Patent Publication Number: US-2011077534-A1

Title: Blood pressure information measurement device capable of obtaining index for determining degree of arteriosclerosis

Description:
TECHNICAL FIELD 
     This invention relates to a blood pressure information measurement device and an index acquisition method. More particularly, the invention relates to an apparatus for measuring blood pressure information by using a cuff including a fluid bag and a method for obtaining an index for determining a degree of arteriosclerosis from the blood pressure information. 
     BACKGROUND ART 
     Measuring blood pressure information such as blood pressure and pulse wave is useful for determining a degree of arteriosclerosis. 
     Conventionally, for example, Japanese Unexamined Patent Publication No. 2000-316821 (hereinafter referred to as Patent Document 1) discloses an apparatus for determining the degree of arteriosclerosis by checking a velocity at which a pulse wave ejected from a heart (hereinafter referred to as PWV: pulse wave velocity). The pulse wave transmission velocity increases as the degree of arteriosclerosis advances. Therefore, the PWV serves as an index for determining the degree of arteriosclerosis. The PWV is calculated by attaching cuffs and the like for measuring pulse waves at at least two or more positions such as an upper arm and a lower limb, measuring the pulse waves at a time, and calculating the PWV based on a difference of times at which the pulse waves emerge at respective positions and a length of an artery between the two points at which the cuffs and the like for measuring the pulse waves are attached. The PWV differs according to measurement positions. Typical examples of PWVs include baPWV obtained from measuring positions of an upper arm and an ankle and cfPWV obtained from measuring positions of a carotid artery and a femoral artery. 
     As a technique for determining the degree of arteriosclerosis from a pulse wave at an upper arm, Japanese Unexamined Patent Publication No. 2007-44362 (hereinafter referred to as Patent Document 2) discloses a technique having a double structure including a blood pressure measuring cuff and a pulse wave measuring cuff. 
     Japanese Unexamined Patent Publication No. 2004-113593 (hereinafter referred to as Patent Document 3) discloses a technique for separating an ejection wave ejected from a heart and a reflection wave reflected by a stiffened portion in an artery and an iliac artery branching portion, and determining the degree of arteriosclerosis based on amplitude differences, amplitude ratios, and emerging time differences thereof. 
     Patent Document 1: Japanese Unexamined Patent Publication No. 2000-316821 
     Patent Document 2: Japanese Unexamined Patent Publication No. 2007-44362 
     Patent Document  3 : Japanese Unexamined Patent Publication No. 2004-113593 
     SUMMARY OF THE INVENTION 
     However, in order to measure a PWV using the apparatus disclosed in Patent Document 1, it is necessary to attach the cuffs and the like to at least two positions such as an upper arm and a lower limb as described above. Therefore, it is difficult to easily measure a PWV at home even when the apparatus disclosed in Patent Document 1 is used. 
     In contrast, Patent Document 2 discloses a technique for determining a degree of arteriosclerosis from a pulse wave at an upper arm. The apparatus disclosed in Patent Document 2 has the double structure including the blood pressure measuring cuff and the pulse wave measuring cuff. However, with the pulse wave measuring cuff alone, a reflection from a periphery is overlapped. Accordingly, a reflection wave may not be correctly separated. Therefore, it is difficult to determine the degree of arteriosclerosis with high accuracy. 
     Further, depending on a subject, it may be difficult to find a feature point for determining the degree of arteriosclerosis based on a pulse wave obtained by avascularizing a peripheral side which is measured by the apparatus disclosed in Patent Document 3. 
     One or more embodiments of the present invention provides a blood pressure information measurement device and an index acquisition method capable of obtaining an index for accurately determining the degree of arteriosclerosis from measured blood pressure information. 
     According to an aspect of the present invention, a blood pressure information measurement device includes a first fluid bag and a second fluid bag, a first sensor and a second sensor for respectively measuring internal pressures of the first fluid bag and the second fluid bag, a first adjusting unit for adjusting the internal pressure of the second fluid bag, and a control unit for controlling calculation for calculating an index for determining a degree of arteriosclerosis and adjustment of the first adjusting unit, wherein the control unit performs calculation for detecting a first pulse wave of a measurement portion based on a change of the internal pressure of the first fluid bag in a first state in which the first fluid bag is wrapped around the measurement portion, the second fluid bag is wrapped at a peripheral side with respect to the first fluid bag, and the second fluid bag presses the peripheral side with respect to the measurement portion around which the first fluid bag is wrapped with an internal pressure higher than a systolic blood pressure, calculation for detecting a second pulse wave based on a change of the internal pressure of the first fluid bag in a second state in which the first fluid bag is wrapped around the measurement portion, the second fluid bag is wrapped at a peripheral side with respect to the first fluid bag, and the second fluid bag presses the peripheral side with respect to the measurement portion around which the first fluid bag is wrapped with an internal pressure lower than at least the systolic blood pressure, and calculation for calculating the index using at least one of a first feature point extracted from the first pulse wave and a second feature point extracted from the second pulse wave. 
     According to another aspect of the present invention, a blood pressure information measurement device includes a first fluid bag and a second fluid bag, a first sensor and a second sensor for respectively measuring internal pressures of the first fluid bag and the second fluid bag, a first adjusting unit for adjusting the internal pressure of the second fluid bag, and a control unit for controlling calculation for calculating an index for determining a degree of arteriosclerosis and adjustment of the first adjusting unit, wherein the control unit performs calculation for detecting a pulse wave of a measurement portion based on a change of the internal pressure of the first fluid bag in which the first fluid bag is wrapped around the measurement portion, the second fluid bag is wrapped at a peripheral side with respect to the first fluid bag, and the second fluid bag presses the peripheral side with respect to the measurement portion around which the first fluid bag is wrapped, calculation for comparing a systolic blood pressure with the internal pressure of the second fluid bag when the pulse wave is detected, and determining whether the detected pulse wave is a first pulse wave detected in a first state in which the peripheral side of the measurement portion is pressed while the internal pressure of the second fluid bag is higher than the systolic blood pressure or a second pulse wave detected in a second state in which the peripheral side of the measurement portion is pressed while the internal pressure of the second fluid bag is lower than at least the systolic blood pressure, and calculation for calculating the index using at least one of a first feature point extracted from the first pulse wave and a second feature point extracted from the second pulse wave. 
     According to still another aspect of the present invention, an index acquisition method for obtaining an index for determining a degree of arteriosclerosis from a pulse wave measured by a blood pressure information measurement device, wherein the blood pressure information measurement device includes a first fluid bag and a second fluid bag, a first sensor and a second sensor for respectively measuring internal pressures of the first fluid bag and the second fluid bag, and a first adjusting unit for adjusting the internal pressure of the second fluid bag, and the index acquisition method includes the steps of controlling the internal pressure of the second fluid bag such that the internal pressure of the second fluid bag attains a pressure higher than a systolic blood pressure, detecting a first pulse wave of a measurement portion based on a change of the internal pressure of the first fluid bag in a first state in which the first fluid bag is wrapped around the measurement portion, the second fluid bag is wrapped at a peripheral side with respect to the first fluid bag, and the second fluid bag presses the peripheral side with respect to the measurement portion around which the first fluid bag is wrapped with an internal pressure higher than the systolic blood pressure, calculating the index from the first pulse wave, performing control to reduce the internal pressure of the second fluid bag in a case where the index is not calculated from the first pulse wave, detecting a second pulse wave of the measurement portion based on a change of the internal pressure of the first fluid bag in a state in which the first fluid bag is wrapped around the measurement portion, the second fluid bag is wrapped at a peripheral side with respect to the first fluid bag, and the second fluid bag presses the peripheral side with respect to the measurement portion with a pressure lower than at least the systolic blood pressure, and calculating the index from the second pulse wave. 
     By using the blood pressure information measurement device according to one or more embodiments of the present invention, it is possible to obtain an index for accurately determining the degree of arteriosclerosis based on the measured blood pressure information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating a specific example of external appearance of a measurement device according to a first embodiment. 
         FIG. 2A  is a diagram illustrating a specific example of a measuring posture when the measurement device according to the first embodiment is used to measure blood pressure information. 
         FIG. 2B  is a schematic cross sectional view illustrating a specific example of a configuration of an arm band according to the first embodiment. 
         FIG. 3  is a diagram illustrating a relationship between a pulse wave waveform and an index for determining a degree of arteriosclerosis. 
         FIG. 4  is a diagram illustrating a specific example of correlation between a PWV and a time difference Tr between an ejection wave and a reflection wave. 
         FIG. 5  is a diagram representing a pulse wave measured when a peripheral side is avascularized and a pulse wave measured when the peripheral side is not avascularized. 
         FIG. 6  is a diagram illustrating functional blocks of the measurement device according to the first embodiment. 
         FIG. 7  is a flowchart illustrating a first specific example of a measuring operation performed by the measurement device according to the first embodiment. 
         FIG. 8  is a diagram illustrating pressure change within each air bladder during the measuring operation performed by the measurement device according to the first embodiment. 
