Definition of Sinus Bradycardia

Introduction

Sinus bradycardia is a type of slow heart rhythm (bradyarrhythmia) where the heart rate falls below 60 beats per minute (bpm) while maintaining a normal sinus rhythm. This condition originates from the sinus node, which is the heart’s natural pacemaker.

Causes of Sinus Bradycardia

Sinus bradycardia can occur due to various physiological and pathological conditions:

Physiological Causes

• Athletic Heart Syndrome – Well-trained athletes often have lower resting heart rates due to increased cardiac efficiency.
• Sleep – Heart rate naturally slows down during sleep due to reduced sympathetic activity.

Pathological Causes

• Sick Sinus Syndrome (SSS) – A dysfunction of the sinus node.
• Myocardial Infarction (Heart Attack) – Especially involving the inferior wall, which affects the vagal tone.
• Hypothyroidism – Reduced thyroid hormone levels slow down metabolism and the heart.
• Electrolyte Imbalances – High potassium (hyperkalemia) or low potassium (hypokalemia) can disturb cardiac electrical activity.
• Medications – Beta-blockers, calcium channel blockers, and digoxin can slow the heart rate excessively.

Symptoms of Sinus Bradycardia

In many cases, sinus bradycardia is asymptomatic. However, if the heart rate is too slow, it may cause: - Fatigue - Dizziness or lightheadedness - Fainting (syncope) - Shortness of breath - Chest discomfort

Diagnosis

• Electrocardiogram (ECG) – Shows a normal P-wave in front of every QRS complex with a slowed heart rate.
• Holter Monitoring – A 24-hour ECG to diagnose intermittent bradycardia.
• Blood Tests – To check for thyroid disorders or electrolyte imbalances.

Treatment

• No Treatment (if asymptomatic) – If sinus bradycardia is due to fitness or sleep, treatment is usually not needed.
• Medication Review – Adjust or stop medications that cause bradycardia.
• Treatment of Underlying Causes – Correct electrolyte imbalances, treat hypothyroidism, or manage heart disease.
• Pacemaker – For severe cases with symptoms like syncope or heart failure, a pacemaker may be needed.

Conclusion

Sinus bradycardia is often a benign condition, especially in healthy individuals. However, when it causes symptoms or results from pathological conditions, medical evaluation and intervention may be necessary.

Source recommendations

1. American College of Cardiology/American Heart Association (ACC/AHA) Guidelines on Bradycardia and Conduction Disorders

1. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628
2. https://www.jacc.org/guidelines/bradycardia
3. https://www.acc.org/latest-in-cardiology/ten-points-to-remember/2018/11/05/15/12/2018-acc-aha-hrs-guideline-on-bradycardia
4. https://pubmed.ncbi.nlm.nih.gov/30412709/
5. https://www.hrsonline.org/guidance/clinical-resources/2018-accahahrs-guideline-evaluation-and-management-patients-bradycardia-and-cardiac-conduction

2. European Society of Cardiology (ESC) Guidelines for 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/