Gas diffusion across the tympanic membrane in chinchillas: effect of repeated perforations

Earlier studies documented a slow diffusion of gas across the tympanic membrane (TM) in humans and animals. However, structural changes caused by repeated TM perforations could affect gas diffusion rates. This possibility was evaluated using a chinchilla model.

In six chinchillas, the right TM was perforated four times at 2-week intervals; the left TM served as a control. Approximately 12 weeks after the fourth perforation and when the right TMs had healed, a probe was introduced into the external canal and sealed to the environment. For 120 min, pressure in the probe was continuously monitored and probe gas was sampled at 10 min intervals and analyzed for composition by online mass spectrometry. Percent compositions for CO(2) and O(2) were calculated and these data were recast as partial-pressure versus time functions. The information contained in those functions was used to estimate time-constants for transTM O(2) and CO(2) diffusion.

In all experiments, CO(2) percent composition in the probe increased as a curvilinear function of time while that for O(2) decreased linearly. For all animals and both gases, the rate of change in probe partial-pressure was greater for the right TM. Average values of the right and left time-constants were (6.7 +/- 2.4) x 10(-3) and (4.2 +/- 2.2) x 10(-3)min(-1)TM(-1) for CO(2), and (2.3 +/- 0.8) x 10(-3) and (1.5 +/- 0.5) x 10(-3)min(-1)TM(-1) for O(2). Between side differences in these time-constants were statistically significant (P < 0.05, 2-tailed, paired, Student's t-test). The average right/left, transTM time-constant ratio was 1.66 +/- 0.43 for CO(2) and 1.61 +/- 43 for O(2); both were significantly different from a value of 1.0 (P < 0.05, 2-tailed Student's t-test).

These results document an increased rate of diffusive gas exchange across TMs that had been repeatedly perforated. This effect may be caused by structural thinning secondary to scar formation and could have implications for middle ear pressure regulation in ears with a history of repeated myringotomies and/or tympanostomy tube insertions.