Dialysis patients are all too familiar with the routine of their treatments: Go to the clinic, get weighed, have their temperature and blood pressure taken, have tubes connected from their access to the dialyzer and then sit in the chair until it is time to go home. While waiting, have you ever wondered how a dialysis machine works?
The dialysis machine works by filtering a patient’s blood and removing waste products and excess fluids. The patient’s blood is passed through the AV Fistula into the dialysis machine through plastic tubing. The dialysis machine contains a large canister called the dialyzer, containing thousands of small fibers that form a semi-permeable membrane that filters out waste products. Fresh dialysis solution is pumped around the filters while the patient’s blood runs through it, separated by the thin membrane.
The dialyzer works on the principle of diffusion; therefore, the waste products and excess fluids pass through the fibers in the membrane into the dialysis solution. The dialysis solution, also called the dialysate, helps to carry away the waste products and excess fluid from the blood and is then discarded. The newly filtered blood is passed through a bubble trap to ensure there are no air bubbles in the blood, and the blood is then returned to the patient’s body through the vein of the AV Fistula.
The dialysis machine mixes and monitors the dialysate. Dialysate is the fluid that helps remove the unwanted waste products from your blood. It also helps get your electrolytes and minerals to their proper levels in your body. The machine also monitors the flow of your blood while it is outside of your body. You may hear an alarm go off from time to time. This is how the machine lets us know that something needs to be checked.
The plastic jugs hold the liquids used to mix the dialysate. The machine mixes the dialysate, which is made up of an acidified solution, bicarbonate and purified water. The acidified solution contains electrolytes and minerals. You may hear it referred to as “acid.” The other solution is bicarbonate or bicarb, which is like baking soda. Both are mixed inside the machine with purified water. While you are dialyzing, dialysate and your blood flow through the dialyzer (but they never touch). Fresh dialysate from the machine enters your dialyzer throughout your treatment. Impurities are filtered out of your blood into the dialysate. Dialysate containing unwanted waste products and excess electrolytes leave the dialyzer and are washed down the drain.
Blood tubing carries your blood from your access to the dialyzer. The blood tubing is threaded through the blood pump. You’ll see the blood pump turning in a circular motion. The pumping action of the blood pump pushes your blood through the dialyzer and back into your body.
Blood tends to clot when it moves through the blood tubing. To prevent this the nurse will give you a drug called “heparin.” Your doctor orders the amount of heparin you get at each treatment. That amount of heparin is drawn up into a syringe then placed on the machine into the “heparin pump.” The heparin pump is programmed to release the right amount of heparin into your blood tubing during your treatment. The heparin prevents your blood from clotting.
One problem that may occur during dialysis is that air gets into the blood tubing. To prevent this from happening, blood tubings have two air traps built into them. One trap is before the dialyzer and the other is after it. These traps catch any air that may get into the system. If air does get past these traps an internal machine air sensor shuts down the blood pump and an alarm will sound. All blood flow is stopped until the air is removed.
The machine continuously monitors the pressures created by your blood inside the blood tubing and dialyzer. It also monitors the blood flow, temperature and proper mixture of the dialysate. If any of these go out of range, the machine lets us know by sounding an alarm, blinking lights and shutting down blood or dialysate flow. It also lets us know if your blood pressure is too low or high. Oh yes, it also alarms when it’s time to go home.
As aforementioned, the dialysis machine works on the principle of diffusion. The dialyzing fluid contains less solutes than the blood. Therefore, the excess fluid, waste products and electrolytes will pass from the blood to the dialysate due to the concentration gradient. The blood and dialyzing fluid flow constantly through the dialyzer and increasing the blood flow and the flow of the dialyzing fluid will optimize the diffusion process even further. The dialysate has the same electrolytes and non-electrolytes as blood, but in a lower concentration than the blood of a patient with kidney disease. This blood is called the uremic blood. However, the dialysate contains no phosphate, urea, urate, sulfate or creatinine to create a lower gradient of these electrolytes in order for them to pass from the blood into dialysate and cleanse the blood. This, however, is not the case for electrolytes and non-electrolytes such as magnesium, bicarbonate, lactate and glucose which may be at the same levels as uremic blood or even higher since the intention is not to remove it.
The dialysis machine contains less than 500 milliliters of blood at any given time, and the blood flow back into the patient’s body may be continuous or intermittent. A small amount of heparin is added to the dialysis machine to prevent the blood from clotting and this also helps the blood flow.
Modern-day dialysis machines are very efficient and remove about 100ml to 200ml of urea from the blood per minute. A healthy kidney, however, removes 70ml of urea per minute. The healthy kidney works continuously while dialysis is performed only 2 to 3 times a week. Furthermore, the regulation of water, electrolyte and non-electrolyte levels with a dialysis machine is not able to match the kidney’s ability as this is in a constant state of flux to maintain the optimum levels and therefore homeostasis.