Patent Publication Number: US-2021169347-A1

Title: Pulse transit time measurement device and blood pressure measurement device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national stage application filed pursuant to 35 U.S.C. 365(c) and 120 as a continuation of International Patent Application No. PCT/JP2019/029018, filed Jul. 24, 2019, which application claims priority from Japanese Patent Application No. 2018-156199, filed Aug. 23, 2018, which applications are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a pulse transit time measurement device that non-invasively measures pulse transit time and a blood pressure measurement device using the pulse transit time measurement device. 
     BACKGROUND ART 
     It is known that there is a correlation between blood pressure and a pulse transit time (PTT), which is a time required for a pulse wave to propagate between two points in an artery. A blood pressure measurement device utilizing the correlation described above measures a pulse transit time of a user (subject) and calculates a blood pressure value of the user, using the measured pulse transit time and a blood pressure calculation equation representing the correlation described above. 
     As a method for measuring the pulse transit time, there is known a method of measuring and acquiring an electrocardiographic signal and a pulse wave signal representing pulse waves at a specific site (for example, ears, upper arms, and the like) of the user and calculating the pulse transit time on the basis of the acquired electrocardiographic signal and pulse wave signal. In this method, the electrocardiographic signal is generally acquired using a plurality of electrodes disposed on the body so as to sandwich the heart of the user. 
     However, Patent Document 1 discloses that electrocardiographic signals can be acquired at any site (for example, an upper arm) of a user. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP 2007-504917 T 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in a method for acquiring electrocardiographic signals using a plurality of electrodes arranged at a single site of a user as disclosed in Patent Document 1, since the signal representing the electrical activity of the heart is small and is easily confused with noise, and the electrocardiographic waveform is different depending on the combination of electrodes, it is difficult to acquire accurate electrocardiographic information. Thus, when the pulse transit time is calculated on the basis of electrocardiographic signals acquired using a plurality of electrodes disposed at a single site of a user, the drive timing of the heart may not be detected correctly and the pulse transit time may not be accurately measured. 
     The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a pulse transit time measurement device capable of measuring the pulse transit time more accurately and a blood pressure measurement device using the pulse transit time measurement device. 
     Solution to Problem 
     The present invention adopts the following configurations in order to solve the above problems. 
     A pulse transit time measurement device according to an aspect includes: a belt unit wound around a target measurement site of a user; a plurality of first electrodes provided on an inner circumferential surface of the belt unit; a second electrode provided on the inner circumferential surface of the belt unit; a third electrode provided on an outer circumferential surface of the belt unit; a first electrocardiographic signal acquisition unit that acquires a first electrocardiographic signal of the user using the plurality of first electrodes; a second electrocardiographic signal acquisition unit that acquires a second electrocardiographic signal of the user using the second electrode and the third electrode in a period; a feature amount parameter calculation unit that calculates a feature amount parameter related to a waveform feature point of the first electrocardiographic signal acquired in the period based on a waveform feature point of the second electrocardiographic signal; a pulse wave signal acquisition unit that includes a pulse wave sensor provided in the belt unit and acquires a pulse wave signal representing a pulse wave of the user using the pulse wave sensor; and a pulse transit time calculation unit that detects a waveform feature point of the first electrocardiographic signal acquired later than the period using the feature amount parameter and calculates a pulse transit time based on a time difference between the waveform feature point of the first electrocardiographic signal that is detected and a waveform feature point of the pulse wave signal. 
     According to the configuration above, for example, when the belt unit is wound around the upper left arm of the user, the first electrode and the second electrode contact the upper left arm. When the user touches the third electrode with the right hand, a state in which the second electrode and the third electrode are positioned so as to sandwich the heart is created. Since the second electrocardiographic signal is acquired using the second electrode and the third electrode arranged so as to sandwich the heart, the second electrocardiographic signal is more accurate than the first electrocardiographic signal acquired using the first electrode disposed on the upper left arm. The first electrocardiographic signal and the second electrocardiographic signal are acquired simultaneously, and a feature amount parameter related to a waveform feature point of the first electrocardiographic signal is calculated on the basis of a waveform feature point of the second electrocardiographic signal. Then, when measuring the pulse transit time, the first electrocardiographic signal and the pulse wave signal are acquired, a waveform feature point of the first electrocardiographic signal is detected using the feature amount parameter, and a time difference between the detected waveform feature point of the first electrocardiographic signal and the waveform feature point of the pulse wave signal is calculated. By using the feature amount parameter calculated in advance, the waveform feature point (for example, a peak point corresponding to the R-wave) of the first electrocardiographic signal that is considered as the drive timing of the heart can be detected correctly and the pulse transit time can be measured accurately. 
     In one aspect, the feature amount parameter calculation unit may detect a peak with a maximum amplitude of the first electrocardiographic signal in a time range determined based on the waveform feature point of the second electrocardiographic signal and acquire an amplitude value of the peak that is detected or a sign of the amplitude value as the feature amount parameter. According to this configuration, the waveform feature point of the first electrocardiographic signal for calculating the pulse transit time can be detected correctly. 
     In one aspect, the second electrode may be one of the plurality of first electrodes. According to this configuration, it is not necessary to provide a dedicated electrode which comes into contact with the target measurement site, for acquiring the second electrocardiographic signal. As a result, the manufacturing cost can be reduced. 
     In one aspect, the above described pulse transit time measurement device may further include an electrode selection unit that selects two first electrodes that provide the first electrocardiographic signal having a greatest amplitude of an R-wave among the plurality of first electrodes, and the first electrocardiographic signal acquisition unit may acquire the first electrocardiographic signal based on a potential difference between the two first electrodes that are selected. 
     According to the configuration described above, the time of the R-wave peak point (the peak point corresponding to the R-wave) of the first electrocardiographic signal can be identified accurately. As a result, the pulse transit time can be measured more accurately. 
     A blood pressure measurement device according to an aspect includes: the above-described pulse transit time measurement device; and a first blood pressure value calculation unit calculating a first blood pressure value based on the pulse transit time that is calculated. According to the configuration described above, since the pulse transit time can be measured for each beat, it is possible to obtain a blood pressure value for each beat. 
     In an aspect, the blood pressure measurement device may further include a pressing cuff provided in the belt unit; a fluid supply unit supplying a fluid to the pressing cuff; a pressure sensor detecting pressure in the pressing cuff; and a second blood pressure value calculation unit calculating a second blood pressure value based on an output of the pressure sensor. 
     According to the configuration described above, continuous blood pressure measurement wherein a blood pressure value is obtained for each beat and blood pressure measurement using an oscillometric method can be executed with one device. As a result, it is highly convenient for the user. 
     In one aspect, the blood pressure measurement device described above may further comprise a button for initiating blood pressure measurement by the pressing cuff, the fluid supply unit, the pressure sensor, and the second blood pressure value calculation unit, and the third electrode may be provided on the button. 
     According to the above configuration, it is possible to calculate a feature amount parameter while calibrating a blood pressure calculation formula that represents a correlation between the pulse transit time and blood pressure and to improve the convenience of the user. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a pulse transit time measurement device capable of measuring the pulse transit time more accurately and a blood pressure measurement device using the pulse transit time measurement device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a blood pressure measurement device according to an embodiment. 
         FIG. 2  is a diagram illustrating the appearance of the blood pressure measurement device illustrated in  FIG. 1 . 
         FIG. 3  is a diagram illustrating the appearance of the blood pressure measurement device illustrated in  FIG. 1 . 
         FIG. 4  is a diagram illustrating a cross-section of the blood pressure measurement device illustrated in  FIG. 1 . 
         FIG. 5  is a block diagram illustrating a hardware configuration of a control system of the blood pressure measurement device illustrated in  FIG. 1 . 
         FIG. 6  is a block diagram illustrating a software configuration of the blood pressure measurement device illustrated in  FIG. 1 . 
         FIG. 7  is a diagram illustrating an example of a method in which a feature amount parameter calculation unit illustrated in  FIG. 6  calculates a feature amount parameter. 