         FIG. 9  is a flowchart illustrating a second specific example of the measuring operation performed by the measurement device according to the first embodiment. 
         FIG. 10  is a flowchart illustrating a third specific example of the measuring operation performed by the measurement device according to the first embodiment. 
         FIG. 11  is a flowchart illustrating a fourth specific example of the measuring operation performed by the measurement device according to the first embodiment. 
         FIG. 12  is a diagram illustrating functional blocks of the measurement device according to a second embodiment. 
         FIG. 13  is a flowchart illustrating a first specific example of a measuring operation performed by the measurement device according to the second embodiment. 
         FIG. 14  is a diagram illustrating pressure change within each air bladder during the measuring operation performed by the measurement device according to the second embodiment. 
         FIG. 15  is a flowchart illustrating a second specific example of the measuring operation performed by the measurement device according to the second embodiment. 
         FIG. 16  is a flowchart illustrating a modification of the second specific example of the measuring operation performed by the measurement device according to the second embodiment. 
         FIG. 17  is a diagram illustrating pressure change within each air bladder during the measuring operation performed by the measurement device according to the second embodiment. 
         FIG. 18  is a flowchart illustrating a third specific example of the measuring operation performed by the measurement device according to the second embodiment. 
         FIG. 19A  is a diagram illustrating a specific example of a measuring posture when a measurement device according to a third embodiment is used to measure blood pressure information. 
         FIG. 19B  is a schematic cross sectional view illustrating a specific example of a configuration of an arm band according to the third embodiment. 
         FIG. 20  is a diagram illustrating functional blocks of the measurement device according to the third embodiment. 
         FIG. 21  is a flowchart illustrating a first specific example of a measuring operation performed by the measurement device according to the third embodiment. 
         FIG. 22  is a diagram illustrating pressure change within each air bladder during the measuring operation performed by the measurement device according to the third embodiment. 
         FIG. 23  is a flowchart illustrating a second specific example of the measuring operation performed by the measurement device according to the third embodiment. 
         FIG. 24  is a flowchart illustrating a third specific example of the measuring operation performed by the measurement device according to the third embodiment. 
         FIG. 25  is a flowchart illustrating a fourth specific example of the measuring operation performed by the measurement device according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be hereinafter described with reference to the drawings. In embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid obscuring the invention. In the below description, the same reference numerals are attached to the same components and constituent elements. Names and functions thereof are also the same. 
     It should be noted that “blood pressure information” means information related to blood pressure obtained by measuring a living body. More specifically, “blood pressure information” includes a blood pressure value, pulse wave waveform, heart rate, and the like. 
     First Embodiment   
     Referring to  FIG. 1 , a blood pressure information measurement device  1 A according to a first embodiment (hereinafter abbreviated as measurement device) includes a base body  2  and an arm band  9  connected to the base body  2  and attached to an upper arm, i.e., a measurement portion. The base body  2  and the arm band  9  are connected via an air tube  10 . On a front surface of the base body  2 , a display unit  4  and an operation unit  3  are arranged. The display unit  4  displays various kinds of information including a measurement result. The operation unit  3  is operated to give various kinds of instructions to the measurement device  1 A. The operation unit  3  includes a switch  31  operated to turn on and off a power supply and a switch  32  operated to give an instruction to start a measuring operation. 
     When a pulse wave is measured using the measurement device  1 A, an arm band  9  is wrapped around an upper arm  100 , i.e., the measurement portion, as shown in  FIG. 2A . When the switch  32  is pressed down in this state, blood pressure information is measured. 
     Referring to  FIG. 2A , the arm band  9  includes an air bladder, i.e., a fluid bag for pressing a living body. The air bladder includes an air bladder  13 A, i.e., a fluid bag, used for measuring blood pressure as blood pressure information, and an air bladder  13 B, i.e., a fluid bag, used for measuring a pulse wave as blood pressure information. For example, as shown in  FIG. 2B , the size of the air bladder  13 B is about 20 mm×200 mm. According to one or more embodiments of the present invention, an air capacity of the air bladder  13 B is ⅕ or less of an air capacity of the air bladder  13 A as shown in  FIG. 2B . 
     The measurement device  1 A obtains an index for determining the degree of arteriosclerosis based on a pulse wave waveform, i.e., blood pressure information, obtained from one measurement portion. Examples of indexes for determining the degree of arteriosclerosis include Tpp (which is also represented as ΔTp), Tr (Traveling time to reflected wave), and AI (Augmentation Index). Tpp is an index represented by a time interval between an emerging time of a peak (maximum point) of an ejection wave, i.e., a traveling wave, and an emerging time of a peak (maximum point) of a reflection wave. In a waveform of  FIG. 3 , Tpp is represented by a time interval between a point A and a point B. Tr is an index represented by a time interval between an emerging time of an ejection wave and an emerging time of a reflection wave reflected by and returned from a branching point of an iliac artery when a traveling wave is reflected by the branching point. In a waveform of  FIG. 3 , Tr is represented by a time interval between a rising point of the ejection wave and the point A. As shown in  FIG. 4 , the index Tr and a PWV are related with each other. Pages 10 to 19 of “Hypertension 1992 Jul; 20 (1):” by London et al. (issued on Jul. 20, 1992) describe as follows. When a measurement portion is an upper arm, and a reflection wave is a reflection wave from an ankle, i.e., a periphery, a correlation between an index Tr and baPWV, i.e., PWV in a case where the measurement portions are the upper arm and the ankle, provides individual parameters such as height and sex. Therefore, the emerging time difference Tr can be adopted as an index for determining the degree of arteriosclerosis. This is also applicable to Tpp. AI is an index based on a feature quantity reflecting the intensity of reflection of a pulse wave mainly corresponding to arteriosclerosis. The intensity of reflection of a pulse wave is an index representing a reflection phenomenon of the pulse wave and representing the degree of ease of blood pumping and the degree of ease of receiving a blood flow volume. AI is an index represented by a ratio of a reflection wave at the maximum point with respect to an amplitude of an ejection wave, i.e., traveling wave, at the maximum point. In the waveform of  FIG. 3 , AI is represented as a ratio of an amplitude P 2  at the point B with respect to an amplitude P 1  at the point A. 
     In order to obtain these indexes from the measured pulse wave, it is necessary to extract a peak of the ejection wave (point A of  FIG. 3 ) and a peak of the reflection wave (point B of  FIG. 3 ) from the measured pulse wave. The points A and B in  FIG. 3  are inflection points of the pulse wave waveform, and the points A and B will be referred to as “feature points”. The points A and B, i.e., the inflection points, are obtained by performing multi-order differentiation of the measured pulse wave waveform (for example, fourth-order differentiation). 
     In order to obtain the above-described feature points, i.e., the inflection points, from the pulse wave waveform obtained through measurement, it is necessary to obtain a highly accurate pulse wave waveform. Accordingly, in the first embodiment, the air bladder for pressing a living body has a double structure including two air bladders  13 A,  13 B arranged side by side in a direction of an artery of a measurement portion. When the arm band  9  is wrapped around the upper arm  100 , the air bladder  13 A is arranged at a peripheral side of the upper arm  100  (a side far from the heart). When the arm band  9  is wrapped around the upper arm  100 , the air bladder  13 B is arranged at a central side (a side closer to the heart). After the upper arm  100  is pressed and fixed, these air bladders  13 A,  13 B inflate and deflate. When the air bladder  13 A inflates, the air bladder  13 A is pressed onto the upper arm  100 . A change of an artery pressure is detected together with an internal pressure of the air bladder  13 A. Further, when the air bladder  13 A inflates, the peripheral side of the artery is avascularized. When the air bladder  13 B inflates in this state, an artery pressure pulse wave generated within the artery is detected in the avascularized state. That is, the pulse wave can be measured while the peripheral side is avascularized. Therefore, the pulse wave can be measured with high accuracy. As a result, feature points can be accurately obtained from the measured pulse wave waveform, and a highly accurate index can be obtained. 
     However, depending on a subject, it may be difficult to find feature points from a pulse wave detected by avascularizing the peripheral side. That is, when a pulse wave as shown in  FIG. 5  is detected, a peak point A 1  of an ejection wave is extracted from a “pulse wave  1 ” measured in the avascularized state. In contrast, it is difficult to find a peak point B 1  of a reflection wave, and the peak point B 1  is not extracted. However, a reflection wave from the peripheral side in a “pulse wave  2 ” measured in a non-avascularized state affects more greatly than in the avascularized state. Therefore, in the “pulse wave  2 ” measured in a non-avascularized state, a peak point A 2  of the ejection wave as well as a peak point B 2  of the reflection wave are extracted. When these pulse waves are overlaid as shown in  FIG. 5 , the emerging time of the point A 1  and the emerging time of the point A 2  are considered to be the same for the same subject. Likewise, the emerging time of the point B 1  and the emerging time of the point B 2  are considered be substantially the same. 
