WO2010071919A1

WO2010071919A1 - Respiratory aid device

Info

Publication number: WO2010071919A1
Application number: PCT/AU2009/001655
Authority: WO
Inventors: Bredge Mccarren; John Eisenhuth
Original assignee: The University Of Sydney
Priority date: 2008-12-22
Filing date: 2009-12-18
Publication date: 2010-07-01

Abstract

A system is described for stimulating the chest wall of a subject to reduce breathlessness. The system includes a respiration sensor (3) operable to generate an output signal dependent on respiration of the subject; a stimulation device (7) operable to stimulate the chest wall of the subject; and a controller (5) arranged to monitor the output signal and to activate operation of the stimulation device dependent on the output signal. The system provides in-phase stimulation of intercostal muscles during respiration. The stimulation device may apply a mechanical vibration to the patient's chest wall. The controller controls the frequency of the applied vibration.

Description

Respiratory Aid Device

Field of the invention

The present invention relates to a respiratory aid device that stimulates the chest wall of a user during respiration to reduce breathlessness.

Background of the invention

In-phase vibration of the chest wall has been shown to have effects on ventilation and dyspnoea. In-phase vibration has been applied by a vibratory pad to the inspiratory intercostal muscles during inspiration and to the expiratory intercostal muscles during expiration. Both inspiratory and expiratory in-phase vibration have been noted to increase total volume, respiratory flow rates and minute ventilation in normal subjects and patients with chronic obstructive pulmonary disease (COPD). Experimental results have also shown increases in arterial oxygenation in patients with COPD. Studies have noted that in patients with spinal-cord injuries (lower cervical, below C4) and COPD patients, functional residual capacity decreased with in-phase vibration. Investigators have proposed that the in-phase vibration increases the tonic activity of the underlying muscles via the stretch reflex, resulting in enhanced chest-wall movement to increase both inspired and expired volumes. The studies cited below used oscillation frequencies ranging between 100-120 Hz. One exception was the study by Piquet et al (1984), who applied compression with an oscillation frequency of 5 Hz.

The in-phase vibration decreased the sensation of dyspnoea in asthmatics, normal subjects with hypercapnic and resistant loads and COPD patients at rest and during exercise. Vibration may alter the input from the intercostal muscles that contributes to the sensation of dyspnoea. In addition, the increase in ventilation and the improved mechanics and gas exchange may result in a decrease in the muscular effort (afferent information) of breathing and decrease the sensation of dyspnoea. Based on these findings it is possible vibration may increase ventilation and decrease dyspnoea.

Studies of the effect of chest-wall vibration include the following: Cristiano LM and Schwartzstein RM (1997) Effect of chest wall vibration on dyspnea during hypercapnia and exercise in chronic obstructive pulmonary disease. American Journal of Respiratory and Critical Care Medicine 155: 1552-1559;

Fujie T, Tojo N, lnase N, Nara N, Homma I and Yoshizawa Y (2002) Effect of chest wall vibration on dyspnoea during exercise in chronic obstructive pulmonary disease. Respiratory Physiology and Neurobiology 130: 305-316;

Homma I, Eklund G and Hagbarth K-E (1978) Respiration in man affected by TVR contractions elicited in inspiratory and expiratory intercostal muscles. Respiration Physiology 35: 335-348;

Homma I, Nagai T, Sakai T, Ohashi M, Beppu M and Yonemoto K (1981 ) Effect of chest wall vibration on ventilation in patients with spinal cord lesion. Journal of Applied Physiology: Respiratory, Environmental and Exercise Physiology 50: 107-111 ;

Homma I, Obata T, Sibuya M and Uchida M (1984) Gate mechanism in breathlessness caused by chest wall vibration in humans. Journal of Applied Physiology: Respiratory, Environmental and Exercise Physiology 56: 8-11 ;

Manning HI, Basner R, Ringler J, Rand C, Fend V, Weinberger SE, Weiss W and Schwartzstein RM (1991 ) Effect of chest wall vibration on breathlessness in normal subjects. Journal of Applied Physiology 71 : 175-181 ;

Piquet J, Brochard L, lsabey D, de Cremoux H, Chang HK, Bognon J and Harf A (1987) High frequency chest wall oscillation in patients with chronic airflow obstruction. American Review of Respiratory Disease 136: 1355-1359; and

Sibuya M, Yamada M, Kanamaru A, Tanaka K, Suzuki H, Noguchi E, Altose MD and Homma I (1994) Effect of chest wall vibration on dyspnea in patients with chronic respiratory disease. American Journal of Respiratory and Critical Care Medicine 149: 1235-1240; Summary of the invention

It is an object of the present invention to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements.

According to a first aspect of the invention there is provided a respiratory aid device for reducing breathlessness comprising:

(a) a respiration sensor that generates an output signal indicative of at least one of i) inspiration by the user and ii) expiration by the user;

(b) a stimulation device that in use is positioned externally on the user to apply a mechanical vibration to stimulate intercostal muscles of the user; and

(c) a controller that monitors the output signal and activates the stimulation device dependent on the output signal.

