NBME 12 question help

[cpufreak3]

Didn't want to put any question info in the title and spoil it for others.

Anyway, this is the question regarding a graph of "drug x" and two different curves (plasma conc. vs time).

The correct answer was a larger Vd for the obese patient.

What I don't understand is, wouldn't a larger Vd cause a lower C_ss (steady state) since both patients are given the same dose?

So the curve for the obese patient should not peak at the same height (but does in the graph provided).

I answered a "slower absorption rate," since it took longer for the obese person to reach the same STEADY STATE.

Anyone have an input? Thanks

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[Sloo0]

I took that exam too, and I think I picked the same answer as you. I dont remember the details, but I know that as soon as I saw the right answer that it made sense.


HOWEVER, I will say that regardless of that that, what you are saying cant be correct simply because steady state is reliant on half life/

 

[addo]

The curve showed a longer time to get to steady state. The time to get to steady state is usually 4-5 half lives, so the longer time to steady state meant longer half life.
From the equation T1/2=ln2*Vd/CL, you can see that either a higher Vd or lower CL will result in a longer half life.

 

[cpufreak3]

I understand that the time to steady state is reliant on half-life, so in a sense, the "slow absorption" is wrong.

However, the steady state VALUE is also dependent on Vd, so why are they the same? That was really what made me change my answer, because I told myself if you're giving the same dose to both patients, and one has a larger Vd, his steady state conc will be lower...

Here's what I mean (provided graph on left, what I think it should be on right if Vd is higher and both given the same dose):

 

[CrouchingLiger]

But steady state concentration is not dependent on Vd. Maintenance dose = (steady state concentration) * (clearance). Or rearranged, SS=MD/CL. If both people are being given the same dose and the drug has the same clearance, the steady state concentration will be the same.

 

[cpufreak3]

But steady state concentration is not dependent on Vd. Maintenance dose = (steady state concentration) * (clearance). Or rearranged, SS=MD/CL. If both people are being given the same dose and the drug has the same clearance, the steady state concentration will be the same.

But clearance is dependent on Vd

Cl = Vd * Ke