Measuring Lowest Flow Rates in Medical Therapies

Medical procedures and therapies usually pose the same difficulty of the accurate measurement and control of the lowest flow rates down to the microliter per hour range. In a majority of these cases, the result of the therapy and also the well-being or even the survival of the patient are highly dependent on the continuous and reliable drug administration in such low-flow regimes.

Although several countermeasures have been assessed and introduced, there is still scope for enhancement and for establishing new technologies. An innovative advancement with immense potential is a single-use liquid flow sensor that can measure the lowest flow rates and rapidly detect common failure modes in a reliable manner.

Liquid flow sensor LD20 for drug delivery
Liquid flow sensor LD20 for drug delivery

Measuring Flow Rates in Ambulatory Infusion Pumps

Ambulatory infusion pumps have been gaining increasing attention in ambulatory and home care settings due to their ability to continuously deliver highly concentrated medication for a number of days. In this case, the flow rates range from a few milliliters up to a few hundred milliliters per hour.

It has been found that portable ambulatory infusion treatments drastically reduce the length of hospitalization with its associated costs, while also considerably enhancing the quality of life of the patient.

For instance, considering chemotherapy, it has been demonstrated that continuous drug administration has an advantageous pharmacodynamic effect on the efficacy and toxicity, in comparison with conventional bolus injections every 24 hours.

A typical elastomeric pump includes a pressurized reservoir that applies a continuous force on the medication, an IV administration set consisting of a pressure drop element (for example, a thin capillary used as a restrictor) which determines the needed flow rate, and an IV catheter or injection port.

Usually, these pumps are absolutely mechanical and designed as single-use devices for pain management, infusional chemotherapy, or chelation therapy. Their attributive pressure profile is characteristically concave, that is, the flow rate generated at the start and at the end of the therapy is higher than in the middle.

Parameters Affecting Flow Rates of Elastomeric Pumps

Two parameters mainly influence the actual flow rates of elastomeric pumps: the differential pressure between the inlet and outlet of the restrictor and the resistance of the restrictor itself. The pressure profile of the pump’s reservoir governs the inlet pressure, whereas the outlet pressure is affected by various factors: the patient’s particular vein back pressure, the type of the injection site, and the difference in height between the reservoir and the injection site.

The viscosity of the medication has an impact on the resistance of the restrictor and is in turn determined by the molecular composition and temperature of the solution. For maximizing temperature stability regardless of varying ambient temperatures, medical device manufacturers usually recommend to tape the restrictor to the patient’s skin at all times.

Disadvantages of Existing Solutions

The solutions established at present, as elaborated above, have a series of familiar disadvantages.

According to international standards, the needed flow rate accuracy is ±15% under highly particular conditions. At the time of a typical daily routine, it is difficult for a patient to stick to the handling instructions that ensure the specified performance. As a result, the real-life flow rates largely vary from the desired values.

Higher flow rates could lead to an increase in the toxicity of the therapy, whereas lower flow rates affect the efficacy of the treatment. Both variations will hamper an ideal therapeutic result. Moreover, medication not administered as a result of lower flow rates and hence leftover in the reservoir, could mandate specialized waste management.

Specifically at low flow rates, clogging of the very thin restrictor or at the injection site is hard to detect reliably and rapidly in the absence of a liquid flow sensor. In the majority of cases, several hours are needed for an occlusion to be detected by the nursing staff or the patient. This, in turn, will hinder an ideal therapeutic result.

At present, it is not possible to either monitor the effects discussed above for remote support or direct feedback, or record them for later analysis by the clinical staff.

Sensirion’s LD20 Liquid Flow Sensor

Hence, various ambulatory infusion pump manufacturers are developing ingenious versions of their prevalent products. One feasible solution is the seamless combining of a single-use liquid flow sensor, such as Sensirion’s LD20, with the infusion set.

Application to Pediatrics and Neonatology

In the field of pediatrics and neonatology, the little and susceptible patients get a lot of special care and attention from the clinicians, nursing staff, parents and also from the medical industry.

