Abstract:
Disclosed are a blood pressure estimation device, etc., that is capable of highly accurate estimation of blood pressure. The blood pressure estimation device ( 101 ) has a blood pressure estimation unit ( 102 ) that: finds a specific blood pressure related to pulse wave information ( 2002 ), by browsing blood pressure information ( 2001 ) associating the pulse wave information and blood pressure related to pulse wave signals caused by blood pressure during a specific period and measured, the pulse wave information associating blood pressure during the specific period and the pulse wave signals; and estimates the blood pressure relating to the pulse wave information ( 2002 ) on the basis of the found specific blood pressure.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a blood pressure estimation device and the like that estimate a blood pressure. 
       BACKGROUND ART 
       [0002]    A blood pressure is one index for analyzing the presence or absence of a cardiovascular disease. For example, analysis based on a blood pressure of a risk such as a so-called lifestyle-related disease is effective for preventing diseases in the cardiovascular system such as stroke, heart failure, and heart attack. Specifically, at outside of the hospital such as at home, it is desirable to measure a blood pressure over a long period at the same time every day in a fixed (or substantially fixed) environment. A blood pressure measured outside of the hospital is suitable for predicting diseases in the cardiovascular system with a high degree of accuracy. Therefore, a blood pressure measured outside of the hospital such as at home tends to be emphasized in medical practice. Further, it is also very important for curing hypertension to accurately measure a blood pressure. 
         [0003]    A blood pressure fluctuates depending on a living activity pattern in a time period such as one year, one month, or 24 hours. Further, the blood pressure greatly fluctuates depending on various factors such as an activity amount, conversation, psychological anxiety, tension, impatience, astonishment, patience, alcohol-intake, smoking, and coldness other than pathological factors. 
         [0004]    Research has also been conducted in recent years for estimating a risk of occurrence of a disorder in an organ on the basis of a difference among a blood pressure measured while sleeping, a blood pressure measured while being active, and a blood pressure measured when waking up in the morning. 
         [0005]    For example, a blood pressure meter (ABPM) disclosed by PTL 1 and PTL 2 is a blood pressure meter that regularly measures a blood pressure by being carried by a person to be measured for  24  hours in order to measure a blood pressure in various situations. ABPM is an abbreviation for Ambulatory_Blood_Pressure_Monitoring. According to “Guidelines for Usage (ABPM) Standards of Blood Pressure Meters Carried for 24 Hours,” a measurement interval is a time interval ranging from 10 to 30 minutes. Further, according to the guidelines, it is necessary for a person to be measured to record daily activities (e.g. bedtime, wake-up time, sleep depth, meals, excretions, dosing, and the like). 
         [0006]    PTL 3 to PTL 5 disclose common blood pressure meters (blood pressure measurement devices). 
         [0007]    PTL 3 discloses a Non Invasive blood pressure measurement device in which the noise can be eliminated and which has a small burden on a person to be measured. The blood pressure measurement device includes a function for eliminating noise from a measured pulse wave. 
         [0008]    The blood pressure measurement device disclosed in PTL 4 measures pulse waves with a plurality of pulse wave sensors and calculates a starting time of a pulse wave having a largest amplitude of the measured pulse waves. The blood pressure measurement device calculates an internal pressure of a cuff at the calculated point of time as a maximum blood pressure. 
         [0009]    The electronic blood pressure meter disclosed in PTL 5 includes an oscillometric measurement unit for measuring a blood pressure on the basis of a pulse wave and a calculation unit for calculating a waveform parameter representing a waveform corresponding to a relative pressure of the pulse wave to an internal pressure of a cuff. The electronic blood pressure meter further includes an update unit for calibrating the calculated blood pressure on the basis of the calculated waveform parameter. 
       CITATION LIST 
     Patent Literature 
       [0010]    PTL 1: Japanese Laid-open Patent Publication No. 11(1999)-47103 
         [0011]    PTL 2: Japanese Laid-open Patent Publication No. 2010-194111 
         [0012]    PTL 3: Japanese Laid-open Patent Publication No. 2005-288002 
         [0013]    PTL 4: Japanese Laid-open Patent Publication No. S62 (1987)-038137 
         [0014]    PTL 5: Japanese Laid-open Patent Publication No. 2003-250770 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0015]    A blood pressure measurement device (blood pressure meter), for example an ABPM device, sets an internal pressure of a cuff to be higher than a blood pressure (a systolic blood pressure value, a maximum blood pressure, or a Systolic_blood_pressure, expressed also as an “SBP”) in a course of heart contraction. Therefore, the blood pressure measurement device may become a significant burden on a person to be measured. Therefore, pressurization upon measurement of a blood pressure is controlled and a systolic blood pressure is estimated in order to reduce the burden on the person to be measured. 
         [0016]    PTL 2 discloses a Non_Invasive blood pressure measurement device and a blood pressure measurement program that control a pressure in a cuff and thereby reduce a burden on a person to be measured. 
         [0017]    The blood pressure measurement device disclosed by PTL 2 measures a correct blood pressure only in a quiet environment or a particular situation. For this reason, it is difficult for the blood pressure measurement device to accurately measure a blood pressure when, for example, a person to be measured is exercising or a person to be measured stays in a place surrounded by large noise. Therefore, the blood pressure measurement device is unsuitable for applications such as ABPM. 
         [0018]    On the other hand, according to the guidelines for ABPM standards, when a blood pressure measurement is started while walking or engaging in manual labor, it is necessary for a person to be measured to keep an upper arm where a blood pressure is measured rested. However, the person to be measured is not always in a state of rest, and therefore, it is desired for a blood pressure to be measured without being in a state of rest. 
         [0019]    Accordingly, a main object of the present invention is to provide a blood pressure measurement device and the like that accurately measure a blood pressure. 
       Solution To Problem 
       [0020]    As an aspect of the present invention, a blood pressure estimation device including: 
         [0021]    blood pressure estimation means for determining a particular blood pressure associated with particular pulse wave information by referring to blood pressure information where (i) pulse wave information where a pressure in a certain time period and a pulse wave signal measured on the basis of the pressure in the certain time period are associated and (ii) a blood pressure for the pulse wave signal are associated, and estimating a blood pressure for the particular pulse wave information on the basis of the determined particular blood pressure. 
         [0022]    In addition, as another aspect of the present invention, a blood pressure estimation method including: 
         [0023]    determining, by using an information processing device, a particular blood pressure associated with particular pulse wave information by referring to blood pressure information where (i) pulse wave information where a pressure in a certain time period and a pulse wave signal measured on the basis of the pressure in the certain time period are associated and (ii) a blood pressure for the pulse wave signal are associated, and estimating a blood pressure for the particular pulse wave information on the basis of the determined particular blood pressure. 
         [0024]    Furthermore, the object is also realized by a blood pressure estimation program, and a computer-readable recording medium which records the program. 
       Advantageous Effects of Invention 
       [0025]    According to the blood pressure estimation device and the like of the present invention, a blood pressure can be estimated with a high degree of accuracy. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0026]      FIG. 1  is a block diagram illustrating components included in a blood pressure estimation device according to a first exemplary embodiment of the present invention. 
           [0027]      FIG. 2  is a flowchart illustrating a flow of processing in the blood pressure estimation device according to the first exemplary embodiment. 
           [0028]      FIG. 3  is a diagram conceptually illustrating components included in blood pressure information. 
           [0029]      FIG. 4  is a diagram conceptually illustrating blood pressure information as one example of blood pressure information. 
           [0030]      FIG. 5  is a diagram conceptually illustrating one example of a pressure signal and pulse wave signals. 
           [0031]      FIG. 6  is a diagram conceptually illustrating one example of a pressure signal and pulse wave signals. 
           [0032]      FIG. 7  is a diagram conceptually illustrating one example of pulse wave information. 
           [0033]      FIG. 8  is a block diagram illustrating components included in a blood pressure measurement device according to the first exemplary embodiment. 
           [0034]      FIG. 9  is a perspective view of a vicinity of a cuff that is not put on. 
           [0035]      FIG. 10  is a diagram illustrating one example of a state where a cuff is put on a specific region. 
           [0036]      FIG. 11A  is a diagram conceptually illustrating one example of a position relation between a cuff and pulse wave measurement units. 
           [0037]      FIG. 11B  is a diagram conceptually illustrating one example of a position relation between a cuff and pulse wave measurement units. 
           [0038]      FIG. 11C  is a diagram conceptually illustrating one example of a position relation between a cuff and pulse wave measurement units. 
           [0039]      FIG. 12  is a block diagram illustrating components included in a blood pressure estimation device according to a second exemplary embodiment of the present invention. 
           [0040]      FIG. 13  is a flowchart illustrating a flow of processing in a blood pressure estimation device according to the second exemplary embodiment. 
           [0041]      FIG. 14  is a diagram illustrating one example of a relation between a characteristic value and a pressure. 
           [0042]      FIG. 15  is a diagram illustrating one example of pulse wave information. 
           [0043]      FIG. 16  is a diagram conceptually illustrating a position relation between a cuff and three pulse wave measurement units. 
           [0044]      FIG. 17  is a diagram conceptually illustrating a position relation between a cuff and four pulse wave measurement units. 
           [0045]      FIG. 18  is a block diagram illustrating components included in a blood pressure measurement device according to a third exemplary embodiment of the present invention. 
           [0046]      FIG. 19  is a flowchart illustrating a flow of processing in the blood pressure measurement device according to the third exemplary embodiment. 
           [0047]      FIG. 20A  is a diagram illustrating one example of a pressure signal measured by a pressure measurement unit. 
           [0048]      FIG. 20B  is a diagram illustrating one example of a pulse wave signal measured at upstream by a pulse wave measurement unit. 
           [0049]      FIG. 20C  is a diagram illustrating one example of a pulse wave signal measured at downstream by a pulse wave measurement unit. 
           [0050]      FIG. 21A  is a diagram illustrating a phase as one example of a pulse wave value calculated via Fourier transform of a pulse wave signal. 
           [0051]      FIG. 21B  is a diagram illustrating a phase as one example of a pulse wave value calculated via Fourier transform of a pulse wave signal. 
           [0052]      FIG. 22  is a diagram illustrating one example of a relation between a characteristic value and a pressure. 
           [0053]      FIG. 23  is a diagram illustrating one example of a calculated degree of similarity. 
