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dear all,
i understand that there are more than one Vd for a two
compartment model.1.volume of distributon of central
compartment,2.V of peripheral compartment,3.Vd(area)
or Vd (beta)=CL/beta .theoretically speaking,what does
Vd (area)mean? can anyone be kind enough to explain?
thank you in advance.
shobha
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[Two replies - db]
X-Originating-IP: [132.204.55.237]
From: "samia ezzine"
To: PharmPK.aaa.boomer.org
Subject: Re: PharmPK Volumes of distribution
Date: Fri, 19 Nov 1999 08:17:56 PST
Hi,
The volume of distribution by area (Vd)area is also known as Vz or Vd
(beta). After a drug is distributed, the total amount of the drug in
the body during the elimination phaseis calculated by using vd(beta).
In fact, Vd (beta) is obtained at the psudo-distribution phase.
I wish that you have your answer.
---
From: "Melethil, Srikumaran K."
To: "'PharmPK.-a-.boomer.org'"
Subject: RE: Volumes of distribution
Date: Fri, 19 Nov 1999 16:32:27 -0600
This is in response to Shoba Rani Hiremath's question on the meaning
of Vd(beta)
in a 2 comp. model. It is apparent volume of distribution in the beta phase, or
the volume after the drug has achieved distribution equilibrium. Or, the
Vd(beta) * Cp = the amount of drug in the body in the beta phase.
Hope this clarifies the concept.
Sri Melethil, Ph.D.
Professor of Pharmaceutics and Medicine
University of Missouri-KC
School of Pharmacy
Room 203-B, 5005 Rockhilll Road
Kansas City, MO 64110
816-235-1794 (fax: 816-235-5190)
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My 2c:
Vbeta is an utterless worthless parameter.
Why do you want to know any apparent PK volume?
First of all you might want to use it to describe (or predict)
concentrations using a PK model. But Vbeta is not a parameter of a 2
cpt model. It is derived from Mickey Mouse PK i.e. using AUC and
"terminal" half-life. You cannot describe a 2 cpt model with 2
parameters. You need 4 (plus parameters for drug input).
Second, you might be interested in understanding something about the
physicochemical and anatomical basis of what determines the
relationship between amount of drug in the body and its
concentration. Vbeta is poor for this also because it is not
independent of the other processes involved in drug disposition
especially the drug distribution phase. Vss is a much better
parameter for this purpose.
So my advice is to consider Vbeta as a historical quirk that was
invented before people figured out how to estimate volumes that were
independent of other parameters such as Vss. It has no role at all in
contemporary or future pharmacokinetic science.
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, Private Bag 92019, Auckland, New Zealand
email:n.holford.-a-.auckland.ac.nz tel:+64(9)373-7599x6730 fax:373-7556
http://www.phm.auckland.ac.nz/Staff/NHolford/nholford.html
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