CHAPTER 55 Chylothorax

Pathophysiology The thoracic duct develops from outgrowths of the jugular lymphatic sacs and the cisterna chyli. During embryonic life, bilateral thoracic lymphatic channels are present, each attached in the neck to the corresponding jugular sac. As development progresses, the upper third of the right duct and the lower two-thirds of the left duct involute and close. The wide variation in the final anatomic structure of the main ductal system attests to the multiple communications of the small vessels comprising the lymphatic system. The thoracic duct originates in the abdomen at the cisterna chyli located over the second lumbar vertebra. The duct extends into the thorax through the aortic hiatus and then passes upward into the posterior mediastinum on the right before shifting toward the left at the level of the fifth thoracic vertebra. It then ascends posterior to the aortic arch and into the posterior neck to the junction of the subclavian and internal jugular veins. The chyle contained in the thoracic duct conveys ap proximately three-fourths of the ingested fat from the intestine to the systemic circulation. The fat content of chyle varies from 0.4 to 4.0 g/dl. The large fat molecules absorbed from the intestinal lacteals flow through the cisterna chyli and superiorly through the thoracic duct. The total protein content of thoracic duct lymph is also high. The thoracic duct also carries white blood cells, primarily lymphocytes (T cells)—approximately 2,000 to 20,000 cells per milliliter. When chyle leaks through a thoracic duct fistula, considerable fat and lymphocytes may be lost. Eosinophils are also present in a higher proportion than in circulating blood. The chyle appears to have a bacteriostatic property, which ac counts for the rare occurrence of infection complicating chylothorax.