TECHNICAL FIELD

The present invention generally relates to calibration systems, and more particularly relates to a method and apparatus for calibrating a system for noninvasively determining the intra-arterial blood pressure of a wearer.

RELATED APPLICATION

This application is a continuation-in-part of U.S. application Serial No. 07/621,165 filed November 30, 1990.

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 noninvasive techniques. Initially, the most accurate blood pressure measurements were achievable only by use of invasive methods. One such common method involved use of a fluid filled catheter inserted into a patient's artery. While invasive methods provide for accurate blood pressure measurements, the risks of infection and other complications, in many cases, outweigh the advantages of using invasive techniques.

Because of the above-mentioned risks associated with invasive methods, a noninvasive method, known as the Korotkoff method is widely used. The Korotkoff method is known as an auscultatory method because it uses, in part, the characteristic sound made as the blood flows through the artery to aid in measuring blood pressure. Although the Korotkoff method is

noπinvaεive, it only provides a measurement of the highest pressure point (systolic) and the lowest pressure point

(diastolic) along the pressure wave. While, for many purposes, systolic and diastolic pressure are sufficient, there are many applications in which it is desirable to use the entire characteristic curve of the blood pressure wave. In these applications, the Korotkoff method simply is incapable of providing satisfactory information. In addition to this limitation of the Korotkoff method, it necessitates the temporary occlusion 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 associated with invasive blood pressure measurement, and the shortcomings of the Korotkoff method, extensive investigation has been conducted in the area of continuous, noninvasive blood pressure monitoring and recording. Some of these noninvasive techniques make use of tonometric principles which take advantage of the fact that as blood flows through the arterial vessel, forces are transmitted through the artery wall, through the surrounding arterial tissue and, consequently, are externally available for monitoring. Because the tonometric method of measuring blood pressure is noninvasive, 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 the limited systolic and diastolic pressure points provided by the Korotkoff method.

A technique for determining intra-arterial blood pressure involves the method of pressing a sensor against the tissue which overlays an artery of interest thereby flattening, or applanating, the underlying artery. This pressing is increased in intensity until a predetermined state of artery applanation is obtained. In this state, certain assumptions can be made regarding the relationship between the forces transmitted to the sensor (through the tissue overlaying the artery) and the intra-arterial blood pressure.

Thus, in view of the above discussion, it can be seen that it is desirable to provide a calibration apparatus which is effective for calibrating the response of the sensor and its associated support system. Such a calibration system should be easy to operate, and ideally, should be usable in the field. Preferably, such a calibration system should also be capable of detecting the temperature dependence of the sensor and thereby, providing the appropriate data, to the blood pressure system which uses the sensor, for compensating for the effects of temperature on the sensor head.

Thus, it is an object of this invention to provide a sensor calibration system which can be used in the field to calibrate a sensor.

It is also an object of this invention to provide a calibration system which does not necessitate dismantling the sensor from a surrounding support structure.

It is still a further object of this invention to provide a sensor calibration system which provides data to compensate for the temperature dependency of the sensor.

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BUMMARY OF THE INVENTION

In light of the foregoing objects, in one aspect, the present invention provides a calibration apparatus for use in calibrating a tissue stress sensor used in a blood pressure monitoring system. The apparatus comprises a calibration head which is generally adapted to engage a support structure associated with the stress sensor. Retaining means is attached to the calibration head for retaining the head in close proximity to the tissue stress sensor. Beating means is attached to the calibration head for heating a stress sensitive portion of the tissue stress sensor. Preferably, the calibration head includes a "C" shaped body adapted to engage the outer support structure of the tissue stress sensor and preferably the retaining means includes a thumb screw threadedly engaged through the "C" shaped body whereby rotating the thumb screw causes the thumb screw to engage the outer support structure of the tissue stress sensor thereby retaining the "C" shaped body to the outer support structure of the tissue stress sensor.

