How to treat a patient with unstable bradycardia?

Treatment of Unstable Bradycardia

Administer atropine 0.5-1 mg IV immediately as first-line therapy for unstable bradycardia, repeating every 3-5 minutes up to a maximum total dose of 3 mg, while simultaneously preparing for transcutaneous pacing if the patient remains hemodynamically unstable. 1, 2

Initial Assessment and Stabilization

Before initiating pharmacologic therapy, rapidly assess for signs of hemodynamic instability including:

• Altered mental status
• Ischemic chest discomfort
• Acute heart failure
• Hypotension (systolic BP <80 mmHg)
• Signs of shock 2

Establish IV access, initiate cardiac monitoring, obtain a 12-lead ECG, and provide supplemental oxygen if hypoxemic. 2

First-Line Pharmacologic Treatment: Atropine

Atropine is FDA-approved for treating bradyasystolic cardiac arrest and severe life-threatening bradycardia. 3

Dosing Protocol

• Initial dose: 0.5-1 mg IV bolus 1, 2
• Repeat: Every 3-5 minutes as needed 1, 2
• Maximum total dose: 3 mg 1, 2

Critical Dosing Warning

Avoid doses <0.5 mg, as these can paradoxically worsen bradycardia through a bimodal sinoatrial node response. 1, 2 The mechanism involves initial vagal stimulation at lower doses before the expected anticholinergic effect occurs at higher doses. 1

When Atropine is Likely Effective

Atropine works best for:

• Sinus bradycardia
• AV nodal block (first-degree or Mobitz type I second-degree)
• Sinus arrest
• Bradycardia from increased vagal tone 2, 4

When Atropine is Likely Ineffective or Contraindicated

Do not rely on atropine alone for:

• Type II second-degree AV block (Mobitz II)
• Third-degree AV block with wide QRS complex (infranodal block) 2, 5
• Post-cardiac transplant patients without autonomic reinnervation—atropine may cause paradoxical high-degree AV block or sinus arrest in 20% of these patients 1, 2

In infranodal blocks, atropine can paradoxically worsen conduction and precipitate ventricular standstill, as the block occurs below the AV node in the His-Purkinje system. 5

Second-Line Therapy: Chronotropic Agents

If atropine fails or is contraindicated, immediately initiate IV infusion of β-adrenergic agonists:

Dopamine (Preferred Second-Line Agent)

• Initial dose: 5-10 mcg/kg/min IV infusion 1, 2
• Titration: Increase by 5 mcg/kg/min every 2 minutes based on heart rate and blood pressure response 2
• Maximum dose: 20 mcg/kg/min (higher doses cause excessive vasoconstriction and arrhythmias) 1, 2
• Mechanism: Provides both chronotropic and inotropic effects through β-1 adrenergic stimulation at therapeutic doses 1

Epinephrine (Alternative)

• Dose: 2-10 mcg/min IV infusion (or 0.1-0.5 mcg/kg/min) 1, 2
• Titrate: To desired heart rate and hemodynamic response 2
• Preferred over dopamine when: Severe hypotension with shock requires both strong chronotropic and vasopressor support 2

Isoproterenol (Special Circumstances)

• Dose: 2-10 mcg/min IV infusion 1
• Advantage: Pure β-agonist providing chronotropy and inotropy without vasoconstriction 2
• Critical contraindication: Avoid in acute coronary ischemia or MI, as it increases myocardial oxygen demand while decreasing coronary perfusion through β-2 vasodilation 1

Third-Line Therapy: Transcutaneous Pacing

Initiate transcutaneous pacing (TCP) immediately in unstable patients who do not respond to atropine. 1, 2 This is a Class IIa recommendation with Level of Evidence B. 2

When to Use TCP

• Atropine failure with persistent hemodynamic instability
• Infranodal blocks (Mobitz II, third-degree with wide QRS)
• As a bridge to transvenous pacing 2, 4

Practical Considerations

• TCP can be applied rapidly without delays associated with transvenous access 2
• Requires sedation/analgesia in conscious patients due to pain 2
• Serves as temporizing measure only 2

Do not delay TCP while administering additional atropine doses in deteriorating patients. 2

Special Clinical Scenarios

Acute Myocardial Infarction

Use atropine cautiously in inferior MI with bradycardia. Increasing heart rate may worsen ischemia or increase infarct size. 1, 2 Limit total atropine dose to 0.03-0.04 mg/kg in patients with known coronary artery disease. 2 Despite this concern, atropine effectively improved AV conduction in 85% of patients with inferior MI and second- or third-degree AV block in one study. 6

Spinal Cord Injury/Neurogenic Shock

Atropine often fails in neurogenic shock due to the unique pathophysiology. 2 If atropine is ineffective:

• Aminophylline: 6 mg/kg in 100-200 mL IV over 20-30 minutes 1, 2
• Theophylline: Oral 5-10 mg/kg/day (therapeutic effect may occur at serum levels below usual 10-20 mcg/mL range) 1, 2, 7

Beta-Blocker or Calcium Channel Blocker Overdose

Standard atropine and chronotropes may be insufficient. Consider:

• Glucagon: 3-10 mg IV bolus, followed by infusion of 3-5 mg/h 1
• High-dose insulin therapy: 1 unit/kg IV bolus, then 0.5 units/kg/h infusion (monitor glucose and potassium closely) 1
• Calcium (for CCB overdose): IV calcium to increase heart rate 1

Digoxin Toxicity

• Digoxin-specific antibody fragments: Dose depends on amount ingested or serum digoxin level (one vial binds ~0.5 mg digoxin) 1

Common Pitfalls to Avoid

1. Administering atropine <0.5 mg: Can paradoxically worsen bradycardia 1, 2
2. Delaying TCP in unstable patients: Don't wait for full atropine dosing if patient is deteriorating 2
3. Using atropine in post-transplant patients: Risk of paradoxical high-grade block 1, 2
4. Relying on atropine for infranodal blocks: Prepare for pacing immediately 2, 5
5. Exceeding atropine 3 mg total dose: Risk of central anticholinergic syndrome (confusion, agitation, hallucinations) 2
6. Using isoproterenol in acute MI: Worsens ischemia through increased oxygen demand and decreased coronary perfusion 1

Monitoring During Treatment

Continuously monitor:

• Heart rate and rhythm
• Blood pressure
• Mental status
• Resolution of symptoms (chest pain, dyspnea, altered mentation) 2

Prepare for transvenous pacing if pharmacologic therapy and TCP fail. 2, 4