BLOOD PRESSURE MEASURING APPARATUS

Abstract: A blood pressure measuring apparatus having a housing (10) detachably set on a body surface (18) of a subject, a pressure sensor (26, 30) accommodated in the housing such that the pressure sensor is opposed to the body surface when the housing is set on the body surface, the pressure sensor being pressed against the body surface so as to detect pulse wave produced from an arterial vessel (40) of the subject, the pressure sensor generating pulse wave signal representing the detected pulse wave, a pressing device (20, 22, 24, 25) for pressing the pressure sensor against the body surface, and a control device (46) for determining blood pressure in the arterial vessel based on the pulse wave signal supplied from the pressure sensor, the pressing means and the control means being accommodated in the housing.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a blood pressure measuring apparatus and particularly to improvements in such an apparatus which measures blood pressure based on pulse wave produced from a blood vessel of a living body. Related Art Statement Pulse wave is generally defined as pressure wave, or oscillation of the wall of a blood vessel, produced with heart beat of a living body and transmitted through the blood vessel. There has been proposed a device for determining blood pressure based on the pulse wave. The Inventors and others filed Japanese Patent Application No. 62-130879 on May 27, 1987 in which they disclosed a pulse wave detecting device which is capable of determining maximum and minimum blood pressure based on the detected pulse wave. The device has (a) a housing detachably set on a body surface of a subject, (b) a pressure sensor accommodated in the housing such that the pressure sensor is opposed to the body surface when the housing is set on the body surface, the pressure sensor being pressed against the body surface so as to detect pulse wave produced from an arterial vessel of the subject, the pressure sensor generating pulse wave signal representing the detected pulse wave, (c) pressing means for pressing the pressure sensor against the body surface, and (d) a control circuit for determining blood pressure in the arterial vessel based on the pulse wave signal supplied from the pressure sensor. This device permits easier blood pressure measurement than a prior device using an inflatable cuff which occludes blood flow for blood pressure measurement as a result of compressing an upper arm of a subject. The above-indicated Japanese Patent Application was laid open under Publication No. 63-293424 on November 30, 1988. However, the above-indicated pulse wave detecting device is not easy to handle or handy to carry because the pressing means and the control circuit are disposed apart from the housing and connected thereto via air piping and electric wiring, respectively. Furthermore, such air piping and/or electric wiring restricts activity of the subject to some extent. GB-A- 2,118,719 A discloses a proposal for a digital watch having a piezoelectric transducer on its watch band held in contact with the wrist adjacent to the radial artery, the watch containing circuitry to derive and display the systolic and diastolic pressure in the artery and the heart beat rate. US-A-4,423,738 discloses an intra-arterial blood pressure monitoring device comprising an array of individual force sensing elements wherein the individual force sensing element which generates the wave form of maximum amplitude is used to monitor blood pressure. According to the invention there is provided a blood pressure measuring apparatus having a housing detachably set on a body surface of a subject, a pressure sensor accommodated in said housing such that said pressure sensor is opposed to said body surface when said housing is set on said body surface, said pressure sensor being pressed against said body surface so as to detect a pulse wave produced from an arterial vessel of said subject, said pressure sensor generating a pulse wave signal representing the detected pulse wave, pressing means for pressing said pressure sensor against said body surface, and control means accommodated in said housing for determining blood pressure in said arterial vessel based on said pulse wave signal supplied from said pressure sensor, wherein said pressing means comprises an externally threaded member secured to said housing and having an axis, said axis being substantially perpendicular to said body surface when said housing is set on said body surface, an internally threaded member engaged with said externally threaded member, and having a multiplicity of outer teeth formed along an outer circumference thereof, a motor having an output shaft, and a pinion engaged with said outer teeth of said internally threaded member and secured to said output shaft of said motor, a cylindrical member disposed co-axial with said externally and internally threaded members, said cylindrical member being fitted at a first axial end thereof on said externally threaded member such that the cylindrical member is not rotatable