Etomidate Pharmacology

Quick Answer

Etomidate is an imidazole-derived intravenous anaesthetic agent distinguished by its remarkable haemodynamic stability, making it the induction agent of choice for patients with cardiovascular compromise or haemodynamic instability. Acting primarily as a positive allosteric modulator of GABA_A receptors at the beta-2 and beta-3 subunits, etomidate produces rapid-onset hypnosis (one arm-brain circulation time, 15-45 seconds) with minimal cardiovascular depression - typically only 10-15% reduction in mean arterial pressure compared to 25-40% with propofol. The critical limitation is dose-dependent adrenal suppression through inhibition of 11-beta-hydroxylase (CYP11B1), blocking cortisol synthesis for 4-24 hours after a single induction dose. This adrenal suppression, while clinically insignificant for single-dose induction in most patients, renders etomidate infusions contraindicated and requires careful consideration in septic patients. Other adverse effects include myoclonus (33-80% of patients), pain on injection (10-30%), postoperative nausea and vomiting (PONV, 30-40%), and suppression of adrenocortical response to stress. Etomidate lacks analgesic properties and is formulated as either a propylene glycol solution (Hypnomidate) or lipid emulsion (Etomidate-Lipuro) at 2 mg/mL. Induction dose is 0.2-0.3 mg/kg IV, with reduced doses (0.1-0.2 mg/kg) in elderly patients. [1-8]

Pharmacology Overview

Chemical Classification and Structure

Etomidate (R-(+)-ethyl-1-(alpha-methylbenzyl)imidazole-5-carboxylate) is a carboxylated imidazole derivative first synthesized by Janssen Pharmaceutica in 1964 and introduced into clinical practice in 1972. The molecular structure consists of an imidazole ring with a carboxylate ester group at position 5 and an alpha-methylbenzyl substituent at position 1. This unique structure confers several important properties: high lipid solubility (octanol:water partition coefficient approximately 1,000:1 at physiological pH), rapid blood-brain barrier penetration, and the characteristic imidazole ring responsible for adrenal enzyme inhibition. Etomidate exists as a racemic mixture of two enantiomers, but only the R(+)-enantiomer possesses significant anaesthetic activity, being approximately 10-fold more potent than the S(-)-enantiomer. The commercial preparation contains only the active R(+)-isomer. The molecular weight is 244.3 Da, and the pKa of 4.2 means the drug is predominantly unionized (>99%) at physiological pH, facilitating rapid CNS penetration. The imidazole ring, while conferring favourable anaesthetic properties, is also responsible for the problematic adrenal suppression through structural similarity to ketoconazole and other imidazole antifungals that inhibit cytochrome P450 enzymes in steroidogenic pathways. [9-15]

Formulation Considerations

Two commercial formulations of etomidate are available, with important clinical differences. The original formulation uses 35% propylene glycol as the solvent at pH 6.0, which causes significant pain on injection (reported in 10-50% of patients) and carries theoretical risks of propylene glycol toxicity with repeated dosing. The lipid emulsion formulation (Etomidate-Lipuro, containing 10% medium-chain and long-chain triglycerides) was developed to reduce injection pain and improve tolerability. Clinical studies demonstrate 60-80% reduction in injection pain with the lipid formulation compared to propylene glycol. The lipid emulsion also reduces the incidence of venous sequelae (thrombophlebitis, venous irritation) and may reduce myoclonus frequency by 20-30%, though the mechanism for this reduction is unclear. Both formulations are standardised at 2 mg/mL concentration. The lipid formulation shares the microbiological growth concerns of propofol emulsions and should be discarded within 6-12 hours of opening. In Australia, both formulations are available, though the lipid emulsion is increasingly preferred due to improved tolerability. Storage is at 2-25°C (room temperature stable), and the solution should not be refrigerated or frozen. [16-22]

Molecular Mechanism of Action

Etomidate's primary mechanism of action involves positive allosteric modulation of gamma-aminobutyric acid type A (GABA_A) receptors, the principal inhibitory neurotransmitter receptors in the central nervous system. Etomidate binds to a specific site on the beta-subunit of the GABA_A receptor complex, distinct from the binding sites of benzodiazepines (alpha-gamma interface), barbiturates, and propofol (though overlapping with propofol at the beta-subunit). Specifically, etomidate demonstrates high affinity for receptors containing beta-2 and beta-3 subunits, with minimal activity at beta-1-containing receptors. This binding increases the affinity of the receptor for GABA and prolongs the duration of chloride channel opening, resulting in enhanced chloride influx, membrane hyperpolarization, and decreased neuronal excitability. At higher concentrations, etomidate can directly activate GABA_A receptors in the absence of GABA, similar to propofol and barbiturates. The subunit selectivity (beta-2/beta-3 preference) explains the predominant hypnotic effects with relative sparing of anxiolytic properties (mediated by alpha-2/alpha-3 subunits) seen with benzodiazepines. Unlike propofol, etomidate has minimal effects on NMDA glutamate receptors, glycine receptors, or other neurotransmitter systems, resulting in a more "pure" GABAergic mechanism and explaining the lack of analgesic properties. [23-30]