According to a second aspect of the invention there is provided a system for stimulating the chest wall of a subject during respiration to reduce breathlessness, the system comprising:

(a) a respiration sensor operable to generate an output signal dependent on respiration of the subject;

(b) a stimulation device operable to stimulate the chest wall of the subject during respiration; and

(c) a controller arranged to monitor the output signal and to activate operation of the stimulation device dependent on the output signal.

According to a further aspect of the invention there is provided a method for stimulating the chest wall of a subject during respiration to reduce breathlessness comprising:

(a) receiving an input dependent on the subject's respiration; (b) generating, in response to the received input, a trigger signal to trigger a stimulation device operable to stimulate the chest wall of the subject;

(c) receiving a feedback signal indicative of the operation of the triggered stimulation device; and

(d) determining, dependent on the feedback signal, a control signal to drive the stimulation device at a specified setpoint.

According to a further aspect of the invention there is provided a computer program product comprising machine-readable program code recorded on a machine readable recording medium, for controlling the operation of a data processing apparatus on which the program code executes to perform a method of stimulating the chest wall of a subject during respiration comprising:

(a) receiving an input dependent on the subject's respiration;

(b) generating, in response to the received input, a trigger signal to trigger one or more stimulation devices operable to stimulate the chest wall of the subject;

According to a further aspect of the invention there is provided a computer program comprising machine-readable program code for controlling the operation of a data processing apparatus on which the program code executes to perform a method of stimulating the chest wall of a subject during respiration comprising:

(a) receiving an input dependent on the subject's respiration;

(b) generating, in response to the received input, a trigger signal to trigger one or more stimulation devices operable to stimulate the chest wall of the subject; (c) receiving a feedback signal indicative of the operation of the triggered stimulation device; and

Brief description of the drawings

Embodiments of the present invention will now be described with reference to the drawings, in which:

Fig 1A is a schematic block diagram of a system that includes a respiration sensor to detect respiration and a stimulation device to stimulate a user's chest wall;

Fig 1 B shows a schematic diagram of an embodiment of the system of Fig 1A in which the stimulation device is a vibrator;

Fig 2 shows a schematic diagram of a system controller for use in the system of Fig 1 B;

Fig 3 shows a schematic diagram of a vibrator controller for use in the system of Fig 1 B;

Fig 4 is a flow diagram of a method for generating a trigger signal based on an output of a respiration sensor;

Fig 5A is a flow chart of a method for generating a vibrator drive signal to maintain a vibrator frequency at setpoint;

Fig 5B is a flow chart of a method for determining the period of vibration of the vibrator of Fig 1 B;

Fig 5C shows a flow chart of a method for generating the vibrator drive signal using fuzzy logic;

Fig 6A shows an example of fuzzy sets that may be used with the error signal in the method of Fig 5C; Fig 6B shows an example of fuzzy sets that may be used with the error rate signal in the method of Fig 5C;

Figs 7A and 7B illustrate how membership of the fuzzy sets may be determined for error and error rate signals respectively;

Fig 8 shows an example of fuzzy output sets that may be used in the method of Fig 5C; and

Fig 9 shows how the fuzzy output sets of Fig 8 may be used to generate a crisp output in the method of Fig 5C.

Detailed description of the embodiments

A system is described for providing in-phase stimulation of the chest wall of a subject to reduce breathlessness during respiration. The term 'in-phase' indicates that the inspiratory intercostal muscles are stimulated during inspiration and/or the expiratory intercostal muscles are stimulated during expiration. In one embodiment a vibrator is used to apply a mechanical vibration to the intercostal muscles used in inspiration or expiration. A control method is described for controlling the frequency of vibration of the vibrator.

Fig 1A shows a system 1 that includes a respiration sensor 3, which is operable to determine whether the subject is inhaling or exhaling. The respiration sensor 3 is in data communication with a system controller 5 that monitors the subject's respiration and sends control signals to a stimulation device 7 that stimulates the subject's chest wall in phase with the subject's respiration.

In the embodiment described in more detail below the stimulation device 7 is a vibrator. However, other stimulating devices may be used. For example, a transcutaneous electrical neural stimulation (TENS) device may be used to apply an electrical stimulus to the subject's intercostal muscles instead of applying a mechanical vibration. It is noted, however, that the stimulation applied in the present arrangements is not designed to cause muscle contraction and alter the normal respiration of the subject. Instead, the stimulation provides an enhanced chest wall movement during respiration to assist in reducing breathlessness.

Fig 1 B shows a system 2 in which the stimulation device is one or more vibrators 11. In system 2, two control units are provided. A vibrator controller 9 generates a drive signal to maintain the vibrator 11 at a desired frequency of vibration. The system controller 5 monitors the respiration sensor 3 and generates a trigger signal based on the subject's respiration. The trigger signal is provided to the vibrator controller 9.

