BackgroundSelf-reported physical activity following pediatric pulmonary embolism(PE) is reduced after diagnosis. However, objectively measured exercise capacity and mechanisms of exercise pathophysiology following PE are unknown. Research QuestionDoes exercise capacity one year after acute PE in children differ from controls? Study Design and MethodsOur case-control study compared exercise capacity and responses to maximal exercise in PE survivors with controls. We also investigated the association of low exercise capacity following PE with prespecified clinical/radiological features at PE diagnosis and elucidated the cause of functional limitations. The primary study outcome was exercise capacity defined by peak oxygen uptake (peak VO2) as a percent of predicted on cardiopulmonary exercise testing (CPET), with <80% predicted peak VO2 considered abnormal/low. Ventilatory inefficiency was defined as VE/VCO2 slope >30 and abnormal stroke volume augmentation as oxygen pulse <10 mL O2.min -1 at peak exercise. Logistic regression was performed to assess the association of prespecified variables with low peak VO2. ResultsWe compared 25 consecutive pediatric PE survivors who completed CPET one-year post-diagnosis with 25 controls who underwent CPET within the same period and were otherwise healthy. Exercise capacity was reduced in 8 of the 25 PE survivors (32%) at one-year post-diagnosis vs. 2 of the 25 control participants (8%) (p:0.034). PE survivors with low exercise capacity demonstrated elevated VE/VCO2 slope (p: 0.01) and a decreased oxygen pulse at peak exercise (p:0.001) consistent with cardiovascular limitation. In univariable analysis, PE category, pulmonary vascular obstruction by Qanadli index, or RV dysfunction at diagnosis was not associated with low exercise capacity. InterpretationAbnormal exercise capacity of cardiopulmonary origin occurred in 1 of 3 pediatric PE survivors despite anticoagulation and irrespective of PE severity, degree of pulmonary vascular obstruction, or RV dysfunction at diagnosis. Cardiorespiratory fitness should be formally considered to develop rehabilitation interventions following pediatric PE.