GB2575516A

GB2575516A - Respiratory diagnostic tool using impulse oscillometry

Info

Publication number: GB2575516A
Application number: GB1811583.2A
Authority: GB
Inventors: Douglas Graeham
Original assignee: Arete Medical Technologies Ltd
Current assignee: Arete Medical Technologies Ltd
Priority date: 2018-07-14
Filing date: 2018-07-14
Publication date: 2020-01-15
Anticipated expiration: 2038-07-14
Also published as: GB2575516B; GB201811583D0

Abstract

A device 100 for performing an impulse oscillometry test (e.g. to indicate the mechanical properties of a subject’s airways) comprises a housing defining an airflow channel (402, fig 4) from a mouthpiece 118 to an air inlet, and an occluder 115. The channel is occluded to create an acoustic impulse by moving the occluder from a first position (fig. 1) to a second position (fig. 2), where in the second position the occluder blocks the airflow channel more than in the first position. A sensor assembly is provided to measure an airflow parameter of the air in the channel following creation of the acoustic impulse. The sensor assembly may include a pressure sensor (404, fig. 4) and a flow sensor (404, fig. 4) The airflow parameter may be used to monitor/diagnose respiratory conditions e.g. COPD. The device may be modular having a primary component 102 and secondary component 112.

Description

RESPIRATORY DIAGNOSTIC TOOL USING IMPULSE OSCILLOMETRY

Field of the Invention

The invention relates to devices for performing tests whose results can be used in the diagnosis of, or monitoring of, respiratory conditions and more particularly to a device capable of performing an impulse oscillometry test.

Background of the Invention

There are many people who suffer from conditions that affect their respiratory systems. These conditions can make it difficult to breathe and can have a negative impact day to day for those who suffer from them.

In order to diagnose and monitor conditions in a subject it is necessary to characterise the subject’s airways. Many respiratory diagnostic tests that can be used to characterise airways require specific breath manoeuvres, which need to be performed accurately in order for the results to be reliable. Some people struggle, or find it impossible, to perform such manoeuvres. A spirometry test is an example of a test that is used to characterise a subject’s airways but that requires a specific manoeuvre of forced, high-flow rate breathing. It may be very difficult to communicate to a young child how to perform the test. They may lack the skill or effort required to be able to reliably perform the test. Adults may also find it difficult to perform the specific breath manoeuvre, particularly those suffering from severe respiratory problems. There is a clear benefit to having a respiratory diagnostic test capable of characterising the airways of a subject that requires minimal skill and effort to perform.

One such respiratory diagnostic test that has been developed is impulse oscillometry. In impulse oscillometry tests, an acoustic impulse is generated that propagates into the airways of the subject. The parameters of the acoustic impulse, such as flow and pressure, can be measured to give an indication of the mechanics of the subject’s airway. This can be used to characterise the airway. The advantage of impulse oscillometry is that only normal, tidal, breathing is required to perform the test. This is easier for the subject.

Current impulse oscillometry test devices use loudspeakers to generate the acoustic impulse in the airways of the subject. This has several disadvantages. Generally, the loudspeaker is positioned in a branch channel from the main flow channel, between a mouthpiece and the air inlet. The branch channel has a closed end i.e. it is a blind channel.

Ideally the acoustic impulse is directed toward the mouthpiece. However, much of the energy from the speaker is lost out of the air inlet. Therefore, a large speaker with high power requirements is required to achieve a large enough acoustic impulse at the mouthpiece. This results in devices that are bulky and that are not portable.

Because the impulse oscillometry devices are large and bulky they are often only found in hospitals and specialist lung laboratories. This means that access to such devices is limited for the general population and only a small proportion of people suspected of having asthma or other respiratory diseases are tested with such devices. It also makes it difficult to continually monitor symptoms over a series of testing sessions. Without objective testing, subjects may be given an incorrect treatment for their condition. Incorrect allocation of treatment wastes resources, may not prevent subjects’ symptoms or attacks, and may cause unnecessary side effects from a treatment.

There is a need for a device for performing oscillometry diagnostic tests that can be used to characterise the airways of a subject that are compact and have the potential to be portable.

Summary of the Invention

The invention provides a device for performing an impulse oscillometry test, a method for generating an impulse during an oscillometry test and a method for performing an oscillometry test according to the appended independent claims, to which reference should now be made. Preferred or advantageous features of the invention are defined in the dependent claims.

In a first aspect of the invention there is a provided a device for performing an impulse oscillometry test comprising a housing defining an airflow channel from a mouthpiece to an air inlet wherein the air inlet is open to the atmosphere, an occluder and a means to move the occluder between a first position and a second position.

In the second position the occluder occludes the airflow channel to a greater extent than in the first position. The occluder is configured such that movement of the occluder from the first position to the second position pushes air in the airflow channel from the occluder in the direction of the mouthpiece to create an acoustic impulse in the air in the airflow channel.

The device further comprises a sensor assembly configured to measure an airflow parameter of air in the airflow channel following creation of the acoustic impulse.

