directory of Chem Help ASAP videos: [ Ссылка ]

Okay. Volume of istribution, let's first talk/remind ourselves what distribution actually is. So distribution is the movement of a drug both to and from the plasma and its site of action. So the drug is continuously going from the plasma, to its site of action, and back into the plasma. The apparent volume of distribution, which I will typically just call volume of distribution, or Vd, is quantification of a drug's distribution. We're going to put a number that will describe the distribution of our drug. The units for volume of distribution are typically L/kg. You might see a drug reported as just liters but just like clearance, we typically divide this by the mass of the patient. Volume of distribution is calculated from clearance and half-life. Volume of distribution equals the product of half-life and clearance divided by 0.693. As always, be careful with units. In the previous video we calculated a molecule's clearance. As it turns out, that molecule was metoprolol. Metoprolol, we calculated its clearance as being 15.4 mL/min/kg patient mass, and the half-life of this molecule is 3.2 hours. With these values and some unit conversions, we can calculate the molecule's volume of distribution. So here is our math to determine the volume of distribution, This looks like a lot of numbers. It's not. Here is our half-life. There is our clearance, and then the second line I added a couple terms to make sure our units work properly. Always be careful with units in any of these calculations. Out pops at the end our number, our volume of distribution of 4.3 L/kg. That's liters of plasma per kg of patient mass. Okay. The typical range for volume of distribution for oral drugs is about 0.1 to 100 L/kg. Drugs with smaller Vd values tend to concentrate mostly in the plasma. As you get higher volume of distribution values, the drug is more and more concentrated in the tissues. There is no ideal volume of distribution value, but a drug must be able to distribute to its target protein. A blood thinner may act in the blood, so a small volume of distribution of 0.2 L/kg would be fine. A drug acting on nuclear receptors within a cell would need a much higher volume of distribution. I'd like to close with just a couple final thoughts on volume of distribution. One, distribution is not uniform. A drug is not uniformly in a tissue. It may concentrate in skin and not muscle, or it may concentrate in bone but not fat. So drugs have different properties, and they tend to concentrate in certain tissues, and the tissue may include the blood. Maybe it concentrates in the plasma. The second idea, volume of distribution is a number, but it's purely a hypothetical number. We call this apparent volume of distribution because it's not real. It's a variable that we determine, an important variable and useful, but we use it to interpret and understand how a drug is behaving in the body. But this is not a real value.

свернуть