Go with the flow: How to integrate sensors in medical devices
Susanne Jungmann, Sensirion, explains the importance of sensors for measuring and controlling lowest flow rates in medical devices and how they can be integrated effectively by manufacturers.
Medical procedures and therapies are often faced by the same challenge: accurately measuring and controlling lowest flow rates down to the microliter per hour range. In most of these cases the outcome of the therapy, as well as the well-being or even the survival of the patient strongly rely on the reliable and continuous drug administration in such low flow regimes. While various countermeasures have been evaluated and introduced, there is still room for improvement and for new technologies to be established. One novel development with tremendous potential is a single-use liquid flow sensor that is able to measure lowest flow rates and detect common failure modes quickly and reliably.
Ambulatory infusion pumps are experiencing increasing popularity in ambulatory and home care settings for the continuous delivery of highly-concentrated medication over several days. Flow rates range from single millilitres up to a few hundred millilitres per hour (ml/h). Portable ambulatory infusion treatments have been able to dramatically reduce the length of hospitalisation with its related costs, while at the same time greatly improving patient quality of life. For example, in the case of chemotherapy, it has been shown that continuous drug administration has a beneficial pharmacodynamic impact on efficacy and toxicity, compared with traditional bolus injections every 24 hours.
A typical elastomeric pump consists of a pressurised reservoir exerting continuous force on the medication, an IV administration set including a pressure drop element (eg a thin capillary used as a restrictor) which determines the required flow rate, and an IV catheter or injection port. These pumps are often purely mechanical and designed as single-use devices for infusional chemotherapy, pain management, or chelation therapy. Their characteristic pressure profile is typically concave, ie. the generated flow rate at the beginning and at the end of the therapy is higher than in the middle.
The actual flow rates of elastomeric pumps are mainly influenced by two parameters: the differential pressure between the inlet and outlet of the restrictor and the resistance of the restrictor itself. While the inlet pressure is determined by the pressure profile of the pump’s reservoir, the outlet pressure is affected by several factors: the type of the injection site, the patient’s specific vein back pressure, as well as the difference in height between reservoir and injection site. The resistance of the restrictor is influenced by the viscosity of the medication which in turn is determined by the solution’s molecular composition and its temperature. In order to maximise temperature stability in spite of changing ambient temperatures, medical device manufacturers typically recommend to tape the restrictor to the patient’s skin at all times.
The currently established solutions, as described above, have a series of well-known shortcomings.
The required flow rate accuracy according to international standards is ±15% under very specific conditions. During a typical daily routine, it is challenging for a patient to adhere to the handling instructions that guarantee the specified performance. In consequence the real life flow rates often deviate drastically from the intended values. While higher flow rates may result in an increased toxicity of the therapy, lower flow rates will reduce the treatment’s efficacy. Both deviations will prevent an optimal therapeutic outcome. Additionally, medication not administered due to lower flow rates and thus leftover in the reservoir, may require specialised waste management.
Especially at low flow rates, clogging of the very thin restrictor or at the injection site is difficult to detect quickly and reliably without a liquid flow sensor. In most cases, it takes hours for an occlusion to get detected by the patient or nursing staff. This again will prevent an optimal therapeutic outcome.
These effects can currently neither be monitored for direct feedback or remote support, nor can they be recorded for later analysis by the clinical staff. Therefore, many ambulatory infusion pump manufacturers are designing smart versions of their existing products. One possible solution is the seamless integration of a single-use liquid flow sensor, like the LD20 from Sensirion, into the infusion set.
In paediatrics and neonatology, small and vulnerable patients receive special care and attention, not only from the nursing staff, clinicians and their parents, but also from the medical industry.
In this context, flow rates of 1 ml/h or even lower are typical due to the patients’ low body weights. Consequentially, the known and inherent issues of the current infusion technology (start-up delays, flow-discontinuation and dependence on hydrostatic pressure) may have even more detrimental effects on the therapy outcome.
Administration of life-saving drugs with a short half-life, for example adrenalin, must not be delayed or discontinued by the infusion technology in question. However, this happens frequently due to the compliance of the tubing and syringe plunger, the movement and vertical displacement of syringe pumps during transport of the patient, or the loading of a new syringe into the pumping device.
The fluidic balance of a child or a neonate has to be observed and calculated on a daily basis. The sum of any fluid injected or ingested, be it drugs or nutrition, and the excreted fluid is not only critical to the well-being of the child but also limited. Single-use liquid flow sensors, integrated into infusion sets, will enable medical devices not only to monitor the administered flow rates accurately, but also to detect the above mentioned common failure modes quickly and reliably.
Continuous urine flow measurements of critically ill patients require flow rates to be measured in the range of single millilitres up to 100 millilitres per hour. Acute Kidney Injury (AKI) is a common complication (occurrence 20% - 30%) in critically ill patients in the ICU. The early detection and correct diagnosis of AKI currently receives a lot of attention in medical research and development. Patient mortality and the risk to develop a chronic kidney disease drastically increases with AKI onset.
Nowadays, AKI can be diagnosed on the basis of laboratory data, including a change in serum creatinine or in the excretion of waste products of the kidney’s nitrogen metabolism. Research showed that the urine output of a catheterised patient is more sensitive than biochemical markers to changes in renal function. It exhibits a faster response than changes to the biochemical composition of the urine.
The standardized RIFLE classification (risk, injury, failure, loss, end-stage) is used by clinicians to increase the sensitivity and specificity of their AKI diagnosis. This classification states, that a decrease in urine output below 0.5 ml per kilogram body weight per hour for more than six hours is a first indication for an increased risk of acute renal failure.
The biggest issue faced today, when determining the urine output of a patient, is that it has to be measured and recorded manually in regular intervals by the ICU’s nursing staff. Only then can a further trend analysis be performed. The possibility of automatically and continuously measuring and recording urine outputs would allow a timelier diagnosis and enable clinicians to recognise or even prevent AKI onset. Again the LD20 single-use liquid flow sensor can solve this problem.
Common to all of the medical fields and applications described above, is that by using a single-use liquid flow sensor, important measurement data and vital signs could be acquired and stored automatically in the electronic patient data management system (PDMS). This solves the issue of time-consuming, inaccurate, and cumbersome hand written medical charts and allows the timely detection of adverse effects. The data would be easily available for a retrospective analysis. Above all, it would free-up time of the nursing staff to be spent on other tasks which still require personal attention.