Blood pressure monitoring method and apparatus

I claim:

1. A machine implemented method for determining correct hold-down pressure to be applied to an external pressure transducer that includes a pressure sensitive element applied to a subject at a location overlying an artery, the determined correct hold-down pressure identifying the hold-down pressure to be employed to obtain correct blood pressure measurements using said transducer, said method comprising,

(1) obtaining a continuous measurement of blood pressure with the external pressure transducer while changing the hold-down pressure on the transducer over a range from where the underlying artery is substantially unflattened to where the artery is substantially occluded,

(2) obtaining from the blood pressure measurement of step 1 a set of at least one of the diastolic pressure, systolic pressure, and pulse amplitude values versus hold-down pressure over said range of hold-down pressures,

(3) fitting a polynomial to at least one set of values obtained in step 2,

(4) using the polynomial obtained in step 3, obtaining an indication of the correct hold-down pressure required for accurate blood pressure measurements, and

(5) setting the transducer hold-down pressure at substantially the indicated correct hold-down pressure obtained in step 4 in preparation for obtaining accurate blood pressure measurements from said transducer.

2. A method as defined in claim 1 wherein a set of diastolic pressure versus hold-down pressure values are obtained in step 2, and in step 3 a polynomial of at least third-order is fitted to the set of diastolic pressure versus hold-down pressure values.

3. A method as defined in claim 2 wherein step 4 includes locating the point of minimum slope of the graph of the polynomial fitted in step 3 using coefficients of the polynomial, the hold-down pressure at the point of minimum slope providing an indication of the correct hold-down pressure.

4. A method as defined in claim 1 wherein a set of systolic pressure versus hold-down pressure values are obtained in step 2, and in step 3 a polynomial of at least third-order is fitted to the set of systolic pressure versus hold-down pressure values.

5. A method as defined in claim 4 wherein step 4 includes locating the point of minimum slope of the graph of the polynomial fitted in step 3 using coefficients of the polynomial, the hold-down pressure at the point of minimum slope providing an indication of the correct hold-down pressure.

6. A method as defined in claim 1 wherein a set of pulse amplitude versus hold-down pressure values are obtained in step 2, and in step 3 a polynomial of at least second-order is fitted to the set of pulse amplitude versus hold-down pressure values.

7. A method as defined in claim 6 wherein step 4 includes locating a pair of hold-down pressure points at which pulse amplitude is substantially sixty percent of maximum pulse amplitude of the graph of the polynomial, the hold-down pressure at substantially the midpoint of said pair of hold-down pressure points providing an indication of the correct hold-down pressure.

8. A method as defined in claim 6 wherein step 4 includes locating the point of maximum pulse amplitude on the graph of the polynomial fitted in step 3 using coefficients of the polynomial, the hold-down pressure at the point of maximum pulse amplitude providing an indication of the correct hold-down pressure.

9. A blood pressure monitoring system for the continuous external measurements of blood pressure in an underlying artery comprising,

an external pressure transducer applied to a subject at a location overlying an artery, said pressure transducer including a pressure sensitive element having at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured and substantially centrally positioned over the underlying artery, said pressure sensitive element producing a continuous waveform having an amplitude which is a function of blood pressure in the underlying artery,

means for applying a variable hold-down pressure to the transducer, the hold-down pressure being variable over a range of at least from where the underlying artery is substantially unflattened to where the artery is substantially occluded,

means responsive to the waveform for obtaining a set of at least one of the diastolic pressure, systolic pressure, and pulse amplitude values over a range of hold-down pressures,

means for fitting a polynomial to a set of at least one of the diastolic pressure, systolic pressure, and pulse amplitude versus hold-down pressure values to obtain coefficients of the polynomial,

means employing the polynomial for obtaining an indication of the correct hold-down pressure required for making accurate blood pressure measurements, and

means for maintaining the transducer hold-down pressure at substantially the indicated correct hold-down pressure for obtaining accurate blood pressure measurements from the transducer.

10. A blood pressure monitoring system as defined in claim 9 wherein said means responsive to the waveform comprises means for obtaining a set of diastolic pressure versus hold-down pressure points,

said fitting means includes means for fitting a polynomial of at least a third-order to the set of diastolic versus hold-down pressure points where the polynomial is of the following type:

P.sub.m =a.sub.0 +a.sub.1 P.sub.h +a.sub.2 P.sub.h.sup.2 +a.sub.3 P.sub.h.sup.3

wherein:

P.sub.m =measured diastolic pressure,

P.sub.h =hold-down pressure, and

a.sub.0, a.sub.1, a.sub.2, and a.sub.3 are coefficients of the polynomial, and,

said means for obtaining an indication of correct hold-down pressure comprises means for locating the hold-down pressure, P3, at the point of minimum slope of the graph of the polynomial in accordance with the following equation;

P3=-a.sub.2 /3a.sub.3.

