Middle Ear Pressure Variations During 50% N2O Anesthesia as a Function of Mastoid Pneumatization

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Hypothesis: Mastoid size is a factor in middle ear (ME) pressure regulation.

Background: In a study investigating ME pressure variations during nitrous oxide (N2O) anesthesia, particularly high values of ME pressure increase rate (PIR) were observed in four patients with sclerotic mastoids. The current study is aimed at systematically assessing this observation.

Methods: Middle ear pressure was measured periodically in 30 patients during 50% N2O anesthesia using tympanometry. For each patient, a curve representing ME pressure during anesthesia was plotted. From the curve steepness, the PIR was calculated. Extent of mastoid pneumatization was assessed planimetrically using mastoid x-rays. Ears then were divided by the median into two groups: ears with small mastoids and ears with large mastoids. The difference between the mean PIR of both ear groups was statistically analyzed.

Results: A significant difference between the PIR was found among the two groups. In ears with mastoids smaller than 9.475 cm2 (the median), the PIR was significantly higher than in ears with mastoids > 9.475 cm2.

Conclusions: These findings support the concept that the mastoid has a ME pressure buffering capability: the larger its volume, the better its buffering capability. It is therefore suggested, that the mastoid plays a role in ME pressure regulation.

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