Abstract BACKGROUND:The six-minute walk test, commonly used to assess exercise capacity and response to therapy in pulmonary arterial hypertension (PAH), has many well-described limitations. Sedentary time is associated with adverse cardiovascular outcomes and reduced quality of life, and measuring sedentary time and physical activity using accelerometry is another potential way to quantify exercise capacity in PAH. Whether sedentary time in different in PAH versus control subjects is unknown. METHODS:Physical activity was measured in 20 patients with PAH and 30 matched healthy controls using accelerometry for seven consecutive days. PAH subjects completed standard six-minute walk testing and baseline demographics were recorded for all subjects. Total daily activity counts, sedentary time, and proportion of time at various activity levels were compared between groups. RESULTS:Sedentary time was significantly higher in PAH subjects compared with controls (92.1% daily activity, 95% CI, 89.5-94.8% versus 79.9% daily activity, 95% CI, 76.4-83.5%, P < 0.001) and all levels of physical activity were reduced in PAH subjects compared with controls (P < 0.01 for all). Daily moderate to vigorous physical activity was reduced in PAH (7.5 minutes, 95% CI, -0.8-15.6 minutes) compared with controls (64.7 minutes, 95% CI, 51.1-78.2 minutes), P < 0.001. Activity counts correlated with six-minute walk distance in PAH (r(s) = 0.72, P < 0.001). CONCLUSIONS:Sedentary time is increased in PAH and may lead to increased risk for metabolic and cardiovascular morbidity. Quantitation of daily activity and sedentary time using accelerometry may be a novel endpoint for PAH management and clinical trials.