Disappearance curves and single-breath DLCO from consecutive samples during slow expiration

The disappearance of CO in three expirates consecutively sampled after various breath holding periods was measured in seven normal subjects. The CO disappearance curves in the three expired portions were nearly linear and parallel each other, but the curves for later sampled expirates were lower, and even those for earlier sampled expirates passed under unity at time zero when extrapolated on a logarithmic graph of relative expired alveolar CO concentration against breath holding time. The slope of each disappearance curve shows "the true diffusing capacity at breath holding lung volume", eliminating an effect of rapid CO absorption at a lower lung volume during expiration. This effect is considered the cause of the downward shift of the decay curve, and the shift is magnified in slow expiration. In addition, the three parallel lines indicate a homogeneous distribution of DL/VA within the lungs. Subsequently, DLCO in conventional calculations had greater values in shorter breath holding time and later sampling.