Influence of increased alveolar PCO2 on thoracic gas volume measurements

We examined the possibility that cyclic gas exchange between alveolar space and the surrounding tissue, induced by alveolar pressure variations, could interfere with plethysmographic measurements of thoracic gas volume (TGV). A model study suggested that TGV could be overestimated by up to 100 ml and that the phenomenon could account for some of its previously reported negative frequency dependence (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 52: 739-747, 1982). As the error would be mainly due to CO2 exchange and be proportional to its partial pressure (PCO2), we studied in nine normal subjects the influence of increasing alveolar PCO2 from 30 to 50 Torr on TGV measurements at panting frequencies (f) of 0.5 and 2-2.5 Hz. Contrary to model predictions, CO2 tended to decrease TGV estimates at low frequency and to increase them at high frequency. As a consequence, frequency dependence of TGV (delta TGV/delta f) was less at high than at low PCO2 (-39 +/- 32 vs. -84 +/- 49 ml x Hz-1, P less than 0.001). The data are not satisfactorily explained but suggest that gas exchange is not an important factor in TGV measurements.