Submitted on February 25, 2004
Accepted on August 10, 2004
ON MORPHOMETRIC MEASUREMENT OF OXYGEN DIFFUSING CAPACITY IN MIDDLE EAR GAS EXCHANGE
Stephen C Kanick1*, William J Doyle2, Samir N Ghadiali3, and William J Federspiel4
1 Department of Pediatric Otolaryngology, Children's Hospital of Pittsburgh, Pittsburgh, PA, USA; Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA, USA
2 Department of Otolaryngology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA
3 Department of Pediatric Otolaryngology, Children's Hospital of Pittsburgh, Pittsburgh, PA, USA; Department of Otolaryngology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA; Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA, USA
4 Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA, USA; McGowan Institute for Regenerative Medicine, Pittsburgh, PA, USA
* To whom correspondence should be addressed. E-mail: chad.kanick{at}gmail.com.
An accurate mathematical model of transmucosal gas exchange is prerequisite to understanding middle ear (ME) physiology. Current models require experimentally measured gas species time-constants for all extant conditions as input parameters. Weibel and colleagues showed that a morphometric model that incorporates more fundamental physiochemical and anatomical parameters accurately simulates pulmonary gas exchange from which the species time-constants can be derived for all extant conditions. Here, we applied a variant of that model to ME gas exchange. Implementation of the morphometric model requires the measurement of diffusional length (
) for the ME mucosa which contributes to the mucosal diffusing capacity, a measure of the resistance to gas flow between airspace and capillary. Two methods for measuring have been proposed: the linear distance between air-mucosal boundary and capillary as described by Ars and colleagues, and the harmonic mean of all contributing pathway lengths as described by Weibel and colleagues. Here, oxygen diffusing capacity was calculated for different ME mucosal geometries using the two measures, and the results were compared to those predicted by a detailed, 2-dimensional finite element analysis. Predictive accuracy was improved by incorporating the measure described by Weibel which captures important information regarding variations in capillary shape and distribution. However, when compared to the oxygen diffusing capacity derived from the finite element analysis, both measures yielded non-linear, positively biased estimates. The morphometric techniques underestimate diffusion length by failing to account for the curvilinear gas flow pathways predicted by the finite element model.