     Referring to  FIG. 6 , the measurement device  1 A includes an air system  20 A connected to the air bladder  13 A via the air tube  10 , an air system  20 B connected to the air bladder  13 B via the air tube  10 , and a CPU (Central Processing Unit)  40 . 
     The air system  20 A includes an air pump  21 A, an air valve  22 A, and a pressure sensor  23 A. The air system  20 B includes an air valve  22 B and a pressure sensor  23 B. 
     The air pump  21 A is driven by a drive circuit  26 A receiving an instruction from the CPU  40 , and pumps compressed gas to the air bladder  13 A. Thereby, the air bladder  13 A is pressurized. 
     The open/close states of the air valves  22 A,  22 B are controlled by the drive circuits  27 A,  27 B receiving instructions from the CPU  40 . The pressures in the air bladders  13 A,  13 B are controlled by controlling the open/close states of the air valves  22 A,  22 B. 
     The pressure sensors  23 A,  23 B respectively detect the pressures in the air bladders  13 A,  13 B, and output signals to amplifiers  28 A,  28 B according to the detected values thereof. The amplifiers  28 A,  28 B respectively amplifies the signals outputted from the pressure sensors  23 A,  23 B, and outputs the amplified signals to ND converters  29 A,  29 B. The A/D converters  29 A,  29 B respectively digitalize analog signals outputted from the amplifiers  28 A,  28 B, and output the digital signals to the CPU  40 . 
     The air bladder  13 A and the air bladder  13 B are connected by a two-port valve  51 . The two-port valve  51  is connected to a drive circuit  53 , which controls opening and closing of the valve. The drive circuit  53  is connected to the CPU  40 , and controls opening and closing of the above two valves of the two-port valve  51  according to a control signal given by the CPU  40 . 
     The CPU  40  controls the air systems  20 A,  20 B and the drive circuit  53  based on instructions inputted to the operation unit  3  on the base body  2  of the measurement device. Measurement results are outputted to the display unit  4  and a memory  41 . The memory  41  stores the measurement results. The memory  41  also stores programs executed by the CPU  40 . 
     A first specific example of an operation performed by the measurement device  1 A will be described with reference to  FIG. 7 . The first specific example is an example of a measuring operation when calculation is performed by a first arithmetic algorithm. The operation shown in  FIG. 7  is started when a subject or the like presses down a measurement button on the operation unit  3  of the base body  2 . This operation is achieved by the CPU  40 . The CPU  40  reads a program stored in the memory  41  and controls each unit as shown in  FIG. 6 . In  FIG. 8 , a portion (A) illustrates a temporal change of a pressure P 1  in the air bladder  13 B, and a portion (B) illustrates a temporal change of a pressure P 2  in the air bladder  13 A. In the portions (A) and (B) of  FIG. 8 , S 3  to S 17  attached to temporal axes correspond to respective operations of the measuring operation performed by the measurement device  1 A. 
     Referring to  FIG. 7 , when the operation starts, first, the CPU  40  performs initialization of each unit (step S 1 ). Subsequently, the CPU  40  starts to pressurize the air bladder  13 A by outputting a control signal to the air system  20 A, and measures a blood pressure during the pressurizing process (step S 3 ). The measurement of the blood pressure in step S 3  may be performed by a measurement method used in an ordinary sphygmomanometer. More specifically, the CPU  40  measures a systolic blood pressure (SYS) and a diastolic blood pressure (DIA) based on a pressure signal obtained from the pressure sensor  23 A. In the example of (B) of  FIG. 8 , the pressure P 2  in the air bladder  13 A increases to a pressure more than the systolic blood pressure in a period of step S 3 . As shown in (A) of  FIG. 8 , the pressure P 1  in the air bladder  13 B is maintained at an initial pressure in the above period. 
     When measuring of the blood pressure is finished in step S 3 , the CPU  40  outputs a control signal to the drive circuit  53  to open both of the valves of the two-port valve  51  on the side of the air bladder  13 A and on the side of the air bladder  13 B (step S 5 ). Thereby, a portion of the air in the air bladder  13 A moves to the air bladder  13 B to pressurize the air bladder  13 B. 
     In the example of (A) of  FIG. 8 , the valves of the two-port valve  51  are opened in step S 5 , whereby a portion of the air in the air bladder  13 A moves to the air bladder  13 B, and the pressure P 2  is reduced. At the same time, as shown in (B) of  FIG. 8 , the pressure P 1  in the air bladder  13 B rapidly increases. Then, when the pressure P 1  and the pressure P 2  become the same, that is, when the internal pressures of the air bladders  13 A,  13 B are balanced, the moving of air from the air bladder  13 A to the air bladder  13 B is finished. At this point, the CPU  40  outputs a control signal to the drive circuit  53  and closes the valves of the two-port valve  51  that were opened in step S 5  (step S 7 ). In (A) and (B) of  FIG. 8 , it is shown that the pressure P 1  and the pressure P 2  are the same in step S 7 . 
     Thereafter, the CPU  40  outputs a control signal to the drive circuit  27 B to adjust and reduce the pressure P 1  in the air bladder  13 B (step S 9 ). The amount of reduction adjustment at this time according to one or more embodiments of the present invention, is about 5.5 mmHg/sec. Alternatively, the pressure P 1  is reduced and adjusted to a pressure appropriate for pulse wave measurement, i.e., 50 to 150 mmHg. On the other hand, at this time, the pressure P 2  of the air bladder  13 A is maintained at a pressure higher than at least the systolic blood pressure, i.e., maximum pressure. Thereby, the air bladder  13 A avascularizes the artery at the peripheral side of the measurement portion. This state is called the avascularized state. In other words, the avascularized state is a state in which the pressure P 2  in the air bladder  13 A presses the peripheral side of the measurement portion with a pressure higher than at least the systolic blood pressure. Thereafter, in the avascularized state, the CPU  40  measures the pressure P 1  in the air bladder  13 B based on a pressure signal given by the pressure sensor  23 B and thereby measures the pulse wave, thus extracting feature points (step S 11 ). In the example of  FIG. 5 , the pulse wave  1 , i.e., the pulse wave during the avascularization, is measured in step S 11 , and features points A 1  and B 1  are extracted based on the pulse wave  1 . In the below description, the pulse wave measured in step S 11  is adopted as the pulse wave  1 , and the extracted feature point is adopted as a feature point  1 . 
     In a case where the feature point  1  is not extracted from the pulse wave  1  in step S 11  (NO in step S 13 ), the CPU  40  performs the following control. Herein, as described above, there is a possibility that in particular the point B 1 , i.e., the peak of the reflection wave, might not be extracted. Accordingly, the CPU  40  outputs a control signal to the drive circuit  27 A to adjust and further reduce the pressure P 2  in the air bladder  13 A (step S 15 ). Alternatively, the air valve  22 A may be opened. In step S 15 , the CPU  40  adjusts and reduces the pressure P 2  to a pressure less than at least the systolic blood pressure, i.e., about 55 mmHg, for example. Thereby, the air bladder  13 A attains a state in which the artery is not avascularized or an avascularized state having a pressure weaker than that of step S 11 . These states are called the non-avascularized state. In other words, the non-avascularized state is a state in which the pressure P 2  in the air bladder  13 A presses the peripheral side of the measurement portion with a pressure lower than at least the systolic blood pressure. In the example of (B) of  FIG. 8 , the pressure P 2  in the air bladder  13 A decreases to a pressure less than the systolic blood pressure in a period of step S 15 . Thereafter, in the non-avascularized state, the CPU  40  measures, in the same manner as step S 11 , the pressure P 1  in the air bladder  13 B based on a pressure signal given by the pressure sensor  23 B and thereby measures the pulse wave, thus extracting feature points (step S 17 ). In the example of  FIG. 5 , the pulse wave  2 , i.e., the pulse wave during the non-avascularization, is measured in step S 17 , and features points A 2  and B 2  are extracted based on the pulse wave  2 . In the below description, the pulse wave measured in step S 17  is adopted as the pulse wave  2 , and the extracted feature point is adopted as a feature point  2 . It should be noted that in step S 17 , the CPU  40  may extract, from the pulse wave  2 , only the feature points that have not been extracted in step S 11 . In step S 11 , there is a possibility that the point B 1  might not be extracted from the pulse wave  1 . In this case, in step S 17 , the CPU  40  may extract only the point B 2  as the feature point  2  from the pulse wave  2 . Steps S 15 , S 17  are skipped when all the feature points  1  are extracted in step S 11  (YES in step S 13 ). 
     When the feature point  1  is extracted in step S 11 , the CPU  40  calculates the above index from the feature point  1 . When the feature point  1  is not extracted in step S 11 , and the feature point  2  is extracted in step S 17 , the CPU  40  calculates the index from the feature point  2 . Then, the CPU determines the degree of arteriosclerosis based on the index (step S 19 - 1 ). Thereafter, the CPU  40  outputs control signals to the drive circuits  27 A,  27 B to open the air valves  22 A,  20 B, thereby releasing the pressures of the air bladders  13 A,  13 B to the atmospheric pressure (step S 21 ). In the example of (A) and (B) of  FIG. 8 , the pressures P 1 , P 2  in the air bladders  13 A,  13 B rapidly decrease to the atmospheric pressure in a period of step S 21 . 
