Middle ear pressure variations during 50% N₂O anesthesia as a function of mastoid pneumatization

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.