11. A blood pressure monitoring system as defined in claim 9 wherein said means responsive to the waveform comprises means for obtaining a set of systolic pressure versus hold-down pressure points,

said fitting means includes means for fitting a polymonial of at least third-order to the set of systolic versus hold-down pressure points where the polynomial is of the following type:

P.sub.m =a.sub.0 +a.sub.1 P.sub.h +a.sub.2 P.sub.h.sup.2 +a.sub.3 P.sub.h.sup.3

wherein:

P.sub.m =measured systolic pressure,

P.sub.h =hold-down pressure, and

a.sub.0, a.sub.1, a.sub.2, and 1.sub.3 are coefficients of the polynomial, and,

said means for obtaining an indication of correct hold-down pressure comprises means for locating the hold-down pressure, P3, of the point of minimum slope of the graph of the polynomial in accordance with the following equation;

P3=-a.sub.2 /3a.sub.3

12. A blood prssure monitoring system as defined in claim 9 wherein said means responsive to the waveform comprises means for obtaining a set of pulse amplitude versus hold-down pressure points,

said fitting means comprises means for fitting a polynomial of at least a second-order to the set of pulse amplitude versus hold-down pressure points,

said means for obtaining an indication of the correct hold-down pressure comprises means for locating a pair of hold-down pressure points on a graph of the polynomial at which the pulse amplitude equals substantially sixty percent of maximum pulse amplitude of the graph, the hold-down pressure at substantially the means of said pair of hold-down pressure points comprising the correct hold-down pressure.

13. A blood pressure monitoring system as defined in claim 9 wherein said means responsive to the waveform comprises means for obtaining a set of pulse amplitude versus hold-down pressure points,

said fitting means comprises means for fitting a polynomial of at least second order to the set of pulse amplitude versus hold-down pressure points, and

said means for obtaining an indication of the correct hold-down pressure comprises means employing coefficients of the polynomial for identifying the hold-down pressure at which pulse amplitude is maximum.

14. A machine-implemented method of obtaining a measure of the compliance of a subject's artery underlying an external pressure transducer that includes a pressure sensitive element applied to the subject using a variable hold-down pressure, the method comprising the steps of,

(1) obtaining a continuous measurement of blood pressure with the external pressure transducer while changing the hold-down pressure on the transducer over a range from where the underlying artery is unflattened to where the artery is substantially occluded,

(2) obtaining from the blood pressure measurement of step 1 a set of at least one of the diastolic and systolic pressure versus hold-down pressure values over said range of hold-down pressures,

(3) fitting a polynomial of at least third-order to a set of values obtained in step 2,

(4) determining the slope of a straight line fitted to a sub-set of values obtained in step 2 over a range of hold-down pressures below which flattening of the underlying artery occurs, and

(5) determining the minimum slope of the graph of the polynomial fitted to said set of values, the ratio of slopes determined in steps 5 and 4 providing a measure of compliance of the underlying artery.

15. A method as defined in claim 14 wherein step 5 includes using coefficients of the polynomial in determining the minimum slope of the graph of the polynomial fitted to said set of pressure values.

16. A method as defined in claim 15 including

(6) obtaining from the blood pressure measurement of step 1 a set of pulse amplitude versus hold-down pressure values over said range of hold-down pressures recited in step 2,

(7) fitting a polynomial of at least second-order to said set of pulse amplitude versus hold-down pressure values,

(8) identifying the hold-down pressure on a graph of the pulse amplitude versus hold-down pressure polynomial at which the pulse amplitude substantially equals to sixty percent of maximum pulse amplitude of the graph of the pulse amplitude versus hold-down pressure polynomial, and

(9) employing the lower hold-down pressure identified in step 8 as the upper end of the range of hold-down pressures used in step 4 for determining the slope of said straight line.