     Thereafter, the CPU  40  displays the measurement results upon performing processes for causing the display unit  4  on the base body  2  to display the calculated systolic blood pressure (SYS), the diastolic blood pressure (DIA), the measurement results such as the measured pulse waves, and the determination result of the degree of arteriosclerosis (step S 23 ). 
     In the measuring operation according to the first specific example, when the feature point  2  is not extracted in step S 17 , the internal pressure P 1  of the air bladder  13 B may be adjusted and reduced. That is, the internal pressure P 1  may be repeatedly adjusted and reduced until all the feature points are extracted. Further, at this time, the measuring operation may be terminated when the internal pressure P 1  has reached a predetermined pressure, or the measuring operation may be terminated when the internal pressure P 1  has been reduced and adjusted for a predetermined number of times. 
     The measurement device  1 A achieves the measuring operation according to the first specific example as shown in  FIG. 7 , thus measuring the pulse wave in the non-avascularized state (pulse wave  2 ), in a case where it is difficult to find the feature points and the feature points are not extracted from the pulse wave  1  of  FIG. 5  measured in the avascularized state. Especially when the peripheral side is avascularized, most of the reflection wave from the peripheral side is shielded, which may prevent extraction of the feature point (B 1  point) corresponding to the peak of the reflection wave. However, in such a case, the measurement device  1 A measures the pulse wave at the peripheral side in the non-avascularized state, thus easily extracting the feature point (B 2  point) corresponding to the peak of the reflection wave in particular. Therefore, the index can be accurately calculated, and the index useful for determining the degree of arteriosclerosis can be obtained. 
     A second specific example of the operation performed by the measurement device  1 A will be described with reference to  FIG. 9 . The second specific example is an example of a measuring operation when calculation is performed according to a second arithmetic algorithm. The operation shown in  FIG. 9  is also started when a subject or the like presses down the measurement button on the operation unit  3  of the base body  2 . This operation is achieved by the CPU  40 . The CPU  40  reads a program stored in the memory  41  and controls each unit as shown in  FIG. 6 . In  FIG. 9 , the same measuring operation as that of the first specific example shown in the flowchart of  FIG. 7  is denoted with the same step number. Accordingly, S 3  to S 17  attached to the temporal axes of (A) and (B) of  FIG. 8  correspond to each operation of the measuring operation shown in  FIG. 9 . 
     Referring to  FIG. 9 , in the measuring operation according to the second specific example, the pulse wave  1  is measured in the avascularized state in step S 11 , and the feature point  1  is extracted from the pulse wave  1 . Thereafter, the operation of step S 15  is performed to further reduce and adjust the pressure P 1  in the air bladder  13 B. Then, in step S 17 , the pulse wave  2  is measured in the non-avascularized state, and the feature point  2  is extracted from the pulse wave  2 . Subsequently, in the measuring operation according to the second specific example, different from the measuring operation according to the first specific example, the CPU  40  calculates an average value between the feature point  1  extracted in step S 11  and the feature point  2  extracted in step S 17 , and calculates the index from the average value, thereby determining the degree of arteriosclerosis (step S 19 - 2 ). In other words, when Tpp is calculated as the index, the CPU  40  calculates an average between an emerging time of the point A 1  extracted from the pulse wave  1  in step S 11  and an emerging time of the point A 2  extracted from the pulse wave  2  in step S 17  and an average between an emerging time of the point B 1  extracted from the pulse wave  1  in step S 11  and an emerging time of the point B 2  extracted from the pulse wave  2  in step S 17 , and the CPU  40  obtains Tpp by calculating a difference therebetween. When AI is calculated as the index, the CPU  40  calculates an average between an amplitude of the point A 1  extracted from the pulse wave  1  in step S 11  and an amplitude of the point A 2  extracted from the pulse wave  2  in step S 17  and an average between an amplitude of the point B 1  extracted from the pulse wave  1  in step S 11  and an amplitude of the point B 2  extracted from the pulse wave  2  in step S 17 , and the CPU  40  obtains AI according to a ratio therebetween. Thereafter, the operation of steps S 21 , S 23  is performed. 
     When the measurement device  1 A achieves the measuring operation according to the second specific example as shown in  FIG. 9 , the index is calculated using an average between the feature points (A 1 , B 1 ) extracted from the pulse wave (pulse wave  1 ) measured in the avascularized state and the feature points (A 2 , B 2 ) extracted from the pulse wave (pulse wave  2 ) measured in the non-avascularized state. Therefore, the index can be accurately calculated, and the index useful for determining the degree of arteriosclerosis can be obtained. 
     A third specific example of the operation performed by the measurement device  1 A will be described with reference to  FIG. 10 . The third specific example is an example of a measuring operation when calculation is performed according to a third arithmetic algorithm. The operation shown in  FIG. 10  is also started when a subject or the like presses down the measurement button on the operation unit  3  of the base body  2 . This operation is achieved by the CPU  40 . The CPU  40  reads a program stored in the memory  41  and controls each unit as shown in  FIG. 6 . In  FIG. 10 , the same measuring operation as that of the first specific example shown in the flowchart of  FIG. 7  and that of the second specific example shown in the flowchart of  FIG. 9  is denoted with the same step number. Accordingly, S 3  to S 17  attached to the temporal axes of (A) and (B) of  FIG. 8  correspond to each operation of the measuring operation shown in  FIG. 10 . 
     Referring to  FIG. 10 , in the measuring operation according to the third specific example, the pulse wave  1  is measured in the avascularized state in step S 11 , and the feature point  1  is extracted from the pulse wave  1 . Thereafter, the operation of step S 15  is performed to further reduce and adjust the pressure P 1  in the air bladder  13 B. Then, in step S 17 , the pulse wave  2  is measured in the non-avascularized state, and the feature point  2  is extracted from the pulse wave  2 . Subsequently, in the measuring operation according to the third specific example, different from the measuring operations according to the first and second specific examples, the CPU  40  compares the feature point  1  extracted in step S 11  and the feature point  2  extracted in step S 17 , and determines whether a difference therebetween is equal to or more than an acceptable value (step S 18 A). More specifically, a difference between an emerging time of the point A 1  extracted from the pulse wave  1  in step S 11  and an emerging time of the point A 2  extracted from the pulse wave  2  in step S 17  and/or a difference between an emerging time of the point B 1  extracted from the pulse wave  1  in step S 11  and an emerging time of the point B 2  extracted from the pulse wave  2  in step S 17  are calculated, and determination is made as to whether the difference is equal to or more than the acceptable value. For example, an acceptable value is about 10 ms, and is stored to the CPU  40  in advance. Alternatively, the acceptable value may be registered and updated by predetermined operation (for example, an operation method known to a user such as a doctor specified in advance). As described above, the emerging time of the point A 1  and the emerging time of the point A 2  are considered to be substantially the same for the same subject. Likewise, the emerging time of the point B 1  and the emerging time of the point B 2  are considered to be substantially the same. Accordingly, when the difference between these emerging times is equal to or more than the acceptable value, it is considered that either of the pulse waves is not correctly measured or the feature points are not correctly extracted. 
     Accordingly, in a case where, in step S 18 A, the difference between the feature point  1  and the feature point  2  is determined to be equal to or more than the acceptable value, or one of the feature point  1  and the feature point  2  is not extracted (NO in step S 18 A), the CPU  40  performs an operation for causing the display unit  4  to display a screen for notifying remeasuring. Then, after the CPU  40  notifies remeasuring (step S 18 B), the CPU  40  causes the measuring operation to return to step S 5 , and opens the two-port valve  51  again. 
     In a case where the feature point  1  is extracted in step S 11 , the feature point  2  is extracted in step S 17 , and the difference therebetween is within the acceptable value (YES in step S 18 A), then the CPU  40  calculates an average value between the feature point  1  extracted in step S 11  and the feature point  2  extracted in step S 17 , and calculates the index from the average value, thereby determining the degree of arteriosclerosis (step S 19 - 2 ), in the same manner as the measuring operation according to the second specific example. Alternatively, the index may be calculated using one of the feature point  1  extracted in step S 11  and the feature point  2  extracted in step S 17 , or the index may be calculated using the feature point  1  extracted from the pulse wave  1  measured in the avascularized state in step S 11 . 
     The measurement device  1 A performs the measuring operation according to the third specific example as shown in  FIG. 10 . Accordingly, remeasuring is performed when a difference between the feature points (point A 1 , point B 1 ) extracted from the pulse wave (pulse wave  1 ) measured in the avascularized state and the feature points (point A 2 , point B 2 ) extracted from the pulse wave (pulse wave  2 ) measured in the non-avascularized state is equal to or more than the acceptable value. Therefore, the index can be accurately calculated, and the index useful for determining the degree of arteriosclerosis can be obtained. 
