- Potassium channel blocker
Potassium channel blockers are agents which interfere with conduction through
Potassium channel blockers used in the treatment of
cardiac arrhythmiaare classified as class III antiarrhythmic agents.
Class III agents predominantly block the potassium channels, thereby prolonging repolarization.cite journal |author=Lenz TL, Hilleman DE |title=Dofetilide, a new class III antiarrhythmic agent |journal=Pharmacotherapy |volume=20 |issue=7 |pages=776–86 |year=2000 |month=July |pmid=10907968 |doi= |url=] More specifically, their primary effect is on IKr.cite journal |author=Riera AR, Uchida AH, Ferreira C, "et al" |title=Relationship among amiodarone, new class III antiarrhythmics, miscellaneous agents and acquired long QT syndrome |journal=Cardiol J |volume=15 |issue=3 |pages=209–19 |year=2008 |pmid=18651412 |doi= |url=]
Since these agents do not affect the
sodium channel, conduction velocity is not decreased. The prolongation of the action potential duration and refractory period, combined with the maintenance of normal conduction velocity, prevent re-entrant arrhythmias. (The re-entrant rhythm is less likely to interact with tissue that has become refractory).
Class III antiarrhythmic agents exhibit reverse use dependent prolongation of the action potential duration (Reverse use-dependence). This means that the refractoriness of the ventricular
myocyteincreases at lower heart rates. This increases the susceptibility of the myocardium to early after-depolarizations(EADs) at low heart rates. Antiarrhythmic agents that exhibit reverse use-dependence are more efficacious at preventing a tachyarrhythmia than converting someone into normal sinus rhythm. "Because of the reverse use-dependence of class III agents, at low heart rates class III antiarrhythmic agents may paradoxically be more arrhythmogenic."
Examples and uses
Amiodaroneis indicated for the treatment of refractory VT or VF, particularly in the setting of acute ischemia. Amiodarone is also safe to use in individuals with cardiomyopathyand atrial fibrillation, to maintain normal sinus rhythm. Amiodaroneprolongation of the action potential is uniform over a wide range of heart rates so this drug does not have reverse use-dependent action. Amiodaronewas the first agent described in this class. [cite web |url=http://www.medscape.com/viewarticle/412798_3 |title=Milestones in the Evolution of the Study of Arrhythmias |format= |work= |accessdate=]
Dofetilideblocks only the rapid K channels; this means that at higher heart rates, when there is increased involvement of the slow K channels, dofetilide has less of an action potential-prolonging effect.
Sotalolis indicated for the treatment of atrial or ventricular tachyarrhythmias, and AV re-entrant arrhythmias.
Ibutilideis the only antiarrhythmic agent currently approved by the Food and Drug Administrationfor acute conversion of atrial fibrillationto sinus rhythm.
Nifekalantcite journal |author=Sahara M, Sagara K, Yamashita T, Iinuma H, Fu LT, Watanabe H |title=Nifekalant hydrochloride, a novel class III antiarrhythmic agent, suppressed postoperative recurrent ventricular tachycardia in a patient undergoing coronary artery bypass grafting and the Dor approach |journal=Circ. J. |volume=67 |issue=8 |pages=712–4 |year=2003 |month=August |pmid=12890916 |doi= |url=http://joi.jlc.jst.go.jp/JST.JSTAGE/circj/67.712?from=PubMed]
These agents include a risk of
torsades de pointes.cite web |url=http://www.merck.com/mmpe/sec07/ch075/ch075a.html |title=Introduction: Arrhythmias and Conduction Disorders: Merck Manual Professional |format= |work= |accessdate=]
Potassium channel blockers have also been proposed for use in the treatment of
multiple sclerosis.cite journal |author=Judge SI, Bever CT |title=Potassium channel blockers in multiple sclerosis: neuronal Kv channels and effects of symptomatic treatment |journal=Pharmacol. Ther. |volume=111 |issue=1 |pages=224–59 |year=2006 |month=July |pmid=16472864 |doi=10.1016/j.pharmthera.2005.10.006 |url=http://linkinghub.elsevier.com/retrieve/pii/S0163-7258(05)00228-7]
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