Pressure buffering by the tympanic membrane. In vivo measurements of middle ear pressure fluctuations during elevator motion.

OBJECTIVES

The tympanic membrane (TM) represents a pressure buffer, which contributes to the overall pressure regulation of the middle ear (ME). This buffer capacity is based on its viscoelastic properties combined with those of the attached ossicular chain, muscles and ligaments. The current work presents a set of in vivo recordings of the ME pressure variations normally occurring in common life: elevator motion. This is defined as a situation of smooth ambient pressure increase or decrease on a limited range and at a low rate of pressure change. Based on these recordings, the purpose was a quantitative analysis of the TM buffer capacity including the TM compliance.

METHODS

The pressure changes in seven normal adult ME's with intact TM's were continuously recorded directly inside the ME cavity during four different elevator trips using a high precision instrument. The TM buffer capacity was determined by the ratio between the changes in ME and the ambient pressure. Further, the ME volumes were calculated by Boyle's Law from pressure recordings during inflation-deflation tests; subsequently the TM compliance could also be calculated. Finally, the correlation between the ME volume and buffer function was determined.

RESULTS

Twenty-one elevator trips could be used for the analysis. The overall mean TM pressure buffering capacity was 23.3% (SEM = 3.4), whereas the mean overall compliance was 28.9 × 10-3 μL/Pa (SEM = 4.8). A strong negative linear correlation was found between the TM buffer capacity and the ME volumes (R2 = 0.92).

CONCLUSIONS

These results were in fair agreement with the literature obtained in clinical as well as temporal bone experiments, and they provide an in vivo reference for the normal ME function as well as for ME modeling. The TM buffer capacity was found more efficient in smaller mastoids. Possible clinical implications are discussed.