Abstract:
A pulse wave measuring apparatus can determine an optimum combination of a pair of light-emitting element and light-receiving element out of plural light-emitting elements and plural light-receiving elements on a real time basis according to a difference of signal intensity of a pulse wave signal from the light-receiving element and always measure an stable pulse wave of a wrist artery robustly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-100131, filed on 30 Mar. 2005, the entire contents of which are incorporated herein by reference. 
   TECHNICAL FIELD 
   The invention relates to a pulse wave measuring apparatus and a method therefor that are used for the purpose of autonomous nervous system measurement, sleep state measurement, health care, and the like. 
   BACKGROUND OF THE INVENTION 
   Heart beats representing various states of a human are used as useful indicators in various fields such as health care, sleep state measurement, and medical treatment. There are mainly two methods for measuring heartbeats. One is a method using an electrocardiogram obtained from an electrode between two points flanking the heart and a reference electrode. The other is a method of capturing pulse waves due to blood flows through blood capillaries or the like synchronizing with the heartbeats. 
   It is difficult for ordinary healthy people to use the method using an electrocardiogram in their daily lives. 
   The pulse wave measuring method mainly includes a reflection type method and a transmission type method. Both the reflection type method and the transmission type method use a difference of light absorbing characteristics of intravascular substances. In the reflection type method, light-emitting elements and light-receiving elements are arranged side by side on a surface of an organism. Light is irradiated on peripheral blood vessels and an amount of reflected light is captured by the light-receiving elements (see, for example, JP-A-2000-107147 and JP-A-2002-360530). In the transmission type method, light-emitting elements and light-receiving elements are arranged to sandwich an organism to capture an amount of light transmitted through a blood vessel with the light-receiving elements. 
   Pulse waves are measured for various purposes such as autonomous nervous system measurement, prevention of life-style related diseases, and sleep state measurement. There is an advantage that pulse waves can be measured easily. However, since a measurement site is a fingertip, an earlobe, a wrist, or the like, the measurement of pulse waves is strongly affected by movement and is susceptible to disturbances such as movement in daily lives. 
   Therefore, the invention provides a pulse wave measuring apparatus and a method therefor that copes with a positional deviation of a sensor module and a change in a position of an artery on a real time basis and is robust against disturbances. 
   BRIEF SUMMARY OF THE INVENTION 
   According to an embodiment of the invention, there is provided a measuring apparatus for detecting a pulse wave signal indicating a change in a blood flow in the blood vessel of a patient by using light, comprising: a sensor module including a plurality of light-emitting elements to irradiate the blood vessel, and a plurality of light-receiving elements to receive reflected light as a pulse wave signal from the blood vessel, each element being attached on the surface of the patient; a processor for light emission that causes the plural light-emitting elements to emit light one after another; an autocorrelation value calculating processor that calculates autocorrelation values of respective pulse wave signals corresponding to respective combinations of the light-emitting elements, which have emitted light, and the light-receiving elements, which have received light, respectively; and an optimum position identifying processor that selects a combination of a light-emitting element and a light-receiving element, which has outputted a pulse wave signal with a highest autocorrelation value among the respective autocorrelation values, as an optimum combination. 
