Cardiomegaly as a cause of nonuniform pulmonary artery perfusion.

In patients with cardiomegaly, a common cause of nonuniform pulmonary artery perfusion on pulmonary arteriograms is compression of the left lower lobe arteries by the enlarged heart, an effect which is accentuated by gravity in the supine position. This may impede flow in such a manner as to erroneously suggest the presence of pulmonary emboli. Subselective left lower lobe arteriography with the patient in the right posterior oblique position will allow better anatomic definition of the vessels in this region and result in fewer equivocal or false positive studies.