Patent Publication Number: US-2020297224-A1

Title: Blood pressure estimation apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International application No. PCT/JP2018/043356, filed Nov. 26, 2018, which claims priority to Japanese Patent Application No. 2017-242392, filed Dec. 19, 2017, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to blood pressure estimation apparatuses, and particularly, to a blood pressure estimation apparatus that estimates a blood pressure based on a pulse transit time. 
     BACKGROUND ART 
     Japanese Patent Laying-Open No. 02-177937 (PTL 1) is a prior art literature disclosing a configuration of a blood pressure monitoring apparatus. The blood pressure monitoring apparatus described in PTL 1 includes a housing shaped into a closed cylinder, a pulse wave sensor, and a pulse wave sensor positioning device. The blood pressure monitoring apparatus is detachably attached to a wrist by a band with an opening end of the housing facing the wrist. The pulse wave sensor and the pulse wave sensor positioning device are provided inside the housing. The pulse wave sensor positioning device includes a pair of rubber bags, an electrically powered pump that supplies a fluid to each of the pair of rubber bags, and a switch valve capable of switching between application of pressure and exhaust of pressure of each of the pair of rubber bags. The pulse wave sensor is disposed between the pair of rubber bags. The pulse wave sensor is positioned relative to a radial artery by controlling the switch valve to adjust the pressure of each of the pair of rubber bags. 
     Japanese Patent Laying-Open No. 63-275320 (PTL 2) is a prior art literature disclosing a configuration of a pulse wave apparatus. The pulse wave apparatus described in PTL 2 includes a hollow main body with an opening at its lower end, a diaphragm and a contact maker that detect pulse waves of an artery, and moving means for locating the contact maker immediately above the artery. The pulse wave apparatus is detachably attached to a wrist by a band with its opening facing the wrist. The diaphragm, contact maker, and moving means are provided inside the main body. The moving means includes a plurality of bellows and a pressure regulation valve that supplies regulated air to each of the plurality of bellows. The pressure regulation valve is controlled to regulate the pressure of the air to be supplied to each of the plurality of bellows, thereby adjusting the position of the contact maker relative to the artery. 
     SUMMARY OF INVENTION 
     Technical Problem 
     The blood pressure monitoring apparatus described in PTL 1 and the pulse wave apparatus described in PTL 2 are each attached to a wrist with the opening end of the housing facing the wrist, and the pulse wave sensor positioning device moves the pulse wave sensor in the housing, thereby adjusting the position of the pulse wave sensor relative to the radial artery. Thus, the range in which the position of the pulse wave sensor is adjustable is limited to the inside of the housing. When a preferable position of the pulse wave sensor is located outside the housing, accordingly, the pulse wave sensor cannot be adjusted to the preferable position. 
     The present invention has been made in view of the above problem, and an object thereof is to provide a blood pressure estimation apparatus capable of increasing a range in which the position of a pulse wave detection unit of a pulse wave sensor is adjustable, thus stably estimating a blood pressure. 
     Solution to Problem 
     A blood pressure estimation apparatus according to the present invention includes a belt, a first fluid bag and a second fluid bag, a pulse wave sensor, a fluid supply unit, a first pressure sensor, and a second pressure sensor. The belt surrounds a measurement site. The first fluid bag and the second fluid bag are located side by side along an inner circumference of the belt, expand and contract upon entry and exit of a fluid, and are provided to press the measurement site from therearound while surrounding the measurement site. The pulse wave sensor includes a pulse wave detection unit that detects a pulse wave of an artery passing through the measurement site. The fluid supply unit supplies the fluid to the first fluid bag and the second fluid bag. The first pressure sensor detects a pressure in the first fluid bag. The second pressure sensor detects a pressure in the second fluid bag. The pulse wave detection unit is disposed on an external surface portion of the first fluid bag and provided to press the measurement site upon expansion of the first fluid bag. A position of the pulse wave detection unit relative to the artery passing through the measurement site is adjusted through adjustment of a ratio between a volume of the fluid in the first fluid bag and a volume of the fluid in the second fluid bag by the fluid supply unit. 
     In one embodiment of the present invention, the pulse wave detection unit detects a pulse wave based on a change in an impedance of the artery passing through the measurement site. 
     In one embodiment of the present invention, the fluid supply unit includes a pump that delivers the fluid, a first on-off valve connected between the first fluid bag and the pump, and a second on-off valve connected between the second fluid bag and the pump. 
     Advantageous Effects of Invention 
     The present invention can increase the range in which the position of the pulse wave detection unit of the pulse wave sensor is adjustable, thus stably estimating a blood pressure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view showing an appearance of a blood pressure estimation apparatus according to an embodiment of the present invention. 
         FIG. 2  is a sectional view showing a state in which the blood pressure estimation apparatus according to the embodiment of the present invention is attached to a measurement site. 
         FIG. 3  shows an arrangement of a pulse wave detection unit of a pulse wave sensor with the blood pressure estimation apparatus according to the embodiment of the present invention being attached to the measurement site. 
         FIG. 4  is a block diagram showing a configuration of the blood pressure estimation apparatus according to the embodiment of the present invention. 
         FIG. 5  is a sectional view showing how the blood pressure estimation apparatus according to the embodiment of the present invention, which is attached to the measurement site, measures a blood pressure by the oscillometric method. 
