Assessing breath-by-breath alveolar gas exchange: is the contiguity in time of breaths mandatory?

Experimental studies have established that alveolar gas exchange is inversely relation to the molecular diffusivity of gas in the lung airways. The mechanism underlying this relationship is, however, unclear. To investigate this phenomenon, the conditions relevant to the experimental studies are simulated using a computational model of pulmonary gas transport. Results from these simulations suggest that the inverse relationship found experimentally can largely be explained on the basis of the intra-acinar stratification of blood flow and gas concentrations. Gas having a relatively low molecular diffusivity is not transported as far into the acinus as gas having a higher diffusivity. When these relative intra-acinar gas distributions interact with the blood flow distribution, which has been shown experimentally to be weighted towards the proximal alveoli, more gas exchange occurs in the low molecular diffusivity mixture. Consideration of the various other mechanisms that have been proposed to explain the experimental findings.he inverse dependence suggests that they are of little significance. In particular, our studies remove the need to invoke Taylor diffusion to explain the experimental findings.

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Evaluation of estimates of alveolar gas exchange by using a tidally ventilated nonhomogenous lung model

Journal of Applied Physiology ◽

10.1152/jappl.1997.82.6.1963 ◽

1997 ◽

Vol 82 (6) ◽

pp. 1963-1971 ◽

Cited By ~ 18

Author(s):

Thierry Busso ◽

Peter A. Robbins

Keyword(s):

Gas Exchange ◽

Lung Model ◽

Gas Composition ◽

Breathing Patterns ◽

Alveolar Volume ◽

Gas Concentration ◽

Pulmonary Capillaries ◽

Partial Pressures ◽

End Tidal ◽

Alveolar Gas Exchange

Busso, Thierry, and Peter A. Robbins. Evaluation of estimates of alveolar gas exchange by using a tidally ventilated nonhomogenous lung model. J. Appl. Physiol. 82(6): 1963–1971, 1997.—The purpose of this study was to evaluate algorithms for estimating O2 and CO2 transfer at the pulmonary capillaries by use of a nine-compartment tidally ventilated lung model that incorporated inhomogeneities in ventilation-to-volume and ventilation-to-perfusion ratios. Breath-to-breath O2 and CO2 exchange at the capillary level and at the mouth were simulated by using realistic cyclical breathing patterns to drive the model, derived from 40-min recordings in six resting subjects. The SD of the breath-by-breath gas exchange at the mouth around the value at the pulmonary capillaries was 59.7 ± 25.5% for O2 and 22.3 ± 10.4% for CO2. Algorithms including corrections for changes in alveolar volume and for changes in alveolar gas composition improved the estimates of pulmonary exchange, reducing the SD to 20.8 ± 10.4% for O2 and 15.2 ± 5.8% for CO2. The remaining imprecision of the estimates arose 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 use changes in end-tidal partial pressures as explicit estimates of the changes in alveolar gas concentration. The proposed method yielded significant improvements in estimation for the model data of this study.

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