On the estimation of pulmonary blood flow from CO 2 production time series

Abstract

It may be possible to estimate a nominal pulmonary blood flow ( Q ̇ ) during an exercise stress test via the algorithm used to estimate breath-by-breath alveolar CO 2 production. Recently it has been demonstrated that by relating breath-to-breath fluctuations in alveolar CO 2 production to breath-to-breath fluctuations in end-tidal CO 2, an optimizing parameter related to Q ̇ can be determined that can be used to process the CO 2 production fluctuations and minimize their variation. However, the reported values of Q ̇ using this procedure appear to be biased low. Using a computer simulation of gas exchange, we demonstrate that the estimate of Q ̇ is biased low when the nominal lung volume used in the alveolar gas exchange algorithm is too large. Furthermore, alveolar CO 2 transport is determined by an integral of alveolar CO 2 over the breath time and, thus, is a path-dependent quantity. The use of end-tidal CO 2 fluctuations to approximate fluctuations in this integral contributes to an error in the estimation of Q ̇ which yields estimates that are biased low. Alternatively, the use of mean alveolar CO 2 fluctuations yield more appropriate Q ̇ estimates. These results suggest practical implications for estimating effective pulmonary blood flow during an exercise stress test by using breath-to-breath estimates of mear alveolar CO 2.