EP2034891A1

EP2034891A1 - Breathing monitor apparatus

Info

Publication number: EP2034891A1
Application number: EP07733219A
Authority: EP
Inventors: Rolf Kahrs Hansen; Magne Tvinnereim
Original assignee: MRA Medical Ltd
Current assignee: MRA Medical Ltd
Priority date: 2006-06-15
Filing date: 2007-06-14
Publication date: 2009-03-18
Also published as: GB0611890D0; WO2007144626A1

Abstract

A breathing monitor apparatus comprises a catheter insertable in the airway of a subject. First and second pressure sensors are provided on the catheter and are arranged to supply first and second pressure signals. A processing unit receives the first and second pressure signals and applies a filter to both of the first and second pressure signals. The processing unit further obtains a value indicative of the amplitude of the first filtered signal and the amplitude of the second filtered signal, and also calculates the difference between the obtained values. The processing unit then generates an output based on the obtained values and the calculated difference, which is indicative of obstruction in the airway.

Description

BREATHING MONITOR APPARATUS

This invention relates to an apparatus for monitoring breathing, for example for use in the detection and analysis of conditions such as sleep apnea and snoring. Previously, apneic events, i.e. cessation of breathing, or other restrictions in breathing, have been diagnosed on the basis of measurements from a catheter with pressure sensors which is inserted into the airway of a patient. In order to provide data useful for surgical procedures to remedy such conditions, it is necessary to determine the approximate location of the obstruction or partial occlusion of the airway. This is done by measuring the pressure difference between pressure sensors on the catheter to determine the pressure gradient in different portions of the airway. However, previously this has required at least three or four pressure sensors which makes the device more complex and costly. Also, sound generated by a patient when asleep, caused by partial obstruction of the airway, i.e. snoring, has previously been measured using external microphones attached to the skin, e.g. on the outside of the throat, or by the use of microphones located near the snoring subject. However, these arrangements have the problem that the volume of sound detected by the microphones can vary depending on the position and orientation of the subject, for example whether the microphones are occluded or not. Thus a consistent standard measurement of the severity of snoring is not possible.

The present invention is concerned with reducing or eliminating any of the above problems.

According to one aspect of the present invention there is provided a breathing monitor apparatus comprising: a catheter insertable in the airway of a subject; first and second pressure sensors provided on the catheter arranged to supply first and second pressure signals, S 1 and S2, respectively; and a processing unit for receiving the first and second pressure signals, wherein the processing unit is arranged to: apply a filter to both of the first and second pressure signals; obtain a first value PPl indicative of the amplitude of the first filtered signal and a second value PP2 = ΔPii indicative of the amplitude of the second filtered signal; calculate the difference between the obtained first and second values PPl - PP2 = ΔPi; and generate an output O/Pl, based on the obtained values and the calculated difference, indicative of obstruction in the airway.

A further aspect of the present invention provides a breathing monitor method comprising: receiving first and second pressure signals S 1 and S2 from respective first and second pressure sensors provided on a catheter inserted in the airway of a subject; applying a filter to both of the first and second pressure signals; obtaining a first value PPl indicative of the amplitude of the first filtered signal and a second value PP2 = ΔPii indicative of the amplitude of the second filtered signal; calculating the difference between the obtained first and second values PPl - PP2 = ΔPi; and generating an output O/P 1 , based on the obtained values and the calculated difference, indicative of obstruction in the airway. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates the head of a subject in schematic cross-section showing the catheter of an apparatus embodying the invention inserted into the airway of the subject; and Figure 2 is a schematic block diagram illustrating the operation of the processing unit of an apparatus according to the present invention.

Figure 1 shows the head H of a subject lying on its back. A catheter C has been inserted through the nose N of the subject into the airway AW, i.e. respiratory tract, of the subject. The catheter C passes through the nasal passage NP, throat T and the tip is located at the upper end of the esophagus E. The upper end of the esophagus E is, of course, in communication with the throat T and the trachea TR.

The catheter C is provided with two temperature sensors TSl and TS2 which can be, for example, platinum resistance temperature sensors. These can be used to measure the flow rate of air in the airway AW of the subject, which can provide useful information to clinicians, but is not the subject of the present invention, and will not be described further. Thus, these temperature sensors can be omitted entirely if desired. The catheter C is also provided with two pressure sensors PSl and PS2. The first pressure sensor PSl is located in the vicinity of the upper end of the esophagus E. The second pressure sensor PS2 is located in the vicinity of the soft palate SP of the subject. In this embodiment, the pressure sensors PSl and PS2 are semiconductor strain gauge pressure transducers which provide a frequency response in excess of 1 kHz, however other types of pressure sensor may be employed provided they are sufficiently compact to enable them to be fitted to the catheter C and inserted into the airway AW of the subject. Wires from each of the sensors in the catheter C are connected to a processing unit 10 by wires L. Figure 2 depicts schematically the functioning of the processing unit 10 which will now be described in more detail.