           [0054]      FIG. 24  is a block diagram illustrating components included in a blood pressure measurement device according to a fourth exemplary embodiment of the present invention. 
           [0055]      FIG. 25  is a flowchart illustrating a flow of processing in a blood pressure measurement device according to the fourth exemplary embodiment. 
           [0056]      FIG. 26  is a block diagram schematically illustrating a hardware configuration of a calculation processing apparatus capable of realizing a blood pressure estimation device or a pressure controlling unit according to each exemplary embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0057]    Next, exemplary embodiments of the present invention will be described in detail with reference to the drawings. 
       First Exemplary Embodiment 
       [0058]    Components included in a blood pressure estimation device  101  according to a first exemplary embodiment of the present invention and processing executed by the blood pressure estimation device  101  will be described in detail with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  is a block diagram illustrating the components included in the blood pressure estimation device  101  according to the first exemplary embodiment of the present invention.  FIG. 2  is a flowchart illustrating a flow of processing in the blood pressure estimation device  101  according to the first exemplary embodiment. 
         [0059]    The blood pressure estimation device  101  according to the first exemplary embodiment includes a blood pressure estimation unit  102 . 
         [0060]    The blood pressure estimation device  101  receives pulse wave information  2002  in which a pressure signal in a certain time period and multiple pulse wave signals measured when a pressure indicated by the pressure signal in the certain time period is applied to a person to be measured are associated with each other. 
         [0061]    Hereinafter, for convenience of description, with respect to the multiple pulse wave signals, it is assumed that there are two pulse wave signals. The pulse wave information  2002  according to the present exemplary embodiment may include three or more pulse wave signals as described later. 
         [0062]    With reference to  FIG. 5 , the multiple pulse wave signals and the pressure signal in the pulse wave information  2002  will be described. FIG.  5  is a diagram conceptually illustrating one example of the pressure signal and the multiple pulse wave signals. The horizontal axis of  FIG. 5  represents time, and represents a later time at a rightward position. The vertical axis in the upper figure of  FIG. 5  represents an amplitude of a pressure signal and represents that the pressure signal is stronger toward the upper side. The vertical axis in the middle figure of  FIG. 5  and the lower figure of  FIG. 5  represents an amplitude of a pulse wave signal and represents that the amplitude of the pulse wave signal increases as being closer to the upper end or the lower end, and the amplitude of the pulse wave signal decreases as being closer to a center of the upper end and the lower end. In the example illustrated in  FIG. 5 , the certain time period refers to a (heartbeat) period in which the heart beats at multiple times. The figure illustrated in the upper part of  FIG. 5  illustrates one example of the pressure signal, and the two figures illustrated in the lower part of  FIG. 5  illustrate one example of the pulse wave signal. 
         [0063]    Further, the pulse wave information may be pulse wave information in a certain period, for example, as illustrated in  FIG. 6 .  FIG. 6  is a diagram conceptually illustrating one example of a pressure signal and a pulse wave signal. The pulse wave information illustrated in  FIG. 6  is information on a pulse wave signal measured when pressurization is stopped at a pressure (approximately 100 mmHg in this case) lower than a systolic blood pressure. The figure in the upper part of  FIG. 6  illustrates one example of the pressure signal, and the two figures illustrated in the lower part of  FIG. 6  illustrate one example of the pulse wave signal. 
         [0064]    When in a certain time period, a pressure lies somewhere between a systolic blood pressure and a diastolic blood pressure measured in the course of heart contraction (a diastolic blood pressure value, a minimum blood pressure, or a Diastolic blood pressure, hereinafter, expressed also as a “DBP”), an amplitude in a pulse wave is large and a pulse wave for a person to be measured can be detected. On the contrary, when the pressure has a value larger than the systolic blood pressure or a value smaller than the diastolic blood pressure, an amplitude in the pulse wave specifically in a downstream is small, and therefore, it is difficult to detect a pulse wave for the person to be measured. 
         [0065]    The pulse wave information is information where a pressure signal at a certain timing is associated with multiple pulse wave signals, as illustrated in  FIG. 7 .  FIG. 7  is a diagram conceptually illustrating one example of the pulse wave information. In the pulse wave information, for example, a pressure signal “70,” a pulse wave signal A “aa,” and a pulse wave signal B “ba” are associated with each other. This represents that the pulse wave signal A “aa” and the pulse wave signal B “ba” are measured when a pressure “70” is applied to a person to be measured. 
         [0066]    In the pulse wave information, a pressure signal at a certain timing need not always be associated with multiple pulse wave signals, and may be a value of parameters calculated by regression analysis and the like of a relation between the pressure signal and the multiple pulse wave signals. Further, the pulse wave information may not be multiple pulse wave signals themselves or a pressure signal itself, and may be a value calculated on the basis of the pressure signal or on the basis of the multiple pulse wave signals in accordance with predetermined steps. In other words, the pulse wave information is not limited to the above-described examples. 
         [0067]    The blood pressure estimation device  101  can refer to blood pressure information  2001  (e.g. blood pressure information  2006  as illustrated in  FIG. 3 ) in which pulse wave information and a blood pressure are associated with each other.  FIG. 3  is a diagram conceptually illustrating components included in the blood pressure information  2006 . In the blood pressure information  2006  pulse wave information are associated with a blood pressure, as described above. 
         [0068]    With reference to  FIG. 4 , one example of the blood pressure information  2001  will be described.  FIG. 4  is a diagram conceptually illustrating blood pressure information  2007  as one example of the blood pressure information  2001 . In this case, a blood pressure includes a diastolic blood pressure and a systolic blood pressure. Further, in the example illustrated in  FIG. 4 , in pulse wave information, a pressure signal is associated with multiple pulse wave signals. 
         [0069]    The blood pressure estimation unit  102  reads a blood pressure associated with the received pulse wave information  2002  from the blood pressure information  2001  (step S 201 ). In other words, the blood pressure estimation unit  102  refers to the blood pressure information  2001  and thereby determines a blood pressure associated with the received pulse wave information  2002 . 
         [0070]    In the above-described example, the blood pressure estimation unit  102  searches pulse wave information coincident with the pulse wave information  2002  in the blood pressure information  2001 . However, the blood pressure estimation unit  102 , for example, may calculate a degree of similarity (or a degree of correlation) representing an extent that the pulse wave information  2002  and pulse wave information in the blood pressure information  2001  are similar to each other and search similar (or coincident) pulse wave information on the basis of the calculated degree. Further, there may be a plurality of pieces of blood pressure information. Alternatively, the blood pressure estimation unit  102  may select pulse wave information having a highest (or approximately highest) degree of similarity and read a blood pressure associated with the selected pulse wave information. 
         [0071]    For example, “the approximately highest” can be defined as a value that falls within a certain range from a maximum. The certain range may be a predetermined value. 
         [0072]    Further, it is not always necessary for the blood pressure estimation unit  102  to calculate degrees of similarity between all pieces of data of pulse wave information in the blood pressure information  2001  and the pulse wave information  2002 . The blood pressure estimation unit  102  may calculate degrees of similarity on the basis of a part of the pieces of date of the pulse wave information in the blood pressure information  2001 . In other words, in this case, it is not always necessary for a pressure in the pulse wave information to include a diastolic blood pressure and a systolic blood pressure. 
         [0073]    Then, the pressure estimation device  101  estimates a blood pressure (hereinafter, expressed as a “first blood pressure” for convenience of description) for the pulse wave information  2002  on the basis of the read blood pressure (step S 202 ). When, for example, there is one read blood pressure, the blood pressure estimation unit  102  may estimate the read blood pressure as the first blood pressure. Further, when estimating a blood pressure read in accordance with a degree of similarity or a degree of correlation, the blood pressure estimation unit  102  may estimate the first blood pressure by determining a weighted average value and the like in accordance with a degree of similarity for the read blood pressure. 
         [0074]    The blood pressure information  2001  includes pulse wave information where a pressure signal with multiple pulse wave signals previously measured and a blood pressure about a person to be measured are associated with each other. The blood pressure information (information of a pressure signal, multiple pulse wave signals, and a blood pressure)  2001  may be prepared before a blood pressure estimation is started. In a method for previously measuring blood pressure information, in manner similar that in the present exemplary embodiment, pulse wave information (a pressure signal and multiple pulse wave signals) is acquired, a blood pressure (a systolic blood pressure and a diastolic blood pressure) is measured in accordance with an oscillometric method or a Korotkoff method, and the measured blood pressure and the acquired pulse wave information are associated with each other. As another method, in a manner similar to that in the present exemplary embodiment, it is possible to acquire pulse wave information, calculate a blood pressure on the basis of a characteristic of the pulse wave information, and associate the calculated blood pressure with the acquired pulse wave information to generate blood pressure information. The method for previously measuring blood pressure information is not limited to the above-described methods. In this case, there may be a plurality of persons to be measured. 
         [0075]    Further, the pulse wave information  2002  need not be a table form as exemplified in  FIG. 7 . The pulse wave information  2002  may be, for example, a relation equation representing a relation (e.g.  FIG. 5  and  FIG. 6 ) between a pressure and multiple pulse wave signals or a parameter representing the relation equation. 
         [0076]    A large degree of similarity represents, for example, a small difference between a value included in the blood pressure information  2001  and a value included in the pulse wave information  2002 . A large degree of correlation represents, for example, a large correlation coefficient between a value included in the blood pressure information  2001  and the pulse wave information  2002 . The degree of similarity and the degree of correlation are not limited to the above-described examples. 
         [0077]    The blood pressure estimation device  101  according to the present exemplary embodiment selects a blood pressure associated with the pulse wave information  2002 where a blood pressure signal is associated with multiple pulse wave signals from the blood pressure information  2001 , and estimates a blood pressure for the pulse wave information  2002  on the basis of the read blood pressure. Therefore, even when a pulse wave or a pressure value includes noise, the blood pressure estimation device  101  reads a blood pressure from pulse wave information including multiple pulse wave signals and blood pressure information and can thereby estimate a blood pressure with reducing an influence of the noise. 
