Scaling Peak Oxygen Consumption for Body Size and Composition in People With a Fontan Circulation

Background Peak oxygen consumption (peak $$) is traditionally divided ("ratio-scaled") by body mass (BM) for clinical interpretation. Yet, it is unknown whether ratio-scaling to BM can produce a valid size-independent expression of peak $$ in people with a Fontan circulation. Furthermore, people with a Fontan circulation have deficits in lean mass, and it is unexplored whether using different measures of body composition may improve scaling validity. The objective was to assess the validity of different scaling denominators (BM, stature, body surface area, fat-free mass, lean mass, and appendicular lean mass using ratio and allometric scaling). Methods and Results Eighty-nine participants (age: 23.3±6.7 years; 53% female) with a Fontan circulation had their cardiorespiratory fitness and body composition measured by cardiopulmonary exercise testing and dual-energy x-ray absorptiometry. Ratio and allometric (log-linear regression) scaling was performed and Pearson correlations assessed scaling validity. Scaling denominators BM (r=-0.25, P=0.02), stature (r=0.46, P<0.001), and body surface area (0.23, P=0.03) were significantly correlated with their respective ratio-scaled expressions of peak $$, but fat-free mass, lean mass, or appendicular lean mass were not (r≤0.11; R²=1%). Allometrically expressed peak $$ resulted in no significant correlation with any scaling denominator (r=≤0.23; R²=≤4%). Conclusions The traditional and accepted method of ratio-scaling to BM is invalid because it fails to create a size-independent expression of peak $$ in people with a Fontan circulation. However, ratio-scaling to measures of body composition (fat-free mass, lean mass, and appendicular lean mass) and allometric techniques can produce size-independent expressions of peak $$ in people with a Fontan circulation.