Blood pressure monitoring method and apparatus

I claim:

1. A system for the continuous external measurement of blood pressure in an underlying artery of a subject including an array of individual pressure sensitive elements, the length of which array exceeds the diameter of the underlying artery,

means employing outputs from all elements of the array for identifying one element at a center of a region to be searched,

user information input means for entering information concerning the subject related to the diameter of the underlying artery into the system,

means employing said information concerning the subject related to the diameter of the underlying artery for estimating an extent of said region to be searched, and

means responsive to outputs from only those elements within said region for selecting one element having a local minimum of at least one of the diastolic and systolic pressures on a plot of diastolic or systolic pressure versus transducer element number.

2. A system as defined in claim 1 wherein said selecting means comprises means for selecting the one element having a local minimum of diastolic pressure.

3. A system as defined in claim 1 wherein said selecting means comprises means for selecting the one element having a local minimum of systolic pressure.

4. A system as defined in claim 1 wherein said selecting means includes means for selecting the one element that has both a local minimum of diastolic pressure and a local minimum of systolic pressure.

5. A system for continuous external measurement of blood pressure in an underlying artery of a subject including an array of individual pressure sensitive elements, the length of which array exceeds the diameter of the underlying artery,

means employing outputs from all elements of the array for identifying one element at the center of a region to be searched, said means including means for finding the center of gravity of the pulse amplitude values for the transducer elements in the array,

means employing information concerning the subject related to the diameter of the underlying artery for estimating the extent of said region to be searched, and

means responsive to outputs from only those elements within said region for selecting the one element having a local minimum of at least one of the diastolic and systolic pressures on a plot of diastolic or systolic pressure versus transducer element number.

6. A system for continuous external measurement of blood pressure in an underlying artery of a subject including an array of individual pressure sensitive elements, the length of which array exceeds the diameter of the underlying artery,

means employing outputs from all elements of the array for identifying one element at the center of a region to be searched, said means including means for identifying two local maxima of the pulse amplitude values and for identifying the element substantially midway between the two local maxima as the element at the center of the region,

means employing information concerning the subject related to the diameter of the underlying artery for estimating the extent of said region to be searched, and

means responsive to outputs from only those elements within said region for selecting the one element having a local minimum of at least one of the diastolic and systolic pressures on a plot of diastolic or systolic pressure versus transducer element number.

7. A system for continuous external measurement of blood pressure in an underlying artery of a subject including an array of individual pressure sensitive elements, the length of which array exceeds the diameter of the underlying artery,

means employing outputs from all elements of the array for identifying one element at the center of a region to be searched, said means including means for fitting a polynomial of at least second order to the pulse amplitude versus transducer element values and for identifying the point on the graph of the polynomial at which it is maximum as the location of the element at the center of the search region,

means employing information concerning the subject related to the diameter of the underlying artery for estimating the extent of said region to be searched, and

means responsive to outputs from only those elements within said region for selecting the one element having a local minimum of at least one of the diastolic and systolic pressures on a plot of diastolic or systolic pressure versus transducer element number.

8. A system for continuous external measurement of blood pressure is an underlying artery of a subject including an array of individual pressure sensitive elements, the length of which array exceeds the diameter of the underlying artery,

means employing outputs from all elements of the array for identifying one element at the center of a region to be searched,

means employing information concerning the subject related to the diameter of the underlying artery for estimating the extent of said region to be searched, which information includes at least one of the subject's sex, age, height, weight, arm diameter and wrist diameter, and

means responsive to outputs from only those elements within said region for selecting the one element having a local minimum of at least one of the diastolic and systolic pressures on a plot of diastolic or systolic pressure versus transducer element number.

9. A system for the continuous external measurement of blood pressure in an underlying artery of a subject comprising,

a pressure sensitive transducer that includes an array of pressure sensitive elements, a group of which elements overlie the artery and with elements extending beyond both edges of the artery, elements of the array producing continuous waveforms which are a function of blood pressure in the underlying artery,

means responsive to outputs from the array for obtaining a set of pulse amplitude values for elements of the array, and

means for selecting one element which is substantially located adjacent the center of the underlying artery, said selecting means including means for finding the center of gravity of the set of pulse amplitude values.

