Positive inotropic: Inhibits Na/K-ATPase, Increases intracellular calcium --> Increases CO in HF due to systolic dysfunction
Negative chronotropic: Parasympathetic activity and vagal tone, shortens atrial contraction and prolongs AV nodal refractory period--> used in atrial fibrillation and flutter
What is the bioavailability of the capsule dosage form of digoxin?
90%
What is the bioavailability of the tablet dosage form of digoxin?
70%
What is the bioavailability of the elixir dosage form of digoxin?
80%
If you need to change from IV to oral digoxin, how do you adjust the dose?
Increase by 25%
If you need to change from oral to IV digoxin, how do you adjust the dose?
Decrease by 25%
Where is the major site of absorption?
Small intestine
What factors can decrease absorption of digoxin?
High fiber meals (tablets and capsules)
Malabsorption syndromes
Chemotherapy and radiation
Drugs: Antacids, cholestyramine, metoclopramide
What drugs can increase the absorption of digoxin?
Amiodarone
Verapamil
Quinidine
Does digoxin have long or short distribution?
Long
What type of tissues does digoxin like?
Lean organ tissues (muscle, heart, kidneys, and liver)
What is the serum to cardiac tissue ratio for digoxin?
1:100
What percentage of digoxin is protein bound?
25%
Does physical exercise increase or decrease the distribution of digoxin to skeletal and heart muscles?
Increases
True or False: The Volume of Distribution of digoxin is affected by obesity.
False
What are two things that can decrease the volume of distribution of digoxin?
Renal Failure
Hyperkalemia
Where is the major site of metabolism for digoxin?
GI tract
What is a potential interaction between macrolides and digoxin?
Digoxin is metabolized by E. lentum which may be eradicated with the use of macrolide antibiotics and therefore will increase the absorption of digoxin by 30%
What are three ways that digoxin is metabolized?
1. Hydrolyzed by stomach acid
2. Liver (minor 3A4)
3. Reduced by GI bacteria to inactive metabolites
What percentage of digoxin is excreted unchanged via the kidneys?
75%
How is 25% of digoxin excreted?
biliary and heptaic elimination
What is the 1/2 life of digoxin?
36 hours
What are factors that decrease the clearance of digoxin?
Heart failure
Hypothyroidism
Verapamil
Amiodarone
Itraconzole
Hypokalemia
What are factors that increase the clearance of digoxin?
Hyperthyroidism
Digoxin Immune fab (Digibind)
What weight is used to calculate a digoxin dose?
Ideal Body Weight
What is the LD for oral digoxin?
1-1.5 mcg
What is the LD for IV digoxin?
0.5-1 mcg IV
How should a LD of digoxin be split?
50% Initially
25% at 6 hours
25% at 12 hours
True or False: A LD is typically not administered when treating HF.
True
What is the MD of digoxin?
0.125-0.5 mcg daily
What is the therapeutic range of digoxin for CHF?
0.5-1.0 mcg/L
What is the therapeutic range of digoxin for AF?
0.8-2.0 mcg/L
When should you check digoxin levels?
Clinical deterioration
Assess compliance
Acute changes in renal function
Toxicity is suspected
New interaction (drug or condition)
When should a level be drawn when given a LD?
12-24 hours after the initial dose
When should a level be drawn when the patient does not receive a LD?
3-5 days
Following IV MD, when should a level be drawn?
No sooner than 6 hours after dose
Or obtain a trough level before next dose
If a digoxin dose is adjusted, when should serum concentration be drawn?
5-7 days
Although toxicity may occur at therapeutic concentrations, at what concentration is toxicity most often seen?
> 2.0 mcg/L
What are cardiac toxicites associated with digoxin?
Bradycardia
Atrial tachycardia with 2nd and 3rd degree AV block
Fatal ventricular arrhythmias
What non-cardiac toxicites are associated with digoxin?
Blurred, yellow-green halo vision
GI: N/V/D, abdominal pain
CNS: headache, fatigue, confusion, vertigo
All due to inhibition of Na/K-ATPase
What are the effects of digoxin with hypokalemia?
Hypokalemia potentiates effects of digoxin
What are the effects of calcium abnormalities on digoxin?
Hypercalcemia facilitates toxicity
What are the effects of magnesium abnormalities on digoxin?
hypomagnesemia potentiates proarrhythmic effects
How do you calculate how much digibind is needed to correct an overdose of digoxin?
