Pharmacology
Inhaled anesthetics
MAC is the alveolar concentration at which 50% of patients don’t move to surgical incision — the common currency for volatile dosing.
| Agent | MAC (≈ age 40, in O₂) | Key traits |
|---|---|---|
| Sevoflurane | ≈ 2.0% | Sweet, non-pungent → inhalational induction; compound A (label limits on low fresh-gas flow) |
| Desflurane | ≈ 6.0% | Pungent (airway irritation); fastest washout; sympathetic surge with rapid increases; heated vaporizer; highest global-warming impact — many systems phasing it out |
| Isoflurane | ≈ 1.2% | Pungent, slower, inexpensive |
| Nitrous oxide | ≈ 104% | Expands closed gas spaces; PONV; diffusion hypoxia at emergence; inactivates vitamin B12 |
MAC goes down with
- Age (≈ 6% per decade after 40)
- Opioids, benzodiazepines, α2 agonists
- Hypothermia, pregnancy
- Acute alcohol intoxication
MAC goes up with
- Chronic alcohol use
- Hyperthermia
- Hyperthyroidism
- Infants (peak ≈ 6 months)
Related: MAC-awake ≈ 0.3–0.4 MAC · MAC-BAR (blunts adrenergic response) ≈ 1.5 MAC. Age-adjusted MAC calculator →
Nitrous oxide: avoid with closed air spaces
Pneumothorax, bowel obstruction, middle-ear surgery, intraocular gas bubble (retinal surgery), air embolism, pneumocephalus. Give 100% O₂ at the end to prevent diffusion hypoxia.
Pimp questions
Pimp question 1
Why is sevoflurane used for inhalational inductions but not desflurane?
Answer
Sevo is non-pungent; des irritates the airway (coughing, laryngospasm).
Pimp question 2
What is diffusion hypoxia?
Answer
At the end of an N₂O anesthetic, N₂O floods out of the blood into the alveoli, diluting O₂. Prevent with 100% O₂ for several minutes.
Pimp question 3
Why does N₂O expand a pneumothorax?
Answer
N₂O is ≈ 30× more soluble than nitrogen: it diffuses into the air space faster than nitrogen can leave.