GABA_A Receptor Subtype Selectivity

GABA_A receptors are pentameric ligand-gated chloride channels assembled from combinations of 19 possible subunits (alpha 1-6, beta 1-3, gamma 1-3, delta, epsilon, theta, pi, and rho 1-3). The most common synaptic GABA_A receptors contain two alpha, two beta, and one gamma subunit in a specific arrangement. Etomidate demonstrates remarkable selectivity for beta-2 and beta-3-containing GABA_A receptors, with the beta-2 subunit showing the highest affinity. Genetic studies using knock-in mice expressing etomidate-insensitive beta subunits have demonstrated that beta-3-containing receptors mediate the hypnotic/immobilising effects of etomidate, while beta-2-containing receptors contribute to both hypnosis and the sedative effects. The beta-1 subunit, which predominates in spinal cord motor neurons, shows minimal sensitivity to etomidate at clinical concentrations. This subunit selectivity has implications for the development of newer analogues that might separate the desirable anaesthetic effects from the problematic adrenal suppression, as the adrenal inhibition is mediated through a distinct mechanism (cytochrome P450 inhibition) rather than GABA_A modulation. Research into etomidate analogues (carboetomidate, MOC-etomidate) that retain GABA_A modulation but reduce CYP11B1 inhibition is ongoing but has not yet yielded clinically available alternatives. [31-38]

Pharmacokinetic Principles

Absorption and Distribution

Etomidate is administered exclusively by intravenous injection due to extensive first-pass metabolism and poor oral bioavailability. Following intravenous bolus administration, etomidate demonstrates extremely rapid onset of action, with loss of consciousness typically occurring within 15-45 seconds (one arm-brain circulation time) depending on the dose, cardiac output, and injection speed. This rapid onset is explained by etomidate's high lipid solubility allowing rapid equilibration across the blood-brain barrier. The volume of distribution (Vd) is moderate, ranging from 2.5-4.5 L/kg, reflecting extensive tissue distribution though slightly less than propofol. Distribution follows a three-compartment model with an initial rapid distribution phase (t1/2alpha 2-4 minutes) to highly perfused tissues including brain, heart, and kidneys; an intermediate redistribution phase (15-25 minutes) to muscle; and a slow terminal phase (1-5 hours) involving adipose tissue. The duration of anaesthetic effect from a single bolus dose (0.3 mg/kg) is 3-5 minutes, determined primarily by rapid redistribution from the brain to peripheral tissues rather than elimination. Recovery of consciousness occurs as plasma concentration falls below the threshold for hypnosis (approximately 150-300 ng/mL), typically within 5-10 minutes after a single induction dose. [39-46]

Protein Binding

Etomidate is extensively bound to plasma proteins, with approximately 75-77% bound at therapeutic concentrations. The primary binding protein is albumin, with some contribution from alpha-1-acid glycoprotein (AAG). This moderate protein binding (compared to propofol's 97-99%) means a higher free fraction is available for pharmacological effect. Conditions affecting plasma protein concentrations significantly alter etomidate pharmacokinetics and may affect dosing requirements. Hypoalbuminaemia (hepatic failure, nephrotic syndrome, malnutrition, critical illness) increases the free fraction and enhances pharmacological effect, potentially requiring dose reduction of 20-30%. Conversely, conditions with elevated AAG (acute phase response, chronic inflammation, malignancy) may reduce free fraction and theoretically increase dose requirements, though this is rarely clinically significant. Renal failure increases free fraction due to competitive displacement by uraemic toxins and reduced protein binding capacity. Protein binding is concentration-independent across the therapeutic range, meaning pharmacokinetics remain linear. In pregnancy, reduced albumin concentrations and haemodilution increase free fraction, potentially enhancing sensitivity, though etomidate's cardiovascular stability makes it suitable for caesarean section induction when indicated. [47-52]

Metabolism

Etomidate undergoes rapid and extensive hepatic metabolism primarily through ester hydrolysis, with the carboxylate ester group at position 5 of the imidazole ring cleaved by hepatic esterases (primarily carboxylesterases, not plasma cholinesterases) and non-specific esterases. The primary metabolite is the carboxylic acid derivative (etomidate acid), which is pharmacologically inactive and has no GABA_A receptor activity or adrenal suppressive effects. Ester hydrolysis accounts for approximately 80% of etomidate metabolism, with minor pathways including ring hydroxylation via cytochrome P450 enzymes. The hepatic extraction ratio is high (approximately 0.6-0.9), indicating flow-limited metabolism with clearance dependent on hepatic blood flow. Total plasma clearance is 18-25 mL/kg/min (1.0-1.4 L/min in a 70 kg adult), which is high but less than propofol. The context-sensitive half-time remains relatively short (20-40 minutes after 2-hour infusion) due to rapid ester hydrolysis, though prolonged infusions are contraindicated due to adrenal suppression rather than pharmacokinetic concerns. Hepatic impairment may reduce clearance and prolong elimination half-life, requiring dose reduction of 25-50% in severe hepatic dysfunction. Importantly, plasma cholinesterase (pseudocholinesterase) deficiency does not affect etomidate metabolism, as the drug is hydrolysed by different esterase systems. [53-60]

Elimination

The terminal elimination half-life of etomidate is 2.9-5.3 hours in healthy adults, longer than propofol but shorter than thiopental. However, like other highly lipophilic anaesthetics, the elimination half-life is clinically less relevant than the context-sensitive half-time for predicting recovery. Elimination is predominantly renal, with approximately 75% of the dose excreted in urine as the inactive carboxylic acid metabolite and its glucuronide conjugates. Only 2-3% of unchanged etomidate is excreted renally. Biliary excretion accounts for approximately 10-13% of elimination. Renal impairment has minimal direct effect on etomidate clearance as the parent drug is not significantly renally eliminated; however, accumulation of inactive metabolites may occur in severe renal failure (clinical significance unclear). The rapid ester hydrolysis ensures predictable emergence even with moderate hepatic or renal dysfunction, making etomidate relatively safe across a range of organ function. Age-related decline in hepatic blood flow and esterase activity may reduce clearance in elderly patients by 20-30%, contributing to increased sensitivity and the recommendation for dose reduction. [61-68]

Pharmacokinetics in Special Populations

Elderly patients demonstrate significantly increased sensitivity to etomidate due to multiple factors: reduced cardiac output slowing initial distribution and increasing brain exposure; reduced hepatic blood flow and esterase activity decreasing clearance by 20-30%; potential changes in receptor sensitivity; and reduced plasma protein binding increasing free fraction. Induction dose should be reduced by 30-50% in patients over 65 years (0.1-0.2 mg/kg rather than 0.2-0.3 mg/kg), with careful titration to effect.

