Background: The 6-minute walk test (6MWT) is used to measure exercise capacity and assess prognosis in interstitial\nlung disease (ILD). Although the 6MWT is usually considered to be a test of submaximal exercise capacity in ILD, the\nphysiological load imposed by this test is not well described and 6MWT outcomes are poorly understood. This study\naimed to compare cardiorespiratory responses to 6MWT and cardiopulmonary exercise test (CPET) in people with ILD.\nMethods: 47 participants with ILD (27 idiopathic pulmonary fibrosis (IPF), mean age 71 (SD 12) years, diffusing capacity\nfor carbon monoxide (TLCO) 49(15) %predicted) undertook CPET and 6MWT on the same day in random order.\nOxygen uptake (VO2), ventilation (VE) and carbon dioxide production (VCO2) were assessed during each test using a\nportable metabolic cart.\nResults: The VO2peak during the 6MWT was lower than during CPET (1.17(0.27) vs 1.30(0.37) L.min?1, p=0.001),\nrepresenting an average of 94% (range 62-135%) of CPET VO2peak. Achieving a higher percentage of CPET VO2peak on\n6MWT was associated with lower TLCO %predicted (r = ?0.43, p = 0.003) and more desaturation during walking (r = ?0.46,\np = 0.01). The VEpeak and VCO2peak were significantly lower during 6MWT than CPET (p < 0.05). However, participants\ndesaturated more during the 6MWT (86(6)% vs 89(4)%, p < 0.001). The degree of desaturation was not affected by the\npercent of peak VO2 achieved during the 6MWT. Responses were similar in the subgroup with IPF.\nConclusions: On average, the 6MWT elicits a high but submaximal oxygen uptake in people with ILD. However\nthe physiological load varies between individuals, with higher peak VO2 in those with more severe disease that\nmay match or exceed that achieved on CPET. The 6MWT is not always a test of submaximal exercise capacity in\npeople with ILD.
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