Sodium channel blockers are divided into various classes.
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 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; 2nd / 3rd 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).