Paper I
2019 February (2010 Scheme) · 40 marks · 120 min

Question

Apparent volume of distribution

Q33 marksShort Notes

Answer

Apparent volume of distribution (Vd) is a theoretical/hypothetical volume of fluid into which a drug appears to be uniformly distributed, that would be required to account for the total amount of drug in the body at the concentration observed in the plasma.

Formula: Vd = Total amount of drug in the body / Plasma drug concentration

Interpretation

  • Low Vd (close to plasma volume, ~4 L): drug largely confined to plasma/vascular compartment (e.g., highly plasma-protein-bound drugs like warfarin)
  • High Vd (much greater than total body water, e.g., >100 L): drug extensively distributed into tissues, sequestered outside plasma (e.g., digoxin, chloroquine)

Clinical significance

  • Used to calculate the loading dose required to rapidly achieve a target plasma concentration
  • Reflects the extent of tissue binding/distribution of a drug
  • Important in determining the effectiveness of dialysis in poisoning (drugs with high Vd are poorly removed by dialysis)

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