         FIG. 6A  is a sectional view showing how the blood pressure estimation apparatus according to the embodiment of the present invention, which is attached to the measurement site, measures blood pressure propagation times, and  FIG. 6B  shows pulse transit times of a radial artery detected by a first pulse wave detection unit and a second pulse wave detection unit of the blood pressure estimation apparatus according to the embodiment of the present invention. 
         FIG. 7  is a graph showing experimental results of the calculation of a cross-correlation coefficient between a pulse wave signal detected by the first pulse wave detection unit and a pulse wave signal detected by the second pulse wave detection unit by changing strengths of pressing the first pulse wave detection unit and the second pulse wave detection unit against a palm lateral surface of a left wrist. 
         FIG. 8  is a sectional view showing a state in which a ratio between a fluid volume in the first fluid bag and a fluid volume in the second fluid bag is adjusted in the blood pressure estimation apparatus according to the embodiment of the present invention. 
         FIG. 9  is a flowchart showing an operation flow in estimation of a blood pressure by the blood pressure estimation apparatus according to the embodiment of the present invention based on a pulse transit time. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A blood pressure estimation apparatus according to an embodiment of the present invention will now be described with reference to the drawings, in which the same or corresponding parts are designated by the same reference numerals, and description thereof will not be repeated. 
       FIG. 1  is a perspective view showing an appearance of a blood pressure estimation apparatus according to an embodiment of the present invention.  FIG. 2  is a sectional view showing a state in which the blood pressure estimation apparatus according to the embodiment of the present invention is attached to a measurement site.  FIG. 2  shows a cross-section perpendicular to the longitudinal direction of a left wrist. In the present embodiment, the measurement site is the left wrist. The measurement site may be a right wrist. 
     As shown in  FIGS. 1 and 2 , a blood pressure estimation apparatus  1  according to an embodiment of the present invention includes a display unit  10 , a belt portion  20 , and a pulse wave sensor. Display unit  10  displays the result of blood pressure estimation of blood pressure estimation apparatus  1 . Belt portion  20  is connected to display unit  10  and surrounds a left wrist  90 , which is a measurement site. The pulse wave sensor includes a pulse wave detection unit  40 E, which detects a pulse wave of an artery passing through the measurement site. 
     Blood pressure estimation apparatus  1  is mainly composed of belt portion  20  surrounding left wrist  90 , which is the measurement site, and display unit  10  connected to belt portion  20 . 
     As shown in  FIG. 1 , display unit  10  has an outer shape of a truncated quadrangular pyramid which projects outwardly from belt portion  20 . Display unit  10  preferably has a small size and a low profile so as not to hinder activities of a subject. 
     Display unit  10  is provided with a display  50  and an operation unit  52 . Display  50  is disposed on a top surface portion  10   a  of display unit  10 . Operation unit  52  is disposed on a lateral surface portion  10   f  of display unit  10 . 
     Display unit  10  is provided integrally with one end  20   e  of belt portion  20  through integral molding. In another configuration, belt portion  20  and display unit  10  may be formed separately and connected to each other by, for example, an engaging member such as a hinge. As shown in  FIG. 1 , a bottom surface  10   b  of display unit  10  and an end  20   f  of belt portion  20  are connected to each other by a buckle  15 . 
     Buckle  15  includes a plate-shaped member  25 , which is disposed on the outer circumferential side, and a plate-shaped member  26 , which is disposed on the inner circumferential side. One end  25   e  of plate-shaped member  25  is pivotally attached to display unit  10  via a coupling rod  27  running in a width direction Y. The other end  25   f  of plate-shaped member  25  is pivotally attached to the other end  26   f  of plate-shaped member  26  via a coupling rod  28  running in width direction Y. One end  26   e  of plate-shaped member  26  is fixed to the vicinity of end  20   f  of belt portion  20  by a fixing portion  29 . 
     For the circumferential direction of belt portion  20 , the position of attachment of fixing portion  29  is adjusted in advance in accordance with the circumferential length of left wrist  90  of a subject. Blood pressure estimation apparatus  1  has a substantially annular shape in its entirety. A portion between bottom surface  10   b  of display unit  10  and end  20   f  of belt portion  20  is configured to be opened/closed by buckle  15  in the direction of arrow B in  FIG. 1 . 
     Belt portion  20  includes a belt  23 , and a first fluid bag  21  and a second fluid bag  22 , which are provided on the inner circumferential side of belt  23  and are capable of expanding and contracting. The dimension of belt portion  20  in width direction Y is, for example, approximately 30 mm. Belt  23  is an elongated band-shaped member surrounding left wrist  90  in the circumferential direction. Belt  23  has an outer circumferential portion  20   b . Belt  23  is formed of a plastic material that is flexible in the thickness direction and is not elastic in the circumferential direction. 
     First fluid bag  21  and second fluid bag  22  are attached to belt  23 . First fluid bag  21  and second fluid bag  22  are positioned side by side on an inner circumferential portion  23   a  of belt  23 . The inner circumferential portion of belt portion  20  which contacts left wrist  90  is composed of a first inner circumferential portion  21   a  and a second inner circumferential portion  22   a . First fluid bag  21  has an external surface portion forming first inner circumferential portion  21   a . Second fluid bag  22  has an external surface portion forming second inner circumferential portion  22   a.    
     Each of first fluid bag  21  and second fluid bag  22  is formed through welding of the circumferential portions of two stretchable polyurethane sheets overlaid with each other, thus being shaped into a bag capable of receiving a fluid. The fluid includes both of liquid and gas, and for example, water, air, or the like can be used as the fluid. First fluid bag  21  and second fluid bag  22  expand and contract upon entry and exit of the fluid and are provided to press left wrist  90  from therearound while surrounding left wrist  90 . 