Oscillometry tests are diagnostic tests, the results of which indicate mechanical properties of the airways of the subject performing the test. These properties can be used to aid the diagnosis of respiratory diseases such as COPD. Oscillometry tests are advantageous as they do not require the subject to perform a specific breath manoeuvre, such as forced breathing. The subject need only perform normal, tidal, breathing through the device. This is advantageous as it means the test is easy and comfortable to perform. This is particularly important for children and for subjects who are suffering from a respiratory disease that would make breathing in another manner, such as high-flow forced breathing, difficult and uncomfortable.

While performing an oscillometry test using the device, a subject may breathe into and/or out through the device, such that air passes through the air channel of the device and into/out of the subject’s lungs. The configuration of the occluder, and the means to move the occluder, advantageously provides a compact and efficient way of generating an acoustic impulse that passes through the air in the airflow channel of the device.

As used herein, an acoustic impulse is a pressure wave comprising a plurality of sinusoidal frequencies. The acoustic impulse is created by the push of the of the air into the airflow channel by the occluder as it is moved from the first position to the second position. The push of air creates pressure fluctuations. The pressure fluctuations ideally take the form of, or approximate, a square wave. In an ideal case the impulse would be created by instantaneous movement of the occluder from the first position to the second position. In reality, this instantaneous movement is not possible resulting in asymmetrical, bell shaped impulse.

The acoustic impulse travels at the speed of sound. Parameters of the acoustic impulse may be sensed by the sensor assembly as the acoustic impulse travels through the airflow channel from the occluder. It is advantageous that the occluder occludes the airflow channel to a greater extent in the second position than in the first position, as this reduces loss of energy from the pressure wave in a direction away from the mouthpiece. Energy can escape through the gap between the occluder and the airflow channel so any reduction in the that gap (achieved by the increased occlusion) ensures less energy escapes.

The characteristics of the acoustic impulse are dependent on the configuration of the occluder, motion of the occluder, and the properties of the air channel including the subject’s lungs. As the configuration of the occluder, motion of the occluder, and properties of the airflow channel within the device can be known or calibrated for, the parameters sensed by the sensor assembly can be used to characterise airway mechanics of the subject’s lungs.

The device may further comprise control circuitry configured either to control the means to move the occluder or to receive signals from the sensor assembly, or both to control the means to move the occluder and to receive signals from the sensor assembly.

The control circuitry may advantageously control the means to move the occluder such that the occluder moves between the first position and the second position at a speed and through a distance such that an acoustic impulse with desired properties is created. This may mean flow rate and pressure fluctuations have a desired amplitude. Alternatively, the means to move the occlude may be driven by the action of the subject breathing.

Once created, the acoustic impulse travels through the airflow channel in the direction of the mouthpiece. This is advantageously in the direction of the airways of the subject using the device. In other words, advantageously, the airflow channel does not include any bends or branches between the occluder and the mouthpiece. This means that a maximal amount of energy will reach the subject’s airways. The acoustic impulse passes through the airflow channel out of the mouthpiece into the airways of the subject. The acoustic impulse interacts with the airways of the subject, resulting in changes in the pressure and flow rate of the air passing the sensor.

The sensor assembly is configured to measure a flow rate of air in the airflow channel or a pressure of air in the airflow channel or both a flow rate of air in the airflow channel and a pressure of air in the airflow channel. The control circuitry may be configured to calculate a parameter that characterises a respiratory system based on a frequency domain analysis of the flow rate of air in the airflow channel or the pressure of air in the airflow channel or both the flow rate of air in the airflow channel and the pressure of air in the airflow channel. The flow rate and pressure are measured by the sensor assembly as required.

The control circuitry may receive measurements of flow and pressure from the sensor assembly, and based on these measurement may form a number of metrics that describe the mechanical properties of the airways. These metrics may be parameters that characterise the respiratory system of the subject. The effect of an acoustic impulse may be different for different frequencies, and different frequencies can penetrate the airways to different depths. Because the impulse comprises multiple frequencies, the metrics can indicate the characteristics of the upper and lower airways as well as the airway system as a whole. The metrics may give an indication of airway resistance and airway reactance and other mechanical properties of the airways. The mechanical properties of the airways may indicate that a subject has a respiratory disease and so these metrics can aid a trained medical person in the diagnosis of respiratory diseases.

The control circuitry may comprise a microcontroller. The microcontroller may be configured to process the data received from the sensors. The processing of data may include performing a Fast Fourier Transform. The result of the processing of data may be a test score which can be used to help monitor or diagnose a respiratory disease. The control circuitry may instead be in communication with a portable computer comprising a microcontroller such as a laptop or smartphone. In these cases, the microcontroller of the portable device performs the data processing. The communication between the control circuitry and the portable device may be wireless a connection. The wireless connection may be a Bluetooth connection.

