EVALUATION OF ESTIMATES OF ALVEOLAR GAS-EXCHANGE BY USING A TIDALLY VENTILATED NONHOMOGENOUS LUNG MODEL

The purpose of this study was to evaluate algorithms for estimating O- 2 and CO2 transfer at the pulmonary capillaries by use of a nine-compa rtment tidally ventilated lung del that incorporated inhomogeneities i n ventilation-to-volume and ventilation-to-perfusion ratios. Breath-to -breath O-2 and CO2 exchange at the capillary level and at the mouth r e simulated by using realistic cyclical breathing patterns to drive th e model, derived from 40-min recordings in six resting subjects. The S D of the breath-by-breath gas exchange at the mouth around the value a t the pulmonary capillaries was 59.7 +/- 25.5% for O-2 and 22.3 +/- 10 .4% for CO2. Algorithms including corrections for changes in alveolar volume and for changes in alveolar gas composition improved the estima tes of pulmonary exchange, reducing the SD to 20.8 +/- 10.4% for O-2 a nd 15.2 +/- 5.8% for CO2. The remaining imprecision of the estimates a rose almost entirely from using end-tidal measurements to estimate the breath-to-breath changes in end-expiratory alveolar gas concentration . The results led us to suggest an alternative method that does not us e changes in end-tidal partial pressures as explicit estimates of the changes in alveolar gas concentration. The posed method yielded signif icant improvements in estimation for the model data of this study.