What are the potential causes of bradycardia in adults?

Bradycardia Etiology in Adults

Bradycardia in adults results from either intrinsic cardiac disease (primarily degenerative fibrosis of the conduction system) or extrinsic/reversible causes (medications, metabolic derangements, ischemia), with degenerative disease being the predominant etiology in elderly patients. 1

Primary Classification of Causes

Bradycardia etiologies are best understood through two fundamental categories that guide clinical approach:

Intrinsic (Primary Cardiac) Causes

Degenerative fibrosis of the conduction system is the most common intrinsic cause in elderly patients, typically affecting those in their 70s and 80s. 1, 2 This pathologic process manifests as:

• Sinus node dysfunction (SND): The fibrotic milieu damages the sinus node, causing sinus bradycardia (<50 bpm), sinus pause (>3 seconds), sinoatrial exit block, or sinus arrest. 1, 2
• Atrioventricular conduction disease: The same degenerative process affects the AV node and His-Purkinje system, causing varying degrees of AV block. 1
• Tachy-brady syndrome: Degenerative fibrosis creates substrate for both atrial arrhythmias and bradycardia, with patients alternating between the two. 1

Ischemic heart disease and myocardial infarction represent critical cardiac causes, particularly inferior MI affecting the AV node blood supply. 1, 2 Associated cardiac conditions include heart failure, valvular heart disease, and atrial fibrillation. 1

Extrinsic (Secondary/Reversible) Causes

Multiple pathophysiologic processes can compromise impulse initiation and propagation, and identifying these is the essential first step before considering permanent interventions. 1

Medications (Most Common Reversible Cause)

• Beta-blockers and calcium channel blockers are the most frequent drug culprits. 2
• Digoxin, antiarrhythmic drugs, and ivabradine can all cause bradycardia. 2
• Drug effects accounted for 21% of emergency department presentations for compromising bradycardia. 1

Metabolic and Endocrine Disorders

• Hypothyroidism should be screened with thyroid function tests in all bradycardic patients. 2 It characteristically produces the "mosque sign" on ECG (dome-shaped symmetric T wave without ST segment). 1
• Electrolyte abnormalities, particularly hyperkalemia and hypomagnesemia, are common and immediately reversible. 2
• Hypoxemia is a critical reversible cause requiring immediate assessment. 2

Acute Cardiac Events

• Acute myocardial infarction caused 14% of emergency bradycardia presentations. 1
• Myocarditis (viral, HIV-related) can cause both sinus bradycardia and AV block. 1

Neurologic and Systemic Conditions

• Increased intracranial pressure, meningitis, and CNS abnormalities affect autonomic regulation. 1, 2
• Obstructive sleep apnea should be screened when bradycardia occurs during sleep or risk factors are present. 2

Infiltrative and Inflammatory Diseases

• Collagen vascular disease, infiltrative diseases (amyloidosis, sarcoidosis), and surgical trauma can damage conduction tissue. 1
• Maternal anti-Ro/SSA and anti-La/SSB antibodies cause congenital complete heart block (relevant for neonatal bradycardia but included for completeness). 1

Other Causes

• Intoxication accounted for 6% of emergency bradycardia cases. 1
• Autonomic effects and neuromuscular disorders can impair conduction. 1

Critical Clinical Distinction

The key clinical question is not the absolute heart rate, but whether bradycardia correlates with symptoms. 2 The clinical manifestations of intrinsic and extrinsic causes are identical, making etiology determination dependent on systematic evaluation rather than presentation. 1

Common Pitfall to Avoid

Failing to identify reversible causes (medications, thyroid dysfunction, electrolytes, hypoxemia) before considering permanent pacing is the most critical error. 2 In emergency settings, 45% of compromising bradycardia cases had reversible etiologies (21% drugs, 14% MI, 6% intoxication, 4% electrolytes). 1