relative to the externally threaded member and is movable relative to the externally threaded member along the co-axis, said cylindrical member being engaged at said one axial end with said internally threaded member such that the cylindrical member is rotatable relative to the internally threaded member about said co-axis and is not movable relative to the internally threaded member along said co-axis. In the blood pressure measuring apparatus constructed as described above, all of the pressure sensor, pressing means and control means are accommodated in the housing. Accordingly, blood pressure measurement is easily performed by setting the housing (apparatus) on a body surface of a subject and detaching it therefrom. Also, the present apparatus permits easier use and conveyance thereof than the conventional apparatus of the type having a housing and air piping and electric wiring connecting between the housing and other external elements. Therefore, the present apparatus is free from the conventionally encountered problem that the activity of the subject is limited by the air piping and/or electric wiring. According to a preferred embodiment of the present invention, the blood pressure measuring apparatus further comprises a display device for displaying the blood pressure, the display device being provided on one of opposite outer surfaces of the housing, the other of the opposite outer surfaces being opposed to the body surface while the housing is set on the body surface. Thus, the display device is externally visible or observable. In the apparatus of the invention, the pressing means includes an externally threaded member secured to the housing and having an axis, the axis being substantially perpendicular to the body surface when the housing is set on the body surface, an internally threaded member engaged with the externally threaded member, and having a multiplicity of outer teeth formed along an outer circumference thereof, a motor having an output shaft, and a pinion engaged with the outer teeth of the internally threaded member and secured to the output shaft of the motor. Upon operation of the motor the internally threaded member is moved toward, and retracted away from, the body surface while being rotated around the externally threaded member as a result of rotation of the pinion driven by the motor. In a preferred embodiment, the pressure sensor includes a semiconductor chip, a plurality of pressure-sensitive semiconductor elements arranged along a line on the semiconductor chip, each of the semiconductor elements generating the pulse wave signal, and a cylindrical member co-axial with the externally and internally threaded members, the cylindrical member being fitted at one of axial ends thereof on the externally threaded member such that the cylindrical member is not rotatable relative to the externally threaded member and is movable relative to the externally threaded member along the co-axis, the cylindrical member being engaged at the one axial end thereof with the internally threaded member such that the cylindrical member is rotatable relative to the internally threaded member about the co-axis and is not movable relative to the internally threaded member along the co-axis, the semiconductor chip being secured to the other axial end of the cylindrical member. Upon the operation of the motor, the semiconductor chip is moved toward the body surface togther with the cylidrical member. In this case, the control means may be adapted to select one of the plurality of pressure-sensitive semiconductor elements which is located right above the arterial vessel, the control means monitoring the blood pressure of the subject based on the pulse wave signal from the selected semiconductor element. In another modified form of the above-indicated embodiment, the control means controls pressing force of the pressing means applied to the pressure sensor, by changing operation amount of the motor based on the pulse wave signal from the pressure sensor. BRIEF DESCRIPTION OF THE DRAWINGS The above and optional objects, features and advantages of the present invention will be better understood by reading the following detailed description of the presently preferred embodiment of the invention, when considered in connection with the accompanying drawings, in which: Fig. 1 is a cross-sectional view of a preferred embodiment of the blood pressure measuring apparatus of the present invention; Fig. 2 is a perspective view partially showing a pressure sensor of the apparatus of Fig. 1; Fig. 3 is a diagrammatic view of a control device of the apparatus of Fig. 1; Fig. 4 is a flow chart partially illustrating the operation of the apparatus of Fig. 1; and Fig. 5 is a graph illustrating varying trends in amplitude and upper peak value of pulse wave signals from a plurality of pressure-sensitive diodes of the pressure sensor of Fig. 2 which diodes are arranged along a line perpendicular to an arterial vessel from which blood pressure is detected. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to Fig. 1, there is shown a blood pressure measuring apparatus of the present invention. In the figure, reference numeral 10 designates a housing which is detachably set on a surface 18 of a wrist 14 of a subject with the help of a band 12. The housing 10 is separated into an upper and a lower room 10a, 10b by an intermediate partition wall 16. In the lower room 10b that is opposed to the wrist surface 18 when the apparatus (housing 10) is set on the surface 18, an externally threaded member 20 is secured to the intermediate partition wall 16 such that an axis of the externally threaded member 20 is substantially perpendicular to the wrist surface 18 when the housing 10 is set on the surface 18. An internally threaded nut member 22 is threadedly engaged with the externally threaded member 20. The nut member 22 has a multiplicity of teeth along an outer circumference thereof which teeth engage a pinion 25 secured to an output shaft of a motor 24. A cylindrical presser member 26 is fitted at an upper axial end thereof on a lower axial end of the externally threaded member 20, such that the presser member 26 is not rotatable relative to the externally threaded member 20 and is movable relative to the same 20 vertically as viewed in Fig. 1, namely, along the axis of the member 20. This arrangement is assured by engagement between a groove 28 axially extending in an outer threaded surface of the externally threaded member 20, and a mating ridge (not shown) formed in an inner surface of the presser member 26. Moreover, the presser member 26 is engaged at the upper end thereof with the nut member such that the presser member 26 is rotatable relative to the nut member 22 and is not movable relative to the same 22. Thanks to this arrangement, upon operation of the motor 24, the nut member 22 is moved downward and upward while being rotated around the externally threaded member 20, and the presser member 26 is slid on the externally threaded member 20 substantially perpendicularly to the body surface 18 together with the nut member 22. A presser plate 30 is fixed to a lower axial end of the presser member 26 such that, with the housing 10 set on the body surface 18, the presser plate 30 extends perpendicular to the direction in which the presser member 26 is moved, namely, in a direction substantially parallel to the body surface 18. The presser plate 30 is moved through a lower opening 13 formed through a bottom wall 15 of the housing 10 when the motor 24 is operated. As shown in Fig. 2, the presser plate 30 includes a semiconductor chip 32 formed of monocrystalline silicon or the like, and a multiplicity of pressure-sensitive diodes 34 formed on an upper surface 33 of the chip 32. Electrical signals representing pressure variation at interfaces between the chip 32 and the diodes 34 are produced between a common terminal 36 and individual terminals 38 of the diodes 34. With the housing 10 set on the wrist 14, the multiplicity of pressure-sensitive diodes 34 are in row along a line substantially perpendicular to a radial artery 40 (Fig. 1), such that the diodes 34 are equally spaced apart from each other by a suitable distance. The width of each diode 34 as measured in the direction perpendicular to the artery 40 and the equal distance between each pair of adjacent two diodes 34, are pre-determined such that at least three diodes 34 can be located within a body-surface portion right above the artery 40 which portion has a length equal to the diameter or width of the artery 40. The shape or configuration of each diode 34, and the dimension (length) of the same 34 as measured in a direction parallel to the artery 40 are appropriately pre-determined. At locations on a lower surface 42 of the semiconductor chip 30 which locations are opposite to the locations on the upper surface 33 of the chip 30 where a number of pressure-sensitive diodes 34 are provided, corresponding number of rubber fillers 44 are provided. The rubber fillers 44 are embedded in recesses 43 such that the fillers 44 do not apply any load to the corresponding diodes 34 and such that outer surfaces of the fillers 44 are aligned with the lower surface 42 of the chip 30, namely, the lower surface 42 has no unevenness. With the presser plate 30 pressed against the body surface 18 as a result of operation of the motor 24, the body surface 18 right above and near the radial artery 40 is flattened under the presser plate 30. Thus, the pressure oscillation or pulse wave produced from the artery 40 is transmitted to the pressure-sensitive diodes 34 via the rubber fillers 44. The semiconductor chip 32 has an extremely small thickness, for example 15 µm, at the locations where the recesses 43 for the ruber fillers 44 are provided. The pressure oscillation transmitted to the rubber fillers 44, causes pressure variation at the interfaces between the pressure-sensitive diodes 34 and the semiconductor chip 32. Each diode 34 generates an electrical signal representing the pressure variation, the electrical signal being utilized as pulse wave signal SM. In the instant embodiment, the pressure-sensitive diodes 34 serve as the pressure-sensitive