NBME 18 Low blood solubility (NBME Answers)

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Low blood solubility 🔍 / 📺 / 🌳 / 📖
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submitted by ∗match95(56)

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Inhaled anesthetics that have low blood:gas partition coefficient/blood solubility dissolve in blood more easily. Therefore, there is LESS gas needed to saturate blood -- aka faster saturation of blood. This leads to faster saturation of brain.

tldr; low solubility --> faster onset --> faster recovery

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pelparente great answer; just to add to it. Lipid solubility determines potency, not onset/offset (that is determined by blood solubility as stated above). The more lipid soluble the more potent the drug. The more lipid soluble the drug the higher the oil:gas partition (directly proportional to potency) and the lower the MAC (inversely proportional to potency). Source: Boards and Beyond General Anesthesia Neurology +6

cbreland I really went down a rabbit hole on this one... Convinced myself that it had something to do with mask v. IV anesthesia even though I haven't seen/heard that anywhere. Taking step in a week🙏🏼 +1

jer040512 I thought a low blood:gas partition coefficient meant that it has a low solubility and therefore DOESN'T dissolve in the blood that easily. +2

dhpainte22 Think onset/offset is about blood solubility and potency of drug higher with higher lipid solubility so low blood gas coefficient has faster onset and recovery. +

jurrutia Just to drill point home further: consider halothane, high lipid solubility (so high potency) and high blood solubility (slow onset). If you want fast onset, low solubility is the key. +2

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submitted by ∗hello_planet(43)

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The other comment is partially incorrect. If the drug molecules dissolve in blood, they do not have an anesthetic effect. If they have a low blood solubility, they do NOT dissolve in blood well which leads to a faster anesthetic effect.

The potency is related to the lipid solubility because the drug has to be lipid soluble to cross the blood-brain barrier. A drug with a higher lipid solubility will be able to reach a therapeutic dose in the brain at a lower administered dose, so lower lipid solubility = more potent.

This question asked about a quick induction time, so we want a drug with a low blood solubility so the drug will not dissolve in the blood and will more quickly cross into the brain.

In other terms, a low blood:gas partition coefficient = low blood solubility = short induction time.

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hello_planet Ugh, it appears you can't edit comments. I meant that a higher lipid solubility = more potent. +1

drdoom nice explanation! +1

ringarosie agree, with explanation and "aside" +

freebie Correct. Anyone confused on this should go watch the Boards & Beyond anesthesia video covering this topic. +

kcyanide101 The explanation also applies to the blood gas coefficient. Another factor that affects induction is Aterio/Venous conc. The higher it is the lower the induction as it shows the peripheral tissue is taking it up.......... Here's a youtube video that simplify's it https://www.youtube.com/watch?v=Zsjs_mDc_AM +

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submitted by ∗an1(114)

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↑Blood solubility: slow speed

↓Blood solubility: fast speed

↑Lipid solubility: very potent

↓Lipid solubility: not potent

Potency = 1/ MAC

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