Low Heart Rate (Bradycardia) on ECG: Causes, Evaluation & When to Worry

Definition

Bradycardia = heart rate <60 bpm on ECG. Not inherently pathological—context determines significance.

Physiologic (Normal) Causes

• Athlete's heart: Resting HR 40–60 bpm; high vagal tone, stroke volume ↑. Asymptomatic; normalizes with exercise.
• During sleep: HR drops 10–20 bpm; lowest in deep NREM.
• Relaxation/meditation: Transient vagal dominance.

Pathologic Causes

Symptoms That Make It Concerning

• Syncope, near-syncope, dizziness
• Fatigue, exercise intolerance (new)
• Confusion, memory issues
• Cold intolerance, weight gain (thyroid)
• Chest pain, dyspnea

Evaluation Steps

1. Correlate with symptoms: Asymptomatic athlete? Likely benign.
2. Medication review: Hold/adjust offending agents if possible.
3. ECG + rhythm strip: Assess P-QRS relationship, block type.
4. Labs: TSH, K⁺, Ca²⁺, Mg²⁺, troponin, digoxin level.
5. Ambulatory monitoring: 24–48 hr Holter or event recorder.
6. Echo: If structural disease suspected.
7. EPS: If syncope + conduction disease.

Treatment

• Reversible causes: correct electrolytes, thyroid, adjust meds.
• Symptomatic bradycardia: atropine (acute), transcutaneous pacing, permanent pacemaker (Class I: symptomatic 2°/3° AV block, sinus node dysfunction).

Bottom Line

Bradycardia on ECG = pause and assess. Asymptomatic + athletic = reassure. Symptomatic or unexplained = investigate promptly.

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Category	Examples	Mechanism
Conduction Disease	AV block (1°, 2°, 3°), SA node dysfunction, bundle branch block	Impaired impulse generation/conduction
Medication Effects	Beta-blockers, CCBs (non-DHP), digoxin, clonidine, opioids, anticoagulants (rare)	↓ SA/AV node automaticity; ↑ vagal tone
Metabolic/Endocrine	Hypothyroidism, hyperkalemia, hypercalcemia, hypothermia	Cellular electrophysiology alteration
Ischemia/Infarction	Inferior MI (RCA → AV node), RVI	AV node ischemia → block
Infiltrative/Inflammatory	Sarcoidosis, Lyme, amyloid, Chagas	Conduction system infiltration
Neurologic	Increased ICP, Cushing reflex (HTN + bradycardia)	Vagal surge from brainstem compression