Background
Fontan palliation has transformed the prognosis of children born with single-ventricle heart disease, allowing many to survive into adolescence and adulthood. Yet survival is only one part of the story. Patients with Fontan circulation live with a physiology that is fundamentally different from a normal biventricular heart: systemic venous blood is routed directly to the pulmonary arteries without a subpulmonary ventricle, so the circulation depends on low pulmonary vascular resistance, stable rhythm, and carefully balanced preload and afterload. Over time, this configuration can predispose patients to arrhythmia, thrombosis, exercise limitation, liver disease, lymphatic complications, stroke, and psychosocial stress.
Despite decades of clinical experience, the long-term trajectory of Fontan survivors remains incompletely defined, especially across the full age spectrum and across diverse care settings. This is precisely the gap the Fontan Outcomes Network was designed to address. The network functions as a learning health system across 38 congenital heart centers in the United States and Canada, aiming not only to collect data but to generate actionable insights that can improve care quality in real time. The report of the first 1,121 participants enrolled over a 2-year period provides an important baseline snapshot of this growing population.
Study Design
This study was a cross-sectional analysis of participants who had undergone Fontan palliation and were enrolled in the Fontan Outcomes Network between August 2022 and August 2024. The investigators collected demographic, clinical, imaging, procedural, and medication data and summarized them descriptively overall and by age group: younger than 12 years, 12 to younger than 18 years, and 18 years or older.
Because this was an observational registry analysis rather than an interventional trial, the study does not test a treatment effect or compare management strategies. Its value lies in scale, multicenter representation, and the breadth of clinically relevant variables captured. The analysis is best understood as a foundation-setting effort: it describes who is being cared for, what complications are present, and where future quality improvement and research priorities may lie.
Key Findings
Who was enrolled?
A total of 1,121 participants were included. The mean age was 16.3±10.2 years, and 471 participants were female, representing 42% of the cohort. This age distribution is notable because it spans childhood through adulthood, reflecting the success of modern congenital heart care in moving Fontan patients into later life while also showing that pediatric care remains central to this population.
From an anatomic standpoint, hypoplastic left heart syndrome was the most common primary diagnosis, present in 431 participants (38.5%). Right ventricular-dominant anatomy was even more common overall, affecting 615 participants (54.9%), particularly in younger age groups. This pattern is clinically important because it underscores the predominance of complex developmental cardiac anatomy in Fontan programs and suggests that many younger patients in the registry represent the more contemporary era of single-ventricle management.
What Fontan type was most common?
Extracardiac conduit Fontan was the most frequent surgical type, seen in 749 participants (66.8%). This is consistent with the broad shift toward extracardiac conduit strategies in modern practice, reflecting concerns about arrhythmia risk and long-term hemodynamic performance with older atriopulmonary connections. The registry therefore captures a contemporary Fontan cohort rather than a predominantly historical surgical population, although older surgical eras remain represented.
How common were Fontan-related adverse events?
Typical Fontan-related adverse events were noted in 662 participants, or 59% of the cohort. This is a strikingly high cumulative burden and reinforces the concept that Fontan palliation creates a chronic disease state rather than a one-time surgical cure. Among the reported complications, chylous pleural effusions occurred in 108 participants (9.6%), Fontan thrombosis in 79 (7.1%), and nonperioperative stroke in 73 (6.5%).
These numbers deserve careful interpretation. They likely reflect a combination of lifetime complication burden, referral to specialized centers, and the broad ascertainment methods of the registry. Still, they highlight the persistent vulnerability of Fontan patients to venous, lymphatic, and thromboembolic complications. Even in the era of improved surgical techniques and closer surveillance, these events remain common enough to warrant systematic prevention strategies and consistent follow-up.
How common were arrhythmias and other chronic comorbidities?
A history of arrhythmia was present in 463 patients, or 41.3% overall, and its prevalence increased with age. This age gradient is expected biologically and clinically. As Fontan patients age, atrial dilation, surgical scar burden, conduction abnormalities, and hemodynamic stress can all contribute to atrial tachyarrhythmias and other rhythm disturbances. The finding is a reminder that rhythm surveillance is not optional in this population; it is a core element of longitudinal care.
Acquired comorbidities were present in 645 participants (57.5%). Among these, asthma was reported in 132 patients (12%) and sleep apnea in 103 (9%). The burden of acquired disease increased with age, which is clinically plausible because older Fontan patients have had more time to accumulate respiratory, metabolic, renal, hepatic, and sleep-related problems. The registry therefore captures not only congenital heart disease but also the broader syndrome of chronic multisystem morbidity that often emerges across the lifespan.
What medications were patients taking?
Antithrombotic medication use was nearly universal, reflecting widespread clinical recognition of thrombosis risk in Fontan circulation. This is an important finding because it suggests that, across centers, clinicians continue to regard thromboprophylaxis as foundational. However, the abstract does not specify the exact regimens, adherence, or whether anticoagulation versus antiplatelet therapy was selected according to individualized risk. Those distinctions matter clinically and will likely be important in future analyses.
Use of other medications varied widely by age and included β-blockers, angiotensin-converting enzyme inhibitors, and pulmonary vasodilators. This heterogeneity likely reflects the absence of a single dominant pharmacologic strategy for Fontan physiology and the reality that treatment is often symptom-directed or lesion-specific. The registry’s medication data may eventually help clarify which subgroups are most likely to receive particular therapies and whether practice patterns align with outcomes.
