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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
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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
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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
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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
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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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Copyright 1995-2010 David W. A. Bourne (david@boomer.org)