Electrophysiology of heart

Cardiac Action Potential

A. Fast-response cells (Atrial muscle, Ventricular muscle, His–Purkinje)

Phases

1. Phase 0 → Fast Na⁺ influx  → rapid upstroke

2. Phase 1 → Transient K⁺ out

3. Phase 2 → Ca²⁺ plateau (L-type)

4. Phase 3 → K⁺ repolarisation

5. Phase 4 → Stable resting potential

Key point:

Conduction velocity depends on the Phase 0 slope

Anything that blocks Na⁺ channels slows conduction → promotes re-entry block or termination

Drugs acting here

• Class I (flecainide, lidocaine)

• Class III (amiodarone via phase 3)

B. Slow-response cells (SA node, AV node)

Phases (No phase 1 or 2)

1. Phase 0 → Ca²⁺ influx (slow)

2. Phase 4 → Spontaneous depolarisation (automaticity)

Key point:

Rate depends on slope of Phase 4

AV nodal delay is physiological protection

Drugs acting here

• β-blockers

• Verapamil / diltiazem

• Digoxin (↑ vagal tone)

Arrhythmias arise from:

1️⃣ Re-entry (circuits)

2️⃣ Automaticity (pacemaker misfire)

3️⃣ Triggered activity (after-depolarisations)

1. Re-entry (common). Classic examples

• Atrial flutter (macro-reentry)

• AVNRT

• AVRT (WPW)

• Scar-related VT (post-MI)

• Many cases of AF (micro-reentry)

Drugs:

Flecainide, amiodarone

Ablation (definitive)

2. ↑ d automaticity: Cells fire faster than they should. Phase 4 slope becomes steeper. Triggered by sympathetic tone. Examples

• Sinus tachycardia

• Ectopic atrial tachycardia

• Junctional tachycardia

Drugs:

β-blockers

Ivabradine (sinus node)

Caution: Automatic tachycardias do not terminate with cardioversion

3. Triggered activity (excitability problem). This is about after-depolarisations. Causes Prolonged QT or Bradycardia. Example

• Torsades de pointes

• Digoxin-induced VT

• Catecholamine-mediated arrhythmias

Drug

Magnesium

β-blockers

Treat cause (Digoxin Fab if toxic)

Drugs for arrhythmia

β-blockers

• ↓ automaticity

• ↓ AV nodal conduction

Digoxin

• ↑ vagal tone → AV node

• No effect on re-entry circuits

Flecainide

• Slows fast Na⁺ conduction

• Excellent for re-entry

• Dangerous in scarred myocardium

Amiodarone

• Hits all three mechanisms

• Effective but toxic (lung, liver, and thyroid) + vortex keratopathy