Kidney dialysis is the process of removing excess water, solutes, and toxins from the blood in people whose kidneys can no longer perform these functions naturally. Along with kidney transplantation, it is a type of renal replacement therapy.
Dialysis may need to be initiated when there is a sudden rapid loss of kidney function, known as acute kidney injury (previously called acute renal failure), or when a gradual decline in kidney function, chronic kidney failure, reaches stage 5. Stage 5 chronic renal failure is reached when the glomerular filtration rate is less than 15% of the normal, creatinine clearance is less than 10 mL per minute, and uremia is present.AMGEN Canada Inc. Essential Concepts in Chronic Renal Failure. A Practical Continuing Education Series. Mississauga, 2008: p. 36.
Dialysis is used as a temporary measure in cases of acute kidney injury or for those awaiting kidney transplant and as a permanent measure in those for whom a transplant is not indicated or not possible.Pendse S, Singh A, Zawada E. "Initiation of Dialysis". In: Handbook of Dialysis. 4th ed. New York; 2008:14–21
In West European countries, Australia, Canada, the United Kingdom, and the United States, dialysis is paid for by the government for those who are eligible. The first successful dialysis was performed in 1943.
Background
The kidneys have an important role in maintaining health. When the person is healthy, the kidneys maintain the body's internal equilibrium of water and minerals (sodium, potassium, chloride, calcium, phosphorus, magnesium, sulphate). The acidic metabolism end-products that the body cannot get rid of via respiration are also excreted through the kidneys. The kidneys also function as a part of the endocrine system, producing erythropoietin, calcitriol and renin. Erythropoietin is involved in the production of red blood cells and calcitriol plays a role in bone formation.Brundage D. Renal Disorders. St. Louis, MO: Mosby; 1992 Dialysis is an imperfect treatment to replace kidney function because it does not correct the compromised endocrine functions of the kidney. Dialysis treatments replace some of these functions through diffusion (waste removal) and ultrafiltration (fluid removal). Dialysis uses highly purified (also known as "ultrapure") water.
Principle Dialysis works on the principles of the diffusion of solutes and ultrafiltration of fluid across a semipermeable membrane. Diffusion is a property of substances in water; substances in water tend to move from an area of high concentration to an area of low concentration. Mosby's Dictionary of Medicine, Nursing, & Health Professions. 7th ed. St. Louis, MO; Mosby: 2006 Blood flows by one side of a semipermeable membrane, and a dialysate, or special dialysis fluid, flows by the opposite side. A semipermeable membrane is a thin layer of material that contains holes of various sizes, or pores. Smaller solutes and fluid pass through the membrane, but the membrane blocks the passage of larger substances (for example, red blood cells and large proteins). This replicates the filtering process that takes place in the kidneys when the blood enters the kidneys and the larger substances are separated from the smaller ones in the glomerulus.
The two main types of dialysis, hemodialysis and peritoneal dialysis, remove wastes and excess water from the blood in different ways. Hemodialysis removes wastes and water by circulating blood outside the body through an external filter, called a dialyzer, that contains a semipermeable membrane. The blood flows in one direction and the dialysate flows in the opposite. The counter-current flow of the blood and dialysate maximizes the concentration gradient of solutes between the blood and dialysate, which helps to remove more urea and creatinine from the blood. The concentrations of solutes normally found in the urine (for example potassium, phosphorus and urea) are undesirably high in the blood, but low or absent in the dialysis solution, and constant replacement of the dialysate ensures that the concentration of undesired solutes is kept low on this side of the membrane. The dialysis solution has levels of minerals like potassium and calcium that are similar to their natural concentration in healthy blood. For another solute, bicarbonate, dialysis solution level is set at a slightly higher level than in normal blood, to encourage the diffusion of bicarbonate into the blood, to act as a pH buffer to neutralize the metabolic acidosis that is often present in these patients. The levels of the components of dialysate are typically prescribed by a nephrologist according to the needs of the individual patient.
In peritoneal dialysis, wastes and water are removed from the blood inside the body using the peritoneum as a natural semipermeable membrane. Waste and excess water move from the blood, across the visceral peritoneum due to its large surface area and into a special dialysis solution, called dialysate, in the peritoneal cavity within the abdomen.
Types There are three primary and two secondary types of dialysis: hemodialysis (primary), peritoneal dialysis (primary), hemofiltration (primary), hemodiafiltration (secondary) and intestinal dialysis (secondary).