     The fourth specific example of the operation performed by the measurement device  1 A will be described with reference to  FIG. 11 . The fourth specific example is an example of a measuring operation when calculation is performed according to a fourth arithmetic algorithm. The operation shown in  FIG. 11  is also started when a subject or the like presses down the measurement button on the operation unit  3  of the base body  2 . This operation is achieved by the CPU  40 . The CPU  40  reads a program stored in the memory  41  and controls each unit as shown in  FIG. 6 . In  FIG. 11 , the same measuring operation as the measuring operation of the first specific example shown in the flowchart of  FIG. 7 , the measuring operation of the second specific example shown in the flowchart of  FIG. 9 , and the measuring operation of the third specific example shown in the flowchart of  FIG. 10  is denoted with the same step number. Accordingly, S 3  to S 17  attached to the temporal axes of (A) and (B) of  FIG. 8  correspond to each operation of the measuring operation shown in  FIG. 11 . 
     Referring to  FIG. 11 , in the measuring operation according to the fourth specific example, in a case where in step S 18 A, the difference between the feature point  1  and the feature point  2  is determined to be equal to or more than the acceptable value, or one of the feature point  1  and the feature point  2  is not extracted (NO in step S 18 A), the CPU  40  performs processing for causing the display unit  4  to display a screen for notifying that the determination result has a low reliability. Then, the CPU  40  performs the measuring operation after notifying to that effect (step S 18 C). In the same manner as the measuring operation according to the second specific example and the measuring operation according to the third specific example, the CPU  40  calculates an average value between the feature point  1  extracted in step S 11  and the feature point  2  extracted in step S 17 , and calculates the index from the average value, thereby determining the degree of arteriosclerosis (step S 19 - 2 ). 
     The measurement device  1 A achieves the measuring operation according to the fourth specific example as shown in  FIG. 11 . Accordingly, even when a difference between the feature points (point A 1 , point B 1 ) extracted from the pulse wave (pulse wave  1 ) measured in the avascularized state and the feature points (point A 2 , point B 2 ) extracted from the pulse wave (pulse wave  2 ) measured in the non-avascularized state is equal to or more than the acceptable value, the measurement device  1 A notifies that the determination result has a low reliability and calculates the index using these feature points. Therefore, although the calculated index has a lower reliability than the index obtained from the measuring operation according to the third specific example, remeasuring is not performed, and the index is calculated from one measuring operation, whereby the degree of arteriosclerosis can be determined in a shorter time. 
     Further, as described above, in the measurement device  1 A, the air bladder  13 A and the air bladder  13 B are connected via the two-port valve  51 . Then, when measuring of the blood pressure is finished in step S 3 , the two-port valve  51  is opened in step S 5 , whereby the air in the air bladder  13 A is moved to the air bladder  13 B. When the two-port valve  51  is opened, the air in the air bladder  13 A rapidly blows into the air bladder  13 B in order to eliminate a pressure difference. Therefore, a time needed to blow air into the air bladder  13 B using a pump can be greatly reduced, and the overall measuring time can be reduced. This can reduce the strain imposed on the subject. In general, when it takes a long time to perform measurement, an artery is pressed for a long time, which stimulates sympathetic nerves and may deteriorate the characteristics of blood vessels. In contrast, an artery is pressed for a shorter time, when the measurement is performed in a shorter time. In general, body movement is more likely to occur as the measuring takes a longer time. However, when the measurement is performed in a shorter time, the body movement is less likely to occur. Therefore, blood pressure information such as pulse waves can be measured with higher accuracy. In addition, the accuracy of the index of arteriosclerosis obtained from the measurement result can also be improved. 
     As shown in  FIG. 6 , a mechanism for blowing air into the air bladder  13 B (air pump, air pump drive circuit) may not be arranged. This can contribute to making the apparatus smaller, lighter, and inexpensive. 
     However, the above measuring operation can be performed not only by the measurement device having the configuration as shown in  FIG. 6  but also by the measurement device having an ordinary configuration as shown in  FIG. 12 . Accordingly, the second embodiment will be described. In the second embodiment, the measuring operation is performed by the measurement device  1 B having the configuration as shown in  FIG. 12 . 
     Second Embodiment   
     The measurement device  1 B is generally the same as the measurement device  1 A shown in  FIG. 1 . Referring to  FIG. 12 , in the measurement device  1 B, an air system  20 B includes an air pump  21 B, and the measurement device  1 B includes a drive circuit  26 B for driving the air pump  21 B, in place of the two-port valve  51  and the drive circuit  53  of the configuration of the measurement device  1 A as shown in  FIG. 6 . The air pump  21  B is driven by the drive circuit  26 B receiving an instruction from the CPU  40 , and blows compressed gas into the air bladder  13 B. 
     A first specific example of an operation of the measurement device  1 B will be described with reference to  FIG. 13 . The first specific example represents a measuring operation when calculation is performed according to the first arithmetic algorithm described in the first embodiment. The operation shown in  FIG. 13  is started when a subject or the like presses down the measurement button on the operation unit  3  of the base body  2 . This operation is achieved by the CPU  40 . The CPU  40  reads a program stored in the memory  41  and controls each unit as shown in  FIG. 12 . In  FIG. 14 , a portion (A) represents a temporal change of the pressure P 1  in the air bladder  13 B, and a portion (B) represents a temporal change of the pressure P 2  in the air bladder  13 A. In the portions (A) and (B) of  FIG. 14 , S 103  to S 121  attached to temporal axes correspond to respective operations of the measuring operation performed by the measurement device  1 B. 
     Referring to  FIG. 13 , when the operation starts, the CPU  40  performs initialization of each unit (step S 101 ). Subsequently, the CPU  40  outputs a control signal to the air system  20 B, and pressurizes the air bladder  13 B to a predetermined pressure (step S 103 ). In the example of (A) of  FIG. 14 , the pressure P 1  in the air bladder  13 B increases within a period of step S 103 . Then, the pressure P 1  is thereafter maintained. In step S 103 , the pressure P 1  is increased so that the pressure P 1  attains a pressure appropriate for pulse wave measurement, i.e., 50 to 150 mmHg. When the pressure P 1  attains the predetermined pressure, the CPU  40  outputs a control signal to the air system  20 A, increases the pressure P 2  of the air bladder  13 A to a predetermined pressure, and causes the air bladder  13 A to pressurize the peripheral side of the measurement portion (step S 105 ). In the example of (B) of  FIG. 14 , the pressure P 2  in the air bladder  13 A increases within a period of step S 105 . In step S 105 , the CPU  40  increases the pressure P 2  until the pressure P 2  attains a pressure higher than the general systolic blood pressure value. According to one or more embodiments of the present invention, the pressure P 2  is increased to about the systolic blood pressure value +40 mmHg. Therefore, the air bladder  13 A avascularizes an artery. Thereafter, the CPU  40  outputs a control signal to the air system  20 A, and starts reducing the pressure P 2  in the air bladder  13 A (step S 107 ). In this case, the amount of pressure reduction adjustment is about 4 mmHg/sec, and the pressure P 2  is gradually reduced. 
     While the pressure P 2  in the air bladder  13 A changes from the maximum pressure to the systolic blood pressure during the pressure reduction process of the pressure P 2  in the air bladder  13 A (YES in step S 111 ), namely, in the avascularized state, the CPU  40  measures a pulse wave by measuring the pressure P 1  in the air bladder  13 B based on a pressure signal given by the pressure sensor  23 B, thereby extracting a feature point (step S 109 ). In a period shown in step S 109  in (A) and (B) of  FIG. 14 , the pulse wave is measured, and the feature point is extracted. In the example of  FIG. 5 , the pulse wave  1 , i.e., the pulse wave during the avascularization, is measured in step S 109 , and features points A 1  and B 1  are extracted based on the pulse wave  1 . It should be noted that, for the sake of the below description, the pulse wave measured in step S 109  will be referred to as the pulse wave  1 , and the extracted feature point will be referred to as the feature point  1 . 
     In a case where the feature point  1  is not extracted from the pulse wave  1  (NO in step S 113 ) while the pressure P 2  in the air bladder  13 A changes to the systolic blood pressure value during the pressure reduction process of the pressure P 2  in the air bladder  13 A, the CPU  40  measures a pulse wave by measuring the pressure P 1  in the air bladder  13 B based on a pressure signal given by the pressure sensor  23 B and thereby extracts a feature point while the pressure P 2  in the air bladder  13 A is less than the systolic blood pressure value during the pressure reduction process of the pressure P 2  in the air bladder  13 A, namely, in the non-avascularized state (step S 115 ). In a period shown in step S 115  in (A) and (B) of  FIG. 14 , the pulse wave is measured, and the feature point is extracted. In the example of  FIG. 5 , the pulse wave  2 , i.e., the pulse wave during the non-avascularization, is measured in step S 115 , and features points A 2  and B 2  are extracted based on the pulse wave  2 . For the sake of the below description, the pulse wave measured in step S 115  will be referred to as the pulse wave  2 , and the extracted feature point will be referred to as the feature point  2 . Step S 115  is skipped when all the feature points  1  are extracted in step S 109  (YES in step S 113 ). 