     Blood pressure estimation apparatus  1  is provided with a fluid supply unit that supplies a fluid to first fluid bag  21  and second fluid bag  22 . Blood pressure estimation apparatus  1  is provided with a first pressure sensor that detects a pressure in first fluid bag  21  and a second pressure sensor that detects a pressure in second fluid bag  22 . 
     A pulse wave detection unit  40 E of the pulse wave sensor is provided on first inner circumferential portion  21   a  of belt portion  20 . In the present embodiment, pulse wave detection unit  40 E of the pulse wave sensor is provided on the external surface portion of first fluid bag  21  of first inner circumferential portion  21   a  of belt portion  20 . Pulse wave detection unit  40 E is provided to press left wrist  90  upon expansion of first fluid bag  21 . 
     Pulse wave detection unit  40 E of the pulse wave sensor is composed of six electrodes spaced from each other in width direction Y of belt portion  20 . Specifically, a current electrode  41 , a detection electrode  42 , a detection electrode  43 , a detection electrode  44 , a detection electrode  45 , and a current electrode  46  are arranged side by side in a row in order from one side of width direction Y. Detection electrode  42  and detection electrode  43  constitute a first pulse wave detection unit. Detection electrode  44  and detection electrode  45  constitute a second pulse wave detection unit. 
     Each of a spacing between detection electrode  42  and detection electrode  43  and a spacing between detection electrode  44  and detection electrode  45  in width direction Y of belt portion  20  is, for example, 2 mm. Each of current electrode  41 , detection electrode  42 , detection electrode  43 , detection electrode  44 , detection electrode  45 , and current electrode  46  has a rectangular outer shape and is formed with a low profile and flexibility. 
     With blood pressure estimation apparatus  1  attached to left wrist  90 , pulse wave detection unit  40 E is disposed corresponding to a radial artery  91  of left wrist  90 . Radial artery  91  passes through the vicinity of a palm lateral surface  90   a  of left wrist  90 , which is the surface on the palm side, within left wrist  90 . In the present embodiment, pulse wave detection unit  40 E detects a pulse wave based on a change in the impedance of radial artery  91  passing through left wrist  90 . 
     The method of detecting pulse waves by the pulse wave detection unit is not limited to the method of detecting a pulse wave based on changes in the impedance of an artery. For example, the pulse wave sensor may include a light emitting element that radiates light toward an artery passing through a corresponding portion of the measurement site and a right receiving element that receives reflected light or transmitted light of the light and detect a change in the volume of the artery as a pulse wave. 
     The pulse wave sensor may include a piezoelectric sensor held in contact with the measurement site and detect a distortion due to a pressure of the artery passing through a corresponding portion of the measurement site as a change in electrical resistance. Further, the pulse wave sensor may include a transmission element that transmits a radio wave toward an artery passing through a corresponding portion of the measurement site and a reception element that receives a reflected wave of the electric wave and detect a change in the distance between the artery and the sensor due to a pulse wave of the artery as a phase deviation between a transmission wave and a reflective wave. 
     In attachment of blood pressure estimation apparatus  1  to left wrist  90 , the subject passes the left hand through belt portion  20  from the direction indicated by arrow A in  FIG. 1  with buckle  15  being opened for an increased annular diameter of belt portion  20 . Then, as shown in  FIG. 2 , the subject adjusts the angular position of belt portion  20  around left wrist  90  to position pulse wave detection unit  40 E of the pulse wave sensor such that pulse wave detection unit  40 E faces radial artery  91  passing through left wrist  90 . 
     Thus, pulse wave detection unit  40 E of the pulse wave sensor is held in contact with a portion  90   a   1  of palm lateral surface  90   a  of left wrist  90 , which corresponds to radial artery  91 . In this state, the subject closes and fixes buckle  15 . Consequently, the subject attaches blood pressure estimation apparatus  1  onto left wrist  90 . With blood pressure estimation apparatus  1  attached to left wrist  90 , display unit  10  is disposed corresponding to a rear surface  90   b  of left wrist  90 , which is the surface on the back side of the hand. 
       FIG. 3  shows the arrangement of the pulse wave detection unit of the pulse wave sensor with the blood pressure estimation apparatus according to the embodiment of the present invention being attached to the measurement site. As shown in  FIG. 3 , with blood pressure estimation apparatus  1  attached to left wrist  90 , pulse wave detection unit  40 E of the pulse wave sensor is preferably located along radial artery  91 . 
     A second pulse wave detection unit  402  composed of detection electrode  44  and detection electrode  45  is disposed downstream of a first pulse wave detection unit  401  composed of detection electrode  42  and detection electrode  43  in a bloodstream of radial artery  91 . A spacing between first pulse wave detection unit  401  and second pulse wave detection unit  402  in width direction Y of belt portion  20  is, for example, 20 mm. In other words, a distance between a midpoint between detection electrode  42  and detection electrode  43  and a midpoint between detection electrode  44  and detection electrode  45  in width direction Y of belt portion  20  is, for example, 20 mm. 
     The components of blood pressure estimation apparatus  1  will now be described in detail.  FIG. 4  is a block diagram showing a configuration of the blood pressure estimation apparatus according to the embodiment of the present invention. 
