# PharmPK Discussion - Volumes of distribution

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• On 2 Nov 2000 at 11:35:40, "manish issar" (missar.aaa.rediffmail.com) sent the message
`Hi AllI have a confusion regarding the apparent volume of distribution andpseudo-steady state volume of distribution. could someone clarifythis.manish`
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• On 2 Nov 2000 at 22:12:12, bvatul.at.ufl.edu sent the message
`Hello ManishThe term Vdss is used to describe the volume that adrug (obeying two compartment model) would occupy atequilibrium. OF course it should be homogenous. Theother two methods which speak of overall distributionvolumes are Vd(exp) and Vd(beta)For a multi comp modelvp(volume of centalcompt)=D0/A+B  (as obtained byfeathering technique)Vd(ss)=Vp+k12/k21*VpVd(extrapolated)=D0/BVd(beta)=K*Vp/betaVd(beta) is influence by changes in K. Vdss is notinfluenced by changesin drug elmination. It reflectsthe true distributional volume when equilibrium isreached. Vdss is greater than Vp. Vp is used in thecalculations of clearance.Hope this helpsAtul`
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• On 3 Nov 2000 at 13:04:04, ml11439.-a-.goodnet.com sent the message
`Manish,     For a drug which confers one compartment characteristics onthe 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 ofthe amount of drug in the body and the plasma concentrationat steady state:                 Vss= Xss/Css     The amount of drug in the body at steady state (Xss) is equalto the amount of drug in the central compartment (Xcss) and in theperipheral 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 thatthe equation for Vss becomes:           Vss=  Vc + Vp     The volume of distribution at steady state is a constant reflectingtotal amount of drug in the body in relation to the plasma concentration.For a drug which confers multicompartment characteristics, the Vdssrepresents the sum of Vc and Vp. For a drug which confers one compartmentcharacteristics, Vdss is equivalent to the one compartment Vd calculatedin equation 1.                                      Mike Leibold, PharmD, RPh                                      ML11439.at.goodnet.comReferemces:1) Gibaldi, M., Perrier, D., Pharmacokinetics, New York, Marcel    Dekker 19752) Gibaldi, M., Perrier, D., Pharmacokinetics, New York, Marcel    Dekker 1982`
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• On 3 Nov 2000 at 20:44:36, ml11439.at.goodnet.com sent the message
`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 thatthe equation for Vss becomes:           Vss=  Vc + Vp     The volume of distribution at steady state is a constant reflectingtotal amount of drug in the body in relation to the plasma concentration.For a drug which confers multicompartment characteristics, the Vdssrepresents the sum of Vc and Vp. For a drug which confers one compartmentcharacteristics, Vdss is equivalent to the one compartment Vd calculatedin equation 1.                                      Mike Leibold, PharmD, RPh                                      ML11439.at.goodnet.comReferemces:1) Gibaldi, M., Perrier, D., Pharmacokinetics, New York, Marcel    Dekker 19752) Gibaldi, M., Perrier, D., Pharmacokinetics, New York, Marcel    Dekker 1982`
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• On 6 Nov 2000 at 12:58:17, "Paul S. Collier" (p.collier.-a-.qub.ac.uk) sent the message
`Manish,You might like to refer to the following paper for a more detailedconsideration of apparent volume of distribution:Collier, P.S. (1983).  Some considerations on the estimation of steadystate apparent volume of distribution and the relationships betweenvolume terms.  J. Pharmacokinetics and Biopharmaceutics, 11, 93-105.PaulDr P.S. CollierSchool of PharmacyThe Queen's University of Belfast97 Lisburn RoadBelfast BT9 7BLN. Ireland,  U.KEmail: p.collier.at.qub.ac.uk`
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