Andre La Gerche, MD, PhD et al
American Heart Association

Background-—Patients with a Fontan circulation achieve lower peak heart rates (HR) during exercise. Whether this impaired
chronotropic response reflects pathology of the sinoatrial node or is a consequence of altered cardiac hemodynamics is uncertain.
We evaluated the adequacy of HR acceleration throughout exercise relative to metabolic demand and cardiac output in patients
with a Fontan circulation relative to healthy controls.

Methods and Results-—Thirty subjects (20 healthy controls and 10 Fontan patients) underwent cardiac magnetic resonance
imaging with simultaneous invasive pressure recording via a pulmonary and radial artery catheter during supine bicycle exercise to
near maximal exertion. Adequacy of cardiac index, stroke volume, and HR reserve was assessed by determining the exerciseinduced increase (Δ) in cardiac index, stroke volume, and HR relative to the increase in oxygen consumption (VO2). HR reserve was
lower in Fontan patients compared with controls (7121 versus 9215 bpm; P=0.001). In contrast, increases in HR relative to
workload and VO2 were higher than in controls. The change in cardiac index relative to the change in VO2 (Δcardiac index/ΔVO2)
was similar between groups, but Fontan patients had increased ΔHR/ΔVO2 and reduced Δ stroke volume/ΔVO2 compared with
controls. There was an early and marked reduction in stroke volume during exercise in Fontan patients corresponding with a
plateau in cardiac output at a low peak HR.

Conclusions-—In Fontan patients, the chronotropic response is appropriate relative to exercise intensity, implying normal sinoatrial
function. However, premature reductions in ventricular filling and stroke volume cause an early plateau in cardiac output beyond
which further increases in HR would be physiologically implausible. Thus, abnormal cardiac filling rather than sinoatrial node
dysfunction explains the diminished HR reserve in Fontan patients. (J Am Heart Assoc. 2019;8:e012008. DOI: 10.1161/JAHA.

Key Words: cardiac magnetic resonance imaging • chronotropic incompetence • exercise physiology • Fontan procedure •
heart rate

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