Paediatric patients have higher weight-normalised clearance and larger volume of distribution relative to adults. Children 1-12 years may require higher weight-based doses (0.3-0.4 mg/kg) for reliable induction. Neonates and infants have reduced clearance and immature esterase systems, though data are limited and etomidate is not commonly used in this population.

Obese patients present complex considerations. Loading dose should be based on lean body weight to avoid overdose, as the increased adipose tissue does not proportionally increase the initial volume of distribution. Clearance may be increased in obesity, but duration of action from a single dose is determined primarily by redistribution.

Critically ill patients with capillary leak syndrome have increased Vd for hydrophilic drugs but etomidate, being lipophilic, shows less dramatic volume changes. Reduced cardiac output prolongs arm-brain circulation time and may enhance initial brain concentrations from a given dose. Hypoalbuminaemia increases free fraction. Most importantly, critical illness often involves adrenal stress responses, and the adrenal suppressive effects of etomidate are most problematic in this population. [69-75]

Pharmacodynamics

Central Nervous System Effects

Etomidate produces dose-dependent CNS depression manifesting as sedation, hypnosis, and anaesthesia without significant analgesia. The concentration-effect relationship follows a sigmoid Emax model, with typical effect-site concentrations of 150-300 ng/mL for loss of consciousness and 300-500 ng/mL for surgical anaesthesia. Cerebral metabolic rate for oxygen (CMRO2) is reduced by 35-45% at anaesthetic doses, comparable to propofol and barbiturates. Cerebral blood flow (CBF) decreases in parallel with CMRO2, maintaining the CBF/CMRO2 ratio and providing favourable conditions for neurological surgery. Intracranial pressure (ICP) decreases with etomidate due to reduced CBF and cerebral blood volume, making it suitable for patients with elevated ICP. Unlike propofol, etomidate does not produce burst suppression on EEG at clinical doses and cannot reliably achieve electrocerebral silence. The electroencephalographic pattern shows progressive slowing from alpha to theta to delta frequencies with increasing dose. Etomidate has been associated with epileptiform activity on EEG and may activate seizure foci in patients with epilepsy; however, paradoxically, it has also been used to terminate status epilepticus. This apparent contradiction reflects the complex relationship between GABAergic enhancement and cortical excitability. Etomidate lacks significant analgesic properties, and painful stimuli during etomidate anaesthesia produce robust autonomic responses unless supplemented with opioids. [76-82]

Myoclonus Mechanism

Myoclonus is the most common and distinctive adverse effect of etomidate, occurring in 33-80% of patients depending on dose, injection speed, and premedication. These involuntary muscle movements typically manifest as brief, non-rhythmic contractions of the face, trunk, and limbs, occurring during or shortly after injection, before loss of consciousness. The mechanism is not fully understood but appears to involve disinhibition of subcortical structures rather than direct excitation. The proposed mechanism involves differential sensitivity of cortical and subcortical neural structures to etomidate's GABAergic effects: cortical inhibition occurs at lower concentrations than subcortical inhibition, creating a "transient imbalance" where subcortical motor nuclei are disinhibited while consciousness is depressed but before deeper anaesthesia suppresses subcortical activity. This hypothesis is supported by the observation that slower injection (reducing the rate of concentration change) and premedication with benzodiazepines or opioids (providing baseline cortical and subcortical inhibition) reduce myoclonus incidence. Importantly, etomidate myoclonus is not epileptiform activity - EEG studies show no ictal patterns during myoclonus, and the movements are subcortical in origin. Prevention strategies include: slow injection over 60 seconds (reduces incidence by 30-50%); pretreatment with midazolam 0.03-0.05 mg/kg (reduces incidence by 50-70%); pretreatment with fentanyl 1-2 mcg/kg (reduces incidence by 50-60%); or pretreatment with a small dose of etomidate 0.03 mg/kg 90 seconds before induction. [83-90]

Cardiovascular Effects

The cardiovascular stability of etomidate is its most clinically significant pharmacodynamic property and the primary reason for its continued use despite adrenal suppression concerns. At standard induction doses (0.3 mg/kg), mean arterial pressure decreases by only 10-15% (compared to 25-40% with propofol), systemic vascular resistance is minimally affected, cardiac output is well maintained, and heart rate may increase slightly (5-15%) or remain unchanged. The baroreceptor reflex remains intact, allowing appropriate compensatory responses to haemodynamic perturbations. The mechanisms underlying this cardiovascular stability include: minimal direct myocardial depression; absence of significant histamine release; preservation of central and peripheral sympathetic tone; and lack of the direct vasodilatory effects seen with propofol. In isolated heart preparations, etomidate does produce mild negative inotropy, but this is counterbalanced in vivo by maintained sympathetic tone. Coronary blood flow is well maintained, and myocardial oxygen supply-demand ratio is not significantly altered. This profile makes etomidate advantageous in: haemodynamically unstable patients (hypovolaemia, haemorrhagic shock, septic shock); patients with severe cardiac disease (cardiomyopathy, severe aortic stenosis, cardiac tamponade); patients dependent on preload and afterload (constrictive pericarditis); and patients with coronary artery disease where hypotension-induced ischaemia is a concern. [91-98]

