Dialysis

:This article is about renal dialysis; for the laboratory technique, see dialysis (biochemistry); for the treatment for liver failure, see liver dialysis

Hemodialysis

The principle of hemodialysis (UK: haemodialysis) is somewhat different. It works by having the blood flow along one side of a semipermeable membrane, with the dialysis solution flowing along the other side, usually in the opposite (countercurrent) direction. Due to the difference in osmolarity between the two solutions, solutes diffuse across the membrane along their concentration gradient. A difference in pressure drives water across the membrane, which also pulls along some solutes (solvent drag).

Related Topics:
Semipermeable membrane - Osmolarity - Drag

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The dialysis solution is used at about the body temperature, and consists of a solution of glucose, amino acids and mineral ions. The solution is sterilized. Urea diffuses into the dialysis solution, which does not contain the compound. However, concentrations of glucose, amino acids and minerals are either similar to those of normal plasma to prevent loss, or higher than those of plasma to act as nutrient supplements. If a strong glucose solution is used, osmosis of water from plasma could occur.

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Dialysis is conducted in a dedicated facility, either a special room in a hospital or a clinic that specializes in hemodialysis. Nurses and technicians working in the facility have special training specific to dialysis. Dialysis can also be done in the patient's home. Although this is currently rare in the USA, home hemodialysis has numerous advantages over institutional dialysis.

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A prescription for dialysis by the renal physician (nephrologist) will specify various parameters for setting up dialysis machines, times and durations of dialysis sessions. In the US and UK, 3-4 hour sessions, 3 times a week are typical, although there are patients who dialyse 2 or 4 or 5 per week. There are also a small number of patients who undergo nocturnal dialysis for 8 hours per night 6 nights per week.

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How much dialysis to prescribe is controversial. For 3 times per week sessions, the amount of dialysis given is based on the ratio of the urea concentration in the blood after dialysis compared to the urea concentration in the blood before dialysis. This is referred to as the URR or urea reduction ratio. Current guidelines in the United States as well as in Europe have set a minimum URR of 65% for three times per week dialysis sessions.

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Step-by-step description of hemodialysis:

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• Before or around the time the patient arrives for his/her scheduled session, a dialysis machine will be prepared. There are many models of dialysis machines, but typically in modern machines there will be a computer, CRT, a pump, and facility for disposable tubing and filters. The filters (the actual artificial kidneys) are cylindrical, clear plastic outside with the filter materiel visible inside (looks like thick paper). They are perhaps 15-18 inches long, and 2-3 inches thick. They have tubing connectors at both ends. The technician or nurse will set up plumbing on the machine in a moderately complex pattern that has been worked out to move blood through the filter, allow for saline drip (or not), allow for various other medications/chemicals to be administered. How the plumbing is set up may vary between models of machine and they types of filters. For some filters, it is necessary to clear sterilizing fluid from the filter before connecting the patient. This is done by altering the plumbing to push saline through the filter, and carefully checked with a type of litmus test.
• The pump does not directly contact the blood or fluid in the plumbing ? it works by applying pressure to the tubing, then moving that pressure point around. Think of a disk with a protrusion in it. Put this into a close fitting 270 degree enclosure. Put plastic tubing between the enclosure and the disk, entering and exiting in the 90 open degrees. Now imagine the disk turning. It will put pressure on the tubing, and the pressure point will roll around through the 270 degrees, forcing the fluid to move. It is characteristic of dialysis machines that most of the blood out of the patients body at any given time is visible. This facilitates troubleshooting, particularly detection of clotting.
• The patient arrives and is carefully weighed. Standing and sitting blood pressures are taken. Temperature is taken.
• Access is set up. For patients with a fistula (a surgical modification to an arm or leg vein to make it more robust, and therefore usable for high capacity blood movement required by dialysis) this means inserting two large gauge needles into the fistula. This is painful for the patient but there are various methods of numbing the entry sites before the needles are inserted ? the two most common are lignocaine (lidocaine), a local anaesthetic injected under the skin, and there is also a cream called EMLA which is applied to the skin 45 minutes before the needles are inserted. Fistulas are widely considered the desirable way to get access for hemodialysis, but they take time to set up and mature (anywhere between 5 weeks to 15 weeks). For other patients, access may be via a catheter installed to connect to large veins in the chest. Other arrangements can be made as well.
• When access has been set up, the patient is then connected to the preconfigured plumbing, creating a complete loop through the pump and filter. The pump and a timer are started. Hemodialysis is underway.
• Periodically (every half hour, nominally) blood pressure is taken. As a practical matter, fluid is also removed during dialysis. Most dialysis patients are on moderate to severe fluid restrictive diets (in addition to other dietary restrictions), since kidney failure usually includes an inability to properly regulate fluid levels in the body. A session of hemodialysis may typically remove 2-5 kilograms (5-10 pounds) of fluid from the patient. The amount of fluid to be removed is set by the dialysis nurse according to the patient's "estimated dry weight." This is a weight that the care staff believes represents what the patient should weigh without fluid built up because of kidney failure. Removing this much fluid can cause or exacerbate low blood pressure. Monitoring is intended to detect this before it becomes too severe. Low blood pressure can cause cramping, nausea, shakes, dizziness, lightheadedness, and unconsciousness.
• At the end of the prescribed time, the patient is disconnected from the plumbing (which is removed and discarded, except perhaps for the filter, which may be sterilized and reused with the same patient at a later date). Needle wounds (in case of fistula) are bandaged with gauze, held for up to 1 hour with direct pressure to stop bleeding, and then taped in place. The process is similar to getting blood drawn, only it is lengthier, and more fluid or blood is lost.
• Temperature, standing and sitting blood pressure, and weight are all measured again. Temperature changes may indicate infection. BP discussed above. Weighing is to confirm the removal of the desired amount of fluid.
• Care staff verifies that the patient is in condition suitable for leaving. The patient must be able to stand (if previously able), maintain a reasonable blood pressure, and be coherent (if normally coherent). Different rules apply for in-patient treatment.