17. A blood pressure monitoring system for continuous external measurement of blood pressure in an underlying artery and for obtaining a measure of compliance of said underlying artery comprising

a pressure transducer that includes a pressure sensitive element having at least one dimension smaller than the lumen of the underlying artery and substantially centrally positioned over the artery, said pressure sensitive element producing a continuous waveform having an amplitude which is a function of blood pressure in the underlying artery,

means for applying a variable hold-down pressure to the transducer,

means responsive to the waveform for obtaining a set of diastolic pressure values, a set of systolic pressure values, and a set of pulse amplitude values over a range of hold-down pressures,

means for fitting a polynomial of at least third-order to a set of at least one of the diastolic pressure and systolic pressure versus hold-down pressure values and for fitting a polynomial of at least second-order to said pulse amplitude versus hold-down pressure values,

means for identifying the lowest hold-down pressure at which the pulse amplitude substantially equals sixty percent of the maximum pulse amplitude on the graph of the pulse amplitude versus hold-down pressure polynomial,

means for determining the slope of a straight line fitted to a sub-set of at least one of the diastolic pressure and systolic pressure values over a range of hold-down pressures below said hold-down pressure identified as the lowest hold-down pressure at which the pulse amplitude substantially equals sixty percent of the maximum, and

means for determining the minimum slope of the graph of at least one of the polynomials of diastolic pressure and systolic pressure versus hold-down pressure values, the ratio of said minimum slope to the slope of said straight line providing a measure of compliance of the underlying artery.

18. A system as defined in claim 17 including means for setting the hold-down pressure of the transducer at substantially the hold-down pressure at which the slope of at least one of the graphs of the diastolic pressure and systolic pressure versus hold-down pressure polynomial is minimum.

19. A system as defined in claim 17 including means for setting the hold-down pressure of the transducer at substantially the hold-down pressure midway between the pair of hold-down pressures at which the pulse amplitude polynomial substantially equals sixty percent of the maximum pulse amplitude on the graph of the pulse amplitude versus hold-down pressure polynomial.

20. A system as defined in claim 17 including means for setting the hold-down pressure of the transducer at substantially the hold-down pressure at which the pulse amplitude polynomial is maximum.

Description Submit all comments and votes

TECHNICAL FIELD

This invention relates to method and apparatus for non-invasively monitoring blood pressure through use of a transducer array of individual pressure or force sensing elements, and to method and means for ascertaining the correct transducer hold-down pressure required for obtaining accurate blood pressure measurements.

BACKGROUND OF THE INVENTION

The continuous measurement of blood pressure by use of arterial tonometer transducers is well known as shown in U.S. Pat. Nos. 3,123,068 to R. P. Bigliano, 3,219,053 to G. L. Pressman, P. M. Newgard and John J. Eige, 3,880,145 to E. F. Blick, 4,269,193 to the present inventor, and 4,423,738 to P. M. Newgard, and in an article by G. L. Pressman and P. M. Newgard entitled "A Transducer for the Continuous External Measurement of Arterial Blood Pressure" (IEEE Trans. Bio-Med. Elec., April 1963, pp. 73-81).

In a typical tonometric technique for monitoring blood pressure, a transducer which includes an array of pressure sensitive elements is positioned over a superficial artery, and a hold-down force is applied to the transducer so as to flatten the wall of the underlying artery without occluding the artery. The pressure sensitive elements in the array typically have at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured, and the transducer is positioned such that at least one of the individual pressure-sensitive elements is over at least a portion of the underlying artery. The output from one of the pressure sensitive elements is selected for monitoring blood pressure. In some prior art arrangements, the pressure sensitive element having the maximum pulse amplitude output is selected, and in other arrangements the element having a local minimum of diastolic or systolic pressure which element is within substantially one artery diameter of the element which generates the waveform of maximum pulse amplitude is selected.

The pressure measured by the selected pressure sensitive element, i.e. the element centered over the artery, will depend upon the hold-down pressure used to press the transducer against the skin of the subject. Although fairly accurate blood pressure measurements are obtained when a hold-down pressure within a rather wide pressure range is employed, it has been found that there exists a substantially unique value of hold-down pressure within said range for which tonometric measurements are most accurate. This so-called "correct" hold down pressure varies among subjects. With prior art tonometric type transducers the correct hold-down pressure often is not determined thereby leading to inaccuracies in the blood pressure measurements.

SUMMARY AND OBJECTS OF THE INVENTION

An object of this invention is the provision of an improved tonometric type method and apparatus for non-invasively monitoring blood pressure with a high degree of accuracy.

Another object of this invention is the provision of such a blood pressure measuring method and apparatus which includes the use of a transducer having an array of individual arterial riders (pressure sensitive elements) and wherein means are provided for determining the correct pressure required to hold the transducer against the subject to assure accuracy of the blood pressure readings.

