Blood pressure monitoring with improved noise rejection

• US 5,873,836 A
• Filed: 07/09/1997
• Issued: 02/23/1999
• Est. Priority Date: 07/09/1997
• Status: Expired due to Fees

• Current Assignee: Bpsure LLC
• Original Assignee: BP Sure LLC
• Inventors: Bahr, Dennis E., Allen, Kurt W., Kahn, Alan R.
• Primary Examiner(s): Nasser, Robert L.
• Application Number:
  US08/890,402
• Time in Patent Office: 594 Days
• Field of Search: 600/490, 600/485, 600/493-6, 600/500
• US Class Current: 600/493
• CPC Class Codes: A61B 5/02141 Details of apparatus construction, e.g. pump units or housings therefor, cuff pressurising systems, arrangements of fluid conduits or circuits A61B5/02233, A61B5/0235 take precedence
• CPC Class Codes: A61B 5/02208 using the Korotkoff method
• CPC Class Codes: A61B 5/7214 using signal cancellation, e.g. based on input of two identical physiological sensors spaced apart, or based on two signals derived from the same sensor, for different optical wavelengths

Abstract

A method and apparatus is provided for distinguishing blood pressure sounds from noise in an automatic blood pressure monitoring system using the information contained in two microphone signals. Two microphones are placed on a patient along an axis of an artery to pick up blood pressure sounds. Pressure is applied to the artery such that the artery opens and closes during each heart cycle, the opening snap of the artery producing a blood pressure sound. The two microphone signals are sampled, filtered, and multiplied together to produce a microphone signal product. The microphone signal product is wave width filtered to remove therefrom portions of the signal having a wave width which is less than a selected minimum wave width value. A blood pressure sound is indicated as being detected when an amplitude of this wave width filtered microphone signal product exceeds a noise signal threshold level. The microphone signal product is also wave width filtered to remove therefrom portions of the signal having a wave width which is greater than a selected maximum wave width value. The amplitude of the resulting noise amplitude signal is used to adjust dynamically the noise signal threshold level in response to changing noise conditions. The detection of blood pressure sounds at a range of applied pressure levels is used to determine the systolic and diastolic blood pressure levels of the patient.

Claims

1. A method for detecting a blood pressure sound in a patient, comprising the steps of:

(a) placing a first microphone on the patient near the location of an artery, the first microphone producing a first microphone signal in response to blood pressure sounds and noise picked up by the first microphone;

(b) placing a second microphone on the patient near the location of the artery and separated a distance from the first microphone, the second microphone producing a second microphone signal in response to blood pressure sounds and noise picked up by the second microphone;

(c) applying pressure to the artery such that the artery will be forced closed during part of a heart beat cycle and will be forced open during part of the heart beat cycle when the patient'"'"'s blood pressure exceeds the pressure applied to the artery, the opening of the artery causing a blood pressure sound to be produced;

(d) multiplying the first and second microphone signals to generate a microphone signal product;

(e) wave width filtering the microphone signal product to remove therefrom portions of the microphone signal product having a wave width which is less than a selected minimum wave width value to generate a wave width filtered blood pressure sound detection signal;

(f) comparing the wave width filtered blood pressure sound detection signal to a noise signal threshold level; and

(g) indicating the detection of a blood pressure sound when the wave width filtered blood pressure sound detection signal exceeds the noise signal threshold level.

2. The method of claim 1 wherein the second microphone is placed on the patient such that a center of the second microphone is separated a distance from a center of the first microphone such that a blood pressure sound picked up by the first microphone will be picked up out of phase by the second microphone.

3. The method of claim 1 comprising additionally the step of selecting portions of the first and second microphone signals having characteristics corresponding to blood pressure sounds before multiplying the first and second microphone signals.

4. The method of claim 3 wherein the step of selecting portions of the first and second microphone signals includes the step of filtering the first and second microphone signals with filters having pass bands corresponding approximately to the frequency range of blood pressure sounds.

