Cardiac Arrhythmias: Class III: K+ channel blockers

• primarily block phase III repolarizing outward K+ currents, hence prolonging effective refractory period

• importantly, phase 4 K+ current is not affected (I_K1)

• reverse use dependence: greater effect at lower heart rates

• effective for treating several atrial and ventricular arrhythmias

• Amidarone

  • most effective antiarrhythmic; functions in all classes

  • has severe side effects, some of which are irreversible

    • pulmonary fibrosis—completely irreversible, potentially fatal. 4-9%

    • visual side effects: irreversible retinal degeneration or reversible corneal deposits

    • hypothyroidism

    • liver enzyme elevations

    • peripheral neuropathy

    • photosensitivity—skin turns blue

  • physicians should consider annual lung function, eye, thyroid, and liver function tests if a patient is on this drug

• Sotalol

  • very, very useful medicine to prevent atrial fibrillation and suppress ventricular arrhythmias

  • however, require serial ECGs to monitor QT interval (must be <>

  • clearance is via kidneys, so in reduced kidney function, reduce dose or choose another medication

  • β-blocker effects, so take this into account in patients with heart failure or on other β-blockers

• Dofetilide

  • primarily used to treat atrial fibrillation

  • must have serial ECGs to make sure that QTc <>

  • cleared by kidneys

  • UNLIKE sotalol, there is no β-blocking effect

Category: Pharmacology Notes