Tissue Oxygen Saturation of the Quadriceps Muscle in Patients with Interstitial Lung Disease

<bold>Background:</bold> The 6-minute walk test (6MWT) is an essential tool for evaluating patients with interstitial lung disease (ILD). Skeletal muscle function and exercise-induced hypoxemia are independent predictors of prognosis in ILD patients. This study aimed to investigate the differences between resting and minimum tissue oxygen saturation (StO₂) and peripheral oxygen saturation (SpO₂) during the 6MWT in ILD patients. <bold>Methods:</bold> This was a prospective, observational cohort study. We measured the 6MWT in stable ILD patients without long-term oxygen therapy. During the 6MWT, SpO₂ was continuously monitored, and StO₂ in the quadriceps muscle was measured using continuous-wave near-infrared spectroscopy. Both SpO₂ and StO₂ were recorded at rest and at their minimum values (nadir) during the test. <bold>Results:</bold> A total of 40 ILD patients were recruited (mean age: 68.2 years; % predicted forced vital capacity: 66.1%). The mean 6-minute walk distance (6MWD) was 381.7 ± 85.9 m. Dyspnea and leg fatigue scores during the 6MWT were 4.4 ± 1.7 and 3.2 ± 1.8, respectively. The mean StO₂ values were 62.1 ± 4.5% at rest and 53.7 ± 6.5% at minimum during the 6MWT. The mean SpO₂ values were 95.8 ± 2.2% at rest and 85.7 ± 7.4% at minimum during the test. Minimum StO₂ was significantly correlated with quadriceps muscle strength, the Medical Research Council dyspnea scale, and 6MWD (p < 0.05). Minimum SpO₂ was significantly correlated with 6MWD (p < 0.05). <bold>Conclusion:</bold> These findings suggest that the minimum StO₂ of the quadriceps muscle is a useful parameter for assessing muscle oxygenation during the 6MWT in ILD patients.