Ch14 VOLUME OF DISTRIBUTION

Question 1

Volume of Distribution Vd

Answer 1

an expression of the proportion of drug in the plasma compared to the rest of the body.

Drugs with a high Volume of Distribution will be mostly in the tissue, not the plasma

Vd will often be reported as litres per kilogram (L/kg). In this case, the actual mass of the person has to be multiplied by the Vd to be used in drug calculations. There is no standard correction for gender, adiposity, or age.

Question 2

drugs that stay in the extracellular water only

Answer 2

Drugs that have low lipid solubility, such as the antibiotic amoxicillin, tend to stay outside of the cells. If the drug never goes into the cells, the volume of distribution is equal to the plasma plus interstitial fluid together, or 14 L in a 70 kg person

Question 3

Drugs with a high volume of distribution

Answer 3

will penetrate the tissues.

These drugs will need to have high lipid solubility or high tissue binding so that they will distribute to intracellular, plasma, and interstitial volume. The volume of distribution for these drugs would be about 42 L in a 70 kg person

Question 4

a Vd greater than all the water in the person

Answer 4

What a very high Vd indicates is that it would take that volume of plasma to recover all of the drug that has been administered, not that the person has suddenly increased in volume. For example, if a drug has a Vd of 168L (such as diazepam), there is so little drug in the plasma that it would take 168 L to recover
all of the drug.

Question 5

Drugs that have a really high Vd

Answer 5

will easily cross the placenta because of their lipid solubility and may accumulate in the fetus

Question 6

Calculating Volume of Distribution

Answer 6

Vd = D / C0

D= dose in mg
C0 = drug concentration before clearance and after distribution is called the concentration at time zero

The main clinical utility of Vd is to calculate the dose needed to immediately reach the desired drug plasma concentration