Pharmaceutical & medical

Flow sensors in ventilation devices

Medical ventilators are used in a diverse range of application areas. As such, the flow sensors that are an essential part of these devices have to meet differing application requirements. In this article, Sensirion’s Daniel Träutlein discusses the various measurement principles for these sensors


ensors are used to measure gas flows for various purposes in modern medical ventilation technology. A variety of measuring principles are used, depending on the manufacturer and the intended use. Medical ventilation technology has made rapid progress since the invention of the “iron lung” about 80 years ago - the first device used for patient ventilation. Along with pressure sensors, flow sensing technology and gas flow sensors play a significant role today. The original iron lung used negative pressure to achieve patient ventilation, but modern devices use positive pressure in the patient’s lungs for this purpose. Along with air, other gases - primarily oxygen - are also used for ventilation. The quantity and mixture of the various gases are precisely controlled and monitored using sensor systems. The requirements on sensor systems vary greatly, depending on where and how the ventilation device is used.

DIVERSE APPLICATION AREAS Medical ventilation devices can basically be divided into three application areas: devices for emergency treatment, devices for intensive care, and devices for home use. Naturally, each area and each individual device has its own specific features. The key considerations with emergency treatment devices are size and weight, since it must be possible to bring them to patients quickly and easily. They usually operate from battery power to make them independent of the AC power line. Ventilation devices for intensive care are used in hospitals and operated by medical staff. They have a significantly greater functional scope than devices for emergency


treatment. The main consideration with these devices is their performance capability. Finally, ventilation devices for home use must ensure proper breathing for patients who are not able to breathe adequately on their own.

MANIFOLD REQUIREMENTS FOR FLOW SENSING TECHNOLOGY The previously mentioned three application areas for ventilation technology lead to diverse requirements for flow sensors. In emergency treatment devices, the pressure drop due to the flow resistance of the sensor in the gas stream should be as small as possible. This is because a high pressure drop means more work for the turbine that generates the positive pressure for the patient. A low pressure drop therefore translates into longer battery service life or a smaller battery, which has a beneficial effect on size or weight. In intensive care, the desire to be able to use the device in the greatest possible variety of setting imposes additional requirements on flow sensing technology. For instance, it should be possible to use the same device for all patients, regardless of whether they are infants or adults. The greatest possible dynamic range and high sensor resolution ensure that the device can be used for patients of different sizes. For example, modern flow sensors should ideally be able to measure flow volumes up to 250 liters per minute, while still being able to detect volumes less than one litre with high precision. Along with this wide dynamic measuring range, measurement accuracy with low gas flows in particular is a crucial aspect. A short sensor response time is also necessary, since various ventilation modes are used in intensive care.

A common aspect of all areas is that the sensors must be robust and have the best possible long- term stability, in order to minimise recalibration and maintenance effort. Other requirements for sensor technology result from the various functions that are used to control or monitor the sensors. A distinction is made between inspiration sensors, expiration sensors and patient-side sensors (spirometry). Sensors on the inspiration side (breathing in) are installed in the device. They are distant from the patient, and the gas always flows from the sensor to the patient (never the other direction). The gas is clean and dry. By contrast, sensors on the expiration side measure the air that is breathed out. This air is moist and comes from the patient. The third area - patient-side sensors - has the most severe requirements. Patient-side sensors must either be suitable for reconditioning, which means cleaning and autoclaving, or disposable. In addition, patient-side sensors must be able to measure both inspiration and expiration, which makes bidirectional calibration essential. This is a major challenge for some types of sensors, in particular hot-wire anemometers. If patient ventilation extends over a relatively long time, weaning creates special challenges for the sensor system if the patient’s breathing must be assisted. The onset of breathing must be detected by the device without any time delay.

VARIOUS MEASURING PRINCIPLES A variety of measuring principles are used for flow measurement in ventilation technology. Due to accuracy requirements and the progressive integration of electronic control systems, the floating-body flow gauges previously used on a

June 2020 Instrumentation Monthly

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