Vaughan Williams Classification of Antidysrhythmic Drugs

based on Cell Membrane Action Potential

during Depolarization

 

(fast potential)

 

Phase 0

        Instant depolarization of cell membrane (Systole begins)

        Na+ influx

 

Class I Drug Actions: 

  • Blocks sodium (Na+) channels in fast potential

  • Slows impulse conduction in atria and ventricles

  • Delays repolarization

Class IA Drugs:
Class IB Drugs:

 

Phase 1

        Overshoot of depolarization

        K+ efflux, Cl– influx

 

Phase 2

        Plateau

        Ca++ influx, K+ efflux

 

Class II Drug Actions:

  • Beta-adrenergic receptors blocking agents

  • Negative inotropic agent

  • Negative chronotropic agent

  • Reduce Ca++ influx in Phase 2

  • Depress Phase 4 deplarization

  • Non-specific Beta blockers block both β1 and β2 receptor sites.

  • Cardiac specific Beta blockers block β1 receptor sites only.

Non-specific β-blockers:

  • Propranolol

  • Carvedilol

Cardiac-specific β-blockers:

 

Class IV Drug Actions:

  • Blocks calcium channels in Phase 3 and Phase 0.

  • Negative inotropic agents

  • Negative chronotropic agents

Class IV Drugs: (Calcium Channel Blockers)

 

Phase 3

        Rapid repolarization

        K+ efflux

 

Class III Drug Actions:

  • Blocks potassium channels

  • Delays repolarization and refractoriness during Phase 3

  • Prolongs duration of action potential

  • Anti-fibrillatory action

Class III Drugs:

 

Phase 4

        Resting membrane potential (repolarized = diastole)

        Na+/K+ pump restores ion concentrations

 

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