Volume of distribution
Knowledge of a drug’s half-life (t½) is useful in pharmacokinetic monitoring of patients because it gives information on (1) how long it would take for a drug to be eliminated from the body (eg, cases of drug overdose) or how long a dose must be held to reach a certain concentration; (2) the length of a dosage interval if a desired therapeutic range is known; and (3) how long a drug must be given before steady state is reached, since approximately five half-lives must elapse before steady state has been achieved.
Assume first-order elimination and one compartment distribution. Also assume that concentrations are in the elimination phase for all of the following problems.
Equation 1 can be used to determine a dosage interval, which is, of course, simply a time change.
The apparent volume of distribution is the theoretical volume that would have to be available in which a drug will disperse if the concentration everywhere in the body were the same as that in the plasma or serum. Drug concentration sampling generally occurs in the plasma or serum. Equation 6 provides three useful values: D (dose), CΔ (the desired concentration change after the dose), and V (the apparent volume of distribution). Each can be solved if the other two are known or can be estimated.
CLvanc = 0.689 × CrCl + 3.66 (where CLvanc and creatinine clearance [CrCl] are in mL/min)
Population volume of distribution for adults with CrCl > 60 mL/min is estimated to be 0.72 L/kg of actual body weight.
Estimate the vancomycin volume of distribution (in liters), k (in hr−1), and t½ (in hours) for a 34-year-old male who weighs 90 kg (ideal body weight 70 kg) and has an estimated CrCl of 97 mL/min.
Dose used would be 400 mg
V = 64.8 L
k = 0.065 hr−1
t½ = 10.7 hours
For numbered equations mentioned in this text, see Select Pharmacokinetic Equations, Appendix C.