volume of distribution

Vd = Total amount of drug in body / Plasma drug concentration (Vd = Dose / C0). It is a theoretical volume, not a true anatomical space, that indicates how a drug is distributed between plasma and tissue compartments.

Key concepts: - Low Vd (~4-8L, approximates plasma volume): drugs that are highly protein-bound (e.g., warfarin, heparin) or large/hydrophilic molecules that stay in the vascular space. - Moderate Vd (~15-40L, approximates extracellular fluid or total body water): drugs that distribute into extracellular space but don't heavily penetrate cells. - High Vd (>40L, exceeds total body water, may reach hundreds of liters): lipophilic drugs that extensively distribute into tissues (e.g., digoxin ~500L, chloroquine, TCAs). High Vd drugs are typically NOT easily removed by dialysis because most of the drug resides in tissues, not plasma.

Clinical applications: - Loading dose calculation: Loading dose = Vd × Target plasma concentration / Bioavailability. This allows rapid achievement of therapeutic plasma levels, especially important in emergencies (e.g., digoxin loading in arrhythmias). - Vd affects half-life: t1/2 = 0.693 × Vd / Clearance. Higher Vd (with constant clearance) increases half-life since drug takes longer to be cleared from tissues. - Renal failure/dialysis: Drugs with low Vd and low protein binding are more dialyzable (e.g., lithium, aspirin in overdose). High Vd drugs (e.g., digoxin, TCAs) are poorly dialyzable. - Disease states alter Vd: Edema/ascites increases Vd for hydrophilic drugs; obesity increases Vd for lipophilic drugs; dehydration decreases Vd.

Board-relevant examples: - Digoxin: Vd ~500L (extensive tissue binding, especially muscle) — poorly dialyzable, requires loading dose. - Warfarin: Low Vd (~8L), highly protein bound to albumin. - Aminoglycosides: Vd approximates extracellular fluid volume; dosing must be adjusted in fluid-overloaded or dehydrated patients.

Understanding Vd helps predict drug behavior in special populations (renal/hepatic impairment, pediatrics, elderly) and guides dosing adjustments and toxicity management (e.g., use of hemodialysis in overdose).