Exchange transfusions,1-3 peritoneal dialysis,4-6 and total body washout7,8 have recently been advocated for the treatment of hepatic coma and Reye syndrome. It is often implied that the purpose of this therapy is to lower the blood ammonia level. For example, one pediatric gastroenterology book, in the section on Reye syndrome, states that "Reduction in blood ammonia levels may be expected with exchange transfusion therapy if one uses fresh, heparinized blood."9 Exchange transfusions have also been used to treat infants with defects in the urea cycle,10,11 where the possible benefits are more clearly limited to the removal of ammonia. However, the experience with exchange transfusion for patients with Reye syndrome in Denver has been discouraging. The following calculations show the ineffectiveness of these procedures in removing ammonia, even with assumptions that overestimate the efficiency.
For the illustration that follows we have assumed the case of