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Fabijan DJ, Morris R, Murray GM. 
“The effect of nitrous oxide on hearing”. 
Anaesth Intensive Care. 2000 Oct 06;28(3):270-5.
There is a belief that the administration of nitrous oxide (N2O) increases hearing acuity. This increase can be interpreted as hearing low intensity sounds more loudly. This study examined auditory threshold levels, acoustic impedance, acoustic perception, memory, and discrimination to determine if hearing was altered by end-tidal 10% or 20% nitrous oxide. Subjects also qualitatively interpreted three intensity levels using a subjective intensity test. Observations were made in the breathing mediums of room air in the control stages (Stages 1 and 4), compared to N2O and supplemental oxygen (Stages 2 and 3), in 16 human subjects in a quiet room. Breathing end-tidal 10% and 20% N2O significantly increased middle ear pressure, and produced a significant effect on the subjective interpretation of a sound's intensity. There was no significant effect on auditory threshold at a range of frequencies. It appears that 10% or 20% N2O inhalation does not lead to the commonly held view of increased hearing acuity as measured in terms of auditory threshold. Rather, at these levels of N2O inhalation, subjects experience a state similar to a pre-sleep stage, whereby the hearing diminishes but remains active for loud intensity sounds.
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