Distinguishing massive pulmonary embolism from physiologic perfusion pattern in Fontan circulation

Learning Objectives 1. Understand how the surgical repair of tricuspid atresia affects the perfusion pattern observed on Tc-99m MAA or intravenous contrast studies. 2. Learn a differential diagnosis for false-positive VQ scans including Fontan circulation. 3. Learn why patients with Fontan circulation are at risk for thromboembolic events. 4. Know how to decrease the possibility of a false-positive VQ scan in patients with Fontan circulation.

A 21 year-old female with a history of surgery for congenital heart disease presented with chest pain and dyspnea. CT pulmonary angiography was performed, but did not exclude pulmonary embolism. A ventilation/perfusion (VQ) lung scan was then performed and showed normal lung ventilation. However, the lung perfusion images revealed absence of perfusion to the left lung. This mismatched VQ pattern can be seen with pulmonary embolism. However, given the patient’s cardiac surgical history, the perfusion part of the VQ scan was repeated using a pedal injection to differentiate between thromboembolism and physiologic Fontan circulation. The pedal injection lung perfusion scan revealed normal left lung perfusion. Because venous return to the lungs occurs passively in Fontan circulation, patients with this condition are at risk for thromboembolic events, which may be unilateral. Fontan circulation can mimic pulmonary embolism on perfusion lung imaging; a pedal injection can avoid a false-positive study