         FIG. 8  is a diagram illustrating an example of a method in which a pulse transit time calculation unit illustrated in  FIG. 6  calculates a pulse transit time. 
         FIG. 9  is a flowchart illustrating operation in which the blood pressure measurement device illustrated in  FIG. 1  calculates a feature amount parameter. 
         FIG. 10  is a flowchart illustrating operation in which the blood pressure measurement device illustrated in  FIG. 1  performs blood pressure measurement based on a pulse transit time. 
         FIG. 11  is a flowchart illustrating operation in which the blood pressure measurement device illustrated in  FIG. 1  performs blood pressure measurement using an oscillometric method. 
         FIG. 12  is a diagram illustrating changes in cuff pressure and pulse wave signal during blood pressure measurement using the oscillometric method. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 
     Overview 
       FIG. 1  illustrates a blood pressure measurement device  10  according to an embodiment. In the example of  FIG. 1 , the blood pressure measurement device  10  is a wearable device and is attached to an upper left arm of a user as a target measurement site. The blood pressure measurement device  10  includes a belt unit  20 , a first blood pressure measurement unit  30 , and a second blood pressure measurement unit  40 . 
     The belt unit  20  has an inner circumferential surface and an outer circumferential surface. The inner circumferential surface is a surface that faces (contacts) the upper left arm of the user in a state in which the blood pressure measurement device  10  is attached to the user (hereinafter, simply referred to as an “attachment state”), and the outer circumferential surface is a surface that does not face (does not contact) the upper left arm of the user in the attachment state. The belt unit  20  includes a belt  21  and a body  22 . The belt  21  is a band-like member that is worn around the upper left arm and is sometimes referred to by another name such as a band or a cuff. 
     The body  22  is mounted on the belt  21 . The body  22  accommodates components such as a control unit  501  (illustrated in  FIG. 5 ) described below together with an operation unit  221  and a display unit  222 . The operation unit  221  is an input device that allows a user to input an instruction to the blood pressure measurement device  10 . In the example of  FIG. 1 , the operation unit  221  includes a plurality of push buttons. The display unit  222  is a display device displaying information such as a blood pressure measurement result. As a display device, for example, a liquid crystal display (LCD), an organic light emitting diode (OLED) display, and the like can be used. A touch screen that also serves as a display device and an input device may be used. 
     The first blood pressure measurement unit  30  non-invasively measures a pulse transit time of the user and calculates a blood pressure value based on the measured pulse transit time. The first blood pressure measurement unit  30  can perform continuous blood pressure measurement for obtaining the blood pressure value for each beat. The second blood pressure measurement unit  40  performs blood pressure measurement using a method different from that of the first blood pressure measurement unit  30 . The second blood pressure measurement unit  40  is based on, for example, an oscillometric method or a Korotkoff method and performs the blood pressure measurement at a specific timing, for example, in response to operation performed by the user. The second blood pressure measurement unit  40  can measure the blood pressure more accurately than the first blood pressure measurement unit  30 . 
     The first blood pressure measurement unit  30  includes an internal electrode group  31 , an external electrode  32 , a first electrocardiographic signal acquisition unit  33 , a second electrocardiographic signal acquisition unit  34 , a feature amount parameter calculation unit  35 , a pulse wave signal acquisition unit  36 , a pulse transit time calculation unit  37 , and a blood pressure value calculation unit  38 . 
     The internal electrode group  31  has a plurality of internal electrodes. These internal electrodes are provided on the inner circumferential surface of the belt unit  20 , so that the internal electrodes are in contact with the upper left arm of the user in the attachment state. The internal electrode corresponds to the first electrode of the present invention. In the example described in the present embodiment, the internal electrodes are used by the first electrocardiographic signal acquisition unit  33 , and one of the internal electrodes is also used by the second electrocardiographic signal acquisition unit  34 . The internal electrode used by the second electrocardiographic signal acquisition unit  34  corresponds to the second electrode of the present invention. The external electrode  32  is provided on the outer circumferential surface of the belt unit  20 , so that the external electrode  32  is not in contact with the upper left arm of the user in the attachment state. The external electrode  32  corresponds to the third electrode of the present invention. 
     The first electrocardiographic signal acquisition unit  33  acquires the user&#39;s electrocardiographic signal (ECG signal) using the internal electrode group  31 . The electrocardiographic signal is a waveform signal that represents a change over time in the electrical activity of the heart. Specifically, the first electrocardiographic signal acquisition unit  33  acquires the user&#39;s electrocardiographic signal on the basis of a potential difference between two internal electrodes selected from the internal electrode group  31 . In the following, the electrocardiographic signal obtained by the first electrocardiographic signal acquisition unit  33  is sometimes referred to as a first electrocardiographic signal. 
     The second electrocardiographic signal acquisition unit  34  acquires the user&#39;s electrocardiographic signal using one internal electrode of the internal electrode group  31  and the external electrode  32 . Specifically, the second electrocardiographic signal acquisition unit  34  acquires the user&#39;s electrocardiographic signal on the basis of a potential difference between one internal electrode and the external electrode  32 . The acquisition of the electrocardiographic signal by the second electrocardiographic signal acquisition unit  34  is performed, for example, in a state in which the right hand of the user is in contact with the external electrode  32 , that is, using electrodes disposed on the left and right sides of the heart so as to sandwich the heart. This measurement method is a measurement method called the first lead, which is the lead of looking at the side walls of the left ventricle, and is capable of acquiring more accurate electrocardiographic signals. The electrocardiographic signal obtained by the second electrocardiographic signal acquisition unit  34  is sometimes referred to as a second electrocardiographic signal. 
     The feature amount parameter calculation unit  35  calculates a feature amount parameter related to the waveform feature points of the first electrocardiographic signal, on the basis of the waveform feature points of the second electrocardiographic signal. The waveform feature points may correspond to any of the Q, R, and S-waves. The first electrocardiographic signals acquired using electrodes disposed at a single site (the upper left arm in this example) have a different waveform shape from that of the second electrocardiographic signal that reflects the electrical activity of the heart more accurately. For example, in the first electrocardiographic signal, the amplitude of the waveform feature points is small, and the waveform feature points appear on the positive or negative side depending on the electrode used. Thus, it is difficult to accurately detect a particular waveform feature point in the first electrocardiographic signal. The feature amount parameter calculation unit  35  detects a waveform feature point of the second electrocardiographic signal and determines a time range for detecting the waveform feature point on the basis of the detected waveform feature point. Subsequently, the feature amount parameter calculation unit  35  detects a peak having a maximum amplitude (the absolute value of the amplitude value becomes maximum) in the first electrocardiographic signal acquired simultaneously with the second electrocardiographic signal in the determined time range and acquires the amplitude value of the detected peak as the feature amount parameter. 
     The pulse wave signal acquisition unit  36  includes a pulse wave sensor and acquires a pulse wave signal representing a pulse wave in the upper left arm of the user, using the pulse wave sensor. The pulse wave sensor is provided on the belt unit  20 . For example, the pulse wave sensor is disposed on the inner circumferential surface of the belt unit  20 , so that the pulse wave sensor is in contact with the upper left arm of the user in the attachment state. Note that some types of pulse wave sensors, such as pulse wave sensors based on the radio wave method described below, do not need to be in contact with the skin of the user&#39;s upper left arm in the attachment state. 
     The pulse transit time calculation unit  37  is configured to detect a waveform feature point of the first electrocardiographic signal obtained by the first electrocardiographic signal acquisition unit  33  using the feature amount parameter calculated by the feature amount parameter calculation unit  35  and calculate the pulse transit time on the basis of a time difference between the detected waveform feature points of the first electrocardiographic signal and the waveform feature points of the pulse wave signal obtained by the pulse wave signal acquisition unit  36 . For example, the pulse transit time calculation unit  37  calculates the time difference between the detected waveform feature point of the first electrocardiographic signal and the waveform feature point of the pulse wave signal as the pulse transit time. In the present embodiment, the timing at which either the Q-wave, the R-wave, or the S-wave of the first electrocardiographic signal peaks is regarded as the drive timing of the heart (for example, the timing at which the heart pumps blood). In the present embodiment, the pulse transit time corresponds to a time required for a pulse wave to propagate through the artery, from the heart to the upper left arm (to be exact, the position where the pulse wave sensor is disposed). 
