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Previously, the decision to choose a particular flow sensing technology in respirators and ventilation devices was a meticulous and complex process. Of late, however, there has been a growing interest in flow sensor solutions that provide a completely calibrated and temperature-compensated output signal.
Proximal flow sensors are extensively used in respiratory devices for intubated patients and non-invasive ventilation patients in emergency rooms, hospitals, and home care situations.
With applications ranging from neonatal to adult care, the related requirements for proximal flow sensors are both challenging and diverse. Sensors must be reliable and economical while offering long-term stability – besides a host of other features. Proximal flow sensors also have predominantly high requirements with regards to hygienic sterilization because of the patient’s contact with air, which can possibly be infected with pathogens.
Flow Sensing Technology Used for Proximal and Expiratory Sensing
There are already several commercially available sensor solutions that can be sterilized with autoclaving or other techniques. All of these sensors use one of two different measurement principles: the hot-wire anemometry principle, or differential pressure measurement via an orifice or a variable orifice to boost sensitivity at low flow ranges.
Both measurement principles have specific advantages. However, all sensors – irrespective of which principle is applied – pose problems with regards to sterilization, and a lot of care is needed during the cleaning and sterilization process to prevent damage to the sensors.
One safe and cost-effective alternative (with regards to total cost of ownership) for reusing sensors might be a single-use/single-patient-use proximal flow sensor. Sensirion is currently offering the first MEMS-based single-use proximal flow sensor. Contrary to current technologies, the SFM3300-D sensors are fully calibrated while the hot wire sensors nowadays used need to be calibrated prior to usage. A completely calibrated sensor helps to save time for the hospital stuff so that they can pay attention to other tasks.
Function of Proximal and Expiratory Flow Sensing
Apart from pressure sensing, flow sensing poses one of the key challenges for respiratory devices. For the sake of straightforwardness, the following focuses on positive pressure ventilators, where the patient is connected to the device either through a mask (non-invasive) or through tracheostomy or intubation (both invasive). Advanced ventilators have a wide range of application modes – for instance pressure-controlled, flow-controlled and many more.
For patients who have difficulty breathing on their own, the trigger for the next inhalation can be fixed by a timer. However, things become more complex if a patient is breathing spontaneously. In the latter case, the patient’s breathing effort must be detected as quickly as possible to attain good synchronization between the patient and device. The article will examine the position of the sensor for this purpose and discuss the effect of sensor position on trigger sensitivity later on.
Beyond the triggers to start the next breath, the end of the inspiratory phase also has to be established based on a value – the flow, volume, time, or pressure. This is referred to as the limit variable. It is also essential to establish the flow control between the trigger and the limit based on one of the values.
Triggers for patient-triggered ventilation can be fixed based either on the flow signal or the pressure. If pressure signals are applied as the trigger, attaining the anticipated sensitivity is tough. This is mostly because of the fact that pressure sensors tend to drift over time. Frequent offset correction is thus needed to guarantee reliable trigger sensitivity without false triggers.
Thanks to their outstanding stability, flow sensors positioned in the proximal configuration will produce a very quick response and high sensitivity. With expiratory positioning of the flow sensor, the sensitivity and stability are the same, but signal detection is delayed by the travel time of the flow via the expiratory tube. Expiratory flow sensing also has other benefits over a proximal configuration: for instance, reducing the likelihood of contamination with mucus. For its part, proximal sensing is less impacted by leaks further down the breathing circuit.
As observed earlier, the inspiratory phase may be stopped based on flow, volume, pressure or even time. The same applies to the flow control between the trigger and the limit of the inspiratory phase. For instance, a ventilator setting selected by the hospital staff might use both time-triggered inspiration and a time-triggered limit. Here, the flow needs to be regulated between those two points. Another setting might use varied parameters for the trigger, limit, and control. The combination of parameters might be selected for medical reasons or just based on the preference of the staff member setting up the ventilator. Monitoring the flow, pressure, and volume values over time gives a chance to observe variations in the patient’s condition, such as reduced lung capacity.
Pressure-triggered limits can be significantly impacted by the compliance of the breathing circuit, which can alter if the circuit is exchanged or if the hoses and tubes are placed differently. A more extreme bend in the hose, for instance, can have an impact.
