4 Principles Of Dialysis

[Barb Frison]

HDand PD differ significantly in terms of risks and benefits to the patient. There are also additional requirements that need to be considered, for example whether the patient can attend an outpatient clinic and whether it is possible to create venous access via an arteriovenous fistula for patients receiving HD.

The development of renal replacement therapy (RRT) provides patients with a viable way of carrying out the role of the kidney when they have established renal failure. It was first used successfully in 1924, and became part of the management plan for patients with end-stage renal disease in the 1960s.

A kidney transplant is the preferred management option for patients with established renal failure. A review of the point prevalence of RRT found transplantation is the most common intervention (50% of patients), followed by HD and HDF (43% of cases) and PD in (7% of cases). HDF numbers are included within the HD prevalence due to lack of clarity of reporting from UK centres[2]

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Not every patient is suitable for or wants to have a transplant, and donors may not be available. In these cases, dialysis becomes the treatment of choice, either for long-term management or until a suitable donor is found.

The incidence of patients starting dialysis in the UK is 108 per million population per year. This has remained constant since 2006 to 2012 (the latest year with data available), although improved survival rates means the number of patients receiving dialysis has increased. In 2012, 54,824 people were receiving dialysis[1]

, and the median age for starting dialysis was 64.6 years[2]

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Currently, 73% of patients in the UK starting RRT begin with HD, 19.5% with PD and 7.4% of patients have a kidney transplant without starting dialysis[2]

. Age is an important factor in determining whether to have a transplant or receive dialysis.

There can be a survival benefit in selected elderly patients of having a transplant; however, these patients are at a higher risk of morbidity and mortality following transplant than they would be by using dialysis[3]

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Dialysis needs to be considered earlier in patients who are malnourished despite treatment, on account of there being a higher mortality rate among patients who start dialysis in a malnourished state because of uremic symptoms.

Ultrafiltration ensures excess fluid is cleared from the body through the use of a positive (blood) or negative (dialysate) pressure gradient, moving fluid from a high to low pressure region[1],[6]

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Convection allows effective clearing of larger molecules from the blood by creating a higher hydrostatic pressure in the blood (using a blood pump), leading to the passive movement of solutes dissolved in fluid. Convection relates to solutes in fluids crossing the membrane where ultrafiltration is related to movement of fluid under pressure[1],[5]

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This is achieved by a dialysis machine, which takes blood from the patient and pumps it towards the semi-permeable membrane. The dialysate is pumped from the opposite direction (countercurrent flow) to create a larger concentration gradient (see figure 1).

Without dialysis, lack of kidney function will result in metabolic acidosis as the body fails to excrete excess acid and uses its serum bicarbonate to neutralise the pH. Therefore the dialysate needs to have a high bicarbonate concentration, which ensures a concentration gradient is present to allow bicarbonate to pass into the blood. Dialysis aims to move the patient from a state of mild metabolic acidosis to mild metabolic alkalosis, which prevents the acidosis from getting too severe between dialysis sessions.

Potassium needs to be removed from the blood. However, a small concentration of potassium is included in dialysates to prevent potassium being removed too quickly, minimising the risk of cardiac arrhythmias or cardiac death. The aim of potassium management during dialysis is to treat hyperkalaemia but not result in a significant hypokalaemia[5]

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An anticoagulant is usually used to maintain patency and prevent clots within the extracorporeal circuit used in HD, which may lead to the dialyser becoming blocked[6]

. It is administered through the extracorporeal circuit; the most commonly used anticoagulant is unfractionated heparin, although a number of renal units now use a low molecular weight heparin instead. If these cannot be used (e.g. the patient is at increased risk of bleeding), a continuous infusion of epoprostenol may be used, or the dialysis may be run without adding an anticoagulant; however, this increases the risk of a blockage in the system. If patients do not tolerate a heparin-based anticoagulant then alternative anticoagulants can be considered (e.g argatroban or danaparoid), or regional citrate base anticoagulation. Regional citrate base formulations result in the chelation of calcium resulting in an anticoagulant effect; this is not used commonly on account of difficulties in monitoring the effect, and the risk of hypernatraemia, metabolic alkalosis and changes in calcium ion concentration[3],[6]

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Nocturnal dialysis is an alternative method, with patients having their dialysis overnight for a longer period, normally six to eight hours. This has been shown to have a reduced risk of hypotension as the volume of blood is removed more slowly. There is also evidence to support it reducing hospitalisation and mortality[5]

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A radiocephalic AVF is generally the site of choice, as it means a brachial fistula can still be used if the fistula fails (if a brachial fistula fails a radial fistula cannot be used). However, radiocephalic AVFs have a lower success rate (60%) than the brachiocephalic ones (90%)

[6]

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CAPD dialysate will usually contain glucose, which acts as the osmotic agent to remove excess fluid from the body[6]

. As with HD, there are several different dialysates available with varying compositions of electrolytes, and the osmotic agent strength can vary from weak to strong. The osmotic strength of the dialysate can be altered to determine how much fluid removal will take place; for example, a higher concentration of glucose will provide a greater concentration gradient for fluid to be removed[6]

.

