Unsuccessful lung scan due to major right-to-left shunt through a sinus venosus septal defect

In a patient with a prior history of cerebral abscess and cerebral ischemia, an unsuccessful perfusion lung scan led to a radionuclide angiocardiogram using an arm vein injection. This showed a total right-to-left (R-L) shunt from the superior vena cava (SVC) to the left atrium. Repeat radionuclide study, through a leg vein, demonstrated a moderate R-L shunt and an interpretable lung scan could be obtained. Catheterization and contrast cineangiogram did not provide the exact diagnosis, the preoperative conclusion being anomalous drainage of the SVC into the left atrium, with atrial septal defect (ASD) and partial anomalous pulmonary venous connection to the SVC. The operative diagnosis was high atrial (sinus venosus) septal defect. This example of major but clinically unsuspected R-L shunt emphasizes the value of performing a perfusion lung scan, preferably in conjunction with radionuclide angiocardiography in patients with a prior history of unexplained cerebral abscess or systemic ischemia. Implications of the site of an ASD on quantitation of L-R shunts by radionuclide methods are also discussed.