Understanding the Causes of Bradycardia

Introduction

Bradycardia is a condition in which the heart rate is slower than normal—typically less than 60 beats per minute in adults. While a low heart rate can be normal in well-trained athletes, it can also indicate underlying medical conditions requiring attention.

Causes of Bradycardia

1. Physiological Causes (Normal Variations)

  • Athletic Heart Syndrome: Well-trained athletes often have lower heart rates due to increased vagal tone and improved cardiac efficiency.
  • Sleep-Related Bradycardia: During deep sleep, the heart rate naturally slows down due to decreased sympathetic nervous system activity.

2. Pathological Causes

A. Intrinsic Cardiac Causes (Heart-Related Issues)

  • Sick Sinus Syndrome (SSS): A condition where the sinoatrial node (the heart’s natural pacemaker) malfunctions.
  • Heart Block: A disruption in electrical impulses between the atria and ventricles, categorized into:
    • First-degree heart block (mild slowing)
    • Second-degree heart block (intermittent failure)
    • Third-degree (complete) heart block (dangerous condition requiring a pacemaker)
  • Myocardial Infarction (Heart Attack): Damage to cardiac tissue, especially of the inferior wall of the heart, affecting electrical conduction.
  • Myocarditis: Inflammation of the heart muscle due to infections or autoimmune conditions.

B. Extrinsic Causes (Non-Heart-Related Factors)

  • Medications:
    • Beta-blockers
    • Calcium channel blockers (e.g., diltiazem, verapamil)
    • Digoxin (used in heart failure)
    • Antiarrhythmic drugs (e.g., amiodarone)
  • Electrolyte Imbalances:
    • High potassium levels (hyperkalemia)
    • Low thyroid hormone levels (hypothyroidism)
  • Neurological Causes:
    • Increased vagal tone (e.g., vasovagal syncope)
    • Brain injuries affecting the autonomic nervous system
  • Infections and Systemic Conditions:
    • Lyme disease (can cause heart block)
    • Hypothermia (low body temperature slows the heart rate)

When to Seek Medical Attention

Seek medical help if bradycardia is accompanied by: - Fainting (syncope) - Dizziness or lightheadedness - Severe fatigue - Chest pain - Shortness of breath

Conclusion

Bradycardia can be benign or a sign of an underlying condition requiring medical attention. If you experience symptoms, consult a cardiologist to determine the cause and appropriate treatment.

Source recommendations

1. 2023 American Heart Association Guidelines for the Evaluation and Management of Bradycardia

  1. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628
  2. https://pubmed.ncbi.nlm.nih.gov/30412710/
  3. https://www.ahajournals.org/doi/10.1161/cir.0000000000000499
  4. https://www.jacc.org/doi/10.1016/j.jacc.2018.10.043
  5. https://www.heartrhythmjournal.com/article/S1547-5271(23)02026-X/fulltext

2. 2021 European Society of Cardiology Guidelines on Cardiac Pacing and Cardiac Resynchronization Therapy

  1. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Cardiac-Pacing-and-Cardiac-Resynchronization-Therapy
  2. https://academic.oup.com/eurheartj/article/42/35/3427/6358547
  3. https://pubmed.ncbi.nlm.nih.gov/34455430/
  4. https://link.springer.com/article/10.1007/s12471-024-01927-y
  5. https://pubmed.ncbi.nlm.nih.gov/34455427/

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