Notes – Unstable Bradycardia

Definition:

• Bradycardia: HR < 60 bpm.

• Symptomatic bradycardia: Low HR plus symptoms directly caused by the bradycardia (e.g., hypotension, syncope, chest pain, confusion).

Initial Assessment

1. Monitor

   • BP (hypotensive? normotensive? hypertensive?)

   • HR, rhythm on monitor

   • O₂ saturation, respiratory effort

   • Temperature (rule out hypothermia)

2. Symptoms

   • Syncope, chest pain, dyspnea

   • Confusion, malaise

3. Physical Exam

   • Mental status: confusion, encephalopathy

   • Heart/lungs: murmurs, rales

   • Extremities: perfusion, cap refill, mottling

Physiology

• Cardiac Output = HR × Stroke Volume

• ↓HR → ↓CO → ↓perfusion despite sometimes normal BP (afterload compensation).

• Be alert for false reassurance with "normal" BP.

ECG Evaluation

• Narrow QRS: Problem at or above AV node (sinus node, AV node) → often more benign.

• Wide QRS: Problem distal to AV node (His-Purkinje, ventricles) → serious, may not respond to meds.

• Examples:

  • Severe sinus bradycardia ± sinus arrest.

  • Third-degree AV block (P waves march independently, wide QRS escape).

Causes (Mnemonic: DIE – "Don’t let your patient DIE")

1. Drugs: β-blockers, calcium channel blockers, digoxin, others.

2. Ischemia: Especially STEMI.

   • Inferior STEMI (RCA → AV node): Can cause sinus brady, AV block, usually transient.

   • Anterior STEMI (LAD): Wider QRS, distal block, worse prognosis, often pacing needed.

3. Electrolytes: Hyperkalemia (wide, bizarre rhythms; may require calcium).

Other causes: Increased ICP, neurogenic shock (spinal cord injury), intra-abdominal hemorrhage (ectopic pregnancy).

Management Algorithm (AHA 2018–2020)

1. Atropine

   • 0.5 mg IV bolus, repeat q3–5 min (max 3 mg).

   • Often ineffective (~28% effective).

   • Avoid in cardiac transplant patients (denervated heart).

2. Epinephrine infusion

   • 2–10 mcg/min IV infusion (or push dose 20–30 mcg).

   • Potent β-adrenergic stimulation ↑HR, ↑contractility.

   • Caution: ↑myocardial O₂ demand (be cautious in cardiogenic shock).

3. Dopamine infusion

   • 5–20 mcg/kg/min.

   • Less commonly used, may be institution-dependent.

4. Adjuncts

   • Calcium: Consider if possible hyperkalemia or renal failure (empiric if suspected).

5. Pacing

   • Transcutaneous pacing (TCP): First-line electrical therapy, rapid, AP pad placement often best. Start with higher current to ensure capture. Requires sedation/analgesia.

   • Transvenous pacing: If TCP unsuccessful or not tolerated. More invasive, risks include tamponade, pneumothorax.

Summary

• Approach systematically: Monitor, vitals, ECG, focused exam.

• Think physiology: CO = HR × SV.

• Work through causes: Drugs, ischemia, electrolytes.

• Treat rapidly: Atropine (limited), then epinephrine/dopamine, calcium if suspected hyperK, and pacing if refractory.

• Always prepare for escalation: Especially in STEMI (pads on early).