Which statement explains why sevoflurane has a faster onset than isoflurane?

Which statement explains why sevoflurane has a faster onset than isoflurane?

• A. Sevoflurane has a lower blood-gas partition coefficient than isoflurane
• B. Sevoflurane has a higher fat solubility than isoflurane
• C. Sevoflurane is more potent than isoflurane
• D. Sevoflurane has a slower uptake into tissues than isoflurane

Answer: (A)

Onset speed for inhaled anesthetics is governed by how quickly the agent moves from the lungs into the blood and then to the brain, which mainly depends on its solubility in blood. A lower blood-gas partition coefficient means the drug is less soluble in blood, so it doesn’t stay dissolved and equilibrates with the alveolar gas rapidly. This quick rise in in-brain concentration produces a faster induction. Sevoflurane has a lower blood-gas partition coefficient than isoflurane, so it is taken up into the blood more slowly and delivered to the brain faster, yielding a quicker onset. If an agent were more fat-soluble, uptake into blood would be slower and the onset would be slower; potency doesn’t determine the speed of onset, and slower tissue uptake would also delay onset.

Onset speed for inhaled anesthetics is governed by how quickly the agent moves from the lungs into the blood and then to the brain, which mainly depends on its solubility in blood. A lower blood-gas partition coefficient means the drug is less soluble in blood, so it doesn’t stay dissolved and equilibrates with the alveolar gas rapidly. This quick rise in in-brain concentration produces a faster induction. Sevoflurane has a lower blood-gas partition coefficient than isoflurane, so it is taken up into the blood more slowly and delivered to the brain faster, yielding a quicker onset. If an agent were more fat-soluble, uptake into blood would be slower and the onset would be slower; potency doesn’t determine the speed of onset, and slower tissue uptake would also delay onset.