Fontan circulation

 InitialInitial ventilation/perfusion scan performed with a right upper extremity IV line.

Ventilation images: Homogeneous distribution on a single breath and equilibrium images with no abnormal retention during the washout phase.

Perfusion images: Complete lack of perfusion to the left lung. No segmental perfusion defect in the right lung. 

MRA of the chest performed one year prior illustrates Fontan circulation

1. The superior vena cava is connected to the right pulmonary artery (right-sided cavopulmonary anastomosis).

2. The right-sided cavopulmonary anastomosis (SVC to RPA) allows a passive venous return to the pulmonary arteries. There is preferential venous return to the left pulmonary artery (red arrow) due to venous admixture from the extraatrial conduit (blue arrows).

3. The extraatrial conduit connects the IVC to both the right and left pulmonary arteries. 

4. The extraatrial conduit allows flow from the IVC to both pulmonary arteries without the mechanical help of the right ventricle (bypassing the right ventricle). There is venous admixture to both the RPA and LPA (blue arrows).

5. Blooming artifact within the LPA related to left pulmonary artery stent.

Terms:-

• IVC conduit: Inferior vena cava conduit (synonyms include external cardiac conduit or extraatrial conduit)- SVC
• SVC: superior vena cava- LPA
• LPA: left pulmonary artery- RPA
• RPA: right pulmonary artery

Key Teaching Pointsteaching points:

• Fontan circulation may mimick a pulmonary embolism on a perfusion scan.
• Due to passive venous return to the pulmonary circulation, pedal injection during perfusion scan may reveal normal perfusion to both lungs.

Patient with a history of tricuspid atresia type IA status post right-sided cavopulmonary anastomosis, extracardiac Fontan, and left pulmonary artery stent placement during childhood. 

The initial VQ scan performed with a right upper extremity IV line demonstrates a complete lack of perfusion to the left lung and normal ventilation imaging. Unilateral flow to the right pulmonary artery may be seen, as in this case, mimicking a pulmonary embolism on perfusion scan.

Repeat perfusion images the same day following pedal injection shows homogeneous perfusion to both lungs.

Case prepared with the help of Dr Suzanne Parets and Dr Samuel Valle.