Pericardial fluid absorption into lymphatic vessels in sheep

We estimated the volumetric lymphatic clearance rate of pericardial fluid in sheep. In the first group of studies, (125)I-human serum albumin (HSA) was injected into the pericardial cavity and after 4 h, various lymph nodes and tissues were excised and counted for radioactivity. Several lymphatic drainage pathways existed defined by elevated (125)I-HSA in the middle and caudal mediastinal, intercostal, and the cardiac nodes located near the root of the aorta. In a second group of experiments, the plasma recovery of intrapericardially administered tracer was compared in sheep with intact lymphatics and in animals in which thoracic duct lymph was diverted and other relevant lymphatics ligated. The 4-h plasma recoveries were reduced significantly from an average of 12.2 +/- 3.4% injected dose in the lymph-intact group to 3.0 +/- 1.1% injected dose in the diverted/ligated group (an inhibition of similar to 75%), In order to estimate the volumetric clearance of pericardial fluid through lymphatics in conscious sheep, (125)I-HSA was administered into the pericardial cavity to serve as the lymph flow marker. (131)I-HSA was injected intravenously to permit calculation of plasma tracer loss and tracer recirculation into lymphatics. From mass balance equations, total pericardial clearance into lymphatics averaged 1.50 +/- 0.43 ml/h or similar to 1.13 ml/h if one was to assume that the average 25% recovered plasma tracer in lymph diverted/ligated animals was due to nonlymphatic transport. In conclusion, mediastinal lymphatic pathways remove a volume equivalent to the pericardial volume (8.1 +/- 1.1 ml) every 5.4 to 7.2 h. (C) 1999 Academic Press.