     In the pressure reduction process since around a time when the internal pressure of the air bladder  13 A reaches the systolic blood pressure value after step S 109 , the CPU  40  measures the above pulse wave as well as the blood pressure. The measurement of the blood pressure may be performed by a measurement method used in an ordinary sphygmomanometer. More specifically, the CPU  40  calculates a systolic blood pressure (SYS) and a diastolic blood pressure (DIA) based on a pressure signal obtained from the pressure sensor  23 A. The CPU  40  terminates the measuring of the blood pressure when the systolic blood pressure value and the diastolic blood pressure value are calculated or when the internal pressure of the air bladder  13 A becomes lower than the diastolic blood pressure value (step S 117 ). 
     When the feature point  1  is extracted in step S 109 , the CPU  40  calculates the index from the feature point  1 . When the feature point  1  is not extracted in step S 109 , and the feature point  2  is extracted in step S 115 , the CPU  40  calculates the index from the feature point  2 . Then, the CPU determines the degree of arteriosclerosis based on the index (step S 119 ). Thereafter, the CPU  40  outputs control signals to the drive circuits  27 A,  27 B to open the air valves  22 A,  20 B, thereby releasing the pressures in the air bladders  13 A,  13 B to atmospheric pressure (step S 121 ). In the example of (A) and (B) of  FIG. 14 , the pressures P 1 , P 2  in the air bladders  13 A,  13 B rapidly decrease to the atmospheric pressure in a period of step S 121 . 
     Thereafter, the CPU  40  displays the measurement results upon performing processes for causing the display unit  4  on the base body  2  to display the calculated systolic blood pressure (SYS), the diastolic blood pressure (DIA), the measurement results such as the measured pulse waves, and the determination result of the degree of arteriosclerosis (step S 123 ). 
     The measurement device  1 B achieves the measuring operation according to the first specific example as shown in  FIG. 13 , thus measuring the pulse wave in the non-avascularized state (pulse wave  2 ), in a case where it is difficult to find the feature points and the feature points are not extracted from the pulse wave  1  of  FIG. 5  measured in the avascularized state. Especially when the peripheral side is avascularized, most of the reflection wave from the peripheral side is shielded, which may prevent extraction of the feature point (B 1  point) corresponding to the peak of the reflection wave. However, in such a case, the measurement device  1 B measures the pulse wave at the peripheral side in the non-avascularized state, thus easily extracting the feature point (B 2  point) corresponding to the peak of the reflection wave in particular. Therefore, the index can be accurately calculated, and the index useful for determining the degree of arteriosclerosis can be obtained. 
     A second specific example of the operation performed by the measurement device  1 B will be described with reference to  FIG. 15 . The second specific example represents a measuring operation when calculation is performed according to the second arithmetic algorithm described in the first embodiment. The operation shown in  FIG. 15  is also started when a subject or the like presses down the measurement button on the operation unit  3  of the base body  2 . This operation is achieved by the CPU  40 . The CPU  40  reads a program stored in the memory  41  and controls each unit as shown in  FIG. 12 . In  FIG. 15 , the same measuring operation as that of the first specific example shown in the flowchart of  FIG. 13  is denoted with the same step number. 
     Referring to  FIG. 15 , the measuring operation according to the second specific example is as follows. When the pressure P 2  in the air bladder  13 A begins to be reduced in step S 107 , the CPU  40  measures a pulse wave by measuring the pressure P 1  in the air bladder  13 B based on a pressure signal given by the pressure sensor  23 B in the pressure reduction process (step S 108 ). At this time, the CPU  40  measures the pressure P 2  in the air bladder  13 A based on a pressure signal obtained from the pressure sensor  23 A, and stores the measured pulse wave as well as the pressure P 2  in the air bladder  13 A during the measuring operation to a predetermined region of the memory  41 . In the example of (A), (B) of  FIG. 14 , step S 108  corresponds to periods of steps S 109 , S 115 . 
     When the measurement of the pulse wave in step S 108  is finished, the CPU  40  obtains the systolic blood pressure (SYS). The systolic blood pressure (SYS) may be obtained by performing calculation based on the pressure signal obtained from the pressure sensor  23 A. Alternatively, the systolic blood pressure (SYS) may be obtained by receiving an input with predetermined buttons and the like on the operation unit  3 . Alternatively, the systolic blood pressure (SYS) may be stored to the memory  41  as a general value in advance and may be obtained from the memory  41 . The CPU  40  compares the pressure P 2  in the air bladder  13 A during the measurement process stored in association with the measured pulse wave and the obtained systolic blood pressure, thereby determining whether the measured pulse wave is measured in the avascularized state or measured in the non-avascularized state. In other words, the systolic blood pressure is used as a threshold value for determining whether it is measured in the avascularized state or in the non-avascularized state. It should be noted that the obtained systolic blood pressure may be a case where the pressure P 2  in the air bladder  13 A is lower than the diastolic blood pressure (DIA) lower than the systolic blood pressure. In such a case, the diastolic blood pressure is also used as the threshold value for comparison with the diastolic blood pressure, whereby the measured pulse wave is determined to be measured in the non-avascularized state. 
     Then, the CPU  40  extracts the feature point from the measured pulse wave (step S 118 ), and calculates the index from the feature point, thereby determining the degree of arteriosclerosis (step S 119 ). In this case, when the points A 1  and B 1 , i.e., the feature points, are extracted from the pulse wave  1  measured in the avascularized state, these may be used to calculate the index in the same manner as the above-described calculation performed according to the first arithmetic algorithm. Alternatively, in the same manner as the calculation performed according to the second arithmetic algorithm, the index may be calculated using respective averages between the points A 1  and B 1 , i.e., the feature points, extracted from the pulse wave  1  measured in the avascularized state and between the points A 2  and B 2 , i.e., the feature points, extracted from the pulse wave  2  measured in the non-avascularized state. Alternatively, in the same manner as the calculation performed according to the third arithmetic algorithm, when respective differences between the points A 1  and B 1 , i.e., the feature points, extracted from the pulse wave  1  measured in the avascularized state and between the points A 2  and B 2 , i.e., the feature points, extracted from the pulse wave  2  measured in the non-avascularized state are within the acceptable value, the index may be calculated using either of the feature points or the average value thereof. Hereinafter, the operation of steps S 121 , S 123  is performed. 
     The measurement device  1 B achieves the measuring operation according to the second specific example as shown in  FIG. 15 . Accordingly, it is not necessary to adjust the pressure P 2  in the air bladder  13 A to a predetermined pressure so that the peripheral side of the measurement portion is in the avascularized state or the non-avascularized state. In other words, for example, the pressure P 2  is reduced with a constant pressure reduction adjustment amount such as about 4 mmHg/sec, and determination can be made as to whether the pulse wave measured during the pressure reduction process is the pulse wave (pulse wave  1 ) in the avascularized state or the pulse wave (pulse wave  2 ) in the non-avascularized state by comparing the pressure P 2  during the measurement and the blood pressure value. Therefore, the index can be accurately calculated without any complicated control, and the index useful for determining the degree of arteriosclerosis can be obtained. Further, since it is not necessary to adjust the pressure P 2 , the measuring operation can be performed in a shorter time. 
     As a modification of the measuring operation according to the second specific example, the measurement device  1 B can perform a measuring operation as shown in  FIG. 16 . The modification of the measuring operation according to the second specific example represents a modification of the measuring operation when calculation is performed according to the first arithmetic algorithm described in the second embodiment. The operation shown in  FIG. 16  is also started when a subject or the like presses down the measurement button on the operation unit  3  of the base body  2 . This operation is achieved by the CPU  40 . The CPU  40  reads a program stored in the memory  41  and controls each unit as shown in  FIG. 12 . In  FIG. 17 , a portion (A) illustrates a temporal change of the pressure P 1  in the air bladder  13 B, and a portion (B) illustrates a temporal change of the pressure P 2  in the air bladder  13 A. In the portions (A) and (B) of  FIG. 17 , S 103  to S 121  attached to temporal axes correspond to respective operations of the measuring operation performed by the measurement device  1 B. 
     Referring to  FIG. 16 , in the modification of the measuring operation according to the second specific example, the CPU  40  measures a pulse wave by measuring the pressure P 1  in the air bladder  13 B based on a pressure signal given by the pressure sensor  23 B (step S 104 ) when the pressure P 1  in the air bladder  13 B is in a pressurized state so as to attain a pressure suitable for pulse wave measurement, i.e., a range of 50 to 150 mmHg, in step S 103 , but before the air bladder  13 A pressurizes the peripheral side of the measurement portion in subsequent step S 105 , namely, in the non-avascularized state. The pulse wave measured in step S 105  is a pulse wave in the non-avascularized state as described above. In the description, the measured pulse wave is referred to as the pulse wave  2 . In the example of (A) and (B) of  FIG. 17 , the pulse wave  2  is measured in a period of step S 104 . As shown in (B) of  FIG. 17 , the pressure P 2  in the air bladder  13 A is not pressurized and is maintained at an initial pressure in the period of step S 104 . 
