Thread by @alexvdbruaene

1/n Why the #Fontan circulation fails and how it relates to Fontan physiology matters when caring for #Fontan patients. All pieces of the puzzle are important. A thread on Fontan physiology in 2 pieces. First physiology at rest. frontiersin.org/articles/10.33…

2/n Drs Fontan and Baudet had 2 aims when constructing the Fontan circulation
•Restore pulmonary blood flow &
•Elimate cyanosis

3/n Historically with the atrium as a pump and even with valves on the systemic venous side, the right-atrium-to-pulmonary artery (RAPA) Fontan became obsolete when Dr. de Leval showed that a valveless chamber
•Does not contribute to blood flow
•Causes energy loss ++

4/n Hence the modern Fontan or total cavopulmonary connection. Even if perfectly constructed, any Fontan bears in itself the components of its own failure. Why?

5/n At rest, there is 1 energy source and a series of resistors with low flow and increased systemic venous pressures as main characteristics. Have a look here: onlinelibrary.wiley.com/doi/full/10.11…

6/n Pulmonary blood flow without a subpulmonary pump; is that even possible?
At rest we just need ~15 mmHg to keep the pulmonary vasculature patent (higher than alveolar pressure and distal pulmonary venous pressure).

7/n This is achieved by
•Passive forces: increase in systemic venous pressure
•(Weakly) active forces: muscle pump, respiration and atrial suction
This is an excellent read: doi: 10.1016/j.jacc.2012.08.970

8/n So we don’t need a subpulmonary pump at rest, but this does result in a physiologic adaptation to the Fontan state where cardiac output is ‘maintained’ at the expense of an increase in systemic venous pressures and increased overall blood volume.

9/n Not having an RA/RV at rest results in
-Higher systemic venous pressures and its consequences ( FALD, PLE, …)
-Abnormal pulmonary blood flow (non-pulsatile, less recruitment, asymmetric perfusion)
-Absence of the RA/RV buffer results in higher SV variation

10/n Loss of pulmonary blood flow pulsatility increases the energy needed to propel blood through the lungs
-1/3 of energy generated by the RV is absorbed in systole and restituted in diastole
-Pulmonary impedance increases when hydraulic power converts into a pressure gradient

11/n Venous resistance is now similar to PVR and venous return determined by
-Mean filling pressure (blood volume, venous tone)
-Atrial pressure (compliance, contractility, valve competence)
-Venous resistance (autonomic tone, muscle pump, Fontan energy loss, PA dysplasia, PVR)

12/n I havent said 1 word on the myocardium. Not that its not important. But in contrast to a situation where venous return is ‘un’limited, this is not the case in a Fontan circulation. The pulmonary circulation is in the driver’s seat controlling ventricular filling and CO

Please share if you liked the thread. A thread on Fontan physiology during exercise follows if there is sufficient interest 😉

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