     As shown in  FIG. 4 , display unit  10  is provided with a central processing unit (CPU)  100 , display  50 , a memory  51 , operation unit  52 , a battery  53 , and a communication unit  59 . 
     Display unit  10  is also provided with a first pressure sensor  31 , a second pressure sensor  34 , a pump  32 , a first on-off valve  35   a , and a second on-off valve  35   b . Pump  32  delivers a fluid to first fluid bag  21  and second fluid bag  22 . First on-off valve  35   a  is connected between first fluid bag  21  and pump  32 . Second on-off valve  35   b  is connected between second fluid bag  22  and pump  32 . 
     Further, display unit  10  is provided with a first oscillator circuit  310 , which converts an output of first pressure sensor  31  to a frequency, a second oscillator circuit  340 , which converts an output of second pressure sensor  34  to a frequency, and a pump drive circuit  320 , which drives pump  32 . 
     Pulse wave sensor  40  includes pulse wave detection unit  40 E and a current feed and voltage detection circuit  49 . Each of current electrode  41 , detection electrode  42 , detection electrode  43 , detection electrode  44 , detection electrode  45 , and current electrode  46  is connected with current feed and voltage detection circuit  49 . Current feed and voltage detection circuit  49  is connected with CPU  100  through a signal wire  72 . 
     Display  50  is implemented by, for example, an organic electro luminescence (EL) display, and displays information on estimation of a blood pressure, such as a blood pressure estimation result, and any other information in response to a control signal from CPU  100 . Display  50  is not limited to the organic EL display and may be implemented by any other type of display, such as a liquid crystal display (LCD). 
     Operation unit  52  is implemented by, for example, a push switch, and provides CPU  100  with an operation signal corresponding to an instruction to start or stop the estimation of a blood pressure by a subject. Operation unit  52  is not limited to the push switch and may be implemented by, for example, a touch panel switch, such as a pressure-sensitive switch or a proximity touch panel switch. Alternatively, display unit  10  may be provided with a microphone, and an instruction to start or stop the estimation of a blood pressure by voice of the subject may be provided to CPU  100  through the microphone. 
     Memory  51  stores in a non-transitory manner a program for controlling blood pressure estimation apparatus  1 , data used for controlling blood pressure estimation apparatus  1 , setting data for setting various functions of blood pressure estimation apparatus  1 , and data on the results of blood pressure estimation. Memory  51  is also used as a work memory in execution of a program. 
     CPU  100  controls various functions of blood pressure estimation apparatus  1  in accordance with the program for controlling blood pressure estimation apparatus  1  stored in memory  51 . For example, when a blood pressure is measured by the oscillometric method, CPU  100  drives pump  32  based on signals from first pressure sensor  31  and second pressure sensor  34  in response to the instruction to start the measurement of a blood pressure from operation unit  52 , thereby rendering first on-off valve  35   a  and second on-off valve  35   b  open. CPU  100  calculates a blood pressure based on the signals from first pressure sensor  31  and second pressure sensor  34 . 
     When estimating a blood pressure based on a pulse transit time, CPU  100  drives pump  32  based on the signals from first pressure sensor  31  and second pressure sensor  34  in response to the instruction to start the estimation of a blood pressure from operation unit  52 , thereby controlling the open/closed state of first on-off valve  35   a  and second on-off valve  35   b.    
     Communication unit  59  is controlled by CPU  100  to transmit predetermined information to an external device through a network  900  or communicate the information received from the external device through network  900  to CPU  100 . The communications performed by network  900  may be either wireless communications or wired communications. For example, network  900  is the Internet, which is not limited thereto. Network  900  may be any other type of network, such as local area network (LAN), or one-to-one communication using a USB cable or the like. Communication unit  59  may include a micro-USB connector. 
     Pump  32  and first on-off valve  35   a  are connected to first fluid bag  21  through a first air pipe  39   a . Pump  32  and second on-off valve  35   b  are connected to second fluid bag  22  through a second air pipe  39   b . Pump  32  is, for example, a piezoelectric pump. Pump  32  supplies air into first fluid bag  21  through first air pipe  39   a  in order to pressurize first fluid bag  21 . Pump  32  supplies air into second fluid bag  22  through second air pipe  39   b  in order to pressurize second fluid bag  22 . 
     First pressure sensor  31  is connected to first fluid bag  21  through a first air pipe  38   a . First pressure sensor  31  detects the pressure in first fluid bag  21  through first air pipe  38   a . First pressure sensor  31  is, for example, a piezoresistive pressure sensor. For example, first pressure sensor  31  outputs, as time-series signals, pressures detected with atmospheric pressure being defined as a zero point. 
     Similarly, second pressure sensor  34  is connected to second fluid bag  22  through a second air pipe  38   b . Second pressure sensor  34  detects the pressure in second fluid bag  22  through second air pipe  38   b . Second pressure sensor  34  is, for example, a piezoresistive pressure sensor. For example, second pressure sensor  34  outputs, as time-series signals, pressures detected with atmospheric pressure being defined as a zero point. 
     Each of first on-off valve  35   a  and second on-off valve  35   b  operates to be opened and closed based on a control signal supplied from CPU  100 . Pump drive circuit  320  drives pump  32  based on a control signal supplied from CPU  100 . 
     First oscillator circuit  310  outputs, to CPU  100 , a frequency signal with a frequency corresponding to an electrical signal value which is based on a change in the electrical resistance due to the piezo resistance effect from first pressure sensor  31 . The output of first pressure sensor  31  is used to control the pressure in first fluid bag  21  and to calculate a blood pressure by the oscillometric method. 
