Sodium channel blockers are divided into various classes.

Table of Contents

Class I : Sodium channel blockers

Class I : Sodium channel blockers are the membrane stabilizers and functions are-

Bind to open / inactivated Na+ channel –Use dependent blockade
Block rapid depolarizing of tissue
Reduce the rate of Phase 4 depolarization in automatic fibers (SA node)

Contra-indication of sodium channel blockers: Complete heart block

The enhancement of sodium channel block seen in rapidly depolarizing tissue has been termed “use-dependent blockade” and is thought to be responsible for the efficacy of these drugs in slowing and converting tachycardias with minimal effects on conduction in normal tissues stimulated at normal physiological rates.

Sodium channel blocker further classified in three subclasses- class Ia, class Ib and class Ic.

Class I : Sodium channel blockers — Class I : **Sodium channel blockers**

Class Ia

Class Ia subclass include Quinidine, Disopyramide and Procainamide drugs. these functions by Inhibiting pacemaker depolarization and AV conduction, Delay conduction velocity- prevent ectopic pacemaker. Class Ia drugs are used for Ventricular tachyarrhythmia, Paroxysmal recurrent atrial fibrillation (vagal overactivity) and for all types of arrhythmia.

Quinidine

Action of Quinidine

Block Na+ , K+ channel; High dose – inhibits L type Ca2+ channel; Reduces automaticity, excitability; prolongs action potential & ERP; Vagolytic effect and enhances AV conduction (dangerous in AF and AFL)

Cardiac Side Effect

Torsades de pointes (↑ QRS) – Polycentric VT potentiated by bradycardia & electrolyte disturbance (low K+, Mg2+); IV – Vasodilatation (α blockade), Hypotension

Extra cardiac Side effect

GI effects (NVD), headache, CNS effects, Hypersensitivity, Chinconism

Procainamide

Use

Ventricular premature contraction, VT, some SVT (combine with digoxin/ β blocker/ Ca2+ blocker)

Side effect

Worsen Ventricular Arrhythmia in AV block, GI effects, Hypotension, Hypersensitivity, agranulocytosis, Lupus like syndrome

Disopyramide

Prominent cardiac depressant and anticholinergic action (indirect) – No effect on sinus rate; Not routinely used because of short half life; 2^nd / 3^rd line drug for prevention of recurrent VA; Maintenance therapy for AF and AFL

Class Ib

Class Ib of sodium channel blocker include Lignocaine, Phenytoin, and Mexiletin.

Lignocaine

Lignocaine have Least cardiotoxic and no proarrhythmic effect

Adverse effect

CNS excitation, blurring of vision, disorientation, nystagmus, neurotoxic; (CATS – Large IV bolus – sinus arrest with seizure); High dose of lignocaine able to cause Cardiac depression, hypotension.

Class Ic

Class Ic subclass of sodium channel blocker include Propafenone, Flecainide, and Encainide drugs.

Flecainide is used in combine with β blocker to prevent passage of atrial
flutter to ventricle (Pill in the Pocket).