Background Adults with a Fontan circulation tend to have myopenia and elevated adiposity when measured by dual energy x‐ray absorptiometry. Bioelectrical impedance analysis is an alternative validated approach to assess body composition. We used bioelectrical impedance analysis to compare body composition between pediatric patients with a Fontan circulation and control individuals without heart disease. Methods and Results A retrospective chart review identified all patients aged <22 years with a Fontan circulation who presented for cardiopulmonary exercise testing and bioelectrical impedance analysis from April 2019 to January 2020. Data were compared with control subjects tested during the same period. We studied 47 patients with a Fontan circulation (53% boys; 15±3.1 years) and 165 controls (48% boys; 14.4±2.5 years). Fontan status was associated with shorter height, but similar age, sex, and overall body mass. Patients with Fontan had lower lean body mass (−12.0±22%, Z‐score −0.5±1, P =0.005), skeletal muscle mass (−13.6±1.4%; Z‐score, −0.5±1; P =0.004), skeletal muscle indexed to height (−10.3±13.3%; Z‐score, −0.5±1; P =0.005), and higher percent body fat (+13.8±18.6%; Z‐score, 0.4±1.2; P =0.03). Greater skeletal muscle mass was associated with higher peak oxygen consumption ( r 2 =0.52, P <0.0001) and oxygen pulse ( r 2 =0.68, P <0.0001). Patients who had suffered a late complication (ie, heart transplant referral or evidence of extracardiac organ dysfunction) of the Fontan operation (13 of 47, 27.7%) had lower skeletal muscle mass ( P =0.048) and higher body fat percentage ( P =0.003). Conclusions The Fontan circulation is associated with marked myopenia and increased adiposity. Higher muscle mass was associated with better exercise capacity. Fontan complications are associated with lower muscle mass and increased adiposity.

Left ventricular DD is associated with decreased exercise capacity in SCA. Interventions to prevent or delay DD could improve exercise capacity, quality of life, and long-term outcomes in SCA.

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