Antiarrhythmics

Medications to treat or prevent abnormal heart rhythms

Electrical Conduction System of the Heart

Heart is capable of generating its own electrical impulse. SA node is the pacemaker where it all begins. It first stimulates atria which then contract to force blood down into ventricles. The impulse reaches AV node and spreads throughout ventricles via bundle of His and Purkinje fibers. When ventricles contract, blood is pumped out through aorta and pulmonary artery.
Sometimes, this near-perfect coordination of electrical activity becomes not so perfect. Antiarrhythmic(=antidysrhythmic)drugs are prescribed to control such event.

Classifications

Major antiarrhythmics are generally classified by their effects on cardiac conduction tissue.

Class I	moricizine	Sodium Channel Blockers
Class IA	quinidine procainamide disopyramide
Class IB	tocainide lidocaine phenytoin
Class IC	flecainide propafenone
Class II	acebutolol esmolol propranolol metoprolol	Beta-Adrenergic Receptor Blockers
Class III	amiodarone bretylium ibutilide sotalol	Potassium Channel Blockers
Class IV	diltiazem verapamil	Calcium Channel Blockers
Misc	adenosine atropine digoxin

Action and Uses

Class I antiarrhythmics in general block Na⁺ channels in heart cells, which decrease the conduction of electrical signals from cell to cell. There are three subgroups under this class.

Class IA

⁺

Class IB antiarrhythmics are the least effective at blocking sodium channels. Because they increase K⁺ conduction, they shorten the duration of repolarization.
Uses: ventricular arrhythmias

Class IC antiarrhythmics are strong sodium channel blockers. They slow nerve impulses in the heart without affecting repolarization. Uses: supraventricular and some ventricular arrhythmias

Class II antiarrhythmics block excess sympathetic activity. They decreases the rate of SA node discharge, slows conduction through the AV node, and thus, decreases heart rate. Most drugs in this category works on Beta₁ receptors which concerns the heart. Propranolol differs in that it works on both Beta₁ receptors and Beta₂ receptors which affect lungs, blood vessels, etc.
Uses: hypertension, angina pectoris, prevention of additional MI(myocardial infarction), treatment of ventricular arrhythmias due to exercise or emotion

Class III antiarrhythmics block potassium channels and therfore, lengthen the duration of repolarization, increases the duration of action potential.
Uses: treatment/prevention of ventricular tachycardia/ventricular fibrillation, supraventricular arrhythmias

Class IV antiarrhythmics inhibit transport of Ca⁺⁺ across the cell membrane during cardiac depolarization; decreases SA node discharge, decreases conduction speed through AV node.
Uses: hypertension, supraventricular tachycardia due to AV nodal reentry

Miscellaneous antiarrhythmics

Adenosine

Digoxin inhibits Na⁺/K⁺ ATPase and thus decreases conduction speed. It also strengthens cardiac muscle by providing more calcium ions. With more effective and stronger pumping, the heart rate decreases. This drug is often used in congestive heart failure and atrial arrhythmias.

Atropine blocks acetylcholine at parasympathetic receptors. It increases cardiac output and heart rate by blocking vagal stimulation. As an antiarrhythmic drug, it is used in bradycardia under 40-50 bpm.

Glossary and Links

angina pectoris chest pain due to lack of blood supply to the heart muscle cells
AV nodal reentry abnormal conduction of electrical impulses through a self-sustaining circuit in the atrioventricular node
bradycardia abnormally slow heart rate
hypertension high blood pressure
myocardial infarction heart attack
paroxysmal supraventricular tachycardia sporadically occurring arrhythmia with an atrial rate that is usually 160 to 200 bpm, originating above the bundle of His
supraventricular arrhythmia arrhythmia that originates from above ventricles
tachycardia abnormally rapid heart rate

Check out these links for more information

NAPSE Heart Rhythm Society Antiarrhythmic drugs are explained here in an easy-to-understand language.
Internet Medical Education, Inc. Go visit these guys if you are interested in learning about abnormal heart rhythms.
Antiarrhythmics Very detailed information on antiarrhythmics and other stuff as well.

Yesol Kim
mikeyesol@hotmail.com
Human Physiology, SMC