APD works in the same way as CAPD, but the majority of the dialysis occurs at night while the patient is sleeping through use of a programmed machine[5]

. It involves on average six changes of fluid (dialysate) over a ten-hour period, all of which are carried out by the dialysis machine. It is more expensive but many patients find it allows them to carry out their normal activities with less restriction[6]

. However, CAPD can be easier for the patient to learn as no machinery is required[6]

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PD cannot be used indefinitely as patients will develop complications such as peritonitis. Normally, patients are able to remain on PD for up to eight years, although one fifth of individuals continue for more than ten years. Patients should be made aware that they will likely need to change the mode of dialysis at some point to HD[3]

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A small number of patients receiving PD can suffer from back pain because of the large volume of fluid in the abdomen leading to poor posture. To alleviate this pain, the dialysis regimen can be altered to use less dialysate through the day as long as dialysis adequacy is not affected.

Pain can also occur when fluid is being administered into the abdomen, slowing administration rate should help in most cases but a few will still be affected. The patient may also be in pain when fluids are removed from the abdomen, this may occur in the first weeks of therapy or suggest an infection could be present[3]

. Patients can also have a lowering of their appetite (due to the glucose in the dialysate) and have a sense of fullness in the abdomen due to the large fluid volumes used, which can lead to a state of malnutrition[3]

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It is important that the nephrology team involves the patient in the decision-making process, equipped with all the information on the advantages and disadvantages of each option, along with advice on which may be the most suitable[5]

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In practice, some patients find it hard to accept their diagnosis and initially refuse dialysis treatment until their symptoms get progressively worse and they understand dialysis is required[5]

. The patient can go through a multitude of feelings such as grief, denial, anger, bargaining, depression and finally acceptance of condition.

This is a common chain of events, and healthcare professionals should be aware of these stages and aim to reduce the anxiety and fear associated with starting RRT. It is also important to correct any misconceptions the patient has regarding dialysis therapy; for example, many patients believe that dialysis is a highly painful process when in fact pain is not usually experienced[5]

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All patients, if appropriate, should be given the opportunity to consider haemodialysis or peritoneal dialysis as their preferred choice of dialysis. In particular, the following patient groups may benefit from PD[7]

:

End-Stage Renal Disease (ESRD) refers to kidney disease that has resulted in the permanent destruction of a sufficient number of nephron units that renal function (waste and/or water removal) must replaced using an artificial kidney (Renal Replacement Therapy). Chronic dialysis is provided either using Intermittent Hemodialysis (IHD) or Peritoneal Dialysis. Chronic IHD is usually provided 2-4 days per week (depending upon the type of renal dysfunction). Peritoneal dialysis can be provided as Continuous Ambulatory Peritoneal Dialysis (CAPD) or Continuous Cycling Peritoneal Dialysis (CCPD). In CAPD, patients can administer and manage passive exchanges 3-5 times per day. CCPD requires patients to connect to a machine (usually at night) for automated exchanges.

Peritoneal dialysis is rarely used in critical care. Patients who are receiving peritoneal dialysis who are stable may have PD continued in the critical care unit. It will be run by a dialysis nurse who is trained in PD. Most critically ill patients who need Renal Replacement Therapy (CRRT) will receive either IHD or Continuous Renal Replacement Therapy (CRRT). Both intermittent hemodialysis and continuous hemodialysis circuits utilize the same principles. Blood is removed from the patient, pumped through a dialysis filter and returned to the patient following removal of surplus water and wastes. The filter performs many of the functions of the kidney's nephron unit, hence, it is referred to as an "artificial kidney".

The major difference between intermittent and continuous therapies is the speed at which water and wastes are removed. Intermittent hemodialysis removes large amounts of water and wastes in a short period of time (usually over 2-4 hours), whereas, continuous renal replacement therapies remove water and wastes at a slow rate more consistent of that of native renal function. While intermittent dialysis allows chronic renal failure patients to limit the amount of time that they are connected to a machine, the rapid clearance of solutes and fluid can be poorly tolerated when a patient is hemodynamically unstable.

During an acute illness, patients with ESRD often require more frequent renal replacement therapy to manage their increased production of metabolic by-products. Patients who develop Acute Kidney Injury that does not resolve with shock management may also require acute renal replacement therapy. RRT for either group may be provided using either IHD or CRRT. Hemodynamic stability usually determines the method.

Intermittent hemodialysis and SLEDD are both delivered using a conventional hemodialysis machine that creates dialysis fluid (called dialysate) by adding electrolytes and salts to city water that has been dechlorinated and purified using reverse osmosis (RO). Dialysate fluid is not IV sterile, therefore, it cannot be delivered into the blood path. IHD and SLEDD require an IHD and an RO machine and are only run by IHD trained nurses at London Health Sciences Centre.

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