MCQ: Pharmacokinetics Flashcards
Vd =
Clearance x time constant
Context-Sensitive Half-Time is?
It is the time for the plasma levels to fall to 50% of the value when the infusion is stopped.
It reflects the combined effects of distribution and metabolism.
How many half-lives does it take to reach steady state in an infusion?
It takes 5 Half-lives to reach steady state concentration.
steady state Vd is dependant on what?
lipid solubility and clearance.
Loading dose calculation?
Loading Dose = Vd x Desired Plasma Concentration
Maintenance dose calculation?
Maintenance Dose = Steady State Concentration x Clearance.
Clearance calculation?
Clearance = Input (mg/min) / Plasma Concentration (mg/ml)
- Dose/ AUC
- Q.ER (flow x extraction ratio)
- Vd x rate constant for elimination
What important drugs does CYP2E1 involve?
Paracetamol
Fluoride containing volatiles - halothane, enflurane, isoflurane, des, sevo
CYP3A4 metabolises?
midazolam and alfentanil.
What differentiates first and zero order kinetics?
First order: constant proportion of drug is eliminated per unit time. Exponential process.
Zero order: constant amount of drug is eliminated per unit time. Enzyme is saturated.
Equations to calculate rate of elimination?
- concentration x clearance
2. Vd x rate constant of elimination
A drug with a high hepatic extraction ratio:
- is it affected by protein binding?
- is it affected by blood flow?
- Not affected by protein binding or enzyme level
2. Is most sensitive to flow/ perfusion
What affects drugs with low extraction ratio?
Enzyme capacity
What drugs are enzyme limited?
Warfarin phenytoin theophylline most benzodiazepines barbituates
In first order kinetics, is half life constant or variable?
Constant
As elimination is proportional to drug concentration, half life will be proportional and constant.
