Examples
If you administer a dose D of a drug intravenously in one go (IV-bolus), you would naively expect it to have an immediate blood concentration which directly corresponds to the amount of blood contained in the body . Mathematically this would be:
But this is generally not what happens. Instead you observe that the drug has distributed out into some other volume (read organs/tissue). So probably the first question you want to ask is: how much of the drug is no longer in the blood stream? The volume of distribution quantifies just that by specifying how big a volume you would need in order to observe the blood concentration actually measured.
A practical example for a simple case (mono-compartmental) would be to administer D=8 mg/kg to a human. A human has a blood volume of around 0.08 l/kg . This gives a 100 µg/ml if the drug stays in the blood stream only, and thus its volume of distribution is the same as that is 0.08 l/kg. If the drug distributes into all body water the volume of distribution would increase to approximately 0.57 l/kg
If the drug readily diffuses into the body fat the volume of distribution may increase dramatically, an example is chloroquine which has a 250-302 l/kg
In the simple mono-compartmental case the volume of distribution is defined as:, where the in practice is an extrapolated concentration at time=0 from the first early plasma concentrations after an IV-bolus administration (generally taken around 5min - 30min after giving the drug).
| Drug | VD | Comments |
| Warfarin | 8L | Reflects a high degree of plasma protein binding. |
| Theophylline, Ethanol | 30L | Represents distribution in total body water. |
| Chloroquine | 15000L | Shows highly lipophilic molecules which sequester into total body fat. |
| NXY-059 | 8L | Highly-charged hydrophilic molecule. |
Read more about this topic: Volume Of Distribution
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