# Etomidate, Barbiturates, and Induction Agent Selection ## Etomidate Pharmacology ### Chemical Properties Etomidate is a carboxylated imidazole derivative supplied as a 2 mg/mL solution in 35% propylene glycol, which causes pain on injection and venous irritation. It has a pH of 6.9 and is highly lipophilic. It contains a chiral center, with the R(+) isomer being the active hypnotic form. ### Mechanism of Action Etomidate is a positive allosteric modulator of GABA-A receptors, binding at the beta-2/3 subunit — a site distinct from the benzodiazepine binding site. It enhances chloride conductance and neuronal inhibition. Because it is highly selective for GABA-A, it has fewer off-target effects than other induction agents. It has no analgesic properties. ### Pharmacokinetics IV onset is 15-30 seconds (one arm-brain circulation). Duration is 3-8 minutes, driven by redistribution. The elimination half-life is 2-5 hours. Metabolism occurs through hepatic ester hydrolysis and CYP enzymes, with extrahepatic hydrolysis also contributing. Protein binding is 75%, primarily to albumin, and clearance reflects a high hepatic extraction ratio. ### Dosing The standard induction dose is 0.2-0.3 mg/kg IV (typically 0.3 mg/kg). In the elderly, 0.15-0.2 mg/kg is appropriate. The pediatric dose is similar to adults at 0.3 mg/kg. No dose adjustment is needed for renal failure. ## Etomidate: Clinical Effects ### Cardiovascular Hemodynamic stability is etomidate's hallmark — it causes minimal change in heart rate, blood pressure, cardiac output, or SVR. It does not release histamine, does not significantly blunt sympathetic responses, and preserves baroreceptor function. This makes it ideal for patients with compromised cardiac function, hypovolemia, or fixed cardiac output states such as aortic stenosis or cardiac tamponade. ### Respiratory Etomidate produces less respiratory depression than propofol or barbiturates. Apnea occurs less frequently and is shorter in duration than with propofol. Brief hiccups are common. ### Central Nervous System Etomidate reduces CMRO2, CBF, and ICP (similar to propofol and barbiturates) while maintaining CPP due to its hemodynamic stability — a significant advantage for neurosurgical patients with compromised hemodynamics. It lowers seizure threshold and can activate epileptiform activity on EEG, which is actually exploited in epilepsy surgery mapping. Myoclonus occurs in 30-60% of patients but is not seizure activity — these are involuntary skeletal muscle movements of subcortical origin. Pretreatment with a small dose of benzodiazepine, opioid, or lidocaine can reduce myoclonus. It can interfere with neuromonitoring if misinterpreted. ### Adrenal Suppression (The Controversy) Etomidate inhibits 11-beta-hydroxylase, blocking cortisol synthesis. A single induction dose suppresses adrenal cortisol production for 4-8 hours (up to 24 hours in some studies). Continuous infusion produces prolonged adrenal suppression and was associated with increased mortality in ICU patients — an experience from the 1980s that led to abandonment of etomidate infusions. The clinical significance of single-dose suppression is debated. The CORTICUS substudy found that a single etomidate dose was associated with higher 28-day mortality in septic patients, though this was an observational, confounded finding. Multiple RCTs and meta-analyses have not consistently shown harm from a single dose. The 2014 Albert et al. meta-analysis found a marginal increase in mortality in septic patients. Current practice is divided: many practitioners avoid etomidate in sepsis, while others consider it acceptable given its hemodynamic benefits. When etomidate is avoided in hemodynamically unstable patients, ketamine is the most common alternative. ## Barbiturate Pharmacology ### Thiopental (Thiopentone) Thiopental is a thiobarbiturate (with a sulfur atom at C2 that increases lipophilicity and potency). It was historically the most widely used IV induction agent worldwide but is currently unavailable in the US due to manufacturing discontinuation. It is still used internationally. #### Pharmacokinetics Onset is 10-20 seconds due to high lipophilicity and rapid brain uptake. Duration is 5-10 minutes from redistribution to muscle and then fat. The