The control circuitry may be configured to move the occluder such that the acoustic impulse that is created comprises a plurality of frequencies of pressure fluctuations in a range from 5 Hz to 20 Hz. The acoustic impulse may be in the form of, or approximate, a square wave, created by the movement of the occluder from the first position to the second position. The occluder may move in a cycle from the first position, to the second position, and return to the first position more than once per breath. For example, the cycle of the occluder may have a frequency or pulse rate of 3 Hz. The highest frequency contained in the impulse is dependent mainly on the duration of the impulse. For example, an upper frequency of 20Hz contained in the impulse may be attained by the occluder moving from the first position to the second position in 50 ms or less. The impulse will contain lower frequencies, such as 5 Hz. The highest frequency with a detectable amplitude may be 20 Hz. The lower frequencies within the impulse penetrate deeper into the subject’s airways and may penetrate out to the lung periphery, whereas the higher frequency signals may not penetrate as far, only reaching the proximal airways. It is therefore advantageous to probe the airways of the subject with a plurality of frequencies of pressure fluctuations rather than a single frequency, giving a fuller picture of the mechanical properties of the subject’s airways. It allows for the recognition of characteristic respiratory responses at different frequencies.

The control circuitry may be configured to perform a Fast Fourier Transform on both the signals relating to measured flow and measured pressure from the sensor assembly and so produce a frequency domain distribution showing an amplitude for each frequency.

The control circuitry may be configured to move the occluder from the first position to the second position such that the acoustic impulse provides a maximum pressure increase of at least 50 Pa at the sensor assembly and a flow rate increase of at least 0.15 litres min^-1through the airflow channel in the direction of the mouthpiece. The increase of pressure is the amount the pressure is increased above the pressure of air in the airflow channel from the subject breathing normally. The control circuitry may be configured to move the occluder from the first position to the second position within 50 ms or less.

In the second position the occluder occludes the airflow channel to a greater extent than in the first position. This change in occlusion results in a change in resistance to air flowing the airflow channel. When the occluder is in the second position, air flowing in the airflow channel experiences a higher resistance to flow than when the occluder is in the first position. The resistance to flow when the occluder is in the first position is lower than 0.15 kPa s L’¹. While the occluder is in the second position, the higher resistance may cause the pressure in the airflow channel to change compared to when the occluder is in the first position. The pressure change (depending on whether the subject is inhaling or exhaling) in the airflow channel may affect the characteristics of the acoustic impulse.

Both the length of time that the occluder is in the second position for, and the configuration of the occluder, may affect the characteristics of the acoustic impulse. The configuration of the occluder refers particularly to the amount to which the occluder increases the resistance to flow when the occluder is in the second position compared to when it is in the first position. The length of time that the occluder is in the second position for and the configuration of the occluder can therefore be chosen to result in an acoustic impulse that has desired characteristics.

In some embodiments the occluder may be configured such that when it is in the second position air can flow through the airflow channel. This means that even though the air in the airflow channel experiences more resistance when the occluder is in the second position, the airflow channel is not totally occluded. The minimum distance between the air outlet and the occluder, when the occluder is in the second position, may be at least 0.5 mm. At this distance the resistance to flow is sufficiently low that the effect of the change in flow resistance on the characteristics of the acoustic impulse is negligible. A gap of not much more than 0.5 mm may be chosen as this results in a suitably low resistance to flow while also ensuring that acoustic impulse is efficiently directed in the airflow channel toward the mouthpiece.

The test may be more comfortable for the subject and more likely to give accurate results if the pressure change experienced by the subject when the occluder is in the second position does not result in a significant build-up of pressure in the subject’s airways. This can be achieved by ensuring that the occluder is in the second position for only a short time and by ensuring that the occluder does not fully close the airflow channel.

The control circuitry may be configured to move the occluder to the first position after the occluder has been in the second position for no more than 20 ms. This is a time that is advantageously short enough that the pressure change due to the increase in resistance while the occluder is in the second position is not noticeable for the subject. The mathematical model required to calculate the metrics from the behaviour of the acoustic impulse is advantageously simpler when the calculations do not have to take into account a significant pressure change resulting from prolonged occlusion of the airflow channel.

In some embodiments it may be advantageous to have high resistance to flow of air in the airflow channel when the occluder is in the second position. In such embodiments the minimum distance between the air outlet and the occluder, when the occluder is in the second position, may be less than 0.5 mm. In other embodiments the occluder is in the second position for long enough that the pressure build-up in the airflow channel has a significant effect on the test. The length of time the occluder is in the second position for may be longer than 20 ms.

A build-up of pressure in the airflow channel may increase the amplitude of the acoustic impulse. This may be because the pressure change in the airflow channel contributes additional energy to the acoustic impulse. The occluder remaining in the second position may more efficiently direct the energy of the acoustic impulse toward the mouthpiece. This may allow a smaller occluder to be used and having a lower power requirement. However, the characteristics of the acoustic impulse may also be affected such that it is less close to a square wave.

The occluder may be configured so that when it is in the second position air cannot flow past it. This is the extreme case of having high resistance to flow when the occluder is in the second position and ensures that a maximum amount of energy of the acoustic impulse is directed in the airflow channel toward the mouthpiece. The occluder may comprise a resilient sealing component configured to contact the housing when the occluder is in the second position. The sealing component may be made of a material that deforms when it comes into contact with the housing. The sealing component advantageously ensures that an air-tight seal is achieved in the airflow channel, preventing the flow of air past the occluder. It also prevents damage to the housing of the device when the occluder is moved with force from the first position to the second position.

