Breath-by-breath measurement of alveolar gas exchange with a slow-response gas analyser

Abstract

The O₂-paramagnetic or polarographic and CO₂-infra-red expired gas analyser have a response delay which results in an underestimation in breath-by-breath$\dot vO_2$ and$\dot vCO_2$ calculations. In this study, correction for this delay has been made. After measuring the step response of the O₂-polarographic and CO₂-infra-red analyser, the damping factor and the natural angular frequency were determined as well as the time constant, assuming the response was a first-order one.$\dot vO_2$ and$\dot vCO_2$ were calculated when the response of the analyser was corrected for the first- and second-order responses using the inverse Laplace transform. For the uncorrected$\dot vO_2$ and$\dot vCO_2$, values from the breath-by-breath method were 27·5 and 18·1 per cent systematically underestimated (p<0·001) compared with those of the Douglas bag method. When correction for the first-order response was made, values of the breath-by-breath method became equivalent to those of the Douglas bag method for$\dot vCO_2$ whereas there was still a 17·5 per cent systematic underestimation (p<0·001) for$\dot vO_2$. The correction for the second-order response gave equivalence and significant correlation (p<0·001) between the values of both methods for$\dot vO_2$ and$\dot vCO_2$. These results might indicate that breath-by-breath measurement of alveolar gas exchange with a slow-response gas analyser is valid when a second-order response delay correction is used.