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Dialysis: Overview, Procedure, Healthy Eating While on Dialysis, and More | Singapore General Hospital

Dialysis - What it is

Overview of Dialysis

Dialysis refers to an artificial means of removal of the waste substances from the body using a specialised membrane.

There are two forms namely haemodialysis or blood dialysis, and peritoneal dialysis.


In haemodialysis, a mechanised blood pump is needed to draw blood from the patient. The blood is circulated into the machine and passes along a specialised membrane (a dialyser) which in essence functions as an artificial kidney. Waste substances from the blood move across the membrane into the solution called a dialysate by a physical process called diffusion. The cleaned blood exits the dialyser and is returned back to the patient.

In haemodialysis, the patient needs to have a surgically created vein that is large enough and with a high enough blood flow to allow efficient and adequate dialysis. The surgeon will create this specialised vein called an arterio-venous fistula or access by connecting 2 blood vessels together. Usually the operation is done at the wrist initially but other sites along the arm can also be chosen. Sometimes patients' veins are too small for such surgery. Under these circumstances, the surgeon may create the access by placing a graft that is essentially functions as an artificial blood vessel that connects two adjacent vessels. This artificial graft is left permanently under the skin of the patient and is repeatedly needled for dialysis.

In peritoneal dialysis, the lining of the abdomen has a membrane that can similarly function like a dialyser membrane. This lining is used as the artificial kidney in peritoneal dialysis. In peritoneal dialysis, a permanent tube (called a tenchkoff catheter) is placed under sterile conditions into the abdominal cavity. Specialised fluid called a dialysate is placed into the abdomen. The waste substances present in the blood flowing through the blood vessels of the abdomen itself move across this membrane into the dialysate. The dialysate is then removed, discarded, and a fresh bag of dialysate placed back into the abdomen to continue dialysis.

Both haemodialysis (HD) and peritoneal dialysis (PD) can effectively remove waste products from patients. Both however cannot entirely replace the kidney function. Apart from removal of waste products, the normal kidneys also produce substances called hormones, of which the two major ones are erythropoietin and vitamin D. Erythropoetin is essential in maintaining appropriate levels of blood counts in the body. In renal disease, the amount of erythropoetin progressively lessens, and the patient develops the problem of anaemia, which essentially is a problem of low blood counts. The kidney also produces vitamin D. Vitamin D is a hormone that works on the bone and intestine to maintain an appropriate level of calcium for the body. Calcium is essential in maintaining the health of bones and teeth. As with erythropoetin, vitamin D likewise becomes progressively reduced in quantity as the kidneys fail. The bones become brittle and prone to injury. The artificial kidneys used in both HD and PD are unable to produce the hormones like the normal kidney. Because of this, these hormones need to given in the forms of injections or oral medication to patients with renal disease.

Choosing Between the Different Forms of Dialysis

Every patient who has kidney failure and facing the prospect of dialysis will usually ask the following questions. "Which is the best form of dialysis?" and " Which one is the most suitable for me?" The answer is not simple. One form of dialysis is not necessarily superior to another. As each patient is different, the type of treatment that they choose must eventually be acceptable to themselves, their families and caregivers. Thus either treatment may be best form of treatment for an individual patient. Sometimes treatments are chosen because of varying lifestyles, ease of access to a dialysis centre, family support, and often because of medical issues.

PD nevertheless may have certain advantages over HD in certain groups of patients.

  • PD benefits patients with heart disease
  • It is slow continuous therapy. The dialysate remains in the abdomen for 4-6 hours before being exchanged with a fresh dialysate. During the 4-6 hours, dialysis continues to occur in the patient. The patient is free to continue with his routine work. Even while the patient is asleep, PD dialysate is left in the abdomen overnight. As dialysis is slow and continuous, there is less strain on the heart.

  • Dialysis and bleeding tendencies
  • A special group of drugs called anticoagulants are frequently required in long term HD patients. In HD patients, blood is drawn out from the enlarged vein (called an arterio-venous fistula or AVF) into tubing that runs into the dialysis machine. The blood under normal circumstances will thicken (i.e. clot up) into a gel unless the blood is thinned with a special type of drug called an anticoagulant. Not all patients however tolerate these drugs well. With PD, the routine use of these anticoagulants can be avoided and thus some patients who have a higher bleeding tendency are better off on PD.

  • PD allows more versatile dialysis schedules
  • Patients on PD can, within limits, adjust their dialysis schedules more easily to fit their routines. Unfortunately, haemodialysis patients usually have fixed schedules for dialysis. Whether this is an advantage is something for the individual patient to decide and depends on their lifestyles, workplace and motivation.

  • PD avoids blood pressure swings
  • Certain patients have wide swings in blood pressure as they move from a lying to a standing position. This is called orthostatic hypotension. Wide blood pressure swings make conventional haemodialysis difficult. Patients with this condition, for example certain groups of diabetics, would benefit from the more gentle nature of peritoneal dialysis, with lesser swings in blood pressure and more persistent and gradual fluid removal.

