How is it done?
Before haemodialysis can be performed you will need a minor operation to create a fistula or insertion of a tunneled neckline (usually used as temporary measure for immediate use) as an “access” to connect you to the machine. The machine may be in a hospital renal unit, in a satellite dialysis unit or if conditions allow, the patient’s own home.
Haemodialysis is done by taking blood from the body and pumping it around a dialysis machine and through a dialyser. In the dialyser, toxins and excess water – which are equivalent to the urine produced by healthy kidneys – pass from the blood into the dialysis fluid. The cleansed blood is then returned to the body at the same rate at which it is removed. Meanwhile the “used” dialysis fluid (full of toxins and extra water) is pumped out of the dialysis machine and down the drain.
Haemodialysis is usually done three times a week , for three to five hours each session. The exact length of the sessions will depend on the amount of waste that an individual patient produces; bigger people generally need longer dialysis sessions than smaller people. Patients who do not pass any urine may also need longer sessions.
How much dialysis is needed?
In our renal unit a consultant or senior doctor and a dietitian are responsible for working out how long kidney patients need to spend on the dialysis machine, and also what size of dialyser they will use. There are different sizes of dialyser - bigger ones remove more toxins than smaller ones. Longer dialysis sessions will also remove more toxins.
As a rule, the bigger/more muscular the patient, the more dialysis they will need. In order to change the amount of dialysis that a patient receives, the nurse can choose to alter the size of the dialyser and / or the length of time that the patient spends on the machine.
The dialysis dose can be worked out simply by comparing the levels of wastes (such as urea or creatinine) in the patient’s blood before and after dialysis and making sure that there is a significant reduction. Some units still use this method, but it is now more common to use one of the newer methods of working out dialysis doses. The first of these uses a calculation called the urea reduction ratio, the other is a method called urea kinetic modelling (UKM). With each of these methods, dialysis target figures are the same whatever the size of the patient.
The urea reduction ratio is really just a more formal way of comparing urea levels in the blood before and after dialysis. As before, the patient’s urea levels are measured in millimoles per litre (mmol/l) of blood, but now the measurements before and after dialysis are used to calculate a percentage reduction in blood urea. (For example, if the blood urea before dialysis was 30 mmol/l, and after dialysis it was 10 mmol/l, then the percentage reduction in urea during dialysis was 50%). Such information allows adjustments to be made at future dialysis sessions in order to achieve the current urea reduction target of at least 65% per session.
Urea kinetic modelling also compares the levels of urea in the patient’s blood before and after dialysis. However, this method also takes into account the size of the dialyser (called ‘K’), the time the patient will need on the machine (called ‘t’) and a number that reflects the patient’s body weight (called ‘V’). This produces a figure called the Kt/V (pronounced ‘K.t.over V’).
Because a patient’s Kt/V figure refers to the amount of urea cleared from the body, the higher the number the better. Recent recommendations state that Kt/V should be more than 1.2 for each dialysis session.
Some patients on haemodialysis believe that it is the amount of fluid that needs to be removed which determines the length of time that they must spend on the dialysis machine. This is wrong. The most important factor affecting the length of dialysis is the amount of toxins that needs to be removed. However, if a patient has a lot of fluid to remove, they may need to spend extra time on the machine to achieve this.