     The blood pressure value calculation unit  38  calculates a blood pressure value on the basis of the pulse transit time calculated by the pulse transit time calculation unit  37  and a blood pressure calculation formula. The blood pressure calculation formula is a relational formula that represents a correlation between the pulse transit time and the blood pressure. An example of a blood pressure calculation formula is illustrated below. 
       SBP= A   1 /PTT 2   +A   2   (1)
 
     Here, SBP represents systolic blood pressure, PTT represents the pulse transit time, and A 1  and A 2  are parameters. 
     The pulse transit time calculation unit  37  can calculate the pulse transit time for each beat, and thus the blood pressure value calculation unit  38  can calculate the blood pressure value for each beat. 
     As described above, the blood pressure measurement device  10  calculates a feature amount parameter related to a waveform feature point of the first electrocardiographic signal acquired using the internal electrode group  31 , on the basis of the second electrocardiographic signal acquired using one internal electrode of the internal electrode group  31  and the external electrode  32 . The use of the feature amount parameter allows the waveform feature point of the first electrocardiographic signal to be detected correctly and allows the pulse transit time to be measured accurately. As a result, the reliability of the blood pressure value calculated on the basis of the pulse transit time is improved. 
     Hereinafter, the blood pressure measurement device  10  will be described in more detail. 
     Configuration Example 
     Hardware Configuration 
     An example of a hardware configuration of the blood pressure measurement device  10  according to the present embodiment will be described with reference to  FIGS. 2 to 6 . 
       FIGS. 2 and 3  are plan views illustrating the appearance of the blood pressure measurement device  10 . Specifically,  FIG. 2  illustrates the blood pressure measurement device  10  viewed from an outer circumferential surface  211  of the belt  21  in an expanded state of the belt  21 , and  FIG. 3  illustrates the blood pressure measurement device  10  viewed from an inner circumferential surface  212  of the belt  21  in an expanded state of the belt  21 .  FIG. 4  illustrates a cross-section of the blood pressure measurement device  10  in the attachment state. 
     The belt  21  includes an attachment member allowing the belt  21  to be detachably attached to the upper arm. In the example illustrated in  FIGS. 2 and 3 , the attachment member is a surface fastener including: a loop surface  213  including a multiplicity of loops; and a hook surface  214  including a plurality of hooks. The loop surface  213  is disposed on the outer circumferential surface  211  of the belt  21  at a longitudinal end portion  215 A of the belt  21 . The longitudinal direction corresponds to the circumferential direction of the upper arm in the attachment state. The hook surface  214  is disposed on the inner circumferential surface  212  of the belt  21  at a longitudinal end portion  215 B of the belt  21 . The end portion  215 B faces the end portion  215 A in the longitudinal direction of the belt  21 . When the loop surface  213  and the hook surface  214  are pressed against each other, the loop surface  213  and the hook surface  214  are joined. In addition, pulling the loop surface  213  and the hook surface  214  away from each other separates the loop surface  213  and the hook surface  214 . 
     As illustrated in  FIG. 3 , the internal electrode group  31  is disposed on the inner circumferential surface  212  of the belt  21 . In the example of  FIG. 3 , the internal electrode group  31  has six internal electrodes  312  aligned at regular intervals in the longitudinal direction of the belt  21 . The interval between the internal electrodes  312  is set, for example, to a quarter of the circumference of the upper arm of the user expected to have the thinnest arm. In this arrangement, as illustrated in  FIG. 4 , for a user expected to have the thinnest arm, four of the six internal electrodes  312  contact the upper left arm  70  in the attachment state and are positioned at regular intervals on the circumference of the upper left arm  70 , and the remaining two internal electrodes  312  contact the outer circumferential surface  211  of the belt  21 . In  FIG. 4 , a humerus  71  and a brachial artery  72  are illustrated. For a user expected to have the thickest arm, all the six internal electrodes  312  contact the upper left arm  70  in the attachment state. 
     Note that the number of internal electrodes  312  is not limited to six, and may be two to five or seven or greater. If two or three internal electrodes  312  are in contact with the upper left arm, the first electrocardiographic signal may not be successfully measured depending on the attachment state. If the first electrocardiographic signal is not successfully measured, a message may be displayed on the display unit  222 , and the blood pressure measurement device  10  needs to be re-attached to the user. In order to avoid situations in which the first electrocardiographic signal cannot be measured, it is desired that at least four internal electrodes  312  contact the upper left arm in the attachment state. 
     The closer the internal electrode  312  is to the heart in the attachment state, the greater the signal representing the electrical activity of the heart and acquired using the internal electrode  312  becomes, that is, the signal to noise ratio (SN ratio) becomes higher. Preferably, as illustrated in  FIG. 3 , the internal electrodes  312  are disposed in a central side portion  217 A of the belt  21 . The central side portion  217 A is a portion that is located closer to the central side (the shoulder side) than a center line  216  in the attachment state. More preferably, the internal electrode  312  is disposed at a central end portion  218 A of the belt  21 . The central end portion  218 A is an end portion located on the central side in the attachment state, and the width of the central end portion  218 A is, for example, one-third of the full width of the belt  21 . 
     As illustrated in  FIG. 2 , the external electrode  32  is provided on the body  22 . Note that the external electrode  32  may be provided on the outer circumferential surface  211  of the belt  21 . 
     A sensor unit  362  of an impedance measurement unit  361  is further disposed on the inner circumferential surface  212  of the belt  21 . In the example of  FIG. 3 , the sensor unit  362  includes a pair of electrodes  362 A,  362 D for energizing the upper left arm and a pair of electrodes  362 B,  362 C for detecting a voltage. The pair of electrodes  362 B,  362 C form the pulse wave sensor. The electrodes  362 A,  362 B,  362 C,  362 D are arranged in that order in the width direction of the belt  21 . The width direction of the belt  21  corresponds to a direction along the brachial artery  72  in the attachment state. 
     The farther the sensor unit  362  is located from the heart in the attachment state, the longer the pulse transit distance is and the greater the measurement value of the pulse transit time is. If the measurement value of the pulse transit time is large, the error generated in calculating the time difference between the waveform feature point of the first electrocardiographic signal and the waveform feature point of the pulse wave signal is relatively smaller than the pulse transit time, and the pulse transit time can be accurately measured. Thus, preferably, the sensor unit  362  is disposed in a peripheral side portion  217 B of the belt  21 . The peripheral side portion  217 B is a portion that is positioned closer to the peripheral side (the elbow side) than the center line  216  in the attachment state. More preferably, the sensor unit  362  is disposed at a peripheral end portion  218 C of the belt  21 . The peripheral end portion  218 C is an end portion located on the peripheral side in the attachment state, and the width of the peripheral end portion  218 C is, for example, one-third the full width of the belt  21 . A portion  218 B between the central end portion  218 A and the peripheral end portion  218 C is referred to as an intermediate portion. 
     As illustrated in  FIG. 4 , the belt  21  includes an inner cloth  210 A an outer cloth  210 B, and a pressing cuff  401  is provided between the inner cloth  210 A and the outer cloth  210 B. The pressing cuff  401  is a band-like member that is long in the longitudinal direction of the belt  21  such that the pressing cuff  401  can surround the upper left arm. For example, the pressing cuff  401  is configured as a fluid bag by placing two stretchable polyurethane sheets opposite each other in the thickness direction and welding the edge portions of the polyurethane sheets. The internal electrode group  31  and the sensor unit  362  are provided in the inner cloth  210 A such that the internal electrode group  31  and the sensor unit  362  are positioned between the pressing cuff  401  and the upper left arm  70  in the attachment state. 
