Introduction
The different types of renal replacement therapies include haemodialysis, peritoneal dialysis, haemofiltration, and renal transplation.
Haemodialysis (HD)
This can be carried out on an outpatient or an inpatient basis. In the context of this explanation it is considered as an inpatient procedure.
Haemodialysis involves diffusion of solutes across a semi-permeable membrane, i.e. the process normally carried out in the kidneys to remove waste products from blood. The dialysis machine utilises counter current flow whereby the dialysis solution flows in opposite direction to blood flow in an external circuit. The counter current process helps to maintain the maximum possible gradient across the semi-permeable membrane, thus increasing the efficiency of the dialysis.
The
dialysis solution,called
dialysate,contains low concentrations of
urea,
potassium and
phosphate to allow these substances to diffuse out of the blood. The concentration of
sodium and
chloride are similar to that in plasma so as to prevent the loss of these electrolytes. There is often a higher amount of
bicarbonate and
glucose in dialysate than that present in the blood to help correct acidosis and provide energy respectively.
Blood flow during dialysis is usually 200-300 ml/min, whilst dialysate flow is 500 ml/min.
Synthetic membranes allow for a faster rate of dialysis than cellulose-based membranes.
All patients are treated with heparin during dialysis to prevent formation of clots against the foreign surface.
Access
Adequate access requires blood flow of at least 200 ml/min.
There are three main modes of access. The modality used depends on the predicted length of treatment, and the condition of the patient’s vasculature. The three types of access are
cannulation,
arteriovenous (AV) fistula and
synthetic graft. Often initially a
cannula is used, whilst an implanted AV fistula or graft matures.
This is sometimes called a CVC, i.e. central venous catheter. A large vein is used – usually the vena cava, internal jugular or femoral vein. The catheter has two lumens, or sometimes two catheters are used, one to withdraw the blood and the other to deliver it back to the body from the dialysis machine. With this method, the level of blood flow is never as good as that that can be obtained from a fistula or graft.
There are two types of catheter – tunnelled and non-tunnelled. Non-tunnelled tend to be used in the short term, for up to 10 days, but usually only for one session of dialysis. This type of catheter exits the skin at the site of the catheter. It may be used for emergency haemodialysis. The tunnelled variety are often inserted into the internal jugular vein, and
travel a long way under the skin before entering the vein in order to reduce the risk of infection travelling to the site of the vein. Tunnelled catheters are for more long-term use and so are usually used in
ARF where the patient is expected to recover within a few weeks’ time.
Infection and venous stenosis are potential risks of
catheterisation. Stenosis occurs due to inflammation of the vein at the catheter entrance site. This can lead to occlusion and scarring of the vein, rendering it useless due to poor blood flow. Stenosis may also mean that this vein can’t be used for future access, and this can be a major problem in some patients who run out of access sites,
translating into a fatal situation.
AV fistula
This is the preferred access method. A surgeon needs to join an artery and a vein together to allow this type of access. This then means that blood by-passes capillaries, and as a result the blood flow rate is markedly increased. The ‘thrill’ of blood flow can be palpated by running a finger over the fistula site.
The fistula is usually created in the non-dominant arm, and is usually inserted between the radial artery and cephalic vein, resulting in a radiocephalic fistula. It is usually visible on the anterior surface of the wrist. The fistula will take several, i.e. 4-8, weeks to mature before it can be used for haemodialysis. And after it has matured, two needles will be inserted into the vein side of the access site, one to remove blood, and the other to return it to the body. The advantages of a fistula are:
- Increased rate of flow of blood, thereby:
- Increasing the effectiveness of haemodialysis
- Reducing the risk of thrombosis
- Reduced risk of inflammatory reaction through the avoidance of foreign grafts or cannulas.
Generally, there aren’t any complications, but sometimes “steal syndrome” can occur. This is the result of blood travelling through the fistula, and straight into the vein and back out of the arm, bypassing the arm capillaries. This leads to cold extremities, cramps, and sometimes even tissue damage.
Over time, the vein side of the fistula may turn into an aneurysm through the frequent insertion of needles. This can be avoided by using careful needle insertion technique and rotating needle placement sites. Another technique, known as the “buttonhole technique”, involves sticking a blunt needle in the same place each time.
AV graft
This is used when either anatomical variation doesn’t allow for the easy creation of an AV fistula, or when there is arterial disease, such as in
diabetes mellitus. The graft is usually made of the synthetic material
PTFE, although sometimes a vein from an animal is used. It will join an artery and a vein in the same way a fistula does. Grafts do not take as long to mature as fistulas, but they do cause more complications. The vein joined to the graft tends to narrow, and this predisposes the patient to clot formation, as does the fact that foreign material has been inserted into the body.
Two-year graft patency is only 50-60%.
In the case of
CKD that requires dialysis, HD is normally carried out about 3 times a week for 4 hours at a time.
Recall that bicarbonate is used at the buffer in the dialysate.
Complications of HD
The most common complication as a result of dialysis is hypotension. This is caused by excessive removal of fluid during the process, as well as left ventricular hypertrophy and abnormalities of venous tone.
Rarely, patients may have an anaphylactic reaction to ethylene oxide which is used to sterilise dialysis machines.
Haemolytic reactions, air embolus and “hard water syndrome” in areas where the local water supply has a high concentration of
calcium, are other potential complications. In the last scenario, the water used to make up the dialysate is not softened, thereby causing nausea,
hypertension,
headache,
confusion, memory loss, and sometimes seizures.
The
symptoms of underdialysis are vague and include insomnia, itching, fatigue despite adequate correction of
anaemia, peripheral sensory neuropathy and “restless legs”. Optimum results from dialysis are seen when long dialysis sessions, >8 hours in length, are carried out. This allows for better removal of small molecules such as
urea.
Recall that the kidneys carry out this function 24/7, but on dialysis this is only achieved for a few hours each week.
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