The seizure meds are given because of this problem ( taken from here as a copy paste
ACVS - Portosystemic Shunts (PSS))
One of the most problematic post-op complications is development of seizures that are refractory to treatment.* This occurs most frequently in toy breed dogs, in the first 4-5 days post-op.* The cause of these seizures is unknown, but is thought to be due to the fact that in the shunting state, toxins in the circulation bathing the brain prompt the body to produce endogenous(made within the body) benzodiazepines (valium-like substances) that act as anti-seizure medication.* Post-op, it is possible that seizures occur because the brain has adapted to that altered metabolism or because of sudden withdrawal of the anticonvulsant effects of the endogenous benzodiazepines that are no longer being produced since the shunt was ligated. Seizures can be are managed with anti-seizure medication (Phenobarbital, potassium bromide).* In severe cases, intravenous administration of pentobarbital, propofol (anesthetic agent) and/or calium may be required to control seizures.* Post-op development of seizures that are poorly controlled by medication is a very poor prognosis.