Diagnosis of Congenital Cardiac Right‐to‐left Shunts with 99m Tc‐macroaggregated Albumin

Diagnosing right‐to‐left congenital cardiac shunts can be difficult. Cardiac catheterization and angiocardiography represent the traditional gold standard for diagnosis, but they are invasive. Nuclear scintigraphy using 99m Tc‐macroaggregated albumin (MAA) has been employed in humans as an alternate method of diagnosis. This study reviews eight dogs presented for evaluation of a suspect right‐to‐left cardiac shunt that were examined using 99m Tc‐MAA. In all, 2–4 mCi (74–148 MBq) of reduced particle 99m Tc‐MAA were injected IV in a cephalic vein and static images of the whole body, including right and left lateral, dorsal, and ventral views, were acquired for 60 s and stored into a 256×256×16 matrix. Shunt fractions were calculated. One dog with radiopharmaceutical distribution limited to the lungs did not have a shunt. Seven dogs had distribution of the radiopharmaceutical outside the pulmonary capillary bed, indicating bypassing of the pulmonary capillary circulation due to a right‐to‐left shunt. Four dogs had 99m Tc‐MAA within the brain. Three dogs that did not have brain uptake, but instead had a sharp cutoff of radioactivity at the level of the front limbs and neck, were diagnosed with reverse patent ductus arteriosus (PDA). The asymmetric distribution of the radiopharmaceutical is due to the location of the shunt, distal to the brachiocephalic trunk and left subclavian artery. Shunt fractions of dogs with extrapulmonary radioactivity ranged from 40% to 59%. Nuclear scintigraphy with 99m Tc‐MAA is a quick alternative method of diagnosing right‐to‐left cardiac shunts that permits quantification of shunt fraction. Distinguishing between reverse PDA and other right‐to‐left shunts may be possible based on the radiopharmaceutical distribution.