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Module 7 – Cardiac Electrophysiology & Dysrhythmias

Dysrhythmias result from abnormalities in:

• Automaticity • Triggered activity • Conduction

Successful treatment requires understanding the cardiac action potential.

Cardiac Action Potentials

Two major cell types:

Fast-Response Cells (Atrial / Ventricular / Purkinje)

• Phase 0 – Na+ influx • Phase 1 – Early repolarization • Phase 2 – Ca2+ plateau • Phase 3 – K+ repolarization • Phase 4 – Resting potential

Slow-Response Cells (SA & AV Node)

• Phase 0 – Ca2+ influx • Phase 3 – K+ repolarization • Phase 4 – Spontaneous depolarization

Drugs modify specific ion currents within these phases.

Vaughan-Williams Classification

Class I – Sodium Channel Blockers

Block Phase 0 depolarization in fast-response cells.

Subclasses:

• Class IA – Moderate Na block + QT prolongation

• Class IB – Mild Na block (shortens QT)

Lidocaine

• Class IC – Strong Na block (no QT change)

Used for rhythm control.

Class II – Beta-Blockers

Suppress sympathetic stimulation.

Primarily used for rate control.

Class III – Potassium Channel Blockers

Prolong repolarization (Phase 3).

• Amiodarone • Dronedarone • Dofetilide • Ibutilide • Sotalol

Increase QT interval.

Risk: • Torsades de pointes

Class IV – Calcium Channel Blockers (Non-DHP)

Block AV nodal conduction.

• Verapamil • Diltiazem

Used for rate control in atrial fibrillation.

Miscellaneous Antiarrhythmics

• Adenosine – AV reentry (PSVT) • Atropine – Bradycardia / AV block • Ivabradine – Sinus tachycardia • Digoxin – Rate control (adjunct)

Rate Control vs Rhythm Control

Rate Control

Goal: • Slow ventricular response

Agents:

• Beta-Blockers • Non-DHP Calcium Channel Blockers • Digoxin

Rhythm Control

Goal: • Restore and maintain sinus rhythm

Agents:

• Class I drugs • Class III drugs

Higher proarrhythmic risk.

Proarrhythmia

All antiarrhythmics can worsen arrhythmias.

Most notable:

• QT prolongation → torsades de pointes • Class IC drugs in structural heart disease

Never treat ECG without treating the patient.

Clinical Strategy by Scenario

Atrial Fibrillation: • Rate control first • Rhythm control if symptomatic or young

SVT (AVNRT/AVRT): • Adenosine • Beta-blocker • Non-DHP CCB

Ventricular Tachycardia: • Amiodarone • Lidocaine

Bradycardia: • Atropine • Temporary pacing if unstable

Clinical Pearls

✔ Rate control is often safer than rhythm control ✔ QT prolongation increases torsades risk ✔ Class IC drugs contraindicated in structural heart disease ✔ Amiodarone is effective but toxic ✔ Treat the patient, not just the ECG