Near Miss, Good Catch: Role of CMR in Identifying Large Systemic-to-Pulmonary Venous Collateral Not Appreciated by Invasive Angiography in a Fontan Patient (QF_FR_091)

A 9-year-old male with pulmonary atresia with intact ventricular septum and hypoplastic right ventricle status post staged palliation, most recently a non-fenestrated extracardiac Fontan at 4 years of age, was referred to our center for CMR to guide surgical planning for potential Fontan revision. He had central pulmonary artery augmentation 2 years prior to CMR and coiling of venovenous collaterals and his left internal mammary artery a few months prior. However, he remained notably hypoxemic with baseline saturations of 70-80%. He was referred to assess his current Fontan physiology by CMR given our institution’s experience with modeling Fontan pathways.

Diagnostic Techniques and Their Most Important Findings:

Cardiac MRI was performed under sedation using a free breathing technique. In addition to standard sequences, 4-dimensional (4D) phase-contrast imaging was performed.

Imaging demonstrated a widely patent Fontan baffle and bilateral bidirectional Glenn anastomoses, though the right Glenn was smaller than the left. There was relatively symmetric flow of Fontan return into the right and left pulmonary arteries (48% and 52%, respectively). The IVC drained into the right-sided Fontan baffle; however, on angiogram, it was noted that, secondary to elevated Fontan pressures, a large portion of the inferior systemic venous return coursed posteriorly into the paravertebral plexus and azygous vein. There was mild mitral valve regurgitation, normal left ventricular size and systolic function, and no systemic outflow obstruction.

A large systemic venous to pulmonary venous connection was noted; this collateral was fed both inferiorly by the azygous vein draining from the paravertebral veins and superiorly by several dilated venous collaterals from the right neck and upper thorax. The venous structure drained inferiorly and emptied into the ceiling of the left atrium (LA) just rightward of the left upper pulmonary vein ostium (Figure 1A-C). The ratio of pulmonary to systemic blood flow (Qp:Qs) was 0.6:1 by phase contrast imaging. Analysis of the 4D flow data demonstrated a stroke volume in the large decompressing venous collateral of 8.7 cc with an estimated step-up of 6.5 cc from additional left-sided collaterals compared to a total systemic arterial stroke volume of approximately 38 cc. Streaming of flow from the superior margin of the collateral into the ceiling of the LA was seen (Figure 1D).

Learning Points from this Case:

Based on the CMR findings, the patient’s home institution undertook a hybrid procedure with surgical access to the collateral, which was successfully occluded with a transcatheter device. He made an uneventful recovery with increase in his baseline oxygen saturations.  In this case, CMR was central to our ability to delineate the prominent venovenous collaterals contributing to the patient’s profound hypoxemia, which had not previously been seen on other imaging modalities, including invasive angiography. Additionally, 4D flow allowed comprehensive assessment of the flow burden within the collateral relative to flow into the branch pulmonary arteries.