The control circuitry may be configured such that the means to move the occluder moves said occluder from a first position to a second position and back to a first position at least 3 times each second during a test period. Creating multiple acoustic impulses throughout a test advantageously allows a full picture of the airways.

The occluder, in the second position, may be configured to occlude the air inlet of the airflow channel.

The face of the occluder in contact with the airflow channel may be substantially concave in shape. This is efficient for creating the acoustic impulse, and ensures that as much air as possible is pushed toward the mouthpiece as the occluder is moved from the first position to the second position.

The means to move the occluder may be configured to move the occluder in a direction parallel to the airflow through the airflow channel when a subject is inhaling or exhaling through the mouthpiece. This results in the push of air by the occluder, when caused by the occluder moving from the first position to the second position, also being parallel to the airflow channel. The pushed air then moves in the direction of the mouthpiece.

The means to move the occluder may be a linear actuator. The linear actuator may advantageously be positioned such that its axis of movement is parallel to the airflow channel. The linear actuator will then move the occluder from the first position to the second position in a direction parallel to the airflow channel. Other means to move the occluder are possible. For example, the occluder may be on a hinged actuator.

The mouthpiece advantageously has a low resistance to flow, low wasted volumes, and relatively smooth internal surfaces. Avoiding sharp internal features reduces disturbance to the impulse/wave travelling into/out ofthe airways. These features may otherwise damp the signal properties of the wave.

The device may be portable. This advantageously means that the device can be brought to a subject rather than the subject having to visit, for example, a hospital. This advantageously makes monitoring of a condition easier, as a subject would not have to return to, for example, a hospital each time the test is performed. It also allows for environmental triggers to be assessed in the real world.

The device may comprise at least one battery configured to provide power to the control circuitry and the means to move the occluder. The device comprising a battery advantageously allows the device to be portable and not required to be near a power source, such as mains power, when in use.

The device may comprise a capacitor configured to be charged by the at least one battery and to discharge through the means to move the occluder. This advantageously allows the use of a battery that has lower power output than the power output requirement of the means to move the occluder. Such a battery will typically be smaller, allowing the device for performing the oscillometry test to be smaller and more portable. In use, the capacitor is charged by the battery and then discharges through the means to move the occluder when an oscillometry test is to be performed. The current and voltage requirements of the control electronics may be different to those of means to move the actuator. The capacitor can provide a high power burst when discharged through the means to move the occluder. The current and voltage of the small battery may meet the requirements of the control circuitry. Advantageously, the capacitor allows only a single, low power, battery to power the device.

In a second aspect ofthe invention there is provided a method of generating an impulse during an impulse oscillometry test comprising the steps of:

providing an airflow channel defined through a housing of a device for performing the impulse oscillometry test, the airflow channel extending from a mouthpiece to an air inlet, wherein the air inlet is open to the atmosphere, moving an occluder from a first position to second position, wherein in the second position the occluder occludes the airflow channel to a greater extent than in the first position, and wherein moving the occluder pushes air in the airflow channel from the occluder in the direction of the mouthpiece to generate an acoustic wave, and returning the occluder to the first position after it has been moved to the second position.

The occluder may move from the first position to the second position within 50 ms or less. This creates an acoustic impulse with an amplitude that is large enough for an impulse oscillometry test.

The control circuitry may be configured to move the occluder back to the first position after the occluder has been in the second position quickly, to minimise pressure build-up in the airflow channel when the occluder is in the second position. The control circuitry may be configured to move the occluder to first position when the occluder has been in the second position for no more than 20 ms. The occluder may move from the first position to the second position and back to the first position within 120 ms or less. The occluder may be moved from the first position to the second position and back to the position 3 times each second. Creating multiple acoustic impulses throughout a test advantageously allows a full picture of the airways.

The generated impulse may comprise multiple frequencies. The generated impulse may comprise frequencies in the range of 5 Hz to 20 Hz. This range of frequencies can advantageously probe both the lower and upper airways of the subject. The lower frequency signals penetrate deeper into the subject’s airways and may penetrate out to the lung periphery. The higher frequency signals do not penetrate as far, only reaching the proximal airways. It is therefore advantageous to probe the airways of the subject with a plurality of frequencies of pressure fluctuations rather than a single frequency, to give a fuller picture of the mechanical properties of the subject’s airways. This also allows for the recognition of characteristic respiratory responses at different frequencies.

In a third aspect of the invention there is provided a method of performing an oscillometry test comprising the steps of:

providing an airflow channel defined through a housing of a device for performing the impulse oscillometry test, the airflow channel extending from a mouthpiece to air inlet wherein the air inlet is open to the atmosphere, moving an occluder from a first position to second position, wherein in the second position the occluder occludes the airflow channel to a greater extent than in the first position, and wherein moving the occluder pushes air in the airflow channel from the occluder in the direction of the mouthpiece to generate an acoustic impulse, returning the occluder to the first position after it has been moved to the second position, and measuring an airflow parameter of air in the airflow channel during a period following the generation of the acoustic impulse in the airflow channel.