     Thereafter, the CPU  40  outputs a control signal to the air system  20 A, and increases the pressure P 2  in the air bladder  13 A to a predetermined pressure, whereby the air bladder  13 A pressurizes the peripheral side of the measurement portion (step S 105 ). According to one or more embodiments of the present invention, the predetermined pressure is about the systolic blood pressure value +40 mmHg as described above. After the pressure P 2  reaches the predetermined pressure, the CPU  40  outputs a control signal to the air system  20 A, and starts reducing the pressure P 2  in the air bladder  13 A (step S 107 ). The amount of reduction adjustment at this time according to one or more embodiments of the present invention is about 4 mmHg/sec. 
     During the pressure reduction process of the pressure P 2  in the air bladder  13 A, the CPU  40  measures a pulse wave by measuring the pressure P 1  in the air bladder  13 B based on a pressure signal given by the pressure sensor  23 B, thereby extracting a feature point (step S 108 ′). At this time, the CPU  40  measures the pressure P 2  in the air bladder  13 A based on a pressure signal obtained from the pressure sensor  23 A, and stores the measured pulse wave as well as the pressure P 2  in the air bladder  13 A during the measuring operation to a predetermined region of the memory  41 . It should be noted that the measuring operation in step S 108 ′ is performed mainly for the purpose of measuring the pulse wave  1  in the avascularized state since the pulse wave  2  in the non-avascularized state is measured in step S 104 . Accordingly, the measuring operation in step S 108 ′ is performed in a very short period compared with step S 108 . According to one or more embodiments of the present invention, the measuring operation in step S 108 ′ is performed while the pressure P 2  in the air bladder  13 A changes from the maximum pressure to the systolic blood pressure. In the example of (A) and (B) of  FIG. 17 , the pulse wave is measured in a period of step S 108 ′. The period of step S 108 ′ corresponds to a period of step S 109  in the example of (A), (B) of  FIG. 14 . On the other hand, as described above, step S 108  corresponds to periods of steps S 109 , S 115  in the example of (A) and (B) of  FIG. 14 . That is, as shown in  FIG. 14  and  FIG. 17 , the measuring operation of step S 108 ′ is performed in a shorter period than the measuring operation of step S 108 . 
     Thereafter, in the pressure reduction process, namely, in the pressure reduction process in which the pressure P 2  in the air bladder  13 A reaches the diastolic blood pressure, the CPU  40  performs only the blood pressure measurement. Accordingly, in the pressure reduction process after step S 108 ′, the CPU  40  increases the amount of pressure reduction adjustment. The amount of reduction adjustment according to one or more embodiments of the present invention is 4 mmHg/sec or more. When the blood pressure measurement is finished (step S 117 ), the CPU  40  compares the pressure P 2  in the air bladder  13 A during the measurement process stored in association with the pulse wave measured in step S 108 ′ with the obtained systolic blood pressure (SYS) and the diastolic blood pressure (DIA), thereby determining whether the measured pulse wave is measured in the avascularized state or measured in the non-avascularized state (step S 118 ′). Then, the CPU  40  extracts the feature point from the measured pulse wave (step S 118 ), and calculates the index from the feature point, thereby determining the degree of arteriosclerosis (step S 119 ). As described above, in step S 104 , the pulse wave  2  in the non-avascularized state is measured. Therefore, in step S 118 ′, the CPU  40  extracts the pulse wave  1  measured in the avascularized state from among the pulse waves measured in step S 108 ′. Hereinafter, the measuring operation of steps S 119 , S 121 , S 123  is performed. 
     The measurement device  1 B achieves the measuring operation according to the modification of the second specific example as shown in  FIG. 16 . Accordingly, the pressure reduction rate of the pressure P 2  in the air bladder  13 A can be further increased after the measurement of the pulse wave in step S 108 ′ is finished. Therefore, the measuring operation can be performed in a shorter time. 
     A third specific example of the operation performed by the measurement device  1 B will be described with reference to  FIG. 18 . The third specific example represents a measuring operation when calculation is performed according to the fourth arithmetic algorithm described in the first embodiment. The operation shown in  FIG. 18  is also started when a subject or the like presses down the measurement button on the operation unit  3  of the base body  2 . This operation is achieved by the CPU  40 . The CPU  40  reads a program stored in the memory  41  and controls each unit as shown in  FIG. 12 . In  FIG. 18 , the same measuring operation as the measuring operation of the first specific example shown in the flowchart of  FIG. 13  and the measuring operation of the second specific example shown in the flowchart of  FIG. 15  is denoted with the same step number. 
     Referring to  FIG. 18 , in the measuring operation according to the third specific example, the CPU  40  measures the pulse wave during the pressure reduction process of the pressure P 2  in the air bladder  13 A, and stores the measured pulse wave as well as the pressure P 2  in the air bladder  13 A during the measuring operation to a predetermined region of the memory  41 , in the same manner as step S 108 . Then, the CPU  40  compares the pressure P 2  during the measurement process with the obtained systolic blood pressure (SYS) and the diastolic blood pressure (DIA), thereby determining whether the measured pulse wave is measured in the avascularized state or measured in the non-avascularized state, in the same manner as step S 109 . Then, the feature point is extracted from the measured pulse wave (step S 118 ). Further, in the measuring operation according to the third specific example, the CPU  40  compares the feature point  1  extracted from the pulse wave measured in the avascularized state and the feature point  2  extracted from the pulse wave measured in the non-avascularized state, and determines whether a difference therebetween is equal to or more than an acceptable value (step S 118 - 1 ), in the same manner as step S 18 A. In a case where in step S 118 - 1 , the difference between the feature point  1  and the feature point  2  is determined to be equal to or more than the acceptable value (NO in step S 118 - 1 ), the CPU  40  performs processing for causing the display unit  4  to display a screen for notifying that the determination result has a low reliability in the same manner as step S 18 C. Then, the CPU  40  performs the measuring operation after notifying to that effect (step S 118 - 2 ). Then, the CPU  40  calculates the index from the extracted feature point, thereby determining the degree of arteriosclerosis, in the same manner as the measuring operation according to the second specific example. 
     The measurement device  1 B achieves the measuring operation according to the third specific example as shown in  FIG. 18 . Accordingly, even when a difference between the feature points (point A 1 , point B 1 ) extracted from the pulse wave (pulse wave  1 ) measured in the avascularized state and the feature points (point A 2 , point B 2 ) extracted from the pulse wave (pulse wave  2 ) measured in the non-avascularized state is equal to or more than the acceptable value, the measurement device  1 B notifies that the determination result has a low reliability and calculates the index using these feature points. Therefore, remeasuring is not performed, and the index is calculated from one measuring operation, whereby the degree of arteriosclerosis can be determined in a shorter time. 
     It should be noted that in the measurement device  1 A and the measurement device  1 B, the air bladder  13 A serves not only for the purpose of avascularization but also for the purpose of calculation of blood pressure value. Then, the blood pressure value is calculated based on a change of the internal pressure of the air bladder  13 A, and the pulse wave is measured based on a change of the internal pressure of the air bladder  13 B. However, the air bladder  13 A may be used only for avascularization, and the blood pressure value may be calculated based on a change of the internal pressure of the air bladder  13 B. 
     Third Embodiment   
     In some cases, it may be difficult to extract a feature point deriving especially from a reflection wave of the pulse wave (pulse wave  1 ) that is measured while the peripheral side of the measurement portion is avascularized to suppress the effect of the reflection wave. Accordingly, in the first embodiment and the second embodiment, the pulse wave (pulse wave  2 ) is measured in non-avascularized state in which the peripheral side is not avascularized, and the feature point is extracted from the pulse wave in the non-avascularized state. In this case, a pulse wave waveform is measured. The pulse wave waveform is a composite waveform made from an ejection wave emitted from the heart and a reflection wave emitted from a periphery such as a palm portion. However, a length from an upper arm, i.e., a measurement portion, to a palm is different for each subject. The length from the upper arm, i.e., the measurement portion, to the palm affects an arrangement between an ejection wave and a reflection wave, namely, the waveform of the measured pulse wave, i.e., the composite wave. Therefore, the accuracy of the obtained index is affected, and the determination of the degree of arteriosclerosis is also affected. 
     One method for suppressing this effect is as follows: the operation unit  3  and the like is used to input in advance a length between the upper arm, i.e., the measurement portion, and a position at which a large reflection occurs, i.e., the palm, and the measured pulse wave is corrected using the length. Another method is to fix the length between the measurement portion and the reflection position to a certain length. 
