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
Pharm 101 700

Pharmacology 101

Top 200 drugs

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

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