Examqo › Examqo library › Anesthesiology › Anaesthetic pharmacology · IMLE
Inhaled anaesthetics and MAC
An inhaled agent's potency is measured in MAC, and its speed is set mainly by its solubility in blood: the lower the blood/gas partition coefficient, the faster both induction and waking.
Why it matters in the exam
This is one of the most frequently asked topics in anaesthesia: what raises and lowers MAC, which agent is fastest, what is special about N2O (expansion of closed spaces, diffusion hypoxia), and which agent suits inhalational induction. There are also questions on compound A with sevoflurane, on CO with dry CO2 absorbents and on the heated vaporiser for desflurane.
MAC is the alveolar concentration, at one atmosphere, that prevents movement in response to a skin incision in 50% of patients. MAC values add up: 0.5 MAC of N2O plus 0.5 MAC of sevoflurane is roughly 1 MAC. All halogenated agents lower blood pressure, dilate the bronchi and the cerebral vessels, potentiate muscle relaxants, relax the uterus at high concentration and trigger malignant hyperthermia — all except N2O.
In questions on the speed of induction, think of three factors: how much agent goes in (inspired concentration and ventilation), how much the blood "steals" on the way (solubility and cardiac output), and the difference between lung and blood. The less the blood steals, the faster the alveolar concentration rises and the faster the brain falls asleep. By the same logic, fast waking depends on an agent that does not stay in blood and tissues. In questions on MAC, the list of factors that do not change it matters as much as the list of those that do, because most distractors are built from it.
Key points
- MAC = the concentration that prevents movement to incision in 50%.
- Low blood/gas coefficient = fast induction and waking (desflurane and N2O are the fastest).
- MAC falls with age, cold, pregnancy and anaesthetic drugs; it rises in infants, with fever and with chronic alcohol use.
- N2O expands closed spaces and causes diffusion hypoxia.
- Sevoflurane for inhalational induction; desflurane irritates the airway.
Summary table
| Agent | MAC (%) | Blood/gas coefficient | Exam feature |
|---|---|---|---|
| N2O | 104 | 0.47 | Expands air spaces; diffusion hypoxia; does not trigger malignant hyperthermia |
| Desflurane | 6.0 | 0.45 | Fastest of the halogenated agents; irritates the airway; needs a heated vaporiser |
| Sevoflurane | 2.0 | 0.65 | Non-irritant — suits inhalational induction; compound A |
| Isoflurane | 1.2 | 1.4 | Cheaper and slower; mildly irritant |
| Halothane | 0.75 | 2.4 | Arrhythmias with adrenaline; hepatitis |
A free exam question
איך נוצר compound A ? (the question as examined, in Hebrew)
- א. תגובת חיזור של sevoflurane בכבד
- ב. תגובת חיזור של sevoflurane בטובולי הפרוקסימלי בכליה
- ג. תגובה של sevoflurane עם בסיס חזק בסופח CO2
- ד. משתחרר באופן ספונטני כאשר sevoflurane נשמר בטמפרטורת חדר
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Anesthesiology, Medical licensing exam (IMLE).
Subscriber? Read the full topic Plans and prices 25 free questions
Sources
- Miller's Anesthesia, 9th edition (2020) — Inhaled Anesthetics: Mechanisms of Action; Uptake and Distribution
- Morgan & Mikhail's Clinical Anesthesiology, 7th edition (2022) — Inhalation Anesthetics
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.