10. A system for continuous external measurement of blood pressure in an underlying artery of a subject comprising,

a pressure sensitive transducer that includes an array of pressure sensitive elements, a group of which elements overlie the artery and with elements extending beyond edges of the artery, elements of the array producing continuous waveforms which are a function of blood pressure in the underlying artery,

means responsive to outputs from the array for obtaining a set of pulse amplitude values for elements of the array, and

means for selecting one element which is substantially located adjacent the center of the underlying artery, said selecting means including means for identifying the two largest local maxima of the set of pulse amplitude values and for identifying the element substantially midway between the two local maxima as the element which is substantially located adjacent the center of the underlying artery.

11. A system for the continuous external measurement of blood pressure in an underlying artery of a subject comprising,

a pressure sensitive transducer that includes an array of pressure sensitive elements, of group of which elements overlie the artery and with elements extending beyond both edges of the artery, elements of the array producing continuous waveforms which are a function of blood pressure in the underlying artery,

means responsive to outputs from the array for obtaining a set of pulse amplitude values for elements of the array, and

means for selecting one element which is substantially located adjacent the center of the underlying artery, said selecting means including means for fitting a polynomial of at least second order to the set of pulse amplitude values, and means for selecting the point on the graph of the polynomial at which it is maximum as the location of the element which is substantially located adjacent the center of the underlying artery.

12. A method for continuous external measurement of blood pressure in an artery of a subject comprising,

positioning a transducer array that includes a plurality of pressure sensitive elements over an underlying artery such that some elements extend beyond both edges of the artery,

employing output from all of the elements, identifying one element at substantially the center of the artery,

based upon information which includes at least one of the subject's sex, age, height, weight, arm diameter and wrist diameter, estimating the diameter of the underlying artery and the number of elements in a group thereof which substantially overlie the artery, and

employing outputs from only those elements in said group thereof, selecting for monitoring the one element having a local minimum of at least one of the diastolic and systolic pressures, where "local minimum" refers to a local minimum on the plot of diastolic or systolic pressure versus transducer element number.

13. A method for the continuous external measurement of blood pressure in an artery of a subject comprising,

positioning a transducer array that includes a plurality of pressure sensitive elements over an underlying artery with some elements extending beyond both edges of the artery,

for each element of the array obtaining a pulse amplitude value in response to the output from the element to obtain a set of pulse amplitude values, and

using the center of gravity of the set of pulse amplitude values for identifying one element which is located substantially at the center of the underlying artery.

14. A method for the continuous external measurement of blood pressure in an artery of a subject comprising,

positioning a transducer array that includes a plurality of pressure sensitive elements over an underlying artery with some elements extending beyond both edges of the artery,

for each element of the array obtaining a pulse amplitude value in response to the output from that element to obtain a set of pulse amplitude values,

identifying the two largest local maxima in the set of pulse amplitude values, and

identifying the element substantially midway between these two local maxim as the element located substantially at the center of the underlying artery.

15. A method for the continuous external measurement of blood pressure in an artery of a subject comprising,

positioning a transducer array that includes a plurality of pressures sensitive elements over an underlying artery with some elements extending beyond both edges of the artery,

for each element of the array obtaining a pulse amplitude value in response to the output from that element to obtain a set of pulse amplitude values,

fitting a polynomial of at least second order to the set of pulse amplitude values,

identifying the point on the graph of the polynomial at which pulse amplitude is maximum,

identifying the element nearest the point on the graph of the polynomial at which the polynomial is maximum as being the element which is substantially located adjacent the center of the underlying artery.

Description Submit all comments and votes

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 ar 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 an 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 tranducer 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 pressures 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 transducer element obtained from a subject under ideal conditions;

FIG. 7 shows a pressure distribution which is similar to that of FIG. 6 from the same subject but under conditions that are not ideal;

FIG. 8 is a flow chart showing details of the selection of the transducer element to be employed for monitoring blood pressure; and

FIG. 9 is a schematic diagram which is similar to that of FIG. 2 but showing only a transducer array and a large underlying artery such as the artery of an adult male.

