Concepts
Non-clinical sciences/Pharmacology principles · Pharmacokinetics
Volume of distribution (Vd)
What it means
Vd = total drug in body / plasma concentration — an apparent volume. Low Vd (warfarin, heparin) = drug stays in plasma → often dialysable. High Vd (digoxin, amiodarone, TCAs) = extensive tissue binding → long half-life, NOT effectively dialysable. Lipophilic drugs have high Vd and cross the blood–brain barrier/placenta.
Worked example
A high Vd explains why digoxin/amiodarone toxicity isn't cleared by dialysis and why loading doses are large.
In the exam
A massively tissue-bound drug has an apparent distribution volume far exceeding total body water, so haemodialysis removes little of it.
What settles it
High Vd = tissue-bound, long t½, not dialysable; low Vd = plasma-confined, often dialysable.
Classically confused with
Bioavailability & first-pass
Source: StatPearls — Pharmacokinetics