Now we know that wastes are removed mostly by diffusion. But how does dialysis remove extra water? Water removal in dialysis is called ultrafiltration.
In PD, ultrafiltration is done with a gradient. Sugar (glucose) in the dialysate pulls water out of the blood. A stronger glucose solution removes more water. A weaker glucose solution removes less water.
In HD, ultrafiltration is done with pumps. The blood pump is a horseshoe-shaped loop of blood tubing with a wheel in the center. The wheel squeezes blood through the tubing—squeezing out some water at the same time. Another pump is applied to the dialysate, to create a vacuum. The vacuum pulls water out of the blood. This pulling is called negative pressure.
Reading the Blood Pump
Just above the blood pump on some HD machines is a number for the blood pump speed. Dialysis centers often aim for a blood pump speed of between 300 and 400 for a standard HD treatment. Slower speeds make it harder to get adequate dialysis during a 3 or 4 hour treatment. (Slower speeds, which are much gentler and easier on the heart, are used for nocturnal HD, which is done for 8 hours at a time during sleep.) Faster speeds may damage a vascular access by forcing blood through with too much pressure. Your doctor will decide what speed is right for you.
When your kidneys fail, water can build up all over your body. Most people don't know that there are three places, or "compartments" in the body where water can go:
- Inside cells (intracellular compartment)
- In between cells (interstitial compartment)
- In your blood (vascular compartment)
Dialysis can only remove water that is in your blood. Only about 15% of the water in your body is in your blood. Water moves from one compartment to another by diffusion—which takes time. This means if you gain a lot of water weight, dialysis may not be able to remove it all. The extra water can build up in your body and damage your heart. Water can also build up in your lungs and affect your breathing. How well you feel after a dialysis treatment depends on how much—and how quickly—water is removed.
One study found that it may take as long as 6 hours or more to feel "normal" again after a standard in-center HD treatment. The less water weight you gain, the less you will need to take off. And if water can be taken off slowly, it is easier on your heart.
Now that you know more about how dialysis works, we can talk about your prescription. Which treatment do you use, or plan to use?
Dialysis and Medications
If dialysis removes wastes from the blood, does it remove medications, too? Well, it depends on the drug. Some drugs and vitamins are removed by dialysis. Others—like EPO to treat anemia—are not. If you notice that one of your drugs isn't working as well as it did before you started dialysis, ask your doctor and your pharmacist. They may suggest that you change the time that you take the medication.
