About Dialysis
Dialysis is a process that removes waste products and excess fluid that accumulate in the blood as a result of failure of the kidneys to function.
Dialysis is not a cure for kidney failure. It only substitutes for some functions of the kidneys like removing some waste products and excess water from the body. When the kidneys have ceased functioning permanently they will not resume function no matter how many dialysis treatments are given. The patient with permanently damaged kidneys (ESRD) will therefore need dialysis treatment for the rest of his life unless he has a successful kidney transplant. A patient with temporarily damaged kidneys (ARF) will be able to discontinue dialysis once his kidneys recover.
There are different types of dialysis.
Hemodialysis :
This is a process in which blood is passed through a special filter (artificial kidney) which contains a special membrane . This membrane is made of a type of cellophane. The membrane contains millions of tiny holes or pores through which waste products and excess fluid are removed.
How it works : During hemodialysis, two processes occur
Diffusion :
In many types of dialysis filters (artificial kidneys), the membrane is constructed in the form of many long tubes or capillaries. The blood passes through the capillary tubes and dialysis fluid from dialysis machine (dialysate) runs outside the capillary tube. The dialysis fluid contains the same salts as are in the blood (sodium, potassium, chloride, calcium, magnesium) but does not contain the waste products. As the blood passes through the capillary tubes and comes in contact with the inside of the membrane, the waste products urea, creatinine, phosphate etc, pass from the blood through the tiny pores in the membrane into the dialysate that flows between the capillary tubes. These waste products are diffusing from an area of very high concentration in the blood to a fluid (diaiysate) that contains no waste product. The dialysate empties into a drain. Fresh dialysate always bathes the outside of capillary tubes through which blood is flowing. Therefore, the process of removing waste products continues as long as blood flows through dialysis filter.
Ultrafiltration :
This involves the use of pressure by the machine to force excess fluid out of the blood. Pressure can be applied in two ways :It is applied to the inside of the membrane. This forces excess water from the blood into the dialysate. Pressure on the outside of the membrane (dialysate side) is lowered, causing a sucking of excessive water from the blood.
Access for Hemodialysis :
In order to perform hemodialysis, it is necessary to create a method of getting the blood from your body to the dialysis filter. Initially, a temporary access is created by introducing a catheter in femoral or jugular vein. However, a permanent access should soon be created for long term use : shunt, fistula, or graft.
Femoral and jugular vein catheterisation is performed using a double lumen catheter. This catheter although appearing to be a single tube, has inside it two tubes. Through one tube called the arterial line, impure blood from the body is obtained and carried to artificial kidney for purification and through other tube called venous line the purified blood is returned back. The femoral vein is a large vein in the groin while a jugular vein is a large vein in the neck. A large vein is required because blood has to be obtained for dialysis at a high flow rate (250 ml./minute). This vascular access should not be maintained for long as it increases chances of infection. Therefore, a permanent device should soon be created.
A Shunt is made of two special plastic tubes that carry blood directly from an artery to a vein. One of the tubes is connected to the artery in the arm or leg through teflon vessel tip and the other is connected to a vein close to the artery. The two tubings are connected by a small connector. During dialysis, the tube in the artery is attached to a tube which carries blood to the artificial kidney. The tube in the vein is attached to a piece of tubing that carries the blood back to your body from the artificial kidney. The shunt permits the patient to have many treatments without the use of needles. However, with this method there is a risk of infection with repeated usage. This form of access is rarely used now-a-days.
The fistula is an internal (under the skin) connection between an artery and a vein in the arm. It is done by a minor surgical procedure using the local anesthetic. Arteries carry blood at a faster rate under high pressure. Veins normally carry blood at a slow rate under low pressure. When a fistula is created, artery is connected to a vein so that blood flows directly from the artery into the vein. This makes the veins larger. It normally takes 4-6 weeks for the veins to become large and usable for dialysis. It is therefore advisable to get a fistula made well before dialysis has to be initiated. If this planning is done, which is possible in patients with slowly progressive renal disease, one can avoid creating temporarily vascular access. During dialysis, two needles are placed in the enlarged veins; one is attached to the tubing that carries blood to the dialysis machine and other to the tubing that carries blood back to the body.
A graft is an internal connection between an artery and a vein using synthetic material. This is required when the veins are not good for fistula construction
Peritoneal Dialysis :
Peritoneal dialysis is another method of removing waste products and excess fluid from the body. Here, the process of removal of waste products and excess fluid takes place within the patients own body, in the peritoneal or abdominal cavity, rather than outside the body in an artificial kidney as it happens with hemodialysis.
The peritoneum is a thin membrane which covers the intestines and other contents of the abdominal cavity. The peritoneum basically functions the same way as the semipermeable membrane used in the artificial kidney. Since it contains a whole network of tiny blood vessels called capillaries, the peritoneum is constantly able to provide a new supply of blood to be filtered. The access to the peritoneal cavity is gained via a flexible hollow tube (catheter), which is implanted through the wall of abdomen into the abdominal cavity
The process of dialysis works like this :
The dialysate (diaysis fluid similar to that used for hemodialysis except that it has a higher glucose concentration which helps removal of excess fluid by way of osmosis) is introduced into the peritoneal cavity and allowed to stay there for some time.As blood flows through the capillaries in the peritoneum, waste product are filtered through diffusion and excess fluid removed through osmosis.The dialysate with the waste products and excess fluid is then removed. Such cycles of introducing the dialysate in the peritoneal cavity, allowing it to remain for some time and then draining out the fluid are repeated every 4 to 6 hours during the day with each exchange taking approximately 40 minutes (20 minutes to introduce and 20 minutes to drain dialysate). Each day, the last exchange is performed right before the patient goes to bed at night. The process described will make one realise that this dialysis process goes on continuously. Moreover the patient does not have to be hooked to the machine and can remain ambulatory. Therefore this type of dialysis is called continuous ambulatory peritoneal dialysis (CAPD). Since the patient is continually dialyzing, there are usually few dietary restrictions. Further, there is no blood loss with CAPD and so patients on CAPD maintain higher hemoglobin then patients on haemodialysis and thus require less of expensive drug, erythropoietin (Eprex, Zyrop & Recormon)
Continuous Cycling Peritoneal Dialysis (CCPD) is very similar to CAPD. In this case, the exchanges are performed at night while the patient sleeps. To facilitate this process the patient is hooked up to an automatic cycling pump. This pump makes several evenly spaced exchanges during the night and a final one just before the patient gets up in the morning. During the day no exchanges are made.
A potential problem with peritoneal dialysis is peritonitis which is an infection of the peritoneal membrane. Therefore, the process of introducing dialysis fluid into peritoneal cavity and removing it from the cavity must be done under strict aseptic conditions. With CCPD, since only one connection is done in a day compared to four connections in a day required with CAPD, the risk of peritonitis is lower
Source : Narmada Kidney Foundation, http://www.narmadakidney.org/INDEX.ASP?PAGE=DAILYSIS