- Direct sympathomimetic effect
- Acts mainly via ɑ1 adrenoreceptors although does have some β-adrenoreceptor activity
- Indirect sympathomimetic effect
- Triggers noradrenaline release from intracytoplasmic stores
Metaraminol
Metaraminol
- Metaraminol is a synthetic sympathomimetic amine
- It is a phenylethylamine derivative
- Median dose equivalence to noradrenaline is 12.5:1
Presentation
- A clear, colourless solution stored in glass vials at room temperature of concentration:
- 10mg/ml, which requires diluting in saline to desired final concentration (typically 0.5mg/ml or 1mg/ml)
- 0.5mg/ml, which can be drawn up and used as is
- Contains sodium chloride and sodium metabisulfite
- As a prefilled 'minijet' containing 5ml of 0.5mg/ml metaraminol for immediate use
Uses
- It is used to correct hypotension associated with anaesthesia and in the critically ill patient, as either intermittent boluses or via infusion
Absorption
- Oral bioavailability ~20%
- Rapid onset of 1-2mins
- Peak effect at 10mins
Distribution
- 45% protein bound
- Does not cross blood-brain barrier
Metabolism
- Hepatic metabolism although no dose adjustment required in liver failure
Excretion
- Elimination half-life 60mins
- Metabolites excreted in both urine and bile
Cardiovascular
- Increased SVR leading to an increased systolic, diastolic and mean arterial pressure
- Small positive inotropic effect
- Reflex bradycardia means cardiac output often drops a little in the face of increased SVR
- Coronary artery flow increases via increased diastolic blood pressure and bradycardia
- PVR is increased and this leads to increased PA pressure
Uteroplacental
- Can in theory cause foetal hypoxia by constricting the uterine vessels thereby limiting placental perfusion
- However, metaraminol is safe for use in the parturient