Intestinal lymphatic pressure increases during intravenous infusions in awake sheep

Intravenous fluid infusions cause increased venous pressure and increased lymph flow throughout the body. Together the increased lymph flow and increased venous pressure (the outflow pressure to the lymphatic system) should increase the pressure within the postnodal intestinal lymphatics. To test this, we measured the pressure in postnodal intestinal lymphatics and the neck vein pressure in five awake sheep. At baseline, the neck vein pressure was 1.2 +/- 1.5 (SD) cmH2O and the lymphatic pressure was 12.5 +/- 1.7 cmH2O. When we infused Ringer solution intravenously (10% body weight in approximately 50 min), the neck vein pressure increased to 17.3 +/- 0.9 cmH2O and the lymphatic pressure increased to 24.6 +/- 3.8 cmH2O (both P < 0.05). In two additional sheep, the thoracic duct lymph flow rate increased from 0.8 +/- 0.4 ml/min at baseline to 5.5 +/- 2.0 ml/min during the infusions. Our results show that postnodal intestinal lymphatic pressure may increase substantially during intravenous fluid infusions. This is important because increases in postnodal lymphatic pressure may slow lymph flow from the intestine.