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Page 2 of 7 Wang-Giuffre et al. Vessel Plus 2022;6:29 https://dx.doi.org/10.20517/2574-1209.2021.98

Patients with Fontan circuits have significantly decreased exercise tolerance secondary to several factors.
The absence of a subpulmonary ventricle causes a lack of negative pressure pulling blood back into the heart
and driving pressure to overcome the pulmonary vascular resistance with progressive exercise. In addition,
these patients have decreased muscle mass, diminished vascular function, and abnormal autonomic
regulation that can impede oxygen delivery to the muscles, thereby further decreasing the ability to exercise
[3]
efficiently . Factors such as depression, deconditioning, chronic illnesses, and arrhythmia further
complicate the picture .
[4,5]
Cardiopulmonary exercise testing (CPET) is an important non-invasive tool to monitor functional
measures of the Fontan circuit and serves as adjunct information to guide timing and need for
interventions. It is a relatively inexpensive modality to evaluate the Fontan circuit for any significant
desaturations, arrhythmia, oxygen delivery and monitor for any decreased function over time.

In this paper, we will discuss the physiology of exercise in the Fontan circulation and the various CPET
measures that can be used to follow Fontan function as well as guide intervention in the Fontan patient.

PHYSIOLOGY OF EXERCISE IN THE FONTAN POPULATION
Preload dependency
During exercise in a normal biventricular system, the pulsatile flow of the blood through the pulmonary
[6-8]
vasculature stimulates the release of nitric oxide, thereby decreasing pulmonary artery pressures . The
subpulmonary ventricle produces a negative pressure to augment the systemic venous return to the heart
and generates increasing pressure to drive blood through the pulmonary vasculature, thereby increasing
cardiac output to allow adequate oxygen delivery with increasing amounts of exertion.

Fontan circuits lack this subpulmonary pump and depend on the negative pressure of inspiration in order
to overcome pulmonary artery pressures. Stroke volume is limited by the inability to increasingly drive the
blood through the pulmonary vasculature, even in the setting of normal ventricular function and end-
diastolic pressure . In addition, there is some energy loss in the Fontan circuit due to the total
[9]
cavopulmonary connection.

Because of multiple sternotomies, Fontan patients have intrathoracic fibrous adhesions that can decrease
chest wall compliance; therefore, they have decreased forced vital capacity (FVC), which also negatively
affects their ability to increase preload back to the heart during inspiration . These factors contribute to the
[8]
relatively decreased cardiac output with increasing exertion compared to biventricular physiology, which
contributes to exercise intolerance in Fontan patients at high exertion.

Ventricular compliance and atrioventricular valve regurgitation
Fontan circulation is well tolerated in younger patients; however, as the patient ages, the ventricular
compliance decreases secondary to myocardial fibrosis. This then affects ventricular filling and decreases
cardiac output and is most evident at higher heart rates because of the decreased filling time of the ventricle.
Significant atrioventricular (AV) valve regurgitation further decreases forward flow that can affect cardiac
output negatively in a Fontan patient.

Chronotropic incompetence and arrhythmias
Peak heart rate decreases as patients age. Chronotropic incompetence and arrhythmias are well-known
complications in the Fontan population. These patients have compromised cardiac output compared to
biventricular patients when chronotropic incompetence is present. Arrhythmias, such as atrial tachycardias,