What keeps your heart beating in a perfectly timed rhythm? Why do some people develop arrhythmias, while others have a stable heartbeat their entire lives? How does the electrical system of the heart ensure a synchronized contraction?
The answer lies in cardiac electrophysiology, a fascinating interplay of ion channels, conduction pathways, and autonomic regulation. When this delicate balance is disturbed, arrhythmias arise, leading to palpitations, syncope, and even sudden cardiac arrest.
In this article, we will explore:
✔ How the heart generates and propagates electrical impulses
✔ What causes arrhythmias, and how they are classified
✔ Why certain arrhythmias are dangerous, and how they are managed
1. How Does the Heart Generate Electrical Impulses?
The heart is myogenic, meaning it generates its own electrical impulses without needing signals from the brain. This is achieved by a specialized conduction system, where certain cardiac cells act as natural pacemakers.
đŸ”¹ Key Players in Cardiac Electrical Conduction:
| Structure | Location | Function | Intrinsic Rate (bpm) |
|---|---|---|---|
| Sinoatrial (SA) Node | Right atrium (near SVC opening) | Primary pacemaker | 60-100 |
| Atrioventricular (AV) Node | Interatrial septum | Delays impulse, allowing ventricular filling | 40-60 |
| Bundle of His | Interventricular septum | Conducts signals to ventricles | 30-40 |
| Right & Left Bundle Branches | Along septum | Spread impulses to Purkinje fibers | 30-40 |
| Purkinje Fibers | Ventricular walls | Cause ventricular contraction | 20-40 |
đŸ“Œ Why is the SA Node the Natural Pacemaker?
The SA node has the fastest firing rate, so it overrides other pacemakers. However, if the SA node fails, the AV node or ventricles can take over, albeit at a slower rate (escape rhythm).
2. What Creates the Cardiac Action Potential?
Why does the heart contract? The answer lies in ion movement across cell membranes, which generates action potentials that trigger contraction.
đŸ”¹ Phases of the Ventricular Action Potential:
| Phase | Key Ion Movement | Effect |
|---|---|---|
| Phase 0 (Depolarization) | Na⁺ Influx | Rapid upstroke → Contraction |
| Phase 1 (Early Repolarization) | K⁺ Outflux | Slight repolarization |
| Phase 2 (Plateau Phase) | Ca²⁺ Influx & K⁺ Outflux | Sustains contraction |
| Phase 3 (Repolarization) | K⁺ Outflux | Returns cell to resting state |
| Phase 4 (Resting Membrane Potential) | Na⁺/K⁺ Pump Restores Balance | Prepares for next beat |
đŸ“Œ How is the SA Node Different?
The SA node lacks a plateau phase and has a slow, spontaneous depolarization (Phase 4), controlled by the funny current (If)—allowing it to act as a pacemaker.
3. Why Do Arrhythmias Occur?
Arrhythmias arise when there is a disturbance in:
✔ Impulse Generation (Automaticity Problems) – The pacemaker cells misfire.
✔ Impulse Conduction (Reentry Circuits) – Electrical signals get trapped in loops.
✔ Triggered Activity (Afterdepolarizations) – Extra beats sneak in.
4. What Are the Types of Arrhythmias?
Arrhythmias are classified based on:
✔ Where They Originate (Atrial vs. Ventricular)
✔ Heart Rate (Tachycardia > 100 bpm, Bradycardia < 60 bpm)
✔ Regularity (Regular vs. Irregular)
A. Bradyarrhythmias (Slow Heart Rhythms)
đŸ”¹ Sinus Bradycardia – Slow SA node firing (common in athletes).
đŸ”¹ AV Blocks (First, Second, Third Degree) – Delayed or blocked conduction at the AV node.
đŸ”¹ Junctional or Ventricular Escape Rhythms – Backup pacemakers take over.
đŸ“Œ Why is Bradycardia Dangerous?
