Effect of portacaval shunt on the disposition of drugs with and without first-pass effect.

The pharmacokinetic parameters of lidocaine, antipyrine and salicylamide were studied in dogs before and after construction of a portacaval shunt. The systemic availability of antipyrine was not altered significantly by the surgical procedure whereas the availability of lidocaine and salicylamide, drugs with a marked first-pass effect, was increased from 14.8 +/- 2.8 to 81.3 +/- 5.2% and from 21.8 +/- 7.8 to 57.5 +/- 2.5%, respectively. Of the 78% first-pass extraction of salicylamide, 36% is due to hepatic extraction whereas the remaining 42% is accounted for by intestinal wall extraction. The presence of a portacaval shunt also reduced the plasma clearance of lidocaine and antipyrine, but not salicylamide. The apparent volume of distribution was decreased only with salicylamide. Surgical construction of a portacaval shunt in dogs in a good model to evaluate first-pass effect. Physicians should adjust downward the dose of drugs with a first-pass effect. Physicians should adjust downward the dose of drugs with a first-pass effect in patients with a surgical portacaval shunt or endogenous portal systemic shunts such as seen in cirrhosis of the liver.