Typical Number in Hospital: 20 Cost Bands: 5 References: 3,4

This is commonly called an 'artificial kidney'. It is a system of pumps, clamps, timers, tubing circuitry, and heaters, which passes blood from the patient, and a dialysate solution, through a dialyser. Usually the system manufactures the dialysate by mixing a concentrated solution with filtered, softened or de- ionized tap water. This is heated to blood temperature, de- aerated and checked for conductivity before passing through the dialyser where it flows over a semipermeable membrane. The patient's blood is made to pass over the other side of the membrane so that fluids and toxins of low molecular weight can pass from the blood into the dialysate. Other molecules from the dialysate pass into the blood to rectify imbalances before it passes to waste. Blood is drawn from, and returned to the patient at a shunt or fistula between blood vessels of an arm, leg, or other access point.

Haemodialysis machines are used in hospitals in cases of acute renal failure and also in some cases of poisoning (e.g. drug overdoses). Patients with chronic renal failure are usually treated using machines at their homes or at special dialysis centres.

Since the patient's life depends upon the correct functioning of the machine it contains a number of measuring and alarm circuits including a conductivity meter which identifies the correct concentration of the dialysing fluid, a temperature monitor to ensure that the dialysate neither cools nor heats the blood, and a dialysate pressure monitor since the quantity of water withdrawn from the patient will depend upon the pressure differential between the blood and dialysate.

Because of the pressure differential, or incorrect assembly of the dialyser, the membrane may rupture leading to a blood leak into the dialysate. A blood leak detector, which is essentially an optical densitometer, is always included in the dialysate circuit after the artificial kidney.

In the circuit containing the patient's blood, the pressure of the blood returning to the patient (venous pressure) is measured, to give protection against leaks in the blood lines, and a special monitor is required to ensure that no air is passed to the patient. This may be a blood level detector on the bubble trap, or another optical densitometer or ultrasonic bubble detector to identify the presence of bubbles in the return circuit to the patient.

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details