In one arrangement the system controller 5 and vibrator controller 9 are provided in separate housings. However, in other arrangements the functions of the system controller 5 and vibrator controller 9 may be combined into a single unit. The controllers may also be housed within a stimulation device 7.

The vibrator 11 may be an off-the-shelf vibrator of the type used in mobile phones, for example the VPM2 from www.solarbotics.com. Such a vibrator may be approximately 12mm in diameter and 3mm thick. When 3 volts DC is applied to the vibrator 11 it will generally vibrate at between 172 Hz and 200 Hz.

The vibrator unit 11 includes an accelerometer which is operable to sense the frequency of vibration of the vibrator. An example of a suitable accelerometer is the ADXL103 accelerometer manufactured by Analog Devices, which has a maximum range of + 1.7g and a maximum frequency response of 2.5 kHz.

In use, the vibrator unit 11 is positioned on the subject's intercostal muscles and held in position by, for example, elasticised straps. During inspiration the vibration may be applied to the second or third intercostal parasternal spaces by one or more inspiratory vibrators. During expiration the vibration may be applied to the seventh to ninth intercostal midaxilliary spaces by one or more expiratory vibrators. The force required to hold the vibrator 11 in place may affect the frequency of vibration of the vibrator. Accordingly, one of the functions of the vibrator controller 9 is to generate a drive signal that maintains the frequency of the vibrator at a specified frequency.

More than one vibrator 11 may be used. In one arrangement two vibrators are attached to the subject's inspiratory muscles and two vibrators are attached to the subject's expiratory muscles. The vibrator controller 9 may provide a control signal for each set of vibrators.

Respiration sensor

In one arrangement the respiration sensor 3 is a 100K NTC thermistor that is mounted on a plastic face mask. Air flow through the face mask effectively heats and cools the thermistor in phase with the subject's respiration. A constant current passing through the thermistor allows the voltage across the thermistor to vary with temperature. Monitoring the changing voltage enables the system controller 5 to determine whether the subject is inhaling or exhaling. In other arrangements the thermistor may be attached to nasal prongs or, for example, to an apparatus that may be mounted on the subject's head in a similar fashion to audio headphones with an extension that can be positioned in front of the subject's mouth and nose. A sensor may be mounted on the extension.

Other types of respiration sensor may also be used, for example using a flow sensor or pressure sensor. Other methods of monitoring respiration can also be used, for example sensing expansion or contraction of the subject's chest wall.

System controller

Fig 2 shows an example of a system controller 5 that receives input signals from the respiration sensor 3 at input 13. The system controller unit 5 has an on-off switch 21. The controller 5 also includes 2 light-emitting diodes (LEDs) 17, 19. The power LED 17 indicates to the user the battery status of the system controller 5. The LED 17 flashes at 4 Hz to indicate that the battery is good and flashes at 1 Hz to indicate that the battery needs replacing. As the battery state changes from good to low, the LED 17 flash rate may vary between 4 Hz and 1 Hz, providing further indication to the user that the battery may soon need replacing. If the power LED 17 fails to flash then either the battery is flat or a fault has developed in the system.

The inspiration LED 19 indicates to the user that the system controller 5 has detected inspiration. The LED 19 lights up continuously for the duration of inspiration. If the LED 19 fails to light up either no inspiration has been detected or a system fault has developed. If inspiration is not detected the following checks may be performed:

1 check that the thermistor is correctly and securely mounted onto the face mask;

2 check that the face mask of the respiration sensor 3 is correctly and securely attached to the user's face; and

3 check that the cable connecting the respiration sensor 3 to the input 13 of the system controller 5 is in good condition and is properly secured to the respiration sensor 3 and the system controller 5.

An additional LED may be provided that indicates when expiration is detected.

In the described arrangement cables are used for data communication between the components of system 2. However, other means of data communication may be used, for example wireless RF signals.

The controllers 5, 9 may be implemented on a hardware platform incorporating a microprocessor. However, the controller functions may also be implemented as software instructions running on a suitable computing device, for example a personal computer, personal digital assistant (PDA) or mobile phone. It will be appreciated that there are many ways in which the controllers may communicate with the stimulation device 7, for example an analogue connection such as a DAC card, a digital connection such as a USB link or a wireless connection, for example using the Bluetooth protocol.

Fig 4 illustrates a method 40 that may be performed by the system controller 5 to generate a trigger signal for the vibrator controller 9. The input to method 40 is the voltage across the thermistor of the respiration sensor 3. In process 42, the voltage signal from the thermistor is conditioned. In one arrangement, the thermistor voltage signal is initially amplified by a factor of 5 using an instrumentation amplifier in the system controller 5. The resultant amplified signal is then filtered by a high-pass filter (HPF) followed by a low-pass filter (LPF). In one arrangement the HPF has a cut-off frequency of about 0.02 Hz and the LPF has a cut-off frequency of about 20 Hz. Preferably the two filters each have unity gain. The output of the LPF is then digitised. The digitisation may be achieved using an on-board, 10-bit analog-to-digital converter (ADC) of a microcontroller embedded in the system controller 5 and sampling at 100 Hz.

