Causes of Bradycardia
Bradycardia is primarily caused by sinus node dysfunction (sick sinus syndrome) and atrioventricular blocks, with first-degree AV block being common but rarely symptomatic, while aortic stenosis and Wolff-Parkinson-White syndrome are not direct causes of bradycardia. 1
Primary Causes of Bradycardia
Sinus Node Dysfunction (Sick Sinus Syndrome)
Sick sinus syndrome encompasses several manifestations that can cause bradycardia:
- Sinus bradycardia: Heart rate <50 bpm
- Sinoatrial exit block: Blocked conduction between sinus node and atrial tissue
- Sinus pause/arrest: Sinus node fails to depolarize or depolarizes after >3 seconds
- Tachycardia-bradycardia syndrome: Alternating tachycardia and bradycardia
- Chronotropic incompetence: Inability to increase heart rate with activity 1
Atrioventricular (AV) Blocks
AV blocks are classified by severity:
- First-degree AV block: PR interval >200 ms (prolonged conduction but all impulses conduct)
- Second-degree AV block:
- Mobitz Type I (Wenckebach): Progressive PR prolongation until a beat is dropped
- Mobitz Type II: Fixed PR interval with intermittent non-conducted P waves
- Advanced AV block (2:1,3:1 conduction patterns)
- Third-degree (complete) AV block: No impulses pass between atria and ventricles 1
Clinical Significance of Specific Conditions
First-Degree AV Block
- Generally benign but can cause symptoms if PR interval is markedly prolonged (>300 ms)
- Can lead to symptoms due to atrial contraction occurring very early in diastole, affecting ventricular filling 1
- Not typically a direct cause of significant bradycardia unless it progresses to higher-degree block
Sick Sinus Syndrome
- Most common indication for permanent pacemaker implantation
- Can present with symptomatic bradycardia, syncope, or pre-syncope
- May alternate between bradycardia and tachyarrhythmias
- Associated with increased risk of thromboembolism if untreated 1
Aortic Stenosis
- Not a direct cause of bradycardia
- May be associated with conduction system disease in some patients due to calcification extending into the conduction system
- Can cause syncope, but typically from outflow obstruction rather than bradycardia 1
Wolff-Parkinson-White Syndrome
- Not a cause of bradycardia
- Characterized by pre-excitation and tachyarrhythmias
- Associated with accessory pathway conduction that typically causes tachycardia, not bradycardia 1
- Sometimes treated with ablation to prevent tachyarrhythmias 1
Other Important Causes of Bradycardia
Medications:
Acute conditions:
- Myocardial infarction (especially inferior MI)
- Electrolyte disturbances
- Hypoxemia
- Increased intracranial pressure 1
Iatrogenic causes:
- Post-cardiac surgery (4.6% develop iatrogenic sick sinus syndrome) 3
- Post-ablation procedures
Increased vagal tone:
- Athletes (physiologic bradycardia)
- Vasovagal responses
- Sleep 1
Clinical Approach to Bradycardia
When evaluating bradycardia, it's crucial to determine if it's causing symptoms such as:
- Syncope or pre-syncope
- Dizziness or lightheadedness
- Heart failure symptoms
- Confusion from cerebral hypoperfusion 1
Symptomatic bradycardia due to sick sinus syndrome or high-grade AV block typically requires permanent pacemaker implantation, while first-degree AV block rarely requires intervention unless severely prolonged 4.
Important Caveats
- Asymptomatic bradycardia, especially in athletes or during sleep, often requires no treatment
- First-degree AV block can progress to higher-degree blocks in some patients
- Wolff-Parkinson-White syndrome is associated with tachyarrhythmias, not bradycardia
- Aortic stenosis causes syncope through different mechanisms than bradycardia
- Temporary treatments for symptomatic bradycardia include atropine and temporary pacing while awaiting definitive treatment 1, 2