It should be clear that features described in relation to one aspect may be applied to other aspects of the invention.

Brief Description of the Drawings

Embodiments in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a side view of a device for performing impulse oscillometry tests in accordance with the invention, the device comprising an occluder wherein the occluder is in a first position;

Figure 2 is the same side view of the device of Figure 1, the occluder of the device is in a second position;

Figure 3 is a cross-sectional view of the oscillometry module of Figures 1 and 2, showing the linear actuator that moves the occluder from the first position to the second position, shown in Figures 1 and 2 respectively;

Figure 4 is a cross-sectional perspective view of the device of Figure 1, showing the airflow channel that is defined through the device; and

Figure 5 is a flow chart showing a method of using the device of Figure 1 to perform an oscillometry test.

Detailed Description

Figure 1 is a side view of a device 100 for performing an oscillometry test that can be used as a diagnostic test to help with diagnosis of respiratory diseases such as COPD. Oscillometry tests are used to indicate mechanical properties of the airways of a subject. The device comprises a primary component 102. The primary component comprises a flow sensor held within housing portion 105, a sensor assembly held within housing portion 107 and control circuitry held within a housing portion 109. The flow sensor, sensor assembly and control circuitry are not shown in Figure 1. The sensor assembly comprises a pressure sensor. The control circuitry receives signals from the flow sensor and sensors in the sensor assembly.

The primary component 102 comprises a battery. This is not shown in Figure 1. The battery supplies power to the control circuitry.

The primary component further comprises a mounting plate 110. The mounting plate extends from housing portion 105 beyond housing portion 107. Housing portion 109 is attached to the mounting plate 110.

A secondary component 112 is connected to the primary component 102 through the mounting plate 110. The secondary component 112 is an oscillometry module. In order to connect the oscillometry module 112 to the primary component 102 through the mounting plate, the secondary component comprises a threaded knob 114 which passes through the mounting plate 110 of the primary component 102 and screws into the secondary component. This robustly and repeatably aligns the secondary component with the primary component.

The oscillometry module 112 comprises an occluder 115 and a means to move the occluder 120. The means to move the occluder, in this embodiment, is a linear actuator which is not visible in Figure 1. The occluder comprises an O-ring 116. The oscillometry module 112 is in electrical contact with the primary component 102. This means that signals can be sent and received from the control circuitry in the primary component to the oscillometry module 112.

Figure 2 is the same side view of the device 100 as Figure 1, but showing the occluder 115 in the second position rather than in the first position. In Figure 2 the O-ring 116 of the occluder 115 is shown to be in contact with housing 107 of the primary component 102.

The control circuitry can send signals to the linear actuator causing it to move the occluder 115 from the position shown in Figure 1, which is the first position, to the position shown in Figure 2, which is the second position. Power from the battery can also be transferred to the oscillometry module 112 through the electrical contact. This provides power to the linear actuator that can be used to move the occluder between the first and second position. The force of the linear actuator may cause vibrations through the device that cause the oscillometry module 112 to move relative to the primary component 102. The use of a threaded knob 114 to connect the oscillometry module to the primary component 102 reduces this movement.

In some embodiments of the device the oscillometry module 112 comprises an additional battery. The power requirements of the of a linear actuator, such as a solenoid, are higher than what many small batteries are able to produce. By having an additional battery located in the oscillometry module 112, the power supplied to the linear actuator can be increased to the required level.

In other embodiments of the device, the primary component 102 or the oscillometry module 112 comprises a capacitor instead of, or as well as, having the additional battery. The capacitor is configured to be charged by the at least one battery and to discharge through the linear actuator. A high power burst can be created when the capacitor is discharged. This high power burst meets the power requirements of the linear actuator.

The O-ring 116 prevents damage to the housing 107 or the occluder 115 when the occluder is moved from the first position to the second position. In many embodiments of the device, when the occluder is in the second position, the O-ring 116 is not in contact with the housing 107. Instead there is a gap between the housing 107 and the O-ring 116 of at least 0.5 mm. However, in all embodiments the occluder 115 will be closer to the housing 107 in the second position compared to the first position.

The device 100 further comprises a mouthpiece 118. The mouthpiece is connected to the primary component 102. An airflow channel is defined through the device 100 from the mouthpiece through the primary component 102, and so through housing 105 and 107. The airflow channel is not visible in Figures 1 and 2.

It should be clear that the invention is not confined to embodiments of the device 100 with a primary component and a secondary component that connects to the primary component.

For example, the entire device may comprise any number of components connected together or may be integrally formed.

