- On 2 Nov 2000 at 11:35:40, "manish issar" (missar.aaa.rediffmail.com) sent the message

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Hi All

I have a confusion regarding the apparent volume of distribution and

pseudo-steady state volume of distribution. could someone clarify

this.

manish - On 2 Nov 2000 at 22:12:12, bvatul.at.ufl.edu sent the message

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Hello Manish

The term Vdss is used to describe the volume that a

drug (obeying two compartment model) would occupy at

equilibrium. OF course it should be homogenous. The

other two methods which speak of overall distribution

volumes are Vd(exp) and Vd(beta)

For a multi comp model

vp(volume of centalcompt)=D0/A+B (as obtained by

feathering technique)

Vd(ss)=Vp+k12/k21*Vp

Vd(extrapolated)=D0/B

Vd(beta)=K*Vp/beta

Vd(beta) is influence by changes in K. Vdss is not

influenced by changesin drug elmination. It reflects

the true distributional volume when equilibrium is

reached. Vdss is greater than Vp. Vp is used in the

calculations of clearance.

Hope this helps

Atul - On 3 Nov 2000 at 13:04:04, ml11439.-a-.goodnet.com sent the message

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Manish,

For a drug which confers one compartment characteristics on

the body, the Vd is:

Vd= Xo/Co= Xo/K*AUC equation 1

The steady state volume of distribution is employed in

"multicompartment" pharmacokinetics, and is defined in terms of

the amount of drug in the body and the plasma concentration

at steady state:

Vss= Xss/Css

The amount of drug in the body at steady state (Xss) is equal

to the amount of drug in the central compartment (Xcss) and in the

peripheral compartment (Xpss):

Xss= Xcss + Xpss

Substituting this value into the above equation for Vss:

Vss= (Xcss + Xpss)/Css= Xcss/Css + Xpss/Css

But, the term Xcss/Css= Vc, and Cpss/Css= Vp. So that

the equation for Vss becomes:

Vss= Vc + Vp

The volume of distribution at steady state is a constant reflecting

total amount of drug in the body in relation to the plasma concentration.

For a drug which confers multicompartment characteristics, the Vdss

represents the sum of Vc and Vp. For a drug which confers one compartment

characteristics, Vdss is equivalent to the one compartment Vd calculated

in equation 1.

Mike Leibold, PharmD, RPh

ML11439.at.goodnet.com

Referemces:

1) Gibaldi, M., Perrier, D., Pharmacokinetics, New York, Marcel

Dekker 1975

2) Gibaldi, M., Perrier, D., Pharmacokinetics, New York, Marcel

Dekker 1982 - On 3 Nov 2000 at 20:44:36, ml11439.at.goodnet.com sent the message

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Manish,

There was an error in my last email:

Vss= (Xcss + Xpss)/Css= Xcss/Css + Xpss/Css

But, the term Xcss/Css= Vc, and ""Xpss/Css= Vp"". So that

the equation for Vss becomes:

Vss= Vc + Vp

The volume of distribution at steady state is a constant reflecting

total amount of drug in the body in relation to the plasma concentration.

For a drug which confers multicompartment characteristics, the Vdss

represents the sum of Vc and Vp. For a drug which confers one compartment

characteristics, Vdss is equivalent to the one compartment Vd calculated

in equation 1.

Mike Leibold, PharmD, RPh

ML11439.at.goodnet.com

Referemces:

1) Gibaldi, M., Perrier, D., Pharmacokinetics, New York, Marcel

Dekker 1975

2) Gibaldi, M., Perrier, D., Pharmacokinetics, New York, Marcel

Dekker 1982 - On 6 Nov 2000 at 12:58:17, "Paul S. Collier" (p.collier.-a-.qub.ac.uk) sent the message

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Manish,

You might like to refer to the following paper for a more detailed

consideration of apparent volume of distribution:

Collier, P.S. (1983). Some considerations on the estimation of steady

state apparent volume of distribution and the relationships between

volume terms. J. Pharmacokinetics and Biopharmaceutics, 11, 93-105.

Paul

Dr P.S. Collier

School of Pharmacy

The Queen's University of Belfast

97 Lisburn Road

Belfast BT9 7BL

N. Ireland, U.K

Email: p.collier.at.qub.ac.uk

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