Amiodarone

Amiodarone is a class III antiarrhythmic agent used in the treatment of atrial, nodal and ventricular tachycardias.

Mechanism of action

The main mechanism of action is by blocking potassium channels which inhibits repolarisation and hence prolongs the action potential. Amiodarone also has other actions such as blocking sodium channels (a class I effect).

Phases of action potential for cardiac myocyte. Click on image to enlarge

Table 1: Classes of Anti-arrhythmics

Class	Mechanism of action	Example drugs
I	Fast sodium (Na) channel blockers Depress phase 0 prolonging repolarization	Ia -Quinidine, procainamide, disopyramide Ib -Lidocaine, phenytoin, mexiletine Ic -Flecainide, propafenone, moricizine
II	Beta blockers Decreases slope of phase 4	Atenolol Bisoprololol Propranolol Metoprolol
III	Potassium (K) channel blockers Prolongs phase 3*	Amiodarone (also acts on phases 1, 2, and 4) Sotalol ( also decreases slope of phase 4) Ibutilide Dofetilide
IV	Slow calcium (Ca) channel blockers Prolongs phase 2	Verapamil Diltiazem
V	Variable mechanism	Adenosine Digoxin Magnesium sulfate

Indications

Paroxysmal supraventricular, nodal and ventricular tachycardias
Atrial fibrillation and flutter
Ventricular fibrillation
Tachyarrhythmias associated with Wolff-Parkinson-White syndrome
Ventricular fibrillation or pulseless ventricular tachycardia refractory to defibrillation

Factors restricting use
The use of amiodarone is limited by a number of factors

long half-life (20-100 days)
should ideally be given into central veins (causes thrombophlebitis)
has proarrhythmic effects due to lengthening of the QT interval
interacts with drugs commonly used concurrently e.g. Decreases metabolism of warfarin
numerous long-term adverse effects (see below)

Monitoring

TFT, LFT, U&E, CXR prior to treatment
TFT, LFT every 6 months

Adverse effects

thyroid dysfunction
corneal deposits
pulmonary fibrosis/pneumonitis
liver fibrosis/hepatitis
peripheral neuropathy, myopathy
photosensitivity
altered taste
sleep disorders
constipation
movement disorders
'slate-grey' appearance
thrombophlebitis and injection site reactions
bradycardia

Contraindications

Severe conduction disturbances (unless pacemaker fitted)
Sinus node disease (unless pacemaker fitted)
Iodine sensitivity
Sino-atrial heart block (except in cardiac arrest)
Sinus bradycardia (except in cardiac arrest)
Thyroid dysfunction

With intravenous use:

Avoid bolus injection in cardiomyopathy
Avoid bolus injection in congestive heart failure
Circulatory collapse
Severe arterial hypotension
Severe respiratory failure