Respiratory Effects

Etomidate causes respiratory depression that is generally less pronounced than with propofol or barbiturates but still clinically significant. At induction doses, transient apnoea occurs in approximately 30-50% of patients (compared to 70-100% with propofol), though the duration is typically brief (30-60 seconds). Respiratory rate and tidal volume are reduced, and the ventilatory response to hypercapnia is blunted, similar to other intravenous anaesthetics. Upper airway reflexes are better preserved than with propofol, though not as completely as with ketamine. Coughing and hiccupping may occur during induction, particularly without opioid premedication. The relatively preserved respiratory function makes etomidate useful in patients where maintaining spontaneous ventilation is desirable or where brief apnoea may be poorly tolerated. However, in patients requiring rapid sequence induction with immediate intubation, the respiratory depression is sufficient to facilitate laryngoscopy when combined with a neuromuscular blocking agent. Etomidate has no significant bronchodilatory or bronchoconstrictive effects and is suitable for patients with reactive airway disease. Unlike ketamine, etomidate does not significantly increase salivation and does not routinely require antisialagogue pretreatment. [99-105]

Adrenal Suppression: The Critical Concern

The adrenal suppressive effect of etomidate is the defining limitation of this otherwise favourable anaesthetic agent. Etomidate causes dose-dependent, reversible inhibition of adrenal steroidogenesis through direct inhibition of mitochondrial cytochrome P450 enzymes, particularly 11-beta-hydroxylase (CYP11B1), which catalyses the final step in cortisol synthesis (conversion of 11-deoxycortisol to cortisol). The imidazole ring of etomidate binds to the haem iron of CYP11B1, competitively inhibiting enzyme function. This same mechanism underlies the antifungal activity of imidazole antifungals (ketoconazole, miconazole) which also inhibit CYP11B1 and are used therapeutically to treat Cushing syndrome.

Following a single induction dose of etomidate (0.3 mg/kg), cortisol synthesis is suppressed for 4-24 hours, with peak suppression at 1-2 hours post-injection. The adrenal response to ACTH (corticotropin) stimulation is blunted during this period, and stress-induced cortisol elevation is impaired. In most surgical patients, this transient suppression is clinically insignificant, as the surgery-related cortisol requirements do not exceed baseline production, and adrenal function recovers fully within 24 hours.

The controversy and clinical significance arise in two contexts:

1. Continuous infusion: When etomidate was used as an ICU sedative (1980s), mortality was significantly increased (associated with a landmark study showing 2-fold increase in mortality in trauma patients). This led to worldwide abandonment of etomidate infusions for sedation. The sustained adrenal suppression prevents the normal stress response to critical illness.

2. Single-dose induction in sepsis: The use of single-dose etomidate for intubation in septic patients remains controversial. Multiple studies have produced conflicting results regarding mortality impact. The KETASED trial and subsequent meta-analyses suggest possible harm from single-dose etomidate in sepsis, though findings are inconsistent. Current practice varies, with many intensivists avoiding etomidate in suspected sepsis while others consider the cardiovascular stability benefits to outweigh the adrenal suppression risks for a single dose.

Aldosterone synthesis (via CYP11B2) is also inhibited by etomidate, potentially contributing to cardiovascular instability in critically ill patients through impaired sodium retention and volume regulation. [106-118]

Clinical Pharmacology

Indications

Etomidate's primary indication is induction of general anaesthesia in patients where haemodynamic stability is critical. Specific clinical scenarios where etomidate is preferred include:

Cardiovascular instability: Patients with haemorrhagic shock, hypovolaemia, dehydration, or septic shock where propofol-induced hypotension could be catastrophic. The maintenance of blood pressure and cardiac output during induction provides a critical safety margin.

Severe cardiac disease: Patients with severe aortic stenosis (where hypotension reduces coronary perfusion pressure), cardiac tamponade (where preload-dependence makes vasodilation dangerous), constrictive pericarditis, severe cardiomyopathy with reduced ejection fraction, and patients with recent myocardial infarction where hypotension-induced ischaemia is a concern.

Rapid sequence induction in trauma: Emergency induction where cardiovascular stability is essential and the patient may be hypovolaemic. However, if traumatic brain injury is present, the theoretical neuroprotective benefits must be weighed against the lack of evidence for improved outcomes with etomidate compared to alternatives.

Coronary artery disease: Patients at high risk of myocardial ischaemia where maintenance of coronary perfusion pressure is essential.

Electroconvulsive therapy (ECT): Etomidate's minimal effect on seizure threshold and brief duration make it suitable for ECT, where seizure duration is important for therapeutic efficacy.

Etomidate is generally NOT preferred for: routine induction in healthy patients (propofol provides faster recovery and antiemetic effects); known or suspected sepsis (adrenal suppression concerns); adrenal insufficiency (contraindicated); and situations requiring prolonged anaesthesia (single agent use is limited by lack of analgesia and no option for infusion maintenance). [119-126]

Dosage and Administration

Standard induction dose is 0.2-0.3 mg/kg IV for healthy adults, administered over 30-60 seconds. This produces loss of consciousness within 15-45 seconds with duration of anaesthetic effect of 3-5 minutes.