semiconductor elements for the blood pressure measuring apparatus. Further, the externally threaded member 20, nut member 22, motor 24 and other members cooperate with each other to function as the pressing means for pressing the diodes 34 against the body surface 18. The presser plate 30 is secured to the cylindrical pressure member 26 via an electrically insulating supporting member (not shown), so that electric leakage from the pressure plate 30 (chip 32) is prevented. In the upper room 10a of the housing 10, is disposed a control device 46 for processing the pulse wave signals SM from the pressure-sensitive diodes 34. The control device 46 includes an electrical circuit shown in Fig. 3. Each pulse wave singal SM is transmitted to an amplifier (not shown), and a band-pass filter (not shown) which transmits only the frequency component corresponding to the pulse wave signal SM. Subsequently the pulse wave signal SM is supplied to an A/D converter 48, which converts the signal SM into digital signal and applies the signal to a CPU (central processing unit) 50. The CPU 50 also receives activation signal SS from a button switch PB which is manually operable from outside the housing 10 for activating the motor 24 to effect the pressing of the presser plate 30 (diodes 34) against the body surface 18. The CPU 50 is coupled to a RAM (random access memory) 52 and a ROM (read only memory 54) via data bus, and cooperates therewith to form a microcomputer. The CPU 50 processes the received signals according to programs pre-stored in the ROM 54 by utilizing the temporary-storage function of the RAM 52, and generates drive signal SD to a motor drive circuit 58 via an output interface 56. The motor drive circuit 58 adjusts operation amount of the motor 24 based on the received signal SD. Concurrently, the CPU 50 supplies display signal DD representing the maximum and minimum blood pressure determined based on the pulse wave signals SM, to a display 60, which displays numeral characters or bar graphs representing the blood pressure according to the received signal DD. The display 60 is disposed and supported at an upper opening 11 of the housing 10, and accordingly the displayed blood pressure is externally observable from the subject or medical staff. Referring to Fig. 4, there is illustrated the flow chart for the signal processing routine executed by the control device 46. There will be described the operation of the instant blood pressure measuring apparatus, in conjunction with the flow chart. Initially, upon operation of the button switch PB with the housing 10 set on the wrist 14 with the band 12 such that the presser plate 30 covers the body surface 18 right above the radial artery 40, the CPU 50 executes step S1 at which the CPU 50 controls the motor drive circuit 58 to press the presser plate 30 (presser member 26) against the body surface 18 with an optimum pressing force. More specifically described, the CPU 50 controls operation amount of the motor 24 by generating the drive signal SD to the motor drive circuit 58, so as to move the presser plate 30 to an optimum vertical position (as viewed in Fig. 1). The optimum pressing force or optimum vertical position is determined by the CPU 50 based on amplitude or magnitude of the pulse wave signal SM, for example, when the amplitudes of pulse wave signals SM from diodes 34 which are positioned on the body surface right above the artery 40 have become substantially equal to each other, or when the magnitude of a pulse wave signal SM from a diode 34 which is positioned at the middle of the body surface right above the artery 40 has become lower than those from the other diodes 34 located on both sides of the middle diode 34. Step S1 is followed by step S2 at which the CPU 50 determines the amplitudes A of the pulse wave signals SM from the multiplicity of pressure-sensitive diodes 34 arranged along a straight line substantially perpendicular to the radial artery 40, and selects a maximum amplitude A max . Subsequently, at step S3 a reference value A s is calculated by multiplying the maximum amplitude A max by a predetermined coefficient k (1 > k > 0). Step S3 is followed by step S4 at which, as shown in the lower portion of the graph of Fig. 5, a group of pulse wave signals SM (A) whose amplitudes are greater than the reference value A s , are selected. The selected signals SM (A) are the signals SM supplied from a group of pressure-sensitive diodes 34 positioned right above the artery 40. In other words, the amplitudes A of the group of diodes 34 right above the artery 40 are greater than those of the other diodes 34 on both sides of the group of diodes 34. In the present embodiment, the selected signals SM (A) include at least three signals SM from at least three diodes 34 right above the artery 40. Since the reference value A s is determined based on the maximum amplitude A max , the selection of the group of the pulse wave signals SM (A) right above the artery 40 is effected with accuracy, for example without influence from physical or physilological differences among subjects. Step S4 