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.

AimsThe Fontan operation has resulted in improved survival in patients with single-ventricle congenital heart disease. As a result, there is a growing population of teenagers and adults with a Fontan circulation. Many co-morbidities have been increasingly recognized in this population due to the unique features of the Fontan circulation. Standardization of how Fontan co-morbid conditions are defined will help facilitate understanding, consistency and interpretability of research and clinical experience. Unifying common language usage in Fontan is a critical precursor step for data comparison of research findings and clinical outcomes and ultimately accelerating improvements in management for this growing group of patients. This manuscript aimed to create unified definitions for morbidities seen after the Fontan palliation. Methods In association of many congenital heart disease organizations, this work used Delphi methodology to reach a broad consensus among recognized experts regarding commonly used terms in Fontan care and research. Each definition underwent at least three rounds of revisions to reach a final definition through surveys sent to experts in the field of single-ventricle care. Results The process of reaching a consensus on multiple morbidities associated with the Fontan procedure is summarized in this manuscript. The different versions that preceded reaching the consensus are also presented in the Supporting Information. Table 1 represents the final definitions according to the consensus. Conclusions We propose the use of these definitions for clinical care, future research studies, registry development and clinical trials.

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