         [0078]    On the other hand, it is difficult for a common blood pressure estimation device to accurately measure a blood pressure when a measured pulse includes noise, as described above. 
         [0079]    In other words, according to the blood pressure estimation device  101  of the present exemplary embodiment, a blood pressure can be estimated with a high degree of accuracy. 
         [0080]    The blood pressure estimation device  101  receives, for example, pulse wave information where a pressure signal measured by a blood pressure measurement device  408  exemplified in  FIG. 8  is associated with multiple pulse wave signals measured by the blood pressure measurement device  408 .  FIG. 8  is a block diagram illustrating components included in the blood pressure measurement device  408  according to the first exemplary embodiment. 
         [0081]    The blood pressure measurement device  408  includes a cuff  401 , a pulse wave measurement unit  402 , a pulse wave measurement unit  403 , a pressure measurement unit  407 , a pressure control unit  404 , an input unit  405 , a display unit  406 , and the blood pressure estimation device  101 . As exemplified in  FIG. 9 , the cuff  401 , the pulse wave measurement unit  402 , and the pulse wave measurement unit  403  are integrally formed.  FIG. 9  is a perspective view of a vicinity of the cuff  401  that is not put on. 
         [0082]    In the following description, for convenience of description, it is assumed that a shape of the cuff  401  is a rectangle (rectangular shape), a trapezoid, or a shape close to a rectangular shape while being developed as exemplified in  FIG. 9 . The shape close to a rectangular shape is, for example, a shape formed in a tapered or arced manner in a shorter side direction, a longer side direction, or both directions. The shape close to a rectangular shape is not limited to the above-described shape. It is assumed that the longer side direction is a direction where the cuff is wound around a specific region, i.e. a circumference direction while being wound around the specific region. Further, it is assumed that the shorter side direction is a direction orthogonal (or substantially orthogonal) to the longer side direction. Further, it is assumed that the entire cuff applies a pressure to the specific region in a state of pressurization. In this case, it is assumed that an “upstream” represents a portion between the nerve center or the heart and the center of the shorter side direction in an artery. It is assumed that a “downstream” represents a portion between the center of the shorter side direction and a peripheral side (e.g. a hand or foot) in the artery. However, an aspect of the cuff is not limited to the above-described manner. 
         [0083]    First, as shown in  FIG. 10 , a person to be measured winds the cuff  401  around a specific region such as an upper arm, a leg, a wrist, or the like and measures a blood pressure. The person to be measured winds the longer side direction around the specific region to place the cuff  401 .  FIG. 10  is a diagram illustrating one example of a state where the cuff  401  is put on a specific region. In this case, it is conceivable that an artery is parallel (or substantially parallel) to the shorter side direction. In this case, the pulse wave measurement unit  402  measures a pulse wave in a portion between the nerve center or the heart and the center of the shorter side direction. On the other hand, the pulse wave measurement unit  403  measures a pulse wave in a portion between the center of the shorter side direction and a peripheral side (e.g. a hand or foot). 
         [0084]    The pulse wave measurement unit  402  and the pulse wave measurement unit  403  each are, for example, a vibration sensor that detects vibrations caused by a pulse wave or a photoelectric sensor that detects reflected light when a specific region is irradiated with light or transmitted light when a specific region is irradiated with light. The pulse wave measurement unit  402  and the pulse wave measurement unit  403  may be sensors different from each other. It is possible that the pulse wave measurement unit  402  is a photoelectric sensor for a first wavelength (e.g. red light of approximately 660 nm) and the pulse wave measurement unit  403  is a photoelectric sensor for a second wavelength (e.g. near-infrared light of approximately 940 nm) different from the first wavelength. Herein, “nm” represents nanometers. 
         [0085]    Further, the pulse wave measurement unit  402  and the pulse wave measurement unit  403  may be a pressure sensor. In the case of the pressure sensor, the pulse wave measurement unit decomposes a pressure signal into signals having cycles different from each other, for example, via Fourier transformation or the like. When the pressure control unit  404  performs pressurization or depressurization at a constant (or substantially a constant) speed, a cycle for a pressure resulting from the pressure control unit  404  takes a long time. Therefore, when a signal having a short cycle is extracted from the pressure signal via a combination with a filter circuit or the like or an application of a digital filter, the pulse wave measurement unit can extract a pulse wave signal resulting from a pulse wave. The pulse wave measurement unit is not limited to the above-described examples. As the pulse wave measurement unit, for example, a magnetic sensor, an acceleration sensor or the like is applicable. 
         [0086]    The person to be measured operates the input unit  405  to start a measurement. The input unit  405  includes, for example, a measurement start button for starting a measurement, a power button, a measurement stop button for cancelling the measurement after the measurement start, and a left button and a right button used upon selecting an item displayed by the display unit  406  (each thereof being not illustrated). The input unit  405  transmits an input signal received from a person to be measured or the like to the blood pressure estimation device  101 . 
         [0087]    In response to the measurement start, the pressure control unit  404  controls an amount of gas (e.g. air), liquid, or both sealed in the cuff  401  while referring to an internal pressure of the cuff  401  measured by the pressure measurement unit  407  to control a pressure applied to a specific region. The pressure control unit  404  controls, for example, operations of a pump that sends the gas sealed in the cuff  401  or a valve in the cuff  401 . 
         [0088]    The cuff  401  may include a pressure bag (air bag) in which air is sealed or a pressure bag  1006  (e.g.  FIG. 11A  to be described later) such as a gel bag in which gel or liquid is sealed. The cuff  401  applies a pressure to the specific region in accordance with control executed by the pressure control unit  404 . 
         [0089]    The pulse wave measurement unit  402  and the pulse wave measurement unit  403  may be disposed to sandwich a pressurization center (or substantially the center) of the shorter side direction of the cuff  401 .  FIG. 11A  is a diagram conceptually illustrating one example of a position relation between a cuff and pulse wave measurement units. An optimum disposition of pulse wave sensors refers to a case in which the pulse wave measurement unit  402  and the pulse wave measurement unit  403  are located at each end of the pressure bag  1006 . In other words, this disposition is a configuration that is located on a pressure unit in which a pulse wave is easily acquired and is large in a difference. For convenience of description,  FIG. 11A  also illustrates a specific region and a blood flow in the specific region. However, the blood pressure measurement device  408  does not include a specific region or a blood flow in the specific region. 
         [0090]      FIG. 11B  is a diagram conceptually illustrating one example of a position relation between a cuff and pulse wave measurement units, different from  FIG. 11A . A configuration illustrated in  FIG. 11B  is a configuration in which a pulse wave measurement unit  1001  and a pulse wave measurement unit  1002  are disposed at each end of the cuff so that a difference between pulse wave signals measured by the pulse wave measurement unit  1001  and the pulse wave measurement unit  1002  is largest. For convenience of description,  FIG. 11B  also illustrates a specific region and a blood flow in the specific region. However, the blood pressure measurement device  408  does not include a specific region or a blood flow in the specific region. 
         [0091]      FIG. 11C  is a diagram conceptually illustrating one example of a position relation between a cuff and pulse wave measurement units. A configuration illustrated in  FIG. 11C  is a configuration in which the pulse wave measurement unit  1001  and the pulse wave measurement unit  1002  are disposed at a center (or substantially a center) of a pressure unit. In the case of the configuration, the pulse wave measurement unit  1001  and the pulse wave measurement unit  1002  easily measure pulse wave signals. For convenience of description,  FIG. 11C  also illustrates a specific region and a blood flow in the specific region. However, the blood pressure measurement device  408  does not include a specific region or a blood flow in the specific region. 
         [0092]    The position relation between the cuff and the pulse wave measurement units is not limited to the above-described examples and may be a configuration in which easiness in measurement of pulse wave signals or a difference is considered. The pulse wave measurement units easily measure pulse waves as being closer to a center (or substantially a center) of the pressure unit and easily measure a difference as being distant from the center (or substantially the center) of the pressure unit (i.e. as being closer to each cuff end). The configuration may be, for example, a configuration in which the pulse wave measurement unit  402  is disposed at an end of the pressure unit and the pulse wave measurement unit  403  is disposed at an end of the cuff or a configuration in which the pulse wave measurement unit  402  is disposed at an end of the cuff and the pulse wave measurement unit  403  is disposed in a vicinity of the center of the pressure unit. In addition, the configuration may be a configuration in which the pulse wave measurement unit  402  is disposed at an end of the cuff and the pulse wave measurement  403  is disposed at an end of the pressure unit or the pulse wave measurement unit  402  is disposed at the end of the cuff and the pulse wave measurement unit  403  is disposed in a vicinity of the center of the pressure unit. Further, the configuration may be a configuration in which the pulse wave measurement unit  402  is disposed in a vicinity of the center of the pressure unit and the pulse wave measurement unit  403  is disposed at an end of the pressure unit or a configuration in which the pulse wave measurement unit  402  is disposed in a vicinity of the center of the pressure unit and the pulse wave measurement unit  403  is disposed at an end of the cuff. 
         [0093]    For example, the pressure measurement unit  407  discretizes a measured pressure, causes the discretized pressure to be subjected to conversion (analog digital conversion, or A/D conversion) into a digital signal, and transmits the digital signal as a pressure signal. Similarly, for example, the pulse wave measurement unit  402  and the pulse wave measurement unit  403  each discretize a measured pulse wave, converts the discretized pulse wave to a digital signal, and transmits the digital signal as a pulse wave signal. 
         [0094]    Upon A/D conversion, a filter or the like that extracts a particular frequency may be used to extract a part of a pressure (or a pulse wave). Further, the pressure (or the pulse wave) may be amplified to a predetermined amplitude by applying an amplifier or the like. 
         [0095]    Then, the blood pressure measurement device  408  extracts, for example, values at a particular timing in the pressure and the pulse wave, and associates the extracted values with each other to generate pulse wave information  2002 . 
         [0096]    Next, the blood pressure estimation device  101  executes the above-described processing on the basis of the pulse wave information  2002  calculated by the blood pressure measurement device  408  to estimate a blood pressure. At this time, the blood pressure estimation device  101  may transmit a control signal for an instruction of a control content to the pressure control unit  404 . 
