Understanding Class 3 and 4 Antiarrhythmics

Apr 18, 2025

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Lecture Notes: Class 3 and Class 4 Antiarrhythmics

Overview

  • Objectives:
    • Understand usage and effects of Class 3 (anti-dysrhythmic agents) and Class 4 antiarrhythmics (calcium channel blockers).
    • Learn how these agents affect cardiac status and care.

Cardiac Cell Properties

  • Contractility: Ability to shorten and return to original length. Requires calcium.
  • Automaticity: Ability of pacemaker cells to depolarize spontaneously. Requires calcium.
  • Excitability: Resting polarized cells' ability to depolarize in response to impulse.
  • Conductivity: Ability to conduct electrical impulses to adjacent cells.

Class 3 Antiarrhythmics

  • Effects on Myocyte Action Potential:
    • Widen the duration of the action potential.
    • Affect repolarizing potassium currents, increasing refractory period.

Key Drugs

  • Amiodarone: Lengthens action potential duration, affects PR, QRS, QT intervals. Treats ventricular tachycardia and atrial fibrillation.
  • Sotalol: Class 2 & 3 properties, used for atrial fibrillation/flutter maintenance, prevents ventricular arrhythmias.
  • Ibutilide: Prolongs action potential repolarization, converts atrial fibrillation/flutter.
  • Dofetilide: Similar to Ibutilide, used for highly symptomatic atrial fibrillation/flutter.
  • Vernakalant: Atrial-specific, converts atrial fibrillation/flutter.

Adverse Effects

  • Prolongation of QT interval, torsade de pointes, photosensitivity, pulmonary fibrosis, thyroid issues, etc.

Class 4 Antiarrhythmics (Calcium Channel Blockers)

  • Function: Block calcium channels affecting contractility and automaticity.
  • Types:
    • Dihydropyridines (DHP): Greater vascular selectivity, used for hypertension.
    • Non-Dihydropyridines (Non-DHP): Affect pacemaker potential, decrease heart rate, used for supraventricular arrhythmias.

Key Drugs

  • Nifedipine: Powerful vasodilator, interacts with grapefruit juice.
  • Amlodipine: Vasodilator, dose adjustment in elderly, liver dysfunction.
  • Felodipine: Treats hypertension, risk of stroke.
  • Diltiazem: Decreases SA/AV node automaticity, used in angina and hypertension.
  • Verapamil: Moderately potent vasodilator, used for angina, has marked effect on AV node.

Effects & Indications

  • Vasodilation: Decreases afterload, increases cardiac output, balances oxygen supply/demand.
  • Indications: Stable angina, coronary artery spasm, hypertension, supraventricular arrhythmias.

Adverse Effects

  • Hypotension, bradycardia, AV block, heart failure, constipation, edema.

Summary

  • DHP vs. Non-DHP: DHP preferred for hypertension due to greater vascular selectivity.

Questions for Review

  • How do Class 3 antiarrhythmics prevent reentry rhythms?
  • Why are DHP calcium channel blockers preferred for hypertension?

This concludes the session on Class 3 agents and Calcium Channel Blockers.

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