Is it appropriate to administer metoprolol (beta blocker) on a pro re nata (prn) basis for acute exacerbations of atrial fibrillation (a-fib) with symptoms of shortness of breath (dyspnea), tachycardia, and hypertension in a patient already on chronic metoprolol therapy?

PRN Metoprolol for Acute A-Fib Exacerbations

Yes, administering additional metoprolol on a PRN basis during acute symptomatic exacerbations of atrial fibrillation is appropriate and supported by guideline recommendations, provided you screen for contraindications and use careful dosing. 1, 2

Rationale for PRN Beta-Blocker Use

Beta-blockers like metoprolol are explicitly recommended as first-line agents for acute rate control in atrial fibrillation, particularly during high sympathetic states that trigger your patient's flare-ups with tachycardia, hypertension, and dyspnea. 1 The 2016 ESC guidelines specifically endorse beta-blockers for acute rate control due to their rapid onset and effectiveness when sympathetic tone is elevated—exactly the scenario your patient experiences during exacerbations. 1

Metoprolol demonstrates superior exercise rate control compared to digoxin and achieves rate reduction more effectively than calcium channel blockers in many patients, making it ideal for symptomatic breakthrough episodes. 2, 3

Practical Dosing Strategy

For acute PRN use in a patient already on chronic metoprolol:

• Administer metoprolol tartrate 25-50 mg orally when symptoms develop (tachycardia >100 bpm with dyspnea). 1, 2
• Reassess heart rate and blood pressure 30-60 minutes after administration. 4
• Target heart rate reduction to <100 bpm or a 20% reduction from baseline. 2, 4
• Do not exceed total daily metoprolol dose of 400 mg (including baseline chronic therapy). 5, 3

The oral route is appropriate for outpatient management; IV metoprolol (2.5-10 mg bolus) is reserved for emergency department settings. 1, 2

Critical Safety Screening Before Each PRN Dose

Before administering PRN metoprolol, verify the patient does NOT have:

• Systolic blood pressure <100 mmHg (metoprolol can precipitate hypotension). 5, 6
• Heart rate <60 bpm (risk of severe bradycardia or heart block). 5
• Active decompensated heart failure with pulmonary congestion (beta-blockers can worsen acute HF). 5
• Severe bronchospasm or active COPD exacerbation (relative contraindication). 5

The FDA label explicitly warns that beta-blockers can cause depression of myocardial contractility and precipitate heart failure. 5 However, in stable chronic heart failure patients already tolerating baseline metoprolol, PRN dosing is generally safe with monitoring. 1

Optimizing the Chronic Regimen

Rather than relying heavily on PRN dosing, consider these adjustments:

• Increase the standing metoprolol dose if breakthrough episodes occur frequently (>1-2 times weekly). 2, 3
• Switch from metoprolol tartrate to metoprolol succinate (extended-release) 50-200 mg once daily for more consistent 24-hour rate control. 2, 3
• Add digoxin 0.125-0.25 mg daily to the regimen if metoprolol alone provides inadequate control—combination therapy is a Class IIa recommendation. 2

Metoprolol CR/XL has been specifically shown effective in maintaining rate control and reducing AF recurrences in multiple studies. 3, 7

Common Pitfalls to Avoid

• Do not assess rate control adequacy based solely on resting heart rate. Many patients have acceptable resting rates but inadequate exercise rate control, which drives symptoms. 2 Have the patient check heart rate during activity (walking, climbing stairs) to guide dosing.

• Do not abruptly discontinue metoprolol if side effects occur—taper over 1-2 weeks to avoid rebound hypertension and tachycardia. 5

• Do not assume rate control eliminates stroke risk. Continue anticoagulation based on CHA₂DS₂-VASc score regardless of how well rate is controlled. 2

• Monitor for excessive bradycardia if combining metoprolol with other rate-controlling agents (diltiazem, digoxin, amiodarone). 2

When to Seek Emergency Care

Instruct the patient to go to the emergency department rather than taking PRN metoprolol if experiencing:

• Systolic BP <90 mmHg
• Severe dyspnea at rest or chest pain
• Presyncope or syncope
• Heart rate >150 bpm despite PRN dose

These scenarios may require IV rate control, cardioversion, or evaluation for acute precipitants (pulmonary embolism, myocardial infarction, thyrotoxicosis). 1