Pharmacokinetics
Apparent volume of distribution
A theoretical volume describing how widely a drug distributes into the body. Vd = Dose / C₀. Values >42 L in a 70 kg adult indicate extensive tissue distribution beyond plasma.
PharmacokineticsDistributionLoading doseHalf-life
Formula
Vd = D / C₀
D = IV dose; C₀ = initial plasma conc.
Total body water
~42 L
Vd >42 L = tissue distribution
t½ relationship
t½ ∝ Vd/CL
Larger Vd → longer half-life
Loading dose
LD = Cp × Vd
IV; divide by F for oral
Vd reference values for selected drugs
| Drug | Vd (L/kg) | Interpretation | Notes |
|---|---|---|---|
| Warfarin | ~0.14 L/kg (~10 L) | Low — confined largely to plasma (high albumin binding) | >99% plasma protein bound |
| Gentamicin | ~0.25 L/kg | Low — distributes into extracellular fluid only (hydrophilic) | Dose on actual body weight; adjust for renal function |
| Theophylline | ~0.5 L/kg | Moderate — distributes into total body water | Use lean body weight; narrow TI |
| Digoxin | ~7 L/kg (~500 L) | Very large — extensive tissue binding (Na/K-ATPase in heart, skeletal muscle) | Long t½ (~36 h); loading dose required |
| Diazepam | ~1–2 L/kg | Moderate-large — lipophilic; brain, adipose accumulation | Prolonged redistribution; context-sensitive half-time |
| Amiodarone | ~40–60 L/kg | Extremely large — lipophilic; extensive tissue storage (lung, liver, thyroid, fat) | t½ 40–55 days; very large loading dose required |
| Chloroquine | ~200–800 L/kg | Enormously large — high tissue affinity (melanin-containing tissues, red cells) | Extremely prolonged t½; tissue toxicity (retina, myocardium) |
| Ibuprofen | ~0.15 L/kg (~10 L) | Low — high plasma protein binding; limited tissue distribution | Enantiomeric; S-form pharmacologically active |
Vd in compartment models
One-compartment model
Drug distributes instantaneously and uniformly throughout a single body volume. Vd = Dose / C₀. Half-life = 0.693 × Vd / CL. Most oral drugs and many IV drugs approximated as one-compartment. See one-compartment model glossary entry.
Two-compartment model
Central compartment (plasma, rapidly equilibrating organs) + peripheral compartment (slowly equilibrating tissues). Bi-exponential plasma-time curve. Vss = Vc + Vp. Use Vss for loading dose calculation; Vc for initial peak estimation.
Obesity and Vd
Lipophilic drugs: significantly increased Vd in obese patients (adipose accumulation) → use total body weight (TBW) or adjusted body weight (ABW). Hydrophilic drugs: Vd less affected by obesity → use ideal body weight (IBW) or lean body weight (LBW) for dosing.
Frequently asked questions
What is the apparent volume of distribution (Vd)?
Vd (apparent volume of distribution) = Dose / C₀ (IV bolus). It is a mathematical construct, not a true anatomical volume. When Vd > 42 L (total body water in a 70 kg adult), it means the drug is preferentially distributed into tissues. Chloroquine ↗ Vd ~200–800 L/kg; amiodarone Vd ~40–60 L/kg. See the core volume of distribution glossary entry and NCBI StatPearls PK ↗.
How is Vd related to half-life and clearance?
t½ = 0.693 × Vd / CL. Larger Vd → longer half-life for the same clearance. Example: digoxin — Vd ~500 L (extensive tissue binding to Na/K-ATPase), CL ~1.6 mL/min/kg → t½ ~36 h. Despite modest protein binding, the vast tissue distribution drives the long half-life. Amiodarone: Vd ~50 L/kg, CL very low → t½ 40–55 days. Use the half-life calculator to explore Vd/CL/t½ relationships.
How does Vd affect the loading dose?
Loading dose = Cp target × Vd (IV). For oral: LD = Cp target × Vd / F. Digoxin (Vd ~500 L): target 1–2 ng/mL → LD ≈ 0.5–1 mg — distributed across 500 L. This is why digoxin "loads" slowly over 24 h with oral digitalisation rather than a single bolus. High-Vd drugs (chloroquine, amodarone) require large loading doses — see loading dose glossary entry. Low-Vd drugs (e.g., gentamicin Vd ~0.25 L/kg) give high initial plasma levels relative to dose.
What factors increase or decrease the apparent Vd?
Vd ↑ with: high lipophilicity (adipose accumulation), low plasma protein binding, high tissue binding, alkaline urine (basic drugs). Vd ↓ with: high plasma protein binding (drug retained in blood), hydrophilicity, acidic conditions (trapping of weak acids). Practical: obesity significantly increases Vd of lipophilic drugs (propofol, lipophilic opioids, benzodiazepines) → use ideal body weight (IBW) or adjusted body weight (ABW) for dose calculations. See pharmacokinetics basics and renal dosing guide.
What is the difference between the central and peripheral volume of distribution?
Two-compartment model: Vss = Vc + Vp. Vc = blood/plasma + rapidly equilibrating organs. Vp = slowly equilibrating peripheral tissues. After IV bolus, the plasma-time curve shows a rapid distribution phase (alpha, central → peripheral equilibration) then a slower elimination phase (beta). Amiodarone: large Vp (lipophilic tissue storage) → very slow beta phase → t½ 40–55 days. Understanding Vc vs Vss is critical for loading dose (use Vss) vs initial peak concentration estimation (use Vc). See one-compartment model.