dialysis

dialysis Failure of the kidneys, whether acute (rapid onset) or chronic (gradual onset), is life-threatening, since waste products accumulate in the body and there are disturbances of body fluid volume and composition. There are two possible treatment: dialysis, or kidney transplantation. A period of dialysis may enable the kidneys of a patient with acute renal failure to recover. In patients with chronic renal failure, dialysis is generally used until a suitable donor kidney becomes available for transplantation. However, many kidney patients have been maintained for over 20 years, with a reasonable quality of life, by dialysis.

Dialysis is a way of removing toxic substances from the blood, and restoring the body fluid volume and composition to close to normal. Two kinds of dialysis are now common, though both types have long histories. Haemodialysis, using dialysers which are sometimes called artificial kidneys, was pioneered by the Dutch physician Willem Kolff, initially at Gronigen University Hospital, and then at Kampen Hospital. Kolff treated his first patient with an experimental haemodialyser in 1943, and in 1956 introduced the first practical haemodialysis machine.

Peritoneal dialysis has a longer history but a shorter period of practical application. The first peritoneal dialysis of a patient was performed in 1923, but the procedure did not become accepted until 1959, when developments in tubing and catheters had made the technique safer.

Neither peritoneal dialysis nor haemodialysis bear much resemblance to the way the kidneys normally work. The kidneys have a filtration process which is essentially non-selective: with the exception of the proteins, all the constituents of the blood plasma are filtered, whether or not the body needs to excrete them or retain them. The selectivity comes after the filtration process, when the nephrons (kidney tubules) reabsorb some substances into the blood, secrete others, or simply allow the filtered substances to continue along the nephron to escape in the urine. No artificial kidney works like this.

Haemodialysis

The principle of haemodialysis is that blood from the patient passes over a ‘dialysis membrane’ of very large surface area, on the other side of which is a dialysing fluid. Hollow fibres are often used instead of a flat membrane. Molecules pass from the blood into the dialysis fluid (and vice versa) by diffusion. The dialysis membrane is permeable (porous) to all the plasma constituents, with the exception of plasma proteins. If the concentration of a substance (such as urea) is greater in the patient's blood plasma than in the dialysis fluid, there will be net transfer of the substance to the dialysis fluid. Conversely, it is possible to raise the concentration of substances in the patient's blood plasma by having a higher concentration of substances (e.g. glucose or bicarbonate) in the dialysis fluid.

The rate of movement of any individual solute across the dialysis membrane depends on four factors: (i) the permeability of the membrane to the solute; (ii) the surface area of the membrane; (iii) the concentration gradient for the solute across the membrane (the difference in concentration between plasma and dialysing fluid); (iv) the length of time that the plasma and dialysing fluid remain in contact with the membrane.

The maximum rate of solute transfer will occur when the concentration difference is greatest. For urea, for example, this will be when the dialysis is begun, because the patient's plasma concentration is high, but the difference becomes smaller as the solute moves from blood plasma to dialysis fluid. This dissipation of the concentration gradient can be minimized by having high flow rates for blood and/or dialysis fluid.

In modern dialysers, the dialysis membrane or hollow fibre has significant permeability to water, so that water (and solute) can be removed from the blood by ultrafiltration (also called bulk flow). The process of carrying solute across a membrane by bulk flow of solvent (water) is convection. The rate of ultrafiltration will depend on the four factors listed above, and on the hydrostatic pressure difference between the blood compartment and the dialysing fluid compartment of the apparatus. Some artificial kidneys do not use dialysing fluid, and rely on ultrafiltration and bulk flow to remove water and solute across a membrane. This process is termed haemofiltration.

For both haemodialysis and haemofiltration, easy access to the patient's blood supply is essential, and since access may be required every few days over a period of many years, special arrangements are necessary. In general this involves a ‘shunt’ — an artificial connection from an artery to a vein, usually in an arm or leg. One such shunt is shown in Fig. 1. It is then a simple matter to connect the ‘artificial kidney’ to the patient's blood supply.

The total volume of blood in the artificial kidney at any moment is small (about 500 ml), with a flow rate of about 300 ml/min, and a total dialyser membrane area of 1–3 m²; this means that the equivalent of the whole blood volume of about 5 litres in an adult circulates through the dialyser every 15–20 min.

Peritoneal dialysis

This is commonly termed CAPD — continuous ambulatory peritoneal dialysis — since most patients undergo the procedure while going about many of their normal activities. The principle of the method is that the peritoneal membrane, which lines the peritoneal cavity in the abdomen, is used as the dialysis membrane.

The patient has a tube (catheter) inserted through the abdominal wall into the peritoneal cavity, and this remains in place on a semi-permanent basis. Dialysis fluid (0.5–3 litres) is allowed to flow into the peritoneal cavity via the catheter, left in typically for several hours or overnight and then drained and replaced with fresh fluid. Movement of fluid and solute occurs across the peritoneal membrane by diffusion and solvent drag (convection), as described above.

The high urea content of the blood in renal failure creates an osmotic attraction across the peritoneal membrane, so that water would tend to move from the peritoneal cavity into the patient. To prevent this, and ensure that water moves from the patient's blood to the dialysis fluid, an osmotically active substance is incorporated in the dialysis fluid. This is usually dextrose, but amino acids and glucose polymers can also be used.

Chris Lote

Bibliography

Gabriel, R. (1990). A patient's guide to dialysis and transplantation, (4th edn). Kluwer Academic.

See also kidneys; organ donation.