Anesthetic Principles - The Gas Laws
- Mechanism of Action: Primarily enhance GABA-A receptor activity, leading to ↑ chloride influx and neuronal hyperpolarization. Potency is directly proportional to lipid solubility (Meyer-Overton rule).
- Minimum Alveolar Concentration (MAC): Alveolar concentration that prevents movement in 50% of patients in response to surgical incision. It is a measure of potency, where $$MAC \approx 1/potency$$.
| Factors ↑ MAC (↓ Potency) | Factors ↓ MAC (↑ Potency) |
|---|---|
| Hyperthermia, Hypernatremia | Hypothermia, Hypotension |
| Chronic alcohol abuse | Acute alcohol intoxication |
| ↑ CNS catecholamines (anxiety) | Co-administration of opioids, benzos |
| Young age (infants) | Elderly, Pregnancy |
- **Blood/Gas Partition Coefficient:** Determines the speed of onset and recovery. A low coefficient means low blood solubility, leading to rapid induction and emergence.
- **Oil/Gas Partition Coefficient:** Reflects lipid solubility and thus potency. A high coefficient indicates high potency (low MAC).
⭐ Nitrous oxide has a very low blood/gas partition coefficient (0.47) for rapid onset, but a high MAC (>100%), making it a weak anesthetic.
Agent Profiles - The Vapor Lineup
| Agent | Potency (MAC) | Onset Speed (Blood:Gas Coeff.) | Key Features/Uses | Major Adverse Effects |
|---|---|---|---|---|
| Nitrous Oxide | ↓ Potency (MAC >100%) | ↑↑ Fast Onset (0.47) | Strong analgesic; second gas effect | Diffusion hypoxia; B12 inhibition |
| Halothane | ↑ Potency (MAC 0.75%) | ↓↓ Slow Onset (2.4) | Pleasant odor; pediatric use (historical) | 📌 Hepatotoxicity; arrhythmia |
| Desflurane | ↓ Potency (MAC 6%) | ↑↑↑ Fastest Onset (0.42) | Rapid recovery; outpatient surgery | Airway irritation; sympathetic stimulation |
| Sevoflurane | ↑ Potency (MAC 2%) | ↑↑ Fast Onset (0.65) | Bronchodilator; inhalation induction | Nephrotoxicity (Compound A) |
| Isoflurane | ↑ Potency (MAC 1.2%) | ↓ Medium Onset (1.4) | Stable CV profile; pungent odor | Coronary steal (theoretical); respiratory depression |
Toxicity & Antidotes - When Gas Goes Bad
- Malignant Hyperthermia (MH): A life-threatening hypermetabolic state of skeletal muscle triggered by inhaled anesthetics (except N₂O) and succinylcholine.
- Pathophysiology: Due to defective ryanodine receptors (RYR1), leading to massive, uncontrolled Ca²⁺ release from the sarcoplasmic reticulum.
- Signs: Sudden ↑ETCO₂, tachycardia, masseter/generalized muscle rigidity, rapid ↑temperature, hyperkalemia, and metabolic acidosis.
- Acute Management Flowchart:
- Other Major Toxicities:
- Halothane Hepatitis: Fulminant hepatic necrosis (immune-mediated).
- Nephrotoxicity: Sevoflurane can form "Compound A" in CO₂ absorbents, a potential nephrotoxin.
- Desflurane: Rapid increases can cause sympathetic stimulation (tachycardia, hypertension).
⭐ Dantrolene acts by binding to the ryanodine receptor (RYR1), inhibiting further calcium release from the sarcoplasmic reticulum, directly counteracting the MH pathology.
High‑Yield Points - ⚡ Biggest Takeaways
- MAC (Minimum Alveolar Concentration) is inversely proportional to potency; a lower MAC indicates a more potent anesthetic.
- The blood:gas partition coefficient determines induction speed. Low solubility (low coefficient) results in a faster onset and recovery.
- Most trigger malignant hyperthermia (except N₂O), a life-threatening condition treated with dantrolene.
- Common side effects include myocardial and respiratory depression, leading to hypotension.
- Watch for specific toxicities: halothane (hepatotoxicity) and methoxyflurane (nephrotoxicity).
- Nitrous oxide offers good analgesia but has low anesthetic potency.
Unlock the full lesson and continue reading
Signup to continue reading this lesson and unlimited access questions, flashcards, AI notes, and more
