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Volume of Distribution Equation:
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The Volume of Distribution (Vd) is a pharmacokinetic parameter that quantifies the apparent distribution of a drug between plasma and the rest of the body after oral or parenteral dosing. It represents the theoretical volume that would be necessary to contain the total amount of administered drug at the same concentration observed in the blood plasma.
The calculator uses the Volume of Distribution equation:
Where:
Explanation: The equation shows the relationship between the amount of drug in the body and the concentration of drug in the plasma.
Details: Vd helps determine the loading dose required to achieve a target plasma concentration. It also provides insight into a drug's distribution characteristics - whether it remains primarily in the vascular system or distributes extensively into tissues.
Tips: Enter dose in mg and concentration in mg/L. Both values must be positive numbers. The calculator will compute the volume of distribution in liters.
Q1: What does a high Vd indicate?
A: A high Vd (> total body water) suggests extensive tissue binding, while a low Vd suggests the drug is mainly confined to plasma.
Q2: What are typical Vd values?
A: Vd ranges from 3-5 L (plasma volume) to >40 L (extensive tissue distribution). For example, warfarin has Vd ~10 L while digoxin has Vd ~500 L.
Q3: How is Vd used clinically?
A: Vd is used to calculate loading doses (Dose = Vd × desired concentration) and to understand drug distribution characteristics.
Q4: Does Vd change in disease states?
A: Yes, conditions like edema, obesity, or altered protein binding can affect a drug's Vd.
Q5: What's the difference between Vd and actual body volume?
A: Vd is a theoretical concept - it may be much larger than actual body volume if the drug concentrates in tissues.