6. ELECTROPHYSIOLOGY OF HEART AND ARRHYTHMIAS

The heart works like an electrical pump. Each heartbeat is triggered and controlled by electrical impulses produced inside the heart. Understanding how these impulses are formed, transmitted, and altered helps us understand arrhythmias— abnormal heart rhythms.

1. Conduction System of the Heart

The conduction system creates and carries electrical signals that make the heart contract in a coordinated manner. It has four major parts:

a) Sinoatrial (SA) Node

  • Main pacemaker of the heart
  • Located in the right atrium
  • Fires 60–100 times per minute
  • Starts the impulse that causes atrial contraction

b) Atrioventricular (AV) Node

  • Located between atria and ventricles
  • Slows the impulse to allow atria to empty before ventricles contract

c) Bundle of His and Bundle Branches

  • Impulse travels from AV node down the left and right bundle branches
  • Directs signals toward the ventricles

d) Purkinje Fibers

  • Spread the impulse through the ventricles
  • Cause strong, coordinated ventricular contraction

Latent Pacemakers

AV node, bundle of His, and Purkinje fibers can also produce impulses, but at a slower rate. They take over only if the SA node fails.

2. Myocardial Action Potential

Cardiac cells contract because ions (sodium, potassium, calcium) move in and out of the cells, creating electrical changes called the action potential.

Phases:

  • Phase 0 – Rapid Depolarization: Sodium enters the cell; the cell becomes positive.
  • Phase 1 – Early Repolarization: Sodium channels close, potassium leaves the cell.
  • Phase 2 – Plateau: Calcium enters, potassium continues to leave; contraction occurs.
  • Phase 3 – Final Repolarization: Cell returns to resting state as potassium exits.
  • Phase 4 – Resting Phase: Cell gets ready for the next impulse.

Refractory Periods

  • Absolute refractory period: No stimulus can trigger another impulse.
  • Relative refractory period: A very strong stimulus might trigger an impulse.

3. Electrocardiography (ECG)

An ECG records the heart’s electrical activity using electrodes on the skin.

  • P wave: Atrial depolarization
  • PR interval: Atria to ventricles conduction
  • QRS complex: Ventricular depolarization
  • ST segment: Plateau phase (ventricular repolarization starts)
  • T wave: Ventricular repolarization

4. Cardiac Arrhythmias

Definition

Arrhythmias are abnormal heart rhythms caused by problems with impulse formation or impulse conduction. They may make the heart beat too fast, too slow, or irregularly.

Classification

  • Supraventricular arrhythmias: Start above the ventricles (SA node, AV node).
  • Ventricular arrhythmias: Start in the ventricles; often more dangerous.
  • Drug-induced arrhythmias: Caused by medications.

5. Causes (Etiology)

Arrhythmias may be caused by:

  • Heart attack (current or old)
  • Coronary artery disease
  • Heart muscle changes (cardiomyopathy)
  • Thyroid disorders
  • Diabetes
  • Sleep apnea
  • Stress, alcohol, caffeine, smoking
  • Drug abuse
  • Genetics
  • COVID-19 infection
  • Electrolyte imbalances (high K+, low Ca2+, low Mg2+)

6. Mechanisms of Arrhythmias

a) Abnormal Impulse Formation

  • Depressed automaticity: escape beats, bradycardia
  • Increased automaticity: premature beats, tachycardia
  • Triggered activity: repeated abnormal firing

b) Abnormal Impulse Conduction

  • Conduction block or delay
  • Reentry circuits: impulse travels in a loop, reactivating tissue again → causes extra beats or tachycardia

7. Symptoms

Some arrhythmias cause no symptoms, while others produce noticeable or dangerous effects.

  • Palpitations (feeling heart skip or race)
  • Chest pain
  • Dizziness or fainting
  • Shortness of breath
  • Anxiety or confusion
  • Weakness or fatigue
  • Low blood pressure
  • Cyanosis or pale skin
  • Reduced urine output (poor blood flow)
  • Convulsions (in severe cases)

8. Diagnosis

  • ECG: main tool for identifying arrhythmias
  • Electrophysiological (EP) testing: locates abnormal pathways
  • His bundle study: finds the origin of conduction blocks
  • Blood tests: electrolytes (K⁺, Ca²⁺, Mg²⁺), thyroid tests
  • Imaging: if structural disease is suspected

9. Treatment (Overview)

Treatment depends on the type of arrhythmia, severity, and cause. Options include medications, correcting underlying issues, electrical therapy, and procedures.

Pharmacological Options

  • Sodium channel blockers: slow conduction (Class I)
  • Beta blockers: reduce heart rate and control rhythm (Class II)
  • Potassium channel blockers: prolong repolarization (Class III)
  • Calcium channel blockers: slow AV conduction (Class IV)
  • Other agents: Adenosine, digoxin, magnesium

Non-drug Options

  • Pacemaker implantation
  • Defibrillation or cardioversion
  • Radiofrequency ablation to destroy faulty pathways
  • Treating underlying conditions (thyroid disease, electrolyte imbalance, CAD)

Summary

The heart’s electrical system controls every heartbeat, and disturbances in this system lead to arrhythmias. Understanding impulse formation, conduction pathways, ECG patterns, and mechanisms of arrhythmias helps guide effective diagnosis and treatment.

Source: Adapted from the uploaded chapter on ELECTROPHYSIOLOGY OF HEART AND ARRHYTHMIAS. :contentReference[oaicite:0]{index=0}

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