In process 44 the slope of the digitised signal is determined, for example by software running on the microcontroller. Each sample of the thermistor signal is compared to the previous sample to determine the rate of change (slope) of the thermistor signal. In one arrangement the average of the last 4 slopes is calculated to determine the sign and the magnitude of the slope. When the magnitude of the slope exceeds a specified threshold and the sign of the slope changes, a new inspiration or expiration cycle has begun.

In process 46, the system controller 5 generates a trigger signal when an inspiration cycle is detected. The trigger signal may be a 3.3 V low-to-high transition pulse whose duration is equal to the duration of the inspiration cycle. The falling edge of the trigger signal may be at the detected start of an expiration cycle. The trigger signal is active (high) when the inspiration LED 19 is on. The trigger signal may be provided to the vibrator controller 9 by means of a cable connected to output 15 of the system controller 5. A similar method may be used to generate a trigger signal when an expiration cycle is detected. In one arrangement the system controller may generate two output signals, one corresponding to an inspiration and another corresponding to an expiration.

The system controller 5 may alternatively determine the slope of the thermistor signal by other means such as analog electronics.

In one arrangement, when the system controller 5 is powered on using the button 21 , the controller 5 initialises various registers used by the microcontroller and enters a low power mode. In the low power mode the system controller is able to service interrupts arising from a timer within the system controller 5. This enables the system's battery status to be monitored and the power status to be updated with only a minimal power drain. The system controller 5 is also able to service an interrupt from the ADC that causes the controller to exit the low power mode, run the slope-detection algorithm and activate the trigger signal if necessary.

In one arrangement, the printed circuit board (PCB) of the power supply of system controller 5 provides a battery status output, which is monitored by the microcontroller on the main PCB of system controller 5. When the status output goes low, indicating a low battery, the microcontroller changes the flash rate of the power LED 17. The flash rate of LED 17 is handled by a timer on board the microcontroller. When the battery voltage is good the power LED 17 flashes at 4 Hz and when the battery is low the flash rate is 1 Hz. In both cases the flash duration is 75ms.

Vibrator controller

Fig 3 shows an example of the vibrator controller 9. The vibrator controller 9 has an input 33 operable to receive the trigger signal from the system controller 5. The vibrator controller also has a power switch 39 and two red LEDs 35, 37. The LED 37 flashes at 4 Hz to indicate that the battery is good and flashes at 1 Hz to indicate that the battery needs replacing. As the battery transitions from good to low the LED 37 flash rate may vary between 4 Hz and 1 Hz providing further indication to the user that the battery will soon need replacing. If the LED 37 fails to flash either the battery is flat or a fault has developed in the system. In one arrangement the vibrator controller uses a 1.5 volt battery.

The vibration LED 35 indicates to the user that a vibrator frequency has been detected within a specified range of the frequency setpoint. In one arrangement the setpoint is 100 Hz and the vibration LED 35 is switched on when the measured vibration frequency is within +/- 5% of the setpoint. If the LED 35 fails to light up, either vibration within the specified range has not been detected or a system fault has developed. If the detected vibration frequency is near the limits of the range, the LED 35 may flicker.

If no vibration has been detected, the following checks may be performed:

1. check that the system controller 5 and vibrator controller 9 are connected correctly and are switched on with the power LEDs 17, 37 flashing at 4 Hz;

2. check that the inspiration LED 19 of the system controller 5 is illuminating in-phase with the subject's inspiration; and

3. check that the vibrator controller 9 and vibrator 11 are properly connected. Fig 5A shows a method 50 of operation of the vibrator controller 9. When the vibrator controller 9 is powered up, registers used by an on-board microcontroller are initialised and the vibrator controller enters a low power mode. In the low power mode the vibrator controller is able to service interrupts arising from a timer and then return to the low power mode. This enables the system's battery status to be monitored and the power status to be updated with only a minimal power drain. The vibrator controller 9 remains in the low power mode until a rising edge is detected at the trigger input 33 (step 51 ). When the controller 9 detects a rising edge, an interrupt is triggered. On completion of the interrupt service routine the low power mode is exited and in process 53 the vibrator controller sends a vibrator drive signal via the output 31. The drive signal activates the vibrator 11.

As described above, the vibrator has an associated accelerometer that returns an output signal to the vibrator controller 9. In process 55 of method 50 the controller 9 uses the accelerometer signal to determine a cycle period of vibration of the vibrator 11. In one arrangement, the rising edge of each vibration of vibrator 11 generates a timer interrupt. The interrupt service routine saves the time measured between rising edges as the period of vibration of the vibrator 11. The accelerometer and the timer provide a frequency sensor.