   According to the embodiment of the invention, it is possible to select a combination of the plural light-receiving elements and the plural light-emitting elements, which are arranged on the surface of the organism of the subject, on a real time basis and always measure stable robust pulse waves. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a block diagram of a pulse wave measuring apparatus according to a first embodiment of the invention; 
       FIG. 2  is a diagram of a first example of arrangement of light-emitting elements and light-receiving elements in a sensor module of the pulse wave measuring apparatus; 
       FIG. 3  is a diagram of a second example of the arrangement of light-emitting elements and light-receiving elements in the sensor module of the pulse wave measuring apparatus; 
       FIG. 4  is a diagram of a third example of the arrangement of light-emitting elements and light-receiving elements in the sensor module of the pulse wave measuring apparatus; 
       FIG. 5  is a perspective view showing an optimum attaching site of the pulse wave measuring apparatus; 
       FIG. 6  is a flowchart of the pulse wave measuring apparatus; 
       FIG. 7  is a diagram of an optimum combination of a light-emitting element and a light-receiving element in the pulse wave measuring apparatus; 
       FIG. 8  is a sectional view showing positions of a sensor module and an artery before a change of an artery position in a second embodiment of the invention; 
       FIG. 9  is a diagram of the positions of the sensor module and the artery before the change of the artery position viewed from a surface of an organism in the second embodiment; 
       FIG. 10  is a sectional view showing positions of the sensor module and the artery after the change of the artery position in the second embodiment; 
       FIG. 11  is a diagram of the positions of the sensor module and the artery after the change of the artery position viewed from the surface of the organism in the second embodiment; 
       FIG. 12  is a flowchart of a pulse wave measuring apparatus according to a third embodiment of the invention; 
       FIG. 13  is a diagram showing a difference of changes in a pulse wave amplitude in a fourth embodiment of the invention; 
       FIG. 14  is a diagram of an attaching position of a sensor module in a fifth embodiment of the invention; 
       FIG. 15  is a diagram of an attaching position of a sensor module in a sixth embodiment of the invention; and 
       FIG. 16  is a diagram showing a shape of the sensor module in the sixth embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the invention will be explained with reference to the accompanying drawings. 
   FIRST EMBODIMENT 
   A pulse wave measuring apparatus  10  in a first embodiment of the invention will be hereinafter explained with reference to  FIGS. 1 to 7 . 
   (1) Structure of the Pulse Wave Measuring Apparatus  10   
     FIG. 1  is a block diagram showing a structure of the pulse wave measuring apparatus  10 .  FIGS. 2 ,  3 , and  4  are diagrams showing example of arrangement of plural light-emitting elements L and plural light-receiving elements P in a sensor module  11 . As shown in  FIG. 1 , the pulse wave measuring apparatus  10  amplifies a pulse wave signal, which is obtained from the sensor module  11 , with amplifiers  12  to  14  and, then, converts the pulse wave signal into a digital signal with an A/D converter  15 . Thereafter, the pulse wave measuring apparatus  10  processes the digital signal with a CPU  15  and displays a result of the processing on a display  17 . Electric power for the pulse wave measuring device  10  is supplied from a battery  18 . Infrared LEDs are used for the light-emitting elements L of the sensor module  11  and photodiodes are used for the light-receiving elements P. Note that a program for realizing a processing method for measurement of a pulse wave described below is stored in the pulse wave measuring device  10  and is processed by the CPU  15 . 
   A structure of the sensor module  11  will be explained. The plural light-emitting elements L and the plural light-receiving elements P are provided on an array substrate  9  of the sensor module  11 . The light-emitting elements L and the light-receiving elements P are attached to a wrist or the like. 
     FIG. 2  is a diagram of a first example of arrangement of the light-emitting elements L and the light-receiving elements P on the array substrate  9 . This is standard arrangement of the light-emitting elements L and the light-receiving elements P in the sensor module  11 . Light-emitting elements L 1  to L 5  are arranged one after another in a row in the horizontal direction and light-receiving elements P 1  to P 5  are arranged one after another in a row in the horizontal direction in parallel to the light-emitting elements L 1  to L 5 . The light-emitting element L 1  and the light-receiving element P 1  are in a one-to-one relation. The same holds true for the other light-emitting elements L 2  to L 5  and the other light-receiving elements P 2  to P 5 . When the sensor module  11  is attached to a wrist  19 , the plural light-emitting elements L and the plural light-receiving elements P are wound along the periphery of the wrist  19 . 
     FIG. 3  is a diagram of a second example of the attachment, in which two light-emitting elements L are associated with one light-receiving element P. The light-emitting elements L 1  to L 5  are arranged one after another in a row in the horizontal direction and the light-receiving elements P 1  to P 5  are arranged one after another in a row in the horizontal direction such that the light-emitting elements L deviate in the horizontal direction with respect to the light-receiving elements P. With the arrangement shown in  FIG. 3 , the light emitting elements L and the light-receiving elements P are excellent in detection of a position when the sensor module  11  is attached. 
