A pitfall in azygos vein cannulation in cirrhotic patients: mistaken cannulation of the mammary vein. Azygos venous flow can be measured by a thermodilution catheter in patients with cirrhosis. This is a useful technique since azygos flow is thought to reflect the superior portosystemic collateral flow in these patients. The authors report 3 cases in which mistaken internal mammary vein cannulation mimicked azygos vein cannulation in the supine fluoroscopic view. A lateral fluoroscopic view confirmed internal mammary cannulation. They suggest that if the azygos arch is unpronounced or flow measurements are unexpectedly low in patients with portal hypertension due to cirrhosis, a lateral view be performed to rule out internal mammary vein cannulation. 