Sandbox dialysis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

A hemodialysis machine

In medicine, dialysis is primarily used to provide an artificial replacement for lost kidney function (renal replacement therapy) due to renal failure. Dialysis may be used for very sick patients who have suddenly but temporarily, lost their kidney function (acute renal failure) or for quite stable patients who have permanently lost their kidney function (end stage renal failure). When healthy, the kidneys maintain the body's internal equilibrium of water and minerals (sodium, potassium, chloride, calcium, phosphorus, magnesium, sulfate) and the kidneys remove from the blood the daily metabolic load of fixed hydrogen ions. The kidneys also function as a part of the endocrine system producing erythropoietin and 1,25-dihydroxycholecalciferol (calcitriol). Dialysis treatments imperfectly replace some of these functions through the diffusion (waste removal) and convection (fluid removal). Dialysis is an imperfect treatment to replace kidney function because it does not correct the endocrine functions of the kidney.

History

Many have played a role in developing dialysis as a practical treatment for renal failure, starting with Thomas Graham of Glasgow, who first presented the principles of solute transport across a semipermeable membrane in 1854. The artificial kidney was first developed by Abel, Rountree and Turner in 1913, the first Peritoneal Dialysis was by Georg Ganter (1923),the first hemodialysis in a human being was by Hass (February 28, 1924) and the artificial kidney was develop a into clinically useful apparatus by Kolff in 1943 - 1945. This research showed that life could be prolonged in patients dying of renal failure. Yet, the technical problems associated with blood access or access to the peritoneum made dialysis limited to patients with acute renal failure until 1960 (though a chronic renal failure patient was treated in 1956 with peritoneal dialysis). In 1960 work on subcutaneous arteriovenous shunt (a plastic tube connected to an artery and a vein) by Scribner and Quinton made hemodialysis available as a treatment for people with chronic renal failure..

Principle

Dialysis works on the principles of the diffusion and osmosis of solutes and fluid across a semipermeable membrane. Blood flows by one side of a semipermeable membrane, and a dialysate or fluid flows by the opposite side. Smaller solutes and fluid pass through the membrane. The blood flows in one direction and the dialysate flows in the opposite. The concentrations of undesired solutes (for example potassium, calcium, and urea) are 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 diffusion of bicarbonate into the blood, to neutralise the metabolic acidosis that is often present in these patients.

Types

There are two main types of dialysis, hemodialysis and peritoneal dialysis.

Hemodialysis

Hemodialysis schematic

Main article: hemodialysis

In hemodialysis, the patient's blood is pumped through the blood compartment of a dialyzer, exposing it to a semipermeable membrane. 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 removal of several litres of excess salt and water during a typical 3 to 4 hour treatment. Hemodialysis treatments are typically given three times per week, but more frequent sessions, which are usually 2 to 3 hours in duration given 5 or 6 times per week can be sometimes prescribed. Hemodialysis treatments can be given either as an outpatient or as home hemodialysis.

Peritoneal dialysis

Main article: peritoneal dialysis

In peritoneal dialysis, a sterile solution containing minerals and glucose is run through a tube into the peritoneal cavity, the abdominal body cavity around the intestine, where the peritoneal membrane acts as a semipermeable membrane. The dialysate is left there for a period of time to absorb waste products, and then it is drained out through the tube and discarded. This cycle or "exchange" is normally repeated 4-5 times during the day, (sometimes more often overnight with an automated system). Ultrafiltration occurs via osmosis; the dialysis solution used contains a high concentration of glucose, and the resulting osmotic pressure causes fluid to move from the blood into the dialysate. As a result, more fluid is drained than was instilled. 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 and it requires motivation. Although support is helpful, it is not essential. It does free patients from the routine of having to go to a dialysis clinic on a fixed schedule multiple times per week, and it can be done while travelling with a minimum of specialized equipment.

Hemofiltration

Main article: 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, facilitating the transport of dissolved substances, importantly ones with large molecular weights, which are cleared less 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 is a term used to describe several methods of combining hemodialysis and hemofiltration in one process.

Starting indications

The decision to initiate dialysis or hemofiltration in patients with renal failure can depend on several factors, which can be divided into acute or chronic indications.

• Acute Indications for Dialysis/Hemofiltration:
  • 1) Hyperkalemia
  • 2) Metabolic Acidosis
  • 3) Fluid overload (which usually manifests as pulmonary oedema)
  • 4) Uremic pericarditis, a potentially life threatening complication of renal failure
  • 5) And in patients without renal failure, acute poisoning with a dialysable drug, such as lithium, or aspirin.
• Chronic Indications for Dialysis:
  • 1) Symptomatic renal failure.
  • 2) Low glomerular filtration rate (GFR) (RRT often recommended to commence at a GFR of less than 10-15 mls/min/1.73m2)
  • 3) Difficulty in medically controlling serum phosphorus or anaemia when the GFR is very low

Related Chapters

• Apheresis is another extracorporeal technique that selectively removes specific constituents from blood.
• Acute renal failure
• Chronic renal failure
• Hepatorenal syndrome
• Nephrology
• Renal failure

Resources

• Home Dialysis Central - Doing dialysis at home allows more personal control; fewer diet and fluid limits and medications; reduces symptoms and hospitalizations, and makes it more possible to keep a job. Learn all about it.
• Dialysis Tips - Resource for dialysis personnel and general background with good understanding of the basic problems of dialysis therapy.
• Global Dialysis - Resource and community for dialysis patients and professionals
• Virtual Dialysis Museum - History and pictures of dialysis machines through time
• The Renal Unit - News and resources for those undergoing dialysis
• HDCN Online journal - Free medical lectures pertaining to various aspects of dialysis and nephrology; intended for physicians and nurses, not for patients.

References

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