====== 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  
  - [[cardio:arrhythmias:quinidine|Quinidine]]
  - [[cardio:arrhythmias:procainamide|Procainamide]]

• Class IB – Mild Na block (shortens QT)  
  - [[cardio:arrhythmias:lidocaine|Lidocaine]]

• Class IC – Strong Na block (no QT change)  
  - [[cardio:arrhythmias:propafenone|Propafenone]]
  - [[cardio:arrhythmias:flecainide|Flecainide]]

Used for rhythm control.

--------------------------------------------------------------------

==== Class II – Beta-Blockers ====

Suppress sympathetic stimulation.

  * • [[cardio:beta_blockers:metoprolol|Metoprolol]]
  * • [[cardio:beta_blockers:propranolol|Propranolol]]
  * • [[cardio:beta_blockers:esmolol|Esmolol]]
  * • [[cardio:beta_blockers:sotalol|Sotalol]]

Primarily used for rate control.

--------------------------------------------------------------------

==== Class III – Potassium Channel Blockers ====

Prolong repolarization (Phase 3).

  * • [[cardio:arrhythmias:amiodarone|Amiodarone]]
  * • [[cardio:arrhythmias:dronedarone|Dronedarone]]
  * • [[cardio:arrhythmias:dofetilide|Dofetilide]]
  * • [[cardio:arrhythmias:ibutilide|Ibutilide]]
  * • [[cardio:beta_blockers:sotalol|Sotalol]]

Increase QT interval.

Risk:
• Torsades de pointes

--------------------------------------------------------------------

==== Class IV – Calcium Channel Blockers (Non-DHP) ====

Block AV nodal conduction.

• [[cardio:ccb:verapamil|Verapamil]]
• [[cardio:ccb:diltiazem|Diltiazem]]

Used for rate control in atrial fibrillation.

--------------------------------------------------------------------

===== Miscellaneous Antiarrhythmics =====

  * • [[cardio:arrhythmias:adenosine|Adenosine]] – AV reentry (PSVT)
  * • [[cardio:arrhythmias:atropine|Atropine]] – Bradycardia / AV block
  * • [[cardio:arrhythmias:ivabradine|Ivabradine]] – Sinus tachycardia
  * • [[cardio:arrhythmias:digoxin|Digoxin]] – Rate control (adjunct)

--------------------------------------------------------------------

===== Rate Control vs Rhythm Control =====

==== Rate Control ====

Goal:
• Slow ventricular response

Agents:

  * • [[cardio:beta_blockers:start|Beta-Blockers]]
  * • [[cardio:ccb:non_dhp|Non-DHP Calcium Channel Blockers]]
  * • [[cardio:arrhythmias:digoxin|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):
  * • [[cardio:arrhythmias:adenosine|Adenosine]]
  * • Beta-blocker
  * • Non-DHP CCB

Ventricular Tachycardia:
  * • [[cardio:arrhythmias:amiodarone|Amiodarone]]
  * • [[cardio:arrhythmias:lidocaine|Lidocaine]]

Bradycardia:
  * • [[cardio:arrhythmias:atropine|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  

--------------------------------------------------------------------

Related:

→ [[cardio:beta_blockers:start|Beta-Blockers]]
→ [[cardio:ccb:non_dhp|Non-DHP Calcium Channel Blockers]]
→ [[cardio:intro:start|Return to Cardiovascular Modules]]