Sodium Channel Blockers [Class 1]

QUINIDINE (Class 1A)

• One of most commonly used oral antiarrhythmic

Effects

• Inhibit fast Na+ channels in atria & ventricles

• Depresses excitability of cardiac muscle

• Slows rate of spontaneous rhythm

• Decreases vagal tone

• Prolongs conduction & effective refractory period

• Useful for both supraventricular & ventricular arrhythmias

• alpha-adrenergic blocker —> hypotension

• antimalarial

Toxicity

• Antimuscarinic action inhibits vagal effects which may override its direct membrane effects ie increased AV conduction & sinus rate

Indications

• Effective in nearly every kind of arrhythmia

Contraindications

• Heart blocks

PROCAINAMIDE (Class 1A)

Effects

• Similar to quinidine

• Less prominent antimuscarinic action than quinidine

Toxicity

• More negative inotropic than quinidine; may ppt heart failure

• Hypotension 2° reduce peripheral resistance

• Lupus like syndrome

Indications

• Effective against most atrial & ventricular arrhythmia

Contraindications

• Heart blocks

DISOPYRAMIDE (Class 1A)

Effects

• Similar to quinidine

• More prominent antimuscarinic action than quinidine therefore should be administered with eg digoxin

Toxicity

• Antimuscarinic action inhibits vagal effects which may override its direct membrane effects ie increased AV conduction & sinus rate

• Atropine like activity accounts for dry mouth, constipation etc

• More negative inotropic than quinidine; may ppt heart failure

Indications

• Supraventricular arrhythmias & ventricular arrhythmias

• 3rd line drug after quinidine & procainamide

Contraindications

• Heart blocks

MEXILETINE (Class 1B)

Effects

• Class 1B agents have little effect on conduction velocity

• Little effect on refractory period?

• Decrease automaticity

• More effective in ventricular arrhythmias

Toxicity

• Predominantly neurologic [tremor, lethargy]

Indications

• Structurally resembles lignocaine except may be given orally

Contraindications

• Heart blocks

PHENYTOIN (Class 1B)

Effects

• Anticonvulsant with antiarrhythmic effects

Indications

• Limited efficacy

• Suppresses ventricular ectopic pacemaker activity

• Especially effective against digitalis induced arrhythmias

Contraindications

• Heart blocks

FLECANIDE (Class 1C)

Effects

• Class 1C agents have electrophysiological properties of both 1A & 1B agents

Indications

• Suppression of ventricular arrhythmias

• WPW reentry supraventricular tachycardias

Contraindications

• Heart blocks

Beta Adrenergic Blockers [Class II]

PROPRANOLOL

Effects

• Reduces excess sympathetic nervous stimulation on heart:

reduce HR; reduce contractility; reduced cardiac output; reduced myocardial oxygen demand

• Counteract increased conduction velocity

Indications

• Suppression of ventricular arrhythmias

• WPW reentry supraventricular tachycardias

Contraindications

• Heart blocks

[Class III}

Includes Bretylium & Amiodarone

Calcium Channel Blockers [Class IV}

Include Verapamil and Diltiazem

Mixed Class

BRETYLIUM

Effects

• Interferes with the neuronal release of catecholamines but also has direct antiarrhythmic properties [class III]

• Lengthens ventricular (but not atrial) action potential duration & thus refractory period

• Causes an initial release of catecholamines that may have some inotropic effect but may also ppt arrhythmias

Indications

• Usually used in emergency setting for IV use after resuscitation from VF when lignocaine & cardioversion has been unsuccessful

• VT

Adverse effects

• Due to its sympathoplegic effects: hypotension

SOTALOL

Effects

• Possess class III activity

• Also non selective beta-blocker that also prolongs AP duration

• An effective antiarrhythmic [class II]

• Reduction in HR & mild reduction in contractility with reduced myocardial work

Indications

• Supraventricular and ventricular arrhythmias

Adverse effects

• Associated with beta blockade eg reduces contractility, bradycardia, bronchospasm

• Associated with prolongation of repolarisation including torsade-de- pointes