Incidental Detection of a Right-to-Left Atrial Shunt Without Pulmonary Hypertension

Right-to-left interatrial shunting permits venous blood to bypass the pulmonary circulation and enter the systemic circulation, potentially contributing to hypoxemia and paradoxical embolization. Although sustained right-to-left shunting is commonly associated with elevated right-sided or pulmonary arterial pressures, it may also occur in patients with normal pulmonary pressures because of transient pressure gradients or preferential flow across an interatrial communication. We present a 57-year-old man without known cardiac or pulmonary disease who developed progressive exertional dyspnea. Because his D-dimer result did not exclude pulmonary embolism, planar ventilation/perfusion imaging was performed following intravenous administration of technetium-99m macroaggregated albumin. Pulmonary perfusion was normal without any segmental defects; however, unexpected radiotracer uptake was identified in the brain and kidneys. This raised suspicion for a right-to-left shunt. CT pulmonary angiography subsequently demonstrated contrast passage from the right atrium to the left atrium. Transthoracic echocardiography with a bubble study confirmed a large right-to-left shunt at the atrial level, with normal right ventricular size and thickness, an estimated systolic pulmonary artery pressure of 15 mmHg, and an estimated right atrial pressure of 8 mmHg. The patient was referred for cardiology follow-up. This case demonstrates that systemic uptake of technetium-99m macroaggregated albumin during lung perfusion imaging should prompt evaluation for right-to-left shunting, even in the absence of pulmonary hypertension.