Hemodialysis
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In medicine, hemodialysis, also haemodialysis, is a method for removing waste products such as potassium and urea, as well as free water from the blood when the kidneys are incapable of this (i.e. in renal failure). It is a form of renal dialysis and is therefore a renal replacement therapy.Hemodialysis is typically conducted in a dedicated facility, either a special room in a hospital or a clinic (with specialized nurses and technicians) that specializes in hemodialysis. Although less typical, dialysis can also be done in a patient's home as home hemodialysis.
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[edit] Principle
The principle of hemodialysis is the same as other methods of dialysis; it involves diffusion of solutes across a semipermeable membrane. In contrast to peritoneal dialysis, in which transport is between fairly static fluid compartments, hemodialysis relies on convective transport and utilizes counter current flow, where the diasylate is flowing in the opposite direction to blood flow in the extracorporeal circuit. Counter-current exchanges maintain the concentration gradient across the membrane at a maximum and increase the efficiency of the dialysis.
The efficiency of waste clearance during hemodialysis is much higher than in natural kidneys. Therefore, dialysis treatments do not have to be continuous and can be performed intermittently, typically three times a week.
Fluid removal (ultrafiltration) is achieved by altering the hydrostatic pressure of the dialysate compartment, causing free water to move across the membrane along a pressure gradient.
The dialysis solution that is used is a sterilized solution of mineral ions. Urea and other waste products, such as potassium and phosphate, diffuse into the dialysis solution. However, concentrations of most mineral ions (e.g. sodium) are similar to those of normal plasma to prevent loss.
Note that this is a different process to the related technique of hemofiltration.
[edit] Prescription
A prescription for dialysis by a physician who specializes in the kidney (nephrologist) will specify various parameters for setting up dialysis machines, such as time and duration of dialysis sessions, size of dialyser and rate of blood flow on dialysis. In general the larger the body size of an individual, the more dialysis they will need. In the North America and UK, 3-4 hour sessions, 3 times a week are typical, although there are patients who dialyse 2, 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.
[edit] Side-effects and complications
Hemodialysis usually also involves the removal (ultrafiltration) of extra fluid, because most patients with end-stage renal failure pass little or no urine. The sudden removal of fluid on dialysis may cause side effects, which are usually proportionate to the amount of fluid which is removed. These potential side effects include low blood pressure, fatigue, chest pains, leg-cramps and headaches.
Since hemodialysis requires access to the circulatory system, patients undergoing hemodialysis have a portal of entry for microbes, which could lead to septicemia or an infection affecting the heart valves (endocarditis) or bone (osteomyelitis). The risk of infection depends on the type of access used (see below). Bleeding may also occur, again the risk depending on the type of access used.
Blood clotting in the tubing and dialyser was a frequent cause of complications until the routine use of anticoagulants.<ref>Janssen M, van der Meulen J (1996). "The bleeding risk in chronic haemodialysis: preventive strategies in high-risk patients.". Neth J Med 48 (5): 198-207. PMID 8710039.</ref> While anti-coagulants have improved outcomes, they are not without risks and can lead to uncontrolled bleeding. Occasionally, people have severe allergic reactions to anticoagulants. In this case dialysis is done without anticoagulation<ref>Raja R, Kramer M, Rosenbaum J, Bolisay C, Krug M. "Hemodialysis without heparin infusion using Cordis Dow 3500 hollow fiber.". Proc Clin Dial Transplant Forum 10: 39-42. PMID 7346852.</ref> or the patient is switched to an alternate anticoagulant.
Heparin is the most commonly used anticoagulant in hemodialysis patients, as it is generally well tolerated and can be quickly reversed with protamine. A common alternative to heparin is citrate, which sees use in the ICU and in patients allergic to heparin.
