The Fontan Procedure is a palliative surgical procedure used in pediatric patients with one functional ventricle. The procedure, a series of stepwise operations that alter cardiorespiratory physiology, separate the systemic and pulmonary circulations to create Fontan physiology, where the systemic venous blood flows passively and without ventricular thrust into the pulmonary circulation. The hallmark of the Fontan circulation is a sustained, abnormally elevated central venous pressure combined with decreased cardiac output, especially during periods of increased demands. Results of several studies in Fontan patients have shown reduced parasympathetic and sympathetic activity compared to controls. In children with congenital heart disease, a differential diagnosis of autonomic dysfunction may be part of their pathophysiology, a compensatory mechanism, a consequence of surgical procedures or a combination of these.

In children, measurement of ANS function is equally important. Children with single ventricle physiology (and other cardiac conditions) have routine surveillance and cardiac magnetic resonance (CMR) imaging to monitor for disease progression. While autonomic data is routinely collected and is available from these scans, these data are rarely, collected and analyzed; however, our group has shown feasibility. Therefore, autonomic data is usually unavailable in children. Despite the availability of agerelated normal values, the predictive power of autonomic activity is understudied in children and there are no published studies of quantification of autonomic data in this population.

Inclusion Criteria:

• Ages 0 to ≤ 30 years
• Baseline sinus rhythm (interventricular conduction delays and bundle branch blocks allowed); and undergoing CMR scan in which sequences containing timing of aortic valve opening (cine), vector cardiogram (VCG) and respiratory bellows use will be obtained
• Fontan physiology - only for the Fontan group. Chest wall deformity or arrhythmogenic right ventricular cardiomyopathy for the healthy control group

Exclusion Criteria:

• Patient or family refusal;
• Contraindication to study procedures
• Prior participation in the study (ie, we plan to sample without replacement)

Patients presenting for clinically indicated CMR scans with planned cine sequence and use of a respiratory bellows.