What is the recommended treatment and dosage of Metaraminol for hypotension?

Metaraminol for Hypotension: Treatment and Dosage Recommendations

Metaraminol is recommended as a second-line vasopressor for hypotension at an initial bolus dose of 0.5 mg (0.25-1.0 mg range), followed by infusion at 0.1-2.0 μg/kg/min when necessary, titrated to desired blood pressure effect. 1

Indications and Mechanism

Metaraminol is an alpha-adrenergic agonist vasopressor indicated for:

• Hypotension unresponsive to fluid resuscitation 1
• Secondary management of anaphylactic reactions 1
• Severe provocable left ventricular outflow tract obstruction (LVOTO) presenting with hypotension and pulmonary edema 1
• Hypotension during subarachnoid anesthesia 2

The drug works primarily through peripheral vasoconstriction, increasing systemic vascular resistance and blood pressure.

Dosing Guidelines

Bolus Administration

• Initial bolus dose: 0.5 mg (range 0.25-1.0 mg per 50 kg adult) 2
• For anaphylaxis: Consider after initial adrenaline and fluid resuscitation 1
• For LVOTO with hypotension: Use with oral or IV beta-blockers 1

Continuous Infusion

• Infusion rate: 0.1-2.0 μg/kg/min, titrated to desired effect 1
• Start at the lowest dose and titrate upward based on blood pressure response
• Monitor blood pressure continuously, preferably with arterial line

Clinical Considerations

Administration Route

• Preferably administer through a central venous catheter when possible 1
• When using peripheral IV, monitor closely for extravasation
• If extravasation occurs, infiltrate 5-10 mg of phentolamine diluted in 10-15 mL of saline into the site to prevent tissue necrosis 1

Specific Clinical Scenarios

1. Anaphylactic reactions:

   • Use after initial adrenaline and fluid resuscitation has failed 1
   • Consider when blood pressure remains low despite adrenaline infusion

2. Brain-injured patients with hypotension:

   • After correction of hypovolemia, use small boluses followed by infusion 1
   • Target blood pressure should be maintained at the level of the tragus

3. Hypertrophic cardiomyopathy with LVOTO:

   • Use with beta-blockers for severe provocable LVOTO presenting with hypotension and pulmonary edema 1

4. Hypotension during subarachnoid anesthesia:

   • Initial dose of 0.25 mg, increasing to 0.5 mg if necessary 2
   • Target a 25% elevation in systolic blood pressure

Monitoring and Precautions

• Continuous monitoring: Blood pressure (preferably invasive), heart rate, ECG
• Avoid in: Active hemorrhage without control of bleeding source 3
• Caution with:
  • Prolonged use at high doses may lead to excessive vasoconstriction 4
  • May increase myocardial oxygen requirements 1

Potential Benefits and Risks

Benefits

• Rapid blood pressure elevation within minutes of administration 5, 6
• May reverse myocardial ischemia associated with hypotension 5, 6

Risks

• Tissue necrosis if extravasation occurs 1
• Excessive vasoconstriction with prolonged high-dose infusion 4
• Rebound hypotension after discontinuation

Alternative Vasopressors

If metaraminol is ineffective or unavailable, consider:

• Norepinephrine: 0.1-0.5 μg/kg/min (first-line in many guidelines) 1
• Epinephrine: 0.05-0.5 μg/kg/min 1
• Phenylephrine: 0.5-2.0 μg/kg/min 1

Practical Approach to Hypotension Management

1. First steps:

   • Ensure adequate fluid resuscitation
   • Identify and treat underlying cause
   • Consider norepinephrine as first-line vasopressor

2. When to use metaraminol:

   • As second-line agent when norepinephrine is unavailable
   • For rapid temporary blood pressure support while preparing other agents
   • In specific scenarios like anaphylaxis or LVOTO with hypotension

3. Dosing strategy:

   • Start with 0.25-0.5 mg bolus
   • If continuous support needed, initiate infusion at 0.1 μg/kg/min
   • Titrate to desired blood pressure effect, not exceeding 2.0 μg/kg/min

Despite its widespread use in the UK and Australia, metaraminol has limited high-quality evidence supporting its use as a first-line vasopressor 7. Current guidelines generally position it as a secondary option after norepinephrine for most clinical scenarios.