elimination half-life is 11-12 hours — long because of slow release from fat stores. The context-sensitive half-time is very long, with significant accumulation from repeated doses. Metabolism is hepatic oxidation (slow) with zero-order kinetics at high doses. Protein binding is 80-85%. #### Clinical Effects Cardiovascularly, thiopental causes dose-dependent hypotension through venodilation, myocardial depression, and blunted baroreflexes, accompanied by baroreceptor-mediated tachycardia. Respiratory depression is dose-dependent with apnea at induction doses. In the CNS, it produces rapid unconsciousness and decreases CMRO2, CBF, and ICP. It is a potent anticonvulsant with debated cerebral protective effects against ischemia. It provides no analgesia and may be hyperalgesic at sub-anesthetic doses. It causes histamine release with bronchospasm risk (avoid in asthma) and is absolutely contraindicated in porphyria because it induces aminolevulinic acid synthase and can precipitate an acute attack. #### Dosing Induction dose is 3-5 mg/kg IV. For status epilepticus, 2-5 mg/kg bolus followed by infusion. For cerebral protection, titration to burst suppression on EEG. ### Methohexital Methohexital is an oxybarbiturate (oxygen at C2) with a shorter elimination half-life than thiopental (3-5 hours). It is more potent than thiopental, with an induction dose of 1-1.5 mg/kg. It causes more excitatory phenomena (involuntary movements, hiccups). Critically, it lowers seizure threshold, which makes it the preferred induction agent for ECT to facilitate seizure duration. It causes pain on injection and is not commonly used for routine induction. It remains available in the US with limited use, primarily for ECT. ### Pentobarbital Pentobarbital is a longer-acting barbiturate used for refractory status epilepticus and refractory intracranial hypertension in the ICU. The infusion is titrated to burst suppression on EEG. It causes significant hemodynamic depression that usually requires vasopressor support and leads to prolonged recovery. | Property | Propofol | Etomidate | Ketamine | Thiopental | Midazolam | |----------|---------|-----------|----------|------------|-----------| | Induction Dose | 1.5-2.5 mg/kg | 0.2-0.3 mg/kg | 1-2 mg/kg | 3-5 mg/kg | 0.1-0.3 mg/kg | | Onset | 15-30 sec | 15-30 sec | 30-60 sec | 10-20 sec | 1-2 min | | Blood Pressure | Decreased (significant) | Minimal change | Increased | Decreased | Mild decrease | | Heart Rate | Minimal change | Minimal change | Increased | Reflex tachycardia | Minimal change | | Respiratory Depression | Significant; apnea common | Mild | Minimal | Significant; apnea | Mild-Moderate | | ICP Effect | Decreased | Decreased | Variable (see text) | Decreased | Decreased | | Analgesia | None | None | Yes | None (may be hyperalgesic) | None | | Pain on Injection | Yes | Yes | No | No | No | | Key Advantage | Antiemetic, fast recovery | Hemodynamic stability | Analgesia, hemodynamic support | Anticonvulsant | Amnesia, anxiolysis | | Key Concern | Hypotension | Adrenal suppression | Emergence phenomena | Porphyria, histamine | Slow onset | ## Induction Agent Selection: Clinical Decision-Making ### Hemodynamically Unstable Patient The first choice is ketamine (1-2 mg/kg IV) for its sympathomimetic properties, keeping in mind the risk in catecholamine-depleted states. The second choice is etomidate (0.3 mg/kg IV), which has the best hemodynamic profile despite the adrenal suppression concern. Reduced-dose propofol (0.5-1 mg/kg) with a vasopressor ready is another option. Full-dose propofol or thiopental should be avoided. ### Rapid Sequence Induction Propofol is the most common choice due to its predictable onset and familiar pharmacology. Etomidate is used for hemodynamically unstable patients undergoing RSI. Ketamine is appropriate for trauma, sepsis, or bronchospasm with hemodynamic instability. Thiopental was historically the standard but is now rarely available. ### Cardiac Tamponade / Fixed Cardiac Output Etomidate is preferred because it maintains SVR, heart rate, and contractility. Ketamine is acceptable for its sympathomimetic support. Propofol should be avoided because afterload reduction and negative inotropy can cause cardiovascular collapse. ### Raised ICP Propofol, etomidate, and