Sinus Bradycardia: When a Slow Heart Rate Is Normal vs. Concerning

Definition

Sinus bradycardia = sinus rhythm with heart rate <60 beats/min in adults. It originates from the SA node but at a slower rate.

Common Benign Causes

• Athletes/endurance training: High vagal tone, efficient stroke volume—resting HR 40–60 bpm is normal.
• During sleep: Physiologic; HR <60 expected.
• Medications: Beta-blockers, calcium channel blockers (non-DHP), digoxin, clonidine, ivabradine.

Potentially Pathologic Causes

• Electrolyte imbalance (hyperkalemia, hypercalcemia)
• Hypothyroidism
• Increased intracranial pressure (Cushing reflex)
• Myocardial infarction (inferior/posterior → vagal stimulation)
• Infiltrative diseases (amyloid, sarcoid)
• SA node dysfunction (sick sinus syndrome)

When to Worry

Symptomatic bradycardia requires urgent evaluation:

• Dizziness, presyncope, syncope
• Exertional intolerance, fatigue disproportionate to activity
• Confusion, memory lapses (cerebral hypoperfusion)
• Cold sweats, dyspnea

Evaluation

1. History: medications, symptoms, exercise level.
2. ECG: confirm sinus mechanism, rule out blocks.
3. Labs: TSH, electrolytes, troponin if indicated.
4. Ambulatory monitoring if intermittent symptoms.

Management

• Asymptomatic, physiologic (athlete) → reassurance.
• Medication-induced → dose adjustment or substitution.
• Pathologic → treat underlying cause; pacemaker if symptomatic intrinsic SA node disease.

Context is everything: a marathoner at 48 bpm is healthy; a sedentary elder at 48 bpm on metoprolol needs review.