       FIG. 5  illustrates an example of a hardware configuration of a control system of the blood pressure measurement device  10  according to the present embodiment. In the example of  FIG. 5 , in addition to the operation unit  221  and the display unit  222  described above, the body  22  includes the control unit  501 , a storage unit  505 , a battery  506 , a switch circuit  333 , a subtraction circuit  334 , an analog front end (AFE)  335 , a subtraction circuit  344 , an AFE  345 , a pressure sensor  402 , a pump  403  as a fluid supply unit, a valve  404 , an oscillation circuit  405 , and a pump drive circuit  406 . The body  22  may be provided with a sound emitter such as a speaker or a piezoelectric sounder. The body  22  may be provided with a microphone to allow the user to input instructions by sounds. In addition to the sensor unit  362  described above, the impedance measurement unit  361  includes an energization and voltage detection circuit  363 . In this example, the energization and voltage detection circuit  363  is provided on the belt  21 . 
     The control unit  501  includes a Central Processing Unit (CPU)  502 , a Random Access Memory (RAM)  503 , a Read Only Memory (ROM)  504 , and the like and controls each component according to information processing. The storage unit  505  is an auxiliary storage device such as, for example, a hard disk drive (HDD) or a semiconductor memory (for example, a flash memory) and non-transitorily stores: programs executed by the control unit  501  (including, for example, a pulse transit time measurement program and a blood pressure measurement program), settings data necessary for executing the programs, the blood pressure measurement result, and the like. A storage medium included in the storage unit  505  is, to enable computers, other devices, machines, or the like to read information such as recorded programs, a medium that stores information such as the programs, by using electrical, magnetic, optical, mechanical, or chemical actions. Note that some or all of the programs may be stored in the ROM  504 . 
     The battery  506  supplies electric power to components such as the control unit  501 . The battery  506  is, for example, a rechargeable battery. 
     The six internal electrodes  312  are connected to an input terminal of the switch circuit  333 . The two output terminals of the switch circuit  333  are connected to two input terminals of the subtraction circuit  334 . The switch circuit  333  receives a switch signal from the control unit  501  and connects the two internal electrodes  312  designated by the switch signal to the subtraction circuit  334 . The subtraction circuit  334  subtracts, from the potential input from one input terminal, the potential input from the other input terminal. The subtraction circuit  334  outputs, to the AFE  335 , a potential difference signal that represents the potential difference between the two interconnected internal electrodes  312 . The subtraction circuit  334  is, for example, an instrumentation amplifier. AFE  335  includes, for example, a low-pass filter (LPF), an amplifier, and an analog-to-digital converter. The potential difference signal is filtered by the LPF, amplified by the amplifier, and converted to a digital signal by the analog-to-digital converter. The potential difference signal converted to the digital signal is provided to the control unit  501 . The control unit  501  acquires, from the AFE  335 , the potential difference signal output in a time-series manner as the first electrocardiographic signal. 
     One of the six internal electrodes  312  is further connected to one input terminal of the subtraction circuit  344 . The external electrode  32  is connected to the other input terminal of the subtraction circuit  344 . The subtraction circuit  344  outputs, to the AFE  345 , a potential difference signal representing the potential difference between the internal electrode  312  and the external electrode  32 . The subtraction circuit  344  is, for example, an instrumentation amplifier. The AFE  345  includes, for example, an LPF, an amplifier, and an analog-to-digital converter. The potential difference signal is filtered by the LPF, amplified by the amplifier, and converted to a digital signal by the analog-to-digital converter. The potential difference signal converted to the digital signal is provided to the control unit  501 . The control unit  501  acquires, from the AFE  345 , the potential difference signal output in a time-series manner as the second electrocardiographic signal. 
     The energization and voltage detection circuit  363  allows a high-frequency constant current to flow between the electrodes  362 A,  362 D. In this example, the current has a frequency of 50 kHz and a current value of 1 mA. The energization and voltage detection circuit  363  detects the voltage across the electrodes  362 B,  362 C and generates a detection signal, in a state in which a current flows between the electrodes  362 A,  362 D. The detection signal represents a change in electrical impedance due to a pulse wave that propagates through a portion of the artery that faces the electrodes  362 B,  362 C. The energization and voltage detection circuit  363  performs signal processing including rectifying, amplifying, filtering, and analog-to-digital conversion on the detection signal and supplies the detection signal to the control unit  501 . The control unit  501  acquires, from the energization and voltage detection circuit  363 , the detection signal output in a time-series manner as a pulse wave signal. 
     The pressure sensor  402  is connected to the pressing cuff  401  via a pipe  407 , and the pump  403  and the valve  404  are connected to the pressing cuff  401  via a pipe  408 . The pipes  407 ,  408  may be a single common pipe. The pump  403  is, for example, a piezoelectric pump and feeds air as a fluid to the pressing cuff  401  through the pipe  408  in order to increase the pressure inside the pressing cuff  401 . The valve  404  is mounted on the pump  403 , and opening and closing of the valve  404  is controlled according to an operation state (on/off) of the pump  403 . Specifically, the valve  404  is in a closed state when the pump  403  is turned on, and the valve  404  is in an open state when the pump  403  is turned off. When the valve  404  is in an open state, the pressing cuff  401  is in communication with the atmosphere, and air in the pressing cuff  401  is discharged into the atmosphere. The valve  404  has a function of a check valve, and air does not flow back through it. The pump drive circuit  406  drives the pump  403  on the basis of a control signal received from the control unit  501 . 
     The pressure sensor  402  detects the pressure in the pressing cuff  401  (also referred to as cuff pressure) and generates an electric signal representing the cuff pressure. The cuff pressure is, for example, pressure based on the atmospheric pressure as a reference. The pressure sensor  402  is, for example, a piezoresistive pressure sensor. The oscillation circuit  405  oscillates on the basis of the electric signal from the pressure sensor  402  and outputs, to the control unit  501 , a frequency signal having a frequency corresponding to the electric signal. In this example, the output of the pressure sensor  402  is used for controlling the pressure of the pressing cuff  401  and for calculating a blood pressure value (including a systolic blood pressure and a diastolic blood pressure) using an oscillometric method. 
     The pressing cuff  401  may be used for adjusting the contact state between the upper left arm and the internal electrode  312  or the sensor unit  362  of the impedance measurement unit  361 . For example, during execution of the blood pressure measurement based on the pulse transit time, the pressing cuff  401  is maintained in a state in which some air is accommodated therein. As a result, the internal electrode  312  and the sensor unit  362  of the impedance measurement unit  361  are reliably in contact with the upper left arm of the user. 
     In the example illustrated in  FIGS. 2 to 5 , the switch circuit  333 , the subtraction circuit  334 , and the AFE  335  are included in the first electrocardiographic signal acquisition unit  33  illustrated in  FIG. 1 , the subtraction circuit  344  and the AFE  345  are included in the second electrocardiographic signal acquisition unit  34  illustrated in  FIG. 1 , and the impedance measurement unit  361  (including the electrodes  362 A to  362 D and the energization and voltage detection circuit  363 ) is included in the pulse wave signal acquisition unit  36  illustrated in  FIG. 1 . Also, the pressing cuff  401 , the pressure sensor  402 , the pump  403 , the valve  404 , the oscillation circuit  405 , the pump drive circuit  406 , and the pipes  407 ,  408  are included in the second blood pressure measurement unit  40  illustrated in  FIG. 1 . 
     Also, with respect to a specific hardware configuration of the blood pressure measurement device  10 , components can be omitted, replaced, or added as appropriate in accordance with embodiments. For example, the control unit  501  may include a plurality of processors. The blood pressure measurement device  10  may include a communication unit  507  for communicating with an external device such as a portable terminal of the user (for example, a smartphone). The communication unit  507  includes a wired communication module and/or a wireless communication module. As a wireless system, for example, Bluetooth (trade name), Bluetooth Low Energy (BLE), or the like can be adopted. 
