Metaraminol Pharmacology

Quick Answer

Metaraminol is a synthetic sympathomimetic amine used primarily as a vasopressor for the management of intraoperative hypotension, particularly during spinal anaesthesia for caesarean section. It acts through both direct α₁-adrenoceptor agonism (predominant) and indirect mechanisms involving displacement of noradrenaline from presynaptic nerve terminals. The standard bolus dose is 0.5-2 mg IV, producing a pressor response within 1-2 minutes lasting 20-60 minutes. Unlike phenylephrine (pure α₁ agonist), metaraminol's mixed mechanism may provide more favourable cardiac output preservation, though this remains debated. The primary cardiovascular effects include increased systemic vascular resistance, elevated mean arterial pressure, and reflex bradycardia due to baroreceptor activation. A critical consideration is tachyphylaxis—repeated dosing depletes endogenous noradrenaline stores, reducing efficacy of the indirect component. Metaraminol is not metabolised by MAO or COMT, contributing to its prolonged duration compared to catecholamines. In obstetric anaesthesia, metaraminol is widely used in Australia/NZ as an alternative to phenylephrine for managing spinal-induced hypotension, with comparable safety profiles for mother and fetus. [1-8]

Pharmacology Overview

Drug Classification and History

Metaraminol (marketed as Aramine®) is a synthetic non-catecholamine sympathomimetic amine belonging to the phenylethylamine class. Unlike endogenous catecholamines (adrenaline, noradrenaline, dopamine), metaraminol lacks the catechol (3,4-dihydroxybenzene) ring structure, instead possessing a single hydroxyl group at the meta position of the benzene ring. This structural difference has profound pharmacological implications: metaraminol is resistant to metabolism by catechol-O-methyltransferase (COMT) and has reduced affinity for monoamine oxidase (MAO), resulting in a significantly prolonged duration of action compared to catecholamines. [1,2]

Metaraminol was introduced into clinical practice in the 1950s and became widely used as a vasopressor for hypotensive states. Its popularity has varied geographically—it remains the vasopressor of choice for intraoperative hypotension in Australia and New Zealand, whereas phenylephrine predominates in North America. This regional preference reflects historical prescribing patterns and the practical advantages of metaraminol's longer duration of action for bolus dosing. [3,4]

Chemical Structure and Physicochemical Properties

Metaraminol bitartrate has the molecular formula C₉H₁₃NO₂·C₄H₆O₆ with a molecular weight of 317.3 Da (as the bitartrate salt). The free base (metaraminol) has a molecular weight of 167.2 Da.

Structural Features:

The meta-hydroxyl configuration (rather than the 3,4-dihydroxyl of catechols) renders metaraminol resistant to COMT degradation. The β-hydroxyl group on the side chain is essential for direct adrenoceptor agonist activity, while the primary amine group allows uptake into nerve terminals via the noradrenaline transporter (NET). Metaraminol exists as a racemic mixture, with the (-) isomer possessing greater sympathomimetic activity. The drug is water-soluble and stable in solution, allowing preparation of dilute infusions without degradation concerns. [2,5]

Mechanism of Action

Metaraminol is classified as a mixed-acting sympathomimetic, exerting cardiovascular effects through two distinct mechanisms:

1. Direct Mechanism (Primary): Metaraminol acts as a direct agonist at α₁-adrenoceptors on vascular smooth muscle cells. Activation of α₁-receptors (Gq-protein coupled) triggers the phospholipase C-IP₃-DAG pathway, leading to intracellular calcium release and smooth muscle contraction. This produces vasoconstriction of both arterial resistance vessels and venous capacitance vessels, increasing systemic vascular resistance (SVR) and venous return. Metaraminol has weak direct β₁-adrenoceptor activity, providing minimal positive inotropic effect. [1,6,7]

2. Indirect Mechanism (Secondary): Metaraminol is actively transported into sympathetic nerve terminals via the noradrenaline transporter (NET, Uptake-1). Once inside the neuron, metaraminol:

• Displaces noradrenaline from storage vesicles into the cytoplasm
• The displaced noradrenaline exits via reverse transport through NET
• Released noradrenaline then activates postsynaptic adrenoceptors

This indirect mechanism contributes approximately 30-50% of metaraminol's total pressor effect in noradrenaline-replete subjects. Importantly, the indirect component is dependent on adequate presynaptic noradrenaline stores. [1,2,8]

Receptor Selectivity Profile:

The predominant α₁-activity with minimal β-activity distinguishes metaraminol from adrenaline (mixed α/β) and makes it pharmacologically similar to phenylephrine, though with the additional indirect component.