A first input I/PI receives the pressure signal Sl from the first pressure sensor PSl . This signal is filtered by a band pass filter (BPF) 20 which passes frequencies in the range 0.1 Hz to 1 Hz or which has a centre frequency of substantially 0.2 Hz on a logarithmic frequency scale. The band-passed signal is then processed by the peak-to-peak unit 22 which produces an output value PP 1 representing the maximum pressure difference from the peak to the trough of the filtered signal from the first pressure sensor PSl. A second input I/P2 receives the second pressure signal S2 from the second pressure sensor PS2 which is then filtered by band pass filter 30 and processed by the peak-to-peak determining unit 32 to produce a peak-to-peak value PP2. The band pass filter 30 and peak-to-peak determining unit 32 are identical in performance to the corresponding band pass filter 20 and peak-to-peak determining unit 22 which operate on the first pressure signal.

A difference unit 40 subtracts the value PP2 from the value PPl to produce a value ΔPi which represents the difference between the peak-to-peak esophageal pressure and the peak-to-peak pressure below the soft palate. This value ΔPi is indicative of the pressure difference across the base of the tongue of the subject. A second value ΔPii is obtained which is indicative of the difference between the peak- to-peak pressure from below the soft palate and the ambient pressure external to the patient. The value ΔPii in fact corresponds to the peak-to-peak signal PP2 because the ambient pressure in the room external to the subject can be considered to be constant and so the band pass filter 30 which removes any DC offset in the second pressure signal S2 effectively already subtracts the ambient pressure. In this way, by using the filtered signal a "virtual sensor" that always has a pressure value of 0 and which is external to the patient has been established. Thus the breathing monitor apparatus can measure two pressure differentials ΔPi and ΔPii using only two physical pressure sensors PSl and PS2.

An analysis unit 42 receives the values ΔPi and ΔPii and also a presetable constant value K. The analysis unit performs a comparison to determine whether the following first condition is satisfied:

ΔPii > ΔPi (1 + K) (1) This is equivalent to taking the ratio of ΔPii to ΔPi and seeing whether ΔPii exceeds ΔPi by a factor of K, for example if K is 0.1, then the condition is that ΔPii is greater than ΔPi by 10%. If the above first condition is satisfied, then it is determined that there is an obstruction in the airway AW of the subject and the obstruction is classified as "upper" i.e. above the tongue base (between the second pressure sensor PS2 and the nostril of the subject.

The analysis unit 42 also performs a comparison to determine whether the following second condition is satisfied:

ΔPii < ΔPi (1 - K) (2)

In this case, for a value of K of 0.1, the condition is whether the value ΔPii falls below the value ΔPi by a factor of 10%. If this second condition is satisfied, then it is determined that there is an obstruction in the airway AW of the subject, and the obstruction is classified as "lower", i.e. below the tongue base towards the trachea TR.

If neither of the above two conditions is satisfied, then it is determined that no obstruction has been detected. The analysis unit 42 produces a first output O/Pl to indicate whether or not an obstruction has been detected, and if so whether it is classified as an upper obstruction or a lower obstruction. The information in the output O/Pl is useful, firstly for avoiding unnecessary corrective surgery in the event that no obstruction is detected, and secondly because surgery can usually successfully be applied in the case of an upper obstruction, but not for a lower obstruction. As can be seen above, the value of the constant K acts as a threshold to determine the proportional pressure difference that should be classified as an obstruction. The value of the constant K can be adjusted as required to provide higher or lower sensitivity to classifying an obstruction. Typically the value of K is at least 0.05, for example 0.1 or above. According to a further enhancement of this embodiment, two different values of K are used, one for the first condition and one for the second condition, such that the threshold for determining an obstruction can be set differently for the cases of upper and lower obstructions.

A further embodiment of the invention will now be described whose features can be used in conjunction with the previously described embodiment. The features of this further embodiment are illustrated in the lower portion of Figure 2 and comprise a further band pass filter 50 and an amplitude detection unit 52. The band pass filter 50 receives the signal Sl or S2 from one or other of the pressure sensors PSl or PS2, or optionally receives both signals Sl and S2. In a version embodying the former option, a catheter can be used which simply has a single pressure sensor. In previous breathing monitor apparatus, low pass filters are used to obtain signals representing pressure variations due to breathing, and higher frequency pressure variations are discarded. In the present embodiment, the band pass filter 50 passes frequencies in the range of 50 to 250 Hz to produce a signal comprising pressure variations due to snoring, i.e. acoustic pressure variations. The lower cut off of the band pass filter 50 could take a higher value, for example 60 Hz and/or the upper cut off frequency of the band pass filter 50 could take a lower value such as 200 Hz and still provide a signal adequate for the measurement of the sound of snoring.

