Pediatric Fontan hearts show diffuse remodeling at moderate altitude on T1 mapping

Children born with single-ventricle heart defects who undergo the Fontan procedure face a lifetime of unique cardiovascular challenges, and researchers are increasingly turning to advanced cardiac imaging to detect the earliest signs of trouble. A new commentary published in Pediatric Radiology by Amol Pednekar, Murat Kocaoglu, and Cara E. Morin of Cincinnati Children’s Hospital Medical Center and the University of Cincinnati College of Medicine examines how two sophisticated magnetic resonance imaging measurements — myocardial T1 relaxation time and extracellular volume fraction, or ECV — should be interpreted in children living with Fontan circulation at moderate altitude. The piece is written as a companion to a recent original study by Arnouts and colleagues that reported elevated myocardial extracellular volume and reduced exercise capacity in pediatric Fontan patients living at moderate altitude, and its central message is that imaging numbers must never be read in isolation from the clinical, physiological, and developmental context of these remarkable children.

The Fontan procedure is the final stage of palliative surgery for children born with a functional single ventricle — a heart with only one effective pumping chamber instead of two. Rather than repairing the underlying anatomy, the operation reroutes venous blood from the body directly to the pulmonary arteries, bypassing the heart and allowing the single ventricle to dedicate itself entirely to pumping oxygenated blood to the body. The result is a circulation that works, but without the normal pressure gradient that in a healthy heart drives blood through the lungs. Systemic venous pressure rises, cardiac output reserves are limited, and the entire cardiovascular system must adapt to a state of chronic preload restriction and passive pulmonary flow. Children who have undergone this procedure now survive well into adulthood in growing numbers, which makes it increasingly important to identify which patients are quietly developing myocardial dysfunction before symptoms appear.

The imaging techniques at the heart of the discussion come from a field known as T1 mapping, which has revolutionized cardiac magnetic resonance imaging over the past decade. In conventional cardiac MRI, radiologists look at pictures of the heart and evaluate its shape, size, and motion. T1 mapping goes further by measuring a fundamental physical property of the heart muscle itself: how quickly hydrogen protons in the tissue relax after being excited by radiofrequency pulses in the MRI scanner’s magnetic field. Native T1 — measured without any contrast agent — reflects the mix of water, fat, protein, and iron inside the myocardium. When fibrosis develops, collagen deposition lengthens the relaxation time, producing a higher T1 value. As Kellman and Hansen noted in their foundational work on the accuracy and precision of the technique, the measurement is exquisitely sensitive but also vulnerable to errors from scanner drift, motion, and the specific mapping sequence used, which is why standardized acquisition protocols matter so much when comparing patients across institutions or, as in this case, across altitudes.

The extracellular volume fraction builds on native T1 by adding a second measurement after intravenous gadolinium contrast has distributed into the space between heart muscle cells. By comparing pre-contrast and post-contrast T1 values in both the myocardium and the blood pool, the technique calculates the fraction of heart tissue occupied by extracellular matrix — the scaffolding material between cells. In a healthy child’s heart, this fraction is typically around 25 percent or slightly lower. When diffuse fibrosis, edema, or other interstitial expansion occurs, ECV rises. Consensus guidelines published by the Society for Cardiovascular Magnetic Resonance and endorsed by the European Association for Cardiovascular Imaging established the clinical recommendations for T1, T2, and ECV mapping that practitioners now follow, but as the Cincinnati authors emphasize, those reference values were largely derived from patients at sea level, living with structurally normal hearts.

Herein lies the crux of the commentary’s argument. Arnouts and colleagues studied pediatric Fontan patients living at moderate altitude and found elevated myocardial ECV alongside diminished exercise capacity. The Cincinnati team does not dispute the finding — indeed, they consider it clinically meaningful — but they caution against a simplistic interpretation in which elevated ECV automatically equals established fibrosis. In children, tissue composition is not static. The growing heart remodels continuously, and myocyte size, matrix content, and water distribution all shift with development, training status, and hemodynamic load. Chronic hypoxia, which is an everyday reality for Fontan patients at altitude even more than at sea level, can increase plasma volume, alter hydration status, and potentially raise ECV through mechanisms that are physiological adaptation or reversible change rather than permanent collagen deposition. Diffuse myocardial remodeling, the phrase the authors choose deliberately, encompasses a spectrum of processes — interstitial fibrosis, myocyte hypertrophy, chronic edema, and microvascular changes — that T1 and ECV cannot disentangle on their own.

