Presribing in liver disease

Question 1

extraction ratio

Answer 1

the percentage of the drug removed from the perfusing blood during its passage through the organ

ie the ratio of hepatic clearance of a drug to the hepatic blood flow

>0.7 = high
0.3-0.7 = intermediate
<0.3 = low

the higher the extraction rate, the higher the first pass metabolism, and a low bioavailability
therefore in cirrhotic patients, dose monitoring and reduction is vital

Question 2

prescribing high extraction drugs

Answer 2

oral: reduce initial dose and maintenance dose

IV: normal initial dose, reduced maintenance dose according to hepatic blood flow

reduced dose = normal dose x bioavailability/100

ie an extraction ratio of 0.8 would mean a bioavailability of 0.2

Question 3

prescribing in low extraction ratio

Answer 3

low albumin binding: reduce maintenance dose, or increase dosing interval

Question 4

general guidelines

Answer 4

use drugs that are not metabolised by the liver
avoid hepatotoxic drugs
avoid sedatives, diuretics or drugs that cause constipation, as this can lead to encephalopathy
avoid drugs that alter haemostasis
avoid drugs that cause Na retention as this can exacerbate ascites/fluid retention