How cardiac output is controlled in a Fontan circulation: an update

Abstract

After creating a Fontan circuit, control of the circulation is shifted upstream from the ventricle to the newly created Fontan portal system. The goal of this review was to illustrate that the customary laws of biventricular cardiac output no longer apply and explain why standardized cardiac failure treatment regimens have little or no effect on a failing Fontan patient. A Fontan circulation is, in effect, a circulation in series regulated by the basic rules of any hydrodynamic circuit. We developed a formula that elucidates how flow through the critical bottleneck, and therefore through the whole circuit, is controlled. The critical bottleneck in a hydrodynamic model is the prime determinant of overall flow; other (less critical) bottlenecks may control local upstream congestion, but not overall flow. Once relieved, control of flow shifts to the next most significant bottleneck. The available options for improving flow in a hydrodynamic model are identical to those applicable to any dam: tackle the obstruction (the most impactful approach), push harder upstream (the easiest action) or pull/suck further downstream of the bottleneck (the least efficient strategy). In the early stages, the Fontan neo-portal circulation plays a pivotal role in the pathophysiology. The ventricle has little effect and has an impact only at a late stage. The Fontan formula in the present article stands as a valuable tool, aiding physicians in comprehending the pathophysiological and hydrodynamic intricacies of the Fontan circuit within the context of everyday clinical practice.

Keywords: Bidirectional Glenn; Bottleneck; Circulation; Flow; Fontan.

Graphical abstract

Figure 1:

Scheme of the normal cardiovascular circulation and the Fontan circulation. Left: Typical biventricular circulation: the pulmonary circulation is connected in series to the systemic circulation. The right ventricle ensures that the right atrial pressure remains low, typically lower than the left atrial pressure, and delivers the driving force to the blood to overcome pulmonary impedance. Right: Fontan circuit: the caval veins are directly connected to the pulmonary artery; systemic venous pressures are markedly elevated compared to the normal biventricular circulation. The fenestration (F) bypasses the critical bottleneck (green box). Ao: aorta; CC: coronary circulation; CV: caval veins; F: fenestration; LA: left atrial; LV: left ventricle; P: pulmonary circulation; PA: Pulmonary artery; RA: right atrial; RV: right ventricle; S: systemic circulation; V: single ventricle. Line thickness reflects output, colour reflects oxygen saturation.

Figure 2:

Temporal changes within a Fontan circuit. Primary changes encompass elevated caval vein pressure and reduced flow; secondary changes induce elevated left atrial pressure and rising pulmonary vascular resistance (red arrow); the primary and secondary changes encompass further reductions in cardiac output and the ejection fraction, coupled with further increased central venous pressure and systemic vascular resistance (orange arrow when relevant and present in formula). AO: aorta; CO: cardiac output; CV: caval veins; CVP: central venous pressure; δP: pressure difference; LA: left atrium; LAP: left atrial pressure; PA: pulmonary artery; PVR: pulmonary vascular resistance; R_cpc: resistance cavo-pulmonary connection; ΣR: sum of resistances; V: single ventricle.