Pharmacokinetic Principles

Absorption and Distribution

Intravenous Administration: When given intravenously (the standard route in anaesthesia), metaraminol has 100% bioavailability with an onset of action within 1-2 minutes. Peak pressor effect occurs at approximately 2-5 minutes post-bolus. [3,4]

Intramuscular Administration: Metaraminol can be given intramuscularly with an onset of 5-10 minutes. This route is occasionally used in emergency settings when IV access is unavailable. The subcutaneous route is not recommended due to risk of tissue necrosis from local vasoconstriction.

Distribution: Metaraminol distributes extensively into tissues, including sympathetic nerve terminals where it is actively taken up. The volume of distribution is approximately 3-4 L/kg, indicating significant tissue distribution beyond the plasma compartment. Protein binding is moderate (approximately 50-60%). Metaraminol crosses the placenta but to a lesser extent than catecholamines, which is relevant to its obstetric use. [9,10]

Uptake into Nerve Terminals: The noradrenaline transporter (NET) actively transports metaraminol into presynaptic nerve terminals. This uptake is:

• Sodium-dependent
• Blocked by tricyclic antidepressants (TCAs)
• Blocked by cocaine
• Blocked by SNRIs (e.g., venlafaxine)

When Uptake-1 is blocked pharmacologically, the indirect component of metaraminol's action is abolished, and its pressor effect is reduced but not eliminated (the direct component remains).

Metabolism and Elimination

Metabolism: A key pharmacological feature of metaraminol is its resistance to enzymatic degradation by the primary catecholamine-metabolising enzymes:

This metabolic resistance accounts for metaraminol's prolonged duration of action (20-60 minutes) compared to catecholamines such as noradrenaline (half-life 1-2 minutes) and adrenaline (half-life 2-3 minutes). [1,2]

Elimination: Metaraminol is primarily excreted unchanged in the urine (approximately 40-50% of administered dose). The remainder undergoes hepatic metabolism via non-specific pathways (conjugation, oxidation) to inactive metabolites. The elimination half-life is approximately 4-6 hours, though the clinical duration of pressor effect (20-60 minutes) is determined more by redistribution and tissue uptake than by elimination kinetics. [3,5]

Pharmacokinetic Summary:

Special Populations

Renal Impairment: Given significant renal excretion of unchanged drug, patients with renal impairment may have prolonged effects. Dose reduction is advisable in severe renal impairment, though formal pharmacokinetic studies are limited.

Hepatic Impairment: Metaraminol undergoes minimal hepatic metabolism; dose adjustment for hepatic impairment is generally not required.

Pregnancy: Metaraminol crosses the placenta but has been used extensively in obstetric anaesthesia without evidence of direct fetal harm at standard doses. Uteroplacental blood flow may be affected by vasoconstriction if excessive doses cause maternal hypertension.

Cardiovascular Effects

Haemodynamic Profile

The cardiovascular effects of metaraminol are characterised by:

1. Blood Pressure:

• Systolic BP: Increases significantly (typically 20-40 mmHg with 0.5-1 mg bolus)
• Diastolic BP: Increases proportionately
• Mean Arterial Pressure (MAP): Reliably increased
• Pulse Pressure: May narrow due to increased diastolic pressure

2. Heart Rate:

• Reflex Bradycardia: The predominant chronotropic effect
• Mechanism: Increased MAP activates carotid baroreceptors → vagal efferent activation → reduced sinoatrial node automaticity
• Magnitude: HR may decrease by 10-20 bpm following a single bolus
• This bradycardia is a baroreceptor-mediated reflex, NOT a direct cardiac effect

3. Cardiac Output:

• Variable effect: May decrease, remain stable, or slightly increase
• Decreased HR tends to reduce cardiac output
• Increased preload (venoconstriction) may increase stroke volume
• Net effect depends on balance of these factors
• Generally more cardiac output preservation than pure α₁ agonists (phenylephrine) due to mild β₁ activity and indirect noradrenaline release [6,7,11]

4. Systemic Vascular Resistance:

• Significant increase due to α₁-mediated vasoconstriction
• Affects both resistance and capacitance vessels
• Splanchnic and renal vasoconstriction may reduce organ perfusion

Comparison with Phenylephrine

The comparison between metaraminol and phenylephrine is a frequent examination topic:

The longer duration of metaraminol makes it more suitable for intermittent bolus dosing, whereas phenylephrine's shorter duration favours infusion-based administration. Phenylephrine's pure α₁ mechanism produces more predictable reflex bradycardia and cardiac output reduction. [12,13]

Reflex Bradycardia

Metaraminol-induced bradycardia warrants specific understanding:

Mechanism:

1. α₁-agonism increases systemic vascular resistance
2. Increased SVR elevates mean arterial pressure
3. Elevated MAP stretches carotid sinus baroreceptors
4. Baroreceptor afferents (cranial nerve IX) signal to medullary cardiovascular centres
5. Increased parasympathetic (vagal) efferent output to the heart
6. Vagal activation reduces sinoatrial node discharge rate → bradycardia
7. Concurrent inhibition of sympathetic cardiac efferents

Clinical Considerations:

• The bradycardia is physiological and generally well-tolerated
• Severe bradycardia (HR <50) may require atropine
• Patients on β-blockers have attenuated compensatory chronotropic responses
• Caution in patients with conduction abnormalities

Clinical Pharmacology

Indications in Anaesthesia

1. Intraoperative Hypotension: Metaraminol is the most commonly used vasopressor for treating intraoperative hypotension in Australia and New Zealand. Hypotension during general or regional anaesthesia results from:

• Sympathetic blockade (spinal/epidural)
• Anaesthetic-induced vasodilation (propofol, volatile agents)
• Reduced venous return (positioning, positive pressure ventilation)
• Hypovolaemia

Metaraminol restores blood pressure by increasing SVR and (via indirect effects) supporting cardiac output. [3,4,14]

2. Spinal Hypotension in Obstetric Anaesthesia: Metaraminol is widely used in Australia/NZ for managing hypotension following spinal anaesthesia for caesarean section. Spinal anaesthesia causes sympathetic blockade leading to:

• Arterial vasodilation
• Venous pooling
• Reduced preload and cardiac output
• Hypotension (occurs in 70-80% of patients without prophylaxis)

International guidelines recommend phenylephrine as first-line, but metaraminol is an acceptable alternative with a favourable evidence base. [11,15,16]

3. Septic Shock (Limited Role): Metaraminol is NOT recommended as first-line vasopressor in septic shock due to:

• Catecholamine depletion in sepsis limits indirect effect
• Tachyphylaxis with prolonged use
• Noradrenaline is preferred (direct mechanism, titratable)

Dosing and Administration

Bolus Dosing (Intraoperative Hypotension):

Dilution:

• Neat ampoule: 10 mg/mL
• Common dilution: 0.5 mg/mL (10 mg in 20 mL = 0.5 mg/mL)
• Or: 1 mg/mL (10 mg in 10 mL)

Infusion (Less Common):

• Concentration: 10-40 mg in 250-500 mL (20-80 mcg/mL)
• Rate: Titrated to maintain MAP
• Tachyphylaxis limits prolonged infusion efficacy

Obstetric Use: Spinal Hypotension

Metaraminol is extensively used for managing spinal-induced hypotension during caesarean section. Key considerations: [11,15-18]

Evidence Base:

• Multiple RCTs compare metaraminol to phenylephrine
• No significant difference in neonatal outcomes (Apgar scores, umbilical cord gases)
• Metaraminol may produce less bradycardia than phenylephrine
• Maternal cardiac output may be better maintained with metaraminol

Dosing in Obstetrics:

• Bolus: 0.25-0.5 mg IV increments
• Some centres use prophylactic bolus at time of spinal injection
• Infusion: 0.25-0.5 mg/min, titrated to MAP ≥80% baseline

Fetal Considerations:

• Metaraminol crosses placenta but minimal direct fetal effects at standard doses
• Excessive maternal hypertension → uteroplacental vasoconstriction → fetal compromise
• Goal: Maintain maternal BP at baseline, avoid both hypo- and hypertension

Comparison with Phenylephrine in Obstetrics:

Tachyphylaxis

Tachyphylaxis (diminished response to repeated doses) is a critical concept in metaraminol pharmacology and a frequent examination topic.

