REBREATHING TL(CO) VERSUS SINGLE-BREATH TL(CO) AT DIFFERENT DEGREES OF UNEQUAL VENTILATION

The influence of unequal ventilation on the differences between the re breathing (RB) and the single breath (SB) measurement of the transfer factor of the lung for carbon monoxide (TL(CO)) was investigated. An a pparatus was developed which measured both T(LCO)RB and TL(CO)SB. Uneq ual ventilation increases with lung volume, caused by the combined eff ect of increasing ratio of total volume to RV from apex to basis, the fact that apex inspires before basis and asymmetric intra-acinar branc hing patterns. Therefore measurements were done at three different val ues of inspired volume (VI) in 10 normal subjects. To separate the eff ects of gas mixing and diffusion, residual volume (RV) was measured by He dilution in addition to TL(CO). We found that RV and TL(CO) increa se with increasing VI. The increase is larger with RB. We also found t hat RV(RB) is larger than RV(SB) at the three values of VI. TL(CO)RB i s smaller than TL(CO)SB except at vital capacity (VC). We concluded fr om the different behavior of RV(RB) and RV(SB) as a function of VI tha t the SB measurement increasingly underestimated RV with increasing un equal ventilation. This is also reflected in the measurement of TL(CO) SB. From the different behavior of TL(CO)RB and TL(CO)SB as a function of VI we concluded that TL(CO)RB was smaller than TL(CO)SB when unequ al ventilation was minimal. This is caused by the smaller mean alveola r volume during RB. But increasing unequal ventilation causes an incre asing underestimation with the SB method, resulting in TL(CO)SB being equal to TL(CO)RB at VC.