     Software Configuration 
     An example of a software configuration of the blood pressure measurement device  10  according to the present embodiment will be described with reference to  FIG. 6 .  FIG. 6  illustrates one example of the software configuration of the blood pressure measurement device  10 . In the example of  FIG. 6 , the blood pressure measurement device  10  includes a first electrocardiographic signal measurement control unit  601 , a first electrocardiographic signal storage unit  602 , a second electrocardiographic signal measurement control unit  603 , a second electrocardiographic signal storage unit  604 , the feature amount parameter calculation unit  35 , a pulse wave measurement control unit  606 , a pulse wave signal storage unit  607 , the pulse transit time calculation unit  37 , the blood pressure value calculation unit  38 , a first blood pressure value storage unit  610 , a blood pressure measurement control unit  611 , a second blood pressure value storage unit  612 , a display control unit  613 , an instruction input unit  614 , and a calibration unit  615 . The first electrocardiographic signal measurement control unit  601 , the second electrocardiographic signal measurement control unit  603 , the feature amount parameter calculation unit  35 , the pulse wave measurement control unit  606 , the pulse transit time calculation unit  37 , the blood pressure value calculation unit  38 , the blood pressure measurement control unit  611 , the display control unit  613 , the instruction input unit  614 , and the calibration unit  615  execute the following processing when the control unit  501  of the blood pressure measurement device  10  executes the programs stored in the storage unit  505 . When the control unit  501  executes the program, the control unit  501  loads the program in the RAM  503 . Then, the control unit  501  causes the CPU  502  to interpret and execute the program loaded in the RAM  503  to control each component. The first electrocardiographic signal storage unit  602 , the second electrocardiographic signal storage unit  604 , the pulse wave signal storage unit  607 , the first blood pressure value storage unit  610 , and the second blood pressure value storage unit  612  are implemented by the storage unit  505 . 
     The first electrocardiographic signal measurement control unit  601  controls the switch circuit  333  to acquire the first electrocardiographic signal. Specifically, the first electrocardiographic signal measurement control unit  601  generates a switch signal for selecting two internal electrodes  312  from among the six internal electrodes  312  and provides the switch signal to the switch circuit  333 . The first electrocardiographic signal measurement control unit  601  acquires the potential difference signal acquired using the two selected internal electrodes  312  and stores the time-series data of the acquired potential difference signal in the first electrocardiographic signal storage unit  602  as the first electrocardiographic signal. 
     The first electrocardiographic signal measurement control unit  601  operates as an electrode selection unit to determine an internal electrode pair optimal for acquiring electrocardiographic signals. The selection of the electrode pair is executed, for example, when the blood pressure measurement device  10  is attached to the upper left arm of the user. For example, the first electrocardiographic signal measurement control unit  601  acquires an electrocardiographic signal for each possible pair of internal electrodes and determines an internal electrode pair that provides an electrocardiographic signal with the greatest amplitude of the R-wave as the optimal electrode pair. Thereafter, the first electrocardiographic signal measurement control unit  601  acquires the first electrocardiographic signal using the optimal internal electrode pair. 
     The second electrocardiographic signal measurement control unit  603  acquires a potential difference signal acquired using one internal electrode  312  and the external electrode  32  and stores the time-series data of the acquired potential difference signal in the second electrocardiographic signal storage unit  604  as a second electrocardiographic signal. The second electrocardiographic signal is acquired in synchronization with the first electrocardiographic signal to calculate the feature amount parameter. At least a portion of the period in which the first electrocardiographic signal is measured may overlap at least a portion of the period in which the second electrocardiographic signal is measured. 
     The feature amount parameter calculation unit  35  reads the second electrocardiographic signal from the second electrocardiographic signal storage unit  604 , detects a waveform feature point of the second electrocardiographic signal, and determines a time range centered on the detected waveform feature points. The feature amount parameter calculation unit  35  reads the first electrocardiographic signal acquired in synchronization with the second electrocardiographic signal from the first electrocardiographic signal storage unit  602 , detects a peak point with the maximum amplitude of the first electrocardiographic signal in the determined time range, and calculates the amplitude value of the detected peak point as the feature amount parameter. Note that the feature amount parameter is not limited to the amplitude value of the detected peak point and may be the sign (positive or negative) of the amplitude value of the detected peak point. 
     Referring to  FIG. 7 , an example of a method of calculating the feature amount parameter will be described. In  FIG. 7 , four internal electrodes  312  are illustrated and designated as internal electrodes  312 - 1 ,  312 - 2 ,  312 - 3 , and  312 - 4  to distinguish between these four internal electrodes  312 . The second-stage graph is the first electrocardiographic signal acquired using the internal electrodes  312 - 1 ,  312 - 3 , and the first-stage graph is the second electrocardiographic signal acquired simultaneously with the first electrocardiographic signal on the second stage. The fourth-stage graph is the first electrocardiographic signal acquired using the internal electrodes  312 - 2 ,  312 - 4 , and the third-stage graph is the second electrocardiographic signal acquired simultaneously with the first electrocardiographic signal on the fourth stage. As illustrated in  FIG. 7 , the first electrocardiographic signal acquired with the internal electrode pair  312 - 1 ,  312 - 3  has a different waveform shape from that of the first electrocardiographic signal acquired using the internal electrode pair  312 - 2 ,  312 - 4 . In the first electrocardiographic signal acquired using the internal electrode pair  312 - 1 ,  312 - 3 , the R-wave peak point has a positive amplitude value. In contrast, in the first electrocardiographic signal acquired with the internal electrode pair  312 - 2 ,  312 - 4 , the R-wave peak point has a negative amplitude value. 
     The feature amount parameter calculation unit  35  detects the R-wave peak point of the second electrocardiographic signal and determines a time range (indicated as a double-sided arrow in  FIG. 7 ) centered on the time of the detected R-wave peak point. Then, the feature amount parameter calculation unit  35  detects, in the determined time range, a peak point with the maximum amplitude of the first electrocardiographic signal and acquires the amplitude value of the detected peak point as the feature amount parameter. 
     Note that the feature amount parameter calculation unit  35  may calculate the feature amount parameter related to a peak point corresponding to a Q-wave or an S-wave without being limited to the R-wave. Since the R-wave appears more clearly than the Q-wave or the S-wave, the peak point corresponding to the R-wave can be identified more accurately than the peak point corresponding to the Q-wave or the S-wave. Therefore, preferably, the feature amount parameter calculation unit  35  calculates the feature amount parameter for the R-wave peak point. 
     Referring again to  FIG. 6 , the pulse wave measurement control unit  606  controls energization and voltage detection circuit  363  to acquire the pulse wave signal. Specifically, the pulse wave measurement control unit  606  instructs the energization and voltage detection circuit  363  to flow a current between the electrodes  362 A,  362 D and acquires a detection signal indicating the voltage between the electrodes  362 B,  362 C detected with the current flowing between the electrodes  362 A,  362 D. The pulse wave measurement control unit  606  stores the time-series data of the detection signal in the pulse wave signal storage unit  607  as a pulse wave signal. 
     The pulse transit time calculation unit  37  reads the first electrocardiographic signal acquired using the optimal internal electrode pair from the first electrocardiographic signal storage unit  602 , reads the pulse wave signal from the pulse wave signal storage unit  607 , and receives the feature amount parameter from the feature amount parameter calculation unit  35 . The pulse transit time calculation unit  37  detects the R-wave peak point of the first electrocardiographic signal with reference to the feature amount parameter and calculates a pulse transit time on the basis of a time difference between the detected R-wave peak point of the first electrocardiographic signal and the rising point of the pulse wave signal. The pulse transit time calculation unit  37  can identify an amplitude value that the R-wave peak point can take on the basis of the feature amount parameter and thus can correctly detect the R-wave peak point of the first electrocardiographic signal. For example, when detecting the R-wave peak point, the S-wave peak point will not be detected erroneously. For example, as illustrated in  FIG. 8 , the pulse transit time calculation unit  37  detects the time of the R-wave peak point from the first electrocardiographic signal, detects the time of the rising point from the pulse wave signal, and calculates a time difference obtained by subtracting the time of the R-wave peak point from the time of the rising point as the pulse transit time. 
