Estimation of the VA/(Q+VTIS) distribution from single-breath alkane uptake

The authors investigated if the ventilation-perfusion distribution can be estimated from the uptake (U) of inert gases with different solubilities during the single-breath maneuver. A model was implemented that describes U as a function of solubility for inhomogeneously distributed alveolar volume (VA) versus blood and tissue volume (Q+VTIS). The VA/(Q+VTIS) distribution describes the relative contribution of gas-exchange units with different VA/(Q+VTIS) ratios to the expiratory volume. U was derived as the sum of uptakes corresponding to different modes in the distribution, weighted with the relative contribution to the expiratory volume. This permits an estimation of the distribution parameters by fitting U as a function of solubility. The n alkanes were used because of their different solubilities. Analysis of the sensitivity of the estimated VA/(Q+VTIS) distribution parameters to measurement errors showed that mostly two modes can he discerned. The influence of fixed model parameters appeared relatively small. The model could well explain U in normal and emphysematous subjects, with a larger contribution of high V _A/(Q+VTIS) ratios in the emphysematous subjects. It was concluded that the V _A/(Q+VTIS) distribution can be estimated noninvasively from single-breath alkane uptake.