Oxygen desaturation during a 6-minute walk test as a predictor of maximal exercise-induced gas exchange abnormalities in sarcoidosis

Abstract

Background: Common tests for evaluating gas exchange impairment have different strengths and weaknesses. Alveolar-to-arterial oxygen pressure difference (AaDO2) at peak exercise is a sensitive indicator but it cannot be measured repeatedly. Diffusing capacity of the lung for carbon monoxide (DLco) is measured at rest and may be too insensitive to predict the effects of exercise on gas exchange impairment. Oxygen desaturation during a 6-minute walk test (∆SpO2-6MWT) can be measured repeatedly, but its value in sarcoidosis is unknown. Here, we evaluated the ability of ∆SpO2-6MWT and DLco to predict gas exchange impairment during exercise in sarcoidosis.

Methods: This retrospective study of 130 subjects with sarcoidosis investigated the relationship between DLco, ∆SpO2-6MWT, and peak AaDO2 using correlation tests, inter-test reliability analyses, and predictive values. For the analyses of inter-test reliability and predictive values, DLco, peak AaDO2, and ∆SpO2-6MWT were considered as binary variables (normal/abnormal) according to previously defined thresholds.

Results: Correlation coefficients between DLco, ∆SpO2-6MWT, and peak AaDO2 were intermediate (0.53-0.67, P<0.0003) and Kappa coefficients were low (0.21-0.42, P=0.0003-0.02). DLco predicted (I) increased peak AaDO2 with a positive predictive value (PPV) of 66% and a negative predictive value (NPV) of 78% and (II) increased ∆SpO2-6MWT with a PPV at 36% and an NPV at 88%. Normal DLco was a good predictor of the absence of severe desaturation during the 6MWT (94% NPV) and at peak exercise during cardiopulmonary exercise test (CPET) (100% NPV). ∆SpO2-6MWT predicted peak AaDO2 increase with a PPV of 74% and an NPV of 60%.

Conclusions: In a large population of sarcoidosis patients, neither ∆SpO2-6MWT nor DLco was a good predictor of increased peak AaDO2. In contrast, normal DLco was a good predictor of the absence of severe desaturation during the 6MWT and at peak exercise during CPET.

Keywords: 6-minute walk test (6MWT); Sarcoidosis; alveolar-arterial oxygen gradient; cardiopulmonary exercise test (CPET); diffusing capacity; pulmonary gas exchange.

Figure 1

Correlations between DLco, ∆SpO₂-6MWT, and peak AaDO₂. DLco, diffusing capacity of the lung for carbon monoxide; ∆SpO₂, oxygen desaturation [(resting − nadir) SpO₂]; 6MWT, 6-minute walk test; AaDO₂, alveolar-to-arterial oxygen pressure difference.

Figure 2

ROC curves. ROC curves determining the cut-off of DLco to predict ∆SpO₂-6MWT (left) and peak AaDO₂ (middle) and the cut-off of ∆SpO₂-6MWT to predict peak AaDO₂ (right). DLco, diffusing capacity of the lung for carbon monoxide; ∆SpO₂, oxygen desaturation [(resting − nadir) SpO₂]; 6MWT, 6-minute walk test; AaDO₂, alveolar-to-arterial oxygen pressure difference.