What about mental health?
One of the most clinically meaningful findings was the burden of psychological morbidity. A clinical diagnosis of an anxiety disorder was present in 382 participants (34.1%) overall, including 194 of 403 adults (48.1%). Antidepressants were prescribed in 132 participants (11.8%) overall, including 95 of 403 adults (23.6%). These numbers are difficult to ignore. They show that Fontan care is not only about hemodynamics, oxygen saturation, and imaging results; it also involves the emotional and behavioral consequences of living with chronic disease, repeated procedures, and uncertain prognosis.
The mental health findings are especially relevant because anxiety may affect adherence, symptom reporting, transition to adult care, employment, education, and quality of life. In a population where self-management and consistent surveillance are essential, unrecognized anxiety can become a clinical risk multiplier. The registry thus makes a strong case for integrating behavioral health into Fontan programs rather than treating it as an optional add-on.
Clinical Interpretation
The major contribution of this report is not that it uncovers a single unexpected complication, but that it provides a coherent, multicenter picture of the contemporary Fontan population. Several themes emerge. First, the cohort is young on average, which means many patients have not yet reached the ages at which Fontan failure, end-organ dysfunction, and severe arrhythmia often become most apparent. Second, the burden of adverse events is already high by adolescence and early adulthood. Third, comorbidity extends well beyond the heart, with substantial respiratory and psychological disease captured in the registry.
For clinicians, these findings reinforce several practice points. Lifelong surveillance remains essential. Arrhythmia monitoring should be routine, especially as patients age. Thrombotic risk assessment must continue even in apparently stable patients. Sleep disorders and other acquired comorbidities should not be overlooked, because they can worsen functional status and may be modifiable. Mental health screening should become standard in pediatric and adult congenital heart clinics, not only because anxiety is common, but because it may influence the success of the entire care plan.
For health systems, the report supports the value of a learning health network model. Rare diseases often suffer from fragmented care and small sample sizes that limit evidence generation. By pooling data across 38 centers, the Fontan Outcomes Network can identify patterns that no single center could detect alone. In the long term, such a network may support benchmarking, risk prediction, implementation of best practices, and more efficient enrollment into future interventional studies.
Strengths and Limitations
The strengths of the study are substantial. The registry includes a large sample size for a rare disease, broad geographic representation, and a multicenter design that improves generalizability relative to single-center reports. The prospective clinical registry structure also enhances the potential for longitudinal follow-up and quality improvement. Importantly, the age-stratified analysis makes the findings useful across pediatric and adult congenital heart care.
That said, there are important limitations. The analysis is cross-sectional and descriptive, so it cannot determine causality or prognosis. It is possible that the cohort is enriched for patients with more complex disease or greater engagement with specialty care, which could overestimate complication rates relative to the broader Fontan population. The abstract does not provide details on data completeness, ascertainment of historical events, uniformity of definitions across sites, or inter-center variation in imaging, anticoagulation, or mental health screening. Those factors can materially influence prevalence estimates.
Another limitation is that many of the reported adverse events are cumulative or historical, rather than indicators of current physiologic status. A patient with a remote episode of thrombosis or arrhythmia may be clinically stable now, but still counted as having the complication history. This is not a flaw, but it does mean the data should be interpreted as lifetime burden rather than current disease activity.
Why This Matters Now
The Fontan population is growing older, and the field is moving from a paradigm of survival after surgery to one of durable chronic disease management. That shift requires better data. The Fontan Outcomes Network is important because it turns a rare, heterogeneous condition into a shared knowledge base. In practical terms, this can improve how clinicians counsel families, anticipate complications, prioritize screening, and coordinate multidisciplinary care.
The most actionable message from the first 1,121 enrollees is that Fontan care must be comprehensive. Cardiology remains central, but it must be paired with anticoagulation strategy, arrhythmia surveillance, respiratory evaluation, sleep assessment, neurodevelopmental support, liver and lymphatic monitoring, and behavioral health services. No single specialty can manage the full burden alone.
Conclusion
The Fontan Outcomes Network provides one of the largest prospective, multicenter data sets ever assembled for patients with Fontan circulation. Its early findings show a cohort with substantial cumulative morbidity, frequent arrhythmia, widespread use of antithrombotic therapy, meaningful rates of thrombotic and cerebrovascular events, and a high burden of anxiety and other acquired comorbidities. The study does not answer treatment-effect questions, but it does establish a crucial foundation for future research and quality improvement.
For clinicians, the message is clear: Fontan physiology requires lifelong, multidisciplinary follow-up, and mental health deserves a place alongside cardiac surveillance. For researchers and health systems, the network demonstrates how collaborative data infrastructure can transform care for a rare and complex disease.
Funding and Clinical Trial Registration
The provided abstract does not report funding details or a ClinicalTrials.gov registration number. Because this was a prospective clinical registry and cross-sectional descriptive analysis, it should be interpreted as a learning health network report rather than a therapeutic clinical trial.
References
1. Glenn TM, Kasparian NA, Amdani S, et al; SV-ONE Investigators. The Fontan Outcomes Network: Findings After 2 Years and 1121 Participants. Circulation. 2026;153(16):1182-1195. PMID: 41878814.
2. Rychik J, Atz AM, Celermajer DS, et al. Evaluation and Management of the Child and Adult With Fontan Circulation: A Scientific Statement From the American Heart Association. Circulation. 2019;140(6):e234-e284.