The filtered acoustic signal is then processed by amplitude detection unit 52 to determine whether the acoustic snoring signal exceeds a predetermined threshold, and if so the output O/P2 indicates the occurrence of snoring. The amplitude detection unit can employ a peak-to-peak measurement unit or other suitable signal processing unit to determine the amplitude of the acoustic signal and then compare that amplitude with a predetermined threshold value to produce the output O/P2 indicating either the absence or occurrence of snoring.

In this way, snoring can be more reliably detected than using external microphones, and if used in conjunction with the first embodiment of the invention, no further sensors are needed because snoring can be measured using existing pressure transducers on a catheter for breathing obstruction monitoring.

A further optional enhancement of this embodiment of the invention is to include frequency analysis of the snoring signal. The signal Sl and/or S2 is processed to produce a spectrum of amplitude versus frequency across a range of frequencies relevant to the diagnosis of snoring. The spectrum could be produced by a specific frequency analysis module, for example employing a suitable transform, or could be achieved by using a bank of band-pass filters of different frequency ranges. The spectrum is then output for use by a clinician. The processing unit 10 illustrated schematically in Figure 2 is shown comprising specific functional blocks, such as band pass filters and various other signal processing units. Whilst it is possible to implement the processing unit 10 using dedicated hard- wired electronic circuits for each of the various units, the preferred embodiment is to implement the signal processing in software rather than in dedicated hardware and so the various units illustrated in Figure 2 are preferably implemented in modules of code executable by a suitable computer microprocessor. The processing unit 10 according to one embodiment is simply a conventional personal computer (PC) with suitable interfaces for receiving the signals from the catheter, a storage medium for storing the signals before or after processing, and a display for showing the output results. In a variation of this embodiment, the catheter C is connected to a dedicated battery-powered unit which can be conveniently worn by the subject and which logs the pressure signals as a function of time. This portable unit can later be connected to a computer, such as a PC, either with a cable or wirelessly, to download the stored signals, which can then be processed and analysed off-line.

The processing unit 10 can be implemented using either analog or digital signal processing units, or a combination thereof. In the preferred embodiment, the signal processing is in the digital domain performed by a digital computer. The different band pass filters used either for airway obstruction identification or snoring detection can operate at different sampling rates, because these filters produce signals in different frequency bands. The sampling rate corresponds to the inverse of the time interval between taking digital samples of analog signals such as S l and S2. The sampling can be done by the filters or by dedicated analog-to-digital converters (not shown). The signal paths for snoring detection and obstruction identification can also have different presetable gain factors, for example using adjustable amplifiers (not shown). The output or outputs can indicate the information on breathing obstruction events and/or snoring events as a function of time, such that it is possible to determine the time of occurance during sleeping of such events and also the duration of any of these events.

Claims

1. A breathing monitor apparatus comprising: a catheter insertable in the airway of a subject; first and second pressure sensors provided on the catheter arranged to supply first and second pressure signals, S 1 and S2, respectively; and a processing unit for receiving the first and second pressure signals, wherein the processing unit is arranged to: apply a filter to both of the first and second pressure signals; obtain a first value PPl indicative of the amplitude of the first filtered signal and a second value PP2 = ΔPii indicative of the amplitude of the second filtered signal; calculate the difference between the obtained first and second values PPl - PP2 = ΔPi; and generate an output O/Pl, based on the obtained values and the calculated difference, indicative of obstruction in the airway.

2. An apparatus according to claim 1, wherein the first and second pressure sensors are provided at locations spaced apart along the length of the catheter.

3. An apparatus according to claim 1 or 2, wherein the first sensor is arranged to be located in use in the vicinity of the esophagus of the subject.

4. An apparatus according to claim 1 , 2 or 3, wherein the second sensor is arranged to be located in use in the vicinity of the soft palate of the subject.

5. An apparatus according to any one of the preceding claims, wherein the filter applied by the processing unit passes frequencies in the range 0.1 Hz to 1 Hz.

6. An apparatus according to any one of the preceding claims, wherein the filter applied by the processing unit is a band-pass filter with a centre frequency of substantially 0.2 Hz.

7. An apparatus according to any one of the preceding claims, wherein each value indicative of the amplitude is a peak-to-peak value.

8. An apparatus according to any one of the preceding claims, wherein the output is indicative of the presence or absence of an obstruction, and the location of an obstruction.