A typical application of the transducer array for arterial tonometry is illustrated in FIG. 1 wherein the transducer housing, or case, 10 which 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 22-1 through 22-19 which constitute the arterial riders, is positioned so that one or more of the riders are entirely over an artery 24. The individual riders 22-1 through 22-19 are small relative to the diameter of the artery 24 thus assuring that a plurality of the riders overlie the artery. The skin surface 26 and artery underlying the transducer must be flattened by application of a holddown pressure to the transducer. One rider overlying the center of the artery is identified as the "centered" rider, from which rider pressure readings for monitoring blood pressure are obtained. The present invention is directed to novel method and means for selecting the "centered" rider, and is described in detail hereinbelow. For present purposes it will be understood that by using the rider selecting means of this invention, one of the riders, such as rider 22-10, may be selected as the "centered" rider, in which case the remainder of the riders, here riders 22-1 through 22-9 and 22-11 through 22-19 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 be selected as the "centered" 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 array, 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 for which the artery is flattened but not occluded, but that most accurate blood pressure readings are obtained when a hold-down pressure is selected that is substantially midway between the pressure at which 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 in the above-mentioned U.S. patent application Ser. No. 927,843.

With the illustrated system, the transducer case 10 and mounting strap 14, together with air pressure applied to a bladder 54, supply the required compression force and hold the riders 22-1 through 22-19 in such a manner that arterial pressure changes are transferred to the riders which overlie the artery 24. This is illustrated schematically in FIG. 2 by showing the individual riders 22-1 through 22-19 backed by rider spring members 30-1 through 30-19, respectively, a rigid spring backing plate 32, and 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 22-1 through 22-19 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 (in the present example) a pneumatic loading system, is included to keep constant the force applied by the mounting strap system 38 to riders 22-1 through 22-19. 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 and 4,269,193, and the Pressman and Newgard IEEE article.

In order to insure that the riders 22-1 through 22-19 flatten the artery and provide a true blood pressure measurement, they must be rigidly mounted to the backing plate 32. Hence, the rider springs 30-1 through 30-19 of the device 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 simulated by rider springs 30-1 through 30-19 having a spring constant on the order of about ten times the corresponding constant for the artery-skin system, so that the deflection of riders 22-1 through 22-19 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 fabrication 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 transducers, 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, respectively. 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 bladder 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 bladder 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 bladder 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 transducer array against the skin of the subject is adjustable by control of the air pressure. (In the diagrammatic mechanical model shown in FIG. 2 the hold-down force F.sub.1 is generated by hold-down force generator 36.)

As noted above, electrical signals related to pressure sensed by the individual transducers 22-1 through 22-19 of transducer 22 are supplied as inputs to an analog multiplexer 43. From the multiplexer, the signals are digitized by an analog-to-digital (A-D) converter 60, and the digitized signals are supplied to a digital computer 62 having memory 62A and a clock 62B. Other information, such as the subject's name, sex, weight, height, age, arm and wrist dimensions, and the like, are supplied to the computer through user information input means such as a a keyboard 64. As described hereinbelow, this information is used to make an estimate of the diameter of the artery of the subject, which then is used to establish the range of transducer elements that will be employed in selecting the "centered" rider after the center of the search region has been established.

Output from the computer is supplied to data display and recorder means 66 which may include a recorder, cathode ray tube monitor, a solid state display, or the like. If the desired, at least a portion of the visual display may be included in transducer case 10. Obviously, the computer output may be supplied to a printer, an audible alarm, or the like, as desired. Also, an output from the computer is supplied over line 68 to the pressure controller for control of the transducer hold-down pressure.

In FIG. 4, to which reference now is made, the signal waveform of the output from one of the pressure sensitive elements 22-1 through 22-19 which overlies artery 24 is shown. Other elements of the transducer array which overlie the artery will have waveforms of similar shape. With a correct hold down pressure and correct selection of the "centered" arterial rider (i.e. the rider substantially centered over the artery) the waveform is representative of the blood pressure within the underlying artery. Systolic, diastolic and pulse amplitude pressures are indicated on the waveform, wherein pulse amplitude is the difference between the systolic and diastolic pressures for a given heartbeat.

