The design of safety-critical medical infusion devices
Intravenous infusion devices - commonly known as infusion pumps - provide clinicians with mechanisms to automate the accurate dosing of potent fluid therapies to critically ill patients. In critical care applications, fluid dosing must be both accurate and safe since unwanted flow disturbance can cause physiological harm to the patient. This study consists of three discrete projects based on these vital themes of safe device design and accurate fluid delivery. The first project, commissioned by Medical Magnetics Ltd during the period 1998 onwards, proposed that the fail-safe design philosophy universally used in the design of infusion pumps, and implemented in embedded software, is lengthy and provides the manufacturer with difficulties in demonstrating the exhaustive fail-safe validation needed for an instrument to be released speedily for sale. An alternative and innovative strategy employing the design of hardware modules and using re-configurable VLSI, is proposed and shown to offer a significant reduction of the design and validation phase of development with consequent financial benefit to the manufacturer. The second project conducted as part of the Manukau Institute Research Programme for 2003 examined the manner in which dosing accuracy is assessed for infusion pumps. The International Standard used by clinicians to select apparatus suitable for treatment of 'critically-ill' patients is shown to be flawed and potentially misleading - a finding of international significance. An innovative mathematical simulation model is described that enables prediction of flow accuracy for various expected operating scenarios previously impossible to investigate using current laboratory measurement techniques. Use of this simulation model indicates that various mechanical design factors influencing system compliance and hence dosing accuracy have been previously ignored by designers and suggests that contemporary infusion pump designs are far from optimum. These findings offer an explanation for instances of dosing error previously reported in the clinical literature and are of international value. The third project of the study utilises the findings of, and is subsequent to, the second project. An innovative design is proposed for an infusion therapy device in which dosing accuracy may be maintained under operating conditions such as height change and patient venous pressure variation that cause unwanted errors in conventional equipment designs. This design is the subject of patent application, commercial exploitation and further development.
Drug infusion pumps, Medical instruments and apparatus
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