thiopental all reduce ICP. The choice depends on hemodynamics: etomidate when blood pressure is tenuous, propofol when hemodynamically stable. Ketamine is increasingly accepted in intubated and ventilated patients. ### Status Epilepticus Midazolam, propofol, or thiopental/pentobarbital are used for refractory cases. Etomidate should be avoided because it may activate seizure foci. Methohexital should also be avoided in epilepsy because it lowers seizure threshold. ### Reactive Airway Disease Propofol is bronchodilatory and safe in asthma. Ketamine is a potent bronchodilator and a good choice. Thiopental should be avoided due to histamine release and bronchospasm risk. Etomidate has a neutral effect on airways and is acceptable. ### Porphyria Safe agents include propofol, opioids, nitrous oxide, succinylcholine, and ketamine (debated). All barbiturates (thiopental, methohexital, pentobarbital) must be avoided because they precipitate acute porphyric crisis. Etomidate is generally considered safe. ### The Pediatric Patient Propofol is the standard choice with a higher dose requirement (2.5-3.5 mg/kg). Ketamine via the IM route is useful for an uncooperative child (4-5 mg/kg). Thiopental was historically standard in pediatrics (5-6 mg/kg) but is now unavailable in many countries. A comparison table formatted as an infographic showing the five major induction agents (propofol, etomidate, ketamine, thiopental, midazolam) across key parameters: induction dose, onset time, cardiovascular effects (blood pressure arrows, heart rate arrows), respiratory depression severity, ICP effect, pain on injection, unique advantages, and key contraindications. Color-coded cells indicate favorable (green), neutral (yellow), and unfavorable (red) properties for each parameter. A decision algorithm for induction agent selection in the hemodynamically compromised patient. Starting with assessment of hemodynamic status (stable vs. unstable), then branching by clinical scenario: sepsis (etomidate vs. ketamine debate with evidence summary), cardiac tamponade (etomidate preferred), hemorrhagic shock (ketamine with volume resuscitation), and severe aortic stenosis (etomidate or cautious dose of any agent). Each endpoint shows recommended agent, dose, and key monitoring considerations. A diagram illustrating the etomidate adrenal suppression controversy: molecular structure of etomidate shown next to the adrenal steroidogenesis pathway with 11-beta-hydroxylase highlighted as the blocked enzyme. Timeline showing cortisol suppression after single dose (4-24 hours) versus infusion (days). Sidebar showing key studies (CORTICUS, Albert meta-analysis) with conflicting evidence icons and current guideline positions. ## Clinical Pearls Etomidate is the most hemodynamically stable induction agent available. The clinical relevance of single-dose adrenal suppression remains debated, but many practitioners avoid it in sepsis when alternatives exist. Thiopental and methohexital are absolutely contraindicated in porphyria — a classic board-examination question. Myoclonus with etomidate is not seizure activity; it is subcortical in origin and does not require anticonvulsant treatment. Ketamine and etomidate both maintain hemodynamic stability, but through completely different mechanisms: ketamine through sympathetic stimulation, etomidate through a neutral cardiovascular profile. Methohexital lowers seizure threshold and is the preferred induction agent for ECT — another board-favorite point. When selecting an induction agent, the four key factors to consider are hemodynamic status, airway reactivity, ICP concerns, and specific contraindications (porphyria, malignant hyperthermia, adrenal insufficiency). ## References - Bruder EA, 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. - Albert SG, et al. The effect of etomidate on adrenal function in critical illness: a systematic review. *Intensive Care Med*. 2011;37(6):901-910. - Sprung CL, et al. Hydrocortisone therapy for patients with septic shock (CORTICUS). *N Engl J Med*. 2008;358(2):111-124. - Stollings JL, et al. Ketamine for analgesia in the critically ill. *Pharmacotherapy*. 2016;36(2):189-203. - Miller RD, et al. *Miller's Anesthesia*, 9th edition. Chapter on Intravenous Anesthetics.