Increased circulating catecholamines are associated with worse exercise performance in adult heart failure patients. Patients with Fontan physiology have increased circulating catecholamines and theoretically could benefit from beta blockade. We hypothesized that carvedilol would improve exercise performance in Fontan patients. A double-blind, placebo-controlled, crossover trial of carvedilol was performed. Single ventricle patients between the ages of 10 and 35 years with a previous Fontan operation who were able to complete a maximal exercise test (respiratory exchange ratio > 1.0) were included. Two 12-week treatment arms were separated by a 6-week washout period. Exercise testing was performed at beginning and end of each treatment arm. Primary endpoint was improvement in peak oxygen consumption/kg (pVO₂) from baseline. Of the 26 subjects enrolled, 23 completed the study. Four subjects did not reach goal maximum carvedilol dose, vs. 1 for placebo (p = 0.14). The mean change in pVO₂ between treatments was not different (carvedilol = − 2.1 mL/kg/min v. placebo = − 1.42, p = 0.28). Carvedilol therapy decreased peak heart rate by 4.2 ± 20.2 bpm, (p < 0.01) leading to an increase in peak oxygen pulse (p < 0.01). Serum N-terminal-proBNP increased with carvedilol therapy (mean change of + 23.77 pg/mL) compared to placebo (mean change of − 5.37 pg/mL, p = 0.03). There were no serious adverse events related to study drug. Carvedilol was not associated with improved exercise performance and was associated with mildly increased N-terminal-proBNP. This study does not support the routine administration of carvedilol to healthy Fontan patients.

Clinical Trials Registration ClinicalTrials.gov Identifier: NCT02946892. Registered October 27, 2016. Retrospectively Registered. https://​clinicaltrials.​gov/​ct2/​show/​NCT02946892