     Similarly, second oscillator circuit  340  outputs, to CPU  100 , a frequency signal with a frequency corresponding to an electrical signal value which is based on a change in the electrical resistance due to the piezo resistance effect from second pressure sensor  34 . The output of second pressure sensor  34  is used to control the pressure in second fluid bag  22  and to calculate a blood pressure by the oscillometric method. 
     Blood pressures calculated by the oscillometric method include a systolic blood pressure (SBP) and a diastolic blood pressure (DBP). 
     Battery  53  supplies electric power to various elements mounted on display unit  10 . Battery  53  also supplies electric power to current feed and voltage detection circuit  49  of pulse wave sensor  40  through a line  71 . Line  71  is provided to extend between display unit  10  and pulse wave sensor  40  in the circumferential direction of belt portion  20  while being sandwiched between belt  23  and first fluid bag  21  of belt portion  20  together with signal wire  72 . Battery  53  is also connected with CPU  100 . 
     Voltage detection circuit  49  of pulse wave sensor  40  operates based on a control signal supplied from CPU  100 . Specifically, voltage detection circuit  49  includes an analog filter  403 , an amplifier  404 , and an analog/digital (A/D) converter  405 . Voltage detection circuit  49  may further include a step-up circuit that boosts a power supply voltage and a voltage regulation circuit that regulates the boosted voltage to a predetermined voltage. 
     Following will describe an operation of blood pressure estimation apparatus  1  in estimation of a blood pressure with blood pressure estimation apparatus  1  according to the embodiment of the present invention. 
     First, blood pressure estimation apparatus  1  measures a blood pressure by the oscillometric method.  FIG. 5  is a sectional view showing how the blood pressure estimation apparatus according to the embodiment of the present invention, which is attached to a measurement site, measures a blood pressure by the oscillometric method.  FIG. 5  shows a cross-section taken in the longitudinal direction of the left wrist. 
     Upon receipt of an instruction to start the measurement of a blood pressure from operation unit  52 , CPU  100  of blood pressure estimation apparatus  1  renders first on-off valve  35   a  and second on-off valve  35   b  open and drives pump  32  through pump drive circuit  320 , thereby supplying air into first fluid bag  21  and second fluid bag  22 . This expands first fluid bag  21  and second fluid bag  22  and gradually pressurizes first fluid bag  21  and second fluid bag  22 . As shown in  FIG. 5 , first fluid bag  21  and second fluid bag  22  extend in the circumferential direction of left wrist  90 , and are pressurized by pump  32  to press the circumference of left wrist  90  uniformly at a pressure Pc 1 . 
     During pressurization, in order to calculate a blood pressure, CPU  100  monitors pressure Pc 1  in first fluid bag  21  with first pressure sensor  31  and also pressure Pc 1  in second fluid bag  22  with second pressure sensor  34  and obtains a fluctuation component of the volume of the artery, which occurs in radial artery  91  of left wrist  90 , as a pulse wave signal. CPU  100  is not necessarily required to obtain a pulse wave signal based on both of pressure Pc 1  in first fluid bag  21  and pressure Pc 1  in second fluid bag  22  and may be only required to obtain a pulse wave signal based on at least one of pressure Pc 1  in first fluid bag  21  and pressure Pc 1  in second fluid bag  22 . 
     Based on the obtained pulse wave signal, CPU  100  applies a publicly known algorithm by the oscillometric method and begins to calculate each of a systolic blood pressure and a diastolic blood pressure. When CPU  100  has not yet calculated a blood pressure due to lack of data, unless pressure Pc 1  in first fluid bag  21  and pressure Pc 1  in second fluid bag  22  have not reached an upper limit pressure, for example, approximately 300 mmHg, CPU  100  begins to boost pressure Pc 1  in first fluid bag  21  and pressure Pc 1  in second fluid bag  22  further and calculate a blood pressure again. 
     When CPU  100  has successfully calculated a blood pressure, CPU  100  stops pump  32  through pump drive circuit  320 . CPU  100  displays the result of blood pressure measurement on display  50  and also records the result in memory  51 . A blood pressure may be calculated not only during pressurization but also during decompression. 
     Since only pulse wave detection unit  40 E is located between the external surface portion of first fluid bag  21  of first inner circumferential portion  21   a  of belt portion  20  and left wrist  90 , pressing by first fluid bag  21  is not hindered by any other member, so that a blood vessel can be closed sufficiently. Since any other member is not located between the external surface portion of second fluid bag  22  of second inner circumferential portion  22   a  of belt portion  20  and left wrist  90 , pressing by second fluid bag  22  is not hindered by the other member, so that a blood vessel can be closed sufficiently. A blood pressure can thus be measured by the oscillometric method with high accuracy. 
     Blood pressure estimation apparatus  1  then measures a pulse transit time.  FIG. 6A  is a sectional view showing how the blood pressure estimation apparatus according to the embodiment of the present invention, which is attached to the measurement site, measures blood pressure propagation times, and  FIG. 6B  shows pulse transit times of a radial artery which are detected by the first pulse wave detection unit and the second pulse wave detection unit of the blood pressure estimation apparatus according to the embodiment of the present invention.  FIG. 6A  shows a cross-section taken in the longitudinal direction of the left wrist. In  FIG. 6B , the vertical axis represents voltage (V), and the horizontal axis represents time. 
