To compare the measurement of the alveolar volume according the ATS guidelines (VAst) to that obtained at different intervals of expired volume, during the single breath CO test. This is done in order to evaluate the consequencies of the sequential emptying of the alveoli on the real entity of the alveolar volume in the presence of maldistribution of ventilation and perfusion in COPD patients.
To this end 30 normals and 32 patients with severe COPD were studied. The alveolar volume has been calculated subdividing the total volume exaled, subtracted from anatomic and instrumental dead space, in 5 quintiles, by considering the mean expired fractions of methane, read instantaneuosly during the emptying phase from the total lung capacity to residual volume (VAq). VAq allowed to derive an index representative of the uniformity of the distribution of ventilation, expressed as change of alveolar volume per liter of volume of gas exaled (VA/VE).
Mean age of normals was 56±11.3 and 70±6.6 for patients with COPD (FEV1% pred. 40±8.2; FEV1/VC% 38±7.9; TLC% pred 105±11.5). Significant differences were found among each VAq; VA/VE resulted significantly increased in COPD than in normals (13.8±5.7 vs 0.8±1.1, p<.0001). By comparing VAst to VAq measured on the first quintile of expirate volume, a statistical difference was observed between the two indexes in COPD patients (4.77±0.97 vs 4.45±0.93, p=0.0009) than in normals (6.14±1.34 vs 6.15±1.34, p=ns).
The presence of an unequal distribution of convective ventilation, due mostly to the sequential emptying of the alveoli, in COPD, may influence the real estimate of pulmonary diffusion measured during the single breath test. It turns out that the measure of the alveolar volume during this test should be standardized for the whole expirate volume of the subject.
The single breath CO test should be reevaluated since it is crucial that the sampling of volume taken at the beginning of the expirate is not representative of the real alveolar gas concentrations, especially in the presence of VA/Q mismatch.
R. Prediletto, None.