     The peak point corresponding to the R-wave is an example of a waveform feature point of an electrocardiographic signal. The waveform feature point of the electrocardiographic signal may be a peak point corresponding to the Q-wave or a peak point corresponding to the S-wave. Since the R-wave appears with a clearer peak than the Q-wave or the S-wave, the time of the R-wave peak point can be identified more accurately. Thus, preferably, the R-wave peak point is used as the waveform feature point of the electrocardiographic signal. Additionally, the rising point is an example of a waveform feature point in the pulse wave signal. The waveform feature point in the pulse wave signal may be the peak point. 
     The blood pressure value calculation unit  38  calculates a blood pressure value on the basis of the pulse transit time calculated by the pulse transit time calculation unit  37  and a blood pressure calculation formula. The blood pressure value calculation unit  38  uses Formula (1) above as a blood pressure calculation formula, for example. The blood pressure value calculation unit  38  stores the calculated blood pressure value in the first blood pressure value storage unit  610  in association with time information. 
     Note that the blood pressure calculation formula is not limited to Formula (1) above. The blood pressure calculation formula may be, for example, the following formula. 
       SBP= B   1 /PTT 2   +B   2 /PTT+ B   3 ×PTT+ B   4   (2)
 
     Here, B 1 , B 2 , B 3 , and B 4  are parameters. 
     The blood pressure measurement control unit  611  controls the pump drive circuit  406  to execute the blood pressure measurement using the oscillometric method. Specifically, the blood pressure measurement control unit  611  drives the pump  403  via the pump drive circuit  406 . In this way, supply of air to the pressing cuff  401  starts. The pressing cuff  401  is inflated, whereby the upper left arm of the user is compressed. The blood pressure measurement control unit  611  monitors the cuff pressure using the pressure sensor  402 . The blood pressure measurement control unit  611  calculates the blood pressure value using the oscillometric method on the basis of a pressure signal output from the pressure sensor  402  in the pressurizing process of supplying air to the pressing cuff  401 . Although the blood pressure value includes the systolic blood pressure (SBP) and the diastolic blood pressure (DBP), it is not limited thereto. The blood pressure measurement control unit  611  stores the calculated blood pressure value in the second blood pressure value storage unit  612  in association with time information. The blood pressure measurement control unit  611  can calculate a pulse rate at the same time as the blood pressure value. The blood pressure measurement control unit  611  stops the pump  403  via the pump drive circuit  406  when calculation of the blood pressure value is completed. Thus, air is exhausted from the pressing cuff  401  through the valve  404 . 
     The display control unit  613  controls the display unit  222 . For example, the display control unit  613  displays the blood pressure measurement result on the display unit  222  after the blood pressure measurement by the blood pressure measurement control unit  611  has been completed. 
     The instruction input unit  614  receives an instruction input from the user through the operation unit  221 . For example, when operation instructing execution of blood pressure measurement is performed, the instruction input unit  614  provides the blood pressure measurement control unit  611  with an initiation instruction of the blood pressure measurement. The blood pressure measurement control unit  611  starts the blood pressure measurement upon receiving an initiation instruction of blood pressure measurement from the instruction input unit  614 . 
     The calibration unit  615  calibrates the blood pressure calculation formula on the basis of the pulse transit time obtained by the pulse transit time calculation unit  37  and the blood pressure value obtained by the blood pressure measurement control unit  611 . The correlation between the pulse transit time and blood pressure values varies from individual to individual. Additionally, the correlation also varies depending on the state in which the blood pressure measurement device  10  is attached to the upper left arm of the user. For example, even within an identical user, the correlation varies between positioning of the blood pressure measurement device  10  closer to the shoulder and positioning of the blood pressure measurement device  10  closer to the elbow. To reflect such a variation in correlation, the blood pressure calculation formula is calibrated. The calibration of the blood pressure calculation formula is performed, for example, when the blood pressure measurement device  10  is attached to the user. The calibration unit  615  acquires a plurality of sets of measurement result for the pulse transit time and measurement result for the blood pressure to determine parameters A1 and A2, on the basis of the plurality of sets of the measurement result for the pulse transit time and the measurement result for the blood pressure. In order to determine the parameters A1 and A2, the calibration unit  615  uses a fitting method such as, for example, a least squares method or a maximum likelihood method. 
     Also, the present embodiment describes an example in which all the functions of the blood pressure measurement device  10  are realized by a general-purpose processor. However, some or all of the functions may be implemented by one or more dedicated processors. 
     Operation Example 
     Selection of Internal Electrode Pair Used for Acquiring First Electrocardiographic Signal 
     When the blood pressure measurement device  10  is attached to the user, first, a process of selecting an optimal internal electrode pair to acquire the first electrocardiographic signal is executed. In this process, the control unit  501  operates as the first electrocardiographic signal measurement control unit  601 . In this example, it is assumed that the internal electrode group  31  includes four internal electrodes  312 , and the internal electrodes are designated as the internal electrodes  312 - 1 ,  312 - 2 ,  312 - 3 ,  312 - 4  to distinguish between these four internal electrodes  312 . The control unit  501  provides a switch signal for selecting the internal electrodes  312 - 1 ,  312 - 2  to the switch circuit  333  and acquires the first electrocardiographic signal using the pair of internal electrodes  312 - 1 ,  312 - 2 . Subsequently, the control unit  501  provides a switch signal for selecting the internal electrodes  312 - 1 ,  312 - 3  to the switch circuit  333  and acquires the first electrocardiographic signal using the pair of internal electrodes  312 - 1 ,  312 - 3 . Similarly, the control unit  501  acquires the first electrocardiographic signal using the pair of internal electrodes  312 - 1 ,  312 - 4 , the pair of internal electrodes  312 - 2 ,  312 - 3 , the pair of internal electrodes  312 - 2 ,  312 - 4 , and the pair of internal electrodes  312 - 3 ,  312 - 4 . The control unit  501  determines an internal electrode pair that provides the first electrocardiographic signal having the greatest R-wave amplitude as an optimal internal electrode pair. 
     Calculation of Feature Amount Parameter 
       FIG. 9  illustrates an operation flow when the blood pressure measurement device  10  calculates the feature amount parameter. The control unit  501  starts calculating the feature amount parameter immediately after the above-described selection process is completed, for example. Moreover, the control unit  501  may calculate the feature amount parameter before starting the blood pressure measurement based on the pulse transit time in response to receiving an operation signal from the operation unit  221  indicating that the user has instructed to start blood pressure measurement based on the pulse transit time. That is, the process illustrated in  FIG. 9  may be executed between steps S 21  and S 22  of  FIG. 10 . 
     In step S 11  of  FIG. 9 , the control unit  501  instructs the user to touch the external electrode  32  with the right hand. Here, the blood pressure measurement device  10  is attached to the upper left arm of the user. For example, the control unit  501  displays a message “Please touch the electrodes on the body with the index finger of the right hand” on the display unit  222 . The message may be output as sound through a speaker. 
     In step S 12 , the control unit  501  determines whether the user is touching the external electrode  32 . The determination of whether the user is touching the external electrode  32  can be made, for example, on the basis of the output of the AFE  345 . Upon detecting that the user is touching the external electrode  32 , the control unit  501  proceeds to step S 13 . 
     In step S 13 , the control unit  501  acquires the first electrocardiographic signal and the second electrocardiographic signal at the same time. For example, the control unit  501  operates as the first electrocardiographic signal measurement control unit  601  and acquires the first electrocardiographic signal using the optimal internal electrode pair. Furthermore, the control unit  501  operates as the second electrocardiographic signal measurement control unit  603  and acquires the second electrocardiographic signal using the internal electrode  312  and the external electrode  32 . 
     In step S 14 , the control unit  501  operates as the feature amount parameter calculation unit  35  and calculates the feature amount parameter for the R-wave peak point of the first electrocardiographic signal on the basis of the second electrocardiographic signal. For example, the control unit  501  detects the R-wave peak point of the second electrocardiographic signal, determines a time range on the basis of the detected R-wave peak point, detects a peak point in the first electrocardiographic signal in the determined time range, and calculates an amplitude value of the detected peak point as a feature amount parameter. 
