Sinus Bradycardia: Most Common Causes

Introduction

Sinus bradycardia is a condition where the heart rate drops below 60 beats per minute. While it can be normal in some individuals (such as athletes), it may also indicate an underlying health issue.

Common Causes of Sinus Bradycardia

1. Physiological Causes (Normal Variants)

• Athletic Heart Syndrome – Well-trained athletes often have lower resting heart rates due to increased parasympathetic (vagal) tone.
• Sleep – During sleep, especially deep sleep phases, the heart rate naturally slows down due to reduced sympathetic stimulation.

2. Pathological Causes

• Intrinsic Sinus Node Dysfunction (Sick Sinus Syndrome) – A disorder of the heart’s natural pacemaker leading to bradycardia, which may require pacemaker implantation.
• Increased Vagal Tone (Vasovagal Response) – Conditions like vasovagal syncope or excessive vagal nerve stimulation can slow the heart rate significantly.
• Ischemic Heart Disease – Coronary artery disease or myocardial infarction affecting the sinus node can cause bradycardia.
• Hypothyroidism – Low thyroid hormone levels slow down metabolism, leading to a lower heart rate.
• Electrolyte Imbalances – High potassium (hyperkalemia) or calcium abnormalities can affect cardiac conduction, leading to bradycardia.
• Infectious Diseases – Conditions like Lyme disease, endocarditis, or myocarditis can impair heart electrical activity.

3. Medication-Induced Causes

Some medications can cause bradycardia, including: - Beta-blockers (e.g., metoprolol, atenolol) – Reduce heart rate by blocking sympathetic stimulation. - Calcium channel blockers (e.g., diltiazem, verapamil) – Slow down the conduction through the AV node. - Digoxin – Increases vagal tone, reducing heart rate. - Antiarrhythmic drugs (e.g., amiodarone) – Can depress the sinus node function. - Opioids and sedatives – Can suppress autonomic activity leading to bradycardia.

When to Seek Medical Attention?

If sinus bradycardia is accompanied by symptoms like dizziness, fatigue, syncope (fainting), shortness of breath, or chest pain, medical evaluation is necessary. Severe cases may require a pacemaker.

Conclusion

Sinus bradycardia has both benign and pathological causes. A comprehensive assessment, including an ECG, blood tests, and medication review, is essential to determine whether treatment is needed.

Source recommendations

1. 2023 American Heart Association Guidelines on Bradycardia

1. https://cpr.heart.org/en/resuscitation-science/cpr-and-ecc-guidelines/algorithms
2. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628
3. https://cpr.heart.org/-/media/cpr-files/cpr-guidelines-files/algorithms/algorithmacls_bradycardia_200612.pdf
4. https://www.ahajournals.org/doi/10.1161/CIR.0000000000001193
5. https://www.heart.org/en/health-topics/high-blood-pressure/the-facts-about-high-blood-pressure/all-about-heart-rate-pulse

2. 2021 European Society of Cardiology Guidelines for Cardiac Pacing

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/