Hemodialysis
From Free net encyclopedia
Nephron (Talk | contribs)
+home hemo caps, clarify for US (its quite common in Australia)
Next diff →
Current revision
Image:Artificialkidney.jpg 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. Dialysis can also be done in the patient's home. Although less typical (especially in the USA), dialysis can be done in a patient's home. Home hemodialysis has numerous advantages over in-center dialysis, such as greater patient control over the therapy and better symptom control due to longer and/or more frequent dialysis sessions.
Contents |
The principle of hemodialysis
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 utilizes counter current flow, where the diasylate is flowing in the opposite direction to blood flow in the extra-corporeal circuit .This mechanism maintains the concentration gradient across the membrane at a maximum, allowing the dialysis to be very efficient. As a result, hemodialysis can be performed intermittently (usually 3 times a week). Fluid removal (ultrafiltration) is achieved by altering the hydrostatic pressure of the blood compartment, causing free water to move across the membrane along a pressure gradient.
The dialysis solution that is used is kept at approximately body temperature and consists of a sterilized solution of mineral ions. Urea and other uremic toxins, as well as potassium and phosphate, diffuse into the dialysis solution. However, concentrations of most mineral ions (eg sodium) are similar to those of normal plasma to prevent loss.
Dialysis 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. 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.
Side-effects and complications
Hemodialysis usually also involves the removal (ultrafiltration) of extra fluid, because most patients with end-stage renal failure pass 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 potenital 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.
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.
Hemodialysis access
There are three primary modes of access to the blood in hemodialysis: an intravenous catheter, an arteriovenous (AV) fistula, or 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.
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.
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.
AV fistula
AV (arteriovenous) 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 in opposite directions, 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 back 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.
AV graft
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 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').
Hemodialysis equipment
Hemodialysis machine
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.
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.
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.
Step-by-step description of hemodialysis
Pre-dialysis
- 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 material 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.
Dialysis
- 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 weight 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.
- During dialysis, occasionally, patients have low blood pressure. Often this is temporary and passes after the head is placed down (Trendelenburg position) for a short time.
Post-dialysis
- 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.
Post-dialysis washout
Following haemodialysis, patients may experience a syndrome called "washout". The patient feels weak, tremulous, extreme fatigue. Patients report they "are too tired, too weak to converse, hold a book or even a newspaper." It may also vary in intensity ranging from whole body aching, stiffness in joints and other flu-like symptoms including headaches, nausea and loss of appetite. The syndrome may begin toward the end of treatment or minutes following the treatment. It may last 30 minutes or 12-14 hours in a dissipating form. Patients though exhausted have difficulty falling to sleep. Eating a light meal, rest and quiet help the patient cope with washout until it has 'worn away.'
See also
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 Canadapt: Hemodiálise