Question about Vd.

Patients have different volumes of distribution for a compound X, what is the major reason for
difference?

 

First you should understand the true definition of Vd. Vd is the volume needed to contain a drug at the same concentration as it would be in the plasma. Remember that this is a theoretical volume.

This is partly dependent on the total body water and the property of the administered drug, but you're right that body composition can affect Vd. Thus, there may be variation between patients.

Key to remember Vd in those with renal/liver/heart failure, as fluid retention affects Vd.

From a testing standpoint, you should be aware of the formula and how to apply it to general scenarios and how to work the equation forwards and backwards. Remember that Step 1 will never try and trick you, so don't overthink these. It's easy points! It's not the most common equation to see on your exam, but an easy one to know and score points on if you're asked about it. Hope that this answered your question, but let me know if you would like more specifics about Vd.

If you want more info on Vd, go ahead and knock yourself out with this paper I found doing a quick search
http://www.ncbi.nlm.nih.gov/pubmed/15601439

 

Thanks Orthopod! Now it's clear