What is sinus bradycardia?

Sinus Bradycardia: Definition, Clinical Significance, and Management

Sinus bradycardia is defined as a sinus rhythm with a heart rate less than 50 beats per minute (bpm) originating from the sinoatrial node. 1

Definition and Electrocardiographic Features

Sinus bradycardia is characterized by:

• Heart rate below 50 bpm
• Normal P wave morphology preceding each QRS complex
• Regular rhythm with normal PR interval
• Normal QRS complexes

Physiological vs. Pathological Bradycardia

Physiological Bradycardia

• Athletic Conditioning: Common in trained athletes, reflecting increased vagal tone and cardiac adaptation
• Sleep: Heart rates may drop to 30-40 bpm during sleep, particularly in athletes
• Normal Variant: May be seen in young, healthy individuals without clinical significance

Pathological Bradycardia

Pathological causes include:

• Sinus node dysfunction (sick sinus syndrome)
• Medication effects (beta-blockers, calcium channel blockers, digoxin)
• Hypothyroidism
• Increased intracranial pressure
• Obstructive jaundice
• Hypothermia
• Electrolyte abnormalities

Clinical Presentation

The clinical presentation depends on:

1. Absolute heart rate: Lower rates typically cause more symptoms
2. Duration of bradycardia episodes
3. Underlying cardiovascular disease

Symptoms

When symptomatic, patients may experience:

• Dizziness or lightheadedness
• Syncope or presyncope
• Fatigue
• Exercise intolerance
• Confusion
• Heart failure symptoms
• Shortness of breath

Diagnostic Approach

Key diagnostic elements include:

1. Symptom-rhythm correlation: Establishing that symptoms occur during documented bradycardia episodes
2. 12-lead ECG: To confirm sinus bradycardia and exclude other conduction abnormalities
3. Ambulatory monitoring: For intermittent symptoms (24-hour Holter, event recorder)
4. Exclusion of reversible causes:
   • Medication effects
   • Thyroid dysfunction
   • Electrolyte abnormalities
   • Sleep apnea

Management Considerations

Asymptomatic Bradycardia

• No treatment required in most cases, especially in:
  • Young, healthy individuals
  • Athletes with physiologic bradycardia
  • Patients with gradual onset bradycardia

Symptomatic Bradycardia

For acute symptomatic bradycardia:

• Atropine
• Isoproterenol, dopamine, dobutamine, or epinephrine if needed
• Temporary pacing for severe symptoms due to reversible causes

For chronic symptomatic bradycardia:

• Permanent pacemaker implantation is indicated for symptomatic sinus node dysfunction when bradyarrhythmia has been demonstrated to account for symptoms 2
• Physiological pacing (atrial or dual-chamber) is preferred over ventricular pacing for sinus node dysfunction

Special Considerations

Athletes

• Sinus bradycardia in athletes is usually physiological and rarely requires intervention
• Only profound sinus bradycardia (heart rate <30 bpm during waking hours) needs to be distinguished from sinus node disease 1
• Sinus node dysfunction can be excluded by demonstrating:
  1. Absence of symptoms like dizziness or syncope
  2. Heart rate normalization during exercise or sympathetic maneuvers
  3. Reversal of bradycardia with training reduction or discontinuation

Vagally-Mediated Bradycardia

• Often occurs during sleep and is typically asymptomatic
• Can be recognized by concomitant sinus node slowing (P-P prolongation)
• Generally does not require intervention

Red Flags Requiring Immediate Attention

• Syncope or near-syncope episodes
• Persistent dizziness with slow heart rate
• Chest pain with bradycardia
• Shortness of breath with bradycardia

Remember that correlation between symptoms and documented bradyarrhythmia is essential when determining the need for treatment, as there is no defined heart rate threshold below which treatment is automatically indicated 2.