The Fontan circulation after 45 years: update in physiology

Abstract

The Fontan operation was first performed in 1968. Since then, this operation has been performed on thousands of patients worldwide. Results vary from very good for many decades to very bad with a pleiad of complications and early death. A good understanding of the physiology is necessary to further improve results. The Fontan connection creates a critical bottleneck with obligatory upstream congestion and downstream decreased flow; these two features are the basic cause of the majority of the physiologic impairments of this circulation. The ventricle, while still the engine of the circuit, cannot compensate for the major flow restriction of the Fontan bottleneck: the suction required to compensate for the barrier effect cannot be generated, specifically not in a deprived heart. Except for some extreme situations, the heart therefore no longer controls cardiac output nor can it significantly alter the degree of systemic venous congestion. Adequate growth and development of the pulmonary arteries is extremely important as pulmonary vascular impedance will become the major determinant of Fontan outcome. Key features of the Fontan ventricle are early volume overload and overgrowth, but currently chronic preload deprivation with increasing filling pressures. A functional decline of the Fontan circuit is expected and observed as pulmonary vascular resistance and ventricular filling pressure increase with time. Treatment strategies will only be successful if they open up or bypass the critical bottleneck or act on immediate surroundings (impedance of the Fontan neoportal system, fenestration, enhanced ventricular suction).

Figure 1

Scheme of the normal cardiovascular circulation (A), and the Fontan circulation at different stages (B and C). (A) Normal biventricular circulation: the pulmonary circulation (P) is connected in series to the systemic circulation (S). The compliance of the right ventricle ensures that the right atrial pressure remains lower than the left atrial pressure and delivers the driving force to the blood to overcome pulmonary impedance. (B) Fontan circuit: the caval veins are directly connected to the PA; systemic venous pressures are markedly elevated. (C) Fontan circuit late (superimposed on early Fontan circuit): with time, a negative spiral ensues: pulmonary resistance increases resulting in further increase in CV congestion but even more in reduced flow, which increases ventricular filling pressure. Ao, aorta; CV, caval veins; LA, left atrium; LV, left ventricle; PA, pulmonary artery; RV, right ventricle; V, single ventricle. Line thickness reflects output, and colour reflects oxygen saturation.

Figure 2

Steady-state flow through multiple bottlenecks in series. Hourglass with sand demonstrating multiple bottleneck concept. A circuit may consist of multiple bottlenecks (A–C). However, only the most critical obstruction (B) determines the flow of sand through the hourglass. As a result, sand accumulates proximal to the obstruction; flow can be improved by altering the critical bottleneck itself or by exerting more push just above or pull just below. The prime determinant of flow is the obstruction at bottleneck B; improving obstructions at A and C will have no effect.

Figure 3

Output at rest modulated by pulmonary vascular resistance (PVR). In a normal subject (black line), ventricular function has little effect on cardiac output except when very poor. The principal determinant of output in patients with Fontan circulation (coloured lines) is the degree of impedance of the pulmonary vasculature resistance (PVR). The higher the PVR (at rest), the lower the cardiac output even in the presence of normal systolic ventricular function; improving systolic function will have no influence on increasing cardiac output at rest. Adapted with permission from Gewillig and Goldberg.

Figure 4

Effect of various degrees of pulmonary bypassing in a Fontan circuit on systemic output (thick line), saturation (dotted line) and systemic venous Congestion (thin line). A ‘good Fontan’ (green or lighter lines) with low neoportal resistance has a cardiac output (solid green line) of about 80% of normal for BSA, with high saturations (dotted green line) and a slightly raised CVP (thin green line). The ‘bad Fontan’ (red lines) with a high neoportal resistance has comparable saturations (dotted red) but with a very low output (solid red) in the presence of a high CVP (thin red). Partial bypassing of the Fontan portal system by a fenestration consistently increases systemic output and lowers venous congestion but may give rise to clinically intolerable degrees of cyanosis (effects of fenestration size can be viewed at bottom right of graph). CVP: central venous pressure. Adapted with permission from Gewillig and Goldberg.

Figure 5

Exercise and output: normal versus Fontan circulation. Normal subjects with a biventricular circulation can increase their cardiac output up to five times (black line). At rest, patients with Fontan circulation at best already have a cardiac output 80% of normal and with a markedly restricted ability to increase during exercise (green line) allowing only a mild sporting ability. At worst (red line), the output is severely restricted at rest and barely increases during exercise. Adapted with permission from Gewillig and Goldberg.

Figure 6

Pulmonary volume load (and outcome) of Fontan since 1990s. In the normal circulation, pulmonary blood flow increases at birth and remains at 100% of normal for BSA (A, black line). In a single-ventricle circulation, a period of excessive pulmonary flow exists after birth until stage 2 palliation (B, green line). With satisfactory pulmonary vascular growth, a reduction in pulmonary blood flow to about 50% for BSA usually occurs after the superior cavo-pulmonary connection. Following total cavo-pulmonary connection (Fontan), there is an increase in pulmonary flow but still well below normal. If flow to the lungs is too low following initial palliation, inadequate growth will inevitably occur (C, yellow line). Systemic desaturation as a result of low pulmonary blood flow may lead to early referral for superior cavo-pulmonary connection, which then further reduces pulmonary blood flow and growth. Fontan operation in this group with hypoplastic pulmonary vasculature and high resistance will result in a Fontan circulation with low cardiac output and a progressive functional impairment despite normal systolic ventricular function. Adapted with permission from Gewillig and Goldberg.

Figure 7

Evolution of pulmonary vascular resistance (PVR) with age. In normal subjects (black line), PVR remains low for many decades and will increase only during ageing without significant cardiovascular limitation. In ‘good’ patients with Fontan circulation with low PVR (green line), resistance remains low for many decades but are expected to increase (dotted line). Increase in PVR is accelerated in patients with a poor Fontan (yellow and red lines) resulting in poor quality of life. Adapted with permission from Gewillig and Goldberg.