Gas Flow Sensing Technology in Medical Ventilation Devices

In modern medical ventilation technology, sensors are employed for measuring gas flows for different purposes. Based on the manufacturer and the desired use, a range of measuring principles are applied. Currently, several different sensor products are available in the market.

The invention of the “iron lung” nearly 80 years ago—the first device used for patient ventilation—has resulted in a rapid progress in medical ventilation technology. Pressure sensors, gas flow sensors and flow sensing technology have a crucial role at present. In the original iron lung, negative pressure was used to realize patient ventilation.

However, in modern devices, positive pressure in the patient’s lungs is used for this purpose. Together with air, other gases—mainly oxygen—are also used for ventilation. Sensor systems are used for precise control and monitoring of the quantity and mixture of the different gases. The requirements on sensor systems largely differ based on the location and method of using the ventilation device.

Diverse Application Areas

In general, medical ventilation devices can be divided into three application areas: devices for intensive care, devices for emergency treatment, and devices for home use. By nature, each individual device and each area has its own particular features.

Size and weight are the most important considerations when it comes to emergency treatment devices, as they have to be brought to patients quickly and easily. To render the devices independent of the AC power line, they are usually operated using battery power. In hospitals, ventilation devices are used for intensive care and are operated by medical staff.

Compared to emergency treatment devices, they have a considerably higher functional scope. The main consideration in relation to these devices is their performance potential. Lastly, ventilation devices for home use must guarantee proper breathing for patients who lack the ability to breathe adequately on their own.

Manifold Requirements for Flow Sensing Technology

The three application areas for ventilation technology mentioned above result in diverse requirements for flow sensors. In emergency treatment devices, the drop in pressure brought about by the flow resistance of the sensor in the gas stream should be as low as possible. This is because a high pressure drop relates to more work for the turbine generating the positive pressure for the patient. A low pressure drop relates to longer battery service life or a smaller battery, which has an advantageous effect on size or weight.

In intensive care, the intention to be in a position to use the device in the widest possible range of setting exerts additional requirements on flow sensing technology. For example, the same device should be used for all patients, irrespective of whether they are adults or infants. The widest possible dynamic range and high sensor resolution guarantee that the device can be used for all types of patients.

For instance, advanced flow sensors should at best be able to measure flow volumes of up to 250 liters /minute, while still being in a position to detect volumes less than 1 liter with high precision. Besides this broad dynamic measuring range, measurement accuracy with low gas flows is specifically an important feature.

As different ventilation modes are used in intensive care, a short sensor response time is also mandatory. A feature common to all areas is that the sensors have to be powerful and have the best possible long-term stability to minimize recalibration and maintenance effort.

Other sensor technology requirements are the result of the different functions that are applied to control or monitor the sensors. A differentiation is made between expiration sensors, inspiration sensors, and patient-side sensors (spirometry). The inspiration-side sensors (breathing in) are equipped in the device. They are located away from the patient. The gas, which is clean and dry, always flows from the sensor to the patient (and not in the other direction).

In contrast, expiration-side sensors measure the air that is exhaled, which is moist and comes from the patient. The patient-side sensors have the most rigorous requirements; they must be either appropriate for reconditioning, which means cleaning and autoclaving, or disposable. Moreover, patient-side sensors should be in a position to measure both inspiration and expiration, which makes bidirectional calibration unavoidable.

This is a significant difficulty for certain types of sensors, specifically hot-wire anemometers. In case patient ventilation extends over a comparatively long time, weaning leads to specific difficulties for the sensor system if the patient’s breathing has to be assisted. It is crucial that the device detects the start of breathing without any time delay.

Various Measuring Principles

Flow measurement in ventilation technology involves a range of measuring principles. As a result of accuracy requirements and the progressive integration of electronic control systems, the floating-body flow gauges that were earlier used on a wide scale are currently only found in older devices.

In several devices, flow measurement is performed using differential pressure sensors. This enables the differential pressure sensor to be located comparatively far from the patient and still measure the flow close to the patient.

The accuracy of this measuring technique is not directly dependent on the sensor, but rather on the combination of the differential pressure sensor and the component used to acquire the pressure drop, which is generally a linear flow element or an orifice.

The hose between the sensor and the flow element also acts as a significant factor. In general, the hose has an attenuating impact, hence it is best to avoid kinks in the hose.

Sensors using ultrasonic signals to make “time of flight” measurements are also available in the market. The actual sensor performs measurement through a plastic wall. One of the benefits of this method is easy reconditioning. However, it incurs considerably higher manufacturing costs.

Thermal measuring principles can also be applied. The thermal techniques can be used to differentiate between traditional hot-wire anemometers and hot-film anemometers. A disadvantage of hot-wire anemometers is that it is only possible to measure the magnitude of the gas flow and the direction cannot be indicated. This disadvantage can be overcome by using hot-film anemometers with multiple zones.

Sensirion’s CMOSens® Technology

Sensirion’s CMOSens® mass flow sensors incorporate refined thermal measurement techniques. In these sensors, the temperature sensors are positioned symmetrically surrounding a heating element. This enables the determination of the flow direction and allows precise bidirectional measurement. As the sensor and the analog and digital signal processing circuitry are integrated in a microchip, precise calibration and temperature compensation of flow measurement are ensured.

Rapid processing of the measurement signal is ensured by the sensor element and processing with calibration data. It is also possible to collect other information related to the gas flow, based on the gas or gas mixture. In case the gas comprises of a mixture of two pure gases with distinctive thermal properties, apart from the flow, the mixing ratio can also be determined.

In ventilation technology, this signal is used for measuring nitrous oxide/air mixtures or helium/oxygen mixtures. However, it is also possible to use it as a redundant signal if the mixture is regulated by other flow sensors.

Apart from pure flow sensors, mass flow controllers are also available in the market. Mass flow controllers are equipped with a built-in valve, and they are a better option for smaller manufacturers in paticular, since the cost of developing the same capability in-house can be relatively high.

The time to market for the product is also reduced by a ready-made solution. However it is necessary for the mass flow controllers used in ventilation technology to be considerably faster compared to those used in industrial applications, as they must be in a position to follow the patient’s breathing.

All images sourced and provided by Sensirion

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: Oct 4, 2019 at 8:36 AM

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