Volume of Distribution: Definition & Significance

Instructor: Alexandra Unfried

Alexandra earned her master's degree in nursing education and is currently a hospital supervisor/administrator.

The volume of distribution is a hypothetical amount, meaning that it is not a real volume, but instead is a concept of volume. This lesson will discuss the definition and significance of volume of distribution related to drug concentrations.

How Is Volume of Distribution Used?

Joe is a pharmacist and is trying to calculate the loading dose of antibiotic medication for a hospitalized patient with a bacterial infection. A loading dose is a higher amount of medication that is used at the start of medication therapy in order to provide a therapeutic concentration of the medication in the body. The dosage is then decreased over time. Joe needs to calculate the volume of distribution for the medication and patient in order to provide the correct dosage of medication.

A loading dose using volume of distribution guides treatment
A loading dose using volume of distribution guides treatment

Joe knows that volume of distribution is also used to verify the half-life of a medication. A half-life is the amount of time it takes for a medication to reduce by half in the body, usually dependent on the elimination pattern of the medication. It's also used to help determine how well a medication is cleared during dialysis, or how severe a drug overdose is.

Joe wants to review the definition and calculation of volume distribution before he begins to determine the loading dose of medication. He knows that there are different classifications of volume of distribution and he wants to use the correct one for the patient.

What Is Volume of Distribution?

Volume of distribution (Vd) is defined as the arrangement or rate of incidence of a drug in the body in relation to the measured plasma concentration. It's a theoretical measurement because it is only applied to a sample concentration and does not take into account the entire circulatory system. It's measured when there is an equal distribution of the drug between body tissues and plasma. The volume of distribution is equal to the dose of medication administered, divided by the measure of plasma concentration. However, there are different aspects of when and how to measure the volume of distribution.

  • Initial volume of distribution (V initial): Measured using the intravascular compartment (organs), which calculates the distribution volume close to the time of administration. It's useful in determining the volume of the intravascular compartment, measuring total blood volume, and estimating the peak plasma concentration to figure out a safe loading dose.
  • Extrapolated volume of distribution (V extrap): Establishes a delayed concentration of medication dispersed using peripheral compartments (tissue of the skin, fat, and muscle) to see how it's distributed in the tissues. It is only helpful in finding out if a medication is not able to be cleared by dialysis.
  • Non-compartmental volume of distribution (V area): Uses ongoing concentration measurements of the medication in the plasma and tissue toward the end of absorption or excretion to make an average volume of distribution.
  • Steady-state volume of distribution (V ss): Expresses the distribution when there's a steady state of concentration. It's assumed to be the most accurate for calculating a loading dose of medication.

Now that Joe has reviewed the different classifications of volume of distribution he needs to determine which calculation he wants to use to determine the patient's loading dose of medication. Besides looking at the timing of measurement, there are other factors that alter the volume of distribution. Joe looks at several different factors of the medication and the patient.

Influences on the Volume of Distribution

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