5. The method of claim 3 wherein the step of selecting portions of the first and second microphone signals includes the steps of:

(a) gain controlling one of the first and second microphone signals to match the average levels of the first and second microphone signals;

(b) adding the microphone signals together to create a microphone signal sum in which a blood pressure sound component of the two microphone signals is reduced;

(c) least means square filtering the microphone signal sum; and

(d) subtracting the least means square filtered microphone signal sum from each of the first and second microphone signals.

6. The method of claim 1 wherein the selected minimum wave width value is less than a minimum wave width of a blood pressure sound.

7. The method of claim 1 wherein the step of applying pressure to the artery includes the step of inflating an inflatable cuff wrapped around an arm of the patient, and wherein the first and second microphones are placed on the arm of the patient under the inflatable cuff along an axis of the artery.

8. The method of claim 1 comprising additionally the step of adjusting the noise signal threshold level in response to changing noise conditions.

13. The method of claim 1 wherein the step of selecting portions of the first and second microphone signals includes the steps of:

(a) gain controlling one of the first and second microphone signals to match the average levels of the first and second microphone signals;

(b) adding the microphone signals together to create a microphone signal sum in which a blood pressure sound component of the two microphone signals is reduced;

(c) least means square filtering the microphone signal sum; and

(d) subtracting the least means square filtered microphone signal sum from each of the first and second microphone signals.

9. A method for detecting a blood pressure sound in a patient, comprising the steps of:

(a) placing a first microphone on a patient near the location of an artery, the first microphone producing a first microphone signal in response to blood pressure sounds and noise picked up by the first microphone;

(b) placing a second microphone on the patient near the location of the artery and separated a distance from the first microphone, the second microphone producing a second microphone signal in response to blood pressure sounds and noise picked up by the second microphone;

(c) applying pressure to the artery such that the artery will be forced closed during part of a heart beat cycle and will be forced open during part of the heart beat cycle when the patient'"'"'s blood pressure exceeds the pressure applied to the artery, the opening of the artery causing a blood pressure sound to be produced;

(d) multiplying the first and second microphone signals to generate a microphone signal product;

(e) wave width filtering the microphone signal product to remove therefrom portions of the microphone signal product having a wave width which is less than a selected minimum wave width value to generate a wave width filtered blood pressure sound detection signal;

(f) comparing the wave width filtered blood pressure sound detection signal to a noise signal threshold level;

(g) indicating the detection of a blood pressure sound when the wave width filtered blood pressure sound detection signal exceeds the noise signal threshold level;

(h) wave width filtering the microphone signal product to remove therefrom portions of the microphone signal product having a wave width which is greater than a selected maximum wave width value to generate a wave width filtered noise amplitude signal;

(i) comparing the wave width filtered noise amplitude signal to the noise signal threshold level;

(j) increasing the noise signal threshold level by an amount related to the product of a first noise signal threshold level adjustment coefficient and a difference between the noise signal threshold level and the wave width filtered noise amplitude signal if the wave width filtered noise amplitude signal exceeds the noise signal threshold level; and

(k) decreasing the noise signal threshold level by an amount related to the product of a second noise signal threshold level adjustment coefficient and the difference between the noise signal threshold level and the wave width filtered noise amplitude signal if the wave width filtered noise amplitude signal is less than the noise signal threshold level.

10. The method of claim 9 wherein the second microphone is placed on the patient such that a center of the second microphone is separated a distance from a center of the first microphone such that a blood pressure sound picked up by the first microphone will be picked up out of phase by the second microphone.

11. The method of claim 9 comprising additionally the step of selecting portions of the first and second microphone signals having characteristics corresponding to blood pressure sounds before multiplying the first and second microphone signals.

12. The method of claim 11 wherein the step of selecting portions of the first and second microphone signals includes the step of filtering the first and second microphone signals with filters having pass bands corresponding approximately to the frequency range of blood pressure sounds.

14. The method of claim 9 wherein the selected minimum wave width value is less than a minimum wave width of a blood pressure sound.

15. The method of claim 9 wherein the selected maximum wave width value is less than a minimum wave width of a blood pressure sound.

16. The method of claim 9 wherein the selected minimum wave width value and the selected maximum wave width value are equal.

17. The method of claim 9 wherein the first noise signal threshold level adjustment coefficient is larger than the second noise signal threshold level adjustment coefficient.

18. The method of claim 9 wherein the step of applying pressure to the artery includes the step of inflating an inflatable cuff wrapped around an arm of the patient, and wherein the first and second microphones are placed on the arm of the patient under the inflatable cuff along an axis of the artery.

19. The method of claim 9 wherein the step of comparing the wave width filtered blood pressure sound detection signal to the noise signal threshold level is delayed from the steps of increasing or decreasing the noise signal threshold level such that the wave width filtered blood pressure sound detection signal based on the microphone signal product is compared to a noise signal threshold level which is increased or decreased based on a microphone signal product from a future point in time.

20. A method for adjusting a noise signal threshold level in response to changing noise conditions during the detection of blood pressure sounds in a patient, comprising the steps of:

(a) placing a first microphone on a patient near the location of an artery, the first microphone producing a first microphone signal in response to blood pressure sounds and noise picked up by the first microphone;

(b) placing a second microphone on the patient near the location of the artery and separated a distance from the first microphone, the second microphone producing a second microphone signal in response to blood pressure sounds and noise picked up by the second microphone;

(c) applying pressure to the artery such that the artery will be forced closed during part of a heart beat cycle and will be forced open during part of the heart beat cycle when the patient'"'"'s blood pressure exceeds the pressure applied to the artery, the opening of the artery causing a blood pressure sound to be produced;

(d) multiplying the first and second microphone signals to generate a microphone signal product;

(e) wave width filtering the microphone signal product to remove therefrom portions of the microphone signal product having a wave width which is greater than a selected maximum wave width value to generate a wave width filtered noise amplitude signal;

(f) comparing the wave width filtered noise amplitude signal to the noise signal threshold level;

(g) increasing the noise signal threshold level by an amount related to the product of a first noise signal threshold level adjustment coefficient and a difference between the noise signal threshold level and the wave width filtered noise amplitude signal if the wave width filtered noise amplitude signal exceeds the noise signal threshold level; and

(h) decreasing the noise signal threshold level by an amount related to the product of a second noise signal threshold level adjustment coefficient and the difference between the noise signal threshold level and the wave width filtered noise amplitude signal if the wave width filtered noise amplitude signal is less than the noise signal threshold level.

21. The method of claim 20 wherein the second microphone is placed on the patient such that a center of the second microphone is separated a distance from a center of the first microphone such that a blood pressure sound picked up by the first microphone will be picked up out of phase by the second microphone.

22. The method of claim 20 comprising additionally the step of selecting portions of the first and second microphone signals having characteristics corresponding to blood pressure sounds before multiplying the first and second microphone signals.

23. The method of claim 22 wherein the step of selecting portions of the first and second microphone signals includes the step of filtering the first and second microphone signals with filters having pass bands corresponding approximately to the frequency range of blood pressure sounds.

24. The method of claim 22 wherein the step of selecting portions of the first and second microphone signals includes the steps of:

(a) gain controlling one of the first and second microphone signals to match the average levels of the first and second microphone signals;

(b) adding the microphone signals together to create a microphone signal sum in which a blood pressure sound component of the two microphone signals is reduced;

(c) least means square filtering the microphone signal sum; and

(d) subtracting the least means square filtered microphone signal sum from each of the first and second microphone signals.

25. The method of claim 20 wherein the selected maximum wave width value is less than a minimum wave width of a blood pressure sound.

26. The method of claim 20 wherein the first noise signal threshold level adjustment coefficient is larger than the second noise signal threshold level adjustment coefficient.

27. The method of claim 20 wherein the step of applying pressure to the artery includes the step of inflating an inflatable cuff wrapped around an arm of the patient, and wherein the first and second microphones are placed on the arm of the patient under the inflatable cuff along an axis of the artery.

28. A method for detecting the blood pressure levels of a patient, comprising the steps of:

(a) placing a first microphone on the patient near the location of an artery, the first microphone producing a first microphone signal in response to blood pressure sounds and noise picked up by the first microphone;

(b) placing a second microphone on the patient near the location of the artery and separated a distance from the first microphone, the second microphone producing a second microphone signal in response to blood pressure sounds and noise picked up by the second microphone;

(c) applying pressure to the artery at a plurality of pressure levels ranging from above a systolic blood pressure level of the patient to below a diastolic blood pressure level of the patient, the artery being forced open during part of the heart beat cycle when the patient'"'"'s blood pressure exceeds the pressure applied to the artery, the opening of the artery causing a blood pressure sound to be produced;

(d) multiplying the first and second microphone signals to generate a microphone signal product;

(e) wave width filtering the microphone signal product to remove therefrom portions of the microphone signal product having a wave width which is less than a selected minimum wave width value to generate a wave width filtered blood pressure sound detection signal;

(f) comparing the wave width filtered blood pressure sound detection signal to a noise signal threshold level;

(g) indicating the detection of a blood pressure sound when the wave width filtered blood pressure sound detection signal exceeds the noise signal threshold level;

(h) repeating the steps of multiplying the first and second microphone signals, wave width filtering the microphone signal product, comparing the wave width filtered blood pressure sound detection signal to the noise signal threshold level, and indicating the detection of a blood pressure sound at each of the plurality of pressure levels; and

(i) displaying a systolic blood pressure level of the patient as a highest of the plurality of pressure levels at which a blood pressure sound is indicated as being detected and displaying a diastolic blood pressure level of the patient as a lowest of the plurality of pressure levels at which a blood pressure sound is indicated as being detected.

29. The method of claim 28 wherein the step of applying pressure to the artery at a plurality of pressure levels includes the step of slowly reducing pressure in an inflatable cuff from above a maximum probable systolic blood pressure level of the patient to a pressure level below the diastolic blood pressure level of the patient.

30. The method of claim 28 wherein the second microphone is placed on the patient such that a center of the second microphone is separated a distance from a center of the first microphone such that a blood pressure sound picked up by the first microphone will be picked up out of phase by the second microphone.

31. The method of claim 28 comprising additionally the step of selecting portions of the first and second microphone signals having characteristics corresponding to blood pressure sounds before multiplying the first and second microphone signals.

32. The method of claim 31 wherein the step of selecting portions of the first and second microphone signals includes the step of filtering the first and second microphone signals with filters having pass bands corresponding approximately to the frequency range of blood pressure sounds.

33. The method of claim 31 wherein the step of selecting portions of the first and second microphone signals includes the steps of:

(a) gain controlling one of the first and second microphone signals to match the average levels of the first and second microphone signals;

(b) adding the microphone signals together to create a microphone signal sum in which a blood pressure sound component of the two microphone signals is reduced;

(c) least means square filtering the microphone signal sum; and

(d) subtracting the least means square filtered microphone signal sum from each of the first and second microphone signals.

34. The method of claim 28 wherein the selected minimum wave width value is less than a minimum wave width of a blood pressure sound.

35. The method of claim 28 comprising additionally the steps of:

(a) wave width filtering the microphone signal product to remove therefrom portions of the microphone signal product having a wave width which is greater than a selected maximum wave width value to generate a wave width filtered noise amplitude signal;

(b) comparing the wave width filtered noise amplitude signal to the noise signal threshold level;

(c) increasing the noise signal threshold level by an amount related to the product of a first noise signal threshold level adjustment coefficient and a difference between the noise signal threshold level and the wave width filtered noise amplitude signal if the wave width filtered noise amplitude signal exceeds the noise signal threshold level; and

(d) decreasing the noise signal threshold level by an amount related to the product of a second noise signal threshold level adjustment coefficient and the difference between the noise signal threshold level and the wave width filtered noise amplitude signal if the wave width filtered noise amplitude signal is less than the noise signal threshold level.

36. The method of claim 35 wherein the selected maximum wave width value is less than a minimum wave width of a blood pressure sound.

37. The method of claim 35 wherein the selected minimum wave width value and the selected maximum wave width value are equal.

38. The method of claim 35 wherein the first noise signal threshold level adjustment coefficient is larger than the second noise signal threshold level adjustment coefficient.

39. The method of claim 35 wherein the step of comparing the wave width filtered blood pressure sound detection signal to the noise signal threshold level is delayed from the steps of increasing or decreasing the noise signal threshold level such that the wave width filtered blood pressure sound detection signal based on the microphone signal product is compared to a noise signal threshold level which is increased or decreased based on a microphone signal product from a future point in time.

40. An apparatus for detecting blood pressure sounds in a patient, comprising:

(a) an inflatable cuff adapted to be wrapped around a limb of a patient;

(b) a first microphone fixed in the inflatable cuff, the first microphone producing a first microphone signal in response to blood pressure sounds and noise picked up by the first microphone;

(c) a second microphone fixed in the inflatable cuff and separated a distance from the first microphone, the second microphone producing a second microphone signal in response to blood pressure sounds and noise picked up by the second microphone;

(d) means for inflating the inflatable cuff to thereby apply pressure to the limb of the patient such that an artery in the patient'"'"'s limb will be forced closed during part of a heart beat cycle and will be forced open during part of the heart beat cycle when the patient'"'"'s blood pressure exceeds the pressure applied to the artery, the opening of the artery causing a blood pressure sound to be produced;

(e) means for multiplying the first and second microphone signals to generate a microphone signal product;

(f) means for wave width filtering the microphone signal product to remove therefrom portions of the microphone signal product having wave width which is less than a selected minimum wave width value to generate a wave width filtered blood pressure sound detection signal;

(g) means for comparing the wave width filtered blood pressure sound detection signal to a noise signal threshold level; and

(h) means for indicating the detection of a blood pressure sound when the wave width filtered blood pressure sound detection signal exceeds the noise signal threshold level.

41. The apparatus for detecting blood pressure sounds of claim 40 wherein the second microphone is fixed in the inflatable cuff such that a center of the second microphone is separated a distance from a center of the first microphone such that a blood pressure sound picked up by the first microphone will be picked up out of phase by the second microphone.

42. The apparatus for detecting blood pressure sounds of claim 40 comprising additionally means for selecting portions of the first and second microphone signals having characteristics corresponding to blood pressure sounds before multiplying the first and second microphone signals.

43. The apparatus for detecting blood pressure sounds of claim 42 wherein the means for selecting portions of the first and second microphone signals includes band pass filters having pass bands corresponding approximately to a frequency range of blood pressure sounds.

44. The apparatus for detecting blood pressure sounds of claim 42 wherein the means for selecting portions of the first and second microphone signals includes:

(a) means for gain controlling one of the first and second microphone signals to match the average levels of the first and second microphone signals;

(b) means for adding the microphone signals together to create a microphone signal sum in which a blood pressure sound component of the two microphone signals is reduced;

(c) means for least means square filtering the microphone signal sum; and

(d) means for subtracting the least means square filtered microphone signal sum from each of the first and second microphone signals.

45. The apparatus for detecting blood pressure sounds of claim 40 wherein the selected minimum wave width value is less than a minimum wave width of a blood pressure sound.

46. The apparatus for detecting blood pressure sounds of claim 40 comprising additionally:

(a) means for wave width filtering the microphone signal product to remove therefrom portions of the microphone signal product having a wave width which is greater than a selected maximum wave width value to generate a wave width filtered noise amplitude signal;

(b) means for comparing the wave width filtered noise amplitude signal to the noise signal threshold level;

(c) means for increasing the noise signal threshold level by an amount related to the product of a first noise signal threshold level adjustment coefficient and a difference between the noise signal threshold level and the wave width filtered noise amplitude signal if the wave width filtered noise amplitude signal exceeds the noise signal threshold level; and

(d) means for decreasing the noise signal threshold level by an amount related to the product of a second noise signal threshold level adjustment coefficient and the difference between the noise signal threshold level and the wave width filtered noise amplitude signal if the wave width filtered noise amplitude signal is less than the noise signal threshold level.

47. The apparatus for detecting blood pressure sounds of claim 46 wherein the selected maximum wave width value is less than a minimum wave width of a blood pressure sound.

48. The apparatus for detecting blood pressure sounds of claim 46 wherein the selected minimum wave width value and the selected maximum wave width value are equal.

49. The apparatus for detecting blood pressure sounds of claim 46 wherein the first noise signal threshold level adjustment coefficient is larger than the second noise signal threshold level adjustment coefficient.

50. The apparatus for detecting blood pressure sounds of claim 40 wherein the means for multiplying the first and second microphone signals, the means for wave width filtering the microphone signal product, the means for comparing the wave width filtered blood pressure sound detection signal to a noise signal threshold level, and the means for indicating the detection of a blood pressure sound include a programmable microprocessor.

51. An apparatus for detecting the blood pressure levels of a patient, comprising:

(a) an inflatable cuff adapted to be wrapped around a limb of the patient;

(b) a first microphone fixed in the inflatable cuff, the first microphone producing a first microphone signal in response to blood pressure sounds and noise picked up by the first microphone;

(c) a second microphone fixed in the inflatable cuff and separated a distance from the first microphone, the second microphone producing a second microphone signal in response to blood pressure sounds and noise picked up by the second microphone;

(d) means for inflating and deflating the inflatable cuff to thereby apply pressure to the limb of the patient at a plurality of pressure levels ranging from above a systolic blood pressure level of the patient to below a diastolic blood pressure level of the patient, an artery in the patient'"'"'s limb being forced open during part of the heart beat cycle when the patient'"'"'s blood pressure exceeds the pressure applied to the limb, the opening of the artery causing a blood pressure sound to be produced;

(e) means for measuring the cuff pressure at each of the plurality of pressure levels;

(f) means for multiplying the first and second microphone signals to generate a microphone signal product;

(g) means for wave width filtering the microphone signal product to remove therefrom portions of the microphone signal product having a wave width which is less than a selected minimum wave width value to generate a wave width filtered blood pressure sound detection signal;

(h) means for comparing the wave width filtered blood pressure sound detection signal to a noise signal threshold level;

(i) means for indicating the detection of a blood pressure sound when the wave width filtered blood pressure sound detection signal exceeds the noise signal threshold level;

(j) means for multiplying the first and second microphone signals, wave width filtering the microphone signal product, comparing the wave width filtered blood pressure sound detection signal to the noise signal threshold level, and indicating the detection of a blood pressure sound at each of the plurality of pressure levels; and

(k) means for displaying a systolic blood pressure level of the patient as a highest of the plurality of pressure levels at which a blood pressure sound is indicated as being detected and displaying a diastolic blood pressure level of the patient as a lowest of the plurality of pressure levels at which a blood pressure sound is indicated as being detected.

52. The apparatus for detecting blood pressure levels of claim 51 comprising additionally:

(a) means for wave width filtering the microphone signal product to remove therefrom portions of the microphone signal product having a wave width which is greater than a selected maximum wave width value to generate a wave width filtered noise amplitude signal;

(b) means for comparing the wave width filtered noise amplitude signal to the noise signal threshold level;

(c) means for increasing the noise signal threshold level by an amount related to the product of a first noise signal threshold level adjustment coefficient and a difference between the noise signal threshold level and the wave width filtered noise amplitude signal if the wave width filtered noise amplitude signal exceeds the noise signal threshold level; and

(d) means for decreasing the noise signal threshold level by an amount related to the product of a second noise signal threshold level adjustment coefficient and the difference between the noise signal threshold level and the wave width filtered noise amplitude signal if the wave width filtered noise amplitude signal is less than the noise signal threshold level.

53. The apparatus for detecting blood pressure levels of claim 52 wherein the means for multiplying the first and second microphone signals, the means for wave width filtering the microphone signal product, the means for comparing the wave width filtered blood pressure sound detection signal to a noise signal threshold level, the means for indicating the detection of a blood pressure sound, the means for increasing the noise signal threshold level, and the means for decreasing the noise signal threshold level include a programmable microprocessor.