5. Apparent volume of distribution, V
We can also use the equations above to calculate the plasma concentration at any time when we know kel and Cp0. However, usually we don't know Cp0 ahead of time, but we do know the dose. A dose in mass units, maybe in mg. To calculate Cp0 we need to know the volume that the drug is distributed into. That is, the apparent volume of the mixing container, the body. This apparent volume of distribution is not a physiological volume. It won't be lower than blood or plasma volume but it can be much larger than body volume for some drugs. It is a mathematical 'fudge' factor relating the amount of drug in the body and the concentration of drug in the measured compartment, usually plasma.
Equation IV-11. Definition for Apparent Volume of Distribution
Equation IV-12. Relationship between Amount and Concentration
Immediately after the intravenous dose is administered the amount of drug in the body is the dose. Thus:-
Equation IV-13. Volume calculated from Cp0
Or
Equation IV-14. Initial Concentration
Table IV-2. Example values for apparent volume of distribution2
| Drug | V (l/kg) | V (l, 70 kg) |
| Sulfisoxazole | 0.16 | 11.2 |
| Phenytoin | 0.63 | 44.1 |
| Phenobarbital | 0.55 | 38.5 |
| Diazepam | 2.4 | 168 |
| Digoxin | 7 | 490 |
The last figure, for digoxin, is much larger than body volume. Drug must be extensively distributed into tissue, leaving low concentrations in the plasma, thus the body as a whole appears to have a large volume, of distribution. Remember, this is not a physiological volume.
Diagram IV-3. Apparent Volume of Distribution
In summary these equations apply:
Equation IV-15. Concentration as a Function of Time
since
(Equation IV-14, from above)
2Gibaldi, M. 1984 "Biopharmaceutics and Clinical Pharmacokinetics", 3rd ed., Lea & Febiger, Chapter 12, page 214
This page was last modified: 12 February 2001
Copyright 2001 David W.A. Bourne