The volume of distribution depends on the physiochemical properties of the drug

Drugs have Vd values ranging from 3-5 liters to more than 25 L/kg.

Drugs that are distributed extensively to tissues have higher Vd.

Typical examples of Vd values for some drugs:

Gentamicin 0.25 L/kg
Theophylline 4.5 L/kg
Ampicillin 0.3 L/kg
Digoxin 7.0L/kg

The volume of distribution is the parameter that relates the plasma concentration of the drug to the amount of drug in the body

Amount of drug in the body = Cp x Vd

Example: What is the amount of a drug in the body if the plasma drug concentration is 2 mg/L, and the Vd of this drug is 50 liters ?

Answer:

Amount = Cp x Vd

Amount = 2 mg/L, x 50 L =100 mg

The volume of distribution allows the calculation of the dose required to achieve specific plasma drug concentration.

Dose of the drug = desired Cp x Vd

Example: What is the theophylline iv dose required to achieve plasma concentration of 15 mg/L in a patient who has Vd of 30 L ?

Answer:

Dose = desired Cp x Vd

Dose = 15 mg/L x 30 L = 450 mg