How pressure sensors are helping asthma sufferers

^{Manufacturer of sensor components and systems, First Sensor discusses how its technology has use of an accurate pressure sensor with a wide dynamic range helps SmartAir’s Airbrio system teach asthma sufferers correct inhaler use.}

The familiar treatment for asthma – an anti-inflammatory medication delivered by a Metered Dose Inhaler (MDI) – is known to be highly effective, when correctly administered. Unfortunately, as medical professionals are all too aware, patients’ adherence to the strict guidelines for use of an asthma inhaler is very variable.

SmartAir, a manufacturer of  health related equipment based in Northern Ireland, has developed Airbrio, which records data associated with an asthma sufferer’s use of an inhaler and provides a ‘compliance score’ which enables the user to teach themselves to improve their inhalation technique.

Airbrio respiratory management technology has at its heart an accurate, robust, differential pressure sensor supplied by First Sensor. This application example describes Airbrio’s operation, and the important attributes of the pressure sensor which helped SmartAir to implement successfully the technology.

Patients’ inconsistent and incorrect inhalation

The instruction manual for an MDI clearly describes the proper process for administering a standard asthma medication. Correct technique calls for slow, steady and deep inhalation over a set period of time in order to fully exhaust the MDI’s intake chamber, and convey the entire dose to the lungs.

Clearly a doctor or nurse cannot be at the patient’s side every time a dose is required: patients’ use of an MDI is normally unsupervised. So if a patient fails to properly learn inhalation technique when first given an MDI, or develops bad habits in their use of an MDI, or forgets one or more of the requirements for proper inhalation, there has to date been no method of alerting the user to their mistakes. This means that neither the patient nor their physician will know that they are potentially receiving only a small fraction of each puff of medication from an MDI.

Clearly these risks adversely affect the physician’s judgement about whether the medication is effective or appropriate for the patient, or about the quantity of the prescribed dose.

Common faults in usage of an MDI include:

• failing to inhale deeply, with the result that some of the dose is left in the intake chamber and not drawn into the airways.
• failing to inhale from the intake chamber for long enough. This has the same effect of leaving some of the dose in the chamber.
• inconsistent use, meaning that a variable dose is conveyed to the airways over a series of inhalation episodes

Simple warning light system

SmartAir combines sensing and electronics technology with a simple user interface. Airbrio can be used with standard MDIs and market-leading spacer devices, and records data relating to the patient’s inhalations. 

It then converts the recorded data into a ‘traffic light’-style guide for the user: when inhaling correctly, green lights are displayed on the Airbrio unit, directly in the user’s eye line. The lights change to orange or red if the inhalation rate falls below its optimal level. The lights also display a ‘Stop’ signal when the drug administration process has been correctly completed.

Airbrio includes a Bluetooth wireless link to a smartphone application which enables the patient and their doctor to conveniently track user’s actual MDI usage versus prescribed frequency over time as well as Inhalation Scores for each MDI use.

Accurate differential pressure measurement

The key to the Airbrio’s operation is accurate sensing of the flow of air from outside through the intake chamber into the patient’s airway. 

To achieve this, Smartair decided on the use of a differential air pressure sensor: by measuring over time the drop in air pressure in the intake chamber relative to the atmosphere during inhalation, the system can calculate the volume of air inhaled and the speed of airflow. Algorithms developed by SmartAir convert the recorded data into real-time guidance lights and the online compliance score.

To provide the pressure sensing function, SmartAir chose an LDE series integrated differential sensor module from First Sensor. Modules in the LDE series are supplied either as a DIP through-hole package, or as a surface-mount unit for assembly on the user’s board. They are fully calibrated at the factory, and have built-in temperature compensation.

The Airbrio design uses a version with an analogue (varying voltage) output, but a digital output over a serial peripheral interface is also available.

The attraction of the LDE series in the Airbrio design was firstly its ease of use: the module is supplied with two ports ready-made for connection to the host device’s tubing, and requires no additional external electronic components.

The high performance of the sensor was the other important reason for its selection by SmartAir: it combines high sensitivity with accuracy over a wide dynamic range.

These attributes were required because SmartAir’s design for the Airbrio is required to be suitable for use by patients of any age: this enables it to avoid the cost of developing two separate models, one for adults and one for children.

But the single design is more challenging to implement: Airbrio has to be able to record data for the relatively large pressure drop in the intake chamber created when a full-grown adult inhales deeply, and the much smaller drop generated by the lighter inhalations of a small child.

And the recorded pressure variations over time, which the Airbrio processor converts into a value for the volume of air inhaled, have to maintain a guaranteed total accuracy of at least ±5%. This is a system-level value, so the accuracy budget for the sensor alone needed to provide some headroom for errors and noise elsewhere in the system.

The analogue output of the sensor module chosen by SmartAir, the LDES250B, maintains accuracy better than ±0.75% over its full-scale measurement range of 0 Pa to ±250 Pa (see Figure 2). The effect of temperature changes on accuracy is limited to a maximum ±1.75% over a range from 5°C to 55°C.

Airbrio now helping improve users’ inhalation technique

Airbrio is now in production, and its  ‘traffic light’ UI and related smartphone app are enabling users to manage their use of their MDI in real time, and to keep an online record over time for periodic review with their physician.

Dr. Jim Harkin, co-founder and chief technology officer of SmartAir, said: ‘SmartAir is a start-up company, and our success depends on getting to market fast with a high-quality, breakthrough product. The First Sensor module helped us achieve this goal: not only was it very easy to integrate into the Airbrio, both electrically and mechanically, but its excellent accuracy gave us a large noise budget to work within for the rest of the system design, giving us great flexibility over the choice and use of the other components in the system.'