Role of the mastoid in middle ear pressure regulation.

OBJECTIVES/HYPOTHESIS: Determine the role of mastoid volume in middle ear pressure (MEP) regulation. The hypothesis was that inert gas exchange between blood and middle ear (ME) is slower for larger mastoid volumes.

STUDY DESIGN

Prospective.

METHODS

For 21 enrolled subjects, the bilateral surface areas and volumes of the mastoid and tympanum were measured from computed tomography scans in 20 subjects with a wide range of mastoid volumes. Then, 19 subjects were reclined in a chair, fitted with a non-rebreathing mask and breathed room air for 20 minutes (acclimation), a gas composition of 25% N(2)O, 20% O(2), balance N(2) for 30 minutes (experiment), and room air for 30 minutes (recovery). Bilateral MEPs were recorded by tympanometry every 2 minutes. The slopes of the MEP-time functions during N(2)O breathing were calculated to the first observation of eustachian tube opening and divided by the estimated blood-ME N(2)O gradient to yield a N(2)O time constant. Sufficient data were available for 16 right and 11 left MEs to calculate the time constant.

RESULTS

MEP did not change during the baseline period, but within 10 minutes of breathing the N(2)O mixture showed a progressive increase. The right-left correlation for the time constant was 0.87 (n = 10 ears, P = .001). Regression of the time constants on ME volume showed an inverse relationship (n = 23 ears, r = -41, P = .05). A better data fit was the curvilinear relationship predicted by a mathematical model of the mastoid acting as a ME ear gas reserve.

CONCLUSIONS

These results support the tested hypothesis that the mastoid could serve as ME gas reserve.