Impaired preload reserve during exercise limits exercise capacity in the better Fontan patients

Background: Factors controlling cardiac output in a Fontan circulation are still poorly understood. This study aimed at evaluating changes in exercise hemodynamics using a novel cardiac magnetic resonance (CMR) methodology during mild, moderate and high intensity exercise. Methods: Ten Fontan patients (6 male, age 20±4 years, NYHA I-II) underwent CMR at rest and during supine exercise on a programmable cycle ergometer. Systemic ventricular volumes were obtained at rest (heart rate 72±14 bpm) and during mild (100±10 bpm), moderate (122±15 bpm) and high intensity (144±15 bpm) exercise. Images were acquired using an ungated, free-breathing real-time CMR sequence (12-18 contiguous 8mm slices). Software was developed to allow for synchronization of short and long-axis images with compensation for respiratory phase translation. Endocardial borders were delineated using a bi-plane model. Simultaneously, radial and pulmonary artery pressures were measured. Results: Cardiac output (CO) increased from rest to moderate intensity exercise, but failed to increase further during high intensity exercise (6.8±1.6 vs 10.0±3.2 vs 11.8±3.2 vs 12.5±3.1 l/min; P<0.0001). The increase in CO depended on a 106±49% increase in heart rate as stroke volume (SV) decreased during high intensity exercise (95±19 vs 100±21 vs 96±20 vs 87±16 ml; P<0.0001). End-diastolic volume (EDV) decreased during high intensity exercise (169±38 vs 174±39 vs 170±38 vs 162±36 ml; P=0.002), whereas end-systolic volume (ESV) did not change during exercise (74±31 vs 74±31 vs 73±32 vs 75±32 ml; P=0.715). Ejection fraction (EF) decreased during exercise (57±10 vs 58±10 vs 59±11 vs 55±11%, P=0.037). Pulmonary artery pressures (9±3 vs 14±4 vs 17±4 vs 21±5 mmHg; P<0.0001) and mean arterial pressures (81±7 vs 92±7 vs 100±6 vs 106±4 mmHg; P<0.0001) increased during exercise. Total pulmonary resistance increased during high intensity exercise (P=0.004) whereas systemic vascular resistance decreased during exercise (P=0.003). Arteriovenous oxygen difference increased from 5.5±1.2 to 9.6±1.9 ml/100ml (P=0.001). Conclusions: A decreasing SV and EDV despite a blunted heart rate response during exercise reflect a limited ventricular preload which is presumably related to the increase in total pulmonary resistance. The decreased preload reserve in a Fontan circuit is an important determinant limiting exercise capacity.