Haemodialysis was introduced in Hong Kong in the late 1960s. Haemodialysis requires a dialyser, which replicates the kidney functions of removing excess waste and water from the blood, and a haemodialysis machine. The haemodialysis machine has several key functions. The main function is to prepare dialysate by diluting one part dialysate concentrate with 34 parts of treated water. Dialysate concentrate is used because during dialysis, dialysate is used at 500 mL per minute; a 5-hour dialysis session would consume 150 L. Storage and transportation of such a large volume of dialysate would be problematic and expensive. The haemodialysis machine also delivers the dialysate to the dialyser and drains the waste dialysate. Finally, the haemodialysis machine monitors dialysate pressure in the arterial and venous chambers, conductivity of the dialysate, presence of air in the bloodstream, and blood leakage. Thus, from an engineering perspective, a dialysis machine is more appropriately called a ‘dialysate delivery and monitoring system’.

In the early days, there were two types of haemodialysis machines: a batch tank machine and a proportioning pump machine. Batch tank systems are very simple; the batch tank has a capacity of about 120 L, which the user manually fills with one part of dialysate concentrate and 34 parts of treated water to make the dialysate. Although simple, because of the large space requirements, it is unsuitable for dialysis centres where there are often multiple machines. It is more commonly used in home haemodialysis, where simplicity and low cost are important. Proportioning pump machines have two pumps; one pump draws one part of dialysate concentrate and another pump draws 34 parts of water. In this way, the dialysate is made continuously online. This saves a lot of space in a dialysis centre, and the preparation time needed is markedly reduced because there is no need to wait for filling up the batch tanks.1

Early haemodialysis machines only prepared the dialysate. In addition to the haemodialysis machine, separate pumps were needed for blood, to deliver the blood from the patient to the dialyser, and heparin, for continuous heparinisation. Moreover, the operator had to calculate the fluid removal rate using the ‘transmembrane pressure’ and the ‘ultrafiltration coefficient’ of the dialyser; however, this could only give an estimate. One example of this early type of machine was the Centry One (COBE Laboratories, Inc., Lakewood [CO], United States), which was used in Princess Margaret Hospital, Hong Kong, when it opened in 1977.

In the 1980s, the early machines were replaced with newer models like the Centry 2 machines (COBE Laboratories, Inc.) [Fig a] and the AK10 models (Gambro Company). The Centry 2 machines were more widely used. These offered several upgrades over the older models. They were compact and robust with the blood pump and heparin pump built in. They also included an indicator to show the rate of fluid removal from the dialyser (‘ultrafiltration meter’) in real time, obviating the need for the operator to calculate this and increasing the accuracy (Fig b