Dose modifications:

• Elderly (>65 years): 0.1-0.2 mg/kg (30-50% reduction)
• Cardiovascularly unstable: 0.1-0.2 mg/kg with careful titration
• Paediatric (rarely used): 0.3-0.4 mg/kg
• Obese: Base dose on lean body weight

Administration technique to reduce adverse effects:

• Administer into large vein (preferably antecubital fossa) to reduce injection pain
• Slow injection over 60 seconds reduces myoclonus by 30-50%
• Consider premedication with midazolam (0.03-0.05 mg/kg) or fentanyl (1-2 mcg/kg) to reduce myoclonus
• Administer through free-flowing IV line with flush to prevent venous irritation
• Lipid emulsion formulation preferred over propylene glycol for reduced injection pain

Supplementation requirements:

• Always supplement with opioid for analgesia (etomidate has no analgesic properties)
• Neuromuscular blocking agent for intubation as usual
• Transition to alternative agent (propofol, volatile anaesthetic) for maintenance as etomidate infusions are contraindicated

Contraindications:

• Absolute: Known adrenal insufficiency, porphyria
• Relative: Sepsis or suspected sepsis (controversial), prolonged critical illness with adrenal suppression risk
• Precautions: Epilepsy (may activate seizure foci), pregnancy (limited data) [127-134]

Drug Interactions

Etomidate demonstrates several clinically relevant drug interactions:

Opioids: Produce synergistic hypnotic effects, allowing dose reduction of etomidate by 20-30%. Opioids also reduce myoclonus incidence by 50-60% through central depressant effects. The combination is commonly used clinically.

Benzodiazepines: Pretreatment with midazolam reduces etomidate requirements by 30-50% and significantly reduces myoclonus incidence (50-70%). This synergy is mediated through complementary GABA_A receptor modulation at different subunit interfaces.

Neuromuscular blocking agents: No direct interaction, but etomidate provides less suppression of the neuromuscular junction compared to propofol, and rocuronium onset may be slightly delayed.

CYP3A4 inhibitors/inducers: Minimal effect on etomidate metabolism as the primary pathway is ester hydrolysis rather than CYP450 metabolism. Ketoconazole and other imidazole antifungals may have additive effects on adrenal suppression.

Drugs affecting esterase activity: Theoretically, drugs that inhibit carboxylesterases could prolong etomidate effect, but this is not clinically significant.

Chronic steroid therapy: Patients on chronic corticosteroids may have suppressed adrenal function, making them more vulnerable to etomidate-induced adrenal inhibition. Consider supplemental hydrocortisone coverage.

Etomidate does not inhibit plasma cholinesterase and does not affect succinylcholine or mivacurium metabolism. [135-142]

Monitoring Requirements

Standard anaesthetic monitoring applies to etomidate administration:

• Continuous ECG with ST-segment analysis in high-risk cardiac patients
• Non-invasive or invasive arterial blood pressure monitoring
• Pulse oximetry
• Capnography once airway secured
• Neuromuscular monitoring when blocking agents used

Specific considerations for etomidate:

• Monitor for myoclonus during induction (distinguish from seizure activity)
• Heart rate monitoring for any paradoxical bradycardia
• Blood pressure monitoring every 1-2 minutes during induction phase

In critical illness settings:

• Consider random cortisol level if sepsis suspected and etomidate used
• ACTH stimulation test may be falsely abnormal for 24 hours post-etomidate
• Monitor for signs of adrenal insufficiency: refractory hypotension, vasopressor requirements, hypoglycaemia, hyponatraemia
• Some clinicians advocate empiric hydrocortisone (50 mg IV) following etomidate in septic patients, though evidence for this practice is limited [143-148]

Adverse Effects and Complications

Myoclonus

Myoclonus occurs in 33-80% of unpremedicated patients receiving etomidate. These involuntary muscle movements are typically brief (seconds to minutes), non-rhythmic, and involve the face, trunk, and limbs. While generally benign, myoclonus can be distressing to observers, may be confused with seizure activity, and can interfere with induction in certain settings (e.g., eye surgery where movement is problematic).

Prevention strategies:

• Slow injection (60 seconds) - reduces incidence 30-50%
• Midazolam premedication (0.03-0.05 mg/kg IV, 2-3 min before) - reduces incidence 50-70%
• Fentanyl premedication (1-2 mcg/kg, 2-3 min before) - reduces incidence 50-60%
• Small priming dose of etomidate (0.03 mg/kg, 90 sec before induction) - reduces incidence 50-70%
• Rocuronium pretreatment (subparalysing dose) - reduces visible movement but not subcortical activity

Management: Usually no treatment required as myoclonus is self-limiting. If severe or prolonged, deepening anaesthesia with additional etomidate or propofol is effective. [83-90]

Adrenal Suppression

Single-dose etomidate causes transient (4-24 hour) adrenal suppression in all patients. Clinical significance depends on the patient population:

In routine surgical patients: Generally insignificant, with no demonstrated adverse outcomes from single-dose induction.

In critically ill/septic patients: Potentially harmful, with some studies suggesting increased vasopressor requirements and possibly increased mortality. The CORTICUS study found that etomidate exposure was associated with higher 28-day mortality in septic patients, though this was confounded by severity of illness.

Management in high-risk patients:

• Consider alternative induction agents (ketamine) in suspected sepsis
• If etomidate used in sepsis, consider stress-dose hydrocortisone (50 mg IV q6h for 48-72 hours), though evidence is limited
• Avoid interpreting cortisol levels or ACTH stimulation tests within 24 hours of etomidate
• Monitor closely for signs of adrenal insufficiency [106-118]

Postoperative Nausea and Vomiting (PONV)

Etomidate is associated with a higher incidence of PONV compared to propofol, occurring in 30-40% of patients without antiemetic prophylaxis. The mechanism is unclear but may involve effects on the chemoreceptor trigger zone or vestibular apparatus. This contrasts with propofol's antiemetic properties. Prophylactic antiemetics (ondansetron, dexamethasone) should be considered when etomidate is used, particularly in patients with other PONV risk factors. [149-152]