     FIG. 4  is a third example of the arrangement, in which the arrangement in  FIG. 3  is further expanded. The light-emitting elements L and the light-receiving elements P are also arranged in the vertical direction to aim at further improvement of detection accuracy. The light-emitting elements L and the light-receiving elements P are arranged alternately in a row in the horizontal direction. The row in the horizontal direction, in which the light-emitting elements L and the light-receiving elements P are arranged in this way, are arranged in plural stages in the vertical direction. 
   (2) Processing Method of the Pulse Wave Measuring Apparatus  10   
   A processing method of the pulse wave measuring apparatus  10  will be explained using the sensor module  11  shown in  FIG. 2  with reference to a flowchart in  FIG. 6 . 
   First, the sensor module  11  is attached on an artery  20  in a wrist  19  of a patient as shown in  FIG. 5 . 
   In step  1 , the pulse wave measuring apparatus  10  sets processing to be performed for all combinations of the light-emitting elements L and the light-receiving elements P. 
   In step  2 , the pulse wave measuring apparatus  10  causes the light-emitting element L 1  to emit light. 
   In step  3 , the pulse wave measuring apparatus  10  receives reflection intensity of the light from the light-emitting element L 1  in the light-receiving elements P 1  to P 5  and stores a pulse wave signal of a light-receiving element with strongest signal intensity. For example, the pulse wave signal is assumed to be a pulse wave signal Y(t) of the light-receiving element P 3 . 
   In step  4 , the pulse wave measuring device  10  slices a pulse waveform for one heart beat that is obtained when the pulse wave signal Y(t) is measured for a fixed period. The fixed period is 1.0 to 1.5 seconds. 
   In step  5 , the pulse wave measuring apparatus  10  compares a sliced pulse waveform H for one heart beat and the pulse wave signal Y(t), which is continuously inputted, to obtain an autocorrelation S(t). For example, the autocorrelation S(t) is equal to or higher than −1 and equal to or lower than 1 . The autocorrelation S(t) is the highest at 1. 
   In step  6 , the pulse wave measuring apparatus  10  calculates an autocorrelation value S 0  that is a value obtained by averaging the autocorrelation S(t) in a predetermined time (e.g., five seconds). If a value of the autocorrelation S(t) in a fixed time is constant, the autocorrelation value S 0  is high (e.g., S 0 &gt;0.8). A signal intensity of a pulse wave signal from the light-receiving element is stably high and it can be judged that the pulse wave signal is a pulse wave. If the signal intensity is unstable, it can be judged that the pulse wave signal is noise. 
   In step  7 , the calculated autocorrelation value S 0  is an autocorrelation value measured for the first time, the pulse wave measuring apparatus  10  proceeds to step  8 . If the calculated autocorrelation value S 0  is the autocorrelation value S 0  calculated for the second or subsequent time, the pulse wave measuring apparatus  10  proceeds to step  9 . In other words, when combinations of the light-emitting element L 1  and the light-receiving elements P 1  to P 5  are processed first, the autocorrelation value S 0  highest in the combinations is set as the autocorrelation value S 0  at the first time. 
   In step  8 , the pulse wave measuring apparatus  10  stores the calculated autocorrelation value S 0  at the first time as an initial value. 
   In step  9 , if the calculated autocorrelation value S 0  is high compared with the initial value stored in step  8  in advance or the autocorrelation value S 0  updated in processing in step  9  of the last time, the pulse wave measuring apparatus  10  updates the initial value or the updated autocorrelation value S 0  to the calculated autocorrelation value S 0  and stores the calculated autocorrelation value S 0 . If the calculated autocorrelation value S 0  is low compared with the initial value stored in step  8  in advance or the autocorrelation value S 0  updated in processing in step  9  of the last time, the pulse wave measuring apparatus  10  does not update the initial value or the updated autocorrelation value S 0 . 