In a second aspect, the present invention provides, a calibration system for use in calibrating a sensor. The system comprises a tissue stress sensor including a sensor support structure to house the tissue stress sensor. A calibration head is provided to engage the support structure. Retaining means is used to retain the calibration head to the sensor support structure. The calibration head further includes means for heating a stress sensitive portion of the tissue stress sensor. Preferably, the sensor support structure includes a wrist strap for fastening the sensor support structure to a wrist of a wearer. Preferably, the stress sensitive portion of the tissue stress sensor further includes a continuous, stress sensitive diaphragm and a hermetically

sealable chamber, wherein the chamber has a continuous wall adapted to separate an inner chamber pressure from an outer chamber pressure, wherein the continuous, stress sensitive diaphragm comprises at least a portion of the chamber wall, whereby the diaphragm is responsive to a pressure differential between the inner and outer chamber pressure.

In a third aspect, the present invention provides, a calibration system for use in calibrating a tissue stress sensor used in a blood pressure monitoring system. The tissue stress sensor is generally operative for producing an electrical output signal representative of tissue stress. The calibration system comprises means, in thermal communication with a stress sensitive portion of the tissue stress sensor, for heating the stress sensor. Also, means, are coupled to the tissue stress sensor for displacing the stress sensitive portion of the sensor. Further, a calibration means is coupled to the electrical output signal of the tissue stress sensor for detecting a response of the stress sensitive portion of the sensor to the application of a known temperature and stress. The calibration means is effective for calibrating the sensor and correcting for the effects of temperature on the data generated by the stress sensor. Preferably, the calibration means includes means for sensing the temperature of the heating means. Also, the displacing means preferably includes a vacuum pump in fluid communication with the tissue stress sensor. Preferably, the calibration means includes means for sensing a vacuum of the vacuum pump. In its most preferred embodiment, the heating means and displacing means are connected to, and controlled by the calibration means.

In a fourth aspect, the present invention provides, a calibration system for use in calibrating a blood pressure monitoring system, the blood • pressure monitoring system

employing a stress sensor for generating an electrical output signal representative of tissue stress. The calibration system comprises a tissue stress sensor having a stress sensitive diaphragm and a sealed chamber, the sealed chamber has a continuous wall defining an inner chamber volume and the stress sensitive diaphragm makes up a portion of the continuous wall, whereby the displaceable diaphragm is responsive to a pressure differential between the pressure within the inner chamber volume and the pressure outside of the inner chamber volume. Means is provided, in thermal communication with the stress sensitive diaphragm, for heating the diaphragm. Also, vacuum means is provided, in fluid communication with the inner volume of said sealed chamber for applying a controlled vacuum to the inner volume, whereby the diaphragm is displaced in response to the application of the vacuum. Calibration means is coupled to the electrical output signal of the tissue stress sensor for detecting the sensors response to the heating and displacing of the diaphragm and generating corrective data for use in correcting the effects of tissue stress sensor temperature on the data generated by the sensor. Preferably, the calibration means includes means for sensing the temperature of the heating means and means for sensing the vacuum of the vacuum means. In a preferred embodiment, the heating means and the vacuum means are controlled by the calibration means. Preferably, the calibration system includes a calibration head which is adapted to engage a support structure of the tissue stress sensor and wherein the calibration head is adapted to engage the heating means thereby keeping the heating means in thermal communication with the diaphragm of the sensor.

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

Figure 1 is a perspective view of a wrist mount apparatus which incorporates a tissue stress sensor.

Figure 2 is a partial cross-sectional view of a wrist mount apparatus taken substantially through lines 2-2 of Figure 1.

Figure 3 is an amplified view of a tissue stress sensor and its operating environment taken substantially within encircled portion 3 of Figure 2.

Figure 4 is a wrist mount apparatus which has the calibration apparatus of the present invention mounted to the sensor support structure.

Figure 5 is a diagrammatic depiction of the calibration system of the present invention.

TreTATJ.En DESCRIPTION OF THE PREFERRED EMBODIMENTS ^'

Now referring to Figure 1, wrist mount apparatus 10 is applied to the wrist area 12 of a user for the purpose of locating tissue stress sensor 14 in operative engagement with the tissue overlaying an artery of interest.