     Calibration of Blood Pressure Calculation Formula Used in Blood Pressure Measurement Based on Pulse Transit Time 
     Subsequently, calibration of the blood pressure calculation formula is executed. Assuming that N is the number of the parameters included in the blood pressure calculation formula, N or more sets of a measurement value for the pulse transit time and a measurement value for the blood pressure are required. The blood pressure calculation Formula (1) described above includes two parameters A1 and A2. In this case, for example, the control unit  501  acquires a set of measurement value for the pulse transit time and measurement value for the blood pressure when the user is at rest. The control unit  501  acquires the set of the measurement value for the pulse transit time and the measurement value for the blood pressure after varying the user&#39;s blood pressure, such as by causing the user to exercise. Thus, two sets of the measurement value for the pulse transit time and the measurement value for the blood pressure are acquired. The control unit  501  operates as the calibration unit  615  and determines the parameters A1 and A2 on the basis of the acquired two sets of the measurement value for the pulse transit time and the measurement value for the blood pressure. After the calibration of the blood pressure calculation formula is completed, blood pressure measurement based on the pulse transit time can be executed. 
     Blood Pressure Measurement Based on Pulse Transit Time 
       FIG. 10  illustrates an operation flow when the blood pressure measurement device  10  performs blood pressure measurement based on the pulse transit time. 
     In step S 21  in  FIG. 10 , the control unit  501  starts blood pressure measurement based on the pulse transit time. For example, the control unit  501  starts blood pressure measurement in response to receiving an operation signal from the operation unit  221  indicating that the user has instructed to start the blood pressure measurement based on the pulse transit time. Additionally, the control unit  501  may start the blood pressure measurement based on the pulse transit time in response to the completion of calibration of the blood pressure calculation formula. 
     In step S 22 , the control unit  501  operates as the first electrocardiographic signal measurement control unit  601  and acquires the first electrocardiographic signal using the two optimal internal electrodes  312 . In step S 23 , the control unit  501  operates as the pulse wave measurement control unit  606  and acquires the pulse wave signal using the pulse wave sensor. The processing of step S 21  and the processing of step S 22  are executed in parallel. 
     In step S 24 , the control unit  501  operates as the pulse transit time calculation unit  37  and calculates the pulse transit time on the basis of the first electrocardiographic signal acquired in step S 22 , the pulse wave signal acquired in step S 23 , and the feature amount parameter obtained by the processing illustrated in  FIG. 9 . For example, the control unit  501  detects the R-wave peak point of the first electrocardiographic signal using the feature amount parameter and calculates a time difference between the detected R-wave peak point and the rising point of the pulse wave signal as the pulse transit time. 
     In step S 25 , the control unit  501  operates as the blood pressure value calculation unit  38  and calculates a blood pressure value from the pulse transit time calculated in step S 24  using the blood pressure calculation Formula (1) described above. The control unit  501  stores the calculated blood pressure value in the storage unit  505  in association with time information. 
     In step S 26 , the control unit  501  determines whether an operation signal indicating that the user has instructed to end the blood pressure measurement based on the pulse transit time has been received from the operation unit  221 . The processes of steps S 22  to S 25  are repeated until the control unit  501  receives the operation signal. Thus, the blood pressure value for each beat is recorded. When the control unit  501  receives the operation signal, the control unit  501  ends the blood pressure measurement based on the pulse transit time. 
     With the blood pressure measurement based on the pulse transit time, the blood pressure can be continuously measured over an extended period of time with a reduced physical burden on the user. 
     Blood Pressure Measurement Using Oscillometric Method 
       FIG. 11  illustrates an operation flow when the blood pressure measurement device  10  performs blood pressure measurement using the oscillometric method. In the blood pressure measurement using the oscillometric method, the pressing cuff  401  is gradually pressurized and then depressurized. In such a pressurization or depressurization process, the pulse transit time fails to be measured correctly. Thus, during the execution of the blood pressure measurement using the oscillometric method, the blood pressure measurement based on the pulse transit time illustrated in  FIG. 10  may be temporarily stopped. 
     In step S 31  of  FIG. 11 , the control unit  501  starts blood pressure measurement using the oscillometric method. For example, the control unit  501  starts blood pressure measurement in response to receiving an operation signal from the operation unit  221  indicating that the user has instructed to execute blood pressure measurement using the oscillometric method. 
     In step S 32 , the control unit  501  operates as the blood pressure measurement control unit  611  to perform initialization for the blood pressure measurement. For example, the control unit  501  initializes a processing memory area. Further, the control unit  501  stops the pump  403  via the pump drive circuit  406 . Along with this, the valve  404  is opened, and the air in the pressing cuff  401  is exhausted. The control unit  501  sets an output value at the present time of the pressure sensor  402  as a reference value. 
     In step S 33 , the control unit  501  operates as the blood pressure measurement control unit  611  to perform control of pressurizing the pressing cuff  401 . For example, the control unit  501  drives the pump  403  via the pump drive circuit  406 . Along with this, the valve  404  is closed and air is supplied to the pressing cuff  401 . As a result, the pressing cuff  401  is inflated, and a cuff pressure Pc gradually increases as illustrated in  FIG. 12 . The control unit  501  monitors the cuff pressure Pc using the pressure sensor  402  and acquires a pulse wave signal Pm representing a fluctuation component of an arterial volume. 
     In step S 34 , the control unit  501  operates as the blood pressure measurement control unit  611  and attempts to calculate the blood pressure value (including the SBP and the DBP) on the basis of the pulse wave signal Pm acquired at that point in time. In a case where the blood pressure value fails to be calculated due to lack of data at this point in time (No in step S 35 ), the processing of steps S 33  and S 34  is repeated as long as the cuff pressure Pc does not reach an upper limit pressure. The upper limit pressure is predetermined from the viewpoint of safety. The upper limit pressure is set to 300 mmHg, for example. 
     In a case where the blood pressure value can be calculated (Yes in step S 35 ), the processing proceeds to step S 36 . In step S 36 , the control unit  501  operates as the blood pressure measurement control unit  611  and stops the pump  403  via the pump drive circuit  406 . Along with this, the valve  404  is opened, and the air in the pressing cuff  401  is exhausted. 
     In step S 37 , the control unit  501  displays blood pressure measurement results on the display unit  222  and records the blood pressure measurement results in the storage unit  505 . 
     Note that the processing procedure illustrated in  FIG. 9, 10 , or  11  is an example, and the processing order or the content of each processing can be changed as appropriate. For example, in the blood pressure measurement using the oscillometric method illustrated in  FIG. 11 , the calculation of blood pressure values may be executed in the depressurization process in which air is discharged from the pressing cuff  401 . 
     Effects 
     As described above, in the blood pressure measurement device  10  according to the present embodiment, the internal electrode group  31 , the external electrode  32 , and the impedance measurement unit  361  are provided on the belt  21 . Thus, by simply winding the belt  21  around the upper left arm, the internal electrode group  31 , the external electrode  32 , and the impedance measurement unit  361  can be attached to the user. Thus, the blood pressure measurement device  10  can be easily attached to the user. 
     The blood pressure measurement device  10  calculates the feature amount parameter related to the waveform feature points of the first electrocardiographic signal acquired using the internal electrode group  31  on the basis of the second electrocardiographic signal acquired using the external electrode  32 . When measuring the pulse transit time, the blood pressure measurement device  10  acquires the first electrocardiographic signal and the pulse wave signal, detects the R-wave peak point of the first electrocardiographic signal using the feature amount parameter, and calculates a time difference between the detected R-wave peak point and the rising point of the pulse wave signal as the pulse transit time. The use of the feature amount parameter enables the R-wave peak point of the first electrocardiographic signal to be detected correctly. As a result, the pulse transit time can be measured more accurately. Furthermore, blood pressure can be more accurately measured in blood pressure measurement based on the pulse transit time. 