Then, in process 57, software running on the microcontroller of controller 9 compares the measured period to the desired period of 10ms. If the measured period is within a specified range, for example 5% of the setpoint (10ms corresponding to 100 Hz), then the controller 9 switches on the vibration LED 35 to inform the user that the measured vibration falls within the specified bounds.

In process 59 software running on the vibrator controller 9 generates a vibrator drive signal to drive the vibration frequency towards setpoint or to maintain the frequency at the specified setpoint.

If a falling edge from the trigger is detected, the vibrator controller 9 re-enters the low power mode and waits for the next rising edge from the trigger before repeating the processes of method 50. It will be understood that other setpoints for the vibration control may be used. In addition, different upper and lower bounds on the vibration frequency may be specified. In the described embodiment steps 5, 57 and 59 are performed by software running on a microcontroller on controller 9. However, one or more steps of method 50 may be implemented in other ways, for example using an application specific integrated circuit (ASIC) or field programmable gate array.

In one arrangement the vibrator controller 9 generates a pulse-width modulated (PWM) vibrator drive signal. The PWM base frequency and the duty cycle are set by a timer on board the microcontroller. The duty cycle may range from 10 to 100%.

Fig 5B shows more detail regarding process 55, in which the vibrator controller 9 determines the cycle period of the measured vibration. Firstly, in step 51 , the output signal of the accelerometer is conditioned. In one arrangement the accelerometer output is amplified by a factor of 5 before being fed into a unity-gain high-pass filter with a cut-off frequency of 37 Hz. The signal is then applied to a low-pass filter with a cut-off frequency of 160 Hz before being amplified a further 3 times. The amplified signal is fed to a comparator. Hysteresis may be incorporated in the comparator to help reject noise. The output of the comparator is provided to the embedded microcontroller of vibrator controller 9. The conditioned signal output of step 61 enters the microcontroller as a square wave whose frequency is the frequency of vibration of vibrator 11.

In step 63 the microcontroller uses the conditioned signal to measure the period of vibration. The rising edge of the input signal terminates a timer on board the microcontroller that captures the duration of the previous cycle of vibration. The timer is initialised and restarted to measure the duration of the next cycle.

Various factors may influence the vibration provided by the vibrator. For example, the tension of elasticised straps used to attach the vibrator to the subject may vary during use. Accordingly, the vibrator controller generates a vibrator drive signal in step 59 to provide the specified vibration frequency.

Fig 5C shows an implementation of this control step that uses fuzzy logic. It will be appreciated that other feedback control techniques may also be used to control appropriate operating variables of the stimulation device 7. In one arrangement the setpoint of the vibrator period is 10ms (100 Hz) which is represented by 148h clock cycles, operating at 32768 Hz. The suffix 'h' indicates a hexadecimal number.

In step 71 the vibrator controller determines an error signal which measures how close the measurement is to the setpoint as follows

Error = (measured parameter) - setpoint (1 )

If the measured value is less than the setpoint then the error is negative and if the measured value is greater than the setpoint then the error is positive.

In step 71 , the vibrator controller also measures an error rate, which is a measure of the rate of change of error and may be obtained as follows:

Error rate = error - last error (2)

In process 73 a fuzzification process is implemented that determines the signals' membership of specified fuzzy sets. In one arrangement fuzzy sets are defined for the error to cover the range of frequencies from 8.5ms (118 Hz) to 11.5ms (87 Hz). This range represents 6Oh clock cycles. The desired period corresponds to 148h clock cycles while the minimum and maximum periods are represented by 118h and 178h cycles respectively. To eliminate negative errors, an offset of 3Oh is added to the error, resulting in an error range from 0 to 6Oh with the 'zero error' at 3Oh. The defined fuzzy sets for the error signal are illustrated in Fig 6A where the X axis shows a range of errors between Oh and 6Oh. Five fuzzy sets are defined. The zero set (ZE) is triangular and ranges from a value of 0 at an error of 18h to a maximum value of 18h at an error of 3Oh, returning to a value of 0 at an error value of 48h. The positive small (PS) fuzzy set is a triangle with a maximum value of 18h at an error of 48h and a value of Oh at errors of 3Oh and 6Oh respectively. The positive medium (PM) fuzzy set rises linearly from a value of 0 at 48h to a maximum value of 18h.

The fuzzy sets negative small (NS) and negative medium (NM) are symmetrically disposed about the zero fuzzy set ZE. The height of the fuzzy sets (18h) represents the degree of membership a crisp input value may have and the location of the set on the horizontal axis is determined by the magnitude of the crisp input value.

