Bradyarrhythmia: Causes and Mechanisms

Introduction

Bradyarrhythmia is a condition characterized by an abnormally slow heart rate, usually defined as fewer than 60 beats per minute. While this can be normal in some individuals, such as well-trained athletes, it may also indicate an underlying health issue. Understanding the causes of bradyarrhythmia is crucial for proper diagnosis and management.

Causes of Bradyarrhythmia

Bradyarrhythmias can arise due to various reasons, broadly categorized into intrinsic and extrinsic causes:

1. Intrinsic Causes (Related to Heart Abnormalities)

These are related to direct dysfunction of the heart's electrical conduction system, primarily affecting the sinoatrial (SA) node or atrioventricular (AV) node.

• Sinus Node Dysfunction (Sick Sinus Syndrome): The SA node, which is the heart's natural pacemaker, may fire too slowly or irregularly.
• Atrioventricular (AV) Block: This occurs when the electrical signals from the atria to the ventricles are delayed or blocked entirely. AV blocks are classified into:
  • First-degree AV block (slowed conduction but all signals transmitted)
  • Second-degree AV block (some signals fail to reach the ventricles)
  • Third-degree AV block (complete dissociation between atrial and ventricular activity)
• Myocardial Infarction (Heart Attack): Damage to the conduction system due to ischemia can disrupt normal electrical activity, leading to bradyarrhythmia.
• Cardiomyopathies: Conditions such as dilated or restrictive cardiomyopathy can impair electrical signal transmission.

2. Extrinsic Causes (External Influences Affecting the Heart)

• Medications: Beta-blockers, calcium channel blockers, digoxin, and certain antiarrhythmic drugs can slow heart rate.
• Electrolyte Imbalances: High potassium (hyperkalemia) or abnormalities in calcium and magnesium levels can affect conductivity.
• Endocrine Disorders:
  • Hypothyroidism: Reduced thyroid hormone levels can slow metabolism, including heart rate.
  • Adrenal insufficiency: Low cortisol levels may contribute to bradycardia.
• Neurological Causes: Increased vagal tone (e.g., due to carotid sinus hypersensitivity, brainstem trauma) can suppress cardiac conduction.
• Sleep Apnea: Episodes of hypoxia and increased vagal activity can lead to intermittent bradycardia.

When to Seek Medical Attention

Bradyarrhythmia can be asymptomatic or cause symptoms such as dizziness, fainting (syncope), fatigue, shortness of breath, or chest pain. If you experience these, seek medical attention promptly, as severe cases may require pacemaker implantation.

Conclusion

Bradyarrhythmia is a complex condition with multiple potential causes. Proper diagnosis includes evaluating medical history, conducting an ECG, checking for reversible factors, and possibly advanced electrophysiological studies. Addressing underlying conditions and, in some cases, using a pacemaker, can help manage the condition effectively.

Source recommendations

1. American Heart Association Guidelines for Bradyarrhythmias and Conduction Disturbances

1. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/conduction-disorders
2. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628
3. https://www.jacc.org/guidelines/bradycardia
4. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000627
5. https://pubmed.ncbi.nlm.nih.gov/30412709/

2. European Society of Cardiology Guidelines on Cardiac Pacing and 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://academic.oup.com/eurheartj/article/34/29/2281/401445
5. https://pubmed.ncbi.nlm.nih.gov/23801827/