Question:medium
Rapid induction of anaesthesia occurs with which of the following inhalational anaesthetics?
Rapid induction of anaesthesia occurs with which of the following inhalational anaesthetics?
Show Hint
Speed of induction tracks a low blood:gas coefficient, but the drug must also be non irritating to the airway; desflurane is fastest on paper but too pungent for mask induction.
Updated On: Jul 8, 2026
- Isoflurane
- Halothane
- Desflurane
- Sevoflurane
Show Solution
The Correct Option is D
Solution and Explanation
Step 1: Think about the wash in curve.
When a patient breathes an anaesthetic gas, the alveolar concentration climbs toward the inspired concentration over time. The speed of this climb is set by how much of the gas the blood soaks up. A gas that blood holds onto poorly rises fast in the alveoli, and this alveolar level tracks the brain level closely.
Step 2: Rank the drugs by blood solubility.
Halothane dissolves the most in blood, so its wash in curve is the slowest of the four. Isoflurane is next. Sevoflurane and desflurane dissolve the least, so their curves rise fastest. Desflurane technically rises fastest of all, with a blood:gas value near $0.42$.
Step 3: Add the airway tolerance factor.
A fast wash in curve is only useful for induction if the patient can actually breathe the gas comfortably through a mask. Desflurane has a sharp, pungent smell that triggers coughing and airway spasm on inhalation, so despite its numbers it is kept out of mask induction.
Step 4: See why sevoflurane wins in practice.
Sevoflurane has almost as low a blood:gas value as desflurane, around $0.65$, and it smells mild. Patients, especially children, tolerate breathing it in without irritation, so the alveolar level rises quickly and smoothly. This makes it the drug actually used for rapid inhalational induction.
Step 5: Conclude.
Halothane and isoflurane are ruled out on solubility alone, and desflurane is ruled out on tolerability despite its low solubility.
\[ \boxed{\text{Sevoflurane}} \]
When a patient breathes an anaesthetic gas, the alveolar concentration climbs toward the inspired concentration over time. The speed of this climb is set by how much of the gas the blood soaks up. A gas that blood holds onto poorly rises fast in the alveoli, and this alveolar level tracks the brain level closely.
Step 2: Rank the drugs by blood solubility.
Halothane dissolves the most in blood, so its wash in curve is the slowest of the four. Isoflurane is next. Sevoflurane and desflurane dissolve the least, so their curves rise fastest. Desflurane technically rises fastest of all, with a blood:gas value near $0.42$.
Step 3: Add the airway tolerance factor.
A fast wash in curve is only useful for induction if the patient can actually breathe the gas comfortably through a mask. Desflurane has a sharp, pungent smell that triggers coughing and airway spasm on inhalation, so despite its numbers it is kept out of mask induction.
Step 4: See why sevoflurane wins in practice.
Sevoflurane has almost as low a blood:gas value as desflurane, around $0.65$, and it smells mild. Patients, especially children, tolerate breathing it in without irritation, so the alveolar level rises quickly and smoothly. This makes it the drug actually used for rapid inhalational induction.
Step 5: Conclude.
Halothane and isoflurane are ruled out on solubility alone, and desflurane is ruled out on tolerability despite its low solubility.
\[ \boxed{\text{Sevoflurane}} \]
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