Now referring to Figure 2, wrist mount apparatus 10 comprises base 16 and sensor support structure 18. Sensor support structure 18 includes tissue stress sensor 14, sensor displacement actuator 20 and sensor support electronics 22. Sensor displacement actuator 20 is controlled via link 24 and is adapted to be responsive to commands within link 24 to effect a linear movement 26. Sensor 14 is fastened to actuator 20 such

that when actuator 20 effects its linear movement 26, sensor 14 is placed in operative engagement with tissue 28 overlaying an artery of interest 30. Tissue stress pulsations transmitted through tissue 28 from artery 30, are caused by intra-arterial blood pressure pulsations acting within artery 30. The diaphragm (or stress sensitive) portion 34 of sensor 14, is displaced in response to tissue contact stress loading acting upon it from tissue 28. The displacement of stress sensitive diaphragm 30, is transduced into electronic signals which are conditioned by support electronics 22 and communicated, via line 32, to the appropriate display and monitoring systems.

Now referring to Figure 3, sensor 14 is comprised of continuous, stress sensitive diaphragm 34 and hermetically sealable chamber 36. Hermetically sealable chamber 36 has a continuous wall 38 adapted to separate inner chamber volume 56 from the pressure surrounding wall 38 (typically atmospheric pressure). A portion of continuous wall 38 is comprised of responsive diaphragm 34. Thus, as discussed in conjunction with Figure 2, when sensor 14 is moved 26 into operative engagement with tissue 28 which overlies artery of interest 30, artery of interest 30 is applanated or flattened. Once the state is reached whereby artery 30 is applanated to an optimum level, the contact stress sensed by stress sensitive diaphragm 34, is well suited for use in determining the intra-arterial blood pressure of artery 30. Thus it has been demonstrated in conjunction with Figures 1-3 how sensor 14 is operative to engage the tissue overlying an artery of interest and is adapted to be displaced in response to intra-arterial blood pulsations. Now, in conjunction with Figures 4 and 5 the calibration system of the present invention will be described as it applies to calibrating sensor 14, sensor support electronics 22 and other components within a noninvasive blood pressure monitoring system.

Now referring to Figure 4, calibration head 40 has generally a "C" shaped structure which is adapted to engage sensor support structure 18. A portion of calibration head 40 includes fastening means for thumb screw 42, which is threadedly engaged to calibration head 40 and adapted to retain calibration head 40 to sensor support structure 18. Calibration head 40 also includes heater 41 and temperature sensor 44. Although heater 41 will be used throughout the remainder of the specification to refer primarily to a device which adds thermal energy to diaphragm 34, it is also contemplated that heater 34 includes a device which is capable of removing heat energy from diaphragm 34. Cooling of diaphragm 34 may be employed in cases where it is desirable to lower the temperature of diaphragm 34 below room temperature or to simply remove heat energy from diaphragm 34 in order to quickly reduce its temperature to room temperature, etc. Consequently, it is contemplated within this disclosure, that heater 41 includes any thermal device which can operate to either heat or cool diaphragm 34. Heater 41 in preferably an electric-type heater which is provided operating current through line 46. Temperature sensor 44 can be comprised of any type of thermal detecting device such as a thermocouple, thermistor, RTD, solid state sensor or the like. The temperature sensed by sensor 44 is converted into an electric signal and sent to calibration electronics via line 48.

Both calibration head 40 is provided with opening 52 whereby vacuum tube 54 can pass therethrough and transmit a source of vacuum into hermetically sealable chamber 36 (hermetically sealable chamber 36 not shown in Figure 4) of sensor 14.

Now referring to Figures 3 and 4, as was mentioned before, because responsive diaphragm 34 forms a part of continuous wall 38 of hermetically sealable chamber 36, it is

