SINGLE COMPARTMENT MODELS
In this model the whole body behaves as a single compartment on whatever the mode of administration of the drug.
Intra Venous Bolus Injection
In an intravenous bolus injection the whole dosage of the drug enter into the blood directly without any medium of absorption.The drug once entered into he blood distribute and equillibrates into various tissues.The body eliminates the drug in first order kinetic.
The volume of the drug distributed(Vd) multiplied by The total amount of the drug in the body(Db)equals to he bolus dosage.
The intravenous dose divided by the Vd will yield the extrapolated initial drug concentration C0(p).When the first order elimination proceed the slope of the graph is represented by k/2.303 where as k-is the first order elimination constant.k-can be calculated by summing up the first order elimination rate constants by various means such as metabolism and renal excretions as follows,
k = km + ke
where km-- the rate constant by metabolism
ke - the rate constant by excretion.
The elimination half life is calculated by
t1/2 = 0.693/k
The volume of distribution Vd is the imaginary volume of body fluid in which the drug is dissolved.
Vd is needed to estimate the amount of drug in the body relative to the concentration of the drug in plasma as follows,
Vd * CP =Db
where Vd is the apparent volume of distribution,Cp is the plasma concentration and Db is the amount of drug in the body.
The Vd and Cp both are inversely proportional to each other.If more drug is distributed extravascularly in the tissues then the Vd is increased and the plasma concentration Cp is decreased and vice versa.
The above equation can be rearranged to find out the apparent volume of distribution (Vd) as follows,
Vd = Db(0)/Cp(0)
Db(0) is the amount of drug given by intravenous bolus injection and Cp(0) is the initial plasma concentration of the drug assumed by extrapolating the slope at the y-axis at which x-axis is zero.
where km-- the rate constant by metabolism
ke - the rate constant by excretion.
The elimination half life is calculated by
t1/2 = 0.693/k
The volume of distribution Vd is the imaginary volume of body fluid in which the drug is dissolved.
Vd is needed to estimate the amount of drug in the body relative to the concentration of the drug in plasma as follows,
Vd * CP =Db
where Vd is the apparent volume of distribution,Cp is the plasma concentration and Db is the amount of drug in the body.
The Vd and Cp both are inversely proportional to each other.If more drug is distributed extravascularly in the tissues then the Vd is increased and the plasma concentration Cp is decreased and vice versa.
The above equation can be rearranged to find out the apparent volume of distribution (Vd) as follows,
Vd = Db(0)/Cp(0)
Db(0) is the amount of drug given by intravenous bolus injection and Cp(0) is the initial plasma concentration of the drug assumed by extrapolating the slope at the y-axis at which x-axis is zero.
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