The heart is not just a pump; it is a self-regulated electrical system that maintains a coordinated rhythm to ensure effective circulation. The electrical conduction system of the heart initiates, conducts, and regulates impulses, enabling the synchronous contraction of the atria and ventricles.
This article will explore:
✔ The components of the cardiac conduction system
✔ The step-by-step sequence of electrical activation
✔ The role of the autonomic nervous system in heart rate control
✔ Clinical correlations, including arrhythmias and conduction defects
1. The Cardiac Conduction System: Key Components
The conduction system consists of specialized myocardial cells that generate and propagate electrical impulses.
|
Structure |
Location |
Function |
|
Sinoatrial (SA) Node |
Right atrium (near SVC opening) |
Pacemaker of the heart, initiates impulse |
|
Internodal Pathways |
Right atrium |
Conduct impulses from SA node to AV node |
|
Bachmann’s Bundle |
Left atrium |
Rapid conduction for synchronized atrial
contraction |
|
Atrioventricular (AV) Node |
Interatrial septum (near tricuspid valve) |
Delays impulse to allow ventricular filling |
|
Bundle of His |
Interventricular septum |
Rapid conduction to ventricles |
|
Right & Left Bundle Branches |
Along septum |
Conduct impulses to Purkinje fibers |
|
Purkinje Fibers |
Ventricular walls |
Cause ventricular contraction |
🔹 Key Concept: The SA node is the primary pacemaker; however, if it fails, the AV node (backup pacemaker) or the ventricles (escape rhythm) can take over.
2. Step-by-Step Sequence of Electrical Activation
Step 1: Impulse Generation in the SA Node
✔ The SA node depolarizes spontaneously, generating action potentials at a normal rate of 60–100 beats per minute (bpm).
✔ Pacemaker potential (phase 4 depolarization) is driven by funny current (If), calcium influx (ICa), and potassium efflux (IK).
✔ The impulse spreads across both atria, triggering atrial contraction (P wave on ECG).
Step 2: Atrial Conduction & AV Node Delay
✔ The impulse reaches the AV node, which delays transmission (~120 ms) to allow ventricular filling before contraction.
✔ This delay is seen as the PR interval on ECG (0.12–0.20 sec).
Step 3: Ventricular Conduction (Bundle of His & Purkinje Fibers)
✔ The impulse travels rapidly through the Bundle of His → Right & Left Bundle Branches → Purkinje fibers, ensuring coordinated ventricular contraction.
✔ This corresponds to the QRS complex on ECG.
Step 4: Ventricular Repolarization
✔ The ventricles relax as potassium channels open, resetting for the next cycle (T wave on ECG).
3. Autonomic Control of Heart Rate
The autonomic nervous system (ANS) modifies heart rate based on physiological demands:
|
System |
Effect on HR |
Neurotransmitter |
Receptor |
|
Sympathetic (Fight/Flight) |
Increases HR |
Norepinephrine |
Beta-1 adrenergic |
|
Parasympathetic (Rest/Digest) |
Decreases HR |
Acetylcholine |
Muscarinic (M2) |
✔ Increased sympathetic activity (stress, exercise, fever) → Increases HR & contractility.
✔ Increased vagal tone (rest, deep breathing, beta-blockers) → Decreases HR.
4. Clinical Correlations: Conduction Abnormalities & Arrhythmias
|
Condition |
Affected Structure |
ECG Findings |
|
Sinus Bradycardia |
SA Node |
Slow HR (<60 bpm) |
|
Atrial Fibrillation |
Atria |
Irregular, chaotic P waves |
|
AV Block (1st degree) |
AV Node |
Prolonged PR interval (>200 ms) |
|
AV Block (3rd degree) |
AV Node |
No communication between atria &
ventricles |
|
Bundle Branch Block |
Bundle of His/Branches |
Wide QRS complex (>120 ms) |
|
Ventricular Tachycardia |
Ventricles |
Fast, wide QRS complexes |
✔ AV node dysfunction → Heart blocks (bradycardia, syncope).
✔ Bundle branch block → Delayed ventricular activation, wide QRS.
✔ Atrial fibrillation → Rapid, irregular contraction (stroke risk).
✔ Ventricular tachycardia → Life-threatening rhythm, requires immediate intervention.
5. Clinical Tests for Conduction Disorders
✔ ECG (Electrocardiogram): Primary tool to diagnose conduction abnormalities.
✔ Holter Monitoring: 24-hour ECG recording for intermittent arrhythmias.
✔ Electrophysiological Study (EPS): Invasive study to map abnormal conduction.
✔ Tilt Table Test: Evaluates autonomic causes of syncope.
🔹 Treatment options include pacemakers, antiarrhythmic drugs, and ablation procedures.
Conclusion
The electrical conduction system of the heart ensures coordinated contraction and efficient circulation. Understanding normal conduction, autonomic regulation, and clinical abnormalities is essential for diagnosing and managing cardiac arrhythmias.
In the next article, we will explore Hemodynamics: Flow, Pressure & Resistance, covering cardiac pressures, vascular resistance, and key equations.
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.
- 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.
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