Journal of Applied Physiology, Vol 54, Issue 5 1202-1208, Copyright © 1983 by American Physiological Society
Influence of increased alveolar PCO2 on thoracic gas volume measurements
R. Peslin, P. Jardin and A. B. Bohadana
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.
