Medical Pharmacology Chapter 2: General Principles: Pharmacokinetics
Volume of distribution (Vd) is the ratio between the amount of drug in body (dose given) and the concentration of the drug (C) measured in blood or plasma.
Vd = (amount of drug in body)/C where C is the concentration of drug in blood or plasma.
Vd as calculated is an apparent volume of distribution. For example:
Vd for digoxin is 440 L/70 kg (liters per 70 kg person)
Vd for chloroquine is 13,000 L/70 kg (liters per 70 kg person)
Such very large Vd would be consistent with very high tissue binding, leaving little free in plasma or blood
Vd is an apparent volume of distribution, since Vd is the volume needed to contain the amount of drug homogeneously at the concentration found in the blood, plasma, or plasma water.
Many drugs have a much higher concentration in extravascular compartments; therefore these drugs are NOT homogeneously distributed.
Physical volumes (L./kg body weight) for some body compartments
Total Body Water (0.5-0.7 L/kg) or about 35000 to 49000 ml (70 kg individual)
Extracellular Water (0.2 L./kg)
Blood (0.08 L./kg);
Plasma (0.04 L./kg)
0.2 - 0.35 L./kg
Factors influencing the volume of distribution:
Extent of drug-plasma protein binding
Partition coefficient of the drug in fat (lipid solubility)
Vd may be affected by:
Patient's body composition
Example of a poorly lipid soluble agent with a Vd about equal to extracellular fluid volume: nondepolarizing neuromuscular blocking drugs.
Stoelting, R.K., "Pharmacokinetics and Pharmacodynamics of Injected and Inhaled Drugs", in Pharmacology and Physiology in Anesthetic Practice, Lippincott-Raven Publishers, 1999, 1-17.
Dolin, S. J. "Drugs and pharmacology" in Total Intravenous Anesthesia, pp. 13-35 (Nicholas L. Padfield, ed), Butterworth Heinemann, Oxford, 2000