Measuring lowest flow rates in neonatology
Measuring lowest flow rates in neonatology

Under these conditions, flow rates lesser than or equal to 1 ml/hour are normal due to the low body weights of the patients. As a result, the known and intrinsic problems of the existing infusion technology (flow discontinuation, start-up delays, and reliance on hydrostatic pressure) could have even more deleterious impacts on the therapy result.

There must be no delay in or discontinuation of administering life-saving drugs with a short half-life, for instance adrenalin, by the infusion technology that is used. Yet this frequently occurs as a result of the compliance of the tubing and syringe plunger, the vertical displacement and movement of syringe pumps when the patient is transported, or the loading of a new syringe into the pumping device.

It is necessary to observe and calculate the fluidic balance of a child or a neonate on a daily basis. The sum of any ingested or injected fluid, such as nutrition or drugs, and the excreted fluid is not just crucial to the well-being of the child but also limited.

Single-use liquid flow sensors, combined with the infusion sets, will allow accurate monitoring of the administered flow rates as well as the detection of the aforementioned common failure modes by the medical devices in a reliable and quick manner.

In order to continuously measure the urine flow of critically ill patients, flow rates have to be measured in the range of a few milliliters up to 100 ml/hour. Acute kidney injury (AKI) is a common complication that occurs in 20%–30% of the critically ill patients in the ICU.

At present, the correct diagnosis and early detection of AKI have received a lot of attention in medical research and development. The onset of AKI leads to a dramatic increase in patient mortality and the risk of developing a chronic kidney disease.

Continuous urine flow measurements to prevent acute kidney injury (AKI)
Continuous urine flow measurements to prevent acute kidney injury (AKI)

Currently AKI can be diagnosed using laboratory data, such as a change in serum creatinine or in the excretion of waste products of the kidney’s nitrogen metabolism. Studies have demonstrated that the urine output of a catheterized patient is highly sensitive compared to biochemical markers to variations in renal function. Compared to variations in the biochemical composition of the urine, it exhibits a more rapid response.

Clinicians use the standardized RIFLE classification (risk, injury, failure, loss, end-stage) to improve the sensitivity and specificity of their AKI diagnosis. This classification indicates that a decline in urine output below 0.5 ml/kg body weight per hour for more than six hours is a first indication of heightened risk of acute renal failure.

Problems Encountered and Solutions

The major problem encountered at present while determining the urine output of a patient is that it needs to be manually measured and recorded at regular intervals by the nursing staff of the ICU. A further trend analysis can be performed only upon achieving this.

The probability of automatic and continuous measurement and recording of urine outputs would enable a timely diagnosis and allow clinicians to detect or even prevent the onset of AKI. Again, the LD20 single-use liquid flow sensor can be used to overcome this challenge.

A factor common to all the medical fields and applications discussed above is that a single-use liquid flow sensor can be used to obtain important measurement data and vital signs and store them automatically in the electronic patient data management system (PDMS).

This overcomes the problem of inaccurate, time-consuming and burdensome hand-written medical charts and enables the timely diagnosis of adverse effects. Moreover, the data would be easily available for a retrospective analysis. Most importantly it would reduce the time spent by the nursing staff on these tasks.

For the first time, all the above-mentioned medical applications can be revolutionized or enabled with the help of Sensirion’s LD20 single-use liquid flow sensor.

All images sourced and provided by Sensirion

About Sensirion Inc

Medical devices must meet the highest standards in terms of quality and reliability. Doctors, nurses, and patients benefit daily from applications in the field of medical technology that are getting smarter by the day.

The use of proven Sensirion sensor solutions contributes to this and offers the possibility of making applications safer, more reliable and more convenient. Our many years of experience in the field of medical technology make us the preferred experts for leading medical-technology companies.

  • Home Care Devices (Ventilation)
  • Critical Care Devices (Ventilation)
  • Anesthesia
  • Point-of-Care Diagnostic
  • Drug Delivery / Infusion
  • Catheters
  • Smart Inhalers
  • Metabolic measurements
  • E-Health / M-Health

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Last updated: Sep 14, 2020 at 5:46 AM


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