  • PD allows needling to be avoided
  • Some patients have very small veins and it is difficult to create an access that is will last for patients. PD offers an alternate dialysis modality, which does not require vascular access. For some patients, the fear of repeated needling makes PD a more suitable option of renal replacement therapy.

  • The diet allowed on PD is more liberal
  • While dialysis patients irrespective of the modality must be well nourished to maximise their benefits on dialysis, PD patients in general require a higher protein and calorie rich diet than HD patients do. They have a more liberal choice of food, although the restrictions on phosphate-rich foods are similar to those of HD patients. Patients on PD are in general allowed more fruit which is high in potassium, as they tend to lose potassium more liberally than in HD patients.

  • PD patients have the benefit of simple home-based therapy, with the versatility of a relatively greater ease of travel when needed. Patients on HD need to make special arrangements to receive their dialysis if travelling overseas. In the absence of dialysis services, HD patients may find it more difficult to travel.
  • PD patients require less erythropoietin than HD patients
  • Patients on PD usually require only low doses of injections (erythropoetin) to build up their blood levels. Usually patients with kidney failure have the problem of anaemia. This means their blood counts, also called hemoglobin count, are low. This causes them to be tired, feel cold, lethargic, and in the long term can worsen heart disease. This problem is usually treated with injections of erythropoetin, which can be once or twice a week. Patients on PD generally are less prone to anaemia than HD patients, and require fewer injections of erythropoetin to boost their blood counts.

  • PD patients usually need less iron supplements compared to HD patients
  • Iron is a special mineral that the body needs to make adequate blood. Patients on HD need iron because some of this is lost during the process of haemodialysis. In PD, there is no iron loss from the dialysis. PD patients may still need to take the iron but usually in lesser amounts.

    Most patients will be able to do well with either form of therapy. Each patient is different. Do ask your doctor for his opinions and evaluation before making a final choice.

Healthy Eating While On Dialysis

Dialysis patients need to be careful about their choice of foods. Dialysis cannot replace the kidney function totally, and because of this, patients will need to continue to be wary about what they eat.

The type of diets permissible differs between haemodialysis and peritoneal dialysis patients. Individual preferences also vary a great deal. It is important that patients see a dietician to discuss a meal plan that is varied and palatable enough for them to eat well and adequately.

General Principles:

Fluids: It is usually important to restrict fluids to that which matches the fluid output in patients. Excessive fluid intake will accumulate in the body and cause leg-swelling, heart disease and breathlessness.

Salt Restriction: Salt makes a person thirsty and causes him to drink and take in more fluids than he can handle. Salt restriction is an important feature in dialysis patients diet plans.

Protein intake: When a patient starts dialysis, he will need a lot of protein every day as part of his meals. Patients who start dialysis are usually undernourished. They have not eaten well for some time, and may have had repeated hospitalisations and infections that cause them to lose a lot of weight. Protein is usually found in meats like chicken, pork, beef and mutton. Certain foods like soya beans or legumes also contain a lot of protein. However, these particular sources of protein also contain large amounts of phosphate, which the patient on dialysis is unable to handle well (See paragraph on Phosphate).

Carbohydrates: Patients should eat adequate carbohydrates as part of their daily diet to provide them with energy. If they don't, the protein they eat will be used unnecessarily for energy and they will not put on desired body mass.

Potassium: Patients on haemodialysis, in particular, should restrict their potassium as they undergo dialysis only three times a week. In between dialysis, their potassium levels can rise to dangerous levels if they have high potassium meals. Fruits, in particular bananas, durians, grapes and dried fruit, are very high in potassium. Patients on peritoneal dialysis usually have a more liberal diet because they lose potassium constantly through their dialysis and tend to need more potassium supplements.

Phosphate restriction: An average patient on dialysis takes  in about 1.5gm of phosphate a day, of which 60% or 900mg are absorbed if phosphate binders are not taken. Dialysis cannot effectively remove this phosphate from the body. On average, only 300mg of phosphate are removed each day by PD, and an average of 700mg of phosphate removed over a 4-hour session of HD (which also works out to 300mg per day for a thrice per week haemodialysis schedule). Retained phosphate can accumulate to dangerously high levels and cause in the long-term bone disease. Medicines called phosphate binders must be taken to reduce the total amount of phosphate absorbed. These medications need to be taken with the meals simultaneously in order to effectively reduce phosphate uptake into the body. Examples of phosphate binders are calcium carbonate and calcium acetate. Occasionally aluminum- based phosphate binders are prescribed, although these are generally used more sparingly. Just as important, a patient must restrict himself from foods high in phosphate to reduce the total absorbed phosphate. These include legumes (beans, nuts, lentils, and corns).

Common Medicines a Person on Dialysis Will Need To Take

Dialysis cannot correct all the abnormalities that occur with renal failure. Most patients will require several medications to treat the kidney failure. These medications will be in addition to the other medication that they need to take for other non-kidney related disease. Some of the common medications that dialysis patients need to be on are listed below.

Supplementary vitamins: Dialysis causes a loss of certain substances like water-soluble vitamins and folate from the body. These are easily replaced by taking supplementary vitamins daily, which are usually prescribed as part of the list of medications.