         [0097]    The display unit  406  displays the blood pressure calculated by the blood pressure estimation device  101 . The display unit  406  is an LCD (Liquid_Crystal_Display), an OLED (Organic_light-emitting_diode), an electronic paper, or the like. The electronic paper can be realized in accordance with, for example, a microcapsule type, an electron powder fluid type, a cholesteric liquid crystal type, an electrophoretic type, an electrowetting type, or the like. 
         [0098]    According to the blood pressure estimation device  101  of the first exemplary embodiment, a blood pressure can be estimated with a high degree of accuracy. The reason is that even when pulse wave information includes an error to some extent, the blood pressure estimation device  101  reads a blood pressure associated with particular pulse wave information from blood pressure information and thereby reduces the error. 
         [0099]    The blood pressure measurement device  408  may include an aspect in which the blood pressure estimation device  101 , and the pulse wave measurement unit  402  and the pulse wave measurement unit  430  execute transmission/reception of pulse wave signals to/from each other via a communication network. An aspect in which the input unit  405  and the display unit  406  are located in the outside of the blood pressure measurement device  408  and are connected to the blood pressure measurement device  408  via a communication network may be employed. 
         [0100]    Further, the specific region may be an upper arm, a wrist, or the like. When the specific region is, for example, a wrist, the pulse wave measurement unit  402  and the pulse wave measurement unit  403  may detect pulse waves via a radial artery. 
         [0101]    The blood pressure measurement device  408  includes the blood pressure estimation device  101  and can therefore estimate a blood pressure with a high degree of accuracy. 
       Second Exemplary Embodiment 
       [0102]    Next, a second exemplary embodiment of the present invention based on the above-described first exemplary embodiment will be described. 
         [0103]    In the following description, characteristic parts according to the present exemplary embodiment will be mainly described and the same components as in the above-described first exemplary embodiment are assigned with the same reference signs, whereby overlapping description will be omitted. 
         [0104]    With reference to  FIG. 12  and  FIG. 13 , components included in a blood pressure estimation device  1101  according to the second exemplary embodiment and processing executed by the blood pressure estimation device  1101  will be described.  FIG. 12  is a block diagram illustrating the components included in the blood pressure estimation device  1101  according to the second exemplary embodiment of the present invention.  FIG. 13  is a flowchart illustrating a flow of processing in the blood pressure estimation device  1101  according to the second exemplary embodiment. 
         [0105]    The blood pressure estimation device  1101  according to the second exemplary embodiment includes a blood pressure estimation unit  1102 . 
         [0106]    The blood pressure estimation unit  1102  receives pulse wave information  2002  (i.e. a pressure signal  2003 , a pulse wave signal  2004 , and a pulse wave signal  2005 ) (step S 800 ). The blood pressure estimation unit  1102  calculates pulse wave values for the received multiple pulse wave signals (i.e. the pulse wave signal  2004  and the pulse wave signal  2005 ) (step S 801 ). 
         [0107]    The pulse wave values are, for example, a pulse wave value  1  to a pulse wave value  6  listed below. Specifically: 
         [0108]    An extremum (i.e. a local maximum value, a local minimum value, or both thereof) of a pulse wave signal, or a timing of an extremal value vicinity . . . (a pulse wave value  1 ) 
         [0109]    An amplitude value of a pulse wave signal . . . (a pulse wave value  2 ), 
         [0110]    A timing when a pulse wave signal rises (starts increasing) . . . (a pulse wave value  3 ), 
         [0111]    A timing when a pulse wave signal falls (starts decreasing) . . . (a pulse wave value  4 ), 
         [0112]    An amplitude of a pulse wave signal in a frequency space into which the pulse wave signal is converted . . . (a pulse wave value  5 ), and 
         [0113]    A phase of a pulse wave signal in a frequency space into which the pulse wave signal is converted . . . (a pulse wave value  6 ). 
         [0114]    For example, “around the extremum” can be defined as a value that is within a particular rage from an extremum. The certain range may be a value calculated on the basis of a fact that a magnitude of an inclination (determined by calculating a differential or difference) to a target for which an extremum is calculated is smaller than a predetermined value. The certain range is not limited to the above-described example. 
         [0115]    The blood pressure estimation unit  1102  calculates a pulse wave value  1  to a pulse wave value  4  in an area equivalent to one heartbeat in a pulse wave signal. The blood pressure estimation unit  1102  may extract, for example, a pulse wave signal having a particular cycle included in the pulse wave signal and calculate the pulse wave value  1  to the pulse wave value  4 . Features of the pulse wave value  1  to the pulse wave value  4  are simplicity of calculation and short analysis time. 
         [0116]    On the other hand, a pulse wave value  5  and a pulse wave value  6  are pulse wave values for a plurality of frequencies included in the pulse wave signal in the area equivalent to one heartbeat. Features of the pulse wave value  5  and the pulse wave value  6  are that a pulse wave value can be calculated even when various frequency components are included. 
         [0117]    Since a case in which a pulse wave signal or a signal derived from the pulse wave signal via a differentiation, a difference, or the like of the pulse wave signal satisfies a predetermined condition needs to be defined as a pulse wave value, the pulse wave value is not limited to the above-described example. 
         [0118]    Further, a method for converting a pulse wave signal into a frequency space (frequency area) includes, for example, short-time Fourier transformation, wavelet transformation, and the like. Short-time Fourier transformation, wavelet transformation, and the like are common techniques. Therefore, in the present exemplary embodiment, description thereof will be omitted. 
         [0119]    With respect to the pulse wave value calculated by the blood pressure estimation unit  1102 , there may be one pulse wave value or a plurality of pulse wave values. 
         [0120]    Next, the blood pressure estimation unit  1102  calculates a difference between the pulse wave values calculated in step S 801  for the pulse wave signal  2004  and the pulse wave signal  2005  as a characteristic value (step S 802 ). 
         [0121]    The blood pressure estimation unit  1102  calculates a difference between the pulse wave values or a ratio between the pulse wave values as the difference. 
         [0122]    The blood pressure estimation unit  1102  may calculate, as the characteristic value, for example, a characteristic value  1  to a characteristic value  6  as listed below. Specifically: 
         [0123]    A ratio between a pulse wave value  1  for the pulse wave signal  2004  and a pulse wave value  1  for the pulse wave signal  2005  . . . (a characteristic value  1 ), 
         [0124]    A difference between a pulse wave value  2  for the pulse wave signal  2004  and a pulse wave value  2  for the pulse wave signal  2005  . . . (a characteristic value  2 ), 
         [0125]    A difference between a pulse wave value  3  for the pulse wave signal  2004  and a pulse wave value  3  for the pulse wave signal  2005  . . . (a characteristic value  3 ), 
         [0126]    A difference between a pulse wave value  4  for the pulse wave signal  2004  and a pulse wave value  4  for the pulse wave signal  2005  . . . (a characteristic value  4 ), 
         [0127]    A ratio between a pulse wave value  5  for the pulse wave signal  2004  and a pulse wave value  5  for the pulse wave signal  2005  . . . (a characteristic value  5 ), and 
         [0128]    A difference between a pulse wave value  6  for the pulse wave signal  2004  and a pulse wave value  6  for the pulse wave signal  2005  . . . (a characteristic value  6 ). 
         [0129]    Herein, the difference may be an absolute value of the difference. A difference to be detected may be a numerical value representing a gap such as a ratio, a difference, or the like. Therefore, the characteristic value calculated by the blood pressure estimation unit  1102  is not limited to the above-described example. 
         [0130]    The blood pressure estimation unit  1102  associates the calculated characteristic value with a pressure signal  2003  at a timing when a pulse wave value as a basis of the characteristic value is calculated and calculates pulse wave information  2002 . 
         [0131]    In this case, in the pulse wave information  2002 , a characteristic value calculated from multiple pulse wave signals is associated with a pressure unlike the information exemplified in  FIG. 7 . 
         [0132]    The blood pressure estimation unit  1102  reads a blood pressure associated with the calculated pulse wave information  2002  in the blood pressure information  2001  and estimates a first blood pressure for the pulse wave information  2002  on the basis of the read blood pressure (step S 803 ). 
         [0133]    In the present exemplary embodiment, in pulse wave information included in the blood pressure information  2001 , a pressure indicated by a pressure signal is associated with a characteristic value calculated by the blood pressure estimation unit  1102 . In the pulse wave information, a characteristic value need not be always associated with a pressure and multiple pulse wave signals at a certain timing and a pressure signal may be associated with each other. 
         [0134]    When, for example, there is one read blood pressure, the blood pressure estimation unit  1102  may estimate the blood pressure as a first blood pressure. Further, when a read blood pressure is determined in accordance with a degree of similarity, the blood pressure estimation unit  1102  may determine an average value as in determination of the blood pressure as a weighted average value in accordance with a degree of similarity to estimate a first blood pressure. 
         [0135]    The characteristic value varies depending on a pressure applied upon measuring a pulse wave. There is, for example, a relation as illustrated in  FIG. 14  between a characteristic value  6  and a pressure.  FIG. 14  is a diagram illustrating one example of a relation between a characteristic value and a pressure. The horizontal axis of  FIG. 14  represents a pressure indicated by the pressure signal  2003 , and represents that the pressure becomes higher toward the right side. The vertical axis of  FIG. 4  represents the characteristic value  6 , and represents that the characteristic value  6  becomes larger toward the upper side. The characteristic value  6  starts rising from a vicinity where a pressure is a diastolic blood pressure and becomes largest (or substantially largest) in a vicinity where the pressure is a systolic blood pressure. 
         [0136]    A circular mark “o” represents a characteristic value as described above. In other words, the blood pressure estimation unit  1102  uses a value in a circular mark “o” and thereby calculates pulse wave information exemplified in  FIG. 7 . A solid line, and a diastolic blood pressure and a systolic blood pressure (dotted-line portions) mean one piece of blood pressure information. 
         [0137]      FIG. 15  illustrates the pulse wave information illustrated in  FIG. 14  and a degree of correlation obtained based on blood pressure information (a case of first blood pressure information or a case of second blood pressure information). The horizontal axis of  FIG. 15  represents a normalized pressure, and represents that the pressure becomes higher toward the right side. The vertical axis of  FIG. 15  represents a degree of correlation, and represents that the degree of correlation becomes higher toward the upper side. 
         [0138]    For example, the blood pressure estimation unit  1102  calculates a degree of similarity R described below in accordance with Equation A. Specifically: 
         [0000]        R =(Σ i ( x   i   −x   m )×( y   i   −y   m ))/ a    (Equation  A )
 