responsive to a differential pressure between the pressure within chamber 36 and the air pressure surrounding hermetically sealable chamber 36. Accordingly, it can be seen that diaphragm 34 of sensor 14 can be displaced by lowering the pressure within volume 56 with respect to the air pressure outside of chamber 36 (air pressure outside of chamber 36 is typically atmospheric pressure, but this does not have to be the case). Thus, if a vacuum is placed on port 58 of chamber 36, diaphragm 34 will be displaced in the same manner in which it is displaced during actual use (when it is subjected to tissue stress). Accordingly, by applying a vacuum of known intensity to chamber 56, the fundamental relationship between diaphragm 34 deflection and the pressure necessary to cause that deflection can be established. It is important to note, that in this respect, the design of sensor 14 plays an important role in the calibration system of the present invention. Specifically, by designing hermetically sealable chamber 36 so as to make responsive diaphragm 34 a part of continuous wall 38, a calibrated vacuum can be used to, in essence, simulate the application of a calibrated pressure across responsive diaphragm 34 of sensor 14. This is seen as a substantial advantage of the disclosed calibration system, inasmuch as the application of a vacuum to an open faced sensor of the type used in tonometry, is generally considered to be much easier to administer than the alternative which would be to attempt to present a calibrated pressure across the external face of diaphragm 34. Two obvious disadvantages with using the external pressure approach include providing a simple and effective way to seal the pressure source against the external face of diaphragm 34 and gaining necessary access to sensor 14 to apply the calibrated pressure without necessitating removing sensor 14 from its support structure 18. Also, when using the external pressure approach to seal the pressure source against the external face of diaphragm 34, care must be taken to exert sufficient force in order to seal the

pressure source against the face of the diaphragm while preventing the application of excessive force against the diaphragm stress structure which tends to stress diaphragm 34 thereby introducing measurement errors.

Now referring to Figure 5, heater 41 is mounted within "C" shaped calibration head 40 such that it is in close proximity to, but does not directly contact, responsive diaphragm 34. In this manner, heater 41 is effective for heating diaphragm 34 but does not transmit any contaminants thereto, nor does it expose diaphragm 34 to damage caused by bumping heater 41 against diaphragm 34. Preferably, heater 41 operates to uniformly heat the entire stress sensitive (or diaphragm) portion 34 of stress sensor 14. This uniform heating is preferably accomplished by extending heater 41 along the full length of diaphragm 34. Temperature sensor 44 is placed in close proximity to heater 41 thereby sensing the approximate temperature of diaphragm 34 and sending the sensed temperature along line 48 to calibration electronics 60. Vacuum tube 54 fluidly couples vacuum from controllable vacuum pump 62 into inner chamber 56. Accordingly, aε has been discussed in conjunction with Figure 3, a vacuum applied by pump 62 is effective for displacing diaphragm 34. Pump 62 is controlled by calibration electronics via line 64. Accordingly, voltage signals sent out from calibration electronics 60 to pump 62 can control the amount of vacuum applied to diaphragm 34 thereby controlling its displacement. Vacuum transducer 66 senses the vacuum in tube 54 and transduces that vacuum into an electric signal and presents it to calibration electronics 60 via line 68. Now the calibration system set out in Figure 5 will be used to explain the general procedure for calibrating transducer 14 and support electronics 22.

Calibration electronics 60 first sends out a signal to controllable voltage supply 70 to generally heat diaphragm 34 to a predetermined temperature. This temperature is preferably a temperature within the temperature range experienced by diaphragm 34 during actual operating conditions. Sensor 44 is responsive to the actual temperature of heater 41 and calibration electronics 60 can constantly monitor and control the diaphragm temperature and use this temperature information in generating corrective data for use in correcting the effects of tissue stress sensor temperature on the data generated by the sensor. After diaphragm 34 has stabilized at a predetermined temperature, various vacuum settings can be applied to diaphragm 34 thereby giving rise to various degrees of diaphragm deflection. Diaphragm deflection is sensed by sensing electronics (sensing electronics not shown) and further conditioned by support electronics 22. Support electronics 22 encode the deflection of diaphragm 34 into electronic signals and pass them along to calibration electronics 60 along line 74. Thus it can be seen that the calibration system set out in Figure 5 is effective for measuring the displacement of diaphragm 34 in response to a known vacuum (which can be used to calculate the equivalent pressure across diaphragm 34) and a known temperature. Thus, by setting various vacuum and temperature points and monitoring the electrical signals produced on line 74, a collection of characteristic data can be generated to generally define the calibration characteristics of diaphragm 34, sensor support electronics 22 and any other system which intervenes between calibration and electronics and diaphragm 34. Thus, it can be easily seen that the function accomplished by calibration electronics 60 can be implemented by using either hardware or software techniques. If software techniques are utilized, a single system may be designed to function both in a calibration mode and an operation mode. Thus, the same system would be used for calibrating the sensor

and monitoring its output during actual use conditions. The following description sets out a preferred methodology for calibrating the sensor system set out in Figure 5.