The present invention includes a transducer array for generation of electrical waveforms indicative of blood pressure in an artery. Using the selected pressure sensing element that is determined to be positioned substantially over the center of the underlying artery, a set of at least one of the diastolic pressure, systolic pressure, or pulse amplitude pressure versus hold-down pressure values over a range of hold-down pressures between which the underlying artery is unflattened and is occluded is obtained. A polynomial is fitted to at least one of the sets of values from which polynomial the correct hold-down pressure is determined. The hold-down pressure at the point of minimum slope of graphs of the polynomials fitted to the systolic or diastolic versus hold-down pressure values provides an indication of the correct hold-down pressure. An indication of the correct hold-down pressure using the pulse amplitude measurements is provided by locating the point where the slope of the polynomial is zero or the midpoint of a pair of hold-down pressures at which the pulse amplitude is substantially sixty percent of maximum pulse amplitude of the graph of the polynomial fitted to the pulse amplitude versus hold-down pressure values. A measure of compliance of the underlying artery is obtained from the ratio of the minimum slope of the graph of the polynomial fitted to one of the systolic or diastolic pressure versus hold-down pressure values to the slope of a straight line fitted to a subset of one of the systolic or diastolic versus hold-down pressure values over a range of hold-down pressure below which flattening of the underlying artery begins or it may be obtained directly from the polynomial coefficients. The hold-down pressure at which flattening of the underlying artery begins is taken as the lowest of the above-mentioned pair of hold-down pressures at which the pulse amplitude is substantially sixty percent of maximum pulse amplitude of the graph of the polynomial fitted to the pulse amplitude versus hold-down pressure values.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with the above and other objects and advantages thereof will be better understood from the following description when considered with the accompanying drawings. It will be understood, however, that the illustrated embodiments of the invention are by way of example only and that the invention is not limited thereto. The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. In the drawings, wherein like reference characters refer to the same parts in the several views:

FIG. 1 shows the external appearance of a blood pressure transducer case, typically positioned over a superficial artery such as the radial artery, for providing a continuous external measurement of arterial blood pressure;

FIG. 2 is a schematic diagram illustrating the force balance between the artery and the multiple transducer elements (arterial riders), with the artery wall properly depressed to give accurate blood pressure readings;

FIG. 3 is a combination simplified vertical sectional view taken through the transducer case of FIG. 1 and block diagram of a system which may be employed therewith in the practice of this invention;

FIG. 4 is a waveform of human blood pressure versus time of the type which may be obtained using the present invention for illustrating systolic and diastolic pressure and pulse amplitude of the blood pressure wave;

FIGS. 5A and 5B together show a flow chart for use in explaining overall operation of this invention;

FIG. 6 shows plots of diastolic pressure and pulse amplitude versus hold-down pressure for a typical subject; and

FIG. 7 is a flow chart showing details of the computation of a correct hold-down pressure.

A typical application of the transducer array for arterial tonometry is illustrated in FIG. 1 wherein the transducer housing, or case, 10 which may have the appearance of an ordinary wristwatch case, is held in place over the radial artery in a human wrist 12 by a band 14. A cord 16 extends from the transducer housing 10 through which electrical wiring for the transducer array within the housing, together with a small tube that connects the housing to an air pressure source, extend. The wiring 18 and tube 20 are shown in FIG. 3, but not in FIG. 1.

Reference now is made to FIG. 2 wherein a diagrammatic mechanical model is shown which is representative of factors to be considered in the physical system. The illustrated model is that shown in the above-mentioned J. S. Eckerle U.S. Pat. No. 4,269,193 which was adapted from the G. L. Pressman and P. M. Newgard article entitled "A Transducer for the Continuous External Measurement of Arterial Blood Pressure". In brief, an array 22 of individual pressure sensitive elements or transducers 22A through 22J which constitute the arterial riders, is positioned so that one or more of the riders are entirely over an artery 24. The individual riders 22A-22J are small relative to the diameter of the artery 24 thus assuring that at least one of the riders in its entirety is over the artery. The skin surface 26 and artery underlying the transducer must be flattened by application of a hold-down pressure to the transducer. One rider overlying the center of the artery is identified as the "centered" arterial rider, from which arterial rider pressure readings for monitoring blood pressure are obtained. Means for selecting the arterial rider are disclosed in the above-mentioned J. S. Eckerle patent and G. L. Pressman and P. M. Newgard article. Using the above-mentioned rider selecting means, rider 22E, for example, may be selected as the "centered" arterial rider, in which case the remainder of the riders, here riders 22A- 22D and 22F through 22J comprise side plates which serve to flatten the underlying skin and artery. Depending upon the positioning of the transducer on the subject, a different transducer element may be positioned over the center of the artery and thereby be selected as the "centered" arterial rider.