Figure 3 is a cross-sectional view of the oscillometry module 112 in isolation from the rest of the device 100. In this cross-sectional view the linear actuator 120 is visible. The linear actuator 120 comprises a main body 302 and a solenoid. The linear actuator is a Johnson Electric Model 51 STA Push DC Tubular Solenoid available from RS components at: https://uk.rs-online.com/web/. When a first voltage is applied to the solenoid 304 of the linear actuator a force is applied to the occluder 115 such that the occluder moves from the first position to the second position. When the power supply is switched off the occluder 115 will move back to the first position. This may be due to a biasing spring or magnetic return system, not shown. In some embodiments the linear actuator is a push-pull linear actuator. In these embodiments the occluder is returned to the first position by forces applied by the linear actuator when a voltage of different polarity to the first voltage is applied to the linear actuator.

The control circuitry is configured to cause the linear actuator 120 to move the occluder from a first position to a second position and back to a first position again at least 3 times each second during a testing period. Creating multiple acoustic impulses throughout a test allows a full picture of the airways and shows how the airways changes throughout the duration of a breath.

Figure 4 is a cross-sectional perspective view of the device 100. Figure 4 shows the airflow channel represented by dotted line 402 through the device 100. The airflow channel is defined through the mouthpiece and housing 105 and 107 of the primary component. The airflow channel 402 terminates at an air outlet. The air outlet is at the end of housing 107 nearest the occluder 115. If the direction of flow of air in the airflow channel is toward the mouthpiece then the air outlet is instead an air inlet. In either case, the air outlet or air inlet are the same thing and positioned at the end of housing 107 nearest the occluder 115. The direction of movement of the occluder from the first position to the second position is parallel to the airflow channel. Flow sensor 404 and the sensor assembly 406, comprising the pressure sensor, are visible in Figure 4. Both the flow sensor and sensor assembly protrude into the airflow channel 402. They are therefore able to measure properties of the air passing through the airflow channel. The flow sensor 404 is a Sensirion SFM3000. The pressure sensor in the sensor assembly 406 is an All Sensors Co DLHR-L02D. The flow sensor and the pressure sensor are available for purchase from the Digi-Key electronics website: https://www.digikey.com/.

If, when the occluder is in the second position the O-ring 116 is in contact with the housing 107, then the airflow channel 402 is sealed at the outlet. In this case, air cannot escape the outlet from the airflow channel when the occluder is in the second position.

However, when the occluder is in the second position the O-ring 116 may not be in contact with the housing 107 and there may be in a gap of at least 0.5 mm between the O-ring 116 and the housing. The airflow channel is then not sealed at the outlet. In this case, air can escape the outlet from the airflow channel when the occluder is in the second position.

A method for using the device 100 in order to perform an oscillometry test will now be described in relation to Figure 5. At step 502 the subject breathes through the device. The breathing required is normal, tidal, breathing. The subject continues to breathe through the device for the duration of the test. The subject breathes in and out through the mouthpiece 118 of the device. The subject’s breath will pass in and out of their airways, through the airflow channel. As the subject inhales, air will enter the airflow channel 402 through the air inlet, positioned at the end of housing 107 nearest the occluder. As the subject exhales, air will exit the airflow channel out through the air inlet, which is now considered an air outlet.

In some embodiments, the subject may only breathe in through the device 100. In these embodiments air enters the device through the air inlet, passes through the airflow channel 402 toward the mouthpiece 118 and into the subject’s airways. The subject would then breathe out directly into the atmosphere and so exhaled breath would not pass through the airflow channel 402 of the device.

In some embodiments, the subject may only breathe out through the device 100. In these embodiments the subject would breathe in air directly from the atmosphere. The subject would then exhale through the mouthpiece 118 of the device 100. The exhaled breath would pass through the airflow channel 402 of the device 100 and out of the air inlet.

At step 504 an acoustic impulse is created in the air in the airflow channel. The acoustic impulse is created when the occluder 115 is moved from the first position to the second position by the linear actuator 120. The direction of movement of the occluder 115 from the first position to the second position is in the direction of the mouthpiece 118. This movement has the effect of pushing the air in the airflow channel 402 in the direction of the mouthpiece 118. The result of the push of air is an impulse of increased flow that passes through the airflow channel 402 in the direction of the mouthpiece. The occluder is moved by the linear actuator 120 with enough force that the push on the air in the airflow channel 402 results in increased flow of at least 0.15 litres per second in the direction of the mouthpiece.

The flow impulse that passes through the airflow channel results in a pressure transient which has an amplitude of at least 50 Pa. The acoustic impulse travels through the airflow channel 402 and into the airway of the subject resulting in changes in the pressure and flow rate of the air passing the sensor. As the acoustic impulse progresses it experiences damping. This causes a reduction in the amplitudes of pressure and flow. A flow rate increase of 0.15 litres per second and a pressure increase of 50 Pa in the direction of the mouthpiece is enough to provide a significant and measurable response.

The acoustic impulse is made up of sinusoidal pressure fluctuations having frequencies between 5 Hz and 20 Hz.

At the end of step 504, after the acoustic impulse has been created, the occluder is returned to the first position.

