Drug distribution: The reversible transfer of a drug between the blood and the extravascular fluids and tissues of the body.
Volume of Distribution: A hypothetical volume of body fluid into which a drug is distributed.
The Volume of distribution (VD), also known as the apparent volume of distribution, is used to quantify the distribution of a drug between plasma and the rest of the body after oral or parenteral dosing.
It is called the Apparent Volume because all parts of the body equilibrated with the drug do not have equal concentration.
It is defined as the volume in which the amount of drug would be uniformly distributed to produce the observed blood concentration.
The apparent volume of distribution is a proportionality constant relating the plasma concentration to the total amount of drug in the body.
XαC
X = VdC
Vd = X/C
Apparent volume of distribution = amount of drug in the body/ plasma drug concentration.
It is expressed in liters or liters /kg body weight.
Apparent volume of distribution is dependent on concentration of the drug in plasma.
Drugs with a large apparent volume are more concentrated in extravascular tissues and less concentrated intravascular.
Applications of volume of distribution:
Vd provides qualitative information.
Amount of drug in the body, DB can be determinedIn case the target C1 is known, dosage regimen can be fixed for therapy.
Loading dose required to achieve steady stage C1 can be estimated.
Dose required for individual patients can be calculated.
Drug – drug interaction can be explained.
Total body clearance can be calculated.
Commonly Asked Question.
Write a note on the volume of distribution.
Define Volume of Distribution. Describe the importance of volume of distribution in detail.