Pulse rate sensor system - Patent Review 5243992

Claims

What is claimed is:

1. An apparatus for noninvasive measurement of pulse rate comprising:

a sensor, wherein said sensor senses at least one pressure or force and produces at least one sensor element signal indicative of said pressure or force;

a gimbal device operably connected to said sensor, said gimbal device having at least one plate member and at least two axes perpendicular to each other and parallel to said at least one plate member, wherein said sensor is rotatable about said at least two axes by a predetermined angle;

a spring element operatively connected at a first end to said gimbal device, said spring element pressing said sensor against a radial artery of a subject;

a case, operatively connected to a second end of said spring element;

a flexible printed circuit board operatively connected at a first end to said sensor;

a central processing unit located in said case and operatively connected to a second end of said flexible printed circuit, said central processing unit determining a pulse rate based on said at least one sensor element signal received from said sensor;

a display located in said case connected to said central processing unit, said display displaying said pulse rate; and playing said pulse rate; and

a strap operatively connected to said case on at least two sides for holding said case on said subject, said strap at no time contacting said spring element.

2. The apparatus of claim 1, wherein said predetermined angle is about 20 degrees.

3. The apparatus of claim 1, wherein said sensor is a tonometer sensor having a plurality of pressure sensing elements disposed in an array for sensing a blood pressure on at least one of said pressure sensing elements and producing a plurality of sensor element signals, at least one of said sensor element signals being indicative of said blood pressure.

4. The apparatus of claim 1, wherein said strap further comprises:

a first side having a first end operatively connected to said case;

a protective band having a first end operatively connected to said case;

a buckle operatively connected to a second end of said protective band, said buckle being operable to grip a second end of said first side to hold said case on said subject.

5. The apparatus of claim 4, wherein said protective band further comprises:

a first and a second mounting pad located at said first and second ends of said protective band, respectively;

a first and a second edge operatively connected to said first and second mounting pads, said first and second edges being disposed on opposite sides of said spring element while said case is held on said subject by said strap.

6. The apparatus of claim 1, wherein said spring element further comprises:

a sensor position adjustment element operatively connected between said gimbal element and said first end of said spring element, and means for slidably mounting said gimbal element with respect to said sensor position adjustment element.

7. An apparatus for noninvasive measurement of pulse rate comprising:

a sensor wherein said sensor senses at least one pressure and produces at least one sensor element signal indicative of said pressure;

a case;

a spring element coupled at a first end to said sensor and at a second end to said case, said spring element pressing said sensor against a subject near a radial artery;

a flexible printed circuit board having a plurality of conductors operatively connected at a first end to said sensor;

a central processing unit located in said case and operatively connected to a second end of said flexible printed circuit board, said central processing unit determining a pulse rate based on said at least one sensor element signal received from said sensor;

a display located in said case and operatively connected to said central processing unit, said display displaying said pulse rate; and

a strap operatively connected to said case on at least two sides for holding said case on a subject.

8. The apparatus of claim 7, wherein said spring element includes:

a gimbal device operably connected to said sensor, said gimbal device having at least one plate member and at least two axes perpendicular to each other and parallel to said at least one plate member, wherein said sensor is rotatable about said at least two axes by a predetermined angle; and

a spring operatively connected at a first end to said gimbal device and at a second end to said case, said spring pressing said sensor against said subject near a radial artery.

9. The apparatus of claim 8, wherein said predetermined angle is about 20 degrees.

10. The apparatus of claim 8, wherein said spring element further comprises:

a sensor position adjustment element operatively connected between said gimbal element and said first end of said spring element, and means for slidably mounting said gimbal element with respect to said sensor position adjustment element.

11. The apparatus of claim 8, wherein said spring has predetermined radius of curvature.

12. The apparatus of claim 7, wherein said sensor is a tonometer sensor having a plurality of pressure sensing elements disposed in an array for sensing a blood pressure on at least one of said pressure sensing elements and producing a plurality of sensor element signals, at least one of said sensor element signals being indicative of said blood pressure.

13. The apparatus of claim 7, wherein said strap further comprises:

a first side having a first end operatively connected to said case;

a protective band having a first end operatively connected to said case;

a buckle operatively connected to a second end of said protective band, said buckle being operable to grip a second end of said first side to hold said case on said subject.

14. The apparatus of claim 13, wherein said protective band further comprises:

a first and a second mounting pad located at said first and second ends of said protective band, respectively;

a first and a second edge operatively connected to said first and second mounting pads, said first and second edges being disposed on opposite sides of said spring element while said case is held on said subject by said strap.

15. An apparatus for noninvasive measurement of pulse rate comprising:

a sensor wherein said sensor senses at least one pressure or force and produces at least one sensor element signal indicative of said pressure or force;

a gimbal device operably connected to said sensor, said gimbal device having at least one plate member and at least two axes perpendicular to each other and parallel to said at least one plate member, wherein said sensor is rotatable about said at least two axes by a predetermined angle;

a spring element operatively connected at a first end to said gimbal device, said spring element pressing said sensor against a subject near a radial artery;

a case, operatively connected to a second end of said spring element;

a central processing unit located in said case and operatively connected to said sensor, said central processing unit determining a pulse rate based on said at least one sensor element signal received from said sensor;

a display located in said case and operatively connected to said central processing unit, said display displaying said pulse rate; and

a strap operatively connected to said case on at least two sides for holding said case on said subject, said strap at no time contacting said spring element.

16. The apparatus of claim 15, further comprising:

a flexible printed circuit operatively connected at a first end to said sensor and at a second end to said central processing unit and having at least one conducting path for conducting said at least one sensor element signal to said central processing unit.

17. The apparatus of claim 15, wherein said predetermined angle is about 20 degrees.

18. The apparatus of claim 15, wherein said sensor is a tonometer sensor having a plurality of pressure sensing elements disposed in an array for sensing a blood pressure on at least one of said pressure sensing elements and producing a plurality of sensor element signals, at least one of said sensor element signals being indicative of said blood pressure.

19. The apparatus of claim 15, wherein said strap further comprises:

a first side having a first end operatively connected to said case;

a protective band having a first end operatively connected to said case;

a buckle operatively connected to a second end of said protective band, said buckle being operable to grip a second end of said first side to hold said case on said subject.

20. The apparatus of claim 19, wherein said protective band further comprises:

a first and a second mounting pad located at said first and second ends of said protective band, respectively;

a first and a second edge operatively connected to said first and second mounting pads, said first and second edges being disposed on opposite sides of said spring element while said case is held on said subject by said strap.

21. The apparatus of claim 15, wherein said spring element further comprises:

a sensor position adjustment element operatively connected between said gimbal element and said first end of said spring element, and means for slidably mounting said gimbal element with respect to said sensor position adjustment element.

22. An apparatus for noninvasive measurement of pulse rate comprising:

a sensor means for sensing at least one pressure or force and producing at least one sensor element signal indicative of said pressure or force;

means for pivoting said sensor means about an axis;

means for pressing said sensor means against a subject near a radial artery;

means for anchoring said pressing means;

central processing means for determining a pulse rate based on said at least one sensor element signal received from said sensor means;

means for displaying said pulse rate; and

means for holding said anchoring means on said subject, said holding means at no time contacting said pressing means.

23. The apparatus of claim 22, wherein said pivoting means allows said sensor to pivot through a predetermined angle of about 20 degrees.

24. The apparatus of claim 22, wherein said sensor means is a tonometer sensor means having a plurality of pressure sensing elements disposed in an array for sensing a blood pressure on at least one of said pressure sensing elements and for producing a plurality of sensor element signals, at least one of said sensor element signals being indicative of said blood pressure.

25. The apparatus of claim 22, wherein said holding means further comprises means for protecting said sensor means and said pressing means.

26. The apparatus of claim 22, wherein said pressing means further comprises means for operatively connecting said pivoting means and said pressing means, wherein said connecting means adjusts at least one characteristic of said pressing means.

27. The apparatus of claim 22, wherein said central processing means further comprises:

means for determining a plurality of average pulse rates based on said sensor element signal;

means for computing a value corresponding to an autocorrelation of said sensor element signal over a predetermined time period; and

means for calculating a display pulse rate based on at least two of said average pulse rates, each of said at least two pulse rates having a corresponding said value within a predetermined range.

28. The apparatus of claim 22, wherein said central processing means further comprises:

means for calculating a plurality of slopes based on said sensor element signal;

means for determining a plurality of average pulse rates based on said sensor element signal having corresponding said slopes greater than a predetermined slope threshold;

means for computing a value corresponding to an autocorrelation of said sensor element signals over a predetermined time period; and

means for calculating a display pulse rate based on at least two of said average pulse rates, each of said at least two pulse rates having a corresponding said value within a predetermined range.

29. An apparatus for noninvasive measurement of pulse rate comprising:

a tonometer sensor means having a plurality of pressure sensing elements disposed in an array, for sensing blood pressure on at least one of said pressure sensing elements and producing a plurality of sensor element signals, at least one of said sensor element signals being indicative of said blood pressure;

means for correcting said sensor element signals using a correction factor based on at least one characteristic of said sensor element signals;

means for determining a plurality of average pulse rates based on said corrected sensor element signals;

means for computing a value corresponding to an autocorrelation of said corrected sensor element signals over a predetermined time period;

means for calculating a display pulse rate based on at least two of said average pulse rates, each of said at least two pulse rates having a corresponding said value within a predetermined range; and

means for generating an output signal corresponding to said calculated display pulse rate.

30. An apparatus for noninvasive measurement of a cardiopulmonary parameter comprising:

means for sensing a physiological parameter response to blood pressure;

means for producing at least one sensor element signal indicative of said physiological parameter;

means for determining a plurality of average cardiopulmonary parameters based on said at least one sensor element signal;

means for determining a plurality of valid average cardiopulmonary parameters;

means for calculating a display cardiopulmonary parameter based on at least two of said valid average cardiopulmonary parameters; and

means for generating an output signal corresponding to said display cardiopulmonary parameter.

31. The apparatus of claim 30, wherein said means for sensing comprises a tonometer sensor means.

32. The apparatus of claim 30, wherein said means for determining further comprises:

means for computing a value corresponding to an autocorrelation of said at least one sensor element signal;

means for identifying said cardiopulmonary parameters having a corresponding said value within a predetermined range.

33. An apparatus for noninvasive measurement of a cardiopulmonary parameter comprising:

a sensor means for sensing at least one cardiopulmonary parameter responsive to blood pressure and producing at least one sensor element signal indicative of said blood pressure;

means for pivoting said sensor means about an axis;

means for pressing said sensor means against a radial artery of a subject;

means for anchoring said pressing means;

central processing means for determining a cardiopulmonary parameter based on said at least one sensor element signal received from said sensor means;

means for displaying said cardiopulmonary parameter;

means for holding said anchoring means on said subject, said holding means at no time contacting said pressing means.

34. The apparatus of claim 33, wherein said pivoting means allows said sensor means to pivot through a predetermined angle of about 20 degrees.

35. The apparatus of claim 33, wherein said holding means further comprises means for protecting said sensor means and said pressing means.

36. The apparatus of claim 33, wherein said pressing means further comprises means for operatively connecting said pivoting means and said pressing means, wherein said connecting means adjusts at least one characteristic of said pressing means.

FIELD OF THE INVENTION

The present invention relates to pulse monitors with visual readouts of pulse rate and more particularly to a tonometer sensor pulse rate monitor which employs multiple noise and motion artifact rejection methods to determine an accurate pulse rate.

BACKGROUND OF THE INVENTION

The present invention relates generally to a method and apparatus for measuring and displaying pulse rate with increased accuracy. More specifically, the present invention provides a method for increasing the accuracy of a pulse rate sensing system by means of a novel pressure sensing array and multiple methods for identification and elimination of artifacts.

Other methods and apparatus are known for measuring pulse rates and for rejecting pulse artifacts. For example, U.S. Pat. No. 4,409,983 shows a pulse measuring device which employs multiple transducers connected to averaging circuits and differential amplifiers. This invention helps separate signals corresponding to motion artifacts from the signal corresponding to a heartbeat pulse. Other apparatus and methods for removing motion artifacts are disclosed in U.S. Pat. Nos. 4,307,728, 4,202,350, 4,667,680, 4,239,048, 4,181,134 and 4,456,959. Methods used to reduce signal errors include the use of windowing and averaging techniques and auto correlation algorithms.

The pulse rate sensor systems described above are subject to several sources of inaccuracies. First, it is difficult to reject motion and noise artifacts in many of these systems. This is especially true for systems employing a single sensor element. (See U.S. Pat. Nos. 4,202,350 and 4,239,048.) These systems have no physical means for receiving both a pulse-plus-artifact signal and a separate artifact signal. Other means are required to compensate for, or eliminate, the error caused by artifacts such as motion artifacts. Signal processing techniques such as filtering and windowing are often used.

Even those systems or methods which employ multiple sensor elements inaccurately measure pulse rate because only a single method is used for enhanced signal processing. For example, different types of motion artifacts can occur simultaneously, and with other pertubations, on the pulse sensor. It is also not unusual for signal errors to be interpreted as pulses or for actual pulses to be missed by the pulse sensor. Methods of pulse rate determination which do not compensate for these errors are inherently inaccurate under real-world conditions ere artifacts are present.

For example, if a pulse rate system detects a "pulse" caused by noise, several adverse results may be seen. The pulse rate system could use the noise as the basis for windowing the signal. The pulse rate system could simply use this "pulse" as part of the overall pulse rate calculation. In addition, the pulse rate system could recognize the noise as noise and subtract out the noise, in some cases subtracting out a valid signal as well.

Another source of inaccuracy that occurs using pulse measuring devices that measure pressure variations caused by a subject's pulse (see U.S. Pat. No. 4,409,983 for example) is inverted pulse waveforms. An inverted waveform can occur when the housing that holds the pressure sensitive element(s) is located on the artery, but the pressure sensitive element(s) itself is located off the artery. In this case the subject's pulse can push up on the housing and lessen the pressure on the pressure sensitive element. The result is that a pulse waveform is still received, but it is inverted and shows a negative relative pressure. Pulse measuring devices which rely on pressure measurements but can correctly interpret only positive pressure waveforms must be placed and held accurately on the artery, creating additional demands on attachment of the device and/or lowering comfort to the user.

Additionally, pronounced dichrotic notches can be found in the pulse of many people. When dichrotic notches are present there are two rises and two falls in blood pressure during a single heartbeat. These can be mistakenly interpreted as two heartbeats, leading to a major inaccuracy in pulse rate measurement.

The present invention overcomes the problems encountered with other pulse rate sensors by applying the principles of arterial tonometry for signal acquisition for a pulse rate sensor. In the invention, multiple algorithms are used in signal processing and pulse rate calculation to compensate for multiple signal errors which could occur during pulse rate measurement.

The principles of arterial tonometry are described in several U.S. Patents including: U.S. Pat. Nos. 3,219,035; 4,799,491 and 4,802,488. These principles are also described in several publications including an article entitled "Tonometry, Arterial," in Volume 4 of the Encyclopedia of Medical Devices and Instruments. (J. G. Webster, Editor, John Wiley & Sons, 1988). All of these references discuss arterial tonometry as used for the measurement of blood pressure.

For blood pressure measurement, it is desirable to flatten a section of the arterial wall as described in these references. Flattening is produced by exerting an appropriate hold down force on the tonometer sensor. For pulse sensing, significant flattening of the arterial wall is not necessary and a lower hold down force can be used. This results in greater comfort for the wearer.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been developed to overcome the foregoing shortcomings of existing pulse rate sensor systems.

It is therefore an object of the present invention to provide a method and an apparatus for measuring pulse rates using arterial tonometry techniques including a sensor array with multiple sensing elements disposed in an array, in order to provide increased accuracy in the determination of pulse rate.

Another object of the present invention is to increase the accuracy of the displayed pulse rate by calculating the displayed pulse rate using only pulse rates determined to be valid.

A further object of the present invention is to determine whether pulses detected are valid, based on the correlation between the present pulse and the previous pulse.

A still further object of the present invention is to remove motion artifacts from the sensor element signals by subtracting a value from all these signals based on a spatially weighted average of these signals.

An additional object of the present invention is to cancel out artifacts from a sensor element which exceed a level predetermined to be the maximum level of a valid blood pressure signal.

Still another object of the present invention is to accurately process inverted waveforms caused by misalignment or shifting of the sensor elements relative to an underlying artery.

These and other objects and advantages are achieved in accordance with the present invention by the steps of: sensing at least one blood pressure waveform signal at a predetermined sampling period using a tonometer sensor having a plurality of sensor elements disposed in an array; producing a plurality of sensor element signals, at least one of the sensor element signals corresponding to the at least one blood pressure signal; correcting the sensor element signals using a correction factor based on one characteristic of the sensor element signals; calculating a plurality of slopes based on the corrected sensor element signals; selecting a corrected sensor element signal corresponding to one of the sensor elements, the selected sensor element signal having slopes greater than a predetermined slope threshold; determining a plurality of pulse rates based on the selected sensor element signal; computing a value corresponding to the autocorrelation of the corrected sensor element signal over a predetermined time period; and calculating a display pulse rate based on at least two of the pulse rates, each of the two pulse rates having the value within a predetermined range.

These and other objects and advantages are achieved in accordance with the preferred embodiment of the present invention comprising: a tonometer sensor means having a plurality of pressure sensing elements disposed in an array, for sensing a blood pressure waveform on at least one of the pressure sensing elements and producing a plurality of sensor element signals, at least one of the sensor element signals being indicative of the blood pressure acting on at least one of the pressure sensing elements; means for pivoting the tonometer sensor means about a pair of axes; means for pressing the tonometer sensor means against a radial artery of a subject; means for anchoring the pressing means on a dorsal side of the subject; central processing means for determining a pulse rate based on at least one of the sensor element signals received from the tonometer sensor means; means for displaying the pulse rate; and means for holding the anchoring means on the subject, the holding means at no time contacting the pressing means.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments are described with reference to the drawings in which:

FIG. 1 is a general arrangement of a pulse rate sensor system illustrating a pulse rate sensor connected to a case assembly;

FIG. 2 is a perspective view of the tonometer sensor supported in a gimbal assembly;

FIGS. 3A, B and C show low, medium and high curvature spring profiles, respectively.

FIG. 4 is a block diagram of the pulse rate processing circuitry in accordance with the preferred embodiment of the present invention;

FIGS. 5A, B, C and D are a flowchart describing a pulse rate measurement and compensation method in accordance with one embodiment of the present invention;

FIG. 6 is a table illustrating the effect of differential enhancement on raw sensor element signal levels;

FIG. 7 is a table illustrating the calculations of the correlation algorithm; and

FIGS. 8A and B are graphical representations of normal and inverted blood pressure waveforms, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a pulse rate monitor 1 in accordance with the preferred embodiment of the present invention is shown having a tonometer sensor 2 and a case 3 housing processing and display circuitry. The tonometer sensor 2 is mounted in a gimbal assembly 9 which in turn is connected to case 3 by a spring 4. Spring 4 includes a sensor position adjustment 5 section at the connection point between gimbal assembly 9 and spring 4.

Further details of this arrangement are shown in FIG. 2, which uses the same component designations found in FIG. 1, where possible. The tonometer sensor 2 is attached to a sensor adapter 12 which, in turn is pinned to the gimbal assembly 9 at axis number 2. A spring mounting pad 11 is pinned along axis number 1 of gimbal assembly 9, mounting pad 11 being the point at which sensor position adjustment 5 connects to gimbal assembly 9. Flexible printed circuit 10 connects the tonometer sensor 2 to circuitry (shown in FIG. 4) in case 3.

Tonometer sensor 2 is an array of pressure or force sensitive elements fabricated into a single structure. Standard photolithographic manufacturing techniques can be used to construct the tonometer sensor. Experimental testing indicates that about 3 to 6 individual sensor elements are necessary for good accuracy in pulse rate measurement but even a single sensor element adapted for use with the method and apparatus of the present invention will produce more accurate pulse rate determinations.

Referring again to FIG. 1., the remainder of the pulse rate monitor 1 will be described in terms of wearing the pulse rate monitor 1. In the preferred embodiment, the pulse rate monitor is worn on the operator's wrist like a wrist watch. When the wearer dons the pulse rate monitor 1, the tonometer sensor 2 is positioned above a radial artery and the case 3 is positioned on the opposite side of the wrist. The case 3, which anchors one end of spring 4, is held in place by strap 8 by cinching an flexible portion 8a of a strap 8 and locking the strap in position by means of a buckle 7. Strap 8 is prevented from directly contacting the tonometer sensor 2, the gimbal assembly 9 and the spring 4 by a protective band 6 which is part of the strap 8. Protective band 6 has a box shaped cutout section which allows it to fit around the tonometer sensor 2, the gimbal assembly 9 and the spring 4, without contacting any of these elements.