The resistance to flow will be higher when the occluder 115 is in the second position than when the occluder is in the first position. While the occluder is in the second position, the higher resistance may cause the pressure in the airflow channel 402 to change. The change in pressure may affect the characteristics of the acoustic impulse. Both the length of time that the occluder is in the second position for and the configuration of the occluder may affect the amount the pressure changes in the airflow channel and so the characteristics of the acoustic impulse. The configuration of the occluder refers particularly to the amount to which the occluder increases the resistance to flow when the occluder is in the second position compared to when it is in the first position.

If the resistance to flow in the airflow channel is sufficiently low when the occluder is in the second position the pressure build-up in the airflow channel will be negligible. Therefore, the change to the characteristics of the acoustic impulse will also be negligible. In this case the occluder in the second position is not in contact with the housing 107 of the primary component 102 and there is a gap of at least 0.5 mm between the occluder and the housing 107. When the occluder is in the second position, air is still able to flow through the device and the resistance with the occluder in the second position is not as high as if the occluder was closer to, or in contact with, the housing. Changes in pressure when the occluder is in the second position can also be reduced by limiting the amount of time the occluder is in the second position for. The occluder remains in the second position for no more than 20 ms before being returned to the first position.

A negligible pressure change in the airflow channel as a result of the occluder being in the second position, means that a relatively simple mathematical model is required. This simplifies the mathematics, particularly calculations using Fast Fourier Transforms. A minimal pressure build-up is also more comfortable for the subject using the device. If the change in pressure in the airflow channel is too substantial the subject will notice the change while breathing through the device which may cause discomfort and interrupt tidal breathing.

If the resistance to flow in the airflow channel is significantly higher when the occluder is in the second position and the occluder is in the second position for a significant period of time, then there will be a non-negligible pressure change in the airflow channel. Therefore, the changes to the characteristics of the acoustic impulse will also be non-negligible. When the occluder is in the second position there may be a gap of less than 0.5 mm between the occluder and the housing 107 of the primary component 102. There may be no gap between the occluder and the housing 107. The length of time the occluder is in the second position for may be longer than 20 ms. The change in characteristics of the acoustic impulse may be an increase in the amplitude of the acoustic impulse. This is because a pressure build-up in the airflow channel contributes additional energy to the acoustic impulse. It is also because the occluder in the second position more efficiently directs the energy of the acoustic impulse into the airflow channel and toward the mouthpiece.

At step 506 the flow and pressure of air in the airflow channel are sensed by the flow sensor 404 and the pressure sensor in the sensor assembly 406. This sensing determines the flow and pressure due to the subject’s breathing which is normal, tidal, breathing. The control circuitry receives the measurements from the flow and pressure sensors. The control circuitry will analyse the signals by performing a Fourier Transform. The amplitude of the pressure fluctuations at frequencies from 5 Hz to 20 Hz can then be continuously measured. The control circuitry stores the results of the measurements and the Fourier Transform.

At step 508 an acoustic impulse is created by the device by moving the occluder from the first position to the second position. The flow and pressure of air in the airflow channel are sensed continuously by the flow sensor 404 and the pressure sensor in the sensor assembly 406 for a period of 150 ms. The 150 ms sensing period begins momentarily before the occluder begins its movement from the first position to the second position. 150 ms is long enough to include measurements after the acoustic impulse has entered the subject’s airways as well as immediately after creation of the acoustic impulse. The flow and pressure measurements are a superposition of the flow and pressure resulting from the subject’s breathing with the flow and pressure resulting from the acoustic impulse. The control circuitry performs a Fourier Transform on the pressure and flow signals and stores the measurements and the results of the Fourier Transform. The measurements of the superimposed flow and pressure can be compared to the measurements of flow and pressure in step 506. This allows the separation of the pressure and flow signal resulting from the subject’s breathing from the portion of the signal that is caused by and in response to the acoustic impulse. The control circuitry can use the response of the acoustic impulse to calculate metrics indicating characteristics of a subject’s airway.

Steps 504 to 508 may be repeated any number of times during a test, while a subject breathes through the device. These steps may be repeated at least three times every second. The measurements taken each time that step 506 and 508 is performed are stored in the control circuitry. Repeating steps 504 to 508 means that measurements can be taken throughout the duration of a breath and so any change in the characteristics of the airways of the subject can be detected.

At step 510 the control circuitry outputs the results of the oscillometry test. This may be done by wireless data transfer to a smart phone or computer, for example. The wireless data transfer may be over Bluetooth. In some embodiments a display may be fitted into the device itself. In some embodiments software either stored in the control circuitry or on a computer or portable device that may be used to process the results of the test and provide a simplified test score or result. In some embodiments the results of the test may be stored to maintain a record of past tests. This record of past tests can be used in a diagnosis process and to monitor changes in a subject’s health over time.

Claims

1. A device for performing an impulse oscillometry test comprising:

a housing defining an airflow channel from a mouthpiece to an air inlet wherein the air inlet is open to the atmosphere, an occluder, a means to move the occluder between a first position and a second position;

wherein in the second position the occluder occludes the airflow channel to a greater extent than in the first position, wherein the occluder is configured such that movement of the occluder from the first position to the second position pushes air in the airflow channel from the occluder in the direction of the mouthpiece to create an acoustic impulse in the air in the airflow channel; and a sensor assembly configured to measure an airflow parameter of air in the airflow channel following creation of the acoustic impulse.

