Classification of Antiarrhythmic Drugs by Their Action

The standard classification of antiarrhythmic drugs was developed by Singh and Vaughan Williams based upon the drug's electrophysiological mechanisms of action:

• Class I drugs, those that act by blocking the sodium channel, are subdivided into 3 subgroups, IA, IB, and IC based on their effects on repolarization and potency towards blocking the sodium channel

  • Subclass IA drugs have high potency as sodium channel blockers (prolong QRS interval), and also usually prolong repolarization (prolong QT interval) through blockade of potassium channels

  • Subclass IB drugs have the lowest potency as sodium channel blockers, produce little if any change in action potential duration (no effect on QRS interval) in normal tissue, and shorten repolarization (decrease QT interval)

  • Subclass IC drugs are the most potent sodium channel blocking agents (prolong QRS interval), and have little effect on repolarization (no effect on QT interval)

• Class II drugs act indirectly on electrophysiological parameters by blocking beta-adrenergic receptors (slow sinus rhythm, prolong PR interval, no effect on QRS or QT intervals)

• Class III drugs prolong repolarization (increase refractoriness) by blocking outward potassium conductance (prolong QT interval), with typically little effect on the rate of depolarization (no effect on QRS interval)

• Class IV drugs are relatively selective AV nodal L-type calcium-channel blockers (slow sinus rhythm, prolong PR interval, no effect on QRS interval)

• Miscellaneous In addition to the standard classes, IA-C, II, III, and IV, there is also a miscellaneous group of drugs that includes digoxin, adenosine, magnesium, alinidine (a chloride channel blocker) and other compounds whose actions don't fit the standard four classes

Category: Pharmacology Notes