P286 Is there diurnal or seasonal variation in the 6-minute walk test distance in patients with pulmonary vascular disease?

Introduction The 2002 American Thoracic Society (ATS) 6-Minute Walk Test (6MWT) guidelines advise ‘ repeat testing should be performed about the same time of day to minimize intraday variability’. It is currently unknown if there is diurnal and/or seasonal variation in the 6MWT in pulmonary vascular disease (PVD).Aims To investigate the association between 6-Minute Walk Distance (6MWD), time of day and season of testing.Method Retrospective 6MWT data was collected from 2019–2025. The 6MWT was conducted in accordance with ATS 6MWT guidelines (2002). Independent sample t-test was performed to determine morning vs afternoon differences in 6MWD. One way ANOVA was performed to determine seasonal differences. Time of day and seasonal effects were further investigated using multivariate linear regression analyses, adjusted for age, sex, BMI and WHO functional class. Time was modelled as a continuous variable (per 1 hour increment) and as a binary predictor of morning (08:00–12:00 am) vs afternoon (12–18:00 pm).Results The cohort consisted of 1618 patients, WHO PH Groups 1 (n= 322), 2 (n=72), 3 (n= 51), 4 (n = 946), and chronic thromboembolic pulmonary disease (n= 227). 6MWD was significantly higher in the morning (363 ± 134 meters; n=684) than in the afternoon (333 ± 144 meters; n=865) (p<0.001). 6MWD was not significantly impacted by season (p=0.75). Multivariable linear regression analyses demonstrated that the 6MWD decreased by 4.1 meters (95% CI 1.57–6.64) (p=0.002) for every hour increment in the time of the day, and by 16.34 meters (95% CI 5.72–29.95) (p=0.0003) in the afternoon compared to the morning.Conclusion The time of day was found to impact 6MWD, with patients with PVD achieving higher 6MWD in the morning. 6MWTs should be conducted at the same time to eliminate this possible fatiguing effect and optimise interpretation of intertest differences in clinical practice and trials. Prospective studies are needed with repeated measurements in the same individuals at different times of the day to validate these findings.