Beyond Body Mass Index: Skeletal Muscle Mass as a Marker of Physiologic Reserve in Fontan Circulation

Body mass index (BMI) incompletely captures body composition and functional reserve in congenital heart disease. We assessed whether skeletal muscle mass (SMM) relates more closely than BMI to stroke volume (SV), strength, and peak oxygen uptake (VO₂max) in Fontan physiology versus healthy controls. Cross-sectional assessment included multifrequency bioimpedance SMM, noninvasive SV (NICaS), handgrip dynamometry, and cardiopulmonary exercise testing. Associations of SMM and BMI with SV, grip strength, and VO₂max were evaluated within cohorts using Pearson correlations, and impedance SMM was compared with Lee-equation SMM. Forty-two participants were studied: Fontan group ( n = 21; mean age 28.3 ± 9.7 years; 8 women) and control group ( n = 21; mean age 29.5 ± 5.9 years; 7 women). BMI was lower in the Fontan group than in the control group (24.0 ± 4.4 vs. 26.4 ± 3.4 kg/m²), as was SMM (27.8 ± 6.2 vs. 34.0 ± 7.7 kg). SMM correlated with SV in the control group ( r = 0.770, p < 0.0001) and Fontan group ( r = 0.573, p = 0.007), whereas BMI showed a weaker association in the control group ( r = 0.465, p = 0.034) and no significant association in the Fontan group ( r = 0.365, p = 0.104). SMM correlated with handgrip strength in both groups (control r = 0.754; Fontan r = 0.807), whereas BMI did not (controls r = 0.197; Fontan r = 0.242). SMM correlated with VO₂max in the control group ( r = 0.503, p = 0.020), but not in the Fontan group ( r = 0.106, p = 0.65). BMI correlated with SMM in the control group ( r = 0.448, p = 0.042), but not in the Fontan group ( r = 0.361), suggesting greater body composition heterogeneity in Fontan patients. Impedance-derived and Lee-estimated skeletal muscle mass showed strong agreement in both groups (control r = 0.903; Fontan r = 0.864). SMM outperformed BMI in reflecting SV and strength; in Fontan circulation, VO₂max appears more centrally constrained than muscle-mass limited.