What is the equivalent IV dose of metoprolol (beta-blocker) when converting from oral metoprolol 25mg twice daily (bid)?

Equivalent IV Dose of Metoprolol When Converting from Oral Metoprolol 25mg BID

When converting from oral metoprolol 25mg twice daily to intravenous dosing, the equivalent IV dose is 5mg given every 6 hours, or administered as three 5mg IV boluses given at 2-minute intervals followed by 5mg IV every 6 hours.

Conversion Rationale

The conversion from oral to IV metoprolol requires understanding the pharmacokinetic differences between routes of administration:

• Oral metoprolol undergoes significant first-pass metabolism, resulting in approximately 50% bioavailability
• IV administration bypasses first-pass metabolism, requiring dose adjustment
• Guidelines consistently recommend lower IV doses compared to oral doses

Recommended IV Dosing Protocol

According to the 2013 ACC/AHA guidelines for management of STEMI and the 2014 AHA/ACC/HRS guidelines for management of atrial fibrillation, the appropriate IV dosing is:

1. Initial IV dosing:

   • 5mg IV bolus over 2 minutes 1
   • May repeat up to 3 doses (total 15mg) at 5-minute intervals if needed and tolerated 1

2. Maintenance IV dosing:

   • 5mg IV every 6 hours 1
   • Titrate based on heart rate and blood pressure response

Clinical Considerations

• Monitoring: During IV administration, closely monitor:

  • Heart rate
  • Blood pressure
  • ECG
  • Signs of heart failure or bronchospasm

• Contraindications to IV metoprolol:

  • Marked first-degree AV block (PR interval >0.24 sec)
  • Second or third-degree AV block without functioning pacemaker
  • Severe left ventricular dysfunction or heart failure
  • Cardiogenic shock or high risk for shock
  • Hypotension (systolic BP <90 mmHg)
  • Significant sinus bradycardia (heart rate <50 bpm)
  • Reactive airway disease 1

• Transition back to oral therapy:

  • After stabilization on IV therapy, transition to oral metoprolol 25-50mg every 6 hours
  • Then convert to maintenance dose of 25-100mg twice daily 1

Special Populations

• Hepatic impairment: Reduce dose as metoprolol blood levels may increase substantially 2
• Elderly patients: Consider lower initial doses due to increased risk of adverse effects
• Renal impairment: No specific dose adjustment required for IV metoprolol 2

Pitfalls to Avoid

1. Avoid rapid IV administration - Always administer IV metoprolol slowly over 2 minutes to prevent severe hypotension

2. Don't overlook contraindications - Patients with decompensated heart failure, cardiogenic shock, or significant bradycardia should not receive IV metoprolol

3. Beware of drug interactions - Concomitant use of other negative chronotropic or inotropic agents may potentiate metoprolol's effects

4. Don't use excessive doses - The IV dose should be lower than the oral dose due to bypassing first-pass metabolism

Remember that the goal of therapy is to achieve the desired clinical effect (heart rate control, anti-ischemic effect) while minimizing adverse effects. Titrate carefully based on individual patient response.