Week 2D - Volume of Distribution Flashcards
volume of distribution (V)
the concentration a drug achieves following
absorption/injection depends on dose and properties of drug distribution in the body
• like CL, V is normally a constant for each drug in each individual, independent of the route of administration
• V can exceed any physical volume in the body
V is an ‘apparent’ volume that varies depends on plasma and tissue binding
immediately after the first dose, the amount of
drug in the body is known:
v= dose/Co
the majority of human body is
water (60%)
physical volumes for a 70 kg person
– total body water: 42 L
– extracellular water: 16 L
– blood: 5.6 L
• plasma: 3 L
warfarin has 8 L of volume distribution
largely retained within the plasma compartment you’re measuring. binding to plasma proteins.
drugs with high V are
not homogeneously distributed in the body
drug amount in the blood/plasma is in rapid equilibrium with
higher concentrations in extravascular tissue, such as adipose tissue
• extent of drug binding to
– plasma proteins
– extravascular tissue
- a drug with high plasma protein binding
* a drug with high tissue binding tends to
often has a small volume of distribution
have a large volume of distribution
loading dose LD
dose given at the onset of therapy to rapidly achieve target concentration (Ctarget) using V: LD = Ctarget x V
- sometimes, units are expressed in volume/kg BW, which must be multiplied by individual’s BW
if an IV bolus dose of 150 mg produces an initial plasma concentration of 0.1 mg/L, then V can be estimated:
v = 150 mg/0.1 mg/L = 1500 L this is similar to:
dissolving 150 mg of drug in 1500 L of water which would give a final concentration of 0.1 mg/L
examples of V
– digoxin: 500 L
– chloroquine: 13000 L
– diazepam: 100 L
CL estimates ____ but V estimates ____
Maintenance dose, Loading dose
- gentamicin has a population V of 18 L
* what is the loading dose required to obtain a therapeutic plasma concentration of 6 mg/L?
L.D. = Ctarget •V L.D. = 6mg/L•18L=108mg
