Why does the heart sometimes beat too fast, too slow, or irregularly? How do arrhythmias disrupt cardiac output? Which arrhythmias are life-threatening, and which are benign?
Cardiac arrhythmias are abnormal heart rhythms caused by disruptions in electrical conduction, automaticity, or reentry circuits. Some are harmless, while others can lead to syncope, stroke, or sudden cardiac death (SCD).
This article will explore:
✔ Types of arrhythmias and their underlying mechanisms
✔ How conduction system abnormalities cause arrhythmias
✔ Why some arrhythmias are dangerous and require urgent treatment
✔ Management strategies for different arrhythmias
1. What are Cardiac Arrhythmias?
🔹 Arrhythmias are abnormal heart rhythms that result from electrical disturbances in the conduction system.
✔ Bradyarrhythmias (Slow HR) – HR <60 bpm (e.g., sinus bradycardia, heart blocks).
✔ Tachyarrhythmias (Fast HR) – HR >100 bpm (e.g., atrial fibrillation, ventricular tachycardia).
✔ Irregular Rhythms – Disorganized conduction (e.g., atrial fibrillation).
📌 Why Do Arrhythmias Matter?
✔ They can impair cardiac output, causing hypotension, syncope, or cardiac arrest.
✔ Some, like atrial fibrillation, increase stroke risk due to clot formation.
2. How Does the Cardiac Conduction System Work?
The heart's electrical system coordinates contraction in a precise order:
✔ SA Node (Pacemaker): Initiates normal rhythm (60-100 bpm).
✔ AV Node: Delays impulse to allow ventricular filling.
✔ Bundle of His → Purkinje Fibers: Rapidly distribute impulses for synchronized contraction.
📌 What Happens if the SA Node Fails?
✔ AV node takes over (40-60 bpm), leading to junctional rhythm.
3. What Causes Arrhythmias?
🔹 Arrhythmias arise from three primary mechanisms:
| Mechanism | Pathophysiology | Example |
|---|---|---|
| Abnormal Automaticity | Increased pacemaker firing | Sinus tachycardia, ectopic atrial tachycardia |
| Reentry Circuits | Repetitive electrical impulses trapped in a loop | Atrial flutter, AVNRT, WPW syndrome |
| Conduction Blocks | Delayed or absent impulse transmission | AV blocks, bundle branch blocks |
📌 Why Does Hypokalemia Cause Arrhythmias?
✔ Low potassium prolongs repolarization, leading to QT prolongation and ventricular arrhythmias.
4. Classification of Arrhythmias
🔹 A. Supraventricular Arrhythmias (Above the Ventricles)
✔ Sinus Bradycardia – Slowed SA node activity (e.g., in athletes, hypothyroidism).
✔ Sinus Tachycardia – Normal response to exercise, fever, stress.
✔ Atrial Fibrillation (AFib) – Chaotic atrial impulses → Irregularly irregular pulse, high stroke risk.
✔ Atrial Flutter – Organized atrial reentry (sawtooth waves on ECG).
✔ AV Nodal Reentrant Tachycardia (AVNRT) – Sudden palpitations from reentry in AV node.
✔ Wolff-Parkinson-White (WPW) Syndrome – Pre-excitation due to an accessory pathway (risk of sudden cardiac death).
🔹 B. Ventricular Arrhythmias (More Dangerous)
✔ Ventricular Tachycardia (VT) – Rapid, wide QRS complexes (can lead to cardiac arrest).
✔ Ventricular Fibrillation (VFib) – Chaotic, uncoordinated ventricular contractions (requires immediate defibrillation).
✔ Torsades de Pointes – Polymorphic VT with prolonged QT interval (caused by electrolyte imbalances, drugs).
🔹 C. Conduction Abnormalities (Heart Blocks)
✔ First-Degree AV Block: Prolonged PR interval (>200 ms).
✔ Second-Degree AV Block (Mobitz I & II): Some impulses fail to conduct (Mobitz II is more serious).
✔ Third-Degree (Complete) Heart Block: No atrial impulses reach ventricles (requires pacemaker).
📌 Which Arrhythmias Need Emergency Treatment?
✔ Ventricular fibrillation (VFib) & pulseless VT → Defibrillation immediately!
✔ Bradyarrhythmias with syncope → Pacemaker.
5. How Are Arrhythmias Diagnosed?
✔ ECG (Electrocardiogram): First-line test to identify arrhythmias.
✔ Holter Monitor: 24-48 hour recording for intermittent arrhythmias.
✔ Electrophysiological Study (EPS): Maps electrical pathways for ablation therapy.
✔ Echocardiography: Evaluates structural heart disease.
📌 Why is Atrial Fibrillation Dangerous?
✔ Blood stasis in fibrillating atria → Clot formation → Stroke risk.
6. How Are Arrhythmias Treated?
🔹 A. Rate vs. Rhythm Control (For Atrial Fibrillation & Atrial Flutter)
✔ Rate Control: Beta-blockers, calcium channel blockers (diltiazem, verapamil).
✔ Rhythm Control: Antiarrhythmics (amiodarone, flecainide), electrical cardioversion.
🔹 B. Anticoagulation (For AFib Patients at Stroke Risk)
✔ CHA₂DS₂-VASc Score determines stroke risk.
✔ Warfarin or DOACs (apixaban, rivaroxaban) prevent embolism.
🔹 C. Antiarrhythmic Drugs (Classified by Vaughan-Williams System)
| Class | Mechanism | Examples |
|---|---|---|
| Class I (Na⁺ Channel Blockers) | Slows depolarization | Flecainide, Procainamide |
| Class II (Beta-Blockers) | Reduces SNS activation | Metoprolol, Esmolol |
| Class III (K⁺ Channel Blockers) | Prolongs repolarization | Amiodarone, Sotalol |
| Class IV (Ca²⁺ Channel Blockers) | Slows AV conduction | Verapamil, Diltiazem |
🔹 D. Defibrillation & Pacemakers
✔ VFib & pulseless VT → Immediate Defibrillation (Shock).
✔ Pacemaker (for heart blocks, severe bradycardia).
✔ Implantable Cardioverter Defibrillator (ICD) for life-threatening VT or VF.
📌 Why is Amiodarone Used for Ventricular Arrhythmias?
✔ Class III action prolongs repolarization, stabilizing VT/VFib.
✔ Preferred in cardiac arrest (after CPR & defibrillation).
7. Key Takeaways: What You Should Remember
💡 Arrhythmias arise from abnormal automaticity, reentry circuits, or conduction blocks.
💡 Atrial fibrillation is the most common arrhythmia and increases stroke risk.
💡 Ventricular fibrillation is fatal without immediate defibrillation.
💡 Treatment includes antiarrhythmic drugs, rate control, cardioversion, and pacemakers/ICDs.
Conclusion
Cardiac arrhythmias range from benign to life-threatening. Understanding their mechanisms, ECG patterns, and treatment options is essential for preventing complications like stroke and sudden cardiac death.
In the next article, we will explore "Atherosclerosis & Coronary Artery Disease," covering how plaques form and lead to heart attacks.
References
- Guyton AC, Hall JE. Textbook of Medical Physiology. 14th ed. Elsevier; 2020.
- Braunwald E. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. 11th ed. Elsevier; 2018.
- Klabunde RE. Cardiovascular Physiology Concepts. 3rd ed. Lippincott Williams & Wilkins; 2021.
- American Heart Association. Arrhythmia Guidelines. Available at: www.heart.org.
- UpToDate. Management of Cardiac Arrhythmias. Available at: www.uptodate.com.
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