Usefulness of the desaturation-distance ratio from the six-minute walk test for patients with COPD

Purpose: A straightforward, noninvasive method is needed to assess emphysema and pulmonary hypertension (PH) in COPD patients. The desaturation-distance ratio (DDR) is an index derived from the distance traveled and level of desaturation during a six-minute walk test (6MWT); it has previously been shown to be associated with percentage of forced expiratory volume in the first second of expiration (%FEV1.0) and percentage of diffusion capacity of the lung for carbon monoxide (%DLCO). The aim of this study was to examine the associations between DDR and emphysema and PH. Patients and methods: We collected the following data for 74 stable COPD outpatients: lung function tests (%FEV1.0 and %DLCO), 6MWT distance and desaturation, and area of emphysema on computed tomography (percentage of low attenuation area). Enlargement of the pulmonary artery (PA) was assessed by the ratio of the diameter of the PA to that of the aorta (PA:A ratio) as an index of PH. DDR was calculated by the distance traveled and the degree of desaturation reached during a 6MWT. The relationships between study outcomes were assessed with Spearman's rank-correlation analysis. Receiver operating characteristic (ROC) curves were used to determine the threshold values with the optimum cutoff points for predicting severe or very severe airway obstruction, pulmonary diffusing capacity disorder, moderate or severe emphysema, and enlargement of the PA. Results: DDR correlated significantly with %FEV1.0, %DLCO, %LAA, and PA:A ratio. DDR showed high accuracy (area under the ROC curve>0.7) for predicting severe or very severe airway obstruction, pulmonary diffusing capacity disorder, moderate or severe emphysema, and enlargement of the PA. Conclusion: The results suggest that DDR is a good index of emphysema and PH in COPD patients. The 6MWT is widely used to assess COPD, and DDR could help with the early diagnosis of COPD.