthesia . In 18 adult patients without ear diseases who underwent non-otological surgery , middle ear pressure measurements were made with impedance audiometer (Teledyne TA-2C) just before and during N2O anesthesia. The results obtained were as follows: 1) Middle ear pressure began to rise immediately after the inhalation of N2O. Gradual rise and sudden fall of the pressure continued during anesthesia due to the diffusion of N2O and passive venting of the eustachian tube. 2) Rising rate of the middle ear pressure changed rapidly after the change of the inspired N2O concentration. 3) The partial pressure of N2O in the middle ear was calculated from the pressure variations, and the time constant of the N2O diffusion was above 400min. 4) The partial pressure of N2O calculated from the measured middle ear pressure during anesthesia turned out to be different from that calculated from the theoretical N2O concentration in blood, although neither oxygen nor Ethrane was presumed to bring any significant increase on the middle ear pressure. 5) Hence it is conceivable that the time constant of the N2O diffusion into the middle ear is more than the preceding value.