Mechanics of Dialyzabilty & Pharmacokinetic Properties

Let’s talk about the mechanics of dialyzability. There are several factors, one obviously is the blood flow, the rate of drug presentation to the hemodialysis. And then the dialysate flow, rate of the drug clearance into the dialysate. And the permeability of the dialyzer membrane which is fixed once is manufactured. The other factor is configuration of the dialysate flow and the blood flow which can be concurrent, counter current, cross current. And of course dialysis interval also determines the dialyzability. Pharmacokinetic properties also determine dialyzability of a drug. Water solubility. If the solute or the drug is insoluble or very lipid soluble, then likely is not dialyzable. Protein binding. If it’s greater than ninety percent and it’s not dialyzable. Molecular weight.

It’s greater than one thousand and it’s not dialyzable because the membrane is semipermeable membrane. Volume diffusion. If it’s greater than 250 liters, then dialysis contribution is probably not significant Because redistribution from the tissue becomes a rate limiting step. Continue on pharmacokinetic parameters that would affect dialyzability. Dialysis clearance or dialysance of course the greater it is, the more dialyzable the drug is. However, dialysis clearance as a limit of between 10 and 100 milliliters per minutes. Based on blood flow, rate of 300 milliliters per minute that’s translated into an extraction ratio of about 0.3. Clearance. This means the body’s ability to clear the drug.

If it’s greater than 5 liter per hour, then the contribution from the dialysis is likely to be small or insignificant. Half-life. If it’s less than 2 hours that means the body is very efficient in clearing the drug to begin with. Therefore, the contribution from the hemodialysis is small. If you put two factors together, high clearance and the short half-life of the drug. Then the contribution of the hemodialysis becomes insignificant.