Sheep lung lymph shunting.

The caudal mediastinal lymph node (CMN) has several efferent lymph vessels in most sheep. When investigators cannulate one of the CMN efferent vessels in order to collect lung lymph, it is possible that lymph may be shunted between the cannulated vessel and other vessels which drain from the CMN into the systemic veins. If shunting does occur then an increase in venous pressure could cause lymph to be shunted to the cannulated lymph vessel. This would increase the flow of lymph from the cannula and could falsely indicate that lung lymph flow had increased. To test this possibility we cannulated CMN efferent vessels in 7 sheep and then used a balloon to raise the pressure in the superior vena cava. Because uncannulated CMN efferent vessels ultimately drain into the superior vena cava, an increase in pressure may cause lymph to be shunted through the lymph cannula. We found, however, that lymph flow increased in only one of seven sheep and conclude that lymph shunting is uncommon when operative preparation includes meticulous ligation of collateral common efferent lymph vessels.