Different exercise-induced desaturation pattern in six minute walk test between interstitial lung disease (ILD) and chronic obstructive pulmonary disease (COPD)

The 6 minute walk test (6MWT) is a tool to assess the physiologic function in variable pulmonary diseases. Oxygen desaturation during 6MWT is a risk factor for exacerbations and mortality in interstitial lung disease (ILD) and chronic obstructive pulmonary disease (COPD). We investigate whether exercise desaturation pattern during 6MWT is different between ILD and COPD. 53 COPD patients and 51 ILD patients performed 6MWT. Pulmonary function test took before and after 6MWT. The relationship of tidal loop and forced expiratory flow-volume (FEFV) curve, and continuous heart rate and saturation every minute (min) during 6MWT and 1 min after stopping walk were analyzed. Desaturation was defined as a 4% decrease of oxygenation compared to the baseline saturation. There were more encroachments of tidal loop with FEFV curve in both pre-exercise (69.8% vs 15.7%) and post-exercise (62.3% vs 19.6%) in COPD group than in ILD group (p<0.001). A left shift of tidal loop after 6MWT was significantly increased in COPD than in ILD (43.4% vs 7.8%, p<0.001). The heart rate was significantly increased in early 1 min though 4 min and after rest at 1 min in ILD patients than COPD patients. The desaturation happened early at 1 min to 6 min in 6MWT and 1 min stopping walk in ILD patients. However, the desaturation was found later at 4 minute of 6MWT in COPD patients. 6MWT revealed that ventilation/perfusion mismatch may induce early desaturation in ILD, while dynamic hyperinflation leading to late desaturation in COPD. Different patterns of the relationship of heart rate and saturation in 6MWT may be helpful for distinguishing chronic lung diseases.