Definition
The apparent volume of distribution (Vd) is a theoretical pharmacokinetic parameter that describes the distribution of a drug in the body.
It represents the hypothetical volume in which a drug would need to be evenly distributed to match its plasma concentration.
Formula
Vd is expressed in liters (L) and helps determine drug dosing and distribution characteristics.
Interpretation of Vd Values
Low Vd (< 10 L): Drug is primarily confined to plasma (e.g., highly protein-bound, hydrophilic drugs).
Moderate Vd (~10–30 L): Drug distributes into extracellular fluids or body water.
High Vd (> 40 L): Drug extensively distributes into tissues (e.g., lipophilic drugs with high tissue binding).
Clinical Significance of Vd
Dosing Regimen: Vd is used to calculate the loading dose needed to achieve a target plasma concentration quickly
Drug Characteristics: Lipophilic drugs tend to have a high Vd, while hydrophilic drugs with limited tissue penetration have a low Vd.
Factors Affecting Vd: Age, gender, body weight, disease states, and drug interactions can influence Vd, impacting drug dosing and distribution.
