Volume of distribution (Vd)

• Vd
  = dose of drug / (plasma concentration before elimination or when steady state is achieved)
• Alternatively
  --> Vd is a theoretical volume required to produce the observed plasma concentration with a given drug dosage
  * [???]

Factors influencing Vd

• Lipid solubility
• Binding to plasma proteins
• Molecular size

Thus,

• 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