Etomidate — Receptor & Molecular Pharmacology
## 1. Primary receptor target
* **Etomidate is a positive allosteric modulator of the GABA-A receptor** (like propofol and barbiturates).
* At **clinical doses**: enhances the effect of endogenous **GABA**.
* At **higher doses**: can directly activate the GABA-A receptor even without GABA present.
* Binding site: β2 and β3 subunits of GABA-A (slightly different affinity compared to propofol).
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## 2. Mechanism of action
* **Enhances inhibitory neurotransmission** by:
* Increasing **GABA affinity** for its receptor (slows GABA dissociation).
* Increasing the **probability and duration of channel opening**.
* Result → **increased chloride influx**, **neuronal hyperpolarisation**, and **CNS depression**.
* Unlike propofol, etomidate has **minimal action on NMDA or glycine receptors**, making it more “clean” and targeted.
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## 3. Key pharmacodynamic profile
* **Onset:** 30–60 seconds (very rapid).
* **Duration:** 5–10 minutes (redistribution).
* **Hemodynamics:** *minimal effect on sympathetic tone, SVR, myocardial contractility*.
* → Excellent for patients with **poor LV function, hypovolemia, critical AS, cardiogenic shock**.
* **Respiratory depression:** less than propofol but still present (especially if combined with opioids).
* **Cerebral effects:** ↓CMRO₂, ↓CBF, ↓ICP, preserves CPP better than propofol due to stable MAP.
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## 4. Endocrine effect — unique and critical
* **Etomidate inhibits 11β-hydroxylase** in the adrenal cortex.
* This blocks conversion of 11-deoxycortisol → cortisol, and 11-deoxycorticosterone → corticosterone.
* Even a **single bolus dose** transiently suppresses cortisol and aldosterone synthesis for **6–24 h**.
* Continuous infusions → prolonged adrenal suppression → ↑ mortality in septic/critically ill patients.
* → **Never use as ICU sedative infusion.** Use only as **induction agent**.
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## 5. Practical use (Cardiac & ICU)
### **Induction dosing**
* **0.2–0.3 mg/kg IV** bolus (commonly 0.3 mg/kg).
* In **frail/poor EF patients**, start at 0.15–0.2 mg/kg.
* Give with opioid ± lidocaine to blunt laryngoscopy response.
### **Infusion (rare, not routine)**
* Not used for maintenance due to adrenal suppression.
* Historically used for status epilepticus sedation → now avoided.
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## 6. Complications and their management
### Common
* **Myoclonus (30–60%)**
* Mechanism: imbalance of excitatory/inhibitory pathways at induction.
* Harmless but can interfere with induction, increase O₂ demand, or risk dislodging lines.
* **Management:** pretreat with small opioid/benzodiazepine dose or lidocaine.
* **Pain on injection**
* Similar to propofol (less severe).
* Lidocaine pretreatment helps.
### Serious / Clinically important
* **Adrenal suppression** (most critical)
* Avoid in septic, adrenal-insufficient, or long-term ICU sedation.
* *One-off dose in unstable cardiac patient for intubation = still acceptable*.
* **Nausea/vomiting (PONV)**
* Higher incidence vs propofol. Use prophylactic antiemetics.
* **Seizure activity** (rare)
* Can activate epileptiform EEG activity.
* Sometimes used in epilepsy monitoring units for this reason.
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## 7. Comparison with Propofol (key contrasts)
| Feature | Propofol | Etomidate |
| ---------------------- | ------------------------------ | --------------------------- |
| Hemodynamics | ↓↓ MAP, ↓ SVR, ↓ contractility | Stable (best in shock/AS) |
| Cerebral | ↓CMRO₂, ↓CBF, ↓ICP | ↓CMRO₂, ↓CBF, ↓ICP |
| Respiratory depression | Significant | Mild–moderate |
| Antiemetic | Strong | No (PONV risk ↑) |
| Adrenal suppression | None | Yes (11β-hydroxylase block) |
| Myoclonus | Rare | Common |
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## 8. Bedside “pocket lines”
* **Dose:** 0.2–0.3 mg/kg IV for induction.
* **Good for:** *shock, severe LV dysfunction, tight AS, poor reserve*.
* **Avoid for:** *sepsis, adrenal insufficiency, long ICU sedation*.
* **Pearls:** stable MAP, myoclonus common, PONV risk ↑, single dose OK, no infusion in ICU.
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✅ **One-liner for exams/rounds:**
*Etomidate is a GABA-A positive allosteric modulator that prolongs GABA binding and enhances Cl⁻ channel opening, producing hypnosis with cardiovascular stability but causing adrenal suppression via 11β-hydroxylase inhibition.*
Good question — let’s tackle **myoclonus** in a way that’s useful both for viva-style questions and bedside understanding.
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Myoclonus
## 1. Definition
* **Myoclonus = sudden, brief, involuntary muscle jerk** caused by abnormal bursts of electrical activity in the brain or spinal cord.
* Can be single jerks or repetitive, and may look like twitching, flailing, or shivering.
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## 2. Types (general classification)
* **Physiological:** normal, e.g. sleep-start “hypnic jerk.”
* **Essential/idiopathic:** occurs without other neurological disease.
* **Epileptic:** part of seizure disorders (juvenile myoclonic epilepsy).
* **Secondary/symptomatic:** due to metabolic, drug-induced, or structural brain disorders.
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## 3. Etomidate-induced myoclonus
* **Very common:** up to 30–60% of patients during induction with etomidate.
* **Mechanism (theory):**
* Etomidate enhances inhibitory GABA activity in cortex **before** full cortical suppression occurs.
* This causes **imbalance**: inhibitory pathways in the brain are suppressed faster than excitatory subcortical centers → subcortical disinhibition → sudden muscle jerks.
* **Timing:** within 30–90 seconds of injection, usually self-limited.
* **Severity:** from minor twitching of face/extremities to strong limb jerks.
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## 4. Why it matters clinically
* Usually benign, but:
* Can increase **O₂ consumption** and **CO₂ production** in critically ill patients.
* May cause harm if the patient has **open globe injuries, full stomach (risk aspiration), or unstable fractures/lines**.
* In neuro or cardiac patients, unnecessary stress response may be undesirable.
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## 5. Prevention & Management
* **Pre-treatment strategies:**
* Small dose of **opioid** (fentanyl 1–2 mcg/kg).
* **Benzodiazepine** (midazolam 1–2 mg).
* **Lidocaine IV** (0.5–1 mg/kg).
* **Slow injection** of etomidate reduces incidence somewhat.
* **If it occurs:** usually no treatment needed, as it’s self-limited and lasts <1–2 min.
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## 6. Key exam/bedside one-liner
*Myoclonus is a sudden involuntary muscle jerk. With etomidate, it occurs in up to 60% of cases due to subcortical disinhibition before full cortical suppression, is usually benign and self-limiting, and can be reduced by opioid or benzodiazepine pretreatment.*
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