What are the causes of sinus bradycardia in various patient demographics, including young athletes and older adults with underlying medical conditions such as hypothyroidism, sleep apnea, and cardiovascular disease?

Causes of Sinus Bradycardia

Sinus bradycardia results from either intrinsic cardiac dysfunction or extrinsic reversible causes, and the critical first step is always identifying and treating reversible causes—particularly medications, hypothyroidism, electrolyte abnormalities, and increased vagal tone—before considering any permanent intervention. 1, 2

Extrinsic/Reversible Causes (Always Exclude First)

Medications

• Beta-blockers, calcium channel blockers, and digoxin are the most common culprits and should be discontinued or dose-reduced when causing symptomatic bradycardia 2
• Other negative chronotropic agents include sodium-channel blockers and potassium-channel blocking antiarrhythmics 3
• In neonates, drugs transmitted transplacentally or through breast milk can cause bradycardia 1, 2

Metabolic and Endocrine Disorders

• Hypothyroidism is a key reversible cause, often presenting with characteristic ECG changes including the "mosque sign" (dome-shaped symmetric T wave without ST segment), and requires thyroxine replacement 1, 2, 3
• Electrolyte abnormalities including severe hypokalemia, hyperkalemia, hypocalcemia, and systemic acidosis must be corrected 2, 3
• Hypothermia causes dose-dependent bradycardia and requires active rewarming 2, 4

Increased Vagal Tone (Physiologic and Pathologic)

• Physiologic bradycardia occurs in trained athletes (resting heart rates 40-50 bpm, as low as 30 bpm during sleep) due to increased vagal tone and does not require intervention 1
• In neonates, increased vagal tone during feeding, sleep, and defecation causes transient physiologic bradycardia 1, 2
• Gastrointestinal distress and acute abdominal pain trigger vagal reflexes causing transient bradycardia 2

Neurologic Causes

• Increased intracranial pressure from any cause triggers reflex bradycardia through vagal stimulation 1, 2
• Central nervous system abnormalities, meningitis, and cerebrovascular disease are associated with bradycardia 1

Infectious and Inflammatory Causes

• Myocarditis causes bradycardia through direct inflammation of the conduction system, often accompanied by fever, gallop rhythm, T wave changes, and conduction disturbances 1, 2
• Acute Lyme disease can cause transient AV block that typically resolves 1
• Typhoid fever and obstructive jaundice are additional infectious causes 1

Sleep-Related Causes

• Sleep apnea is strongly associated with nocturnal bradyarrhythmias (prevalence 7.2-40% for profound sinus bradycardia), with stereotypical progressive bradycardia during apnea followed by tachycardia during arousal 1
• Treatment with continuous positive airway pressure reduces bradyarrhythmic episodes by 72-89% and eliminates the need for pacing in most patients 1
• Physiologic nocturnal bradycardia is common in young individuals and conditioned athletes, with sinus pauses up to 3 seconds considered normal 1

Intrinsic Cardiac Causes

Sinus Node Dysfunction (Sick Sinus Syndrome)

• Degenerative fibrosis of the sinus node is the most common intrinsic cause, typically occurring in patients in their 70s or 80s 1
• Manifests as sinus bradycardia, sinoatrial block, chronotropic incompetence, and tachy-brady syndrome 1
• Associated concurrent conditions include ischemic heart disease, heart failure, valvular disease, cerebrovascular disease, and atrial fibrillation 1

Ischemic and Structural Heart Disease

• Myocardial ischemia or infarction can compromise sinus node function 1
• Heart failure and valvular heart disease are common concurrent issues in patients with bradycardia requiring treatment 1

Infiltrative and Systemic Diseases

• Sarcoidosis and amyloidosis require prophylactic pacing even if AV block is transient due to risk of disease progression 1
• Collagen vascular diseases can compromise impulse initiation and propagation 1
• Neuromuscular disorders are associated with progressive conduction disease 1

Congenital and Surgical Causes

• True congenital sinus node dysfunction is limited to rare heterotaxy syndromes (polysplenia type lacks sinus node) 1
• Surgical trauma from Mustard, Senning, Glenn, or Fontan operations compromises the sinus node, with tachy-brady syndrome observed in >50% of D-TGA patients after atrial switch 1
• Sinus venosus defects may be associated with sinus node dysfunction 1

Autoimmune Causes

• Maternal anti-Ro/SSA and anti-La/SSB antibodies cause transient sinus bradycardia or complete AV block in neonates of mothers with lupus or connective tissue disease 1

Critical Clinical Pitfalls

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, 3

• Distinguish physiologic bradycardia (athletes, sleep, young individuals) from pathologic bradycardia requiring intervention 1
• Asymptomatic bradycardia, even <50 bpm, has a protective effect with lower mortality and does not benefit from pacing 1, 5
• Screen for sleep apnea in patients with nocturnal bradyarrhythmias, as treatment eliminates the need for pacing in most cases 1
• Document temporal correlation between symptoms and bradycardia before attributing symptoms to heart rate 3