Metoprolol PO to IV Conversion
For patients converting from oral metoprolol to IV, use a conservative approach starting with 2.5-5 mg IV bolus over 1-2 minutes, regardless of the previous oral dose, rather than attempting mathematical conversion. 1, 2
Standard IV Dosing Protocol
- Administer 5 mg IV metoprolol as a slow bolus over 1-2 minutes 1, 2
- Repeat every 5 minutes as needed based on hemodynamic response 1, 2
- Maximum total dose is 15 mg (three 5 mg boluses) 1, 2
The FDA label explicitly states this protocol for acute situations, emphasizing that IV dosing should start conservatively and titrate based on clinical response rather than using the previous oral dose as a guide 2. This approach is critical because oral and IV metoprolol have approximately a 2.5:1 bioavailability ratio, meaning IV administration produces substantially higher effective drug levels 2.
Why Mathematical Conversion Is Not Recommended
- Oral metoprolol has only ~50% bioavailability due to first-pass metabolism, while IV administration bypasses this entirely 2, 3
- The relationship between oral and IV dosing is non-linear and unpredictable 1
- Starting with the "equivalent" IV dose based on oral dosing significantly increases risk of severe hypotension and bradycardia 1
Multiple perioperative studies demonstrate this principle: patients on various oral metoprolol doses (25-100 mg) all received the same conservative IV protocol (2-4 mg during surgery or 5 mg boluses postoperatively) with good outcomes 4.
Critical Contraindications Before IV Administration
Check these absolute contraindications before giving any IV metoprolol: 1, 2
- Signs of heart failure, low output state, or decompensated heart failure
- Systolic blood pressure <100-120 mmHg with symptoms
- Heart rate <50-60 bpm or >110 bpm
- Second or third-degree heart block without functioning pacemaker
- PR interval >0.24 seconds
- Active asthma or severe reactive airway disease
- Evidence of cardiogenic shock risk (age >70, Killip class >1)
The COMMIT trial demonstrated that early IV metoprolol in acute MI increased cardiogenic shock by 30%, particularly in high-risk patients, underscoring the importance of hemodynamic stability before IV administration 1, 5.
Required Monitoring During IV Administration
Continuous monitoring is mandatory: 1, 2
- Heart rate monitoring continuously
- Blood pressure checks after each bolus
- Continuous ECG monitoring
- Auscultation for new rales (pulmonary congestion)
- Auscultation for bronchospasm
Transition Back to Oral Therapy
- Start oral metoprolol tartrate 15 minutes after the last IV dose 1, 2
- Initial oral dose: 25-50 mg every 6 hours for 48 hours 1, 2
- Then transition to maintenance dosing (typically 100 mg twice daily) 2
Do not return directly to extended-release formulations immediately after IV therapy; use immediate-release metoprolol tartrate first to allow for dose titration 1.
Common Pitfalls to Avoid
- Never give the full 15 mg as a single rapid bolus - this dramatically increases adverse event risk 1
- Never attempt 1:1 or mathematical conversion from oral to IV dosing - this causes severe bradycardia and hypotension 1
- Never use IV metoprolol in decompensated heart failure - wait until hemodynamic stabilization 1, 2
- Never combine with other AV nodal blocking agents without careful monitoring - profound bradycardia can develop 1
Alternative for High-Risk Patients
Consider esmolol instead of metoprolol for patients at high risk of adverse effects: 1
- Loading dose: 500 mcg/kg over 1 minute
- Maintenance infusion: 50-300 mcg/kg/min
- Advantage: ultra-short half-life (9 minutes) allows rapid titration and quick reversal if problems occur