FIG 73.7 Theoretical schema to illustrate circulatory pressure changes in normal and Fontan patients at rest (blue) and during exercise (red) In the normal circulation (A), pressure is generated in the systemic ventricle (LV) to produce flow in the aorta (Ao) and systemic circulation (S) Pressure dissipates across the systemic microcirculation such that right atrial (RA) pressure is low The prepulmonary pump (RV) provides the pressure to generate the flow in the pulmonary artery (PA), which then dissipates in the pulmonary circulation (P) but is sufficient to maintain preload in the left atrium (LA) During exercise, systemic vascular resistance falls such that there is little increase in mean LV pressure requirements However, more substantial pressure increases are required in the RV, and these pressure requirements increase with exercise intensity In the Fontan patient (B), the cavopulmonary bypass (CPB) does not provide any contractile force, and therefore flow through the pulmonary circulation is dependent on the pressure difference between the RA and LA During exercise, transpulmonary flow can be augmented only by a reduction in pulmonary vascular resistance Beyond mild to moderate exercise, pulmonary vasodilation is maximal and flow increases require a prepulmonary pump Without this, pulmonary pressure does not rise, transpulmonary flow does not increase, LA pressure (preload) does not increase, and cardiac output cannot supply the metabolic demands of exercise (From La Gerche A, Gewilliq M What limits cardiac performance during exercise in normal subjects and in healthy Fontan patients? Int J Pediatr 2010;2010[5]:1–8.) FIG 73.8 Work versus oxygen uptake (VO2) during exercise In the normal circulation (A) there is a point above which VO2 cannot be increased despite an increase in workload This represents the maximal VO2 (VO2 max), which in this situation is identical to VO2 peak In the Fontan circulation (B) exercise duration workload and VO2 are reduced compared with normal and frequently there is no plateau in VO2, such that the VO2 max is not achieved There are several aspects of the Fontan circulation that contribute to impaired exercise capacity (Fig 73.9) Under normal conditions, cardiac output is augmented by increases in preload, heart rate, and myocardial contractility and a reduction in afterload Stroke work is increased substantially more in the subpulmonary (right) ventricle than the systemic (left) ventricle.43 In the absence of a subpulmonary ventricle many of these adaptive responses are absent or compromised FIG 73.9 Mechanisms of impaired exercise capacity in the Fontan patient The four major cornerstones to impaired exercise tolerance in the Fontan circulation are preload insufficiency, chronotropic incompetence, restrictive lung disease, and underlying and residual lesions Some features of the cornerstones are inherent in the physiology of a Fontan circulation, including the lack of a subpulmonary pump and elevated systemic venous pressure The remainder make a variable contribution to impaired exercise capacity, as do other factors including anemia, neurohormonal activation, arrhythmia, and deconditioning Preload Insufficiency In the Fontan circulation, preload is chronically depleted, and this effect is magnified under exercise conditions In the absence of a subpulmonary ventricle, systemic ventricular filling is dependent on diastolic function and low pulmonary vascular resistance to pull blood through the pulmonary circulation These factors are the primary drivers of exercise capacity in the Fontan circulation Following volume unloading during staged surgical reconstruction, the functionally univentricular heart reduces in size by 25% to 70%.47 Although remodeling could compensate for this change by reducing myofiber length, diastolic dysfunction predominates from early on in the majority of Fontan