Volume of distribution (Vd)

Vd = dose of drug / (plasma concentration before elimination or when steady state is achieved)

Factors influencing Vd

• Lipid solubility
• Binding to plasma proteins
• Molecular size

When protein binding is high or poorly lipid soluble
--> Drug entry into cell is limited
--> Plasma concentration remains high
--> Vd is smaller

For example,

• Non-depolarising neuromuscular blocking agent
  * Has poor lipid solubility
  --> Vd is similar to ECF
• Thiopentone and diazepam
  * Lipid soluble
  --> Vd exceed total body water