First Use Syndrome is a very rare but severe anaphylactic reaction to the dialyzer. Its symptoms include sneezing, wheezing, shortness of breath, back pain, chest pain, or sudden death. It can be caused by residual sterilant in the dialyzer or the material of the membrane itself. In recent years, the incidence of First Use Syndrome has fallen off, due to an increased use of gamma irradiation instead of chemical sterilants, and the development of new dialyzer membranes of higher biocompatibility.
There are specific complications associated with different types of hemodialysis access, listed below.
[edit] Access
There are three primary modes of access to the blood in hemodialysis: an intravenous catheter, an arteriovenous (AV) Cimino fistula, or synthetic graft. The type of access is influenced by factors such as the expected time course of a patient's renal failure and the condition of his or her vasculature. Patients may have multiple accesses, usually because an AV fistula or graft is maturing, and a catheter is still being used.
[edit] Catheter
Catheter access, sometimes called a CVC (Central Venous Catheter), consists of a plastic catheter with two lumens (or occasionally two separate catheters) which is inserted into a large vein (usually the vena cava, via the internal jugular vein or the femoral vein) to allow large flows of blood to be withdrawn from one lumen, to go into the dialysis circuit, and to be returned via the other lumen. However blood flow is almost always less than that of a well functioning fistula or graft.
They are usually found in two general varieties, tunnelled and non-tunnelled.
Non-tunnelled catheter access is for short term access (up to about 10 days, but often for one dialysis session only), and the catheter emerges from the skin at the site of entry into the vein.
Tunnelled catheter access involves a longer catheter, which is tunnelled under the skin from the point of insertion in the vein to an exit site some distance away. They are usually placed in the internal jugular vein in the neck and the exit site is usually on the chest wall. The tunnel acts as a barrier to invading microbes and as such tunnelled catheters are designed for short to medium term access (weeks to months only), as infection is still a frequent problem. + Tunnelled catheter access involves a longer catheter, which is tunnelled under the skin from the point of insertion in the vein to an exit site some distance away. They are usually placed in the in
Aside from infection, venous stenosis is another serious problem with catheter access. The catheter is a foreign body in the vein, and often provokes an inflammatory reaction in the vein wall, which results in scarring and narrowing of the vein, often to the point where it occludes. This can cause problems with severe venous congestion in the area drained by the vein and may also render the vein, and the veins drained by it, useless for the formation of a fistula or graft at a later date. Patients on longterm hemodialysis can literally 'run-out' of access, so this can be a fatal problem.
Catheter access is usually used for rapid access for immediate dialysis, for tunnelled access in patients who are deemed likely to recover from acute renal failure, and patients with end-stage renal failure, who are either waiting for alternative access to mature, or those who are unable to have alternative access.
Catheter access is often popular with patients, as attachment to the dialysis machine doesn't require needles. However the serious risks of catheter access noted above mean that such access should only be contemplated as a long term solution in the most desperate access situation.
[edit] AV fistula
Image:Fistula.gif AV (arteriovenous) cimino fistulas are recognized as the preferred access method. To create a fistula, a vascular surgeon joins an artery and a vein together through anastomosis. Since this bypasses the capillaries, blood flows at a very high rate through the fistula. One can feel this by placing one's finger over a mature fistula. This is called feeling for "thrill", and feels like a distinct 'buzzing' feeling over the fistula. Fistulas are usually created in the non-dominant arm, and may be situated on the hand (the 'snuffbox' fistula'), the forearm (usually a radiocephalic fistula, in which the radial artery is anastomosed to the cephalic vein) or the elbow (usually a brachiocephalic fistula, where the brachial artery is anastomosed to the cephalic vein). A fistula will take a number of weeks to mature, on average perhaps 4-6 weeks. During treatment, two needles are inserted into the fistula, one to draw blood and one to return it.
