MIDDLE-EAR GAS-EXCHANGE IN RHESUS-MONKEYS

The mechanism of gas exchange between the middle ear and tissue is fun damental to understanding middle ear physiology and pathophysiology. I n this study, the middle ears of six rhesus monkeys were inflated on s eparate occasions with nitrogen (N2), carbon dioxide (CO2), and oxygen (O2), and middle ear pressures were recorded at defined times postinf lation for up to 4 hours. From these data, rate constants governing th e exchange of these gases were estimated and compared to those predict ed under both diffusion and perfusion limitations. The results show th at the rate constants for middle ear to tissue exchange of O2 and CO2 are consistent with a diffusion-limited process. In contrast, middle e ar pressure did not decrease over the study period following introduct ion of N2 into the middle ear. This is interpretable as a much slower rate of N2 exchange than that predicted by either perfusion or diffusi on-limited models calibrated to the O2 and CO2 rate constants. These r esults have significant implications for middle ear gas exchange and s uggest that for relatively short observation periods, the behavior of middle ear pressure is controlled by experimentally established O2 and CO2 gradients.