Visualisation of the thoracic duct after popliteal lymph node injection in the pig: comparison of radiographic and thoracoscopic techniques

Radiographic contrast studies have been recommended to identify the thoracic duct (TD) and its branches before and after surgery for total occlusion. The macroscopic identification of the TD and its branches during surgery usually involves injection of methylene blue (MB). Radiographic contrast and methylene blue can be injected into different anatomical structures (lymph node, lymph vessel, s.c.). The purpose of this study was to compare two different techniques (radiographic and thoracoscopic) for visualisation of the TD after intrapopliteal lymph node injection in the pig. Six piglets from the same litter (two males and four females), two months of age were used. Iohexol at 245 mg/ml was injected into the left popliteal lymph node (LN) under general anaesthesia; hindlimb, abdominal, and thoracic radiographs were taken. A 0.25% methylene blue solution was injected into the right popliteal lymph node and the thoracic duct colouration was assessed thoracoscopically. The thoracic duct was visualised radiographically in one out of six pigs after iohexol injection and thoracoscopically in five out of six pigs after methylene blue injection. The difference was statistically significant (P = 0.040). Popliteal LN lymphangiography using iohexol at 245 mg/ml in piglets should not be the recommended method for TD visualisation. Intrapopliteal injection of a 0.25% solution of methylene blue is recommended as a method of TD visualisation prior to thoracic duct ligation.