9. An apparatus according to any one of the preceding claims, wherein the processing unit is arranged to compare the obtained second value indicative of the amplitude of the second filtered signal ΔPii and the difference between the obtained amplitude values ΔPi, and generates a first output if the comparison exceeds a predetermined threshold factor 1+K and generates a second output if the comparison falls below a predetermined threshold factor 1-K.

10. An apparatus according to any one of the preceding claims, wherein the processing unit generates a first output if the following condition is satisfied:

ΔPii > ΔPi (1 + K) where ΔPii is the value indicative of the amplitude of the second signal, ΔPi is the difference between the obtained amplitude values, and K is a constant.

11. An apparatus according to claim 9 or 10, wherein the first output indicates the presence of an obstruction in the airway between the second pressure sensor and the exterior of the subject.

12. An apparatus according to any one of the preceding claims, wherein the processing unit generates a second output if the following condition is satisfied:

ΔPii < ΔPi (1 - K) where ΔPii is the value indicative of the amplitude of the second signal, ΔPi is the difference between the obtained amplitude values, and K is a constant.

13. An apparatus according to claim 9 or 12, wherein the second output indicates the presence of an obstruction in the airway between the first and second pressure sensors.

14. An apparatus according to any one of claims 9 to 13, wherein the value of the constant K is at least 0.05.

15. An apparatus according to any one of claims 9 to 13, wherein the value of the constant K is at least 0.1.

16. An apparatus according to any one of the preceding claims, wherein the processing unit is further arranged to apply a further filter to at least one of the first and second pressure signals to obtain an acoustic signal.

17. An apparatus according to claim 16, wherein the processing unit is arranged to generate a further output O/P2 indicative of whether the acoustic signal exceeds a further predetermined threshold.

18. An apparatus according to claim 17, wherein the further output indicates snoring if the acoustic signal exceeds the further predetermined threshold.

19. An apparatus according to claim 16, 17 or 18, wherein the further filter applied by the processing unit is a band-pass filter which passes frequencies substantially in the range of 50 Hz to 250 Hz.

20. An apparatus according to claim 16, wherein the further filter is arranged to produce a frequency spectrum of the acoustic signal.

21. A breathing monitor method comprising: receiving first and second pressure signals Sl and S2 from respective first and second pressure sensors provided on a catheter inserted in the airway of a subject; applying a filter to both of the first and second pressure signals; obtaining a first value PPl indicative of the amplitude of the first filtered signal and a second value PP2 = ΔPii indicative of the amplitude of the second filtered signal; calculating the difference between the obtained first and second values PPl - PP2 = ΔPi; and generating an output O/P 1 , based on the obtained values and the calculated difference, indicative of obstruction in the airway.

22. A method according to claim 21 , further comprising providing the first and second pressure sensors at locations spaced apart along the length of the catheter.

23. A method according to claim 21 or 22, further comprising locating the first sensor in the vicinity of the esophagus of the subject.

24. A method according to claim 21, 22 or 23, further comprising locating the second sensor in the vicinity of the soft palate of the subject.

25. A method according to any one of claims 21 to 24, wherein the filter applied to the first and second pressure signals passes frequencies in the range 0.1 Hz to 1 HZ.

26. A method according to any one of claims 21 to 25, wherein the filter applied to the first and second pressure signals is a band-pass filter with a centre frequency of substantially 0.2 Hz.

27. A method according to any one of claims 21 to 26, wherein obtaining each value indicative of the amplitude comprises obtaining a peak-to-peak value.

28. A method according to any one of claims 21 to 27, wherein the generated output is indicative of the presence or absence of an obstruction, and the location of an obstruction.

29. A method according to any one of claims 21 to 28, wherein generating an output comprises comparing the obtained second value indicative of the amplitude of the second filtered signal ΔPii and the difference between the obtained amplitude values Δpi, and generating a first output if the comparison exceeds a predetermined threshold factor 1+K and generating a second output if the comparison falls below a predetermined threshold factor 1-K.

30. A method according to any one of claims 21 to 29, comprising generating a first output if the following condition is satisfied:

31. A method according to claim 29 or 30, wherein the first output indicates the presence of an obstruction in the airway between the second pressure sensor and the exterior of the subject.

32. A method according to any one of claims 21 to 31 , comprising generating a second output if the following condition is satisfied:

33. A method according to claim 29 or 32, wherein the second output indicates the presence of an obstruction in the airway between the first and second pressure sensors.

34. A method according to any one of claims 29 to 33, wherein the value of the constant K is at least 0.05.

35. A method according to any one of claims 29 to 33, wherein the value of the constant K is at least 0.1.

36. A method according to any one of claims 21 to 35, further comprising applying a further filter to at least one of the first and second pressure signals to obtain an acoustic signal.