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Correlates of Six-Minute Walk Test in Idiopathic Pulmonary Fibrosis

Medical University of SC, Charleston, SC

PURPOSE: To evaluate the utility of Six-Minute Walk Test (6MWT) for Idiopathic Pulmonary Fibrosis (IPF) and determine correlates with spirometry, diffusion, high resolution CT (HRCT), and dyspnea scores.

METHODS: We retrospectively evaluated all patients with IPF seen at MUSC from 1999 to 2003 to find those with concurrent HRCT, spirometry, diffusion, and 6MWT. HRCT was evaluated by two reviewers and summary scores for fibrosis obtained. Desaturation was defined by > 3% drop in oxygen saturation. Univariate correlations were performed on the initial set of tests with p-values < 0.05 judged significant.

RESULTS: 27 patients had all the required tests for study inclusion performed. Twelve individuals had OLB diagnosis with the remainder meeting the ATS defination of probable IPF. Patient demographics included age 67 ± 12 years, male 66%, FVC% predicted 68 ± 21%, and DLCO% predicted 42 ± 17%. Correlates of 6MWT distance included high FVC % predicted (p<0.01) high FEV1% predicted (p<0.02), low DLCO% predicted (p<0.03), high Borg dyspnea score at rest (p<0.02), and no need for supplemental oxygen (p<0.01). Oxygen desaturation occured in 24 of 27 patients. Correlates of the difference between maximal and minimal oxygen saturation included a high Borg dyspnea score at maximal exercise (p<0.02) and low DLCO% predicted (p<0.005). Nonsmokers had more desaturation (p<0.02) than those who were smokers despite similar measures of FVC, FEV1, and DLCO. The HRCT fibrosis score failed to correlate with any measure of 6MWT (p=NS).

CONCLUSION: The 6MWT measures a walk distance and degree of oxygen desaturation that have independent correlates in IPF. Neither of the measures are strongly correlated with HRCT fibrosis score. The determination of which tests are best as clinical outcome measures remain unknown. The correlation between cigratte smoking and less oxygen desaturation implies lead time bias.

CLINICAL IMPLICATIONS: A 6MWT measures variables that are independent from lung fibrosis. Oxygen desaturation is correlated with measures of vascular patency and dyspnea. Walk distance is more correlated with measures of lung mechanics. A 6MWT integrates important variables in IPF.

DISCLOSURE: R.K. Jain, None.