Functionality
In general, extracorporeal dialysis taking place outside the body can be distinguished from intracorporeal dialysis taking place inside the body. Most cases involve extracorporeal treatment. Here, the patient is connected to the external dialysis machine, which then performs the blood washing.
There are several technical principles for washing the blood. Common to all methods is that access to the patient’s blood must first be created. This is done via catheter (a kind of thin tube) (acute) or dialysis shunts (chronic).
The catheter frequently used in acute dialysis is the Shaldon catheter, which provides access to large peripheral veins in the neck or groin, allowing blood to flow to and from the dialysis machine. If a patient is dependent on dialysis for a longer period of time or permanently, permanent access in the form of an arterio-venous shunt is recommended. Usually located on the forearm, one of the two forearm arteries is connected directly to the adjacent vein in a minor surgical procedure, so that arterial blood flows in this vein (so-called Cimino fistula).
This type of shunt can be recognized by the fact that the vein at the affected area is greatly dilated and thus easy to puncture. You can also feel the blood flow in the shunt with your bare hand and sometimes you can hear a murmur. Two accesses are then placed at this shunt: one leads blood to the dialysis machine, the other collects the cleaned blood and feeds it back to the body.
The steps cleaning the blood then take place in the connected dialysis machine.In addition, the modern dialysis machine has several filters, which prevent the formation of gas bubbles in the blood, for example, which can lead to complications. It is also possible to administer medication during dialysis via additional addition points. The heart of the device, which is usually about the size of a small chest of drawers, is always a semi-permeable membrane.
This means that a membrane is built in that has many microscopic pores and is therefore semi-permeable: water, ions and smaller particles such as the unwanted pollutants can pass through the membrane. The pores are too small for larger particles dissolved in the blood and they remain in the blood. These include above all the blood cells (red, white blood cells and platelets) or even vital proteins that are not filtered out.
At the membrane itself, there are two mechanisms that are used to purify the blood and thus determine the possible variants of dialysis: Hemodialysis and hemofiltration (Greek: haima = blood). The basis of hemodialysis is the principle of osmosis. It describes the behavior of particles dissolved in water, in this case blood, to distribute themselves evenly along a semi-permeable membrane along concentration differences on both sides of the membrane.
To make use of this effect in practice, a specific solution is needed for successful dialysis, the dialysate, which is located on one side of the membrane. On the other side, the patient’s blood is drained off. The composition of the dialysate is precisely tailored to the patient’s needs, thus enabling an easily controlled exchange of substances between blood and dialysate along the membrane.
One example: If there is too much potassium in the patient’s blood, a dialysate with a low potassium concentration is chosen so that during dialysis the excess potassium ions migrate through the membrane out of the blood until a desired level is reached. In this way, it is possible to regulate all substances that can pass through the membrane up or down. Excess water, which leads to oedema, can also be removed from the body in this way.
In contrast to this, hemofiltration is basically the same structure inside the device, but here, a difference in concentration is not responsible for the mass transfer. Instead, a pump creates a slight negative pressure on the semi-permeable membrane so that water and dissolved substances are continuously removed. Both methods offer the possibility of optimizing the dialysis result by adding desired substances or liquid from the dialysis machine to the blood.
A combination of both principles is also used in practice and is known as hemodiafiltration. Until now, the typical, much more frequently used principle of extracorporeal dialysis has been described. In the rather rare application of intracorporeal dialysis, a tube is implanted under the abdominal wall and rinsed with solutions. In this case, however, the body’s own peritoneum, which lies against the inside of the abdominal wall, serves as a membrane. This is called peritoneal dialysis.
