Alveolar ejection volume: a misnomer?

In their paper "Volumetric capnography in patients with acute lung injury: effects of positive end-expiratory pressure", BLANCH et al. [1] use a "new" term: "alveolar ejection volume" (VAE). This is intended as an index of " ... uneven ventilation/ perfusion of heterogeneous alveoli which may be useful to monitor in patients with ARDS [acute respiratory distress syndrome] receiving mechanical ventilation". Figure 1 in the article shows the plots of both expired expiratory carbon dioxide tension (PE,CO 2) and the cumulative plot, instantaneous carbon dioxide production (V©CO 2), against expired volume. The eye suggests that the airway or anatomical deadspace (VD,aw) [2] is about 180 mL in figure 1a, and about 200 mL in figure 1b. The VAE differs greatly between the two figures. In figure 1b it places alveolar ejection about 200 mL later than VD,aw. VAE was described, by the authors of the present paper, in this journal in 1997 [3]. The rationale given for it is that only gas from the last part of the expirate is pure alveolar gas, and therefore this part of the cumulative V©CO 2 plot can be extrapolated to give a volume which better describes alveolar efficiency. In fact, the extrapolation merely reflects the fact that phase III slope [4] continues to change in some patients. The slope of the phase III is not caused by the final washout of airway deadspace gas as this occurs much earlier [5]. Diagrams of this sort, as pioneered by CUMMING and co-workers [2, 6], describe what portion of the expirate contributes to gas exchange, and what portion does not, i.e. forms a deadspace. There is no justification for using an arbitrarily decided computer algorithm to define a place in the middle of the alveolar tidal volume without any physiological basis. I believe that the cumulative V©CO 2 diagrams used in the paper by BLANCH et al. [1] are being incorrectly interpreted. As described by CUMMING and coworkers [2, 6], they provide a useful alternative to, e.g. the method of FOWLER [4] used to derive VD,aw. The remaining part of the tidal volume, i.e. alveolar tidal volume, represents gas exchange at various ventilation/perfusion ratios including zero and infinity [1]. Bohr©s deadspace fraction and phase III slope give a similar sort of information to VAE, a qualitative, but not quantitative, measure of the spread of ventilation/perfusion ratios. The mean slope of phase III, possibly normalized for degree of …