Quantification of the capacity of the liver to remove ammonia from the circulation of dogs with portacaval transposition.

To quantitate the ammonia that the liver removes from the circulation and to investigate the distribution of this substance during and after an exogenous ammonia load, ten dogs with portacaval transposition were studied by placing catheters in the hepatic and portal veins through the external jugular vein, in the portal vein going to the liver and in the infrarenal vena cava through the femoral vein. A catheter also was inserted into the femoral artery. Blood ammonia levels were measured in each catheter, then an infusion of ammonium sulfate, 0.7 milligram per minute per kilogram for 45 minutes through the infrarenal vena cava, was given continuously. All other catheters were simultaneously sampled at 15 minute intervals during the infusion and for a 45 minute period after it was stopped. By using a continuous infusion of indocyanine green and Fick's formula, the total hepatic blood flow was estimated in five of the ten dogs. Thus, knowing the amounts of ammonia in the hepatic inflow and outflow tracts and relating them to the estimated hepatic blood flow, the hepatic extraction ratios of ammonia were calculated. The estimated hepatic blood flow changed minimally before and after portacaval transposition. The blood ammonia levels in all sites where samples were obtained, except for the hepatic vein, followed uniform patterns. In the femoral artery, the portal vein and liver-portal vein now anastomosed to the infrahepatic infrarenal vana cava-the blood ammonia levels during the period of infusion increased by at least 90 per cent. When the infusion was discontinued, the blood ammonia levels decreased but remained elevated, from 30 to 60 per cent of the preinfusion blood ammonia levels. Blood ammonia levels in the hepatic veins increased some but never exceeded 56 micrograms per 100 milliliters. It also was found that the liver removes 80 to 87 per cent of the ammonia reaching it by means of the urea cycle; the other 13 to 19 per cent of the ammonia returns to the circulation through the hepatic veins and is distributed into the circulation, causing the blood ammonia levels to remain higher for at least 45 minutes after the ammonia infusion load was discontinued than the preinfusion control levels in all the sites where samples were obtained.