NEURO 24: Anesthesia

1. Evolution of General Anesthesia

• Ancient History: People have tried to induce unconsciousness for surgery since ancient times using alcohol, opium, and herbal extracts.

• 1800s:

  • Nitrous oxide ("laughing gas") was discovered in the 1700s but was mostly used for entertainment ("ether frolics").

  • 1846: The first public demonstration of ether anesthesia by William Morton.

  • 1847: Chloroform was introduced.

• 1900s-Present: Newer anesthetics like cyclopropane (1929), thiopental (1935), and modern inhalational and IV anesthetics were developed. Now, anesthesia is a specialized field requiring knowledge in pharmacology, physiology, and surgery.

2. Components of General Anesthesia

The "ideal" general anesthetic should provide:

1. Amnesia (memory loss)

2. Analgesia (pain relief)

3. Hypnosis (unconsciousness)

4. Muscle relaxation (to prevent movement)

5. Suppression of autonomic reflexes (to keep vital signs stable)

Since no single drug does all of this perfectly, we use "balanced anesthesia" with multiple agents.

3. What is MAC (Minimum Alveolar Concentration)?

• MAC is a way to measure the potency of inhaled anesthetics.

• Definition: The concentration of a vapor in the lungs that prevents movement in response to a surgical incision in 50% of patients.

• Think of it like ED50 (the dose needed to be effective in 50% of people).

• A higher MAC = lower potency (needs more drug to work).

• A lower MAC = higher potency (needs less drug to work).

• MAC is influenced by: age (higher in young people, lower in elderly), temperature (lower in hypothermia), chronic alcohol use (increases MAC).

4. How Do Volatile Anesthetics Work?

No one knows the exact mechanism, but there are two main theories:

❌ Old Theory: Unitary Hypothesis (Meyer-Overton Rule)

• Assumed all anesthetics work the same way by dissolving into cell membranes and altering function.

• Disproven because it couldn’t explain differences in drug potency or why some anesthetics have different effects on the body.

✅ New Theory: Ion Channel Effects

• Enhance inhibitory neurotransmission (increase GABA and glycine receptor activity → causes sedation & unconsciousness).

• Suppress excitatory neurotransmission (block NMDA, acetylcholine, and serotonin receptors → reduces pain, awareness, and movement).

5. Characteristics of Volatile & IV Anesthetics

• Volatile (Inhaled) Anesthetics:

  • Nitrous Oxide (N₂O): Weak but fast-acting, used in dental offices and labor.

  • Sevoflurane: Fast onset, used in children, bronchodilator (good for asthmatics).

  • Desflurane: Rapid onset/offset but pungent and irritates airways.

  • Xenon: New, very potent, but expensive.

• IV Anesthetics:

  • Propofol: Rapid onset, used for induction, lowers blood pressure, not an analgesic.

  • Ketamine: NMDA antagonist, dissociative anesthesia, increases BP & HR, good for trauma patients.

  • Thiopental: Ultra-short-acting barbiturate for induction only.

  • Etomidate: Used for induction, cardiovascular stability.

  • Dexmedetomidine: α₂ agonist, used for light sedation in ICU (not general anesthesia).

6. Side Effects of General Anesthesia

• Common (Short-Term): Nausea, vomiting, sore throat, dizziness, shivering, confusion.

• Rare but Serious:

  • Anesthesia Awareness: Waking up during surgery (1 in 1000 cases).

  • Postoperative Delirium (POD) in Elderly: Confusion/agitation.

  • Postoperative Cognitive Dysfunction (POCD): Long-term memory issues.

  • Malignant Hyperthermia: Genetic reaction to anesthetics → fever, muscle rigidity, hyperkalemia.

7. Special Considerations

• Children: Some studies suggest anesthesia may affect brain development, but results are inconsistent.

• Elderly: Higher risk of POD and POCD due to brain aging.

• Substance Abusers:

  • Smokers: Higher risk of lung complications.

  • Alcoholics: Faster metabolism of anesthetics, risk of withdrawal.

  • Cocaine Users: Risk of hypertension and arrhythmias.

  • Opioid Users: High pain tolerance → may need more post-op analgesia.