Pain on Injection

The propylene glycol formulation causes injection site pain in 10-50% of patients, related to the osmolality and pH of the solution. The lipid emulsion formulation (Etomidate-Lipuro) reduces this significantly. Administration into large veins and slower injection also reduce pain. Pretreatment with lidocaine (as used for propofol) is not commonly employed but may be effective. [16-22]

Rare Complications

• Anaphylaxis: Rare (<1:10,000) but reported with both formulations
• Seizures: May activate seizure foci in epileptic patients, though has also been used to treat status epilepticus
• Haemolysis: Reported with propylene glycol formulation; very rare with lipid emulsion
• Venous sequelae: Thrombophlebitis, particularly with propylene glycol formulation into small veins

ANZCA Primary Exam Focus

Common MCQ Patterns

ANZCA Primary MCQs frequently test the following etomidate concepts:

Mechanism of action: Questions emphasise GABA_A receptor modulation at beta-2/beta-3 subunits (not alpha-gamma interface like benzodiazepines). Candidates should understand that etomidate is a positive allosteric modulator increasing chloride conductance.

Adrenal suppression: Questions test knowledge of 11-beta-hydroxylase (CYP11B1) inhibition, the enzyme blocked, duration of suppression (4-24 hours), and clinical implications. Key point: the imidazole ring structure is responsible for adrenal enzyme inhibition.

Cardiovascular stability: Comparison questions highlighting etomidate's 10-15% MAP reduction versus propofol's 25-40% reduction. Understand the mechanisms (maintained sympathetic tone, minimal direct cardiac depression, no histamine release).

Myoclonus: Questions on incidence (33-80%), mechanism (subcortical disinhibition, NOT seizure activity), and prevention strategies.

Pharmacokinetics: Ester hydrolysis metabolism (NOT plasma cholinesterase), high hepatic extraction ratio, elimination half-life 2.9-5.3 hours.

Clinical indications: Scenarios involving haemodynamic instability, severe cardiac disease (aortic stenosis, tamponade), and trauma where etomidate would be preferred. [153-158]

Primary Viva Question Themes

Primary vivas on etomidate typically progress through:

1. Mechanism and structure: "Describe the mechanism of action of etomidate" - expect discussion of GABA_A receptor modulation, subunit selectivity, and the relationship between imidazole structure and adrenal suppression.

2. Cardiovascular effects: "Why is etomidate considered haemodynamically stable?" - candidates should compare to propofol quantitatively (10-15% vs 25-40% MAP reduction) and explain mechanisms.

3. Adrenal suppression: "What is the mechanism and clinical significance of etomidate's adrenal effects?" - expect detailed discussion of CYP11B1 inhibition, duration, and the sepsis controversy.

4. Clinical scenario: "A 75-year-old patient with severe aortic stenosis requires emergency laparotomy. What induction agent would you choose?" - etomidate is a reasonable choice with justification based on cardiovascular stability.

5. Adverse effects: "What are the adverse effects of etomidate and how would you manage myoclonus?" - systematic approach to myoclonus prevention and management.

Comparison Questions

Etomidate is frequently compared to other induction agents:

Candidates should be able to justify agent selection for specific clinical scenarios using these comparisons. [159-165]

Australian/NZ Specific Considerations

TGA-Approved Formulations

In Australia, etomidate is available as:

• Hypnomidate (Janssen): Propylene glycol formulation, 2 mg/mL in 10 mL ampoules
• Etomidate-Lipuro (B. Braun): Lipid emulsion formulation, 2 mg/mL in 10 mL ampoules

The lipid emulsion formulation is increasingly preferred due to reduced injection pain and venous irritation. Both formulations are Therapeutic Goods Administration (TGA) registered for induction of anaesthesia. Storage is at room temperature (15-25°C); solutions should not be refrigerated. The lipid formulation should be used within 6-12 hours of opening due to microbial growth potential. Both formulations contain the active R(+)-enantiomer only. [166-170]

PBS Listing and Availability

Etomidate is not listed on the Pharmaceutical Benefits Scheme (PBS) for outpatient prescription but is available through hospital pharmaceutical budgets. It is stocked in most major teaching hospitals, particularly in emergency departments and intensive care units where rapid sequence induction in unstable patients is common. Regional hospitals may have limited stock or may stock only one formulation. Remote facilities served by Royal Flying Doctor Service (RFDS) may carry etomidate for emergency intubation, though ketamine is more commonly used for pre-hospital and retrieval medicine due to cardiovascular stimulation and intramuscular administration option.

In New Zealand, etomidate is available through hospital pharmacy procurement and funded under relevant DHB pharmaceutical budgets. It is stocked in major centres but may be unavailable in smaller peripheral hospitals. [171-175]

Local Practice Patterns

Australian and New Zealand practice regarding etomidate in sepsis varies. Some centres avoid etomidate entirely in suspected sepsis, favouring ketamine for haemodynamic stability without adrenal suppression. Other centres use etomidate for single-dose induction with or without empiric hydrocortisone supplementation. The ANZICS CORE database does not specifically track etomidate use, limiting local outcome data. Professional society guidelines (ANZCA, CICM) do not mandate or prohibit etomidate in any specific population but recommend awareness of adrenal effects. The Australian Resuscitation Council and ANZCOR guidelines do not specifically address etomidate selection for emergency intubation. Individual hospital protocols should be consulted. [176-180]

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Considerations

Limited pharmacogenomic data exist specific to Aboriginal and Torres Strait Islander populations regarding etomidate metabolism or sensitivity. However, several clinical factors warrant consideration when using etomidate in Indigenous patients.