is followed by step S5 at which the CPU 50 determines an upper peak value (maximum value) P max of each of the pulse wave signals SM (A) selected at step S4. The upper peak value P max corresponds to systolic blood pressure in the artery 40. At the following step S6, a pulse wave signal SM (A) whose upper peak value P max is smaller than those of the other signals SM (A) , is selected as signal SM (P) . As shown in the upper portion of the graph of Fig. 5, the selected signal SM (P) is the signal SM (A) from a diode 34 which is positioned at the middle of the body surface right above the artery 40. In other words, the upper peak value P max of the pulse wave signal SM from the diode 34 located at the middle of the body surface right above the artery 40, is smaller than those from the other diodes 34 on both sides of the middle diode 34. Stated further differently, the selected signal SM (P) is the signal SM (A) whose upper peak value P max corresponds to a minimal value of the upper curve of the graph of Fig. 5 which represents a distribution of upper peak values P max of the signals SM A taken in the direction perpendicular to the artery 40. Once the pulse wave signal SM (P) is selected, the CPU 50 continuously reads in the signal SM (P) supplied from the corresponding diode 34, and at step S7 the CPU 50 determines the maximum and minimum blood pressure based on the signal SM (P) , and generates display signal DD representing the determined blood pressure. Thus, the display 60 continuously displays the blood pressure. The reason for determining blood pressure based on the signal SM (P) from the diode 34 located at the middle of the body surface right above the artery 40, is that the signal SM (P) is free from influence from tension of the wall of the artery 40 and accordingly the signal SM (P) accurately reflects actual pressure oscillation (pulse wave) transmitted through the artery 40. Stated differently, regarding absolute value of blood pressure, the value determined based on the signal SM (P) is very approximate to that of the actual blood pressure in the artery 40. Thus, the upper and lower peak values P max and P min of the pulse wave signal SM (P) accurately correspond to the actual maximum and minimum blood pressure. Once the positional relationship of the presser plate 30 relative to the artery 40 is changed due to physical motion of the subject, for example, the previously selected pulse wave signal SM (P) does not accurately represent the actual blood pressure. It is therefore preferred that steps S2-S6 be repeated to update the signal SM (P) by selecting different signals SM (or different diodes 34) at suitable time intervals or each time a predetermined number of pulses have been detected. In that case, the apparatus is capable of continuously measuring accurate blood pressure, even in the case where the positional relationship between the pressure plate 30 (diodes 34) and the body surface 18 (artery 40) is changed. All the elements or parts for the instant blood pressure measuring apparatus are accommodated in the housing 10. Specifically described, the presser plate 30 including the multiplicity of pressure-sensitive diodes 34, is disposed in the lower room 10b of the housing 10. Furthermore, the housing 10 accommodates the pressing means for pressing the presser plate 30 against the body surface 18, namely the motor 24, the nut member 22 and other members, the control device 46 for determining blood pressure based on the pulse wave signal SM, and the display 60 for displaying the blood pressure determined by the control device 46. Accordingly, blood pressure measurement is effected by simply setting the apparatus (housing 10) on the wrist 14 with the band 12 and operating the button switch PB. Further, the apparatus permits easier handling and conveyance than the prior device having air piping and/or electrical wiring for connecting between a housing thereof set on a body surface and external elements thereof. Moreover, the apparatus by no means restricts activity of the subject, in contrast to the above-indicated prior device. While the present invention has been described in its preferred embodiment with detailed particularities, it is to be understood that the invention may be embodied with various modifications. For example, although in the illustrated embodiment the display 60 is provided on the housing 10, it is possible to employ memory or the like for storing blood pressure measurements, in place of or in addition to the display 60. Further, it is possible to concurrently monitor blood pressure regarding a plurality of subjects in a room remote from the subjects, by using a radio telemeter in each blood pressure apparatus (housing 10). Furthermore, it is possible to employ well-known sensors such as semiconductor strain gauges or pressure-sensitive transistors as the pressure sensor, in place of the pressure-sensitive diodes 34 used in the illustrated embodiment. While in the illustrated embodiment the control device 46 selects one pulse wave signal SM (P) from the pulse wave signals SM generated by the