What causes bradycardia?

Bradycardia Causes

Bradycardia results from either intrinsic cardiac dysfunction or extrinsic reversible causes, and identifying reversible causes—particularly medications, electrolyte abnormalities, hypothyroidism, and infections—is the critical first step before considering permanent interventions. 1, 2

Intrinsic Cardiac Causes

Sinus Node Dysfunction (Sick Sinus Syndrome)

• Characterized by inappropriate sinus bradycardia, sinus arrest, sinoatrial block, or alternating bradycardia-tachycardia (tachy-brady syndrome). 1, 3
• Patients with symptomatic sinus node dysfunction have high risk of cardiovascular events including syncope, atrial fibrillation, and heart failure. 4, 3
• Chronotropic incompetence develops with age and is associated with increased cardiovascular death and overall mortality. 4, 3

Atrioventricular Conduction Disorders

• Includes first-degree, second-degree (Mobitz type I and II), and third-degree (complete) heart block. 1
• High second- or third-degree AV blocks typically require permanent pacemaker placement when symptomatic. 5

Structural and Infiltrative Disease

• Infiltrative cardiac diseases such as amyloidosis or lymphoma infiltrating the conduction system can cause bradycardia. 1, 3
• Congenital heart defects, particularly complex malformations, can lead to complete AV block. 1

Ischemic Heart Disease

• Acute myocardial infarction, particularly inferior MI affecting the AV node blood supply, commonly causes bradycardia. 1, 3
• Myocarditis can affect the conduction system through direct inflammation and cause bradycardia. 1, 2, 3

Extrinsic and Reversible Causes

Medications (Most Common Reversible Cause)

• Beta-blockers, calcium channel blockers (especially verapamil and diltiazem), and digoxin are the most common medication culprits and should be discontinued or dose-reduced when causing symptomatic bradycardia. 1, 2, 3
• Antiarrhythmic drugs including sotalol, amiodarone, and lidocaine can cause bradycardia as a side effect. 1
• Chemotherapeutic agents (cisplatin, paclitaxel, 5-fluorouracil, thalidomide, arsenic trioxide) have been associated with bradycardia. 1
• Drugs transmitted transplacentally or through breast milk can cause bradycardia in neonates. 2

Electrolyte Abnormalities

• Severe hypokalemia, hyperkalemia, hypocalcemia, and systemic acidosis must be corrected. 1, 2, 3
• Hypokalemia can lead to bradyarrhythmias, especially in combination with digoxin. 1

Endocrine and Metabolic Disorders

• Hypothyroidism is a key reversible metabolic cause, often presenting with characteristic ECG changes, and requires thyroxine replacement. 1, 2
• Hypopituitarism can lead to bradycardia through multiple mechanisms. 1

Neurologic Causes

• Increased intracranial pressure from any cause triggers reflex bradycardia through vagal stimulation (Cushing reflex). 1, 2
• This mechanism is critical to recognize as it indicates serious underlying pathology requiring urgent intervention. 2

Vagally-Mediated Bradycardia

• Increased vagal tone is common in young athletes, during sleep, or with vagal maneuvers and represents physiologic bradycardia. 1
• Vasovagal reflex triggered by pain, particularly abdominal pain, can cause transient bradycardia. 1, 2
• Gastrointestinal distress and acute abdominal pain trigger vagal reflexes causing transient bradycardia. 2
• Increased vagal tone during feeding, sleep, and defecation causes physiologic bradycardia in neonates. 2

Infectious Causes

• Myocarditis, meningitis, and typhoid fever can cause bradycardia. 1
• Bacterial meningitis can cause persistent symptomatic bradycardia requiring careful evaluation. 6

Environmental and Toxic Causes

• Hypothermia causes dose-dependent progressive slowing of heart rate with decreasing core temperature and requires active rewarming. 1, 2
• Toxins including certain herbal medicines and toad venom can cause bradycardia. 1
• Obstructive jaundice has been associated with bradycardia through unclear mechanisms. 1

Other Causes

• Neoplasms such as vagal paraganglioma and catecholamine-secreting tumors can cause bradycardia. 1
• Maternal antibodies (anti-Ro/SSA and La/SSB antibodies) can cause congenital heart block in neonates. 1
• Sleep apnea may manifest as nocturnal bradycardia. 1

Clinical Evaluation Framework

Symptom Assessment

• Symptoms suggesting pathologic bradycardia include syncope, dizziness, fatigue, reduced exercise capacity, dyspnea, and chest pain. 1
• Correlation between symptoms and bradycardia is considered the "gold standard" of diagnosis. 4
• Asymptomatic sinus bradycardia has not been associated with adverse outcomes and typically does not require treatment. 3

Initial Evaluation

• Initial evaluation should focus on signs of increased work of breathing, oxygen saturation, blood pressure, and ECG findings. 1
• Verify P wave morphology, amplitude, duration, and axis to distinguish between right, left, or combined atrial abnormality. 3
• For patients with infrequent symptoms, long-term monitoring with an implantable cardiac monitor is reasonable if initial non-invasive evaluation is non-diagnostic. 3

Critical Clinical Pitfall

The most important clinical error is failing to identify reversible causes before considering permanent pacing—aggressive investigation for medications, hypothyroidism, electrolyte abnormalities, infections, and increased intracranial pressure must be completed first. 2 Potentially reversible causes should be identified and treated before considering permanent interventions. 4, 1 If a hemodynamically stable patient can be observed safely while being treated for a metabolic or ischemic condition or an adverse drug reaction, permanent pacemaker implantation is not justified, though temporary pacing may be necessary in the interim. 7