In the firmware, fuzzy sets may be represented by two lines of the form Y = mX + b, where Y is the degree of membership, M is the slope, X equals error (or error rate) and b is the intercept on the Y axis. For the fuzzy sets shown in Fig 6A and 6B the maximum degree of membership was chosen to be 18h as the slope becomes 1h. Thus, for example, in the set MS one side of the set is defined by the line of equation 3 below and the other side of the set is defined by the line of equation 4 below.

y = x (3)

y = -x + 18h (4)

The other fuzzy sets are defined in a similar way.

Fuzzy sets are also defined for the error rate variable, as illustrated in Fig 6B. The error rate is the difference between consecutive errors and represents the rate of change of error. Since the error difference is always measured at intervals equal to the sampling period (Ts) it is not necessary to explicitly include Ts in the calculation of the error rate.

Fig 7 A illustrates how a crisp input is fuzzified to determine membership of the fuzzy sets. In example of Fig 7A the error signal has a value of 5h. The crisp input value intersects the line of MS at a value of 08h and intersects the line of fuzzy set NM at a height of 10h. Thus, the crisp error signal 5h belongs to sets NM and NS. The degree of membership of the error value to set NM is 66.7% (10h/18h) and to set NS is 33.3% (08h/18h).

Fig 7B illustrates the fuzzification of a crisp input of an error rate of 3Ch. This error rate belongs equally to sets ZE and PS. The degree of membership of the error rate to set ZE is 50% (0Ch/18h) and to set PS is 50% (0Ch/18h).

Fuzzification takes a crisp value and determines its degree of membership of the fuzzy sets. In the described arrangement the error and error rate may each belong to two sets where membership of set 1 is defined by (set ID 1 , DoM 1 ) and membership of set 2 is defined by (set 2 ID₁ DoM 2).

For example, for an input error of 5h, set 1 = (NM, 1Oh) and set 2 = (NS, 08h).

Next, in step 75, the vibrator controller performs a fuzzy inference process in which predefined fuzzy rules are applied to determine a fuzzy output value.

Fig 8 shows an example of fuzzy output sets that may be used in the calculation of the vibrator drive signal. The sets are labelled:

The quantity on the X axis corresponding to each fuzzy output set in Fig 8 represents the number of clock cycles necessary to produce the desired duty cycle of the vibrator 11. For example, for 1 Ch:

DC = 1Ch/55h x 64h = 33%,

where 55h is the number of clock cycles necessary to produce the vibrator drive frequency.

The fuzzy inference rules capture the significance of the error and error rate signals. For example, if Error = Positive Medium and Last Error = Positive Small then the Error Rate is positive and small. The Error is getting larger at a small rate. If Error = Negative Medium and Last Error = Positive Small then the Error Rate is negative and medium. The Error is getting larger at a medium rate.

If Error = Negative Small and Last Error = Negative Medium the Error Rate is positive and small. The Error is getting smaller at a small rate.

If Error = Positive Small and Last Error = Negative Medium the Error Rate is positive and medium. The Error is getting larger at a medium rate.

If Error = Negative Small and Last Error = Positive Medium the Error Rate is negative and medium. The Error is getting larger at a medium rate.

The example above can be summarized by the following:

If +E and +ER -> Increasing Error

If +E and -ER -> Decreasing Error

If -E and +ER -> Decreasing Error

If -E and -ER -> Increasing Error.

The output control parameter in this case is the duty cycle of the vibrator drive. If the duty cycle is low the vibrator's frequency is low and its period is longer. If the error is negative the vibrator's period is less (vibrator's frequency is higher) than the set point so the duty cycle of the vibrator drive should be reduced to achieve the set point. The output sets shown in Fig 8 are defined as VLDC = Very Low Duty Cycle, LDC = Low Duty Cycle, MDC = Medium Duty Cycle, HDC = High Duty Cycle and VHDC = Very High Duty Cycle.

The fuzzy inference step 75 uses the following rules. Rule 1 is:

If Error is negative (period < set point) and medium and Error Rate is also negative and medium the error is increasing at a high rate so the output must attempt to nullify this condition by increasing the period (reducing frequency) which is achieved by applying a VLDC vibrator drive.

This may be expressed as:

RuIeI : E(NM) & ER(NM) -> Out(VLDC).

The other rules are:

Rule2: E(NM) & ER(NS) -> Out(VLDC)

Rule3: E(NM) & ER(ZE) -> Out(LDC)

Rule4: E(NM) & ER(PS) -> Out(HDC)

Ruleδ: E(NM) & ER(PM) -> Out(MDC)

Rule25: E(PM) & ER(PM) -> Out(VHDC).

Table 1 below shows the Fuzzy rules used in the described implantation of step 75.

Table 1: Fuzzy Rules.

From the example in Fuzzification, if an error of 5h was measured and an error rate of 3Ch was calculated the following Fuzzy sets result:

E1 = (NM, 1Oh), E2 = (NS, 08h), ER1 = (ZE, OCh), ER2 = (PS, OCh).

