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Reducing the risks in using a new dialysis system for children

Key Points

  • The Royal Victoria Infirmary in Newcastle has developed a new system, called NIDUS, for treating babies with kidney problems for which until now there has been no suitable treatment.

  • We have studied the risks involved to show that the machine used cannot lead to certain dangerous situations arising.

  • This risk analysis is part of the process used to convince the regulator to allow the system to be used widely.

A new way to help children with kidney problems
Our kidneys help remove dangerous waste from our bodies, control the levels of other important chemicals in our blood and help control our blood pressure. When they fail, one treatment is dialysis - a machine takes over doing what the healthy kidneys would do. This is normally done by drawing the patient’s blood out of their body and into a machine that acts like a kidney itself. This is called haemodialysis. Another way, called Peritoneal dialysis, actually does this dialysis in a cavity inside the patient. It is filled with a liquid that over time draws out the waste from the surrounding blood vessels, cleaning the blood. All the liquid is later drawn out of the body and disposed of, to be replaced by fresh liquid.

For children with failing kidneys, dialysis is extremely difficult because the volume of blood that they circulate is very small. Where peritoneal dialysis can be used, it is hard to remove the extra liquid accurately. In children where it isn’t possible or fails, haemodialysis, as used with adults, is challenging or impossible. The typical  volume of liquid withdrawn, say 70 mL, is almost the entire blood volume of a small 1 kg baby. There is also no haemodialysis system approved for use in children weighing less than 8 kg. If these children cannot be given an alternative treatment they are likely to die.

Checking and reducing the risk of using the new machine
A new machine called NIDUS is being used experimentally at the The Royal Victoria Infirmary in Newcastle. Before it can be used more widely, the risks of using it must be identified and carefully assessed. It must then be approved for use by the regulator. The RVI team is doing a risk analysis that will be submitted to the regulator to achieve this certification. Part of this analysis, performed by the CHI-MED team,  has involved a careful investigation of all possible things that the machine can do. We have used techniques to check rigorously what the machine’s software will allow to happen. This risk analysis is being used as part of the submission to the regulator.

The program that drives NIDUS is controlled by a table that can be represented as a spreadsheet called a state transition table. It describes the states of the machine and how events change the state of the dialyser. It takes account of both normal events generated by a person operating the machine and events caused by errors such as bubbles in the system. The spreadsheet is used by the software controller to change the state of the dialysis process.

The risk analysis process has shown, for example,  that particular combinations of states cannot occur in some dangerous circumstances. It has also involved showing  that appropriate information is displayed to remind the clinician to, for example, close a clamp before the next phase of the cycle can commence. We have also shown that particular sequences of states are only possible in certain normal and desirable situations when dialysing, washing etc.

The analysis process has been repeated a number of times. Properties of the device that would help reduce risk were determined through meetings between the developers at RVI and the CHI+MED team. When a property failed, suggesting unnecessarily high risk, examples of the failure were presented to the development team to consider. This could be done in seconds for each property. As a result of the analysis, as well as producing the risk analysis required by the regulator, the development team changed the design to improve its overall robustness. This process encouraged the developers to be confident that the device was safe, far beyond what could be achieved through the testing process more normally used. It also provides part of the documentary evidence to the regulator that all reasonable measures have been taken to ensure the safety of the device.