Blood Pressure

Hand-held non-invasive blood pressure measurement device

Blood Pressure Abstract
A hand-held non-invasive blood pressure measurement device allows a varying pressure to be applied to an artery while pressure waveforms are sensed to produce pressure waveform data. Waveform parameters are derived from the sensed pressure waveform data. Blood pressure is then determined using the derived parameters. The user is guided or prompted to apply the varying pressure through audible and visual feedback.

Blood Pressure Claims
What is claimed is:

1. A method for determining blood pressure of an artery having a pulse, the method comprising:

manually applying pressure to the artery;

sensing pressure data produced by the artery;

guiding the manually applying of pressure based upon the sensed pressure data;

deriving a plurality of parameters from the sensed pressure data; and

determining a blood pressure value based upon the parameters.

2. The method of claim 1 wherein guiding the manually applying comprises producing a feedback signal which changes as a function of the sensed pressure data.

3. The method of claim 2 wherein the feedback signal is an audible signal.

4. The method of claim 3 wherein the audible signal changes in frequency as a function of pressure waveform amplitude derived from the sensed pressure data.

5. The method of claim 3 wherein the audible signal changes in volume as a function of pressure waveform amplitude derived from the sensed pressure data.

6. A method for determining blood pressure of an artery, the method comprising:

manually applying pressure to the artery so that the artery exhibits a plurality of pressure waveforms;

producing sensed pressure waveform data representing each of the plurality of pressure waveforms;

guiding the manually applying of pressure based upon the sensed pressure waveform data;

deriving a plurality of parameters from the sensed pressure waveform data; and

determining a blood pressure value based upon the parameters.

7. The method of claim 6 wherein guiding the manually applying comprises producing a feedback signal which changes as a function of the sensed pressure data.

8. The method of claim 7 wherein the feedback signal is an audible signal.

9. The method of claim 8 wherein the audible signal changes in frequency as a function of pressure waveform amplitude derived from the sensed pressure data.

10. The method of claim 8 wherein the audible signal changes in volume as a function of pressure waveform amplitude derived from the sensed pressure data.

11. A non-invasive blood pressure measurement device, the measurement device comprising:

manually-operated pressure means for manually applying pressure to the artery so that the artery exhibits pressure data;

sensing means for sensing the pressure data;

signal producing means connected to the sensing means for producing output signals corresponding to the sensed pressure data;

means for guiding a user to manually apply pressure based upon the output signals; and

processing means for receiving the output signals from the signal producing means, for deriving a plurality of parameters using sensed pressures and for determining a blood pressure value using the derived parameters.

12. The measurement device of claim 11, wherein the means for guiding produces a feedback signal which changes as a function of the sensed pressure data.

13. The measurement device of claim 12 wherein the feedback signal is an audible signal.

14. The measurement device of claim 13 wherein the audible signal changes in frequency as a function of pressure waveform amplitude derived from the sensed pressure data.

15. The measurement device of claim 13 wherein the audible signal changes in volume as a function of pressure waveform amplitude derived from the sensed pressure data.

16. The measurement device of claim 11 wherein the manually-operated pressure means comprises:

a housing shaped to be gripped by a user to apply force toward the artery.

17. The measurement device of claim 16 wherein the sensing means includes:

a transducer having a sensing surface;

a flexible diaphragm for being positioned over the underlying artery; and

interface means between the flexible diaphragm and the sensing surface of the transducer for transmitting pressure pulses from the diaphragm to the transducer.

18. The measurement device of claim 17 wherein the flexible diaphragm is mounted on a sensor interface assembly which is pivotally connected to the housing.

19. A non-invasive blood pressure measurement system comprising:

means for manually applying pressure to an artery;

means for sensing pressure from the artery over time while the pressure is applied to the artery to generate pressure data;

means for guiding a user to manually apply the pressure; and

means for deriving a pressure value based upon a waveform analysis of the pressure data.

20. The measurement device of claim 19, wherein the means for guiding produces a feedback signal which changes as a function of the sensed pressure data.

21. The measurement device of claim 19 wherein the feedback signal is an audible signal.

22. The measurement device of claim 19 wherein the audible signal changes in frequency as a function of pressure waveform amplitude denied from the sensed pressure data.

23. The measurement device of claim 19 wherein the audible signal changes in volume as a function of pressure waveform amplitude derived from the sensed pressure data.

24. The measurement device of claim 19 wherein the manually-operated pressure means comprises:

a housing shaped to be gripped by a user to apply force toward the artery.

25. The measurement device of claim 24 wherein the sensing means includes:

a transducer having a sensing surface;

a flexible diaphragm for being positioned over the underlying artery; and

interface means between the flexible diaphragm and the sensing surface of the transducer for transmitting pressure pulses from the diaphragm to the transducer.

26. The measurement device of claim 25 wherein the flexible diaphragm is mounted on a sensor interface assembly which is pivotally attached to the housing.

27. A method of determining blood pressure, the method comprising:

applying pressure to an artery;

sensing pressure over time while the pressure is applied to the artery to generate pressure waveform data for pressure waveforms representing a plurality of beats;

guiding the applying of pressure based upon the pressure waveform data;

detecting onset of the beats from the waveform data;

extracting waveform parameters using a detected onset of one of the beats; and

determining a blood pressure value based upon the waveform parameters.

28. The method of claim 27 wherein guiding the applying comprises producing a feedback signal which varies as a function of the sensed pressure data.

29. A method for determining blood pressure of an artery having a pulse, the method comprising:

applying pressure to the artery;

sensing pressure data produced by the artery;

guiding the applying of pressure based upon the sensed pressure data;

deriving a plurality of different parameters from the sensed pressure data; and

determining a blood pressure value as a function of the plurality of different parameters.

30. The method of claim 29 wherein guiding the applying comprises producing a feedback signal which varies as a function of the sensed pressure data.

31. A method for determining blood pressure of an artery having a pulse, the method comprising:

sensing pressure data produced by the artery while manually applying pressure to the artery;

guiding the manually applying of pressure based upon the sensed pressure data;

deriving parameters from the sensed pressure data; and

determining a blood pressure value based upon the parameters.

32. The method of claim 31 wherein guiding the applying comprises producing a feedback signal which varies as a function of the sensed pressure data.

33. The method of claim 32 wherein the feedback signal is an audible signal.

34. The method of claim 32 wherein the feedback signal is a visual signal.

35. A method for determining blood pressure of an artery having a pulse, the method comprising:

manually applying pressure to the artery;

sensing pressure data produced by the artery;

guiding the manually applying of pressure based upon the sensed pressure data;

deriving a plurality of parameters from the sensed pressure data; and

determining a blood pressure value based upon the plurality of parameters and a stored set of coefficients.

36. The method of claim 35 including:

selecting a set of data from a plurality of beats including a maximum amplitude beat, wherein at least one parameter is derived from the selected set of data.

37. The method of claim 35 wherein deriving a plurality of parameters from the sensed pressure data includes deriving a relative amplitude value and at least one waveform shape parameter.

38. The method of claim 35 wherein deriving a plurality of parameters includes deriving parameters of a curve generated from the sensed pressure data.

39. The method of claim 35 wherein a first pressure is applied to the artery at a starting point of a waveform and wherein a second pressure is applied to the artery at an ending point of the waveform and wherein deriving a plurality of parameters includes adjusting the waveform so that the starting point and the ending point of the waveform have equal pressure amplitudes.

40. The method of claim 35 including:

scaling a pressure waveform to eliminate gain, wherein the parameters are derived from the scaled pressure waveform.

41. The method of claim 35 wherein deriving a plurality of parameters includes deriving at least one parameter selected from a group consisting of:

(a) rise time of a selected portion of a waveform, (b) slope of a portion of a waveform, (c) applied pressure corresponding to a starting point of a waveform, (d) applied pressure corresponding to an ending point of a waveform, (e) pressure of a waveform at a selected time, (f) a pressure corresponding to a selected point on a waveform, (g) a time value corresponding to a width of a selected portion of a waveform, (h) mean amplitude of a waveform, (i) an applied pressure corresponding to a selected point of a waveform, and (j) mean of a curve generated from data taken from a plurality of waveforms.

42. The method of claim 35 wherein deriving a plurality of parameters includes:

deriving at least one parameter other than pressure waveform amplitude from the sensed pressure data.

43. The method of claim 35 wherein the step of applying a varying pressure to the artery includes:

applying an increasing pressure to the artery.

44. The method of claim 35 including:

creating pressure waveform data of pressure waveforms representing a plurality of beats from the sensed pressure data;

detecting onset of the beats from the waveform data; and

deriving at least one parameter using a detected onset of one of the beats.

45. A non-invasive blood pressure monitoring device, the monitoring device comprising:

manually-operated pressure means for applying a pressure to the artery so that the artery exhibits pressure data;

means for guiding a user in applying pressure to the artery with the manually operated pressure means based upon the pressure data;

sensing means for sensing the pressure data;

signal producing means connected to the sensing means for producing output signals corresponding to the sensed pressure data;

storing means for storing a set of coefficients; and

processing means for receiving the output signals from the signal producing means, for deriving a plurality of parameters using sensed pressures and for determining a blood pressure value using the derived parameters and the stored set of coefficients.

46. The measurement device of claim 45, wherein the means for guiding produces a feedback signal which changes as a function of the sensed pressure data.

47. The measurement device of claim 45 wherein the feedback signal is an audible signal.

48. The measurement device of claim 45 wherein the audible signal changes in frequency as a function of pressure waveform amplitude derived from the sensed pressure data.

49. The measurement device of claim 45 wherein the audible signal changes in volume as a function of pressure waveform amplitude derived from the sensed pressure data.

50. The measurement device of claim 45 wherein the manually-operated pressure means comprises:

a housing shaped to be gripped by a user to apply force toward the artery.

51. The measurement device of claim 50 wherein the sensing means includes:

a transducer having a sensing surface;

a flexible diaphragm for being positioned over the underlying artery; and

interface means between the flexible diaphragm and the sensing surface of the transducer for transmitting pressure pulses from the diaphragm to the transducer.

52. The measurement device of claim 51 wherein the flexible diaphragm is mounted on a sensor interface assembly which is pivotally attached to the housing.

53. A method for monitoring pressure waveform data produced by an artery, the method comprising:

manually applying pressure to the artery beginning with an initial pressure and ending with a final non-occluding pressure so that the artery exhibits a plurality of pressure waveforms; and

sensing pressure waveform data produced by the artery representing each of the plurality of pressure waveforms, wherein the final non-occluding pressure applied to the artery is determined based upon the pressure waveform data sensed while pressure is applied to the artery.

54. The method of claim 53 and further comprising:

producing a feedback signal which changes as a function of the sensed pressure data.

55. The method of claim 54 wherein the feedback signal is an audible signal.

56. The method of claim 55 wherein the audible signal changes in frequency as a function of pressure waveform amplitude derived from the sensed pressure data.

57. The method of claim 55 wherein the audible signal changes in volume as a function of pressure waveform amplitude derived from the sensed pressure data.

58. The method of claim 53 and further comprising:

guiding the manually applying of the increasing pressure based upon the pressure waveform data.

59. A method for monitoring blood pressure parameters of an artery, the method comprising:

sensing pressure data produced by the artery over time representing a plurality of arterial pressure waveforms;

determining a maximum pressure amplitude for each of the plurality of arterial pressure waveforms from the sensed pressure data; and

manually applying pressure to the artery while the pressure data is sensed based upon the maximum pressure amplitudes.

60. The method of claim 59 wherein the step of manually applying an increasing pressure to the artery includes:

manually applying the increasing pressure to the artery until at least one waveform has a maximum pressure amplitude less than a maximum pressure amplitude of a preceding waveform.

61. The method of claim 59 and further comprising:

prompting the manually applying of the increasing pressure based upon the pressure data sensed.

62. The method of claim 61 wherein prompting the manually applying comprises producing a feedback signal which changes as a function of the sensed pressure data.

63. The method of claim 62 wherein the feedback signal is an audible signal.

64. The method of claim 63 wherein the audible signal changes in frequency as a function of pressure waveform amplitude derived from the sensed pressure data.

65. The method of claim 63 wherein the audible signal changes in volume as a function of pressure waveform amplitude derived from the sensed pressure data.

66. A method for monitoring blood pressure data produced by an artery, the method comprising:

sensing pressure data produced by the artery over time representing a plurality of arterial pressure waveforms;

determining a maximum pressure amplitude for each of the plurality of arterial pressure waveforms from the sensed pressure data; and

manually applying pressure to the artery until at least one waveform has a maximum pressure amplitude less than a maximum pressure amplitude of a preceding waveform.

67. The method of claim 66 and further comprising:

guiding the manually applying of increasing pressure based upon the sensed pressure data.

68. The method of claim 67 wherein guiding the manually applying comprises producing a feedback signal which changes as a function of the sensed pressure data.

69. The method of claim 68 wherein the feedback signal is an audible signal.

70. The method of claim 69 wherein the audible signal changes in frequency as a function of pressure waveform amplitude derived from the sensed pressure data.

71. The method of claim 69 wherein the audible signal changes in volume as a function of pressure waveform amplitude derived from the sensed pressure data.

72. A method for determining blood pressure of an artery, the method comprising:

positioning a sensor having a constant volume fluid filled sensing chamber over the artery;

manually applying force to the sensor to press the sensor towards the artery;

sensing pressure data produced by the artery by sensing pressure within the constant volume fluid filled chamber to generate pressure waveform data;

guiding the manually applying force based upon the pressure data;

deriving parameters from the pressure data; and

determining a blood pressure value based upon the parameters.

73. The method of claim 72 wherein the step of manually applying force to the sensor includes:

manually applying an increasing force to the sensor.

74. The method of claim 73 wherein guiding the manually applying comprises producing a feedback signal which changes as a function of the sensed pressure data.

75. The method of claim 74 wherein the feedback signal is an audible signal.

76. The method of claim 75 wherein the audible signal changes in frequency as a function of pressure waveform amplitude derived from the sensed pressure data.

77. A non-invasive blood pressure measurement device comprising:

a sensor having a constant volume fluid filled sensing chamber configured for being positioned over an underlying artery;

manual force applying means for applying a force to the sensor to press the sensor against the underlying artery;

sensing means for sensing pressure within the constant volume fluid filled sensing chamber representing pressure data produced by the underlying artery;

prompting means for prompting a user to apply a force to the manual force applying means;

signal producing means connected to the sensing means for producing output signals corresponding to the sensed pressures within the constant volume fluid filled sensing chamber;

storing means for storing a set of coefficients; and

processing means for receiving the output signals from the signal producing means, for deriving a plurality of parameters using the sensed pressures and for determining a blood pressure value using the derived parameters and the stored set of coefficients.

78. A non-invasive blood pressure measurement device comprising:

a sensor having a constant volume fluid filled sensing chamber configured for being positioned over an underlying artery;

means for manually applying force to the sensor to press the sensor against the underlying artery;

sensing means for sensing pressure within the constant volume fluid filled sensing chamber;

means for prompting a user to manually apply force to the sensor based upon the sensed pressure; and

means for deriving a blood pressure value based upon a waveform analysis of the sensed pressure.

79. A method of non-invasive blood pressure measurement, the method comprising:

positioning a pressure sensor over an underlying artery;

prompting a user to apply force to press the sensor against the underlying artery;

deriving a blood pressure value based upon a waveform analysis of sensed pressure data.

80. The method of claim 79 wherein prompting the user is a function of the sensed pressure data.

81. A device for sensing blood pressure within an underlying artery of a patient, the device comprising:

a fluid filled sensing chamber having a diaphragm;

a transducer fluidly coupled to the fluidly filled sensing chamber, wherein the transducer senses fluid pressure within the chamber;

a flexible body conformable wall proximate the sensing chamber and isolated from the sensing chamber for applying force to the artery while preventing pressure in a direction generally parallel to the artery from being applied to the sensing chamber; and

a housing connected to the wall for applying force to apply pressure to the artery.

82. A device for sensing blood pressure pulses within an underlying artery surrounded by tissue as the underlying artery is compressed, the sensor comprising:

a transducer;

a flexible diaphragm for placement above the underlying artery;

a fluid coupling medium between the transducer and the flexible diaphragm, wherein the fluid coupling medium transmits blood pressure pulse signals from the underlying artery to the transducer;

a flexible, variable height, body conforming sidewall isolated from the fluid coupling medium and positioned for engaging tissue proximate to the underlying artery; and

means for manually applying force to the sidewall.

83. A device for external measurements of blood pressure in an underlying artery surrounded by tissue of a patient, the system comprising:

sensing means for sensing blood pressure pulses in the underlying artery;

means for manually applying a variable pressure to the sensing means; and

means for calculating blood pressure based upon a pressure at maximum energy transfer and shape of the sensed pressure pulses within the underlying artery.

84. The device of claim 83 wherein the sensing means includes:

a transducer having a sensing surface;

a flexible diaphragm for being positioned over the underlying artery; and

interface means between the flexible diaphragm and the sensing surface of the transducer for transmitting pressure pulses from the diaphragm to the transducer.

Blood Pressure Description
BACKGROUND OF THE INVENTION

The present invention relates to systems for measuring arterial blood pressure. In particular, the invention relates to a method and apparatus for measuring arterial blood pressure in an non-invasive manner.

Blood pressure has been typically measured by one of four basic methods: invasive, oscillometric, auscultatory and tonometric. The invasive method, otherwise known as an arterial line (A-Line), involves insertion of a needle into the artery. A transducer connected by a fluid column is used to determine exact arterial pressure. With proper instrumentation, systolic, mean and diastolic pressure may be determined. This method is difficult to set up, is expensive and involves medical risks. Set up of the invasive or A-line method poses problems. Resonance often occurs and causes significant errors. Also, if a blood clot forms on the end of the catheter, or the end of the catheter is located against the arterial wall, a large error may result. To eliminate or reduce these errors, the set up must be adjusted frequently. A skilled medical practitioner is required to insert the needle into the artery. This contributes to the expense of this method. Medical complications are also possible, such as infection or nerve damage.

The other methods of measuring blood pressure are non-invasive. The oscillometric method measures the amplitude of pressure oscillations in an inflated cuff. The cuff is placed against a cooperating artery of the patient and thereafter pressurized or inflated to a predetermined amount. The cuff is then deflated slowly and the pressure within the cuff is continually monitored. As the cuff is deflated, the pressure within the cuff exhibits a pressure versus time waveform. The waveform can be separated into two components, a decaying component and an oscillating component. The decaying component represents the mean of the cuff pressure while the oscillating component represents the cardiac cycle. The oscillating component is in the form of an envelope starting at zero when the cuff is inflated to a level beyond the patient's systolic blood pressure and then increasing to a peak value where the mean pressure of the cuff is equal to the patient's mean blood pressure. Once the envelope increases to a peak value, the envelope then decays as the cuff pressure continues to decrease.

Systolic blood pressure, mean blood pressure and diastolic blood pressure values can be obtained from the data obtained by monitoring the pressure within the cuff while the cuff is slowly deflated. The mean blood pressure value is the pressure on the decaying mean of the cuff pressure that corresponds in time to the peak of the envelope. Systolic blood pressure is generally estimated as the pressure on the decaying mean of the cuff prior to the peak of the envelope that corresponds in time to where the amplitude of the envelope is equal to a ratio of the peak amplitude. Generally, systolic blood pressure is the pressure on the decaying mean of the cuff prior to the peak of the envelope where the amplitude of the envelope is 0.57 to 0.45 of the peak amplitude. Similarly, diastolic blood pressure is the pressure on the decaying mean of the cuff after the peak of the envelope that corresponds in time to where the amplitude of the envelope is equal to a ratio of the peak amplitude. Generally, diastolic blood pressure is conventionally estimated as the pressure on the decaying mean of the cuff after the peak where the amplitude of the envelope is equal to 0.82 to 0.74 of the peak amplitude.

The auscultatory method also involves inflation of a cuff placed around a cooperating artery of the patient. Upon inflation of the cuff, the cuff is permitted to deflate. Systolic pressure is indicated when Korotkoff sounds begin to occur as the cuff is deflated. Diastolic pressure is indicated when the Korotkoff sounds become muffled or disappear. The auscultatory method can only be used to determine systolic and diastolic pressures.

Because both the oscillometric and the auscultatory methods require inflation of a cuff, performing frequent measurements is difficult. The frequency of measurement is limited by the time required to comfortably inflate the cuff and the time required to deflate the cuff as measurements are made. Because the cuff is inflated around a relatively large area surrounding the artery, inflation and deflation of the cuff is uncomfortable to the patient. As a result, the oscillometric and auscultatory methods are not suitable for long periods of repetitive use.

Both the oscillometric and auscultatory methods lack accuracy and consistency for determining systolic and diastolic pressure values. The oscillometric method applies an arbitrary ratio to determine systolic and diastolic pressure values. As a result, the oscillometric method does not produce blood pressure values that agree with the more direct and generally more accurate blood pressure values obtained from the A-line method. Furthermore, because the signal from the cuff is very low compared to the mean pressure of the cuff, a small amount of noise can cause a large change in results and result in inaccurate measured blood pressure values. Similarly, the auscultatory method requires a judgment to be made as to when the Korotkoff sounds start and when they stop. This detection is made when the Korotkoff sound is at its very lowest. As a result, the auscultatory method is subject to inaccuracies due to low signal-to-noise ratio.

The fourth method used to determine arterial blood pressure has been tonometry. The tonometric method typically involves a transducer including an array of pressure sensitive elements positioned over a superficial artery. Hold down forces are applied to the transducer so as to flatten the wall of the underlying artery without occluding the artery. The pressure sensitive elements in the array typically have at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured. The transducer is positioned such that at least one of the individual pressure sensitive elements is over at least a portion of the underlying artery. The output from one of the pressure sensitive elements is selected for monitoring blood pressure. The pressure measured by the selected pressure sensitive element is dependent upon the hold down pressure used to press the transducer against the skin of the patient. These tonometric systems measure a reference pressure directly from the wrist and correlate this with arterial pressure. However, because the ratio of pressure outside the artery to the pressure inside the artery, known as gain, must be known and constant, tonometric systems are not reliable. Furthermore, if a patient moves, recalibration of the tonometric system is required because the system may experience a change in gains. Because the accuracy of these tonometric systems depends upon the accurate positioning of the individual pressure sensitive element over the underlying artery, placement of the transducer is critical. Consequently, placement of the transducer with these tonometric systems is time-consuming and prone to error.

The oscillometric, auscultatory and tonometric methods measure and detect blood pressure by sensing force or displacement caused by blood pressure pulses as the underlying artery is compressed or flattened. The blood pressure is sensed by measuring forces exerted by blood pressure pulses in a direction perpendicular to the underlying artery. However, with these methods, the blood pressure pulse also exerts forces parallel to the underlying artery as the blood pressure pulses cross the edges of the sensor which is pressed against the skin overlying the underlying artery of the patient. In particular, with the oscillometric and the auscultatory methods, parallel forces are exerted on the edges or sides of the cuff. With the tonometric method, parallel forces are exerted on the edges of the transducer. These parallel forces exerted upon the sensor by the blood pressure pulses create a pressure gradient across the pressure sensitive elements. This uneven pressure gradient creates at least two different pressures, one pressure at the edge of the pressure sensitive element and a second pressure directly beneath the pressure sensitive element. As a result, the oscillometric, auscultatory and tonometric methods produce inaccurate and inconsistent blood pressure measurements.

SUMMARY OF THE INVENTION

The present invention is an improved method and device for determining blood pressure of an artery having a pulse. As a varying pressure is manually applied to the artery, pressure waveforms are sensed to produce sensed pressure waveform data. The sensed pressure waveform data are then analyzed to derive waveform parameters. One or more blood pressure values are derived based upon the waveform parameters. The manual application of varying pressure is guided or prompted based upon the sensed pressure waveform data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a blood pressure measuring device positioned over the wrist of a patient.

FIG. 2 is an electrical block diagram of the blood pressure measuring device of FIG. 1.

FIG. 3A is a top view of the sensor interface assembly.

FIG. 3B is a cross-sectional view of the sensor interface assembly along section 3B--3B of FIG. 3A.

FIG. 4 is a graph illustrating blood pressure waveforms.

FIG. 5 is a graph illustrating a curve fit from points taken from the waveforms of FIG. 4.

FIG. 6 is a graph illustrating a corrected and scaled waveform taken from the waveforms of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a hand held blood pressure measurement device being used to measure and display blood pressure within an underlying artery within wrist 12 of a patient. With device 10, a small amount of force is manually applied to the radial artery at the projection of the styloid process bone. As the force is manually applied, cardiac pressure waveforms are recorded and the corresponding hold down pressure which is being manually applied is also recorded. Using the pressure shape of the cardiac pressure, waveform parameters are generated. These parameters, along with universal coefficients, are used to calculate pressure values which then can be displayed.

Blood pressure measurement device 10 includes main housing 14, display panel 16, on/off switch 18, display select switch 20, sensor interface assembly 22, connection cable 24, connection plug 26, and mounting stem 28.

Housing 14 contains all of the electrical components of measurement device 10. In the embodiment shown in FIG. 1, housing 14 is generally cylindrical in shape. Its diameter and length allow housing 14 to be easily held by the user (either medical personnel or the patient) during the measurement process. The hold down force is applied by applying force in an axial direction to wrist 12 which is transmitted from housing 14 through stem 28 to sensor interface assembly 22.

Display panel 16 is preferably a liquid crystal display (LCD). In a preferred embodiment, display panel 16 simultaneously displays a pair of values based upon blood pressure measurements. One pair of values represent systolic and diastolic pressure. Another pair of values represent pulse rate and mean blood pressure. Select switch 20 allows the user to select either pair of values to be displayed on display panel 16.

Power switch 18 is actuated to turn on power to the circuitry within housing 14. Timing circuitry within housing 14 automatically turns power off after a predetermined period of inactivity, or switch 18 may be manually toggled to the off state.

Sensor interface assembly 22 is pivotally mounted to a distal end of stem 28. As pressure is manually applied by moving housing 14 toward the artery, that force is transferred from housing 14 through stem 28 to sensor interface assembly 22. Sensor interface assembly 22 is generally similar in construction to the sensor interface assemblies described in the copending patent application entitled Method and Apparatus for Calculating Blood Pressure of an Artery, Ser. No. 08/388,751, filed Feb. 16, 1995, and assigned to Medwave, Inc., the assignee of the present application.

Cable 24 and connection plug 26 connect sensor interface assembly 22 to signal processing circuitry located within housing 14.

In operation, sensor interface assembly 22 is positioned over an artery such as the radial artery (as illustrated in FIG. 1). Alternatively, device 10 can be used in a number of other locations, such as on the temporal artery or the dorsalis pedis artery. The user then begins to apply force to the artery by applying axial force from housing 14 through stem 28 to sensor interface assembly 22. The force applied to the artery is swept in an increasing fashion so that pressure waveform data from a series of pulses are obtained with different amounts of force being applied. To achieve the desired pattern of variable force, user feedback is preferably provided with device 10. In a preferred embodiment, feedback is in the form of audible tones. As pressure is applied, device 10 emits a tone for each cardiac output waveform. Each tone is modulated and has a higher pitch sound as the amplitude of the cardiac waveform increases. By listening to the tone, the user knows at what rate to apply the pressure to the artery. At the point of maximum energy transfer between the artery and sensor interface assembly 22, the cardiac pressure waveform reaches a peak amplitude and, therefore, the highest frequency tone is produced. As the user continues to apply higher pressure to the artery, the amplitude of the cardiac pressure waveform decreases, and therefore the frequency of the tone decreases. By listening to the tone, the user can perform a variable pressure sweep to measure pressure using device 10.

Feedback to the user can be supplied in other ways as well. For example, an audible tone can be produced using a combination of frequency modulation and amplitude modulation. In other words, as the amplitude of the pressure waveform increases, both pitch (frequency) and amplitude (volume or loudness) of the tone will change.

In another embodiment, visual feedback is displayed to the user so that a correct pressure sweep is applied. The visual feedback can be through display panel 16, or through light emitting diodes positioned on housing 14, such as at the end of housing 14 which contains switches 18 and 20.

FIG. 2 is a electrical block diagram of device 10. Pressure transducer 30 within sensor interface assembly 22 is connected by cable 24 and connector 26 to circuitry within housing 12. Power supply circuit 32 includes battery 34, regulator 36, and switch 18. The output of regulator 36 is electrical power which is used to energize the remaining circuitry, which includes amplifier 40, analog to digital (A/D) converter 42, microprocessor 44, speaker 46, display panel 16 and select switch 20. Microprocessor 44 includes digital signal processing circuitry 50, read only memory (ROM) and electrically erasable programmable read only memory (EEPROM) 52, random access memory (RAM) 54, timer circuitry 56, and input/output ports 58.

Transducer 30 senses fluid pressure communicated to transducer 30 within sensor interface assembly 22 and supplies an electrical signal through cable 24 and connection plug 26. In a preferred embodiment, transducer 30 is a piezoresistive pressure transducer.

The output of transducer 30 is an analog electrical signal representative of sensed pressure. That signal is amplified by amplifier 40 and applied to an input of A/D converter 42. The analog signal to A/D convertor 42 is converted to digital data and supplied to the digital signal processing circuitry 50 of microprocessor 44.

Based upon the pressure data received, microprocessor 44 performs calculations to determine blood pressure values. Those calculations will be described in more detail with reference to FIGS. 4-6. As each pulse produces a cardiac waveform, microprocessor 44 determines a peak amplitude of the waveform. Microprocessor 44 drives speaker 46 to produce audible tones which vary as a function of the sensed waveform. The audible tones vary in frequency or amplitude (or both) to guide the user in applying a variable force to the artery.

When a measurement cycle has been completed, microprocessor 44 performs calculations to determine systolic pressure, diastolic pressure, mean blood pressure, and pulse rate. Depending upon the setting of select switch 20, microprocessor 44 provides display control signals to display 16. With one position of select switch 20, systolic and diastolic blood pressure are displayed. With the other position of select switch 20, pulse rate and mean blood pressure are displayed. Additional data may be displayed when select switch 20 is pressed.

FIGS. 3A and 3B illustrate sensor interface assembly 22 (and the distal end of stem 28) in detail. Sensor interface assembly 22 includes top plate 150, upper cup 152, upper capture 154, diaphragm capture 156, inner mounting ring 158, outer mounting ring 160, side wall diaphragm 162, damping ring 164, inner diaphragm 166 and outer diaphragm 168.

Rings 158 and 160 and the upper outer end of side wall diaphragm 162 are mounted in shoulder 204.

Transducer 30 is placed in inset 272 in top plate 150. Transducer outlet 274 connects with fluid passage 208. Bore 276 also communicates with fluid passage 208 and fluid filled chamber 210. Detent 278 allows stem 28 to be snapped in place and removed as required. Multiconductor cable 24 connects transducer 30 to connector 26.

Fluid passage 208 is in fluid communication with sensor interface chamber 210. A fluid coupling medium fills chamber 210, passage 208, which connects to transducer 30. Ball 28 is pivotally mounted in socket 152a. Because socket 152a is adjacent to sensor interface chamber 210, sensor interface assembly 22 is pivotally coupled to stem 28 about a low pivot point. This permits sensor interface assembly 22 to be stably positioned above the underlying artery. In addition, the low pivot point enables the user to apply a more direct, uniform force on diaphragm 168. Thus, the hold down pressure manually applied by the user (through housing 14 and stem 28) is more uniformly applied to the anatomy above the underlying artery.

An outer surface or perimeter of upper capture 154 projects outwardly to form spar 230. Spar 230 partially supports side wall diaphragm 162, which is partially captured between ring 158 and spar 230. In the preferred embodiment, adhesive is used to bond the surfaces together. Other method such as ultrasonic welding or a press fit could be used. Expansion cavity 240 enables upper diaphragm 166 to initially change shape while only experiencing a small change in volume.

Diaphragm capture 156 is a elongated, annular ring including bore 276 and lower lip 252. Bore 276 extends through diaphragm capture 156 and defines a portion of fluid passage 208. Lip 252 projects outwardly from a lower end of diaphragm capture 156. Diaphragm capture 156 fits within bore 232 of upper capture 154 until an inner edge of diaphragm capture 156 is captured between lower lip 252 and the lower end of upper capture 154. Diaphragm capture 156 is preferably adhesively affixed to upper capture 154. Alternatively, diaphragm capture 156 may be press fit within upper capture 154.

Side wall diaphragm 162 and rings 158 and 160 define an annular deformable chamber 260 coupled to ring 164. Side wall diaphragm 162 is preferably formed from a generally circular sheet of flexible material, such as polyurethane, and is partially filled with fluid. Diaphragm 162 has a hole sized to fit around upper portion 234 of upper capture 154. Diaphragm 162 includes outer edge portion 162a and inner edge portion 162b. Outer edge portion 162a is trapped and held between outer ring 160 and top plate 150. Inner edge portion 162b is trapped and supported between ring 158 and spar 230 of upper capture 154. Diaphragm 162 is made from a flexible material and is bulged outward when chamber 260 is partially filled with fluid. Chamber 260 is compressible and expandable in the vertical direction so as to be able to conform to the anatomy of the patient surrounding the underlying artery. As a result, the distance between top plate 150 and the patient's anatomy can vary around the periphery of side wall diaphragm 162 according to the contour of the patient's anatomy. Furthermore, because fluid is permitted to flow through and around chamber 260, pressure is equalized around the patient's anatomy.

Damping ring 164 generally consists of an annular compressible ring and is preferably formed from a foam rubber or other pulse dampening material such as open celled foam or closed cell foam. Ring 164 is centered about and positioned between side wall diaphragm 162 and diaphragms 166 and 168. Damping ring 164 is isolated from the fluid coupling medium within chamber 210. Because ring 164 is formed from a compressible material, ring 164 absorbs and dampens forces in a direction parallel to the underlying artery which are exerted by the blood pressure pulses on sensor interface assembly 22 as the blood pressure pulse crosses sensor interface assembly 22. Because bottom ring 164 is isolated from the fluid coupling medium, the forces absorbed or received by ring 164 cannot be transmitted to the fluid coupling medium. Instead, these forces are transmitted across ring 164 and side wall diaphragm 162 to top plate 150. Because this path is distinct and separate from the fluid coupling medium, chamber 210 and the fluid coupling medium are isolated from these forces. In addition, ring 164 also presses tissue surrounding the artery to neutralize or offset forces exerted by the tissue.

Upper diaphragm 166 is an annular sheet of flexible material having an inner portion 166a, an intermediate portion 166b, an outer portion 166c and an inner diameter sized to fit around diaphragm capture 156. Inner portion 166a is trapped or captured between lip 252 of diaphragm capture 156 and the bottom rim of upper capture 154. Inner portion 166a is preferably adhesively affixed between lip 252 and upper capture 154.

Intermediate portion 166b lies between inner portion 166a and outer portion 166c. Intermediate portion 166b is adjacent to expansion cavity 240 and is isolated from ring 164 and chamber 260. Because intermediate portion 166b is positioned adjacent to expansion cavity 240, intermediate portion 166b is permitted to initially move upward into expansion cavity 240 as chamber 260, ring 164 and outer diaphragm 168 conform to the anatomy of the patient surrounding the underlying artery while chamber 260 experiences only a small change in volume. As ring 164 is pressed against the anatomy of the patient surrounding the artery to neutralize or offset forces exerted by the tissue, diaphragm 168 is also compressed. However, because intermediate portion 166b is permitted to roll into expansion cavity 240, chamber 210 does not experience a large volume decrease and a large corresponding pressure increase. Thus, sensor interface assembly 22 permits greater force to be applied to the anatomy of the patient through ring 164 to neutralize tissue surrounding the artery without causing a corresponding large change in pressure within chamber 210 as the height of the side wall changes. As a result, sensor interface assembly 22 achieves more consistent and accurate blood pressure measurements.

Outer diaphragm 168 is a generally circular sheet of flexible material capable of transmitting forces from an outer surface to fluid within chamber 210. Outer diaphragm 168 is coupled to inner diaphragm 166 and is configured for being positioned over the anatomy of the patient above the underlying artery. Outer diaphragm sheet 168 includes non-active portion or skirt 168a and active portion 168b. Skirt 168a constitutes the area of diaphragm 168 where inner diaphragm 166, namely outer portion 166c, is heat sealed or bonded to outer diaphragm 168. Skirt 168a and outer portion 166c are generally two heat sealed or bonded sheets of flexible material, forces parallel to the underlying artery are transmitted across skirt 168a and outer portion 166c and are dampened by the compressible material of ring 164.

Active portion 168b is constituted by the portion of outer diaphragm sheet 168 which is not bonded to inner diaphragm 166. Active portion 168b is positioned below and within the inner diameter of ring 164. Active portion 168b is the active area of sensor interface assembly 22 which receives and transmits pulse pressure to transducer 30. Active portion 168b of diaphragm 168, intermediate portion 166b of diaphragm 166 and diaphragm capture 156 define sensor interface chamber 210.

The coupling medium within chamber 210 may consist of any fluid (gas or liquid) capable of transmitting pressure from diaphragm 168 to transducer 30. The fluid coupling medium interfaces between active portion 168b of diaphragm 168 and transducer 30 to transmit blood pressure pulses to transducer 30. Because the fluid coupling medium is contained within sensor interface chamber 210, which is isolated from the side wall of sensor interface assembly 22, the fluid coupling medium does not transmit blood pressure pulses parallel to the underlying artery, forces from the tissue surrounding the underlying artery and other forces absorbed by the side wall to transducer 30. As a result, sensor interface assembly 22 more accurately measures and detects arterial blood pressure.

Sensor interface assembly 22 provides external measurements of blood pressure in an underlying artery. Because sensor interface assembly 22 senses blood pressure non-invasively, blood pressure is measured at a lower cost and without medical risks. Because sensor interface assembly 22 is relatively small compared to the larger cuffs used with oscillometric and auscultatory methods, sensor interface assembly 22 applies a hold down pressure to only a relatively small area above the underlying artery of the patient. Consequently, blood pressure measurements may be taken with less discomfort to the patient. Because sensor interface assembly 22 does not require inflation or deflation, faster, more frequent measurements may be taken.

Furthermore, sensor interface assembly 22 better conforms to the anatomy of the patient so as to be more comfortable to the patient and so as to achieve more consistent and accurate blood pressure measurements. Because chamber 260 is deformable and partially filled with fluid, chamber 260 better conforms to the anatomy of the patient and equalizes pressure applied to the patient's anatomy. Because ring 164 is compressible and because diaphragm 168 is flexible and is permitted to bow or deform inwardly, ring 164 and diaphragm 168 also better conform to the anatomy of the patient. At the same time, however, sensor interface assembly 22 does