     Accordingly, in a measurement device  1 C according to a third embodiment, the length between the measurement portion and the reflection position is fixed to a certain length, and another cuff to be attached to a periphery is arranged in addition to the air bladder for measurement process attached to the measurement portion in order to combine an ejection wave with a reflection wave emitted from the periphery located at the defined length from the measurement portion. 
     Referring to  FIG. 19A , the measurement device  1 C includes, for example, an arm band  8  to be wrapped around a wrist, i.e., a peripheral side with respect to the measurement portion. The arm band  8  includes an air bladder  13 C as shown in  FIG. 19B . As described above, the arm band  8  is attached to a wrist away by the predetermined length to the peripheral side from the arm band  9  including the air bladder  13 A and the air bladder  13 B. The attachment position may be determined by a person who carries out measurement. According to one or more embodiments of the present invention, a member for identifying the attachment position of the arm band  8 , such as a belt having the predetermined length for connecting between the arm band  8  and the arm band  9 , is included. The air bladder  13 C inflates and pressurizes the wrist. 
     Referring to  FIG. 20 , the measurement device  1 C includes an air system  20 C connected to the air bladder  13 C via an air tube in addition to the configuration of the measurement device  1 A shown in  FIG. 5 . 
     The air system  20 C includes an air pump  21 C, an air valve  22 C, and a pressure sensor  23 C. The air pump  21 C is driven by the drive circuit  26 C receiving an instruction from the CPU  40 , and blows compressed gas into the air bladder  13 C. Thereby, the air bladder  13 C is pressurized. 
     The open/close state of the air valve  22 C is controlled by the drive circuit  27 C receiving instructions from the CPU  40 . The pressure in the air bladder  13 C is controlled by controlling the open/close state of the air valves  22 C. 
     The pressure sensor  23 C detects the pressure in the air bladder  13 C, and outputs a signal to an amplifier  28 C according to the detected values thereof. The amplifier  28 C amplifies the signal outputted from the pressure sensor  23 C, and outputs the amplified signal to a converter  29 C The converter  29 C digitalizes analog signals outputted from the amplifier  28 C, and outputs the digital signal to the CPU  40 . 
     The CPU  40  controls the air systems  20 A,  20 B,  20 C and the drive circuit  53  based on instructions inputted to the operation unit  3  on the base body  2  of the measurement device. 
     Further, according to one or more embodiments of the present invention, the measurement device  1 C includes a device for inputting a length of an artery from the air bladder  13 B to the air bladder  13 C. The length of the artery from the air bladder  13 B to the air bladder  13 C may simply be a length of an arm from the air bladder  13 B to the air bladder  13 C, i.e., a length of the arm between the arm band  8  and the arm band  9 . The device for inputting the length is not specifically limited. For example, the device may be a switch for inputting the length, included in the operation unit  3 . When a person who carries out measurement inputs the length using the switch, the length is inputted. Alternatively, for example, the arm band  8  and the arm band  9  may be connected by a belt, and the device may be a mechanism arranged on the belt for detecting the length. By adjusting the length so as not to loosen the belt along the arm after the arm band  8  and the arm band  9  are attached, the length of the arm between the arm band  8  and the arm band  9  is inputted with the above mechanism. 
     A first specific example of a measuring operation performed by the measurement device  1 C will be described with reference to  FIG. 21 . The first specific example represents a measuring operation when calculation is performed according to the first arithmetic algorithm described in the first embodiment. The operation shown in  FIG. 21  is started when a subject or the like presses down the measurement button on the operation unit  3  of the base body  2 . This operation is achieved by the CPU  40 . The CPU  40  reads a program stored in the memory  41  and controls each unit as shown in  FIG. 20 . In  FIG. 22 , a portion (A) represents a temporal change of a pressure P 3  in the air bladder  13 C, a portion (B) represents a temporal change of the pressure P 1  in the air bladder  13 B, and a portion (C) represents a temporal change of the pressure P 2  in the air bladder  13 A. In the portions (A), (B), (C) of  FIG. 22 , S 3  to S 21  attached to temporal axes correspond to respective operations of the measuring operation performed by the measurement device  1 C. 
     Referring to  FIG. 21 , the measurement device  1 C performs the same operation as steps S 1  to S 13  as the first specific example of the measuring operation performed by the measurement device  1 A. As shown in (A) of  FIG. 22 , in the measurement device  1 C, the pressure P 3  in the air bladder  13 C is maintained at an initial pressure during the process. 
     When the feature point  1  is not extracted from the pulse wave  1  during the avascularization in step S 11  (NO in step S 13 ), the CPU  40  reduces and adjusts the pressure P 2  of the air bladder  13 A so that the pressure P 2  becomes lower than at least the systolic blood pressure, for example, about 55 mmHg in step S 15 , and outputs a control signal to the air system  20 C, thereby increases the pressure P 3  in the air bladder  13 C so that the pressure P 3  attains a predetermined pressure (step S 16 ). In step S 16 , for example, the CPU  40  increases the pressure P 3  to about the systolic blood pressure +40 mmHg, so that the pressure P 3  becomes higher than at least the systolic blood pressure. At this time, the air bladder  13 A does not avascularize an artery at the peripheral side close to the measurement portion, but the air bladder  13 C avascularizes the artery at the position of the arm band  8  attached to the position away from the measurement portion by the predetermined length. Thereafter, the predetermined length at the peripheral side with respect to the measurement portion is not avascularized. At this state, the CPU  40  measures the pressure P 1  in the air bladder  13 B based on a pressure signal given by the pressure sensor  23 B and thereby measures the pulse wave, thus extracting feature points in step S 17 . Thereafter, the same measuring operation as that of the measurement device  1 A is performed. 
     Even when the second to fourth arithmetic algorithms described in the first embodiment are performed, the measuring operation of the measurement device  1 C can be performed in the same manner. 
     The second to fourth specific examples of the measuring operation performed by the measurement device  1 C will be described with reference to  FIG. 23  to  FIG. 25 . The measuring operations shown in these flowcharts are almost the same as the measuring operations according to the second to fourth specific examples performed by the measurement device  1 A as shown in  FIGS. 9 to 11 , respectively. In any case, when the pulse wave  2  is measured in the non-avascularized state in step S 17 , the pressure P 3  in the air bladder  13 C is increased to a pressure higher than at least the systolic blood pressure in step S 16 , whereby the air bladder  13 A does not avascularize the artery at the peripheral side close to the measurement portion but the air bladder  13 C avascularizes the artery at the position of the arm band  8  attached to the position away from the measurement portion by the predetermined length. 
     The measurement device  1 C achieves the measuring operations as shown in  FIG. 21  and  FIGS. 23 to 25 . Accordingly, when the pulse wave (pulse wave  2 ) is measured in the non-avascularized state, the position at which the ejection wave is reflected can be adjusted. Therefore, the waveform of the pulse wave measured in the non-avascularized state is less affected by the length, which is different for each subject, from the measurement portion to the position at which the ejection wave is reflected. Therefore, the index can be more accurately calculated, and the index useful for determining the degree of arteriosclerosis can be obtained. 
     In the above example, an upper arm is the measurement portion, and the upper arm is attached with the arm band including the air bladder for avascularization of only the wrist corresponding to the position away from the upper arm by the predetermined length. Alternatively, when, for example, a plurality of reflection positions at the peripheral side are expected due to different measurement portions, a plurality of arm bands including respective air bladders for avascularization may be attached. In this manner, the index can be more accurately calculated. 
     In the above example, the measurement device  1 C includes the air bladder  13 C in addition to the configuration of the measurement device  1 A. However, the measurement device  1 C may include the air bladder  13 C in addition to the configuration of the measurement device  1 B. In this case, when the pressure P 2  in the air bladder  13 A becomes lower than the systolic blood pressure (NO in step S 111 ) or when the pulse wave is measured during the pressure increasing process in step S 104 , the pressure P 3  in the air bladder  13 C is increased to a pressure higher than at least the systolic blood pressure, whereby the position away from the measurement portion by the predetermined length is avascularized. 
     It is to be understood that the embodiments disclosed herein are examples in all respects and are not restrictive. While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. It is to be understood that the scope of the present invention is defined not by the above descriptions, but by the claims, and includes meanings equivalent to the claims and all the modifications and variations within the scope. 
     DESCRIPTION OF SYMBOLS 
       1 A,  1 B,  1 C Measurement device 
       2  Base body 
       3  Operation unit 
       4  Display unit 
       8 ,  9  Arm band 
       10  Air tube 
       13 A,  13 B,  13 C Air bladder 
       20 A,  20 B,  20 C Air system 
       21 A,  21 B,  21 C Air pump 
       22 A,  22 B,  22 C Air valve 
       23 A,  23 B,  23 C Pressure sensor 
       26 A,  26 B,  26 C,  27 A,  27 B,  27 C,  53  Drive circuit 
       28 A,  28 B,  28 C Amplifier 
       29 A,  29 B,  29 C ND converter 
       31 ,  32  Switch 
       40  CPU 
       41  Memory 
       51  Two-port valve 
       100  Upper arm