General anesthesia principles

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🥊 Drug class/MOA - The Knockout Crew

📌 PEK for IV Anesthetics: Propofol, Etomidate, Ketamine.

DrugMOAKey Clinical Points
PropofolPotentiates GABA-ARapid onset/offset. Hypotension, apnea, antiemetic.
EtomidatePotentiates GABA-AHemodynamically stable (cardio-stable). ⚠️ Adrenal suppression.
KetamineNMDA AntagonistDissociative anesthesia. Sympathomimetic (↑HR, ↑BP), bronchodilation.
BarbituratesPotentiates GABA-A↓ICP, neuroprotective. Risk of laryngospasm.

💨 PK/PD - In, Out, How Fast?

  • Inhaled Anesthetics (Uptake/Elimination):
    • Blood:Gas Partition Coefficient: Determines speed.
      • Low solubility (N₂O, Desflurane) = Fast onset/offset.
      • High solubility (Halothane) = Slow onset/offset.
    • Induction is faster with: ↑ ventilation, ↓ cardiac output.
  • IV Anesthetics (e.g., Propofol):
    • Onset: Rapid distribution to vessel-rich groups (brain).
    • Termination: Redistribution to muscle/fat, not metabolism.
  • Potency - MAC (Minimum Alveolar Concentration):
    • Concentration preventing movement in 50% of patients to stimulus.
    • ↓ MAC = ↑ Potency.
    • Factors ↓ MAC: ↑ Age, hypothermia, opioids.

⭐ In shock (↓ CO), inhaled anesthetic induction is faster. Less blood flow removes less agent from the lungs, causing a rapid rise in alveolar partial pressure.

😴 Staging the Sleep: General Anesthesia

General anesthesia depth is classically described in four stages (Guedel's Classification). The goal is to rapidly achieve and maintain Stage III for surgery.

  • Stage I (Analgesia): Drowsy, conscious, reduced pain sensation.
  • Stage II (Excitement): Delirium, irregular breathing, risk of laryngospasm.

⭐ This stage is dangerous and is bypassed quickly with modern agents (e.g., propofol). Uninhibited reflexes can cause vomiting, aspiration, and injury.

  • Stage III (Surgical Anesthesia): Unconscious, regular breathing, loss of reflexes. Ideal for surgery.
  • Stage IV (Overdose): Medullary paralysis, apnea, circulatory collapse. ⚠️ Lethal.

😱 The Unwanted Wake-up (Intraoperative Awareness)

  • Definition: Explicit recall of sensory perceptions (auditory, tactile, pain) during general anesthesia. A devastating complication, can cause PTSD.
  • High-Risk Scenarios:
    • Cardiac surgery, trauma, C-sections
    • Hemodynamic instability requiring ↓ anesthetic
    • History of awareness, substance use
  • Prevention & Monitoring:
    • Vigilant monitoring of vitals & end-tidal anesthetic concentration (ETAC).
    • Processed EEG (e.g., Bispectral Index - BIS) helps titrate anesthetic depth. Target BIS: 40-60.

Warning: The greatest risk is paralysis without amnesia. Always ensure adequate sedation and amnesia (e.g., benzodiazepines, volatile anesthetics) before administering neuromuscular blockers.

BIS monitor sensor placement and signal pathways

⚡ Biggest Takeaways

  • MAC (Minimum Alveolar Concentration) is inversely proportional to potency; it's decreased by age, hypothermia, and other CNS depressants.
  • Low blood:gas solubility (e.g., N₂O, Desflurane) means rapid induction and recovery; high solubility means slow onset.
  • Propofol is common for induction/maintenance; Etomidate is hemodynamically stable but causes adrenal suppression.
  • Ketamine causes dissociative anesthesia and is a sympathomimetic (bronchodilation, ↑HR/BP).
  • Malignant hyperthermia is triggered by succinylcholine/volatile anesthetics; treat with dantrolene.
  • Succinylcholine can cause life-threatening hyperkalemia, especially in burn/crush injury patients.

Practice Questions: General anesthesia principles

Test your understanding with these related questions

Five minutes after arriving in the postoperative care unit following total knee replacement under general anesthesia, a 55-year-old woman is acutely short of breath. The procedure was uncomplicated. Postoperatively, prophylactic treatment with cefazolin was begun and the patient received morphine and ketorolac for pain management. She has generalized anxiety disorder. Her only other medication is escitalopram. She has smoked one pack of cigarettes daily for 25 years. Her temperature is 37°C (98.6°F), pulse is 108/min, respirations are 26/min, and blood pressure is 95/52 mm Hg. A flow-volume loop obtained via pulmonary function testing is shown. Which of the following is the most likely underlying cause of this patient's symptoms?

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Flashcards: General anesthesia principles

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EF < _____% and MI within _____ months are absolute contraindications to non-cardiac surgery

TAP TO REVEAL ANSWER

EF < _____% and MI within _____ months are absolute contraindications to non-cardiac surgery

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