         [0000]    wherein y i  is a difference (e.g. a phase difference) of characteristic values at a particular blood pressure P i included in blood pressure information. The sign x i  is a difference (e.g. a phase difference) of characteristic values at the particular blood pressure P i  included in particular blood pressure information. The sign y m  is an arithmetic average of phase differences {y i }. The sign x m  is an arithmetic average of phase differences {x i }. The sign a is an optional constant. Σ i  represents determining a sum for i. 
         [0139]    The blood pressure estimation unit  1102  may normalize pulse wave information and pulse wave information in blood pressure information on the basis of a systolic blood pressure or the like and thereby calculate a degree of similarity on the basis of the normalized pulse wave information. In this case, the blood pressure estimation unit  1102  also coverts a blood pressure value associated with the pulse wave information on the basis of normalization of the pulse wave information. Therefore, the blood pressure estimation unit  1102  multiplies the blood pressure value and a normalization constant together to estimate a blood pressure for pulse wave information  2002 . 
         [0140]    The normalization makes it possible to reduce redundant pulse wave information in blood pressure information. In other words, the normalization makes it possible to lower a capacity of a database. Accordingly, a processing load in the blood pressure estimation unit  1102  is reduced. 
         [0141]    The blood pressure estimation device  1101  according to the second exemplary embodiment includes components similar to those in the first exemplary embodiment, and therefore, effects similar to those in the first exemplary embodiment can be obtained from the second exemplary embodiment. In other words, according to the blood pressure estimation device  1101  of the second exemplary embodiment, a blood pressure can be estimated with a high degree of accuracy. 
         [0142]    Hereinafter, an effect of reducing noise by calculating a difference between pulse wave values as a characteristic value used for calculating a degree of similarity will be described. 
         [0143]    Movements in a person to be measured, vibrations transmitted from the outside, noise from a surrounding area, and the like are added to a pulse wave signal as noise signals. 
         [0144]    For convenience of description, measured signals including noise signals are denoted by S 1 and S 2 , and pulse wave signals related to the person to be measured are denoted by P 1  and P 2 .    
         [0145]    In this case, the measurement signals and the pulse wave signals have the relationships expressed by Equation 1 and Equation 2 below. Specifically, 
         [0000]        S   1   =P   1   ×a   1   +b   1    (Equation 1)
 
