Core15: Volume of distribution

🧪 High-yield / 要点：Volume of distribution Vd is a theoretical volume: Vd = amount of drug in body / plasma concentration. High Vd usually means good tissue distribution / lipophilicity / tissue accumulation.

Definition / 定義

• Vd / Volume of distribution = hypothetical fluid volume needed to explain the measured plasma concentration.
• Formula:
  • Vd = IV administered dose ÷ plasma concentration
  • Also written as Vd = amount of drug in body / C plasma
• It is not a real anatomical volume; it reflects where the drug “appears” to distribute.

Low vs high Vd / 低Vdと高Vd

Vd	Meaning	Typical pattern
Low Vd	Drug stays mainly in plasma	Large plasma concentration fluctuations
High Vd	Drug leaves plasma and accumulates in tissues/fat	Smaller plasma fluctuations due to tissue/adipose reservoir

Theoretical distribution / 理論的な分布

Example Vd	70 kg equivalent	Main interpretation	Possible reason
20 L/kg	1400 L	Strong tissue accumulation	High lipophilicity, tissue protein binding
0.6 L/kg	42 L	Whole body water distribution	Leaves plasma and enters cells
0.2 L/kg	14 L	Extracellular fluid distribution	No cell penetration
0.04–0.06 L/kg	~3–4 L	Mainly plasma	Strong plasma protein binding / limited distribution

Practical examples / 実例

• Amitriptyline：~19 L/kg → very high tissue distribution.
• Chlorpromazine：~20 L/kg → very high Vd.
• Digoxin：~7 L/kg → high tissue distribution.
• Phenytoin：~0.6 L/kg → intermediate; real value reflects both protein binding and tissue accumulation.
• Warfarin：~0.14 L/kg → low Vd, strong plasma protein binding.
• Heparin：~0.06 L/kg → mainly plasma.

Remember / 覚え方

• High Vd = hidden in tissues
• Low Vd = stays in blood
• Vd increases with lipophilicity and tissue binding
• Vd decreases with strong plasma protein binding