Relation of Systemic Venous Return, Pulmonary Vascular Resistance, and Diastolic Dysfunction to Exercise Capacity in Patients With Single Ventricle Receiving Fontan Palliation

Fontan patients have a reduced exercise capacity, primarily owing to limitations in the ability to augment pulmonary blood flow and stroke volume. To date, the mechanism of peak exercise pulmonary blood flow restriction has not been elucidated. We performed a single-center, prospective, crossover trial of supine and upright exercise in Fontan patients and healthy controls to determine the mechanisms of exercise limitation in the Fontan-palliated patient. A total of 29 Fontan patients and 16 control subjects completed the protocol. The duration of exercise, percentage of predicted peak oxygen consumption (VO2) and peak work were reduced in the Fontan group, regardless of posture (p ≤0.03). The percentage of predicted oxygen pulse, a surrogate for pulmonary stroke volume, was not increased with supine posture in the Fontan cohort (upright, 82.3 ± 18.8% vs supine, 82.4 ± 19.7%; p = 0.6). In both groups, the percentage of predicted peak VO2 was lower with supine exercise than with upright exercise (p ≤0.002). Diastolic dysfunction was present in 57% of the Fontan patients and was associated with a reduced percentage of predicted peak VO2 (p = 0.04) and supine peak work (p = 0.008). Six Fontan patients who underwent supine exercise with indwelling catheters failed to demonstrate the expected decrease in pulmonary vascular resistance characteristically seen with peak exercise (at rest, 2.8 ± 0.7 mm Hg/L/min/m2 vs at peak, 2.8 ± 0.9 mm Hg/L/min/m2; p = 0.9). In conclusion, supine exercise in Fontan patients does not result in an increased VO2 or oxygen pulse, suggesting that inadequate venous return might not be the primary limitation of exercise capacity in this population. Diastolic dysfunction and relatively excessive peak exercise pulmonary vascular resistance might be more important factors in Fontan exercise limitation.