For a Fuzzy rule to be applicable it must contain a non-zero error and error rate. The certainty of the output of a rule can only be as good as the least certain parameter. For this reason, the minimum value of the relevant parameters is the certainty that the output belongs to a particular output set. So, if (E = (NM, 1Oh)) AND (ER = (ZE, OCh)) THEN (OUTPUT = (VLDC, 1Oh)). Table 2 below shows the results of applying the rules.

Table 2: The table shows the results of applying the Fuzzy rules to the error (5h) and error rate (3Ch) of the example above. From Table 2 and by applying the Max of Min rule, for each row of the table the Fuzzy output sets are Outputi = (VLDC, OCh) and Output2 = (LDC, 08h).

Having obtained a fuzzy output using the fuzzy output sets, the best estimate of the output may be determined by the process of defuzzification, which is applied by the vibrator controller in process 77.

In one arrangement, the centre of gravity method of defuzzification is used, utilising the following formula:

Crisp Output = Output Set (i) x Certainty of Membership (i) / Sum of Certainties.

Applying this equation to the example gives:

Crisp Output = ((09h x OCh) + (1 Ch x 08h)) / (OCh + 08h) = 11 h.

Fig 9 illustrates the example of defuzzification in which a crisp output of 11 h is calculated, corresponding to input error and error rate signals of 5h and 3Ch respectively.

The crisp output generated in process 77 represents a duty cycle. The value is assigned to a register of the timer, thereby changing the duty cycle of the vibrator drive signal to a value that would tend to keep the vibrator's frequency at the specified setpoint. The drive signal is applied to the vibrator 11.

The respiratory aid device has been used in a double-blinded, cross over phase 1 (efficacy) repeated measures trial. Subjects undertake two 20-minute cycle sessions during which the order of the placement of the RAD vibrators is randomised. The active position was the third intercostal parasternal space and the sham position was approximately the mid-clavicular line over pectoralis major. The subject's breathlessness was measured every two minutes using the Borg Dyspnoea scale. Seven subjects with severe COPD were recruited and three subjects completed the 20- minute protocol without difficulty. Initial statistical analysis of the results for these subjects indicated that the parasternal stimulation position reduced the sensation of dyspnoea. The placement had a significant effect p=0.02, with sternal placement (active) reducing dyspnoea by a mean of 0.364 points on the 10 point Borg Dyspnoea scale.

The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive. For example, a flow sensor or pressure sensor may be used in the respiration sensor 3 instead of the thermistor, or elasticised bands around the subject's chest may be used to detect the expansion and contraction of the chest, thereby indicating inspiration and expiration.

The vibrator may also be activated in phase with the user's expiration.

A vibration protocol may also be specified, for example, the system may activate the vibrator 11 for 1 minute in each 5 minute interval or 1 respiration cycle in 10 respiration cycles.

A mode may also be specified in which the vibrator 11 is enabled for random intervals. After a time interval (T), optionally selected by the user, the vibrator is disabled. The duration of T may be fixed, variable according to a preset range, or random. Within T the vibration protocol may follow a user-selected profile that may be fixed, variable according to preset values, or may include randomly-selected parameters.

The operation of the vibrator 11 may also be made dependent on respiration rate or some other physiological variable. For example, if the measured respiration rate or blood oxygen level is above or below a specified threshold, the system 2 can enable the use of the vibrator 11.

In further arrangements, the vibration setpoint may be variable. For example, a user may be able to select a setpoint in a range of, for example, 80-120 Hz. The setpoint may alternatively be specified in seconds or, for example, as a percentage of a range between fast and slow.

In one arrangement, the respiration aid system 1 , 2 includes a user interface that allows the user to select various features of the system. For example, the user may select a vibration setpoint or a vibration protocol. The user may also be offered a choice of having the vibration activated in phase with inspiration or expiration or both. The user may, for example, specify a number of stimulation devices to operate.

As mentioned earlier, other stimulation devices may also be used to induce vibration in the chest wall of the user.

In other arrangements the respiratory aid device may be used in conjunction with a ventilator. For example, a patient may be on a mechanical ventilator in intensive care or on non-invasive ventilation at home or in the hospital. The stimulation device 7 may be used to stimulate the intercostal muscles in phase with the operation of the ventilator. In these cases the input signal to the system controller 5 may be triggered by the ventilator. That is, rather than measuring the patient's respiration, the input to the system controller 5 corresponds to the action of the ventilator. With reference to Figs 1A and 1B, the respiration sensor 3 may be an arrangement that provides a signal corresponding to the ventilator cycle.

The controllers 5, 9 may be implemented as software routines running on the ventilator system. Thus, a ventilator may output a drive signal for one or more stimulation devices 7 applied to a patient on the ventilator.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

It will also be understood that the term "comprises" (or its grammatical variants) as used in this specification is equivalent to the term "includes" and should not be taken as excluding the presence of other elements or features.

