Comparison of oxygen saturation of blood in azygos vein and superior vena cava.

Blood samples from the superior vena cava (SVC) are usually considered an acceptable representation of mixed systemic venous return in identifying and quantifying left-to-right atrial shunts. However, if during sampling from the SVC unrecognized entrance of the catheter into the azygos vein takes place, diagnostic errors may result. Blood samples from the azygos vein were collected from 60 children with congenital heart disease and the oxygen saturations ranged from 20% to 92%. These saturations were compared to those of samples taken from the superior vena cava above the entrance of the azygos vein. No substantial difference (saturation within +-4%) was found in 24 patients, while 28 patients had a significantly higher saturation in the azygos vein (from +5% to +21%) and eight patients had a lower saturation (from -5% to -16%). A significant left-to-right shunt may be overlooked or erroneously thought to be present when sampling of blood takes place unknowingly from the azygos vein. Examples are presented. Approaching from the groin it was possible to catheterize the azygos vein in two thirds of the cases when a deliberate attempt was made to do so.