How should I manage severe bradycardia (heart rate 33) in a patient receiving metoprolol?

Management of Severe Metoprolol-Induced Bradycardia (Heart Rate 33 bpm)

Stop metoprolol immediately—symptomatic bradycardia with heart rate <50 bpm is an absolute contraindication to continued beta-blocker therapy, and a heart rate of 33 bpm represents life-threatening bradycardia requiring urgent intervention. 1

Immediate Assessment and Stabilization

Check for signs of hemodynamic instability including altered mental status, chest discomfort, acute heart failure, hypotension (systolic BP <100 mmHg), dizziness, lightheadedness, or other signs of shock. 1 Obtain a 12-lead ECG immediately to identify the rhythm, rule out high-grade AV block (second- or third-degree), and assess QRS width. 1 Ensure adequate oxygenation and establish IV access. 1

Measure blood pressure and assess for signs of hypoperfusion such as oliguria, cool extremities, or altered mental status, which indicate severe hemodynamic compromise requiring more aggressive intervention. 1

Pharmacologic Management

Administer atropine 0.5 mg IV every 3-5 minutes (maximum total dose 3 mg) as first-line therapy for acute symptomatic bradycardia while arranging for definitive management. 1 However, recognize that atropine may be less effective in beta-blocker-induced bradycardia compared to vagally-mediated bradycardia. 1

If the patient remains symptomatic or develops cardiac arrest despite atropine, initiate transcutaneous pacing immediately. 1 Do not delay pacing while giving additional atropine doses in patients showing poor perfusion. 1

Advanced Therapies for Refractory Cases

For severe metoprolol toxicity unresponsive to standard measures:

• High-dose insulin therapy (hyperinsulinemia/euglycemia): Administer 250 units IV bolus followed by continuous infusion, which has been shown to reverse metoprolol-induced cardiac arrest by improving myocardial contractility. 2, 3
• Intravenous lipid emulsion (ILE): Give 20% lipid emulsion 100 mL bolus followed by 200 mL over 30 minutes, which acts as a lipid extractor to lower serum metoprolol levels. 2, 3
• Glucagon: Administer 5-10 mg IV bolus (up to 14 mg has been used), though response may be limited. 3
• Vasopressor support: Epinephrine 0.1 μg/kg/min, norepinephrine 0.5 μg/kg/min, or dobutamine 10 μg/kg/min may be required to maintain perfusion. 3

Critical Warnings About Metoprolol Management

Never abruptly discontinue metoprolol in patients with coronary artery disease without a plan for gradual tapering once stabilized—abrupt withdrawal is associated with a 2.7-fold increased risk of 1-year mortality and can precipitate severe angina, myocardial infarction, and ventricular arrhythmias with up to 50% mortality. 1, 4

Do not restart metoprolol at the original dose after resolution of bradycardia. If beta-blockade is still indicated (e.g., for coronary disease or heart failure), reduce the dose by at least 50% and monitor closely. 1

Evaluation for Contributing Factors

Assess for drug interactions that may have precipitated severe bradycardia:

• CYP2D6 inhibitors (paroxetine, fluoxetine, bupropion) can increase metoprolol levels 4-6 fold and cause severe bradycardia and AV block. 5
• Other rate-lowering drugs including digoxin, diltiazem, verapamil, or amiodarone can have additive bradycardic effects. 1
• Moderate CYP2D6 inhibitors (escitalopram, citalopram, duloxetine) increase metoprolol levels 2-3 fold. 5

Rule out other reversible causes including hypothyroidism, increased intracranial pressure, or infection that may be contributing to bradycardia. 1

Disposition and Monitoring

Admit to an intensive care unit with continuous cardiac monitoring. 1 Monitor heart rate, blood pressure, and rhythm continuously during the acute phase. 1 Watch for signs of worsening heart failure, hypotension, or recurrent bradycardia. 1

Obtain urgent cardiology consultation for consideration of temporary transvenous pacing if transcutaneous pacing is required or if high-grade AV block is present. 1

Long-Term Management Strategy

Once the patient is hemodynamically stable (heart rate >50 bpm, systolic BP >100 mmHg, no symptoms):

• If beta-blockade remains indicated, restart at 50% of the previous dose (e.g., if on metoprolol tartrate 50 mg twice daily, restart at 25 mg twice daily). 1
• Monitor heart rate and blood pressure at 1-2 week intervals during dose adjustments. 1
• Consider switching to metoprolol succinate (extended-release) rather than immediate-release tartrate, as the SR formulation has nearly half the risk of emergent bradycardia compared to IR formulation (12.9 vs 24.1 per 1000 person-years). 6
• Discontinue or substitute any CYP2D6 inhibiting antidepressants—paroxetine, fluoxetine, and bupropion should not be used with metoprolol; consider switching to sertraline, venlafaxine, mianserin, or mirtazapine which do not significantly inhibit CYP2D6. 5

Common Pitfalls to Avoid

Do not rely solely on atropine in beta-blocker toxicity—it is often insufficient and pacing or advanced therapies may be needed. 1

Do not assume the bradycardia is benign even if blood pressure is maintained—a heart rate of 33 bpm represents severe conduction suppression that can rapidly deteriorate to cardiac arrest. 1

Do not give the full 15 mg IV metoprolol dose if considering IV beta-blockade in the future—this significantly increases hypotension and bradycardia risk. 1