Class IB Anti-arrhythmic Drugs

Antiarrhythmic drugs are used to treat abnormal heart rhythm. These drugs are classified based on Vaugha-Williams classification:
- Class I (fast sodium channel blocker)
- Class II (beta adrenergic blocker)
- Class III (potassium channel blocker)
- Class IV (Calcium channel blocker)
- Class V (miscellaneous)
Class I antiarrhythmic drugs are again classified into IA, IB and IC on basis of their impact on length of action potential.
Class IB antiarrhythmic drugs are those which weakly block sodium channel and decrease action potential duration.
Drugs belonging to this class are lignocaine, phenytoin, tocainide and mexiletine.

Mechanism of Action of Class IB Anti-arrhythmic Drugs

Figure 1- Schematic representation of effects of class IB anti-arrhythmic agents (source- Lippincott’s Illustrated Reviews)

They weakly block sodium channel, shorten phase 3 repolarization and decrease action potential duration.
They suppress automaticity and have very less effect on conduction velocity.

Lignocaine is amide type local anesthetic and is also included in list of World Health Organization’s essential drugs list. It is also known as lidocaine.

Pharmacological effects

It blocks both open and inactivated cardiac Na⁺ When given through IV route, it shortens ventricular action potential, selectively act on diseased or ischemic myocardium, depress diastolic depolarization and automaticity in ventricular tissues.
It has little influence on conduction velocity, doesn’t affect electrophysiological functions of atria, SA node and AV node and has no hemodynamic adverse effects.

Pharmacokinetics

IV route is preferred over oral route as it undergoes extensive first pass hepatic metabolism. Its metabolites glycine xylidide (GX) and monoethyl GX are less potent than lignocaine.
It binds to plasma protein (60-80%) and its half life is 15-30 minutes after single injection.
When given in combination with drugs which affect CYP isoenzymes, it should be monitored closely.

Adverse Effects

It is relatively safe drug.
Adverse effects include drowsiness, muscle twitching, blurred vision, confusion and convulsion.

Use

Most useful in abolishing ventricular arrhythmia.
As it doesn’t affect AV nodal conduction velocity, it is used for treating digitalis induce ventricular arrhythmia.
Emergency termination of ventricular tachycardia following recent MI (Myocardial Infarction) and occurring during cardiac surgery.

It is chemical analogue of lignocaine and thus is pharmacologically similar to lignocaine. However, it is effective as oral formulation (it doesn’t undergo first-pass hepatic metabolism).
It has narrow therapeutic index. Common adverse effects are tremor, nausea, vomiting, ataxia, blurred vision and hypotension.
Mexiletine is approved for treating ventricular arrhythmia. It is used in combination with quinidine or sotalol may increase efficacy and reduce side effects.

It is an amine analogue of lignocaine and can be used orally for treating ventricular arrhythmia. It is also reported to be effective in trigeminal neuralgia, tinnitus.
Due to its serious adverse reactions including neurological adverse effects and aplastic anemia, its use is discontinued nowadays. However, its various analogues have been developed as sodium channel inhibitors.

Phenytoin is commonly used anti-epileptic drug.
Phenytoin is mainly used to treat digitalis induced cardiac arrhythmia. It is also useful in prevention of recurring cardiac arrhythmias.
It is mainly used when other agents are contraindicated or unavailable.
Phenytoin was popularly used as antiarrhythmic drug from 1950s to 1970s. Its use as antiarrhythmic drug has lessen in recent decades due to introduction of newer and less toxic agents.
It is administered through oral or slow IV route.

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Meyler’s Side Effects of Drugs (Sixteenth Edition). The International Encyclopedia of Adverse Drug Reactions and Interactions. 2016; Pages 36-38.
Nerves and Nerve Injuries Vol 2: Pain, Treatment, Injury, Disease and Future Directions. 2015; Pages 81-97.
Karthikeyan M. Therapeutic Applications of Phenytoin. Asian Journal of Pharmaceutical and Clinical Research. 2009; 2(3):1-14.
Wang LW, Subbiah RN, Kilborn MJ, Dunn RF. Phenytoin: an old but effective antiarrhythmic agent for the suppression of ventricular tachycardia. Med J Aust. 2013; 199 (3): 209-211.
https://www.amboss.com/us/knowledge/Antiarrhythmic_drugs
Pharmacology and pharmacotherapeutics. 24^th edition.
Goodman and Gillman’s Manual of Pharmacology.
Lippincott’s Illustrated Reviews. 6th edition.