Higher prevalence of cardiovascular disease in Aboriginal and Torres Strait Islander communities (including ischaemic heart disease, cardiomyopathy, and rheumatic heart disease) may actually favour etomidate selection due to its haemodynamic stability. Rheumatic heart disease causing severe valvular lesions (particularly mitral stenosis and aortic regurgitation) is more prevalent in Indigenous populations, and these patients benefit from etomidate's cardiovascular profile.

Chronic kidney disease affects Aboriginal and Torres Strait Islander peoples at 3-4 times the rate of non-Indigenous Australians. While etomidate pharmacokinetics are minimally affected by renal impairment (ester hydrolysis metabolism), associated uraemia may alter protein binding and increase free fraction, potentially requiring modest dose reduction.

Diabetes mellitus and associated comorbidities are highly prevalent. Diabetic autonomic neuropathy may theoretically affect cardiovascular responses, though etomidate's minimal cardiovascular effects make this less clinically relevant than with propofol.

Sepsis considerations: Given higher rates of infectious diseases and sepsis presentations in some Indigenous communities, the etomidate-adrenal suppression controversy is particularly relevant. Alternative agents (ketamine) may be preferred in suspected sepsis, or empiric hydrocortisone may be considered if etomidate is used.

Cultural safety principles apply to all medication administration. Explanation of the anaesthetic process using culturally appropriate language, involvement of Aboriginal Health Workers or Hospital Liaison Officers where available, and accommodation of family presence during induction and recovery can reduce anxiety and improve patient experience. The altered state of consciousness during anaesthetic induction and emergence may have particular cultural significance requiring sensitive discussion.

Māori Health Considerations

For Māori patients in New Zealand, engagement with whānau in pre-anaesthetic discussions recognizes the collective nature of health decision-making. Cardiovascular disease prevalence, diabetes, and associated comorbidities are elevated in Māori populations, with similar implications for drug selection as noted above.

Remote and Rural Considerations

In remote Indigenous communities across Australia, etomidate availability may be limited. Ketamine is often preferred for emergency intubation in retrieval medicine due to cardiovascular stability, intramuscular administration option, and room-temperature stability. RFDS and remote area retrieval teams may carry etomidate but typically reserve it for specific cardiovascular indications where its haemodynamic stability is essential. Limited monitoring capabilities in remote settings favour drugs with wider therapeutic indices and predictable cardiovascular effects. [181-188]

Assessment Content

SAQ Practice Question (20 marks)

Question: A 72-year-old, 65 kg man with known severe aortic stenosis (valve area 0.6 cm², mean gradient 60 mmHg) and chronic stable angina requires emergency laparotomy for a perforated duodenal ulcer. His blood pressure is 95/60 mmHg and heart rate is 90 bpm. He has no known drug allergies. Discuss your choice of induction agent, with particular reference to etomidate pharmacology, and describe your induction strategy.

Model Answer:

Clinical context and haemodynamic goals (4 marks):

• Severe aortic stenosis with critical valve area (<1.0 cm²) represents fixed obstruction to LV outflow [1]
• Haemodynamic goals: Maintain sinus rhythm, avoid tachycardia (reduces diastolic filling time), maintain adequate SVR (coronary perfusion depends on aortic root diastolic pressure), avoid hypotension (already borderline at 95/60) [1.5]
• Emergency surgery precludes extensive optimisation; must balance haemodynamic stability against time urgency [1]
• Already hypotensive - propofol-induced further hypotension (25-40% MAP reduction) could be catastrophic [0.5]

Etomidate pharmacology relevant to this case (6 marks):

• Mechanism: Positive allosteric modulator of GABA_A receptors at beta-2/beta-3 subunits, enhancing chloride conductance and producing hypnosis [1]
• Cardiovascular profile: Only 10-15% reduction in MAP (compared to 25-40% with propofol), maintained SVR, preserved cardiac output, intact baroreceptor reflex [2]
• Mechanisms of stability: Minimal direct myocardial depression, no histamine release, preserved sympathetic tone, no direct vasodilation [1]
• Metabolism: Hepatic ester hydrolysis (not plasma cholinesterase), independent of renal function [1]
• Onset: Rapid (15-45 seconds), duration 3-5 minutes from redistribution [1]

Adrenal suppression considerations (4 marks):

• Etomidate inhibits 11-beta-hydroxylase (CYP11B1), blocking cortisol synthesis for 4-24 hours [1]
• Imidazole ring structure responsible for enzyme inhibition [0.5]
• In this patient: Surgical stress of emergency laparotomy will generate significant cortisol demand [1]
• Risk-benefit: Single-dose adrenal suppression is transient; risk of induction hypotension with propofol likely outweighs adrenal concerns in this haemodynamically unstable patient [1]
• Consider: Empiric hydrocortisone 50 mg IV can be given if prolonged surgery or postoperative vasopressor requirement anticipated [0.5]

Induction strategy (6 marks):

• Pre-oxygenation: 100% O2 for 3-5 minutes, end-tidal O2 >85% [0.5]
• Premedication: Fentanyl 50-100 mcg (1-1.5 mcg/kg) to reduce etomidate requirement and blunt laryngoscopy response, BUT carefully as may reduce SVR [1]
• Monitoring: Invasive arterial line before induction if time permits; otherwise non-invasive BP every minute [0.5]
• Vasopressor availability: Phenylephrine (pure alpha-agonist preferred in AS), metaraminol, or norepinephrine drawn up and ready [1]
• Etomidate dose: 0.15-0.2 mg/kg (10-13 mg for 65 kg), reduced due to age and hypotension, administered slowly over 60 seconds [1]
• Neuromuscular blockade: Rocuronium 1.2 mg/kg for rapid sequence [0.5]
• Intubation: Grade view, minimal laryngoscopy time to reduce stimulation-induced tachycardia [0.5]
• Post-intubation: Immediate BP check, vasopressor as needed, transition to maintenance with volatile or propofol once stable [1]

Total: 20 marks

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Primary Viva Scenario (15 marks)

Opening Stem: You are the anaesthetic registrar called to the emergency department to assist with intubation of a 55-year-old man with suspected bacterial meningitis. He is febrile (39.2°C), tachycardic (HR 120), hypotensive (BP 85/50), and has a GCS of 9 (E2V3M4). The emergency physician asks your opinion on the choice of induction agent.

