Kidney dialysis
In medicine, dialysis is a type of renal replacement therapy which is used to provide an artificial replacement for lost kidney function due to renal failure. It is a life support treatment and does not treat any kidney diseases. Dialysis may be used for very sick patients who have suddenly lost their kidney function (acute renal failure) or for quite stable patients who have permanently lost their kidney function (end stage renal failure). When healthy, the kidneys remove waste products (for example potassium, acid and urea) from the blood and also remove excess fluid in the form of urine. Dialysis treatments have to duplicate both of these functions as dialysis (waste removal) and ultrafiltration (fluid removal).
Principle
Dialysis works on the principle of the diffusion of solutes across a semipermeable membrane. Blood flows by one side of a semipermeable membrane, and a dialysis solution or fluid flows by the opposite side. Smaller solutes pass through the membrane. The concentrations of undesired solutes (for example potassium, urea, and phosphorus) are high in the blood, but low or absent in the dialysis solution and constant replacement of the dialysate ensures that the concentration of undesired solutes is kept low on this side of the membrane. The dialysis solution has levels of minerals like sodium and chloride that are similar to their natural concentration in healthy blood. For another solute, bicarbonate, dialysis solution level is set at a slightly higher level than in normal blood, to encourage diffusion of bicarbonate into the blood, to neutralise the acidosis that is often present in these patients.
Types
There are two main types of dialysis, hemodialysis and peritoneal dialysis. Hemofiltration is not strictly speaking a dialysis treatment, but is extremely similar.
Hemodialysis
In hemodialysis, the patient's blood is pumped through the blood compartment of a dialyzer, exposing it to a semipermeable membrane. Dialysis solution is pumped through the dialysate compartment of the dialyzer, which is configured so that the blood and dialysis solutions flow on opposite sides of the semipermeable membrane. The cleansed blood is then returned via the circuit back to the body. Ultrafiltration occurs by increasing the hydrostatic pressure across the dialyzer membrane. This usually is done by applying a negative pressure to the dialysate compartment of the dialyzer. This pressure gradient causes water and dissolved solutes to move from blood to dialysate, and allows removal of several liters of excess salt and water during a typical 3-4 hour treatment. Dialysis patient weight is measured in kilos: therefore, one kilo of fluid equals 2.2 pounds of body weight. Hemodialysis treatments are typically given three times per week, but more frequent sessions, which are usually 2-3 hours in duration given 5-6 times per week can be sometimes prescribed. Hemodialysis treatments can be given either in outpatient dialysis centers or can be done by a patient at home, providing they have suitable help and accommodation.
Peritoneal dialysis
In peritoneal dialysis, a sterile solution containing minerals and glucose is run through a tube into the peritoneal cavity, the abdominal body cavity around the intestine, where the peritoneal membrane acts as a semipermeable membrane. The dialysate is left there for a period of time to absorb waste products, and then it is drained out through the tube and discarded. This cycle or "exchange" is normally repeated 4-5 times during the day, (sometimes more often overnight with an automated system). Ultrafiltration occurs via osmosis; the dialysis solution used contains a high concentration of glucose, and the resulting osmotic pressure causes fluid to move from the blood into the dialysate. As a result, more fluid is drained than was instilled. Peritoneal dialysis is less efficient than hemodialysis, but because it is carried out for a longer period of time the net effect in terms of removal of waste products and of salt and water are similar to hemodialysis. Peritoneal dialysis is carried out at home by the patient and it requires motivation. Although support is helpful, it is not essential. It does free patients from the routine of having to go to a dialysis clinic on a fixed schedule multiple times per week, and it can be done while traveling with a minimum of specialized equipment.
Hemofiltration
Hemofiltration is a similar treatment to hemodialysis, but it makes use of a different principle. The blood is pumped through a dialyzer or "hemofilter" as in dialysis, but no dialysate is used. A pressure gradient is applied; as a result, water moves across the very permeable membrane rapidly, facilitating the transport of dissolved substances, importantly ones with large molecular weights, which are cleared less well by hemodialysis. Salts and water lost from the blood during this process are replaced with a "substitution fluid" that is infused into the extracorporeal circuit during the treatment. Hemodiafiltration is a term used to describe several methods of combining hemodialysis and hemofiltration in one process.
Starting indications
The decision to initiate dialysis or hemofiltration in patients with renal failure can depend on several factors, which can be divided into acute or chronic indications.
- Acute Indications for Dialysis/Hemofiltration:
- 1) Hyperkalemia
- 2) Metabolic Acidosis
- 3) Fluid overload (which usually manifests as pulmonary oedema)
- 4) Uremic pericarditis, a potentially life threatening complication of renal failure
- 5) And in patients without renal failure, acute poisoning with a dialysable drug, such as lithium, or aspirin.
- Chronic Indications for Dialysis:
- 1) Symptomatic renal failure.
- 2) Low glomerular filtration rate (GFR) (RRT often recommended to commence at a GFR of less than 10-15 mls/min/1.73m2)
- 3) Difficulty in medically controlling serum phosphorus or anaemia when the GFR is very low
See also
- Apheresis is another extracorporeal technique that selectively removes specific constituents from blood.
External links
- Dialysis Tips - Resource for dialysis personnel and general background with good understanding of the basic problems of dialysis therapy.
- Global Dialysis - Resource and community for dialysis patients and professionals
- Online Community for Dialysis Patients by Dialysis Patients
- Virtual Dialysis Museum - History and pictures of dialysis machines through time
- The Renal Unit - News and resources for those undergoing dialysis
- HDCN Online journal - Free medical lectures pertaining to various aspects of dialysis and nephrology; intended for physicians and nurses, not for patients.