The Electrical System of the Heart: How It Works and What Can Go Wrong

By SWA

The heart is more than a muscular pump—it’s an intricately wired electrical system designed to keep blood flowing through the body in a coordinated, rhythmic manner. At the core of this function lies a remarkable ability of heart muscle cells (myocardium) to generate and conduct electrical impulses that trigger each heartbeat. Understanding this process—and what can go wrong—offers valuable insight into many common cardiac conditions.

The Electrical Pathway: How the Heart Beats

The heart contracts in a synchronized way due to the organized flow of electrical signals. These signals originate in a specialized cluster of cells known as the sinoatrial (SA) node, located in the right atrium. Often referred to as the heart's natural pacemaker, the SA node initiates each heartbeat by generating an electrical impulse.

Once fired, this impulse spreads across the atrial muscle like a wave, causing the atria (upper chambers) to contract. This contraction pushes blood into the ventricles (lower chambers). The impulse travels from one heart cell to the next—similar to how electricity travels through a series of connected cables.

The signal then arrives at the atrioventricular (AV) node, positioned between the atria and ventricles. The AV node serves a crucial function: it delays the electrical signal briefly to give the ventricles time to fill with blood from the atria. This delay ensures optimal blood flow with each beat.

After this pause, the impulse travels down a specialized conduction system: the Bundle of His, then into the right and left bundle branches, and finally through the Purkinje fibers. This pathway allows the signal to reach all parts of the ventricles efficiently, prompting a strong and coordinated ventricular contraction that sends blood to the lungs and the rest of the body.

The Electrical Pathway: How the Heart Beats

The heart contracts in a synchronized way due to the organized flow of electrical signals. These signals originate in a specialized cluster of cells known as the sinoatrial (SA) node, located in the right atrium. Often referred to as the heart's natural pacemaker, the SA node initiates each heartbeat by generating an electrical impulse.

Once fired, this impulse spreads across the atrial muscle like a wave, causing the atria (upper chambers) to contract. This contraction pushes blood into the ventricles (lower chambers). The impulse travels from one heart cell to the next—similar to how electricity travels through a series of connected cables.

The signal then arrives at the atrioventricular (AV) node, positioned between the atria and ventricles. The AV node serves a crucial function: it delays the electrical signal briefly to give the ventricles time to fill with blood from the atria. This delay ensures optimal blood flow with each beat.

After this pause, the impulse travels down a specialized conduction system: the Bundle of His, then into the right and left bundle branches, and finally through the Purkinje fibers. This pathway allows the signal to reach all parts of the ventricles efficiently, prompting a strong and coordinated ventricular contraction that sends blood to the lungs and the rest of the body.

When the Signal Fails: Causes of Electrical Conduction Interruptions

While this conduction system is highly efficient, it is not immune to dysfunction. Interruptions in the heart's electrical signals—known as conduction blocks or heart blocks—can significantly affect heart rhythm and performance. Common causes include:

1. Damage to the Heart Tissue

  • A heart attack (myocardial infarction) can injure or destroy conduction pathways.

  • Scar tissue from previous cardiac events may obstruct signal flow.

2. Age-Related Degeneration

  • With age, the conduction system can deteriorate, especially in the SA or AV node.

  • This degeneration, often due to fibrosis, impairs the heart’s ability to conduct impulses effectively.

3. Medication Side Effects

  • Certain drugs, such as beta blockers, calcium channel blockers, and digoxin, can slow or block conduction—especially at the AV node.

4. Electrolyte Imbalances

  • Proper conduction depends on balanced levels of potassium, calcium, and magnesium.

  • Imbalances can disrupt the electrical stability of heart cells.

5. Congenital Heart Disorders

  • Some individuals are born with abnormal conduction systems, leading to lifelong rhythm problems.

6. Inflammatory Conditions

  • Myocarditis or infections like Lyme disease can inflame and impair electrical pathways.

7. Surgical or Procedural Injury

  • Invasive cardiac procedures can unintentionally damage the conduction system.

AV Nodal Reentrant Tachycardia (AVNRT): A Unique Electrical Misfire

While most problems with the AV node involve delayed or blocked conduction, the AV node can also become the center of an abnormally fast rhythm. This condition, known as AV nodal reentrant tachycardia (AVNRT), is one of the most common types of supraventricular tachycardia (SVT).

In AVNRT, the AV node contains two conduction pathways: a fast pathway and a slow pathway. Under certain conditions, a premature electrical impulse can enter the slow pathway and loop back through the fast pathway, creating a reentry circuit. This loop allows electrical signals to circulate rapidly and repeatedly, overriding the normal rhythm and causing the heart to beat very quickly—often between 150 and 250 beats per minute.

Symptoms of AVNRT may include:

  • Sudden onset of a racing heartbeat

  • Lightheadedness or dizziness

  • Shortness of breath

  • Anxiety

  • Chest discomfort

AVNRT is typically not life-threatening, but it can be very uncomfortable and disruptive. Episodes may be brief or prolonged, and they often start and stop abruptly.

Management and Treatment Options:

  • Vagal maneuvers (like bearing down or coughing) can often interrupt the circuit and stop the tachycardia.

  • Medications such as adenosine or beta blockers may help control episodes.

  • Catheter ablation, a minimally invasive procedure, can permanently correct the reentry pathway and prevent recurrence with a high success rate.

Clinical Consequences of Conduction Disruption

Whether caused by blocked conduction or abnormal reentry circuits like AVNRT, disruptions in the heart's electrical activity can lead to bradycardia, tachycardia, or arrhythmias. These can present with symptoms such as:

  • Fatigue

  • Dizziness or fainting

  • Palpitations

  • Chest pain

  • In severe cases, cardiac arrest

Conclusion

The heart’s electrical system is a finely tuned network that ensures precise, rhythmic contractions to sustain life. From the SA node's pacing to the AV node’s relay function, every component must work in harmony. Disorders such as conduction blocks or AV nodal reentrant tachycardia highlight the importance of electrical integrity in cardiac health. With modern diagnostic tools and treatments—from medications to ablation therapy—most rhythm disturbances can be effectively managed, improving quality of life and outcomes for affected individuals.

© 2000-2025 Sieglinde W. Alexander. All writings by Sieglinde W. Alexander have a fife year copy right.
Library of Congress Card Number: LCN 00-192742 ISBN: 0-9703195-0-9

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