     First, in detection of a pulse wave of radial artery  91 , CPU  100  of blood pressure estimation apparatus  1  renders first on-off valve  35   a  and second on-off valve  35   b  open and drives pump  32  through pump drive circuit  320 , thereby supplying air into first fluid bag  21  and second fluid bag  22 . 
     Consequently, first fluid bag  21  and second fluid bag  22  are expanded, and first fluid bag  21  and second fluid bag  22  are gradually pressurized. Each of first fluid bag  21  and second fluid bag  22  is pressurized by pump  32 , so that the inner pressure attains to Pc 2  lower than Pc 1 , as shown in  FIG. 6A . Each of first pulse wave detection unit  401  and second pulse wave detection unit  402  is pressed against palm lateral surface  90   a  of left wrist  90  through expansion of first fluid bag  21 . Specifically, upon receipt of a pressing force corresponding to pressure Pc 2  in first fluid bag  21 , each of first pulse wave detection unit  401  and second pulse wave detection unit  402  is pressed against palm lateral surface  90   a  of left wrist  90 . 
     In order to detect a pulse wave of a radial artery, current feed and voltage detection circuit  49  applies a voltage between current electrode  41  and current electrode  46  to flow a current i, which has, for example, a frequency of 50 kHz and a current value of 1 mA. In this state, current feed and voltage detection circuit  49  detects a voltage signal v 1  between detection electrode  42  and detection electrode  43  and a voltage signal v 2  between detection electrode  44  and detection electrode  45 . 
     Specifically, current feed and voltage detection circuit  49  accepts an input of voltage signal v 1  detected by first pulse wave detection unit  401  and accepts an input of voltage signal v 2  detected by second pulse wave detection unit  402 . 
     Voltage signal v 1  represents a change in the electrical impedance in a portion of palm lateral surface  90   a  of left wrist  90 , which faces first pulse wave detection unit  401 , due to a pulse wave of a bloodstream of radial artery  91 . Voltage signal v 2  represents a change in the electrical impedance in a portion of palm lateral surface  90   a  of left wrist  90 , which faces second pulse wave detection unit  402 , due to a pulse wave of a bloodstream of radial artery  91 . 
     Analog filter  403  of current feed and voltage detection circuit  49  has a transfer function G and performs filtering on the amplified voltage signal v 1  and voltage signal v 2 . Specifically, analog filter  403  removes noise of other than frequencies that characterize voltage signal v 1  and voltage signal v 2  and performs filtering for improving a signal-noise ratio (SN ratio). Amplifier  404  is implemented by, for example, an operational amplifier and amplifies the filtered voltage signal v 1  and voltage signal v 2 . A/D converter  405  converts the amplified voltage signal v 1  and voltage signal v 2  from analog data to digital data and outputs the digital data to CPU  100  through line  72 . 
     CPU  100  performs signal processing on digital data of each of the received voltage signal v 1  and voltage signal v 2 , thereby generating a pulse wave signal PS 1  and a pulse wave signal PS 2  each having a waveform with a crest as shown in  FIG. 6B . CPU  100  further calculates a time difference Δt between a peak A 1  of pulse wave signal PS 1  and a peak A 2  of pulse wave signal PS 2 . Time difference Δt is a pulse transit time (PTT). 
     The voltage value of each of voltage signal v 1  and voltage signal v 2  is, for example, approximately 1 my. Each of peak A 1  of pulse wave signal PS 1  and peak A 2  of pulse wave signal PS 2  is, for example, approximately 1 V. Assuming that a pulse wave velocity (PWV) of a bloodstream of radial artery  91  is in the range of 1000 cm/s or more and 2000 cm/s or less, when a distance D between first pulse wave detection unit  401  and second pulse wave detection unit  402  is 20 mm, a time difference Δt between pulse wave signal PS 1  and pulse wave signal PS 2  is in the range of 1.0 ms or more and 2.0 ms or less. 
     CPU  100  performs calibration between a blood pressure measured by the oscillometric method and a pulse transit time Δt to associate the blood pressure and pulse transit time Δt with each other. As a result, a blood pressure can be estimated based on pulse transit time Δt. 
     In the estimation of a blood pressure based on pulse transit time Δt, the cross-correlation coefficient between pulse wave signal PS 1  and pulse wave signal PS 2  should exceed a threshold for guaranteeing reliability. 
     Description will now be given of an example experiment in which the cross-correlation coefficient between pulse wave signal PS 1  and pulse wave signal PS 2  was calculated by changing the force of pressing of pulse wave detection unit  40 E against palm lateral surface  90   a  of left wrist  90 . 
       FIG. 7  is a graph showing experimental results of the calculation of a cross-correlation coefficient between a pulse wave signal detected by the first pulse wave detection unit and a pulse wave signal detected by the second pulse wave detection unit by changing the force of pressing of the first pulse wave detection unit and the second pulse wave detection unit against the palm lateral surface of the left wrist. In  FIG. 7 , the vertical axis represents a cross-correlation coefficient r between two waveforms of pulse wave signal PS 1  and pulse wave signal PS 2 , and the horizontal axis represents a force (mmHg) of pressing of the first pulse wave detection unit and the second pulse wave detection unit against the palm lateral surface of the left wrist. 