Severe bradycardia can reduce cardiac output, leading to dizziness, syncope, and poor organ perfusion.
B. Tachyarrhythmias (Fast Heart Rhythms)
đŸ”¹ Atrial Fibrillation (AFib) – Irregular atrial signals causing chaotic, uncoordinated contraction.
đŸ”¹ Supraventricular Tachycardia (SVT) – Rapid, regular rhythms originating above the ventricles.
đŸ”¹ Ventricular Tachycardia (VTach) – Life-threatening fast rhythm from the ventricles.
đŸ”¹ Ventricular Fibrillation (VFib) – Disorganized electrical activity leading to cardiac arrest.
đŸ“Œ Why is VTach and VFib Life-Threatening?
- Ventricular Tachycardia can progress to Ventricular Fibrillation, where the heart quivers instead of pumping, leading to sudden cardiac death if untreated.
- Immediate defibrillation is required to reset the rhythm.
5. How Are Arrhythmias Diagnosed?
✔ ECG (Electrocardiogram): Primary tool to classify arrhythmias.
✔ Holter Monitoring: 24-hour ECG to catch intermittent arrhythmias.
✔ Electrophysiological Study (EPS): Maps electrical pathways for ablation therapy.
6. How Are Arrhythmias Managed?
A. Rate vs. Rhythm Control
✔ Rate Control: Slow the heart down (Beta-blockers, Calcium Channel Blockers, Digoxin).
✔ Rhythm Control: Restore normal rhythm (Antiarrhythmic Drugs, Electrical Cardioversion).
B. Defibrillation vs. Cardioversion
✔ Defibrillation (Shock anytime): Used for VFib or pulseless VTach.
✔ Cardioversion (Synchronized shock): Used for AFib, AFlutter, SVT.
C. Pacemakers & ICDs (Implantable Cardioverter-Defibrillators)
✔ Pacemakers: Prevent bradycardia.
✔ ICDs: Detect and shock life-threatening arrhythmias like VFib.
đŸ“Œ What About Ablation?
Catheter Ablation destroys arrhythmic foci (e.g., AFib, SVT) to restore normal conduction.
7. Key Takeaways: What You Should Remember
đŸ’¡ The SA node is the heart’s natural pacemaker, but if it fails, backup pacemakers take over.
đŸ’¡ Action potentials in the heart involve sodium, potassium, and calcium ion channels.
đŸ’¡ Arrhythmias occur due to problems in impulse generation, conduction, or reentry circuits.
đŸ’¡ Some arrhythmias (AFib) are common and manageable, while others (VTach, VFib) require immediate defibrillation.
đŸ’¡ Diagnosis relies on ECG, and treatment options range from medications to pacemakers and ablation.
Conclusion
The heart’s electrical system is a finely tuned orchestra, ensuring perfectly timed contractions. When this system malfunctions, arrhythmias arise, ranging from benign palpitations to life-threatening ventricular fibrillation. Understanding cardiac electrophysiology is essential for diagnosing, managing, and preventing arrhythmias.
In the next article, we will explore "Cardiac Output & Ejection Fraction: Measuring Heart Performance", diving into how we assess the efficiency of the heart.
References
- Braunwald E. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. 11th ed. Elsevier; 2018.
- Guyton AC, Hall JE. Textbook of Medical Physiology. 14th ed. Elsevier; 2020.
- Klabunde RE. Cardiovascular Physiology Concepts. 3rd ed. Lippincott Williams & Wilkins; 2021.
- Zipes DP, Jalife J. Cardiac Electrophysiology: From Cell to Bedside. 7th ed. Elsevier; 2019.
- Mayo Clinic. Heart Rhythm Disorders: Symptoms & Treatments. Available at: www.mayoclinic.org.
- UpToDate. Diagnosis and Management of Cardiac Arrhythmias. Available at: www.uptodate.com.
- ACC/AHA Guidelines. Management of Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death. Circulation. 2017;136:e1-e50.
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