     One internal electrode of the internal electrode group  31  is used for acquiring the second electrocardiographic signal. As a result, there is no need to provide a dedicated electrode for acquiring the second electrocardiographic signal, which makes it possible to reduce the manufacturing cost. 
     The first electrocardiographic signal is acquired using two first electrodes that provide the first electrocardiographic signal having the greatest R-wave amplitude, selected from the internal electrode group  31 . As a result, it is possible to identify the time of the R-wave peak point of the first electrocardiographic signal and to measure the pulse transit time more accurately. 
     A peak point corresponding to the R-wave is used as the waveform feature point of the electrocardiographic signal. Since the R-wave appears more clearly than the Q-wave or the S-wave, the time of the R-wave peak point can be identified more accurately. As a result, the feature amount parameter can be calculated with high accuracy. 
     The blood pressure calculation formula used in the first blood pressure measurement unit  30  needs to be calibrated on the basis of the blood pressure value acquired by a measurement system different from that of the first blood pressure measurement unit  30 . In the present embodiment, the second blood pressure measurement unit  40  is integrated with the first blood pressure measurement unit  30 , and the blood pressure calculation formula is calibrated on the basis of the blood pressure value obtained by the second blood pressure measurement unit  40 . As a result, the blood pressure calculation formula can be calibrated by the blood pressure measurement device  10  alone. For this reason, the blood pressure calculation formula can be calibrated easily. 
     Since the blood pressure measurement based on the pulse transit time and the blood pressure measurement using the oscillometric method can be performed by one device, the user&#39;s convenience is improved. 
     Modified Example 
     The present invention is not limited to the above embodiment. 
     In the embodiment described above, one of the internal electrodes is used for acquiring the first electrocardiographic signal and the second electrocardiographic signal. Instead of this, a dedicated internal electrode may be provided on the inner circumferential surface of the belt unit  20  to measure the second electrocardiographic signal. 
     In the embodiment described above, the pulse wave sensor employs an impedance method in which a change in impedance resulting from a change in volume of the artery is detected. Also, the pulse wave sensor may adopt another measurement method such as a photoelectric method, a piezoelectric method, or a radio wave method. In an embodiment employing the photoelectric method, the pulse wave sensor includes: a light emitting element that radiates light toward the artery passing through a target measurement site; and a photodetector for detecting reflected light or transmitted light of the light, and the pulse wave sensor detects a change in light intensity resulting from a change in volume of the artery. In an embodiment employing the piezoelectric method, the pulse wave sensor includes a piezoelectric element provided on the belt to be in contact with the target measurement site and detects a change in pressure resulting from a change in volume of the artery. In an embodiment employing a radio wave method, the pulse wave sensor includes: a transmission element that transmits a radio wave toward the artery passing through a target measurement site and a receiving element that receives a reflection wave of the radio wave, and the pulse wave sensor detects a phase shift between the transmission wave and the reflection wave associated with the change in volume of the artery. 
     The blood pressure measurement device  10  may further include a pressing cuff, a pump that supplies air to the pressing cuff, a pump drive circuit that drives the pump, and a pressure sensor that detects pressure in the pressing cuff in order to adjust the contact state between the internal electrode  312  and the upper left arm. This pressing cuff is provided at the central end portion  218 A of the belt  21 . In this case, the pressing cuff  401  is provided in the intermediate portion  218 B of the belt  21 , for example. 
     The blood pressure measurement device  10  may further include a pressing cuff, a pump that supplies air to the pressing cuff, a pump drive circuit that drives the pump, and a pressure sensor for detecting the pressure in the pressing cuff in order to adjust the contact state between the sensor unit  362  of the impedance measurement unit  361  and the upper left arm. This pressing cuff is provided at the peripheral end portion  218 C of the belt  21 . In this case, the pressing cuff  401  is provided in the intermediate portion  218 B of the belt  21 , for example. 
     The external electrode  32  may be provided in a start button that initiates the blood pressure measurement (blood pressure measurement by the second blood pressure measurement unit  40 ) using the oscillometric method, included in the operation unit  221 . For example, the start button is formed of a conductive material and the start button serves as the external electrode  32 . When the user depresses the start button, the blood pressure measurement using the oscillometric method starts. At this time, since the user is in touch with the external electrode  32 , it is possible to acquire the electrocardiographic signal by the first lead, and it is possible to calculate the feature amount parameter. Thus, the feature amount parameter can be calculated at the same time as the blood pressure measurement using the oscillometric method is performed. In addition, the blood pressure calculation formula may be calibrated using the blood pressure values obtained by performing the blood pressure measurement using the oscillometric method. That is, the feature amount parameter can be calculated at the same time as the blood pressure calculation formula is calibrated. 
     The blood pressure measurement device  10  may not include the second blood pressure measurement unit  40 . In an embodiment in which the blood pressure measurement device  10  does not include the second blood pressure measurement unit  40 , a blood pressure value obtained by measurement with another blood pressure monitor needs to be input to the blood pressure measurement device  10  for calibration of the blood pressure calculation formula. 
     A portion of the blood pressure measurement device involved in the measurement of the pulse transit time may be implemented as a single device. In an embodiment, a pulse transit time measurement device including the belt unit  20 , the internal electrode group  31 , the external electrode  32 , the first electrocardiographic signal acquisition unit  33 , the second electrocardiographic signal acquisition unit,  34 , the feature amount parameter calculation unit  35 , the pulse wave signal acquisition unit  36 , and the pulse transit time calculation unit  37  is provided. For example, the pulse transit time measurement device may transmit the measurement result of the pulse transit time to an external device, and the external device may calculate a blood pressure value from the measurement result of the pulse transit time. 
     The target measurement site is not limited to the upper arm and may be another site such as the wrist, thigh, or ankle in which the pulse wave signal can be acquired. 
     The present invention is not limited to the embodiment described above as is and can be embodied by modifying the constituent elements within a range not departing from the gist of the invention in an implementation stage. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment described above. For example, some constituent elements may be omitted from the entire constituent elements illustrated in the embodiment. Furthermore, the constituent elements of different embodiments may be combined appropriately. 
     REFERENCE SIGNS LIST 
       10  Blood pressure measurement device 
       20  Belt unit 
       21  Belt 
       22  Body 
       210 A Inner cloth 
       210 B Outer cloth 
       211  Outer circumferential surface 
       212  Inner circumferential surface 
       213  Loop surface 
       214  Hook surface 
       221  Operation unit 
       222  Display unit 
       30  First blood pressure measurement unit 
       31  Internal electrode group 
       32  External electrode 
       33  First electrocardiographic signal acquisition unit 
       34  Second electrocardiographic signal acquisition unit 
       35  Feature amount parameter calculation unit 
       36  Pulse wave signal acquisition unit 
       37  Pulse transit time calculation unit 
       38  Blood pressure value calculation unit 
       312  Internal electrode 
       333  Switch circuit 
       334  Subtraction circuit 
       335  AFE 
       344  Subtraction circuit 
       345  AFE 
       361  Impedance measurement unit 
       362  Sensor unit 
       362 A to  362 D Electrode 
       363  Energization and voltage detection circuit 
       40  Second blood pressure measurement unit 
       401  Pressing cuff 
       402  Pressure sensor 
       403  Pump 
       404  Valve 
       405  Oscillation circuit 
       406  Pump drive circuit 
       501  Control unit 
       502  CPU 
       503  RAM 
       504  ROM 
       505  Storage unit 
       506  Battery 
       507  Communication unit 
       601  First electrocardiographic signal measurement control unit 
       602  First electrocardiographic signal storage unit 
       603  Second electrocardiographic signal measurement control unit 
       604  Second electrocardiographic signal storage unit 
       606  Pulse wave measurement control unit 
       607  Pulse wave signal storage unit 
       610  First blood pressure value storage unit 
       611  Blood pressure measurement control unit 
       612  Second blood pressure value storage unit 
       613  Display control unit 
       614  Instruction input unit 
       615  Calibration unit 
       70  Upper left arm 
       71  Humerus 
       72  Brachial artery