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I am not aware of something called blood/lipid coefficient. All I know and I think all what the USMLE will ask you about is the blood/gas coefficient which is directly related to lipid solubility. The more lipid soluble (high blood/gas ratio) the more it will be trapped in the blood.

Remember it's the gaseous nature of the anesthetic that we want to cross the CNS in order to cause the desired effect, therefore if the agent is highly lipid soluble it will be more difficult to diffuse to the brain tissue in the gaseous form. Therefore such an agent will have slow onset of action.

Do not confuse potency with onset of action, these are different concepts. A high blood/gas ratio means high potency and low MAC but it also means slow onset of action and slow recovery.

Similarly, high lipid solubility (high blood/gas ratio) means that the agent will be easier to go to tissues (on it's way to the CNS) and therefore it will have high arteriovenous concentration gradient and therefore it needs more time until it can starts to affects the CNS (slow onset).

Lastly, the relationship between MAC and lipid solubility is hypothetical and is not been solidly proved or measured. It's called Meyer-Overton hypothesis and it has been rejected in several papers and anesthetic situations.
Hey, so I wanted to post on this really old and seemingly resolved thread because it came up in a google search when I began to research this question after getting what was probably the very same Uworld question that prompted this thread. That was an hour ago so I wanted to save the next person a headache of being misled by Uworld and the post I'm quoting.

I too could not find any such thing as a blood/lipid coefficient, but the blood/gas coefficient is the solubility of the anesthetic in liquid/blood, not lipids. It represents the tendency for the anesthetic to disperse throughout your tissues (which are mostly water), and so is inversely proportional to ONSET.

AV gradient represents how much of the anesthetic went out into the tissues between the arterial and venous systems and thus is proportional to the blood/gas partition coefficient, and both are inversely proportional to onset.

You really should know lipid solubility as well (it's even in first aid), and it is the oil/gas partition coefficient (not blood/gas). It has nothing to do with onset, it has everything to do with potency. Lipid solubility is proportional to potency, while MAC is inversely proportional to potency. Neither affect onset. This makes perfect sense given how anesthetics are thought to work. They infiltrate and perturb membranes and hydrophobic portions of proteins - so of course potency would be related to lipid solubility.
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