Errors due to tissue CO2 capacity in estimation of pulmonary blood flow from single-breath gas analysis

The single-breath method for estimation of pulmonary blood flow (Q) and mixed venous PCO2 (PvCO2) (Kim et al., 1966) is based on the assumption that the carbon dioxide exchange between the CO2 stores in the lung tissue and the alveoli is negligible compared to the exchange between the lung capillaries and the alveoli. Recent investigations have questioned this assumption, and the present study is a quantitative analysis, based on a computer simulation, of errors in the estimated values of Q and PvCO2 caused by changes in the CO2 stores. The result of the analysis is that the presence of exchangeable CO2 stores causes a considerable underestimation of both pulmonary blood flow and mixed venous PCO2. The errors in Q and PvCO2 depend on the lung volume, the tissue volume, the numerical values of Q and PvCO2, and the duration of the single breath maneuver. The last mentioned property of the system is demonstrated experimentally. Furthermore, the analysis suggests a modification of the algorithm utilized in the single-breath method to eliminate the errors due to the CO2 stores. Finally, the analysis indicates that the application of the new algorithm allows estimation of the CO2 distribution volume in the lung tissue.