On the estimation of pulmonary blood flow from CO₂ production time series

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 CO₂ production. Recently it has been demonstrated that by relating breath-to-breath fluctuations in alveolar CO₂ production to breath-to-breath fluctuations in end-tidal CO₂, an optimizing parameter related to Q ̇ can be determined that can be used to process the CO₂ 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 CO₂ transport is determined by an integral of alveolar CO₂ over the breath time and, thus, is a path-dependent quantity. The use of end-tidal CO₂ 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 CO₂ 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 CO₂.