The compliance of the breathing circuit has minimal impact on measurement and integration of the flow signal if the flow sensor is positioned in the proximal configuration. However, this is not the case for expiratory flow sensing. In such cases, having the flow and pressure signal helps illuminate the impact of compliance. Proximal flow sensing is less impacted by leaks – because of connecting nebulizers and humidifiers, for instance – further away from the patient.
While there will certainly be a trend towards more intelligent and adaptive ventilation modes, the standard underlying modes will still be based on the flow-, pressure-, volume- and time-based values mentioned above.
Placement of Flow Sensors: Proximal versus Expiratory Placement
Here, one has to differentiate between single-limb and dual-limb circuits. In a dual-limb circuit, the expiratory path and the inspiratory path each have separate tubes. The expiratory tube and the inspiratory tubes meet at the y-piece, and the last few centimeters to the patient pass through a single tube.
During inspiration, the air flows via the inspiratory tube to the y-piece and from there to the patient. During exhalation, the air flows to the y-piece, closes a flap that prevents the air from flowing back via the inspiratory tube and opens the expiratory tube. In a single-limb breathing circuit, there is just one tube from the ventilator to the patient. Before the patient, there is an expiratory valve that allows the air from the ventilator to pass through to the patient during inhalation. During the expiration phase, the same valve opens and allows the air to be discharged to the surrounding area.
In both scenarios, the flow sensor measuring the inspiratory flow can be positioned in the machine, where the sensor will not come into contact with contaminated or wet air. In the case of single-limb circuits, the expiration flow can only be measured when a proximal flow sensor is used; otherwise only the inspiratory flow is identified, and the number of available ventilation modes is inadequate. With a dual-limb circuit, either an expiratory or proximal flow sensor solution might be used. Proximal flow sensing has a few advantages in terms of trigger sensitivity because of the close proximity to the patient. On the other hand, that close proximity also brings extra challenges, such as contamination with mucus, which might be more easily managed with an expiratory placement.
A majority of ventilator manufacturers presently use the proximal configuration for neonatal patients; where suitable, special neonatal sensors are used. For adult patients, by contrast, certain manufacturers use flow sensors in proximal configurations and some use expiratory setups.
The SFM3300 series flow meters can be used in both expiratory and proximal configurations. Proximal placement offers the maximum-possible trigger sensitivity, while expiratory placement helps regulate several inlet conditions, in turn providing the most accurate flow readings.
Single-Use Sensors Versus Autoclavable Sensors
Proximal configurations may use autoclavable sensors that can be re-used a number of times or a single-use option that is disposed of after use. Both options are equally feasible, based on circumstances. In the end, the cost of ownership – presumably the main factor in deciding which option is better – strongly relies on the cost per autoclave cycle, which consequently relies significantly on labor costs in the respective market. Ventilator manufacturers thus require both options to serve various markets.
Sensirion provides two options. The first option is the re-usable SFM3300-AW, which can be sterilized by different techniques – Sensirion-tested autoclave sterilization at 134 °C or cleaning in a Cidex® Activated Dialdehyde solution. With the SFM3300-D model, Sensirion is adding a single-use option to the SFM3300 mass flow meter series. Both sensors work in the same flow range and realize the same accuracy specifications. Most notably, the two sensors share the same electrical and pneumatic interfaces. This enables ventilator manufacturers to combine the sensors in their designs and offer both solutions to their customers without extra development considerations.
A single-use option needs to be enhanced for cost, while an autoclavable option is enhanced for reliability and stability over the sensor’s lifetime. To optimize costs, the single-use sensor is produced from a low-cost plastic material and is designed to function without extra metal meshes on the inside. In contrast to the autoclavable SFM3300-AW, the single-use SFM3300-D does not have an extra EEPROM to store hourly usage data.
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The new MEMS-based, single-use proximal and expiratory sensors allow ventilator manufacturers and their customers to spend less time on calibration. Hospital staff can currently focus more on their patients instead of calibrating flow sensors.
This information has been sourced, reviewed and adapted from materials provided by Sensirion Inc.
For more information on this source, please visit Sensirion Inc.