Class 1B agents have the lowest affinity for the target sodium channels and decrease the action potential duration (Image B).
Because class 1B agents have low affinity for the sodium channel, they are able to bind and release active channels more efficiently and, thus, target ectopic foci. This is particularly relevant in the post-myocardial infarction (MI) period because their low affinity for the target sodium channels allows them to specifically target cells with high ectopy.
Class 1 antiarrhythmic agents are all sodium channel blockers. There are three groups within class I: Class 1A includes quinidine, procainide, disopyramide. These agents have intermediate binding to Na channels and also have some binding to potassium channels. The result is a prolongation of action potential duration (Figure A). Class 1B agents, described above, include lidocaine, tocainide, and mexiletine. Class 1C includes propafenone, flecainide, and encainide. Class 1C agents exhibit very tight binding and slow release of Na channels. They do not affect the action potential duration (Figure D).
Hebbar et al. describe the management of common arrhythmias. They note that in patients without evidence of structural heart disease, ventricular ectopy (i.e. premature ventricular complexes [PVCs] without sustained ventricular tachycardia) does not need to be treated. However, in individuals with established heart disease and PVCs, there is a greater risk of developing ventricular tachycardia or fibrillation. These patients should be treated with a beta blocker or class I antiarrhythmic drug.
As discussed by Ryan et al. in the AHA guidelines for treatment of MI, lidocaine (class 1B agent) is used as an alternative to other antiarrhythmic drugs (such as amiodarone) for the acute treatment of hemodynamically compromising ventricular ectopy following myocardial ischemia or infarction. It is no longer recommended to use lidocaine for prophylaxis against primary ventricular fibrillation following acute MI.
Figures A to D are explained in the incorrect answer explanations below. Illustration A summarized the effects of Class 1 agents on the ventricular action potential and the electrocardiogram.
Answer 1: Figure A demonstrates the effect on the ventricular action potential of Class 1A antiarrhythmics.
Answer 3: Figure C demonstrates the effect on the ventricular action potential of Class 3 antiarrhythmics. These agents are potassium channel blockers.
Answer 4: Figure D demonstrates the effect on the ventricular action potential of Class 1C antiarrhythmics.
Answer 5: Class 1B antiarrhythmics do affect the ventricular action potential. Figure B demonstrates these effects.
Hebbar AK, Hueston WJ. Management of common arrhythmias: Part II. Ventricular arrhythmias and arrhythmias in special populations. Am Fam Physician. 2002 Jun 15;65(12):2491-6. Review. PubMed PMID: 12086238.
PMID:12086238 (Link to Abstract)
Ryan TJ, Antman EM, Brooks NH, Califf RM, Hillis LD, Hiratzka LF, Rapaport E, Riegel B, Russell RO, Smith EE 3rd, Weaver WD, Gibbons RJ, Alpert JS, Eagle KA, Gardner TJ, Garson A Jr, Gregoratos G, Ryan TJ, Smith SC Jr. 1999 update: ACC/AHA guidelines for the management of patients with acute myocardial infarction. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Management of Acute Myocardial Infarction). J Am Coll Cardiol
PMID:10483976 (Link to Abstract)
USMLE World Step 1 QBank Question #159. Copyright © USMLEWorld, LLC 2012