Example : Quinidine sulfate (F = 0.8, k = 0.1 hr-1, and Vd =180 L) is a class Ia antiarrhythmic drug. Barbiturates are enzyme inducers that can enhance the elimination of quinidine by 50% (k = 0.15 hr-1). What will be the average steady state conc in a patient who is taking 300 mg q 8 hr quinidine alone and in a different patient who is taking the same dose of quinidine with barbiturates ?
Quinidine average steady state
conc is lowed when administered
with barbiturates
Cpaverage =
(0.8) 300 mg
0.1 hr-1 180L 8hr
= 1.67 mg/L
Quinidine + barbiturates :
Cpaverage =
(0.8) 300 mg
0.15 hr-1 180L 8hr
= 1.11 mg/L
Higher quinidine doses are required for patients taking barbiturates.