Hemodialysis
In hemodialysis, the patient's blood is pumped through the blood compartment of a dialyzer, exposing it to a partially permeable membrane. The dialyzer is composed of thousands of tiny hollow synthetic fibers. The fiber wall acts as the semipermeable membrane. Blood flows through the fibers, dialysis solution flows around the outside of the fibers, and water and wastes move between these two solutions.Ahmad S, Misra Hemodialysis Apparatus. In: Handbook of Dialysis. 4th ed. New York, NY; 2008:59-78. The cleansed blood is then returned via the circuit back to the body. Ultrafiltration occurs by increasing the hydrostatic pressure across the dialyzer membrane. This usually is done by applying a negative pressure to the dialysate compartment of the dialyzer. This pressure gradient causes water and dissolved solutes to move from blood to dialysate and allows the removal of several litres of excess fluid during a typical 4-hour treatment. In the United States, hemodialysis treatments are typically given in a dialysis center three times per week (due in the United States to Medicare reimbursement rules); however, as of 2005 over 2,500 people in the United States are dialyzing at home more frequently for various treatment lengths. Studies have demonstrated the clinical benefits of dialyzing 5 to 7 times a week, for 6 to 8 hours. This type of hemodialysis is usually called nocturnal daily hemodialysis and a study has shown it provides a significant improvement in both small and large molecular weight clearance and decreases the need for phosphate binders. These frequent long treatments are often done at home while sleeping, but home dialysis is a flexible modality and schedules can be changed day to day, week to week. In general, studies show that both increased treatment length and frequency are clinically beneficial.
Hemo-dialysis was one of the most common procedures performed in U.S. hospitals in 2011, occurring in 909,000 stays (a rate of 29 stays per 10,000 population).Pfuntner A., Wier L.M., Stocks C. Most Frequent Procedures Performed in U.S. Hospitals, 2011. HCUP Statistical Brief #165. October 2013. Agency for Healthcare Research and Quality, Rockville, MD. .
Peritoneal dialysis
In peritoneal dialysis, a sterile solution containing glucose (called dialysate) is run through a tube into the peritoneal cavity, the abdominal body cavity around the intestine, where the peritoneal membrane acts as a partially permeable membrane.
This exchange is repeated 4–5 times per day; automatic systems can run more frequent exchange cycles overnight. Peritoneal dialysis is less efficient than hemodialysis, but because it is carried out for a longer period of time the net effect in terms of removal of waste products and of salt and water are similar to hemodialysis. Peritoneal dialysis is carried out at home by the patient, often without help. This frees patients from the routine of having to go to a dialysis clinic on a fixed schedule multiple times per week. Peritoneal dialysis can be performed with little to no specialized equipment (other than bags of fresh dialysate).
Hemofiltration
Hemofiltration is a similar treatment to hemodialysis, but it makes use of a different principle. The blood is pumped through a dialyzer or "hemofilter" as in dialysis, but no dialysate is used. A pressure gradient is applied; as a result, water moves across the very permeable membrane rapidly, "dragging" along with it many dissolved substances, including ones with large molecular weights, which are not cleared as well by hemodialysis. Salts and water lost from the blood during this process are replaced with a "substitution fluid" that is infused into the extracorporeal circuit during the treatment.
Hemodiafiltration
Hemodiafiltration is a combination between hemodialysis and hemofiltration, thus used to purify the blood from toxins when the kidney is not working normally and also used to treat acute kidney injury (AKI).
Intestinal dialysis In healthy humans, the intestines both remove uremic toxins (urea, creatine, uric acid) from blood and add uremic toxins (indoxyl sulfate, ammonia, etc.) to blood. More uremic toxins are excreted through the gut (as feces) than through the kidneys (as urine). This exchange of substances is enabled by the massive surface area of the intestinal capillary network and intestinal mucus, together serving as a large semipermeable membrane. In patients with kidney failure, the intestines receive a larger influx of uremic toxins due to a higher concentration in blood, but this does not automatically translate to a benefit in reducing blood toxin levels as gut bacteria use these toxins as food, producing more toxins in the process. The goal of intestinal dialysis is to maximize the removal of uremic toxins into the intestines while minimizing the production of new toxin molecules in the intestines. It serves as a more conservative renal replacement therapy for those unable to tolerate conventional dialysis. There are a few forms of intestinal dialysis:
Small bowel/intestinal dialysis puts the dialysate in the small intestines through a surgical opening (ileostomy), using the intestinal walls as the semipermeable membrane for removing toxins. It did not show any actual survival benefit overall and was replaced by modern dialysis methods. Induced diarrhea has been found to provide toxin removal in small, non-randomly controlled studies. One method uses the ingestion of 1 to 1.5 liters of non-absorbable solutions of polyethylene glycol or mannitol every fourth hour. Colonic dialysis puts the dialysate in the colon, which can be done without a surgical opening: an anal catheter inserted through the anus can reach the ascending colon and inject the dialysate there. The clinical use of colonic dialysis is still in its early stages (as of 2020), being studied mainly in Iran and China. Oral adsorbents are used in hope that they will absorb toxins as they pass through the gut. One of them, AST-120, has undergone two multinational randomized controlled trials in which it failed to show a benefit in slowing CKD progression. However, a post-hoc analysis finds some benefit in a subgroup of patients.
A related development is supplementation with soluble fibres such as acacia fibre into the diet. These fibers are digested by bacteria in the colon. This bacterial growth increases the amount of nitrogen that is eliminated in fecal waste.
Indications The decision to initiate dialysis or hemofiltration in patients with kidney failure depends on several factors. These can be divided into acute or chronic indications.