What are the common causes of bradycardia?

Common Causes of Bradycardia

Bradycardia is most commonly caused by medications (particularly beta-blockers, calcium channel blockers, and digoxin), followed by intrinsic sinus node dysfunction, atrioventricular conduction disorders, and various reversible conditions including electrolyte abnormalities, hypothyroidism, and increased vagal tone. 1

Definition and Classification

Bradycardia is defined as a heart rate below 60 beats per minute, though clinically significant bradycardia is generally considered when the rate is below 50 beats per minute and associated with symptoms 1. Bradycardia can be broadly classified into two main categories:

1. Sinus Node Dysfunction (SND)
2. Atrioventricular (AV) Block

Common Causes of Bradycardia

1. Medications

• Beta-blockers (e.g., atenolol, metoprolol) - can cause marked bradycardia, especially when combined with other negative chronotropic agents 2
• Non-dihydropyridine calcium channel blockers (e.g., diltiazem, verapamil) 3
• Digoxin - especially with toxic levels 1
• Antiarrhythmic drugs (e.g., amiodarone, sotalol) 2
• Other medications: lithium, methyldopa, risperidone, cisplatin, interferon 1

2. Intrinsic Cardiac Causes

• Sinus node dysfunction/Sick sinus syndrome - age-related degeneration of the sinus node 1
• Atrioventricular conduction disorders - first, second, or third-degree heart block 1
• Acute myocardial infarction/ischemia - particularly involving the inferior wall 1
• Cardiac surgery - valve replacement, maze procedure, coronary artery bypass graft 1
• Heart transplantation - denervation and potential rejection 1

3. Increased Vagal Tone

• Vasovagal reactions 1
• Carotid sinus hypersensitivity 4
• Sleep - particularly in young, healthy individuals 1
• Athletic training - common in well-conditioned athletes 1

4. Metabolic/Systemic Causes

• Hypothyroidism - often associated with "mosque sign" T waves 1
• Electrolyte abnormalities - hyperkalemia, hypokalemia, hypoglycemia 1
• Hypothermia - therapeutic or environmental exposure 1
• Hypoxemia, hypercarbia, acidosis 1
• Increased intracranial pressure 1

5. Infectious Causes

• Lyme disease 1
• Viral myocarditis 1
• Other infections: legionella, psittacosis, typhoid fever, typhus, listeria, malaria, leptospirosis, Dengue fever 1

6. Other Causes

• Obstructive sleep apnea 1
• Hypovolemic shock 1
• Central nervous system disorders 1
• Connective tissue diseases - particularly in neonates born to mothers with anti-Ro/SSA antibodies 1

Clinical Manifestations

The clinical presentation of bradycardia varies widely from asymptomatic to severe symptoms including:

• Syncope or presyncope
• Dizziness or lightheadedness
• Fatigue or exercise intolerance
• Dyspnea (particularly with chronotropic incompetence)
• Heart failure symptoms
• Confusion or altered mental status
• Frank syncope 1

Important Clinical Considerations

• Bradycardia may be entirely asymptomatic in young, healthy individuals or during sleep 1
• The severity of symptoms generally correlates with heart rate and pause duration 1
• Establishing correlation between symptoms and bradycardia is considered the "gold standard" for diagnosis 1
• Bradycardia in the setting of acute myocardial infarction may require special attention due to risk of progression 1
• Combined use of beta-blockers and calcium channel blockers can produce profound bradycardia that is more than additive 3

Management Approach

When evaluating bradycardia, it's essential to:

1. Assess for potentially reversible causes first
2. Evaluate the severity of symptoms and hemodynamic stability
3. Consider the risk of progression to asystole 5

For symptomatic patients presenting with bradycardia due to SND or AV block, evaluation and treatment of reversible causes is strongly recommended (Class I recommendation) 1.

Pitfalls to Avoid

• Don't assume all bradycardias require treatment - asymptomatic bradycardia, particularly in healthy individuals or athletes, often requires no intervention 1
• Don't overlook medication effects - they are among the most common and readily reversible causes 1
• Avoid using atropine in heart transplant patients without evidence of autonomic reinnervation 1
• Don't miss the diagnosis of tachy-brady syndrome, which may require both pacemaker and pharmacologic therapy 4
• Remember that nonspecific symptoms like fatigue can be multifactorial and difficult to correlate with bradycardia 1