         [0000]        S   2   =P   2   ×a   2   +b   2    (Equation 2)
 
         [0000]    (where a 1  and a 2  respectively denote multiplication noise for the pulse wave signal S 1  and multiplication noise for the pulse wave signal S 2 , and b 1  and b 2  respectively denote addition noise for the pulse wave signal S 1  and addition noise for the pulse wave signal S 2 ). 
         [0146]    Here, k is defined according to Equation 3 below. Specifically, 
         [0000]        k=b   1   /b   2    (Equation 3)
 
         [0147]    Equation 4 below is established on the basis of Equation 1, Equation 2, and Equation 3 described above. Specifically, 
         [0000]        S   1   −k×S   2   =P   1   ×a   1   −P   2   ×k×a   2    (Equation 4)
 
         [0148]    When a 1  and a 2  are sufficiently close to one (i.e., each multiplication noise is sufficiently small), or when a characteristic value that is not affected by any multiplication noise is extracted, a 1  and a 2  can be ignored, consequently reducing noise. 
         [0149]    Here, m is defined according to Equation 5 below. Specifically, 
         [0000]        m=a   1   /a   2    (Equation 5)
 
         [0150]    Equation 6 below is established on the basis of Equation 1, Equation 2, and Equation 5 described above. Specifically, 
         [0000]        S   1   /m/S   2 =( P   1   +b   1   /a   1 )/( P   2   +m×b   2   /a   1 )   (Equation 6)
 
         [0151]    When b 1  and b 2  are sufficiently small with respect to a 1  and a 2 , respectively, or when a characteristic value that is not affected by any addition noise is extracted, a 1  and a 2  can be ignored, consequently reducing noise. 
         [0152]    Multiplication noise and addition noise are non-independently added to multiple pulse wave signals measured by multiple pulse wave measurement units located at positions close to each other. In this case, even when the values k and m are not determined, noise signal components can be reduced by calculating the difference. 
         [0153]    Hence, the blood pressure estimation device  1101  according to the second exemplary embodiment can estimate blood pressure with a high degree of accuracy. 
         [0154]    Further, with respect to the above-described pulse wave signals, there may be three or more pulse wave signals. In other words, as illustrated in  FIG. 16 , when a cuff  1005  also includes three pulse wave measurement units (a pulse wave measurement unit  1001 , a pulse wave measurement unit  1002 , and a pulse wave measurement unit  1003 ), the blood pressure estimation device  1101  can estimate a blood pressure in a manner similar to that in the above-described example.  FIG. 16  is a diagram conceptually illustrating a position relation between the cuff  1005  and the three pulse wave measurement units. 
         [0155]    For convenience of description,  FIG. 16  includes a specific region and a blood flow and the like in the specific region. However, the blood pressure measurement device  1007  does not include any specific region and any blood flow and the like in a specific region. 
         [0156]    The blood pressure measurement device  1007  includes a pulse wave measurement unit  1001 , a pulse wave measurement unit  1002 , a pulse wave measurement unit  1003 , and the cuff  1005 . The cuff  1005  may include a pressure bag  1006 . At least two pulse wave measurement units of the pulse wave measurement unit  1001 , the pulse wave measurement unit  1002 , and the pulse wave measurement unit  1003  are located at positions so that pressurization center (or substantially center) in the shorter-side direction of the pressure application in the cuff  105  is located between the pulse wave measurement units. 
         [0157]    Each of the pulse wave measurement unit  1001 , the pulse wave measurement unit  1002 , and the pulse wave measurement unit  1003  measures a pulse wave at the specific region. 
         [0158]    Here, for convenience of description, measurement signals including noise are denoted by S 1 , S 2 , and S 3 , and pulse signals are denoted by P 1 , P 2 , and P 3 . 
         [0159]    In this case, the measurement signals and the pulse wave signals have the relationships expressed by Equation 7 to Equation 9 below. Specifically, 
         [0000]        S   1   =P   1   ×a   1   +b   1    (Equation 7)
 
         [0000]        S   2   =P   2   ×a   2   +b   2    (Equation 8)
 
         [0000]        S   3   =P   3   ×a   3   +b   3    (Equation 9)
 
         [0000]    (where a 1 , a 2 , and a 3  each denote multiplication noise for the corresponding pulse wave signal, and b 1 , b 2 , and b 3  each denote addition noise for the corresponding pulse wave signal). 
         [0160]    Here, k 1  is defined according to Equation 10 below, and k 2  is defined according to Equation 11 below. Specifically, 
         [0000]        k   1   ═b   1   /b   2    (Equation 10)
 
         [0000]        k   2   ═b   1   /b   3    (Equation 11)
 
         [0161]    By calculating the difference between Equation 7 and Equation 8 and the difference between Equation 7 and Equation 9, Equation 12 and Equation 13 below are established. Specifically, 
         [0000]        S   1   −k   1   ×S   2   =P   1   ×a   1   −P   2   ×k   1   ×a   2    (Equation 12)
 
         [0000]        S   1   −k   2   ×S   3   =P   1   ×a   1   −P   3   ×k   2   ×a   3    (Equation 13)
 
         [0162]    By calculating (Equation 12)/(Equation 13), Equation 14 below is established. Specifically, 
         [0000]      ( S   1   −k   1   ×S   2 )/( S   1   −k   2   ×S   3 )=( P   1   −P   2   ×k   1   ×a   2   /a   1 )/( P   1   −P   3   ×k   2   ×a   3   /a   1 )   (Equation 14)
 
