Bradycardia Causes and Management

Intrinsic Cardiac Causes

• The American Heart Association defines bradycardia as a heart rate less than 60 beats per minute, though clinically significant bradycardia typically occurs at rates below 50 beats per minute and is associated with symptoms or hemodynamic compromise 1
• Sinus Node Dysfunction (Sick Sinus Syndrome) is characterized by inappropriate sinus bradycardia, sinus arrest, sinoatrial block, or alternating bradycardia and tachycardia 2, 3
• Atrioventricular (AV) Conduction Disorders, including first-degree, second-degree (Mobitz type I and II), and third-degree (complete) heart block, can cause bradycardia 1, 3
• Congenital Heart Defects, such as complex congenital heart malformations, can lead to complete AV block 4
• Infiltrative Cardiac Disease, including conditions like amyloidosis or lymphoma infiltrating the conduction system, can cause bradycardia 5

Extrinsic/Reversible Causes

• Calcium channel blockers, particularly verapamil and diltiazem, can cause bradycardia 6
• Digoxin can cause bradyarrhythmias at therapeutic doses in sensitive individuals or in overdose 6
• Antiarrhythmic drugs, such as sotalol, amiodarone, and lidocaine, can cause bradycardia as a side effect 1, 7
• Chemotherapeutic agents, including cisplatin, paclitaxel, 5-fluorouracil, thalidomide, and arsenic trioxide, have been associated with bradycardia 5
• Hypokalemia can lead to bradyarrhythmias, especially in combination with digoxin 3
• Hypocalcemia can impair cardiac conduction 3
• Increased vagal tone, common in young athletes, during sleep, or with vagal maneuvers, can cause bradycardia 4
• Vasovagal reflex, triggered by pain, particularly abdominal pain, can cause bradycardia 8
• Hypothyroidism can cause bradycardia often with characteristic ECG changes 4
• Hypopituitarism can lead to bradycardia through multiple mechanisms 4
• Increased intracranial pressure can cause bradycardia through the Cushing reflex 4
• Hypothermia can cause progressive slowing of heart rate with decreasing core temperature 4
• Infectious diseases, such as myocarditis, meningitis, and typhoid fever, can cause bradycardia 4, 5
• Obstructive jaundice has been associated with bradycardia through unclear mechanisms 4
• Acute myocardial infarction, particularly inferior MI affecting the AV node blood supply, can cause bradycardia 3, 7
• Myocarditis can affect the conduction system and cause bradycardia 2, 5
• Toxins, including certain herbal medicines and toad venom, can cause bradycardia 6
• Neoplasms, such as vagal paraganglioma and catecholamine-secreting tumors, can cause bradycardia 5
• Maternal antibodies, including anti-Ro/SSA and La/SSB antibodies, can cause congenital heart block in neonates 4

Clinical Significance and Evaluation

• Symptoms suggesting pathologic bradycardia include syncope, dizziness, fatigue, reduced exercise capacity, dyspnea, and chest pain 2
• Initial evaluation should focus on signs of increased work of breathing, oxygen saturation, blood pressure, and ECG findings 1
• Potentially reversible causes should be identified and treated before considering permanent interventions 1, 8

Management Considerations

• For symptomatic bradycardia, treatment should address the underlying cause when possible 1
• Acute management of symptomatic bradycardia may include atropine, especially for vagally-mediated bradycardia 8
• Avoid unnecessary permanent pacemaker implantation for reversible causes of bradycardia 8

Bradycardia Causes and Associations

Sleep-Related Bradycardia

• Sleep apnea may manifest as nocturnal bradycardia, as reported by the American College of Cardiology 9

Bradycardia Causes and Clinical Significance

Intrinsic Cardiac Causes

• The American College of Cardiology notes that sinus node dysfunction, characterized by inappropriate sinus bradycardia, sinus arrest, sinoatrial block, or alternating bradycardia and tachycardia, can manifest symptoms ranging from mild fatigue to frank syncope, with syncope present in 50% of patients who received pacemakers for this condition 10
• Sinus node dysfunction represents a failure of impulse formation or sinoatrial conduction block, and patients may exhibit symptoms due to this condition 10

Extrinsic and Reversible Causes

• Lyme disease can cause periatrioventricular node inflammation, leading to bradycardia, as noted by the American College of Cardiology 11
• Transient injury to the conduction system at the time of open heart surgery can also cause bradycardia, according to the American College of Cardiology 11

Clinical Significance

• The American College of Cardiology defines symptomatic bradycardia as a documented bradyarrhythmia directly responsible for syncope, near syncope, transient dizziness, lightheadedness, or confusional states from cerebral hypoperfusion attributable to slow heart rate 11
• Fatigue, exercise intolerance, and congestive heart failure may also result from bradycardia, as noted by the American College of Cardiology 11
• A definite correlation of symptoms with a bradyarrhythmia is required to fulfill criteria for symptomatic bradycardia, according to the American College of Cardiology 11
• The American College of Cardiology advises caution not to confuse physiological sinus bradycardia with pathological bradyarrhythmias 11