Contamination of caudal mediastinal node efferent lymph in sheep

Many investigators have used the chronic sheep lung lymph preparation to collect caudal mediastinal node (CMN) efferent lymph. These investigators have assumed that the lymph collected with the preparation is almost pure lung lymph. We examined 17 sheep for possible systemic contamination to the lymph, and in each sheep we found one to five lymph vessels that ran from the diaphragm to the CMN. Contamination from these vessels would not be eliminated in the chronic sheep preparation. We estimated the flow rate from these vessels to be 3.0 +/- 2.6 (SD) ml/h in anesthetized sheep. This represents 25-60% of the lymph flow rate in the chronic lymph preparation. In five sheep, we also located 1-4 esophageal lymph vessels that entered th CMN. These results show that lymph collected with the chronic sheep lung lymph preparation contains a significant nonpulmonary contamination.