The first artificial kidney machine was developed in 1943 by Dr.Willem Kolff. Hemodialysis is derived from two words, hemo, meaning blood and dialysis. Today, it has become the most commonly performed type of dialysis. Hemodialysis treatments are required three days per week, and each treatment lasts about four hours.
Figure: A patient undergoing Hemodialysis.
How it works?
During hemodialysis treatment, the patients blood is circulated outside of the body into an artificial kidney machine, through a link to the patients circulatory system. The blood flows into the dialysis machine, is cleansed and then returns to the patients body. The dialysis process is continual, with new blood and dialysate constantly circulating into the machine and waste products and excess fluids constantly circulating out.
Figure: A picture of Dialyser
The artificial kidney machine contains an internal chamber which is divided by a thin semipermeable membrane. It is called semipermeable because only particles of a certain size will pass through it. On one side of the membrane is a special solution called dialysate. On the other side is the blood which needs to be cleansed. Since red and white blood cells are relatively large, they will not pass through the pores of the membrane, but waste products (such as creatinine and urea) are very tiny and will pass through the membrane.
Waste products are removed from the blood through three chemical processes: Diffusion, Osmosis and Ultrafiltration.
- Diffusion works on the principle that liquids of different concentrations tend to equalize when placed next to each other. For example, if you take a cellophane bag and fill it with water mixed with food coloring, seal it and place it in a container of clear water, you will find in a short time that the clear water will have food coloring in it. The food coloring has passed through the tiny holes in the cellophane membrane. The water in the bag still has food coloring in it because the process works both ways.Diffusion continues until there are equal amounts of food coloring on both sides of the membrane. This state is called equilibrium. Waste products are found in the patients blood which are not present in the dialysate. During diffusion the tiny waste particles move out of the blood and into the dialysate until both solutions have reached equilibrium. By constantly introducing clean batches of dialysate the waste products continue to shift towards the waste-free dialysate until a large proportion of waste products have been washed out of the blood. If the patient is low on vital elements, such as calcium or dextrose, they may be added to the dialysate. Just as the waste products shifted to the side where they were lacking, the calcium and dextrose will shift towards the side of the chamber where they are needed, the patients blood.
- Osmosis is very similar to diffusion, but rather than removing waste products, excess fluid is removed. To facilitate this process dextrose is added to the dialysate. This works to pull the excess fluid from the blood.
- Ultra-filtration is the process by which very minute particles are removed from a substance, in this case the blood. In order to speed up the removal of excess fluid, a pump may be used. This pump raises the pressure of the blood flowing through the dialysis machine, thereby forcing excess fluids out of the blood.
Figure: Hemodialysis access site.
Access for Hemodialysis:
In order to be dialyzed there must be a way to connect your circulatory system to the artificial kidney machine. This connection is called a vascular access and initially requires minor surgery. The fistula is created by a surgical procedure which attaches a vein to an artery in an arm. This increases the amount of blood that flows through the vein which in turn causes the vein to enlarge. The larger size of the blood vessel allows two needles to be inserted for treatment. Because this may cause discomfort, the skin can be anaesthetised with Novocain.
These needles are connected to tubes which go to and from the artificial kidney machine. A graft also provides access to the patients circulatory system. When a graft is used, synthetic material is inserted to form a connection between an artery and a vein. The advantage of the graft and fistula, which both require minor surgery, is that they are under the skin and therefore less prone to infection than an external shunt. They do require two needle sticks each time dialysis is performed. Sometimes a person must start dialysis before a permanent access can be created. In this case, a catheter may be inserted into a vein in your chest, neck or leg to gain access to the circulatory system. A catheter may also be used if the patient has used up all other possible sites for a fistula or graft. The advantage of a catheter is that no needles need to be inserted. The disadvantages are numerous, including the possibility of infection and clotting.