Assessment of the alveolar volume when sampling exhaled gas at different expired volumes in the single breath diffusion test

Abstract

BackgroundAlveolar volume measured according to the American Thoracic Society-European Respiratory Society (ATS-ERS) guidelines during the single breath diffusion test can be underestimated when there is maldistribution of ventilation. Therefore, the alveolar volume calculated by taking into account the ATS-ERS guidelines was compared to the alveolar volume measured from sequentiallly collected samples of the expired volume in two groups of individuals: COPD patients and healthy individuals. The aim of this study was to investigate the effects of the maldistribution of ventilation on the real estimate of alveolar volume and to evaluate some indicators suggestive of the presence of maldistribution of ventilation.MethodsThirty healthy individuals and fifty patients with moderate-severe COPD were studied. The alveolar volume was measured either according to the ATS-ERS guidelines or considering the whole expired volume subdivided into five quintiles. An index reflecting the non-uniformity of the distribution of ventilation was then derived (DeltaVA/VE).ResultsSignificant differences were found when comparing the two measurements and the alveolar volume by quintiles appeared to have increased progressively towards residual volume in healthy individuals and much more in COPD patients. Therefore, DeltaVA/VE resulted in an abnormal increase in COPD.ConclusionThe results of our study suggest that the alveolar volume during the single breath diffusion test should be measured through the collection of a sample of expired volume which could be more representative of the overall gas composition, especially in the presence of uneven distribution of ventilation. Further studies aimed at clarifying the final effects of this way of calculating the alveolar volume on the measure of DLCO are needed. DeltaVA/VE is an index that can help assess the severity of inhomogeneity in COPD patients.

Quantification of single-breath underestimation of lung volume in emphysema

“…Since VA SB is measured with a fixed wash out volume, however, we cannot exclude the possibility that some of the measured exhaled gas sample could have included dead space gas, thus underestimating true VA SB (Graham et al 1985). In addition, recent evidence suggests that the collection of exhaled gas relatively early in the expiratory phase as per standard technique may result in an underestimation of VA SB in patients with COPD and non-uniform distribution of ventilation (Prediletto et al 2007). It is therefore fair to assume that our methodology may have exaggerated the differences between single breath and both rebreathe and plethysmographic volumes in the emphysema patients by a similar degree.…”

The effects of lung volume reduction treatment on diffusing capacity and gas exchange

“…The reliability of D LCO testing in emphysema, in order to estimate the anatomical loss of gas exchange area, can be affected in several ways (figure 1). First, inhomogeneous V′ may be present due to the presence of both airways disease and/or emphysema [24,25]. THOMPSON et al [24] developed mathematical models in which they tested different types of inhomogeneous V′ and, when there was inhomogeneity of inspired volume or end-expiratory volume, D LCO was underestimated.…”

Treatment of severe COPD

“…In emphysema, there is loss of gas exchange surface, and an inverse linear relation between DLCO and severity of emphysema on CT has been established [164]. However, in COPD, other factors such as ventilation/perfusion (V/Q) disturbances, inhomogeneous ventilation, and airway obstruction can influence the outcome of the DLCO measurement both negatively and positively [170,171,173]. The measured DLCO for a patient with COPD is therefore likely to be a balance of these factors.…”

“…The reliability of DLCO testing in emphysema, in order to estimate the anatomical loss of gas exchange area, can be affected in several ways (figure 1). First, inhomogeneous V′ may be present due to the presence of both airways disease and/or emphysema [170,171]. THOMPSON et al [171] developed mathematical models in which they tested different types of inhomogeneous V′.…”