pharmacology question: volume of distribution

[amestramgram]

Hi, I have studied and studied the volume of distribution, but I can't for the life of me figure out why we care about this number.

Could somebody explain to me the real life practical meaning of a Vd in non pharmacological terms?

thanks very much!

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[st0w]

Could somebody explain to me the real life practical meaning of a Vd in non pharmacological terms?

Basically, it gives you an idea of how much of the drug is free and available in plasma, as opposed to outside the plasma in various tissues. If the VD is super high, that means most of the drug will be outside the plasma (it doesn't tell you where it will be, just somewhere else). If VD is low, then the drug tends to stay within the plasma.

If you expect a drug to have a particular VD and find the actual value deviates from the expected, this suggests that something is going on that is resulting in more or less of the drug being in plasma vs. tissues. Renal/hepatic failure can affect it (increased fluid retention leads to increased VD because the same concentration of drug is distributed across greater volume), as can being hypovolemic.

 

[amestramgram]

so the liters that describe Vd are not real life liters yes? that brings me to my next question: what is the meaning of a Vd greater/smaller than the volume of the whole body? why does it make sense that the Vd of digoxin = 280L?

thanks

 

[physics junkie]

so the liters that describe Vd are not real life liters yes? that brings me to my next question: what is the meaning of a Vd greater/smaller than the volume of the whole body? why does it make sense that the Vd of digoxin = 280L?

thanks

I can't tell if you're trolling or not. You've "studied and studied" Vd and you don't understand what a Vd of 280L means?

For a 70kg patient:
(3L = plasma volume)
Vd = 6L: drug distributes in the blood volume (includes RBCs+plasma volume)
Vd = 12L: drug distributes in the blood + extracellular fluid
Vd = 42L: drug distributes in the total body water
Vd > 42L: the drug is depositing in areas of the body...digoxin is not distributed in the same concentration throughout your body . This usually means a drug is lipophilic and is passing through cell membranes and accumulating in intracellular spaces.

Here is how you calculate Vd: "I know I gave X grams of drug to the patient IV. Let me take a blood sample from my patient and measure the concentration in terms of Y grams/Liter."

X/Y = Vd.

 

[illixir]

Digoxin has a high Vd because it binds at the Na-K ATPase which maintains the membrane potential and is thus pretty much everywhere in your body. Muscle especially is responsible.