Claims

1. A respiratory aid device for reducing breathlessness comprising:

(a) a respiration sensor that generates an output signal indicative of at least one of i) inspiration by a user and ii) expiration by the user;

2. A respiratory aid device according to claim 1 wherein the stimulation device comprises at least one inspiratory vibrator that in use applies the mechanical vibration to inspiratory intercostal muscles of the user and the controller activates the inspiratory vibrator if the output signal indicates inspiration by the user.

3. A respiratory aid device according to claim 1 or 2 wherein the stimulation device comprises at least one expiratory vibrator that in use applies the mechanical vibration to expiratory intercostal muscles of the user and the controller activates the expiratory vibrator if the output signal indicates expiration by the user.

4. A respiratory aid device according to any one of the preceding claims further comprising an elastic chest belt to hold the stimulation device in position on the user's chest.

5. A respiratory aid device according to any one of the preceding claims comprising a frequency sensor that senses a frequency of the applied mechanical vibration.

6. A respiratory aid device according to claim 5 wherein a frequency setpoint is specified for the mechanical vibration and the controller generates a driving signal to reduce an error between the sensed frequency and the frequency setpoint.

7. A respiratory aid device according to claim 5 or 6 wherein the frequency sensor comprises an accelerometer and a timer to measure a duration between rising edges of an accelerometer output signal.

8. A respiratory aid device according to any one of the preceding claims comprising a user input to enable selection of at least one of:

• a frequency setpoint for the mechanical vibration;

• the application of the mechanical vibration during inspiration;

• the application of the mechanical vibration during expiration;

• a quantity of vibrators to be stimulated to apply the mechanical vibration; and

• a protocol for the application of the mechanical vibration.

9. A respiratory aid device according to any one of the preceding claims wherein the respiration sensor measures a variable selected from the group consisting of:

a temperature;

an air flow rate; and

an air pressure.

10. A system for stimulating the chest wall of a subject during respiration to reduce breathlessness, the system comprising:

(a) a respiration sensor operable to generate an output signal indicative of at least one of inspiration by the subject and expiration by the subject;

(b) a stimulation device operable to stimulate the chest wall of the subject during respiration; and (c) a controller arranged to monitor the output signal and to activate operation of the stimulation device dependent on the output signal.

11. A system according to claim 10 wherein the controller is arranged to activate the stimulation device if the output signal of the respiration sensor indicates at least one of:

commencement of inspiration by the subject; and

commencement of expiration by the subject.

12. A system according to claim 11 that, in use, stimulates inspiratory intercostal muscles of the subject during inspiration.

13. A system according to claim 11 or 12 that, in use, stimulates expiratory intercostal muscles of the subject during expiration.

14. A system according to claim 10 wherein the stimulation device comprises at least one vibrator operable to apply a mechanical vibration to the chest wall.

15. A system according to claim 14 wherein a frequency setpoint is specified for the vibrator and the controller is arranged to generate a driving signal to reduce an error between a measured vibrator frequency and the frequency setpoint.

16. A system according to claim 15 wherein the controller comprises fuzzy logic means to generate the driving signal.

17. A system according to any one of claims 10 to 16 wherein the controller activates the stimulation device dependent on a measured physiological parameter of the subject.

18. A system according to claim 17 wherein the measured parameter is at least one of a rate of respiration and a blood-oxygen level.

19. A system according to any one of claims 10 to 18 wherein the output signal is indicative of the operation of a ventilator assisting the respiration of the subject.

20. A system according to any one of claims 10 to 19 further comprising a user interface to enable selection of one or more parameters of the chest-wall stimulation.

21. A system according to any one of claims 14 to 16 wherein the frequency setpoint lies in a range of 80 to 120 Hz.

22. A respiratory aid device for use with a ventilator, comprising:

(a) an input for receiving a signal indicative of operation of the ventilator on a patient;

(b) a stimulation device that in use is positioned on a chest wall of the user to apply a mechanical vibration to stimulate intercostal muscles of the user; and

(c) a controller that activates the stimulation device dependent on the received signal.

23. A method for stimulating the chest wall of a subject during respiration to reduce breathlessness comprising:

(a) receiving an input dependent on the subject's respiration;

(b) generating, in response to the received input, a trigger signal to trigger a stimulation device operable to stimulate the chest wall of the subject during respiration;

24. A computer program product comprising machine-readable program code recorded on a machine readable recording medium, for controlling the operation of a data processing apparatus on which the program code executes to perform a method of stimulating the chest wall of a subject during respiration to reduce breathlessness comprising: (a) receiving an input dependent on the subject's respiration;

(b) generating, in response to the received input, a trigger signal to trigger one or more stimulation devices operable to stimulate the chest wall of the subject during respiration;

25. A computer program comprising machine-readable program code for controlling the operation of a data processing apparatus on which the program code executes to perform a method of stimulating the chest wall of a subject during respiration to reduce breathlessness comprising:

(a) receiving an input dependent on the subject's respiration;