On the inaccuracy of breath-by-breath metabolic gas exchange systems

This technical note is presenting and discussing a severe limitation of the breath-by-breath (BBB) determination of pulmonary gas exchange routinely used as a surrogate for the metabolic gas exchange rate at rest and during exercise. We are presenting the view that continuous airway gas sampling at the mouth used for the determination of O₂ and CO₂ content is inaccurate in the range of a low-to-medium expiratory flows, due to the discrepancy between the cross section surfaces of the sampling line and the expiratory tube (mask or mouth piece). This difference results in the sampling of a mixed exhaled gas at low expiratory flow, for which any temporal relationship between the instantaneous expired CO2 and O₂ signals and instantaneous expiratory flow is lost. Further analysis of this mechanism points to the difference between the exhalation (ve) and sampling (vs) gas velocities, where the ratio of ve/vs must be equal or higher than 1 to enable proper analysis of the respiratory gas concentrations during exhalation. Moreover, the above requirement is particularly crucial in monitoring the tidal concentration of the respiratory gases in experiments with small animals.