         [0163]    Equation 14 indicates that, when a l  is sufficiently close to a 2  and a 3  after the influences of the addition noises b 1 , b 2 , and b 3  are cancelled, the influences of the multiplication noises can be ignored. This indicates that noise can be reduced. 
         [0164]    Further, the noise signals (a 1 , a 2 , a 3 , b 1 , b 2 , and b 3 ) are non-independently added to multiple pulse signals measured by multiple pulse wave measurement units located at positions close to each other. Accordingly, Equation 14 indicates that the influences of these noises can be reduced by calculating the difference even when the values k 1  and k 2  are not determined. 
         [0165]    Hence, the blood pressure estimation device  1101  according to the second exemplary embodiment can reduce the influences of noise by estimating blood pressure on the basis of three or more pulse wave signals as described above. 
         [0166]    Further, as illustrated in  FIG. 17 , when the cuff  1005  also includes four pulse wave measurement units (a pulse wave measurement unit  1001 , a pulse wave measurement unit  1002 , a pulse wave measurement unit  1003 , and a pulse wave measurement unit  1004 ), the blood pressure estimation device  1101  can estimate a blood pressure in a manner similar to that in the above-described example.  FIG. 17  is a diagram conceptually illustrating a position relation between the cuff  1005  and the four pulse wave measurement units. 
         [0167]    For convenience of description,  FIG. 17  also illustrates a specific region and a blood flow in the specific region. However, a blood pressure measurement device  1008  does not include a specific region or a blood flow in the specific region. 
         [0168]    The blood pressure measurement device  1008  includes the pulse wave measurement unit  1001 , the pulse wave measurement unit  1002 , the pulse wave measurement unit  1003 , the pulse wave measurement unit  1004 , and the cuff  1005 . The cuff  1005  may include a pressure bag  1006 . At least two pulse wave measurement units of the pulse wave measurement unit  1001 , the pulse wave measurement unit  1002 , the pulse wave measurement unit  1003 , and the pulse wave measurement unit  1004  are located at positions that sandwich a pressurization center (or substantially the center) of a shorter side direction in the cuff  1005 . 
         [0169]    The pulse wave measurement unit  1001 , the pulse wave measurement unit  1002 , the pulse wave measurement unit  1003 , and the pulse wave measurement unit  1004  each measure a pulse wave in a specific region. 
         [0170]    The blood pressure estimation device  1101  estimates a blood pressure in a manner similar to the above-described processing by using the pulse wave measurement unit  1001 , the pulse wave measurement unit  1002 , the pulse wave measurement unit  1003 , and the pulse wave measurement unit  1004 . 
         [0171]    Therefore, the blood pressure estimation device  1101  according to the second exemplary embodiment estimates a blood pressure on the basis of four or more pulse wave signals and can thereby reduce an influence of noise on the basis of a reason similar to the above-described reason. 
       Third Exemplary Embodiment 
       [0172]    A third exemplary embodiment of the present invention based on the above-described first exemplary embodiment will be described. 
         [0173]    In the following description, characteristic parts according to the present exemplary embodiment will be mainly described and the same components as in the above-described first exemplary embodiment are assigned with the same reference signs, whereby overlapping description will be omitted. 
         [0174]    With reference to  FIG. 18  and  FIG. 19 , components included in a blood pressure measurement device  1201  according to the third exemplary embodiment and processing executed by the blood pressure measurement device  1201  will be described.  FIG. 18  is a block diagram illustrating the components included in the blood pressure measurement device  1201  according to the third exemplary embodiment of the present invention.  FIG. 19  is a flowchart illustrating a flow of processing in the blood pressure measurement device  1201  according to the third exemplary embodiment. 
         [0175]    The blood pressure measurement device  1201  includes a cuff  401 , a pulse wave measurement unit  402 , a pulse wave measurement unit  403 , a pressure measurement unit  407 , a pressure control unit  1203 , an input unit  405 , a display unit  406 , and a blood pressure estimation device  1202 . 
         [0176]    First, the pressure control unit  1203  executes control for increasing an internal pressure of the cuff  401  in accordance with a start of measurement (step S 1301 ). The pressure measurement unit  407  measures a pressure in a process of pressurization and transmits the measured pressure to the blood pressure estimation device  1202  as a pressure signal  2003 . Further, the pulse wave measurement unit  402  and the pulse wave measurement unit  403  measure pulse waves in specific regions and transmit the measured pulse waves to the blood pressure estimation device  1202  as pulse wave signals (i.e. a pulse wave signal  2004  and a pulse wave signal  2005 ). 
         [0177]    The blood pressure estimation device  1202  then receives the pressure signal  2003  and the pulse wave signals, and calculates a characteristic value on the basis of the received pressure signal  2003  and the received pulse wave signals. Next, the blood pressure estimation device  1202  determines whether the calculated characteristic value is equal to or larger than a predetermined value (step S 1302 ). 
         [0178]    When determining that the characteristic value is equal to or larger than the predetermined value (YES in step S 1302 ), the blood pressure estimation device  1202  estimates a pressure in the pressure signal as a diastolic blood pressure (step S 1303 ). The blood pressure estimation device  1202  transmits a control signal for increasing a pressure to the pressure control unit  1203 , when determining that the characteristic value is smaller than the predetermined value (NO in step S 1302 ). The pressure control unit  1203  receives the control signal and executes control for increasing the internal pressure of the cuff  401  in accordance with the received control signal (step S 1301 ). 
         [0179]    The blood pressure estimation device  1202  transmits a control signal for increasing a pressure to the pressure control unit  1203  after estimating the diastolic blood pressure. The pressure control unit  1203  receives the control signal and executes control for increasing the internal pressure of the cuff  401  in accordance with the received control signal (step S 1304 ). 
         [0180]    Next, the blood pressure estimation device  1202  receives a pressure signal  2003  and pulse wave signals after pressurization and calculates characteristic values on the basis of the received pressure signal and the received pulse wave signals. Then, the blood pressure estimation device  1202  calculates a degree of similarity from the calculated characteristic values and determines whether the degree of similarity is equal to or larger than a reference value (step S 1305 ). 
         [0181]    When determining that the degree of similarity is equal to or larger than the reference value (YES in step S 1305 ), the blood pressure estimation device  1202  estimates a pressure in the pressure signal as a systolic blood pressure (step S 1306 ). The blood pressure estimation device  1202  transmits a control signal for increasing a pressure to the pressure control unit  1203 , when determining that the degree of similarity is not equal to or larger than the reference value (NO in step S 1305 ). The pressure control unit  1203  receives the control signal and executes control for increasing the internal pressure of the cuff  401  in accordance with the received control signal (step S 1304 ). 
         [0182]    The blood pressure estimation device  1202  transmits a second control signal for reducing a pressure to the pressure control unit  1203 , after calculating the systolic blood pressure. The pressure control unit  1203  receives the second control signal and executes control for reducing the internal pressure of the cuff  401  in accordance with the received second control signal (step S 1307 ). 
         [0183]    The blood pressure estimation device  1201  according to the third exemplary embodiment includes components similar to the blood pressure estimation device  101  according to the first exemplary embodiment, and therefore, effects similar to those in the first exemplary embodiment can be obtained from the third exemplary embodiment. In other words, according to the blood pressure estimation device  1201  of the third exemplary embodiment, a blood pressure can be estimated with a high degree of accuracy. 
         [0184]    The pressure measurement unit  407 , the pulse wave measurement unit  402 , and the pulse wave measurement unit  403  measure a pressure signal and pulse wave signals illustrated in  FIG. 20A  to  FIG. 20C , respectively.  FIG. 20A  is a diagram illustrating one example of a pressure signal measured by the pressure measurement unit  407 .  FIG. 20B  is a diagram illustrating one example of a pulse wave signal (hereinafter, expressed as a “pulse wave signal A”) measured by the pulse wave measurement unit  402 .  FIG. 20C  is a diagram illustrating one example of a pulse wave signal (hereinafter, expressed as a “pulse wave signal B”) measured by the pulse wave measurement unit  403 . Further, each horizontal axis in  FIG. 20A  to  FIG. 20C  represents time, and represents a later time at a rightward position. The vertical axis of  FIG. 20A  represents a pressure, and represents that the pressure becomes higher toward the upper side. The vertical axis of  FIG. 20B  and  FIG. 20C  represents an amplitude of a pulse wave, and represents that the amplitude of a pulse wave becomes higher toward the upper side or the lower side and becomes lower toward 0. A pressurization stop time represents a time when pressurization is stopped. 
         [0185]    The blood pressure estimation device  1202  receives the pulse wave signal A and the pulse wave signal B and calculate Fourier transform of the received pulse wave signals. In the present exemplary embodiment, it is assumed that Fourier transform of a pulse wave in one heartbeat is calculated. In this case, the blood pressure estimation device  1202  calculate Fourier transform of the pulse wave signal A and Fourier transform of the pulse wave signal B to calculate pulse wave values illustrated in  FIG. 21A  and  FIG. 21B .  FIG. 21A  and  FIG. 21B  each are a diagram illustrating a phase as one example of a pulse wave value calculated via Fourier transform of a pulse wave signal. 
         [0186]    The blood pressure estimation device  1202  calculates a characteristic value, for example, as exemplified in  FIG. 22  on the basis of the calculated pulse wave value.  FIG. 22  is a diagram illustrating one example of a relation between a characteristic value and a pressure. A circular mark “0” of  FIG. 22  represents one example of the calculated characteristic value. 
         [0187]    The blood pressure estimation device  1202  calculates, for example, a pulse wave value at a predetermined timing on the basis of the pulse wave signal  2004  and further calculates a pulse wave value at the predetermined timing on the basis of the pulse wave signal  2005 . The blood pressure estimation device  1202  calculates characteristic values on the basis of the two pulse wave values, associates the calculated characteristic values with a blood pressure at the predetermined timing, and thereby generates association information. Similarly, the blood pressure estimation device  1202  generates association information (expressed as “first association information” for convenience of description) also for pulse wave information included in blood pressure information  2001 . 
         [0188]    For convenience of description, with respect to the blood pressure information  2001 , it is assumed that there are a plurality of pieces of blood pressure information  2001 . In this case, the blood pressure estimation device  1202  generates first association information for each of the pieces of blood pressure information. 
         [0189]    Thereafter, the blood pressure estimation device  1202  calculates a degree of similarity (e.g.  FIG. 23 ) between the association information and the first association information, for example, in accordance with Equation A.  FIG. 23  is a diagram illustrating one example of the calculated degree of similarity.  FIG. 23  illustrates degrees of similarity calculated for two pieces of blood pressure information (e.g. the first blood pressure information and the second blood pressure information). When, for example, with respect to particular blood pressure information having a highest degree of similarity between the association information and the first association information in a plurality of pieces of blood pressure information, the degree of similarity is equal to or larger than a reference value, the blood pressure estimation device  1202  estimates a systolic blood pressure on the basis of the particular blood pressure information. The pressure control unit  1203  then executes control for reducing the internal pressure of the cuff  401 . 
