Understanding Sinus Bradycardia on ECG

Introduction

Sinus bradycardia is a condition in which the heart rate is slower than normal, typically below 60 beats per minute in adults. It results from slower firing of the sinoatrial (SA) node, which is the natural pacemaker of the heart.

ECG Characteristics of Sinus Bradycardia

To recognize sinus bradycardia on an ECG, look for:

• Heart Rate: Less than 60 beats per minute.
• Rhythm: Regular P-P and R-R intervals.
• P Wave: Normal in morphology and preceding each QRS complex.
• PR Interval: Normal (120-200 ms).
• QRS Complex: Normal duration (≤120 ms).

Causes of Sinus Bradycardia:

Sinus bradycardia can be physiological or pathological. Some common causes include:

• Physiological: Athletes and well-conditioned individuals may have a naturally slow heart rate.
• Pathological:
  • Medication effects (e.g., beta-blockers, calcium channel blockers, digoxin).
  • Hypothyroidism (causes reduced metabolic activity).
  • Hypothermia (decreased body temperature slows heart rate).
  • Sick sinus syndrome (dysfunction of the SA node).
  • Myocardial infarction (especially involving the inferior wall).
  • Increased vagal tone (seen in conditions like vasovagal syncope).

Symptoms and When to Seek Medical Help

Some individuals with sinus bradycardia may have no symptoms, but others might experience:

• Fatigue
• Dizziness or lightheadedness
• Syncope (fainting)
• Shortness of breath
• Chest pain (rare but possible)

Seek immediate medical attention if the slow heart rate is associated with symptoms like fainting or severe dizziness.

Management and Treatment:

The management of sinus bradycardia depends on its cause and whether it is symptomatic:

• Asymptomatic patients often require no treatment.
• Symptomatic bradycardia may require:
  • Stopping or adjusting medications that slow the heart.
  • Treating underlying conditions (e.g., managing hypothyroidism).
  • Pacemaker placement in cases of severe or persistent bradycardia.
  • Emergency treatment with atropine (first-line), dopamine, or epinephrine if the bradycardia leads to instability.

Conclusion

Sinus bradycardia is often benign but can sometimes indicate an underlying problem. A detailed evaluation, including ECG and possibly further cardiac tests, is needed to determine the cause and appropriate treatment.

Source recommendations

1. 2023 American Heart Association Guidelines on Bradyarrhythmias

1. https://cpr.heart.org/en/resuscitation-science/cpr-and-ecc-guidelines/algorithms
2. https://www.ahajournals.org/doi/10.1161/CIR.0000000000001194
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.0000000000000628
5. https://pubmed.ncbi.nlm.nih.gov/30412710/

2. 2022 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/34455427/