multiplicity of pressure-sensitive diodes 34 and determines blood pressure based on the selected signal SM (P) , according to the flow chart of Fig. 4 for the signal processing routine, it is possible to change the number of the diodes 34 and/or the arrangement of the flow chart for the signal processing routine so long as blood pressure in the artery 40 is determined through detection of the pulse wave produced from the artery 40. Although in the illustrated embodiment both maximum and minimum blood pressure is measured, it is possible to adapt the apparatus to measure only one of the maximum and minimum blood pressure or average value of the maximum and minimum blood pressure. While in the illustrated embodiment the microcomputer or software is adapted to select the pulse wave signal SM (P) for blood pressure measurement, it is possible to provide a logic circuit or hardware for effecting the same operation, namely, selection of the signal SM (P) . Furthermore, while the illustrated apparatus is adapted to detect pulse wave from a radial artery (40) to measure blood pressure, the principle of the present invention is applicable to apparatus of other types adapted to detect pulse wave from a cartid artery or a vain for the same purpose. It is possible to measure blood pressure in a vain although it does not produce pulse wave or transmit pressure oscillation. It is to be understood that the present invention may be embodied with other modifications, changes and improvements which may occur to those skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims: A blood pressure measuring apparatus having a housing (10) detachably set on a body surface (18) of a subject, a pressure sensor (26, 30) accommodated in said housing such that said pressure sensor is opposed to said body surface when said housing is set on said body surface, said pressure sensor being pressed against said body surface so as to detect a pulse wave produced from an arterial vessel (40) of said subject, said pressure sensor generating pulse wave signal representing the detected pulse wave, pressing means (20, 22, 24, 25) for pressing said pressure sensor against said body surface, and control means (46) accommodated in said housing for determining blood pressure in said arterial vessel based on said pulse wave signal supplied from said pressure sensor, characterised in that said pressing means comprises an externally threaded member (20) secured to said housing (10) and having an axis, said axis being substantially perpendicular to said body surface (18) when said housing is set on said body surface, an internally threaded member (22) engaged with said externally threaded member, and having a multiplicity of outer teeth formed along an outer circumference thereof, a motor (24) having an output shaft, and a pinion (25) engaged with said outer teeth of said internally threaded member and secured to said output shaft of said motor, a cylindrical member (26) disposed co-axial with said externally and internally threaded members (20, 22), said cylindrical member being fitted at a first axial end thereof on said externally threaded member such that the cylindrical member is not rotatable relative to the externally threaded member and is movable relative to the externally threaded member along the co-axis, said cylindrical member being engaged at said one axial end with said internally threaded member such that the cylindrical member is rotatable relative to the internally threaded member about said co-axis and is not movable relative to the internally threaded member along said co-axis. The apparatus as set forth in claim 1, further comprising a display device (60) for displaying said blood pressure, said display device being provided on one of opposite outer surfaces of said housing (10), the other of said opposite outer surfaces being opposed to said body surface (18) when said housing is set on said body surface. The apparatus as set forth in claim 1 or 2, wherein said pressure sensor comprises a semiconductor chip (32), a plurality of pressure-sensitive semiconductor elements (34) arranged along a line on said semiconductor chip, each of said semiconductor elements generating the pulse wave signal, said semiconductor chip being secured to a second axial end of said cylindrical member. The apparatus as set forth in claim 3, wherein said control means (46) selects one of said plurality of pressure-sensitive semiconductor elements (34) which is located right above said arterial vessel (40), said control means monitoring said blood pressure of said subject based on the pulse wave signal (PM (P) ) from the selected semiconductor element. The apparatus as set forth in any preceding claim, wherein said control means (46) controls pressing force of said pressing means (20, 22, 24, 25) applied to said pressure sensor (26, 30), by changing operation amount of said motor (24) based on said pulse wave signal (SM) from said pressure sensor.

Inventors: HARADA, CHIKAO, YOKOE, HIFUMI
Applicants: COLIN ELECTRONICS CO., LTD.
Application year: 1989
Main classification: A61B5
IPC codes: A61B5