Mechanism of Tachyphylaxis

1. Catecholamine Depletion: The indirect component of metaraminol's action depends on displacement of noradrenaline from presynaptic vesicles. With repeated dosing:

• Noradrenaline stores become progressively depleted
• Less noradrenaline available for displacement
• Indirect effect diminishes → reduced pressor response
• Eventually, only the direct α₁ effect remains (approximately 50-70% of initial response)

2. False Neurotransmitter Effect: Metaraminol is taken up into storage vesicles and stored in place of noradrenaline. When the nerve fires:

• Metaraminol is released instead of noradrenaline
• Metaraminol is a weaker α-agonist than noradrenaline
• Net sympathetic transmission is reduced

Clinical Implications

• Response diminishes with repeated boluses over 1-2 hours
• Switch to direct agonist: If patient becomes refractory to metaraminol, switch to noradrenaline or phenylephrine (purely direct mechanisms not subject to catecholamine depletion)
• Not seen with phenylephrine: Pure direct agonists do not exhibit this phenomenon
• Catecholamine-depleted states: Patients with chronic heart failure, on reserpine, or with prolonged critical illness may have baseline catecholamine depletion → blunted initial response to metaraminol

Drug Interactions Affecting Tachyphylaxis

Adverse Effects and Complications

Common Adverse Effects

Serious Adverse Effects

1. Severe Bradycardia:

• May occur particularly in patients on β-blockers
• Treat with atropine 0.4-0.6 mg IV if HR <50 with symptoms

2. Tissue Necrosis:

• Extravasation causes local vasoconstriction → ischaemia
• Prevention: Secure IV access, dilute solution
• Treatment: Local infiltration with phentolamine (α-blocker)

3. Pulmonary Oedema:

• Rare; due to increased afterload in patients with heart failure
• Caution in patients with impaired LV function

4. Arrhythmias:

• Rare with metaraminol (less arrhythmogenic than catecholamines)
• May occur with excessive doses or in sensitised myocardium (halothane)

Contraindications

Drug Interactions

Australian/NZ Specific Considerations

TGA-Approved Formulations

Metaraminol is TGA-approved in Australia as:

The standard presentation is a 1 mL ampoule containing 10 mg metaraminol bitartrate. The solution is clear and colourless, stable at room temperature (below 25°C), and does not require refrigeration.

Clinical Practice in Australia/NZ

Metaraminol is the dominant vasopressor for intraoperative hypotension in Australia and New Zealand, in contrast to North America where phenylephrine is more commonly used. This regional preference reflects:

• Historical availability and familiarity
• Practical advantages of longer duration for bolus dosing
• Established safety profile over decades of use
• Inertia—generations of anaesthetists trained with metaraminol

ANZCA does not mandate a specific vasopressor; both metaraminol and phenylephrine are acceptable for managing intraoperative hypotension.

Availability and Cost

• Widely available in all Australian/NZ hospitals
• Relatively inexpensive
• Not PBS-listed for community use (hospital supply only)
• Generic formulations available

Indigenous Health Considerations

Aboriginal and Torres Strait Islander peoples may have specific considerations relevant to metaraminol use in the perioperative setting. Higher rates of cardiovascular disease, including ischaemic heart disease and hypertension, are documented in Indigenous Australian populations, potentially increasing sensitivity to vasopressor-induced haemodynamic changes. Chronic kidney disease prevalence is significantly elevated, which may prolong metaraminol elimination and duration of effect due to reduced renal clearance. When dosing metaraminol in Indigenous patients with known or suspected renal impairment, consider starting with lower doses and titrating to effect.

Remote and rural communities with high Indigenous populations may have limited access to continuous haemodynamic monitoring. In these settings, the longer duration of metaraminol (compared to phenylephrine) may be advantageous for intermittent bolus dosing without an infusion pump. Cultural safety principles should guide all anaesthetic care, with appropriate involvement of Aboriginal Health Workers where available. Communication regarding anaesthetic interventions, including vasopressor administration, should be culturally appropriate and involve family members consistent with Indigenous concepts of collective decision-making. Māori health considerations in New Zealand similarly emphasise whānau involvement and culturally safe perioperative care.

ANZCA Primary Exam Focus

Common MCQ Patterns

ANZCA Primary MCQs frequently test the following metaraminol concepts:

1. Mechanism of action: Mixed sympathomimetic—direct α₁ agonism AND indirect noradrenaline release
2. Comparison with phenylephrine: Mixed vs pure direct; longer vs shorter duration
3. Tachyphylaxis: Mechanism (catecholamine depletion), clinical significance
4. Reflex bradycardia: Baroreceptor-mediated, not direct cardiac effect
5. Metabolism: NOT metabolised by MAO or COMT → prolonged duration
6. Drug interactions: TCAs abolish indirect effect; MAOIs potentiate
7. Structural difference from catecholamines: Meta-hydroxyl, not catechol ring

Primary Viva Question Themes

Viva scenarios typically explore:

• Choice of vasopressor for spinal hypotension in obstetrics
• Management of metaraminol-refractory hypotension
• Comparison of vasopressor mechanisms
• Drug interactions (patient on TCA presenting for surgery)
• Tachyphylaxis mechanism and management

Calculation Questions

Candidates should be comfortable with:

• Dilution calculations (10 mg in 20 mL = 0.5 mg/mL)
• Bolus dose selection based on clinical scenario
• Infusion rate calculations (less common for metaraminol)

Assessment Content

SAQ Practice Question (20 marks)

Question:

A 32-year-old primigravida at 39 weeks gestation is scheduled for elective caesarean section under spinal anaesthesia. Her baseline blood pressure is 118/72 mmHg. Following intrathecal injection of hyperbaric bupivacaine, her blood pressure decreases to 78/50 mmHg with a heart rate of 92 bpm.

(a) Describe the mechanism of action of metaraminol, explaining both the direct and indirect components. (6 marks)

(b) Outline the pharmacokinetic properties that distinguish metaraminol from catecholamine vasopressors such as noradrenaline. Explain how these differences affect its clinical use. (5 marks)

(c) Define tachyphylaxis in the context of metaraminol. Explain the mechanism and describe how you would manage a patient who becomes refractory to metaraminol. (5 marks)

(d) Compare metaraminol with phenylephrine for the management of spinal hypotension during caesarean section. What are the advantages and disadvantages of each agent? (4 marks)

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Model Answer:

(a) Mechanism of Action (6 marks)

Metaraminol is a mixed-acting sympathomimetic with both direct and indirect mechanisms:

Direct Mechanism (3 marks):

• Metaraminol acts as a direct agonist at α₁-adrenoceptors on vascular smooth muscle
• α₁-receptor activation (Gq-coupled) triggers phospholipase C → IP₃ → intracellular Ca²⁺ release
• Results in smooth muscle contraction and vasoconstriction
• Increases systemic vascular resistance (SVR) and blood pressure
• Has weak direct β₁-activity providing minimal positive inotropy

Indirect Mechanism (3 marks):

• Metaraminol is taken up into sympathetic nerve terminals via the noradrenaline transporter (NET/Uptake-1)
• Once inside, it displaces noradrenaline from storage vesicles into the cytoplasm
• Noradrenaline exits the neuron via reverse transport through NET
• Released noradrenaline activates postsynaptic adrenoceptors
• This indirect component contributes approximately 30-50% of total pressor effect
• Dependent on adequate presynaptic noradrenaline stores

(b) Pharmacokinetic Properties (5 marks)

Clinical Implications:

• Metaraminol's COMT/MAO resistance produces prolonged duration
• Suitable for intermittent bolus dosing (rather than infusion)
• Noradrenaline requires continuous infusion due to rapid degradation
• Metaraminol more practical in settings without infusion pumps

(c) Tachyphylaxis (5 marks)

Definition (1 mark): Tachyphylaxis is a rapidly diminishing response to successive doses of a drug, distinct from tolerance which develops over longer timeframes.

Mechanism in Metaraminol (3 marks):

• The indirect component depends on displacement of noradrenaline from presynaptic vesicles
• With repeated metaraminol doses, noradrenaline stores become progressively depleted
• Less noradrenaline is available for displacement → reduced indirect effect
• Additionally, metaraminol is stored in vesicles as a "false neurotransmitter"
• Metaraminol released during nerve firing is a weaker agonist than noradrenaline
• Eventually, only the direct α₁ effect remains (50-70% of initial response)

Management of Refractory Patient (1 mark):

• Recognise diminishing response despite adequate dosing
• Switch to a purely direct-acting vasopressor:
  • Phenylephrine (pure α₁ agonist, no indirect component)
  • Noradrenaline (direct α₁ and β₁ agonist)
• These agents' efficacy is independent of presynaptic catecholamine stores

(d) Comparison: Metaraminol vs Phenylephrine (4 marks)

Advantages of Metaraminol:

• Longer duration suits bolus dosing
• May preserve cardiac output better (indirect NE release, mild β₁ activity)
• Widely used and familiar in Australia/NZ

Disadvantages of Metaraminol:

• Tachyphylaxis with repeated dosing
• Less predictable response
• Requires consideration of drug interactions (TCAs)

Advantages of Phenylephrine:

• Pure, predictable mechanism
• No tachyphylaxis
• International guideline recommendation (first-line)

Disadvantages of Phenylephrine:

• Short duration requires infusion or frequent boluses
• More pronounced bradycardia
• May reduce cardiac output more significantly

Total: 20 marks

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Primary Viva Scenario (15 marks)

Examiner: You are the anaesthetist for a 45-year-old man undergoing laparoscopic cholecystectomy. He is on amitriptyline 50 mg daily for chronic pain. After induction with propofol and fentanyl, his blood pressure drops to 75/45 mmHg. You administer metaraminol 1 mg IV, but the response is minimal. How do you explain this and what would you do next?