The tonometer stress contact sensor of the present invention generates a plurality of electrical signals which are each a function of local diaphragm 34 deflection and local diaphragm 34 temperature. First consider the simplest model of this sensor neglecting the contribution of temperature, and assume a linear relationship between output current and stress.

If p = the displayed value of pressure in mHg, then

p = SLOPE • x + OFFSET

where SLOPE and OFFSET are calibration constants, and x is an electrical signal assumed to be a linear function of pressure.

For this model, the two calibration constants are calculated in software by substituting the desired values of p and the measured values of x for the following two known conditions:

0 mmHg applied to sensor, 120 mmHg applied to sensor.

However, the contribution of temperature to the sensor function is significant, and cannot be neglected. The simplest model of the function of pressure and temperature assumes that the OFFSET term is a linear function of temperature, and that the SLOPE term is a constant independent of temperature.

If we let d = the displayed value of pressure in mmHg, then,

d = XSLOPE • x + (RSLOPE • r + OFFSET)

where XSLOPE, RSLOPE, and OFFSET are calibration constants, x is assumed to be a linear function of pressure and temperature and r is assumed to be a linear function of temperature.

The r input is derived from the reference sensor previously mentioned which has an opto electronic structure identical to that which generates the x input, except that it is not modulated by pressure. It is located such that it is in the same environment, and consequently, at the same temperature as the x opto electronics.

For this model, the three calibration constants are calculated in software by inserting the desired values of d and the measured values of x and r for the following three known conditions:

0 mmHg applied to sensor at temperature 1,

120 mmHg applied to sensor at temperature 1,

0 mmHg applied to sensor at temperature 2.

However, the effect of temperature on the SLOPE coefficient is also significant, and cannot be neglected. A model which also accounts for this effect, assuming it is linear, is constructed as follows:

d « [RXSLOPE • r + XOFFSET] • x + [RSLOPE • r + OFFSET]

where d is the displayed value of pressure in mmHg, RXSLOPE, XOFFSET, RSLOPE, and OFFSET are calibration constants, x is assumed to be a linear function of pressure and temperature, and r is assumed to be a linear function of temperature.

For this model, the four calibration constants are calculated in software by inserting the desired values of d and the measured values of x and r for the following four known conditions:

0 mmHg applied to sensor at temperature 1, (pi, tl)

120 mmHg applied to sensor at temperature 1, (p2, tl)

0 mmHg applied to sensor at temperature 2, (pi, t2)

120 mmHg applied to sensor at temperature 2, (p2, t2).

A method of automatically performing these four calibrations in computer software, so that there is no interaction between conditions, i.e., calibration of any condition does not require recalibration of any previously calibrated condition, is derived as follows:

0 mmHp. temperature 1 cal:

From model,

(1) d ■ [RXSLOPE • r + XOFFSET] • x + [RSLOPE • r + OFFSET]

and, since we expect to display 0 mmHg at pi and tl,

(2) 0 = [RXSLOPE • r(tl) + XOFFSET] • x(pl,tl) + [RSLOPE • r(tl) + OFFSET]

Solving (2),

(3) OFFSET = - [RXSLOPE • r(tl) + XOFFSET] • x(pl,tl) - RSLOPE • r(tl)

The input values are saved as:

Rll m r(tl) Xll = x(pl,tl)

where pi = 0 mmHg applied pressure, and tl = temperature 1.

