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NBME 18 Answers

nbme18/Block 1/Question#31 (17.2 difficulty score)
7 yo boy appendectomy
Low blood solubilityπŸ”

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submitted by match95(48),
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lrtd; lwo ulstbiyloi ->-; stafer esnot -t;-&g faetsr rvreceyo

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 +2  
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 weekπŸ™πŸΌ +  
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. +  

submitted by hello_planet(22),

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.

As an aside, I strongly disagree with putting this site behind a large paywall given that all of the content on this site is user-generated. The site owner is profiting entirely off of the work of others, and I refuse to support him. I was able to comment without paying for a subscription, otherwise I would have left entirely at this point.

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