🫀 Sinus bradycardia is a slow but regular rhythm from the sinoatrial (SA) node, defined as HR < 60 bpm.
It can be a benign, physiological finding (e.g., in athletes or during sleep) but in other contexts may reduce cardiac output, causing dizziness, presyncope, or syncope.
Always distinguish between normal adaptation and pathological bradycardia.
📌 About
- Severe sinus bradycardia may impair cardiac output → dizziness, hypotension, presyncope, syncope.
- In elderly patients, may represent sick sinus syndrome 🧓.
- Can be transient (sleep, high vagal tone) or persistent (structural heart disease, drugs).
📉 ECG Features
- Regular sinus rhythm: P wave precedes every QRS.
- Rate < 60 bpm (may drop to 30–40 bpm in sleep or athletes).
- PR interval normal unless concomitant AV nodal disease.
⚠️ Common Causes
- Physiological: ↑ vagal tone (athletes 🏃, sleep 💤).
- Ischaemia: Inferior MI → SA node dysfunction.
- Medications: Digoxin, beta-blockers, verapamil, diltiazem.
- Metabolic/Endocrine: Hypothyroidism 🦋, cholestatic jaundice, electrolyte disturbances (esp. ↑K⁺).
- Neurological: Raised ICP → Cushing’s reflex (HTN + bradycardia + irregular breathing).
- Other: Hypothermia ❄️, sick sinus syndrome in elderly.
🧪 Investigations
- Bedside: 12-lead ECG (to confirm sinus origin, exclude heart block, look for ischaemia).
- Bloods: U&E, TFTs, LFTs, troponin if ACS suspected.
- Other: Holter monitor if intermittent; consider echocardiography if structural disease suspected.
🩺 Management
- Asymptomatic: No treatment if physiological or mild (e.g., in athletes).
- Address Reversible Causes: Stop rate-limiting drugs, correct metabolic/endocrine abnormalities.
- Symptomatic (presyncope/syncope, hypotension):
- Place patient flat, elevate legs → optimise preload.
- Atropine IV (first-line if haemodynamically unstable).
- If refractory → IV isoprenaline, dopamine, or temporary pacing ⚡.
- Correct electrolytes (esp. K⁺, Mg²⁺, Ca²⁺).
- Acute Coronary Syndrome: Treat as ACS → PCI if indicated.
- Chronic Symptomatic Cases: Consider permanent pacemaker implantation (esp. sick sinus syndrome).
💡 Teaching Pearls:
- Always check for drug causes (digoxin, BB, CCB).
- In ACS, sinus bradycardia may be vagally mediated or due to inferior MI.
- Athletic bradycardia is normal if asymptomatic and with normal chronotropic response to exercise.
- Symptomatic bradycardia in the elderly often → pacemaker.