         [0190]    The blood pressure measurement device  1201  applies a pressure near a systolic blood pressure to estimate a systolic blood pressure while increasing the internal pressure of the cuff  401 . The blood pressure measurement device  1201  reduces the internal pressure in accordance with completion of the estimation. 
         [0191]    On the other hand, a common blood pressure measurement device estimates a systolic blood pressure while reducing the internal pressure of the cuff  401  after applying a pressure sufficiently higher than the systolic blood pressure. 
         [0192]    Therefore, according to the blood pressure measurement device  1201  of the present exemplary embodiment, a systolic blood pressure can be measured at a pressure lower than that for the common blood pressure measurement device. 
         [0193]    In other words, according to the blood pressure measurement device  1201  of the present exemplary embodiment, it is possible to shorten a measurement time and further reduce a burden imposed to a person to be measured. 
       Fourth Exemplary Embodiment 
       [0194]    Next, a fourth exemplary embodiment of the present invention based on the above-described third exemplary embodiment will be described. 
         [0195]    In the following description, characteristic parts according to the present exemplary embodiment will be mainly described and the same components as in the above-described third exemplary embodiment are assigned with the same reference signs, whereby overlapping description will be omitted. 
         [0196]    With reference to  FIG. 24  and  FIG. 25 , components included in a blood pressure measurement device  1401  according to the fourth exemplary embodiment and processing executed by the blood pressure measurement device  1401  will be described.  FIG. 24  is a block diagram illustrating the components included in the blood pressure measurement device  1401  according to the fourth exemplary embodiment of the present invention.  FIG. 25  is a flowchart illustrating a flow of processing in the blood pressure measurement device  1401  according to the fourth exemplary embodiment. 
         [0197]    The blood pressure measurement device  1401  includes a cuff  401 , a pulse wave measurement unit  402 , a pulse wave measurement unit  403 , a pressure measurement unit  407 , a pressure control unit  1403 , an input unit  405 , a display unit  406 , and a blood pressure estimation device  1402 . 
         [0198]    First, the pressure control unit  1403  executes control for increasing an internal pressure of the cuff  401  up to a predetermined pressure (a pressure sufficiently higher than a common systolic blood pressure), in accordance with a start of measurement (step S 1501 ). Then, the pressure control unit  1403  executes control for reducing the pressure (step S 1502 ). In a process of the depressurization, the pressure measurement unit  407  measures the internal pressure of the cuff  401  and transmits the measured pressure to the blood pressure estimation device  1402  as a pressure signal  2003 . Further, the pulse wave measurement unit  402  and the pulse wave measurement unit  403  measure pulse waves in specific regions and transmit the measured pulse waves to the blood pressure estimation device  1402  as pulse wave signals. 
         [0199]    The pressure control unit  1403  executes control in step S 1501  and step S 1502 , and therefore, a pressure decreases with a lapse of time. 
         [0200]    Then, the blood pressure estimation device  1402  calculates pulse wave values and calculates characteristic values on the basis of the calculated pulse wave values. 
         [0201]    The blood pressure estimation device  1402  calculates a degree of similarity on the basis of the calculated characteristic values. When the calculated degree of similarity is equal to or larger than a reference value, the blood pressure estimation device  1402  estimates a systolic blood pressure on the basis of the degree of similarity (step S 1503 ) and estimates a diastolic blood pressure on the basis of the calculated degree of similarity compared with the reference value (step S 1505 ). In this case, the pressure control unit  1403  continues to execute control for reducing the pressure (step S 1504 ). 
         [0202]    The blood pressure measurement device  1401  according to the fourth exemplary embodiment includes components similar to those in the first exemplary embodiment, and therefore, effects similar to those in the first exemplary embodiment can be obtained from the fourth exemplary embodiment. In other words, according to the blood pressure estimation device  1401  according to the fourth exemplary embodiment, a blood pressure can be estimated with a high degree of accuracy. 
         [0203]    The blood pressure estimation device  1401  sufficiently performs pressurization and thereafter measures a pulse wave in a depressurization process to estimate a blood pressure. As a result, according to the blood pressure measurement device 1401  according to the present exemplary embodiment, a pulse wave can be measured at a pressure of a wider range, and therefore, a blood pressure can be estimated with a higher degree of accuracy. 
         [0204]    It is not always necessary for the blood pressure measurement device  1401  to estimate a blood pressure only in a process of pressurization and only in a process of depressurization, as described in the third exemplary embodiment and the fourth exemplary embodiment. The blood pressure measurement device  1401  may control a pressure on the basis of a degree of similarity calculated by the blood pressure estimation device  1402 . 
         [0205]    When, for example, a degree of similarity does not exceed a reference value even with a decrease in an internal pressure of the cuff  401 , it is difficult to determine a systolic blood pressure, and therefore, the blood pressure control unit  1403  may execute control for increasing the internal pressure of the cuff  401 . In this case, while the pressure control unit  1403  repeats pressurization and depressurization, the blood pressure estimation device  1402  estimates a systolic blood pressure and a diastolic blood pressure. 
         [0206]    When the above-described processing is executed, a systolic blood pressure can be searched via repetition of a process of pressurization and a process of depressurization, and therefore, according to the blood pressure measurement device  1401 , a blood pressure can be estimated with a higher degree of accuracy. 
         [0207]    Further, the blood pressure measurement device  1401  may estimate a blood pressure in a process of pressurization and further estimate a blood pressure in a process of depressurization. In this case, the blood pressure measurement device  1401  estimates a blood pressure by processing for averaging the two blood pressure or the like. 
         [0208]    Further, the blood pressure measurement device  1401  may estimate blood pressures in the respective processes via repetition of a process of pressurization and a process of depressurization. In this case, the blood pressure measurement device  1401  estimates a blood pressure by processing for averaging the estimated blood pressures or the like. 
         [0209]    When the above-described processing is executed, a blood pressure estimated by averaging blood pressures calculated in a process of repetition of pressurization and depressurization becomes a more accurate. In other words, according to the blood pressure measurement device  1401 , a blood pressure can be estimated with a higher degree of accuracy. 
         [0210]    (Hardware Configuration Example) 
         [0211]    A configuration example of hardware resources that realize a blood pressure estimation device in the above-described exemplary embodiments of the present invention using a single calculation processing apparatus (an information processing apparatus or a computer) will be described. However, the pressure estimation device may be realized using physically or functionally at least two calculation processing apparatuses. Further, the pressure estimation device may be realized as a dedicated apparatus. 
         [0212]      FIG. 26  is a block diagram schematically illustrating a hardware configuration of a calculation processing apparatus capable of realizing the blood pressure estimation device according to each of the first exemplary embodiment to the four exemplary embodiment or a pressure controlling unit in the blood pressure measurement device. A calculation processing apparatus  20  includes a central processing unit (CPU)  21 , a memory  22 , a disc  23 , a non-transitory recording medium  24 , an input apparatus  25 , an output apparatus  26 , and a communication interface (hereinafter, expressed as a “communication I/F”)  27 . The calculation processing apparatus  20  can execute transmission/reception of information to/from another calculation processing apparatus and a communication apparatus via the communication I/F  27 . 
         [0213]    The non-transitory recording medium  24  is, for example, a computer-readable Compact Disc, Digital_Versatile_Disc. The non-transitory recording medium  24  is, for example, Universal Serial Bus (USB) memory, or Solid State Drive. The non-transitory recording medium  24  allows a related program to be holdable and portable without power supply. The non-transitory recording medium  24  is not limited to the above-described media. Further, a related program can be carried via a communication network by way of the communication I/F  27  instead of the non-transitory medium  24 . 
         [0214]    In other words, the CPU  21  copies, on the memory  22 , a software program (a computer program: hereinafter, referred to simply as a “program”) stored by the disc  23  when executing the program and executes arithmetic processing. The CPU  21  reads data necessary for program execution from the memory  22 . When display is needed, the CPU  21  displays an output result on the output apparatus  26 . When a program is input from the outside, the CPU  21  reads the program from the input apparatus  25 . The CPU  21  interprets and executes a blood pressure estimation program present on the memory  22  corresponding to a function (processing) indicated by each unit illustrated in  FIG. 1 ,  FIG. 8 ,  FIG. 12 ,  FIG. 18 , or  FIG. 24  described above or an blood pressure estimation program ( FIG. 2 ,  FIG. 13 ,  FIG. 19 , or  FIG. 25 ). The CPU  21  sequentially executes the processing described in each exemplary embodiment of the present invention. 
         [0215]    In other words, in such a case, it is conceivable that the present invention can also be made using the blood pressure estimation program. Further, it is conceivable that the present invention can also be made using a computer-readable, non-transitory recording medium storing the blood pressure estimation program. 
         [0216]    The present invention has been described using the above-described exemplary embodiments as exemplary cases. However, the present invention is not limited to the above-described exemplary embodiments. In other words, the present invention is applicable with various aspects that can be understood by those skilled in the art without departing from the scope of the present invention. 
         [0217]    This application is based upon and claims the benefit of priority from Japanese patent application No. 2014-025372, filed on Feb. 13, 2014, the disclosure of which is incorporated herein in its entirety. 
       REFERENCE SIGNS LIST 
       [0218]      101  Blood pressure estimation device 
         [0219]      102  Blood pressure estimation unit 
         [0220]      2001  Blood pressure information 
         [0221]      2002  Pulse wave information 
         [0222]      401  Cuff 
         [0223]      402  Pulse wave measurement unit 
         [0224]      403  Pulse wave measurement unit 
         [0225]      404  Pressure control unit 
         [0226]      405  Input unit 
         [0227]      406  Display unit 
         [0228]      407  Pressure measurement unit 
         [0229]      408  Blood pressure measurement device 
         [0230]      1101  Blood pressure estimation device 
         [0231]      1102  Blood pressure estimation unit 
         [0232]      2001  Blood pressure information 
         [0233]      2002  Particular pulse wave information 
         [0234]      2003  Pressure signal 
         [0235]      2004  Pulse wave signal 
         [0236]      2005  Pulse wave signal 
         [0237]      2006  Blood pressure information 
         [0238]      2007  Blood pressure information 
         [0239]      1001  Pulse wave measurement unit 
         [0240]      1002  Pulse wave measurement unit 
         [0241]      1003  Pulse wave measurement unit 
         [0242]      1004  Pulse wave measurement unit 
         [0243]      1005  Cuff 
         [0244]      1006  Pressure bag 
         [0245]      1007  Blood pressure measurement device 
         [0246]      1008  Blood pressure measurement device 
         [0247]      1201  Blood pressure measurement device 
         [0248]      1202  Blood pressure estimation device 
         [0249]      1203  Pressure control unit 
         [0250]      1401  Blood pressure measurement device 
         [0251]      1402  Blood pressure estimation device 
         [0252]      1403  Pressure control unit 
         [0253]      20  Computing device 
         [0254]      21  CPU 
         [0255]      22  Memory 
         [0256]      23  Disk 
         [0257]      24  Nonvolatile recording medium 
         [0258]      25  Input device 
         [0259]      26  Output device 
         [0260]      27  Communication IF