RE-BREATHING VS SINGLE-BREATH TLCO IN PATIENTS WITH UNEQUAL VENTILATION AND DIFFUSION

The single-breath (SE) method for determining the transfer factor for carbon monoxide (TLCO) is of limited value for the detection of diffus ion disorders on the alveolar level, because the results are influence d by unequal distribution of ventilation and diffusion. The rebreathin g method (RE) is thought not to be influenced by these inequalities. T o the authors' knowledge, no study has measured both TLCORB and TLCOSB systematically and compared them with regard to the influence of uneq ual ventilation and diffusion. Therefore, the present study measured t otal lung capacity (TLC) as well as TLCO, both with the RE vital capac ity method and the SE method. using the same apparatus in 10 healthy s ubjects and in 35 patients with chronic obstructive pulmonary disease (COPD). These patients are known to have increased unequal ventilation and diffusion in comparison with healthy subjects. In the healthy sub jects, a small difference was found between TLC measured with the RE m ethod (TLCRB) divided by the predicted value (TLCRB/pred) and TLCSB/pr ed (mean difference 0.07; SE=0.02): no significant difference was foun d between TLCORB divided by the predicted value of TLCOSB (TLCORB/pred ) and TLCOSB/pred. In the COPD patients, however, TLCRB/pred was large r than TLCSB/pred (mean difference 0.17; SE=0.02) and TLCORB/pred was larger than TLCOSB/pred (mean difference 0.23; SE=0.05). Multiple regr ession analysis revealed that in the COPD patients, 54% of the varianc e of the difference between TLCRB/pred and TLCSB/pred, and 76% of the variance of the difference between TLCORB/pred and TLCOSB/pred. were e xplained by parameters related to unequal ventilation and diffusion. I n 25 of the 35 COPD patients, TLCOSB/pred was less than 0.8, whereas i n 11 of these 25 patients, TLCORB/pred was more than 0.8. This differe nce was significant (P=0.0005), In these 11 patients, the SE measureme nt resulted in the incorrect diagnosis of a diffusion disorder on the alveolar level. The RE method, however, never resulted in the diagnosi s of a diffusion disorder when TLCOSB/pred was larger than 0.8. It is concluded that in a significant number of COPD patients, TLCOSB is bel ow the normal range, whereas TLCORB is not below the normal range. Thi s difference between TLCORB and TLCOSB is related to the combined effe ct of unequal ventilation and diffusion, and is of clinical importance for the detection of a diffusion disorder on the alveolar level.