The Pulmonary Vasculature in the Fontan Circulation
Highlights
- In a ‘Fontan circulation’, there is no pump to propel blood into the pulmonary arteries since the systemic veins are directly connected to the pulmonary arteries (figure 1B). The remaining postcapillary energy is harnessed to drive blood through the lungs. However, the pulmonary impedance hampers venous return through the pulmonary vascular bed leading, like any dam wall or bottleneck, to congestion upstream and restricted flow downstream (View Highlight)
- The majority of the clinical and physiological ailments in a Fontan circuit are due to the upstream venous congestion and the downstream decreased output (View Highlight)
- By constructing a Fontan circulation, a man-made neoportal system is formed since one capillary bed pools blood into another capillary bed without receiving energy from a pump. This neoportal system significantly influences output in the cardiovascular system; it thereby becomes the critical bottleneck within this circulation. (View Highlight)
- The body tolerates only a narrow window of increased pressures in the systemic veins (up to 20 mm Hg) and a small range of ventricular filling pressures; this leaves the impedance of the neoportal system as the critical determinant of output in this newly created bottleneck (View Highlight)
- The ventricle, although providing the driving force for the circuit, cannot compensate for the major restriction in the bottleneck: it would have to ‘pull’ blood through the lungs using a degree of suction which is not physiological.8 In this situation, the ventricle is no longer controlling the cardiac output nor can it decrease the extent of congestion in the systemic veins; the role of the ventricle is reduced to pump the output allowed by the Fontan. However, the ventricle can (and will in time) deteriorate as a result of the limited flow through the bottleneck by increasing its end-diastolic pressures, which will worsen systemic venous congestion and further reduce output (View Highlight)
- in patients with Fontan circulation, increases in contractility (systolic function) will not lead to increases in cardiac output at rest: inotropes may make the ventricle squeeze harder but as preload is limited it will not eject more. (View Highlight)
- Impediments in any area of the neoportal system will markedly alter output of the Fontan circuit to a much greater extent than similar impairments in patients with a normal biventricular circulation (View Highlight)
- A successful Fontan circuit with a neoportal system with low impedance is characterised by only a mild decrease in cardiac output associated with a mild increase in systemic venous pressures; there is no need for a fenestration, thus no arterial desaturation (View Highlight)
- In a failing Fontan circuit, the high resistance of the neoportal system will result in a reduction in output and raised venous pressures to intolerable levels; a fenestration will lessen these effects, but a satisfactory compromise between desaturation on the one hand, and congestion and output on the other hand may not be reached (View Highlight)
- as long as a fenestration improves output and congestion, the neoportal system is the critical bottleneck (View Highlight)
- Healthy athletes can increase their pulmonary blood flow significantly during peak exercise. This is accomplished by a reduction in PVR due to vasodilation and recruitment of segments and increased right ventricular work consisting of flow acceleration coupled with increased systolic pressures up to 70 mm Hg (View Highlight)
- In the patient with Fontan circulation, there is no pump to increase and accelerate pulmonary blood flow. Furthermore, pulmonary vascular reactivity and recruitment of vessels are limited or even absent. A patient with Fontan circulation has therefore a restricted ability to boost cardiac output during exercise (View Highlight)
- 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 (View Highlight)
- The Glenn and Fontan connections themselves generate abnormal environments for the pulmonary vascular bed: longstanding diminished flow, desaturation, increased collateral flow, substandard mixing of inferior and superior caval flows, lack of pulsatile flow, endothelial dysfunction and absence of periods of high flow and high pressure with vessel recruitment as normally occur during exercise. These conditions can further influence growth and function with increasing resistances. (View Highlight)
- Early overprotection of the ventricle may over time lead to progressive and late severe diastolic dysfunction with increased filling pressures in the Fontan circuit because of chronic underfilling and resultant remodelling. (View Highlight)
- In a complex circuit, many components can act as a bottleneck and may be improved. However, when multiple bottlenecks are put in series, only changes at and immediately around the critical bottleneck will improve overall flow (View Highlight)
- the critical bottleneck in a Fontan circuit is the neoportal Fontan system. Treatment strategies will therefore only be successful if they open up the bottleneck (impedance of the Fontan neoportal system), bypass the bottleneck (fenestration), increase the pressure before the bottleneck (systemic venous pressure) or enhance run-off after the bottleneck (ventricular suction (View Highlight)
- Regular exercise and adapted breathing patterns may play a role in the ability to transiently increase central venous pressure but especially to lower the pulmonary vascular impedance by repeated vessel recruitment and vasodilation (View Highlight)
- Manipulation of the pulmonary vascular bed has emerged as a logical target in recent years. In a Fontan circuit, PVR is mildly elevated at baseline but does not decrease normally during increased cardiac output (View Highlight)
- ventricular filling is essentially a passive phenomenon and no ventricle will be able to generate adequate suction, let alone build up the negative pressures required to ‘pull’ the blood through the bottleneck (neoportal Fontan system). (View Highlight)
- Creation of a fenestration has proven that cardiac output can be improved and congestion relieved in these patients (View Highlight)
- Closing the fenestration typically results in improved oxygen saturations both at rest and during exercise resulting in improved exercise ability. (View Highlight)
- The discussion is still open whether the increased exercise tolerance outweighs the long-term benefit of decreased congestion, which might delay late hepatic and lymphatic dysfunction. (View Highlight)
- functional decline over time is inevitable as PVRs and ventricular filling pressures increase. (View Highlight)