Superficial arteries, such as the radial artery, are supported from below by bone which, in FIG. 2 is illustrated by ground symbol 28 under the artery. The wall of artery 24 behaves substantially like a membrane in that it transmits tension forces but not bending moments. The artery wall responds to the loading force of the transducer, and during blood pressure measurements acts as if it is resting on the firm base 28. The effective stiffness of an artery wall is small and differs between subjects. In prior art mechanical models of the physical system, the effective stiffness of the artery wall is taken as zero, in which case the actual hold-down pressure employed is not considered to affect accuracy of the blood pressure readings so long as the transducer is pressed against the skin surface with sufficient force to cause compression but not occlusion of the underlying artery. Applicant has found that not only are blood pressure readings dependent upon hold-down pressure within the range of hold-down pressures that the artery is flattened but not occluded, but that most accurate blood pressure readings are obtained where a hold-down pressure is selected that is substantially midway between the pressure where flattening of the artery begins and the minimum pressure required for occluding the same. Novel steps involved in computing the correct hold-down pressure are described in detail hereinbelow following completion of the description of the mechanical model of FIG. 2 and the overall system shown in FIG. 3.

With the illustrated system, the transducer case 10 and mounting strap 14 together with air pressure applied to a bellows, 54, supply the required compression force and hold the riders 22A-22J in such a manner that arterial pressure changes are transferred to the riders which overlie the artery 24. Diagrammatically this is illustrated by showing the individual riders 22A-22J backed by rider spring members 30A-30J, respectively, a rigid spring backing plate 32, and a hold-down force generator 36 between the backing plate 32 and the mounting strap system 38.

If, without force generator 36, the coupling between the mounting strap system 38 and spring backing plate 32 were infinitely stiff to restrain the riders 22A-22J rigidly with respect to the bone structure 28, the riders would be maintained in a fixed position relative to the artery. In practice, however, such a system is not practical, and hold-down force generator 36, comprising a pneumatic or other suitable loading system, is included to keep constant the force applied by the mounting strap system 38 to riders 22A through 22J. In the mechanical model the spring constant, k (force per unit of deflection) of the force generator 36 is nearly zero. Suitable pneumatic loading systems are shown and described in the above-referenced U.S. Pat. Nos. 3,219,035, 4,269,193 and the Pressman-Newgard IEEE article.

In order to insure that the riders 22A through 22J flatten the artery and provide a true blood pressure measurement, they must be rigidly mounted to the backing plate 32. Hence, the rider springs 30A through 30J of the omdel ideally are infinitely rigid (spring constant k=.infin.). It is found that as long as the system operates in such a manner that it can be modeled by rider springs 30A through 30J having a spring constant on the order of about ten times the value for the artery-skin system, so that the deflection of riders 22A through 22J is small; a true blood pressure measurement may be obtained when the correct hold-down pressure is employed.

The actual physical structure of a practical transducer of a type which may be employed for transducer array 22 in the present system is shown in the above-mentioned J. S. Eckerle U.S. Pat. No. 4,269,193. There, a transducer array is shown in which the individual transducers (riders) are formed in a thin monocrystalline silicon substrate which is made using integrated circuit techniques. In FIG. 3, to which reference now is made, a simplified showing of transducer 22 is shown comprising a chip 40 which includes an array of individual tranducers, not shown. Electrical conductors 42 connect the individual transducers to the wiring 18 for connection thereof to a multiplexer 43.

As seen in FIG. 3, case 10 comprises a generally cylindrical, hollow, container having rigid back and side walls 44 and 46, rspectively. The silicon transducer chip 40 is mounted within the face 48 of the case (designated as the front or operative face) in a cylindrical cup-like transducer housing 50. The operative face 48 includes the silicon transducer chip 40 along with its included individual transducers and arterial riders. Chip 40 may be affixed to a conventional ceramic dual in-line package that is plugged into an associated dual in-line socket, neither of which are shown in the drawings. A silicone rubber filler 52 is provided inside the housing 50 and around the dual in-line package and socket to provide a good seal, prevent electrical leakage between the transducer circuits and housing 50, and provide a flat surface to press against the subject. The front face 48 of the transducer includes the lower faces of housing 50 and filler 52.

The transducer housing 50 is fixed to the inside of the transducer case 10 by means of a cup-like silicone rubber bellows 54 which is sealed around the lower outside lip of the cup-shaped transducer housing 50, extends upwardly inside the outer wall of the transducer case 10, and is sealed to a ring 56, which in turn is fixed and sealed to the inside back of the transducer case 10. A chamber is formed inside the bellows which is connected to an air pressure source 58 through tube 20. A pressure controller 58A may be included in the pressure source. Since the flexible bellows 54 is sealed both to the transducer housing 50 and the inside of the transducer case 10, air under pressure from source 58 pneumatically loads the operative face 48. With the transducer strapped to the subject's wrist, the hold-down force F.sub.1 exerted by the tran