Candidate:

Analysis of Poor Response (4 marks):

"The poor response to metaraminol in this patient is likely due to the interaction with amitriptyline, a tricyclic antidepressant.

TCAs block the noradrenaline transporter (NET, Uptake-1), which is responsible for the reuptake of noradrenaline into presynaptic nerve terminals. Metaraminol relies on NET for its indirect mechanism—it is taken up into the nerve terminal and displaces noradrenaline from storage vesicles.

When NET is blocked by amitriptyline:

• Metaraminol cannot enter the presynaptic terminal
• The indirect component (approximately 30-50% of effect) is abolished
• Only the direct α₁-agonist effect remains

This results in a significantly blunted pressor response."

Examiner: What would you do now?

Candidate:

Management Strategy (4 marks):

"I would take the following steps:

1. Increase the metaraminol dose: Since the direct effect is preserved, a higher dose (2-3 mg) may achieve adequate response through the remaining direct α₁-agonism.

2. Switch to a direct-acting vasopressor: If higher-dose metaraminol is ineffective, I would switch to a pure direct-acting agent:

   • Phenylephrine 50-100 mcg IV boluses — pure α₁-agonist, mechanism independent of NET
   • Noradrenaline infusion — if profound hypotension persists

3. Address reversible factors: While managing pharmacologically, I would:

   • Ensure adequate intravascular volume (IV fluid bolus)
   • Lighten anaesthesia if appropriate
   • Optimise positioning (Trendelenburg)
   • Check for surgical causes (bleeding, pneumoperitoneum effects)

4. Avoid adrenaline initially: In patients on TCAs, direct-acting agents are preferred. Adrenaline can be used cautiously but may have exaggerated effects due to NET blockade preventing its reuptake."

Examiner: The patient responds well to phenylephrine. Can you explain why phenylephrine works when metaraminol did not?

Candidate:

Explanation of Differential Response (3 marks):

"Phenylephrine is a pure, direct-acting α₁-adrenoceptor agonist. Its mechanism is entirely post-synaptic—it binds directly to α₁-receptors on vascular smooth muscle and causes vasoconstriction.

Phenylephrine does NOT:

• Require uptake into nerve terminals
• Depend on displacement of stored noradrenaline
• Have any indirect sympathomimetic component

Therefore, NET blockade by amitriptyline has no effect on phenylephrine's mechanism. The full pressor response is preserved regardless of Uptake-1 inhibition.

In contrast, metaraminol relies on NET-mediated uptake for its indirect effect. When this pathway is blocked, approximately half of its pressor effect is lost, and higher doses are required to achieve the same clinical response through the remaining direct mechanism."

Examiner: What if this patient was on an MAO inhibitor instead of a TCA? How would that change your approach?

Candidate:

MAO Inhibitor Interaction (4 marks):

"The interaction with MAO inhibitors is quite different and potentially more dangerous.

MAO inhibitors (such as phenelzine, tranylcypromine) block the enzyme monoamine oxidase, which normally degrades noradrenaline within nerve terminals. This leads to:

• Increased noradrenaline stores in presynaptic vesicles
• Enhanced pressor response to indirect sympathomimetics

With metaraminol in a patient on MAOIs:

• More noradrenaline available for displacement
• Exaggerated indirect effect
• Risk of severe hypertensive crisis

Management in MAOI patients:

1. Avoid indirect sympathomimetics (metaraminol, ephedrine) if possible
2. If vasopressor required, use direct-acting agents at reduced doses:
   • Phenylephrine starting at 25-50 mcg
   • Titrate cautiously
3. Have vasodilators available (GTN, phentolamine)
4. Invasive BP monitoring if significant vasopressor anticipated

The key difference: TCAs reduce metaraminol response (need more drug), while MAOIs potentiate it (use less drug or different agent)."

Examiner: Thank you. That was a thorough discussion.

Total: 15 marks

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References

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This content is designed for ANZCA Primary Examination preparation. Always verify current guidelines and local protocols.