The metabolic basis of portasystemic encephalopathy and the effect of selective vs nonselective shunts.

Portasystemic encephalopathy has been a major deterent to the utilization of total or non-selective shunts. A procedure to determine the maximum rate of urea synthesis (MRUS) has been developed and a depression in the ability to synthesize urea has been shown to correlate closely with the development of encephalopathy. Utilizing this procedure and a modified ammonium tolerance curve, a controlled comparison was instituted between selective and non-selective shunts. Following a non-selective or total shunt, there was a definite deterioration in both the MRUS and the ammonium chloride tolerance curve which was accompanied by a high rate of clinical encephalopathy. In marked contrast, the selective shunt, which maintains portal venous perfusion of the liver, showed little or no change in the MRUS and the ammonium chloride tolerance curve following the selective shunt and there was a very low incidence of encephalopathy. The difference between the non-selective and selective shunt in maintenance of urea synthesis, metabolism of ammonium chloride, and the development of clinical encephalopathy show the selective shunt procedure to be definitively superior in this regard.