Pharm 101: Amiodarone
Class
Class III antiarrhythmic
Pharmacodynamics
- Class III antiarrhythmic. Also has class I, II and IV effects
- Primary action:
- Prolongs action potential duration (and QT interval) due to blockade of rapid component of delayed K current
- Chronic use also blocks slow K rectifier
- Other effects:
- Blocks inactivated Na channels. AP prolonging action reinforces this effect
- Weak adrenergic and Ca channel blocking actions. Causes slowing of HR and AV nodal conduction.
- Peripheral vasodilation (after intravenous administration)
Pharmacokinetics
- Variable absorption with bioavailability 35-65%
- Bimodal half-life:
- Rapid component 3-10 days (50% of drug)
- Slower component of several weeks
- After cessation, drug effects are maintained for 1-3 months
- CYP inhibitor
Clinical uses
- Treatment of atrial and ventricular tachyarrhythmias (SVT, AF, VT, VF)
- Used both to revert VT and prevent recurrence
- ALS algorithm after 3rd cycle of shockable rhythm
Adverse effects
- Acute:
- Symptomatic bradycardia and heart block in patients with pre-existing sinus or AV nodal disease
- Hypotension after intravenous administration (due to peripheral vasodilation)
- Prolongs QT interval, but has low incidence of TdP (< 1%). This is likely due to its broad spectrum of antiarrhythmic actions.
- Increases the pacing threshold for ICD
- Chronic:
- Pulmonary fibrosis
- Deranged LFTs and hepatitis
- Hypothyroidism (blocks peripheral conversion of T4 to T3) or hyperthyroidism (source of large amounts of inorganic iodine)
- Skin deposits: photodermatitis and grey-blue discolouration in sun-exposed areas
- Asymptomatic corneal microdeposits
- Optic neuritis (rare)
Precautions/contraindications
- Administration in conjunction with other CYP substrates:
- Increases INR in co-administration with warfarin
- Increases effect of digoxin
- Increases concentration of statins
Further Reading
- Nickson C. Anti-arrhythmic Drugs and Cardiac Arrest
- Nickson C. Ventricular Tachycardia
Pharmacology 101
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Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Co-creator of the LITFL ECG Library. Twitter: @rob_buttner