Expected Viva Progression:

Initial assessment and clinical context (3 marks):

• Suspected bacterial meningitis with signs of sepsis/septic shock (hypotension, tachycardia, fever) [1]
• Reduced GCS likely representing meningeal inflammation/cerebral oedema rather than intrinsic brainstem depression [0.5]
• Urgent need for airway protection (GCS <8) and likely need for CT/LP following intubation [0.5]
• Haemodynamically unstable - traditional teaching would favour etomidate [0.5]
• BUT: Sepsis present - etomidate adrenal suppression controversy [0.5]

Examiner: What are the options for induction agent in this patient?

Induction agent discussion (6 marks):

Etomidate:

• Advantages: Haemodynamic stability (10-15% MAP reduction), rapid onset, favourable cerebrovascular profile (reduces ICP) [1]
• Disadvantages: Adrenal suppression (4-24 hours via CYP11B1 inhibition), concerning in sepsis where cortisol response is critical for survival [1]
• Evidence: Conflicting studies on mortality impact in sepsis; CORTICUS subgroup analysis suggested harm; subsequent meta-analyses inconclusive [0.5]

Ketamine:

• Advantages: Cardiovascular stimulation (increases BP 20-40%), no adrenal suppression, analgesic properties, bronchodilation [1]
• Disadvantages: Historical concern about ICP elevation (now largely refuted with controlled ventilation), emergence phenomena (less relevant in ICU setting) [0.5]
• Modern evidence supports ketamine in sepsis and head injury when ventilation controlled [0.5]

Propofol:

• Advantages: Favourable cerebrovascular profile, antiemetic, rapid recovery [0.5]
• Disadvantages: Significant hypotension (25-40% MAP reduction) - potentially catastrophic in this already hypotensive patient [0.5]
• Would require aggressive fluid loading and vasopressor support [0.5]

Examiner: What would be your choice and how would you manage this induction?

Recommended approach and justification (4 marks):

• Recommendation: Ketamine 1-1.5 mg/kg IV (or reduced dose 0.5-1 mg/kg given hypotension may indicate catecholamine depletion) [1]
• Rationale: Maintains haemodynamics, no adrenal suppression in sepsis, modern evidence supports safety in meningitis/elevated ICP with controlled ventilation [1]
• Alternative acceptable answer: Etomidate 0.2 mg/kg with plan for empiric hydrocortisone 50 mg IV [0.5]
• Management strategy:
  • Pre-oxygenation
  • Fluid bolus (500-1000 mL) running
  • Vasopressor (norepinephrine) available
  • Ketamine 1 mg/kg + rocuronium 1.2 mg/kg
  • Immediate post-intubation BP check and vasopressor as needed
  • Controlled ventilation targeting normocapnia [1.5]

Examiner: If etomidate were used, what would be your post-induction management regarding adrenal function?

Post-etomidate adrenal management (2 marks):

• Consider empiric stress-dose hydrocortisone (50 mg IV q6h) for 48-72 hours [0.5]
• Do not interpret random cortisol or ACTH stimulation test for 24 hours post-etomidate [0.5]
• Monitor for signs of adrenal insufficiency: refractory hypotension despite fluid resuscitation, increasing vasopressor requirements, hypoglycaemia, hyponatraemia [0.5]
• Wean hydrocortisone once haemodynamically stable and sepsis controlled [0.5]

Total: 15 marks

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References

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Additional References

33. Wagner RL, White PF, Kan PB, et al. Inhibition of adrenal steroidogenesis by the anesthetic etomidate. N Engl J Med. 1984;310(22):1415-1421. PMID: 6325910
34. Cuthbertson BH, Sprung CL, Annane D, et al. The effects of etomidate on adrenal responsiveness and mortality in patients with septic shock. Intensive Care Med. 2009;35(11):1868-1876. PMID: 19652948
35. Annane D. ICU physicians should abandon the use of etomidate! Intensive Care Med. 2005;31(3):325-326. PMID: 15605229
36. Jabre P, Combes X, Lapostolle F, et al. Etomidate versus ketamine for rapid sequence intubation in acutely ill patients: a multicentre randomised controlled trial. Lancet. 2009;374(9686):293-300. PMID: 19573904
37. Sprung CL, Annane D, Keh D, et al. Hydrocortisone therapy for patients with septic shock. N Engl J Med. 2008;358(2):111-124. PMID: 18184957
38. Hohl CM, Kelly-Smith CH, Yeung TC, et al. The effect of a bolus dose of etomidate on cortisol levels, mortality, and health services utilization: a systematic review. Ann Emerg Med. 2010;56(2):105-113.e5. PMID: 20346539
39. Chan CM, Mitchell AL, Shorr AF. Etomidate is associated with mortality and adrenal insufficiency in sepsis: a meta-analysis. Crit Care Med. 2012;40(11):2945-2953. PMID: 22971586
40. Bruder EA, Ball IM, Ridi S, et al. Single induction dose of etomidate versus other induction agents for endotracheal intubation in critically ill patients. Cochrane Database Syst Rev. 2015;1:CD010225. PMID: 25568981