Conclusions-The middle ear (ME) surface area/volume ratio (SA/V) is greater than that of the tympanum. The rate of ME pressure decrease between Eustachian tube openings is proportional to the ME SA/V. This analysis showed that the MACS will not function as a ME gas reserve under the assumed conditions, but could, if the blood perfusion/surface area is much greater for the tympanum than the MACS and is lesser for greater MACS volumes.Objective-Measure the surface and volumes for the MACS and tympanum and evaluate if the MACS could function as a ME gas reserve.Methods-Twenty adult subjects with a wide range of MACS volumes had a CT scan of their MEs. Using Image J software, the left and right surface areas and volumes of the tympanum and MACS were reconstructed. These data were entered into a simple perfusion-limited model of transmucosal gas exchange between ME and mucosal blood. The model predicted that the MACS would function as a ME gas reserve if the SA/V for the ME is less that that for the tympanum, or equivalently, if the tympanum SA/V divided by the ME SA/V is less than a critical value of 1.Results-Both MACS and tympanum surface areas were linearly related to their volumes. MACS surface area and volume and the ME SA/V were significantly greater than those for the tympanum. Solving the model equation using the measured values yielded a critical value of 1.4 which was significantly greater than 1.

Objective-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-ProspectiveMethods-For 21 enrolled subjects, the bilateral surface areas and volumes of the mastoid and tympanum were measured from CT scans in 20 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% N2O, 20% O2, balance N2 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 N20 breathing were calculated to the first observation of Eustachian tube opening and divided by the estimated blood-ME N2O gradient to yield a N2O 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 N2O mixture, showed a progressive increase. The right-left correlation for the time-constant was 0.87 (n=10 ears, p=0.001). Regression of the time-constants on ME volume showed an inverse relationship (n=23 ears, r=−41, p=0.05). A better data fit was the curvilinear relationship predicted by a mathematical model of the mastoid acting as a ME ear gas reserve. Conclusion-These results support the tested hypothesis that the mastoid could serve as ME gas reserve.

Introduction-The geometry of mastoid air cells system (MACS) has not been described for adults over a large range of MACS volumes.