Rebreathing TLCO versus single breath TLCO at different degrees of unequal ventilation

The influence of unequal ventilation on the differences between the rebreathing (RB) and the single breath (SB) measurement of the transfer factor of the lung for carbon monoxide (TLCO) was investigated. An apparatus was developed which measured both TLCORB and TLCOSB. Unequal ventilation increases with lung volume, caused by the combined effect of increasing ratio of total volume to RV from apex to basis, the fact that apex inspires before basis and asymmetric intra-acinar branching patterns. Therefore measurements were done at three different values of inspired volume (VI) in 10 normal subjects. To separate the effects of gas mixing and diffusion, residual volume (RV) was measured by He dilution in addition to TLCO. We found that RV and TLCO increase with increasing VI. The increase is larger with RB. We also found that RVRB is larger than RVSB at the three values of VI. TLCORB is smaller than TLCOSB except at vital capacity (VC). We concluded from the different behavior of RVRB and RVSB as a function of VI that the SB measurement increasingly underestimated RV with increasing unequal ventilation. This is also reflected in the measurement of TLCOSB. From the different behavior of TLCORB and TLCOSB as a function of VI we concluded that TLCORB was smaller than TLCOSB when unequal ventilation was minimal. This is caused by the smaller mean alveolar volume during RB. But increasing unequal ventilation causes an increasing underestimation with the SB method, resulting in TLCOSB being equal to TLCORB at VC.