What Limits Cardiac Performance during Exercise in Normal Subjects and in Healthy Fontan Patients?

Abstract

Exercise is an important determinant of health but is significantly reduced in the patient with a univentricular circulation. Normal exercise physiology mandates an increase in pulmonary artery pressures which places an increased work demand on the right ventricle (RV). In a biventricular circulation with pathological increases in pulmonary vascular resistance and/or reductions in RV function, exercise-induced augmentation of cardiac output is limited. Left ventricular preload reserve is dependent upon flow through the pulmonary circulation and this requires adequate RV performance. In the Fontan patient, the reasons for exercise intolerance are complex. In those patients with myocardial dysfunction or other pathologies of the circulatory components, it is likely that these abnormalities serve as a limitation to cardiac performance during exercise. However, in the healthy Fontan patient, it may be the absence of a sub-pulmonary pump which limits normal increases in pulmonary pressures, trans-pulmonary flow requirements and cardiac output. If so, performance will be exquisitely dependent on pulmonary vascular resistance. This provides a potential explanation as to why pulmonary vasodilators may improve exercise tolerance. As has recently been demonstrated, these agents may offer an important new treatment strategy which directly addresses the physiological limitations in the Fontan patient.

Hemodynamic performance of the Fontan circulation compared with a normal biventricular circulation: a computational model study

“…The unique single-ventricle physiology of the Fontan circulation has been extensively addressed in the literature (29,33,42,49,68). A prevalent viewpoint is that CO in a Fontan circulation is determined predominantly by pulmonary vascular resistance rather than by other cardiovascular properties (e.g., ventricular contractility and peripheral vascular resistance) that are generally known as important factors in the control of CO in a biventricular circulation (29,42).…”

“…A prevalent viewpoint is that CO in a Fontan circulation is determined predominantly by pulmonary vascular resistance rather than by other cardiovascular properties (e.g., ventricular contractility and peripheral vascular resistance) that are generally known as important factors in the control of CO in a biventricular circulation (29,42). The viewpoint can give a reasonable explanation as to why pulmonary vasodilators may increase resting CO and improve exercise tolerance in some Fontan patients (27,78,93).…”

Describing right ventricular function

“…In recent years, however, this view has changed dramatically, mainly because it became clear that not only in pulmonary hypertension, but also in other conditions, like acute respiratory distress syndrome and left heart failure, right ventricular function is a prognostic determinant [2]. In addition, the importance of the right ventricle in determining maximal exercise capacity became clear [3]. Most of the studies investigating the right ventricle in different disease conditions and at exercise characterise the right ventricle by parameters obtained by imaging alone.…”

Management of the Fontan circulation in the adult patient

“…This is demonstrated in Figure 5, where pressure in the various stages of the circulation is shown in the resting state and on exercise in a normal subject and in a Fontan patient. (15) In a normal subject cardiac output is increased by pre-load recruitment with the effect of the muscular venous pump, being to increase RV pre-load, and increased heart rate and force of contraction pushing blood through the pulmonary vascular bed, leading to increased pre-load to the left ventricle and a consequent increase in stroke volume, which coupled with increased heart rate leads to an increase in cardiac output. In comparison, the lack of a sub-pulmonary pump and preexistent systemic venous hypertension, a requirement of the Fontan circulation, means that during exercise there is little pre-load…”

“…Many centres advocate aspirin alone (20,21) and although no reports demonstrate superiority, (22) it is clear that either is necessary and without antiplatelet or anticoagulant therapy thromboembolic events are frequent. (23) (15) ] In the normal circulation exercise leads to a rise in aortic pressure and a modest rise in systemic venous pressure (S), RV pressure increases as does PA pressure leading to an increase in pulmonary venous pressure (P) and an increase in pre-load to the LV. In the Fontan circulation S is already high and must remain higher than P to allow transpulmonary blood fl ow and the result is that on exercise there is little if any augmentation of ventricular preload.…”