Combined Analysis by Peak and Submaximal Exercise Parameters Delineates Underlying Mechanisms of Poor Exercise Performance in Fontan Patients

Fontan patients have reduced exercise performance primarily due to the absence of a sub-pulmonary ventricle mitigating ventricular filling during exercise, but other factors may be involved. Comprehensive cardiopulmonary exercise testing (CPET) analysis may delineate the underlying mechanisms. We retrospectively reviewed CPET parameters of Fontan and age-matched control adolescents. Both peak and submaximal parameters were analyzed. Fifty-one Fontan patients (15.4 ± 3.5 years old, male 21) and 65 age-matched controls (14.7 ± 2.0 years old, male 32) were studied. Peak values of heart rate (HR), oxygen consumption (VO2)/kg, and work rate (WR)/kg were significantly lower in Fontan than control patients in both sexes. Peak oxygen pulse (pOP/kg) and submaximal slope of [VO2/kg]/HR (Δ[VO2/kg]/ΔHR), surrogates of stroke volume, were significantly reduced whereas ΔHR/Δ[WR/kg], HR-dependency, was significantly increased in Fontan patients, indicating limited stroke volume reserve during exercise resulting in higher HR increase during exercise despite lower peak HR. When dividing Fontan patients into upper and lower halves at the average pVO2/kg, the upper half showed significantly higher peak WR/kg, ventilatory anaerobic threshold (VAT)/kg, oxygen uptake efficiency slope (OUES)/kg, and ΔVO2/ΔWR (aerobic work efficiency), suggesting the difference in muscle conditioning and breathing efficiency. There were some sex differences in CPET parameters that predict exercise performance among Fontan patients. Our proposed CPET assessment methods can address the underlying exercise physiology of Fontan patients. Enhancing muscle conditioning and ventilatory efficiency by routine exercise may help improve functional reserve of Fontan patients, which can be monitored by these CPET parameters.