Pharmacokinetics of Digoxin

Question 1

If Digoxin concentration is above its therapeutic window (toxicity), what would patients be complaining of?

Answer 1

• Nausea
• Anorexia
• Visual disturbances
• Bradycardia
• Arrhythmias

Question 2

Define bioavailability

Answer 2

The apparent volume in which a drug is dissolved in the body

In L or L/Kg

Pharmaceutical dosage form can affect bioavailability

Question 3

Define volume of distribution

Answer 3

Dose/Conc

Question 4

Define clearance

Answer 4

Clearance = renal clearance + hepatic clearance

First order kinetics state that the rate of elimination is proportional to concentration of drug

C_t = C₀e^-kt

Rate constant = K = Clearance/V_d

t_1/2 = time for conc to decrease by 50% = 0.693/k

Question 5

What is the bioavailability of digoxin?

Answer 5

Vd = 7.3L/kg = 511L

Widely distributed in the body

Avidly bound

Low volume: low binding/distribution

Question 6

When starting a regimen you need to achieve a steady state.

How many half lives does it take to reach steady state?

Answer 6

5 half lives

Question 7

What is a loading dose?

Answer 7

Initial high dose therapy to boost plasma concentration

Loading dose = desired C_ss x V_d

Often given in divided doses to monitor response

Also used for a rapid reponse (e.g. digoxin)

Question 8

Answer 8

Question 9

How do you calculate infusion rate?

Answer 9

Clearance x C_ss

Question 10

How do you calculate the dose for an oral regimen?

Answer 10

Question 11

Which equation indicates renal function?

Answer 11

Cock-croft