The physiological burden of moderate altitude adds another layer of complexity. At elevation, ambient oxygen partial pressure falls, and every Fontan patient already operates with chronically reduced oxygen saturation because of the mixing inherent in the single-ventricle circulation. The lung’s passive vascular bed, which depends on adequate pulmonary blood flow and low resistance, must function under hypoxic conditions that trigger vasoconstriction and can elevate pulmonary vascular resistance over time. For a circulation whose entire design depends on driving blood through the lungs without a pump, even modest increases in pulmonary resistance translate directly into higher systemic venous pressures, reduced cardiac output reserve, and impaired exercise tolerance. Exercise intolerance is, in fact, one of the most consistent and troubling findings in long-term Fontan survivors regardless of where they live, and as Rhodes and colleagues documented in their review of exercise testing and training in children with congenital heart disease, peak oxygen consumption in these patients falls well short of healthy peers — a gap that widens over time and predicts late outcomes.

Rathod, Powell, and Geva, writing in the Circulation Journal, framed the fundamental problem that motivates all of this imaging work: myocardial fibrosis in congenital heart disease is common, clinically consequential, and difficult to detect before it becomes irreversible. Histological studies in adult congenital heart disease and in failing Fontan circulations have demonstrated diffuse interstitial fibrosis that correlates with ventricular dysfunction, arrhythmia, and heart failure. If noninvasive imaging can identify the fibrotic process while it is still developing — or, equally important, distinguish true fibrosis from reversible edema and adaptive remodeling — clinicians could theoretically intervene earlier with medications, exercise prescriptions, or catheter-based therapies before the damage becomes fixed. That is the enormous promise of T1 mapping and ECV quantification, and it is precisely why the Cincinnati authors are so invested in making sure these measurements are interpreted correctly in this population.

The commentary’s practical guidance centers on integrating the numbers with everything else that is known about the patient. A given ECV value in a sedentary fourteen-year-old Fontan patient living at 1,500 meters with resting hypoxemia, elevated systemic venous pressure, and reduced exercise capacity may tell a very different story than the identical number in a well-compensated patient at sea level. The authors urge clinicians to consider hematocrit, hydration status, altitude-driven plasma volume changes, heart rate variability, and the specifics of the patient’s surgical anatomy when weighing T1 and ECV results. They also highlight the importance of technical consistency: values obtained on one scanner with one mapping sequence cannot be compared naively with values from another, and normal ranges must be established for the specific population and environment in question rather than imported from healthy sea-level controls.

What makes this discussion resonate beyond a single subspecialty is the broader principle it illustrates. Quantitative imaging is transforming medicine, replacing the subjective eye of the radiologist with reproducible numbers that can be tracked over years and compared across centers. But numbers are only as meaningful as the physiological and clinical context in which they are measured. In complex, rare diseases like single-ventricle congenital heart disease, where every patient’s anatomy, surgical history, and environment differ, the temptation to apply universal thresholds is strong — and potentially misleading. The Cincinnati commentary is, at its heart, a call for interpretive humility paired with scientific rigor: measure carefully, standardize diligently, and always interpret the result in light of the whole child, including the air that child breathes.

The stakes for these patients could not be higher. Fontan survivors are living longer than ever before, and the field is racing to understand why some hearts fail in adolescence while others remain functional for decades. Imaging biomarkers such as T1 and ECV offer the possibility of surveillance that begins before symptoms, potentially guiding the timing of interventions and the design of clinical trials. The original study by Arnouts and colleagues — examined in pediatric Fontan patients at moderate altitude, published in Pediatric Radiology, spanning pages 1–12 in 2026 — represents one of the first efforts to characterize myocardial tissue composition in this environmental context, and the accompanying commentary by Pednekar, Kocaoglu, and Morin ensures that the findings will be read with the nuance they deserve. As the population of Fontan survivors continues to grow, the collaboration between imaging scientists, cardiologists, and physiologists reflected in these two papers will be essential to turning quantitative MRI data into better, longer lives for children whose hearts were rebuilt from the very beginning.