     In this example experiment, cross-correlation coefficient r between pulse wave signal PS 1  and pulse wave signal PS 2  was calculated while gradually increasing pressure Pc 2  in first fluid bag  21 , which is the force of pressing of each of first pulse wave detection unit  401  and second pulse wave detection unit  402  against palm lateral surface  90   a  of left wrist  90 , from 0 mmHg. A threshold Th of cross-correlation coefficient r was set to 0.99. 
     As shown in  FIG. 7 , as the pressing force increased from 0 mmHg, cross-correlation coefficient r increased to a maximum value r max, and then decreased after reaching maximum value r max. In the range of pressing force of 72 mmHg or more and 150 mmHg or less, cross-correlation coefficient r exceeded threshold Th. This range is an appropriate pressing force range. That is to say, the appropriate pressing force range has a lower limit P 1  of 72 mmHg and an upper limit P 2  of 150 mmHg. When the value of pressing force was P 3  within the appropriate pressing force range, cross-correlation coefficient r had maximum value r max. 
     Although cross-correlation coefficient r between pulse wave signal PS 1  detected by first pulse wave detection unit  401  and pulse wave signal PS 2  detected by second pulse wave detection unit  402  was calculated by changing the force of pressing of first pulse wave detection unit  401  and second pulse wave detection unit  402  against palm lateral surface  90   a  of left wrist  90  in this example experiment, even when the pressing force is uniform, cross-correlation coefficient r fluctuates as the positions of first pulse wave detection unit  401  and second pulse wave detection unit  402  change relative to radial artery  91  of left wrist  90 . 
     Specifically, there is an appropriate position range of each of first pulse wave detection unit  401  and second pulse wave detection unit  402  relative to radial artery  91  of left wrist  90 . When at least one of first pulse wave detection unit  401  and second pulse wave detection unit  402  is located outside of this appropriate position range, cross-correlation coefficient r is equal to or less than threshold Th, resulting in reduced reliability of a blood pressure estimate. 
     In blood pressure estimation apparatus  1  according to the present embodiment, then, when cross-correlation coefficient r is less than or equal to threshold Th even though the pressing force is within the appropriate pressing force range, CPU  100  determines that at least one of first pulse wave detection unit  401  and second pulse wave detection unit  402  is located outside of the appropriate position range and causes the fluid supply unit to adjust the ratio between the volume of the fluid in first fluid bag  21  and the volume of the fluid in second fluid bag  22 , thereby adjusting the positions of first pulse wave detection unit  401  and second pulse wave detection unit  402  relative to radial artery  91  of left wrist  90 . 
       FIG. 8  is a sectional view showing a state in which a ratio between a fluid volume in the first fluid bag and a fluid volume in the second fluid bag is adjusted in the blood pressure estimation apparatus according to the embodiment of the present invention.  FIG. 8  shows a cross-section perpendicular to the longitudinal direction of the left wrist. 
     As shown in  FIG. 8 , as the volume of the fluid in first fluid bag  21  is increased and the volume of the fluid in second fluid bag  22  is decreased, left wrist  90  is pressed by first fluid bag  21  to move toward second fluid bag  22  in the range surrounded by belt  23 . This changes the positions of first pulse wave detection unit  401  and second pulse wave detection unit  402  relative to radial artery  91  of left wrist  90 . 
     CPU  100  calculates cross-correlation coefficient r in this state and, when cross-correlation coefficient r exceeds threshold Th, determines that first pulse wave detection unit  401  and second pulse wave detection unit  402  are located within the appropriate position range relative to radial artery  91  of left wrist  90 . 
     Conversely, when cross-correlation coefficient r becomes further apart from threshold Th, as the volume of the fluid in first fluid bag  21  is decreased and the volume of the fluid in second fluid bag  22  is increased, left wrist  90  is pressed by second fluid bag  22  to move toward first fluid bag  21  in the range surrounded by belt  23 . CPU  100  calculates cross-correlation coefficient r in this state and, until cross-correlation coefficient r exceeds threshold Th, repeatedly adjusts the positions of first pulse wave detection unit  401  and second pulse wave detection unit  402  relative to radial artery  91  of left wrist  90 . 
     Description will now be given of an operation flow in the estimation of a blood pressure based on pulse transit time Δt by blood pressure estimation apparatus  1  according to the embodiment of the present invention.  FIG. 9  is a flowchart showing an operation flow in estimation of a blood pressure by the blood pressure estimation apparatus according to the embodiment of the present invention based on pulse transit time estimates. 
     As shown in  FIG. 9 , CPU  100  of blood pressure estimation apparatus  1  according to the embodiment of the present invention pressurizes first fluid bag  21  and second fluid bag  22  (S 10 ). CPU  100  then calculates cross-correlation coefficient r between pulse wave signal PS 1  detected by first pulse wave detection unit  401  and pulse wave signal PS 2  detected by second pulse wave detection unit  402  in real time (S 11 ). 
     CPU  100  then determines whether cross-correlation coefficient r exceeds threshold Th (S 12 ). When cross-correlation coefficient r is less than or equal to threshold Th, CPU  100  determines whether the pressure in first fluid bag  21  or the pressure in second fluid bag  22  exceeds the upper limit (S 17 ). This upper limit is set to such a pressure that would not put an enormous burden on a subject. 
     When the pressure in first fluid bag  21  and the pressure in second fluid bag  22  do not exceed the upper limit, CPU  100  repeats the processes of steps S 10  to S 12  in order to set the force of pressing of first pulse wave detection unit  401  and second pulse wave detection unit  402  against palm lateral surface  90   a  of left wrist  90  within the appropriate pressing force range. 