The advantages of AV fistula use are lower infection rates, as there is no foreign material involved in their formation, higher blood flow rates (which translates to more effective dialysis), and a lower incidence of thrombosis. The complications are few, but if a fistula has a very high flow in it, and the vasculature that supplies the rest of the limb is poor, then a steal syndrome can occur, where blood entering the limb is drawn into the fistula and returned to the general circulation without entering the capillaries of the limb. This results in cold extremities of that limb, cramping pains, and if severe, tissue damage. One long term complication of an AV fistula can be the development of a bulging in the wall of the vein, or aneurysm, where the vessel wall is weakend by the repeated insertion of needles over time. To a large extent the risk of developing an aneurysm can be reduced by careful needling technique. Aneurysms may necessitate corrective surgery and may shorten the useful life of a fistula.
[edit] AV graft
Image:Hemodialysis graft.gif AV (arteriovenous) grafts are much like fistulas in most respects, except that an artificial vessel is used to join the artery and vein, made of a synthetic material, often PTFE (Gore-Tex). Grafts are used when the patient's native vasculature does not permit a fistula. They mature faster than fistulas, and may be ready to use days after formation. However, they are at high risk for developing narrowing where the graft is sewn to the vein. As a result of the narrowing, clotting or thrombosis often occurs. As foreign material, they are at greater risk for becoming infected. The options for sites to place a graft are larger, due to the fact that the graft can be fashioned quite long. Thus they can be placed in the thigh or even the neck (the 'necklace graft').
[edit] Equipment
Image:Hemodialysis schematic.gif The hemodialysis machine performs the function of pumping the patient's blood and the dialysate through the dialyzer. The newest dialysis machines on the market are highly computerized and continuously monitor an array of safety-critical parameters, including blood and dialysate flow rates, blood pressure, heart rate, conductivity, pH, etc. If any reading is out of normal range, an audible alarm will sound to alert the patient-care technician who is monitoring the patient. Two of the largest manufacturers of dialysis machines are Fresenius and Gambro.
[edit] Water system
An extensive water purification system is absolutely critical for hemodialysis. Since dialysis patients are exposed to vast quantities of water, which is mixed with the acid bath to form the dialysate, even trace mineral contaminants or bacterial endotoxins can filter into the patient's blood. Because the damaged kidneys are not able to perform their intended function of removing impurities, ions that are introduced into the blood stream via water can build up to hazardous levels, causing numerous symptoms including death. For this reason, water used in hemodialysis is typically purified using reverse osmosis. It is also checked for the absence of chlorine ions and chloramines, and its conductivity is continuously monitored, to detect the level of ions in the water. -
[edit] Dialyzer
The dialyzer, or artificial kidney, is the piece of equipment that actually filters the blood. One of the most popular types is the hollow fiber dialyzer, in which the blood is run through a bundle of very thin capillary-like tubes, and the dialysate is pumped in a chamber bathing the fibers. The process mimics the physiology of the glomerulus and the rest of the nephron. Pressure gradients are used to remove fluid from the blood. The membrane itself is often synthetic, made of a blend of polymers such as polyarylethersulfone, polyamide and polyvinylpyrrolidone. Dialyzers come in many different sizes. A larger dialyzer will usually translate to an increased membrane area, and thus an increase in the amount of solutes removed from the patient's blood. Different types of dialyzers have different clearances for different solutes. The nephrologist will prescribe the dialyzer to be used depending on the patient. The dialyzer may either be discarded after each treatment or reused. If it is reused, there is an extensive procedure of sterilization. Dialyzers are not shared between patients in the practice of reuse.
[edit] See also
- Dialysis
- Home hemodialysis
- Peritoneal dialysis
- Hemofiltration
- Extracorporeal therapies
- Renal replacement therapy
- Step-by-step description of hemodialysis
[edit] References
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[edit] External links
- Your Kidneys and How They Work - (American) National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), NIH
- Treatment Methods for Kidney Failure - (American) National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), NIH
- Treatment Methods for Kidney Failure: Hemodialysis - (American) National Kidney and Urologic Diseases Information Clearinghouse, NIH
- What is dialysis? - Kidney Foundation of Canada
- Global Dialysis Global Directory of dialysis centers