120 mmHg. temperature 1 cal:

Substituting the p2 and tl conditions into (1),

(4) 120 = [RXSLOPE • r(tl) + XOFFSET] • x(p2,tl) + [RSLOPE • r(tl) + OFFSET]

Subtracting (4) - (2),

(5) 120 = [RXSLOPE • r(tl) + XOFFSET] • [x(p2,tl) - x(pl,tl)]

(6) 120 = [RXSLOPE • r(tl) + XOFFSET] • [x(p2,tl) - Xll]

Solving (6),

(7) XOFFSET = 120 + [x(p2,tl) - Xll] - RXSLOPE • r(tl)

Readjusting OFFSET from (3),

(8) OFFSET _* = - [RXSLOPE • Rll + XOFFSET] • Xll - RSLOPE • Rll

The input values are εaved as:

R21 = r(tl) X21 = x(p2,tl)

where p2 = 120 mmHg applied pressure, and tl s temperature 1.

0 mmHg. temperature 2 cal:

Substituting the pi and t2 conditions into (1),

(9) 0 = [RXSLOPE • r(t2) + XOFFSET] • x(pl,t2) + [RSLOPE • r(t2) + OFFSET]

Subtracting (9) - (2),

(10) 0 = [RXSLOPE • [r(t2) • x(pl,t2) - r(tl) • x(pl,tl)] +

XOFFSET • [x(pl,t2) - x(pl,tl)] + RSLOPE • [r(t2) - r(tl)]

Solving (10),

(11) RSLOPE = -{RXSLOPE • [r(t2) • x(pl,t2) - Rll • Xll] + XOFFSET • [x(pl,t2) - xll]} ÷ (r(t2) - Rll}

Readjusting OFFSET from (3),

(12) OFFSET - - [RXSLOPE • Rll + XOFFSET] • Xll - RSLOPE • Rll

The input values are εaved as:

R12 = r(t2) X12 m x(pl,t2)

where pi c 0 mmHg applied pressure, and t2 = temperature 2.

120 mmHg. temperature 2 cal:

Substituting the p2 and t2 conditions into (1),

(13) 120 - [RXSLOPE • r(t2) + XOFFSET] • x(p2,t2) + [RSLOPE • r(t2) + OFFSET]

Subtracting (13) - (9),

(14) 120 = [RXSLOPE • r(t2) + XOFFSET] • [x(p2,t2) - x(pl,t2)]

= [RXSLOPE • r(t2) + XOFFSET] • [x(p2,t2) - X12]

(15) RXSLOPE • r(t2) + XOFFSET = 120 ÷ [x(p2,t2) - X12]

From (7) and (8),

(16) XOFFSET = [120 + (x21 - xll}] - RXSLOPE • R21

Substituting (16) into (15),

RXSLOPE • [r(t2) - R21] = [120 + (x(p2,t2) - X12}] -

[120 + [x21 - xll}]

(17) RXSLOPE = [120 + [r(t2) - R21}] • [[1 + (x(p2,t2) - xl2}] - [1 + {x21 - xll}]]

Readjust XOFFSET by computing from (16) with the new value of RXSLOPE inserted.

Readjust RSLOPE from (11) with the new values of RXSLOPE and XOFFSET inserted.

Readjuεt OFFSET from (12) with the new values of RXSLOPE, XOFFSET, and RSLOPE inserted.

The function of the calibration points, in the order discussed above, is as follows:

Point 1 Set pressure offset (zero point).

Point 2 Set pressure slope (gain).

Point 3 Set offset (zero point) compensation.

Point 4 Set εlope (gain) compensation.

As can be seen from the above method, there is no interaction between the calibration points. The points may be calibrated in any order, and not all points need be calibrated. For example, if operation will only occur at temperature 1, only the first two points need be calibrated. If only the offset needs to be corrected, the first point calibration is sufficient. If only the full scale point at temperature 1 needs ^" correction, only the second point need be calibrated.

The ' foregoing detailed description shows that the preferred embodiments of the present invention are well suited to fulfill the objects of the invention. It is recognized that those skilled in the art may make various modifications or additions to the preferred embodiments chosen here to illustrate the present invention, without departing from the spirit of the present invention. Accordingly, it is to be understood, that the subject matter sought to be afforded protection hereby should be deemed to extend to the subject matter defined in the appended claims, including all fair equivalents thereof.