     When the pressure in first fluid bag  21  or the pressure in second fluid bag  22  exceeds the upper limit, CPU  100  determines that at least one of first pulse wave detection unit  401  and second pulse wave detection unit  402  is located outside of the appropriate position range, and temporarily opens the inside of first fluid bag  21  and the inside of second fluid bag  22  to atmospheric pressure (S 18 ). Specifically, CPU  100  opens first on-off valve  35   a  and second on-off valve  35   b  with pump  32  being stopped. 
     CPU  100  then pressurizes first fluid bag  21  to, for example, A mmHg (S 19 ). Specifically, CPU  100  opens first on-off valve  35   a  and closes second on-off valve  35   b  with pump  32  being driven. 
     CPU  100  then pressurizes first fluid bag  21  and second fluid bag  22  (S 20 ). For example, CPU  100  pressurizes first fluid bag  21  to A+B mmHg and pressurizes second fluid bag  22  to B mmHg. Specifically, CPU  100  opens first on-off valve  35   a  and second on-off valve  35   b  with pump  32  being driven. 
     CPU  100  performs the processes of S 11  to S 12  again in order to check whether first pulse wave detection unit  401  and second pulse wave detection unit  402 , the positions of which have been adjusted, are located within the appropriate position range. 
     When cross-correlation coefficient r exceeds threshold Th, CPU  100  stops pump  32  (S 13 ). In this state, CPU  100  calculates, as pulse transit time (PTT), time difference Δt between peak A 1  of pulse wave signal PS 1  and peak A 2  of pulse wave signal PS 2  (S 14 ). 
     CPU  100  then calculates and estimates a blood pressure based on pulse transit time Δt using a correspondence equation Eq between pulse transit time Δt and blood pressure associated with each other through calibration (S 15 ). Correlation equation Eq may be a publicly known fractional function. 
     CPU  100  then checks whether an instruction to stop measurement has been provided from operation unit  52  (S 16 ). When the instruction to stop measurement has not been provided from operation unit  52 , CPU  100  periodically repeats the calculation of pulse transit time Δt (S 14 ) and the estimation of blood pressure (S 15 ) every time pulse wave signal PS 1  and pulse wave signal PS 2  are provided in accordance with a pulse wave. CPU  100  displays the result of blood pressure estimation on display  50  and records the result in memory  51 . Upon receipt of the instruction to stop measurement from operation unit  52 , CPU  100  ends the blood pressure estimation operation. 
     In blood pressure estimation apparatus  1  according to the present embodiment, blood pressure can be estimated based on pulse transit time Δt to continuously monitor a blood pressure for a long period of time while reducing a physical burden on a subject. Blood pressure estimation apparatus  1  can also perform both of the measurement of a blood pressure by the oscillometric method and the estimation of a blood pressure based on pulse transit time Δt, and accordingly can provide improved convenience and easily perform calibration between pulse transit time Δt and blood pressure. 
     In blood pressure estimation apparatus  1  according to the present embodiment, pulse wave detection unit  40 E is disposed on the external surface of first fluid bag  21  for measuring blood pressure by the oscillometric method, and pulse wave detection unit  40 E is pressed against palm lateral surface  90   a  of left wrist  90  upon expansion of first fluid bag  21 , thereby detecting a pulse wave. A fluid supply unit can thus be used in common for the measurement of blood pressure by the oscillometric method and the detection of pulse wave, leading to a simplified configuration of blood pressure estimation apparatus  1 . 
     Blood pressure estimation apparatus  1  according to the present embodiment can adjust, when at least one of first pulse wave detection unit  401  and second pulse wave detection unit  402  is located outside of the appropriate position range, a ratio between the volume of the fluid in first fluid bag  21  and the volume of the fluid in second fluid bag  22  to adjust the positions of first pulse wave detection unit  401  and second pulse wave detection unit  402  relative to radial artery  91  of left wrist  90  within the appropriate position range, thus estimating a blood pressure while guaranteeing reliability based on pulse transit time Δt. Also, a wide range in which the position of pulse wave detection unit  40 E is adjustable can be secured, leading to stable blood pressure estimation. 
     Although first fluid bag  21  and second fluid bag  22  are used and pulse wave detection unit  40 E is disposed on the external surface of first fluid bag  21  in the present embodiment, the present invention is not limited thereto. For example, each of first fluid bag  21  and second fluid bag  22  may be divided at a middle position in width direction Y. In this case, first pulse wave detection unit  401  and second pulse wave detection unit  402  are disposed on the external surfaces of separate fluid bags. Consequently, the positions of first pulse wave detection unit  401  and second pulse wave detection unit  402  can be adjusted separately. 
     Also, although a blood pressure is estimated based on pulse transit time Δt in the present embodiment, the present invention is not limited thereto. For example, a blood pressure may be estimated based on the waveform of pulse wave signal PS 1  detected by first pulse wave detection unit  401 . In this case, the position of first pulse wave detection unit  401  is adjusted such that the maximum amplitude value of pulse wave signal PS 1  is more than or equal to a threshold. 
     It is noted that the embodiments disclosed herein are illustrative in every respect, and do not provide grounds for restrictive interpretation. Therefore, the technical scope of the present invention should not be interpreted by the above embodiments only, and is defined based on the description in the scope of the claims. Further, any modifications within the meaning and scope equivalent to the scope of the claims are encompassed.