Phosphate binders: This type of medication should be taken with meals. This medication binds to excess phosphate in the meal so that there is no excessive absorption of phosphate. Kidneys which function normally can easily remove excess phosphate that is absorbed after a meal. The kidneys of renal failure patients cannot clear phosphate from the body. Furthermore, dialysis cannot remove phosphate very efficiently either. To prevent an excessive buildup of phosphate, dialysis patients should take phosphate binders with their meals. It is important for patients to realise that the medication should be taken together with meals to maximise its efficacy to bind phosphate. There are different kinds of phosphate binders falling into two broad groups. They are calcium-based binders (for example calcium carbonate, calcium acetate, calcium hydroxide and calcium citrate), or non calcium-based binders (for example aluminum hydroxide binder). A patient during the different stages of his life on dialysis may require different types of phosphate binders in different amounts, based on their needs, dietary intake, duration on dialysis and tolerance to the medication. These drugs in general can cause constipation. Calcium-based phosphate binders may in certain patients cause high blood calcium levels, which when occurring with high blood phosphate levels can cause calcium to be deposited outside the bone. This condition causes red eyes, fistula failure, joint pains and skin nodules.

Erythropoetin: This is given in the form of injections usually under the skin at intervals of between 2 and 10 days. This medication brings up the blood stores in the body. A low blood count is a problem for patients with renal failure, and causes them to feel cold, tired, lethargic, unable to think, and in the long term contributes to worsening heart disease. Prior to the development of this medication, patients with renal failure required multiple blood transfusions. With erythropoetin injections, patients feel better and need fewer blood transfusions. Like all medication, it is not without side effects. It can cause a worsening of hypertension or flu-like symptoms of chills and body ache, both of which are usually transient.

Iron supplements: Patients on dialysis usually lack a mineral called iron. This contributes to the problem of low blood count. Most patients on dialysis require a tablet of iron a day, although the requirements may be higher in patients on erythropoetin therapy or those with other reasons for chronic iron losses. There are different preparations, but in general this medication can cause constipation, which can be controlled using laxatives and a high-fibre diet.

Antihypertensive medication: This refers to blood pressure medication. Ideally, a patient on dialysis will not need high blood pressure medication. However, there will remain a group of patients whether on peritoneal dialysis or haemodialysis who will continue to require these medicines. There are multiple types of high blood pressure medication with various limitations and efficacy, which are best discussed with the attending physician. As uncontrolled blood pressure can still lead to heart disease and stroke, it is important for kidney failure patients to take their high blood pressure medicine regularly so that blood pressure can be maintained in the normal range while on dialysis treatment.

Complications of Dialysis

All medical treatment potentially have side effects or complications. Dialysis likewise, whether peritoneal or haemodialysis, similarly have potential problems.

The problems in dialysis include:

Access complications: The arterio-venous fistula can become blocked and non-functioning, infected, ballooned out (aneurysmal) and rupture. In the same way, the peritoneal catheter can be infected at its contact at the skin (exit site infection), or within the tract of the abdomen (tunnel infection), or within the abdomen (peritonitis). It can become blocked or kinked, or may leak. Antibiotic therapy cures most cases of infections. In most instances these complications can be treated with medicines (e.g. antibiotics for infections), or appropriate surgery. However, an alternate temporary access into a new vein in the neck or chest or groin may need to be inserted with a blood catheter until the original problem is solved.

Heart disease: The heart can be affected in many ways in dialysis patients. The lining of the heart can become filled with fluid that compresses the heart (pericarditis); the blood vessels of the heart can become clogged up (atherosclerosis or a heart attack), the valves of the heart can become thickened or infected (bacterial endocarditis). Kidney failure patients also tend to have too much fluid in their body. This if uncorrected by a combination of judicious fluid and salt restriction together with adequate fluid removal by dialysis can lead to overstretching of the heart muscle with permanent damage to the heart (cardiomyopathy).

Vascular Diseases presenting as strokes or gangrene: Patients on dialysis have increased propensity for strokes (a sudden brain attack with paralysis, numbness or coma) and gangrene (inadequate blood supply causing death of the organ) of the lower extremities

Fluid overload: This refers to a condition where there is too much salt and water in the body. This manifests as breathlessness or swelling of the legs, hands and abdomen. This can be avoided with adequate dialysis and restriction of fluids.

Malnutrition: The patient on dialysis is prone to malnutrition. Malnutrition is dangerous to the patient. Patients who are malnourished at the start of dialysis have a much higher risk of death compared to those are not. The key to preventing malnutrition is to ensure that patients have a diet that is adequately high in proteins (for a building block) and calories for energy. While a low protein diet is appropriate in the early stage of renal disease, among patients on dialysis, the converse is true: patients on dialysis should be on a high protein diet. In addition, they should have adequate dialysis. In the elderly particularly, food should be prepared so that they can easily eat, their dentition corrected, and infections of any source treated aggressively.

Dialysis - Symptoms

Dialysis - How to prevent?

Dialysis - Causes and Risk Factors

Dialysis - Diagnosis

Dialysis - Treatments

Dialysis - Preparing for surgery

Dialysis - Post-surgery care

Dialysis - Other Information

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