2. A device for performing an impulse oscillometry test according to claim 1 comprising control circuitry configured to control the means to move the occluder or to receive signals from the sensor assembly or both to control the means to move the occluder and to receive signals from the sensor assembly.

3. A device for performing an impulse oscillometry test according to claims 1 or 2, wherein the sensor assembly is configured to measure flow rate of air in the airflow channel or a pressure of air in the airflow channel or both flow rate of air in the airflow channel and a pressure of air in the airflow channel.

4. A device for performing an impulse oscillometry test according to claim 2, wherein the control circuitry is configured to calculate a parameter that characterises a respiratory system based on a frequency domain analysis of the flow rate of air in the airflow channel or the pressure of air in the airflow channel or both the flow rate of air in the airflow channel and the pressure of air in the airflow channel.

5. A device for performing an impulse oscillometry test according to any preceding claim, wherein the control circuitry is configured to move the occluder such that the acoustic impulse that is created comprises a plurality of frequencies including a range of 5 Hz to 20 Hz.

6. A device for performing an impulse oscillometry test according to any preceding claim, wherein control circuitry is configured to move the occluder from the first position to the second position such that the acoustic impulse provides a maximum pressure increase of at least 50 Pa at the sensor assembly and a flow rate increase of at least 0.15 litres min-1 through the airflow channel in the direction of the mouthpiece.

7. A device for performing an impulse oscillometry test according to any preceding claim, wherein control circuitry is configured to move the occluder from the first position to the second position such that air is pushed in the airflow channel for 50 ms or less.

8. A device for performing an impulse oscillometry test according to any preceding claim, wherein the occluder is configured such that when it is in the second position air can flow through the airflow channel.

9. A device for performing an impulse oscillometry test according to claim 8, wherein the minimum distance between the air outlet and the occluder, when in the second position, is 0.5 mm.

10. A device for performing an impulse oscillometry test according to any one of claims 1 to 7, wherein the occluder is configured such that when it is in the second position air cannot flow past the occluder from the air inlet to the mouthpiece.

11. A device for performing an impulse oscillometry test according to claim 10, wherein the occluder comprises a sealing component configured to contact the housing when the occluder is in the second position.

12. A device for performing an impulse oscillometry test according to any preceding claim, wherein control circuitry is configured such that the means to move the occluder moves said occluder from a first position to a second position and back to a first position at least 3 times each second during a test period.

13. A device for performing an impulse oscillometry test according to any preceding claim, wherein the occluder, in the second position, is configured to occlude the air inlet ofthe airflow channel.

14. A device for performing an impulse oscillometry test according to any preceding claim, wherein the means to move the occluder is configured to move the occluder in a direction parallel to the airflow through the airflow channel when a subject is inhaling or exhaling through the mouthpiece.

15. A device for performing an impulse oscillometry test according to any preceding claim, wherein the means to move the occluder is a linear actuator.

16. A device for performing an impulse oscillometry test according to any preceding claim, wherein the device is portable.

17. A device for performing an impulse oscillometry test according to any preceding claim, wherein the device comprises at least one battery configured to provide power to the control circuitry and the means to move the occluder.

18. A device for performing an impulse oscillometry test according to claim 17, wherein the device comprises a capacitor configured to be charged by the at least one battery and to discharge through the means to move the occluder.

19. A method of generating an acoustic impulse during an impulse oscillometry test comprising the steps of:

providing an airflow channel defined through a housing of a device for performing the impulse oscillometry test, the airflow channel extending from a mouthpiece to an air inlet, wherein the air inlet is open to the atmosphere, moving an occluder from a first position to second position, wherein in the second position the occluder occludes the airflow channel to a greater extent than in the first position, and wherein moving the occluder pushes air in the airflow channel from the occluder in the direction ofthe mouthpiece to generate an acoustic impulse, and returning the occluder to the first position after it has been moved to the second position.

20. A method of generating an impulse during an impulse oscillometry test according to claim 19, wherein the occluder is moved from the first position to the second position such that air is pushed in the airflow channel for 50 ms or less.

21. A method of generating an impulse during an impulse oscillometry test according to claim 19 or 20, wherein control circuitry is configured to move the occluder to the first position when the occluder has been in the second position for no more than 20 ms.

22. A method of generating an impulse during an impulse oscillometry test according claims 19 to 21, wherein the occluder is moved from the first position to the second position and back to the position 3 times each second.

23. A method of generating an impulse during an impulse oscillometry test according claims 19 to 22, wherein the generated impulse comprises multiple frequencies.

24. A method of generating an impulse during an impulse oscillometry test according claims 19 to 23, wherein the generated impulse comprises frequencies in the range of 5 Hz to 20 Hz.

25. A method of performing an oscillometry test comprising the steps of: