Method of determining which portion of a stress sensor is best positioned for use in determining intra-arterial blood pressure

What is claimed is:

1. For use in a non-invasive blood pressure monitoring system, a method of determining which portion of a stress sensitive element of a tissue stress sensor is best located for detecting the stress of tissue overlying an artery of interest, said stress sensitive element having a length that exceeds a lumen of said artery of interest, said method including the steps of:

(A) placing said stress sensitive element of said tissue stress sensor in communication with said tissue overlying said artery of interest,

and orienting said tissue stress sensitive element such that said tissue stress sensitive element spans beyond the lumen of said artery of interest;

(B) obtaining, from said tissue stress sensor, at least one electrical signal representing stress data across said length of said stress sensitive element, said stress data including a plurality of stress datum, each stress datum of said stress data representing stress communicated to a predetermined portion of said stress sensitive element from said tissue overlying said artery of interest, each said predetermined portion of said stress sensitive element lying along said length of said stress sensitive element;

(C) computing from said stress data, a centroid of energy associated with said stress sensitive element; and

(D) using said centroid of energy to determine which portion of said stress sensitive element is best located for determining the blood pressure within said artery of interest.

2. The method of claim 1 wherein, said stress data includes data corresponding to a systolic blood pressure within said artery of interest.

3. The method of claim 1 wherein, said stress data includes data corresponding to a diastolic blood pressure within said artery of interest.

4. The method of claim 1 wherein, said stress data includes data corresponding to a pulsatile blood pressure within said artery of interest.

5. The method of claim 1 wherein, said stress data includes data corresponding to a mean blood pressure within said artery of interest.

6. The method of claim 1, further including the step of:

(E) using each said stress datum value to calculate a corresponding energy value, each said energy value being associated with one of said predetermined portions of said stress sensitive element, and determining which one of said energy values is a maximum, and wherein step (C) further includes the sub-step of,

calculating said centroid of energy using only said stress datum values which have an energy value, as computed in step (E), which exceeds a predetermined percentage of said maximum energy value.

7. The method of claim 6, wherein said predetermined percentage is determined by assigning it a value equal to an empirically established percentage value that indicates a correlation between said centroid of energy and a center of said artery of interest.

8. The method of claim 1, wherein step (C) further includes the sub-steps of:

(i) using each said stress datum value to calculate a corresponding energy value, each said energy value being associated with one of said predetermined portions of said stress sensitive element;

(ii) ordering said energy values according to their respective magnitudes; and

(iii) calculating said centroid of energy by using only said stress datum values associated with a first n energy values of highest magnitude as ordered in sub-step 8(ii).

9. The method of claim 8, wherein n is determined by the sub-steps of:

(i) associating each said energy value ordered in sub-step 8(ii) with a predetermined segment length along the length of said stress sensitive element;

(ii) selecting said energy values of greatest magnitude as ordered in sub-step 8(ii), and totaling the predetermined segment lengths associated with all said selected energy values; and

(iii) setting n equal to the number of energy values selected when the cumulative predetermined segment lengths as totalized in sub-step 9(ii) exceed a predetermined percentage of said length of said stress sensitive element.

10. The method of claim 9, wherein said predetermined percentage is determined by assigning it a value equal to an empirically established percentage value that indicates a correlation between said centroid of energy and a center of said artery of interest.

11. The method of claim 1, wherein step (C) further includes:

using each said stress datum value obtained in step (B) to calculate a corresponding energy value, each one of said energy values being associated with a predetermined portion of said stress sensitive element, and

generating a weighted energy value by attaching a weighting factor to each one of said energy values, and

calculating said centroid of energy, using said weighted energy values.

12. The method of claim 11, wherein step (C) further includes the steps of:

determining which one of said weighted energy values is a maximum, and

calculating said centroid of energy, using only said stress datum values which have a corresponding weighted energy value which exceeds a predetermined percentage of said maximum weighted energy value.

13. The method of claim 11, wherein step (C) further includes:

(i) ordering said weighted energy values according to their respective magnitudes; and

(ii) calculating said centroid of energy by using only said stress datum values associated with a first n weighted energy values of highest magnitude as ordered in sub-step 13(i).

14. The method of claim 13, wherein n is determined by the sub-steps of:

(i) selecting weighted energy values of greatest magnitude as ordered in sub-step 13(i) and totaling a cumulative length of each said predetermined segment associated with all said selected weighted energy values; and

(ii) setting n equal to a value equal to a total number of weighted energy values selected when the cumulative length as totalized in sub-step 14(i) equals said predetermined percentage of said length of said stress sensitive element.

15. The method of claim 11, wherein said centroid of energy is conducted as follows: ##EQU12## where: X=centroid of energy;

x=location along the length of the stress sensitive element;

E(x)=stress energy at location x;

F[E(x)]=weighted function of stress energy; and

b,c=limits of integration in the range of zero to L, where L is the length of the stress sensitive element.

16. The method of claim 15, wherein said stress energy E(x) is computed as follows:

E(x)=(.sigma.(x)).sup.2

where:

.sigma.(x)=stress datum sensed by stress sensitive element at location x.

17. The method of claim 16, wherein said weighted function of stress energy F[E(x)] is computed as follows:

F[E(x)]=[E(x)].sup.N

where:

F[E(x)]=weighted function of stress energy;

N=exponent of predetermined value.

18. The method of claim 17, wherein said stress datum .sigma.(x), includes datum corresponding to at least one of a diastolic blood pressure (x), a systolic blood pressure (x), a pulsatile blood pressure (x), and a mean blood pressure (x) within said artery of interest.

19. The method of claim 1, wherein step (C) includes computing said centroid of energy as follows: ##EQU13## where: X=centroid of energy;

x=location along the length of the stress sensitive element;

E(x)=stress energy at location x; and

b,c=limits of integration in the range of zero to L, where L is the length of the stress sensitive element.

20. The method of claim 19, wherein said stress energy E(x) is computed as follows:

E(x)=(.sigma.(x)).sup.2

where:

.sigma.(x)=stress datum sensed by stress sensitive element at location x.

21. The method of claim 20, wherein said stress datum .sigma.(x), includes datum corresponding to at least one of a diastolic blood pressure (x), a systolic blood pressure (x), a pulsatile blood pressure (x), and a mean blood pressure (x) within said artery of interest.

22. For use in a non-invasive blood pressure monitoring system, a method of determining which portion of a stress sensitive element of a tissue stress sensor is best located along a length of said stress sensitive element for detecting a stress of tissue overlying an artery of interest, said length of said stress sensitive element exceeding the lumen of said artery of interest, said method including the steps of:

(A) placing said stress sensitive element of said tissue stress sensor in communication with said tissue overlying said artery of interest, and

orienting said stress sensitive element such that said stress sensitive element spans beyond the lumen of said artery of interest;

(B) causing said stress sensitive element to act against said tissue overlying said artery of interest thereby causing in said artery, a first artery applanation state, and

obtaining a first artery applanation state index;

(C) obtaining, during said first artery applanation state from said tissue stress sensor, at least one electrical signal representing a first set of stress data across said length of said stress sensitive element, said first set of stress data representing a plurality of stress datum, each said stress datum representing stress communicated to one of a plurality of portions of said stress sensitive element from said tissue overlying said artery of interest, said portions of said stress sensitive element lying along said length of said stress sensitive element;

(D) causing said stress sensitive element to act against said tissue overlying said artery of interest thereby causing in said artery, a second artery applanation state, and

obtaining a second artery applanation state index;

(E) obtaining, during said second artery applanation state from said tissue stress sensor, at least one electrical signal representing a second set of stress data across said length of said stress sensitive element, said second set of stress data representing a plurality of stress datum, each stress datum representing stress communicated to one of said portions of said stress sensitive element from said tissue overlying said artery of interest;

(F) using said first and second sets of stress data and said first and second artery applanation state indexes to construct tissue flexibility data values which define a tissue flexibility function relating the flexibility to x, where flexibility is the flexibility of said tissue overlying said artery of interest and x is a location along said length of said stress sensitive element;

(G) computing, using said tissue flexibility data values, a centroid of tissue flexibility; and

(H) using said centroid of tissue flexibility to determine which portion of said stress sensitive element is best located for determining the blood pressure within said artery of interest.

23. The method of claim 22, wherein said first and second sets of stress data includes data corresponding to a diastolic blood pressure within said artery of interest.

24. The method of claim 22, wherein said first and second sets of stress data includes data corresponding to a systolic blood pressure within said artery of interest.

25. The method of claim 22, wherein said first and second sets of stress data includes data corresponding to a pulsatile blood pressure within said artery of interest.

26. The method of claim 22, wherein said first and second sets of stress data includes data corresponding to a mean blood pressure within said artery of interest.

27. The method of claim 22, further comprising the step of:

(I) determining which of said tissue flexibility data values is a maximum, and wherein step (G) further includes the sub-step of,

(i) calculating said centroid of tissue flexibility using only the tissue flexibility data values which have a magnitude which exceeds a predetermined percentage of said maximum tissue flexibility value.

28. The method of claim 27, wherein said predetermined percentage is determined by empirically establishing a percentage value that indicates a correlation between said centroid of energy and a center of said artery of interest.

29. The method of claim 22, wherein step (G) further includes the sub-steps of:

(i) ordering the tissue flexibility data values according to magnitude, and

(ii) calculating said centroid of tissue flexibility by using only a first n of said tissue flexibility data values of highest magnitude as ordered in sub-step 29(i).

30. The method of claim 29, wherein said n is determined by the sub-steps of:

(i) associating each said tissue flexibility data value ordered in sub-step 30(i) with a predetermined segment length along said length of said stress sensitive element,

(ii) selecting said tissue flexibility data values of greatest magnitude as ordered in sub-step 30(i) and totaling the lengths of each predetermined segment which is associated with a selected tissue flexibility data value, and

(iii) setting said n equal to a number of tissue flexibility data values selected when the cumulative length of said selected segment lengths, as totalized in sub-step 30(ii), equals a predetermined percentage of said length of said stress sensitive element.

31. The method of claim 30, wherein said predetermined percentage is determined by empirically establishing a percentage value that indicates a correlation between said centroid of tissue flexibility and a center of said artery of interest.

32. The method of claim 22, wherein the centroid of tissue flexibility is computed as follows: ##EQU14## where: X=centroid of tissue flexibility;

x=location along the length of the stress sensitive element;

C(x)=tissue flexibility function (which is a measure of the flexibility of the tissue overlying said artery of interest) at location x; and

b,c=limits of integration in the range of zero to L, where L is the length of the stress sensitive element.

33. The method of claim 32, wherein computing said tissue flexibility function C(x), includes: ##EQU15## where: K(x)=tissue foundation modulus and wherein, K(x) is computed as follows: ##EQU16## where: .sigma.(x).sub.AAS.sbsb.1 =stress data sensed by stress sensitive element at location x while undergoing the first artery applanation state;

.sigma.(x).sub.AAS.sbsb.2 =stress data sensed by stress sensitive element at location x while undergoing the second artery applanation state;

x=location along the length of the stress sensitive element;

AAS.sub.1 =First Artery Applanation State;

AAS.sub.2 =Second Artery Applanation State;

AASI.sub.1 =First Artery Applanation State Index; and

AASI.sub.2 =Second Artery Applanation State Index.

34. The method of claim 33, wherein said stress data .sigma.(x), includes data corresponding to one of a diastolic blood pressure (x), systolic blood pressure (x), a pulsatile blood pressure (x), and a mean blood pressure (x) within said artery of interest.

35. The method of claim 33, wherein calculating said first artery applanation state index AASI.sub.1, includes applanating said artery of interest to a first artery applanation state AAS.sub.1, and calculating an average stress data value .sigma..sub.AVG(AAS1), wherein .sigma..sub.AVG(AAS1) is calculated as follows: ##EQU17## where: .sigma..sub.ANG(AAS.sbsb.1)=average stress value across the length of the stress sensitive element while the artery of interest undergoes the first artery applanation state;

O, L=limits of integration (across the length of stress sensitive element).

36. The method of claim 35, wherein said stress data .sigma.(x).sub.AAS1, includes data corresponding to one of a diastolic blood pressure (x), systolic blood pressure (x), a pulsatile blood pressure (x), and a mean blood pressure (x) within said artery of interest.

37. The method of claim 33, wherein calculating said second artery applanation state index AASI.sub.2, includes applanating said artery of interest to a second artery applanation state AAS.sub.2, and calculating an average stress data value .sigma..sub.AVG(AAS2), wherein .sigma..sub.AVG(AAS2) is calculated as follows: ##EQU18## where: .sigma..sub.AVG(AAS.sbsb.2)=average stress value across the length of the stress sensitive element while the artery of interest undergoes the second artery applanation state; and

O, L=limits of integration (across the length of stress sensitive element).

38. The method of claim 37, wherein said stress data .sigma.(x).sub.AAS2, includes data corresponding to one of a diastolic blood pressure (x), systolic blood pressure (x), a pulsatile blood pressure (x), and a mean blood pressure (x) within said artery of interest.

39. The method of claim 22 wherein step (G) further includes attaching a weighting factor to said tissue flexibility data values and computing said centroid of tissue flexibility, using said weighted tissue flexibility data values.

Description Submit all comments and votes

TECHNICAL FIELD

The present invention generally relates to pressure measurement systems, and more particularly relates to a method for non-invasively determining the intra-arterial blood pressure of a wearer.

BACKGROUND OF THE INVENTION

Systems for measuring the intra-arterial blood pressure of a patient can be subdivided into two main groups--those which invade the arterial wall to access blood pressure and those which use non-invasive techniques. Traditionally, the most accurate blood pressure measurements were achievable only by using invasive methods. One common invasive method involves inserting a fluid filled catheter into the patient's artery.

While invasive methods provide accurate blood pressure measurements, the associated risk of infection and potential for complications, in many cases, outweigh the advantages in using invasive methods. Because of these risks associated with invasive methods, a non-invasive method, known as the Korotkoff method is widely used.

The Korotkoff method is known as an auscultatory method because it uses the characteristic sound made as the blood flows through the artery to mark the points of highest (systolic) and lowest (diastolic) blood pressure. Although the Korotkoff method is non-invasive, it only provides a measurement of the highest pressure point and the lowest pressure point along the continuous pressure wave. While systolic and diastolic pressure are sufficient for accurate diagnosis in many instances, there are many applications in which it is desirable to monitor and utilize the entire characteristic curve of the blood pressure wave. In these applications, the Korotkoff method is simply incapable of providing ample information. In addition to this limitation of the Korotkoff method, it necessitates the temporary occlusion (complete closing) of the artery in which blood pressure is being monitored. While arterial occlusion is not prohibitive in many applications, there are occasions where the patient's blood pressure must be monitored continuously (such as when undergoing surgery) and accordingly, the prohibiting of blood flow, even on a temporary basis, is undesirable.

Because of the above-mentioned risks involved with invasive blood pressure measurement, and the shortcomings of the Korotkoff method, extensive investigation has been conducted in the area of continuous, non-invasive blood pressure monitoring and recording. Some of these non-invasive techniques make use of tonometric principles which take advantage of the fact that as blood pressure flows through the arterial vessel, forces are transmitted through the artery wall and through the surrounding arterial tissue and are accessible for monitoring at the surface of the tissue. Because the tonometric method of measuring blood pressure is non-invasive, it is used without the risks associated with invasive techniques. Furthermore, in addition to being more accurate than the Korotkoff method discussed above, it has the capability of reproducing the entire blood pressure wave form, as opposed to only the limited systolic and diastolic pressure points provided by the Korotkoff method.

Because the accuracy of tonometric measurements depend heavily upon the method and apparatus used to sense tissue forces, several sensors have been specifically developed for this purpose. For example, U.S. Pat. No. 4,423,738 issued to Newgard on Jan. 3, 1984 discloses an electromechanical force sensor which is made up of an array of individual force sensing elements, each of which has at least one dimension smaller than the lumen of the underlying artery wherein blood pressure is to be measured. Also, U.S. Pat. No. 4,802,488 issued to Eckerle on Feb. 7, 1989, discloses an electromechanical transducer that includes an array of transducer elements. The transducer elements extend across an artery with transducer elements at the ends of the array extending beyond opposite edges of the artery. Additionally, U.S. patent application Ser. Nos. 07/500,063 and 07/621,165 both disclose tonometric sensors for use in determining intra-arterial blood pressure. Each of the above four mentioned patents/patent applications disclose transducers having sensing portions that span well beyond the lumen opening of the underlying artery. One main reason it is advantageous to construct a sensor in this manner is because the arteries of interest are relatively small and difficult to locate. By constructing tonometric sensors which employ a relatively long sensing area, the placement of the sensor by a technician, is not as critical as it would be if the sensor was capable of only sensing along a narrow region.

Although by constructing a tonometric sensor with a long sensing portion, the technician's task is simplified, it introduces certain complexities into the methodology used for determining intra-arterial blood pressure. For example, because the sensor face is made relatively long as compared to the lumen of the underlying artery, only a small fraction of the sensing portion of the tissue stress sensor is overlying the artery, and it is only this portion which is sensing useful forces (i.e. forces which are related to intra-arterial blood pressure). The remaining portion of the sensing portion is in contact with tissue which does not overlie the artery of interest, and accordingly, does not transmit forces to the sensing portion which can be used for determining intra-arterial pressure.

Therefore, in view of the above complexities, when employing tonometric sensors of the type discussed above, before the accurate intra-arterial blood pressure can be determined, a method must be employed for determining which portion of the sensor is best positioned over the artery of interest for determining the intra-arterial blood pressure. One such method is disclosed in U.S. Pat. No. 4,269,193 issued to Eckerle on May 26, 1981. The method disclosed in the '193 patent includes selecting the transducer element which has a maximum pulse amplitude output and then looking to its neighbors and choosing the neighbor having a spacially local minimum of at least one of the diastolic and systolic pressures. Other methods are disclosed in U.S. Pat. No. 4,802,488 issued to Eckerle on Feb. 7, 1989. In the '488 patent the following methods are disclosed, a curve-fit method, a two-humps method, a center-of-gravity method, and a "catch-all" method which includes using one of the three aforementioned methods in conjunction with externally supplied user information (such as sex, height, age, etc.). Also, in U.S. Pat. No. 4,893,631 issued to Wenzel, et al. on Jan. 16, 1990, discloses a method for determining which sensor in an array of sensors best tracks the pulse in an underlying artery using a spacially weighted averaging method. This method employs the steps of finding local diastolic pressure minimums, selecting the number of transducers spanning the local minimums, computing the spacially weighted average from elements centered about the local minimums and computing a weighted average therefrom.

Although the above-referenced methods may yield some degree of success, the Applicants of the present invention believe that a method which is superior to those heretofore disclosed methods employs the use of stress energy. For example, it is believed, that the area of the sensor which is best positioned to determine intra-arterial pressure is that portion which receives the greatest contact stress energy from the tissue overlying the artery of interest.

In addition to the above-referenced contact stress energy transfer methodology, a second methodology is disclosed which uses a tissue flexibility distribution method to determine which portion of the stress sensitive element is best suited to measure intra arterial blood pressure. This approach is based on the idea that the tissue immediately over the artery of interest is more flexible than the tissue remote from the artery of interest. By employing a method which determines the flexibility of the tissue at each portion along the stress sensitive element, it can be determined which portion of the stress sensitive element is best suited to use in computing intra-arterial pressure.

Thus, it is an object of this invention to provide a method for determining which portion of a stress sensitive element is best suited to determine intra-arterial blood pressure.

Two methods are disclosed for achieving this object. The first method includes determining which portion along the length of the stress sensitive element receives maximum energy transfer from the tissue overlying the artery of interest. The second method involves determining which portion of the tissue overlying the artery of interest is most flexible.

By determining which portion of the stress sensitive element receives the greatest energy transfer or by determining which portion of the tissue underlying the stress sensitive element is most flexible, this information can be used to determine which portion of the stress sensing element is best suited for determining intra-arterial blood pressure of an underlying artery.

SUMMARY OF THE INVENTION

In light of the foregoing objects, the present invention provides a method, for use in non-invasive blood pressure monitoring, of determining which portion of a stress sensitive element of a tissue stress sensor is best located for detecting the stress of tissue overlying an artery of interest, the stress sensitive element having a length that exceeds the lumen of the artery of interest, the method generally including the steps of; placing the stress sensitive element of the tissue stress sensor in communication with the tissue overlying the artery of interest, and orienting the tissue stress sensitive element such that the tissue stress sensitive element spans beyond the lumen of the artery of interest; obtaining from the tissue stress sensor at least one electrical signal representing stress data across the length of the stress sensitive element, said stress data including stress datum communicated to a predetermined portion of the stress sensitive element from the tissue overlying the artery of interest, each predetermined portion of the stress sensitive element lying along the length of the stress sensitive element; computing from the stress data, a centroid of energy associated with the stress sensitive element; and using the centroid of energy to determine which portion of the stress sensitive element is best located for determining the blood pressure within the artery of interest.

In a preferred embodiment, the stress data includes data which corresponds to the systolic blood pressure, diastolic blood pressure, pulsatile blood pressure, or the mean pressure within the artery of interest.

In a preferred embodiment, the disclosed method includes using each stress datum value to calculate a corresponding energy value, each energy value being associated with one predetermined portion of the stress sensitive element, and determining which one of the energy values is a maximum, and calculating the centroid of energy using only stress datum values which have an energy value which exceeds a predetermined percentage of the maximum energy value.

In a further preferred embodiment, the method of the present invention includes using each stress datum value to calculate a corresponding energy value, each energy value being associated with one of the predetermined portions of the stress sensitive element, ordering the energy values according the their respective magnitudes and calculating the centroid of energy by using only stress datum values associated with a first n energy values of highest magnitude. Preferably, n is determined by associating each energy value ordered with a predetermined segment length along the length of the stress sensitive element, selecting the energy values of greatest magnitude and totaling the predetermined segment lengths associated with all of the selected energy values, and setting n equal to the number of energy values selected when the cumulative predetermined segment lengths exceed a predetermined percentage of the length of the stress sensitive element.

A further preferred embodiment of the disclosed method includes using each of the stress datum values to calculate a corresponding energy value, each of the energy values being associated with a predetermined portion of the stress sensitive element, and attaching a weighing factor to each one of the energy values and calculating the centroid of energy using the weighted energy values.

A second method is disclosed for use in a non-invasive blood pressure monitoring system, of determining which portion of a stress sensitive element of a tissue stress sensor is best located along the length of the stress sensitive element for detecting the stress of tissue overlying an artery of interest, the length of the stress sensitive element exceeding the lumen of the artery of interest, the method including the steps of placing the stress sensitive element of the tissue stress sensor in communication with the tissue overlying the artery of interest, and orienting the stress sensitive element such that the stress sensitive element spans beyond the lumen of the artery of interest; causing the stress sensitive element to act against the tissue overlying the artery of interest thereby causing in the artery a first applanation state and obtaining an index of the first artery applanation state; obtaining, during the first applanation state, from the tissue stress sensor at least one electrical signal representing a first set of stress data across the length of the stress sensitive element, said at least one signal representing stress datum communicated to a predetermined portion of the stress sensitive element from the tissue overlying the artery of interest, each predetermined portion of the stress sensitive element lying along the length of the stress sensitive element; causing the stress sensitive element to act against the tissue overlying the artery of interest thereby causing in the artery, a second artery applanation state, and obtaining an index of the second artery applanation state; obtaining, during the second artery applanation state, from the tissue stress sensor, a plurality of electrical signals representing a second set of stress data across the length of the stress sensitive element, each signal of the plurality of electrical signals representing stress datum communicated to one of the predetermined portions of the stress sensitive element from the tissue overlying the artery of interest; using the first and second sets of stress data and the first and second artery applanation state indexes to construct tissue flexibility data values which define a tissue flexibility function relating the flexibility of the tissue overlying the artery of interest to x, where x is a location along the length of the stress sensitive element; computing, using the tissue flexibility data values, a centroid of tissue flexibility; and using the centroid of tissue flexibility to determine which portion of the stress sensitive element is best located for determining the blood pressure within the artery of interest.

In a preferred embodiment, the first and second sets of stress data include using the data corresponding to the diastolic blood pressure, systolic blood pressure, pulsatile blood pressure, or mean blood pressure within the artery of interest.

Other advantages and meritorious features of the present invention will become more fully understood from the following description of the preferred embodiments, the appended claims, and the drawings, a brief description of which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a tissue stress sensor attached to the wrist of a wearer.

FIG. 2 is a cross sectional view taken substantially along lines 2--2 of FIG. 1.

FIG. 3 is an enlarged view of encircled portion 3 of FIG. 2.

FIG. 4 is a cross sectional view of the tissue contact stress sensor of the present invention taken substantially along lines 4--4 of FIG. 3.

FIG. 5 is a cross-sectional view of the tissue contact stress sensor of the present invention taken substantially along lines 5--5 of FIG. 4.

FIG. 6 is a partially exploded view of the tissue contact stress sensor of the present invention.

FIGS. 7A and 7B are diagrammatic views of the emitter and detector portions of the semiconductor assembly of the present invention.

FIG. 8 is an electronic block diagram of the tissue contact stress sensor and associated supporting electronics of the present invention.

FIG. 9 is a detailed schematic of blocks 40 and 42 of FIG. 8.

FIG. 10 is a graphic representation of a typical blood pressure waveform.

FIG. 11 is a graphical representation of contact stress versus distance along the length of a stress sensitive element.

FIG. 12 is a graphical representation of a normalized contact stress energy curve plotted as a function of distance along the stress sensitive element.

FIG. 13 is a graphical representation of a normalized weighted contact stress energy curve plotted as a function of distance long the stress sensitive element.

FIG. 14 is a graphical representation of a normalized tissue foundation flexibility curve plotted as a function of distance along the stress sensitive element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now referring to FIG. 1, wrist mount apparatus 21 includes base 23 and flexible strap 25. Flexible strap 25 is adapted to engage base 23 to the wrist of a user. Tissue stress sensor housing 27 is fastened to base 23 and houses a tissue stress sensor for transducer) 20 (tissue stress sensor not shown) and a means 29 for moving the tissue stress sensor 20 (see FIG. 2) into operative engagement with the tissue overlying an artery of interest. Various electrical signals are derived from the tissue stress sensor located within sensor housing 27 and are made available therefrom via conductors within cable 31. These electrical signals carry data which will be used to derive the intra-arterial blood pressure of the wearer of apparatus 21.

Now referring to FIG. 2, sensor housing 27 is mounted to base 23. Within sensor housing 27 is mounted a fluid operated slave bellows 29. Bellows 29 is attached to, at one of its ends tissue stress sensor 20. As bellows 29 receives a displacement fluid from a source of fluid via tubing 33, it expands downwardly 43 thereby causing tissue stress sensor 20 to engage tissue 24 overlying artery of interest 26.

Now referring to FIG. 3, tissue stress sensor 20 includes wafer 30 which has a nonresponsive portion 32 and a responsive portion (also denoted as a stress sensitive element, a diaphragm, or diaphragm portion) 34. Nonresponsive portion 32 serves mainly to support responsive portion 34. Under conditions when tissue stress sensor 20 is not being applied against tissue 24, radial artery 26' has a generally rounded opening (or lumen) as depicted at 26'. As wafer 30 of tissue stress sensor 20 is pressed against tissue 24, stress is created in tissue 24. This stress loads responsive portion 34 of wafer 30 thereby causing responsive portion 34 to deflect. In addition to causing the deflection of responsive portion 34, the stress created in tissue 24 also causes radial artery 26' to flatten (or applanate) along its top surface 36. As the blood pressure within radial artery 26 changes (i.e. pulsates), stress is created in tissue 24 which disturbs the equilibrium between responsive portion 34 of water 30 and top surface 28 of tissue 24. This disturbance in equilibrium causes movement between diaphragm 34 of wafer 30 and top surface 28 of overlying tissue 24. Such movement exists until a new equilibrium is established. The ability of diaphragm 34 to move and assume a unique displacement position for a given blood pressure within radial artery 26 forms the fundamental mechanism whereby tissue stress transducer 20 is capable of sensing the intra-arterial pressure of radial artery 26.

Because sensor 20 is used to compress or applanate radial artery 26 during blood pressure measurement, as well as measure the contact stress in tissue 24, the geometry of sensor 20 and its surrounding structure are vital to the proper conduction of stresses from radial artery 26 to tissue surface 28. A detailed discussion of sensor 20 and its associated structure now follows.

Now referring to FIG. 4, tissue contact stress sensor 20 is comprised of sensor head 40 and sensor base portion 42. Sensor head 40 comprises the transducer portion of sensor 20 and sensor base portion 42 includes electronic circuitry and other mechanical support structure necessary for properly operating sensor head 40. Sensor head 40 is generally comprised of six elements: sensor wafer 30, spacing structure 44, infrared emitting diodes (typified at 46), photo receivers (typified at 48), emitter/detector substrate 50 and circuit traces 52, 54.

An important feature of sensor 20 centers around the material and construction of sensor wafer 30. Sensor wafer 30 is formed from a wafer of single crystal silicon (SCS). Responsive diaphragm portion 34 of wafer 30 is formed by chemically micro-machining a trough 56 in the face of SCS wafer 30. This trough has a tetragonal-pyramidal geometry due to the crystal lattice structure of the SCS wafer 30. The bottom of the trough area 58 defines responsive diaphragm portion 34 of wafer 30. This portion defines a thin diaphragm region of highly controlled thickness and geometry. A major advantage in using SCS in the construction of diaphragm 34 is its superior engineering properties and its ability to be micro-machined which in turn provides a one-piece structure free of pre-stressing. Additional benefits in using SCS material include its ability to replicate small geometric features precisely and repeatedly, its linear elastic properties (i.e., almost no hysteresis) and its ability to quickly evidence its failed condition (under failure, the SCS diaphragm 34 totally fails thereby immediately evidencing its failed condition). This is to be contrasted with other materials which, under failure, do not fracture as does SCS but rather undergo inelastic deformation. Once the diaphragm undergoes inelastic deformation it loses its calibration but generally does not manifest its extreme, failed condition thereby usually going unnoticed.

Underside 60 of trough 56 is preferably metalized with a reflective material such as aluminum or gold. The thickness of the aluminum or gold is preferably generally 600 angstroms and its purpose will be explained shortly. Responsive portion 34 of wafer 30 changes its geometry with applied stress as a function of the material properties of the diaphragm. It is important to note that a coating of aluminum or gold generally 600 angstroms in thickness does not materially alter the properties of diaphragm portion 34 of wafer 30.

In the construction of tonometry sensors, the elasticity of responsive portion 34 of wafer 30 must be compatible with the characteristics of human tissue. If diaphragm surface 34 deforms excessively when responding to the stress of surf