OVERALL SYSTEM OPERATION

FIGS. 5A and 5B, together show a flow chart of an algorithm for general, overall, operation of the blood pressure monitoring system. Some of the operations indicated therein are under control of computer 62 responsive to programming instructions contained in memory unit 62A. Obviously, several program steps may be involved in the actual implementation of the indicated operations. Since the programming of such steps is well within the skill of the average programmer, a complete program listing is not required and is not included herein.

Preparation for monitoring is begun at START step 100 at which time system power is turned on or a reset operation is performed, by means not shown, and counters, registers, timers, and the like in computer 62 are initialized. At step 102 information concerning the subject, such as the subject's sex, weight, height, age, arm and/or wrist dimensions, and the like, is entered into the computer memory through use of the keyboard 64. Next, at step 104, a nominal hold-down pressure (H-D.P.) is applied wherein air under pressure from source 58 is supplied to the transducer. For example, a hold-down pressure of say 120 mmHg may be supplied to the transducer, which pressure serves to extend the bladder 54 whereby operative face 48 extends outwardly a short distance from the bottom of the case 10. The transducer is attached to the subject at step 106 at a location wherein a centrally located transducer element, such as element 22-10 of transducer array 22 overlies the center of the artery 24. Of course, the exact position of the transducer array relative to the underlying artery generally is not visually apparent to the subject, or operator, and repositioning of the transducer may be required to properly position the same.

With the transducer attached to the subject, step 108 is entered at which point systolic and diastolic pressures are determined from the outputs of each transducer element, and pulse amplitude values are determined by subtraction of the diastolic pressure values from the systolic pressure values. Step 110 then is entered wherein the transducer element to be used in monitoring blood pressure is selected. Novel algorithms which may be used in selecting the proper transducer element are described in detail hereinbelow. For present purposes, it will be understood that the transducer element that is selected for monitoring will be near the center of the array and will be substantially centered over the underlying artery. As will become apparent hereinbelow, step 110 may require respositioning of the transducer for properly locating it relative to the underlying artery.

Following step 110, the hold-down pressure to use to obtain accurate blood pressure measurements is computed at step 116. Algorithms which may be used in computing the correct hold-down pressure are disclosed in the above-mentioned patent application Ser. No. 927,843, filed Nov. 6, 1986. For purposes of the present application, it will be understood that a correct hold-down pressure for accurate blood-pressure monitoring is computed at step 116, following which, at step 118, the computed hold-down pressure is set by control of pressure controller 58A by the computer 62.

With the transducer properly positioned on the subject and the correct hold-down pressure supplied thereto, the system is in condition for obtaining accurate blood pressure readings. At step 120 an indication that the system is operative is provided, as by display of the words "Readings Valid". Obviously, other displays, such as a green indicator light, may be employed for indicating the operating state of the system.

From the output from the selected transducer element, systolic and diastolic pressure values together with pulse amplitude values are readily determined in step 122. Also, pulse rate is readily calculated by determining the time between successive diastolic or systolic pressures. At step 124, values calculated and determined in step 122 are displayed and/or recorded along with the actual waveform. Obviously, the values which are calculated and displayed depend upon the use to be made of the blood pressure monitor, a display of all of the values not being required in many instances. For example, the blood pressure waveform could be recorded without calculation and display of any of the values identified in step 122.

After one or more of the values identified in step 122 are determined, such as systolic and/or diastolic pressure, step 126 is entered wherein the system waits for the next heartbeat cycle. Diastolic or systolic pressure points may be used to identify reference points in the heartbeat cycles. Decision step 128 then is entered at which time a timer in computer 62 is tested to determine whether or not it has reached a predetermined time "M", where M is a time period of, say, 30 minutes. If the elapsed time exceeds the predetermined time period, M, wherein the deci