   In step S 10 , the pulse wave measuring apparatus  10  repeats the processing from step  1  until processing for all combinations of the light-emitting elements L and the light-receiving elements P ends. When the processing for all the combinations ends, the pulse wave measuring apparatus  10  proceeds to step  11 . 
   In step  11 , the pulse wave measuring apparatus  10  determines a combination of the light-emitting element L and the light-receiving element P having the highest autocorrelation value. For example, when it is assumed that the combination is a combination of the light-emitting element L 4  and the light-receiving element P 1 , the combination is as shown in  FIG. 7 . 
   With the processing described above, although the pulse wave measuring apparatus  10  in this embodiment has the plural light-emitting elements L and the plural light-receiving elements P, the pulse wave measuring apparatus  10  can identify an optimum position. Thus, it is possible to realize the wave pulse measuring apparatus that copes with a positional deviation of the sensor module  11  and a change in a position of the artery  20  on a real time basis and is robust against disturbances. 
   SECOND EMBODIMENT 
   A second embodiment of the invention will be explained with reference to  FIGS. 8 to 11 . 
   In this embodiment, a method of selecting an optimum combination of the light-emitting element L and the light-receiving element P, which occurs when a position of the blood vessel (the artery)  20  changes in the pulse wave measuring device  10 , will be explained. 
     FIGS. 8 and 10  are sectional views showing positions of the sensor module  11  and the artery  20  of the wrist  19 .  FIGS. 9 and 11  are schematic diagrams of the sensor module  11  and the artery  20  viewed from a surface of an organism. In  FIGS. 9 and 11 , positions of the sensor module  11  and the artery  20  before and after a change of a position of the artery  20  are shown, respectively. 
   As a characteristic of the light-emitting elements L used in the sensor module  11  in this embodiment, when light is made incident in an organism and the light is reflected to return to the outside of the organism, a distance between the light-emitting elements L and the light-receiving elements P is proportional to a depth of the light made incident in the organism. As the distance of the light-emitting elements L and the light-receiving elements P are larger, the depth of the light made incident in the organism detected by the light-receiving elements P is larger. 
   In  FIGS. 8 and 9 , the artery  20  is close to the surface of the organism. Since the artery  20  is close to the surface of the organism, an interval between the light-emitting elements L and the light-receiving elements P is preferably small. A combination of the light-emitting element L 4  and the light-receiving element P 2  shown in  FIG. 9  is appropriate. 
   In  FIGS. 10 and 11 , the artery  20  is far from the surface of the organism. Since the artery  20  is far from the surface of the organism, an interval between the light-emitting elements L and the light-receiving elements P is preferably large. A combination of the light-emitting element L 5  and the light-receiving element P 1  shown in  FIG. 11  is appropriate. 
   THIRD EMBODIMENT 
   A third embodiment of the invention will be explained with reference to  FIG. 12 . 
   In this embodiment, assuming that an optimum combination of a pair of the light-emitting element L and the light-receiving element P has already been acquired by the method explained in the first embodiment, a method of causing the light-emitting elements P to emit light selectively in selecting an optimum combination again will be explained. 
   When a currently selected combination of a pair of the light-emitting element L and the light-receiving element P is as shown in  FIG. 2 , if intensity of a pulse wave signal weakens in the combination because of influence of movement, the pulse wave measuring apparatus  10  tries to search for an optimum combination again. 
   In this search, the pulse wave measuring apparatus does not cause the light-emitting elements L to emit light in order from the light-emitting element L 1 . Instead, as indicated by steps  20  to  22  in a flowchart in  FIG. 12 , the pulse wave measuring apparatus  10  causes the light-emitting elements L to emit light in order from the light-emitting element L near the currently selected light-emitting element L. This makes it possible to select an optimum combination by performing light emission as a small number of times as possible. In that case, as indicated by step  21 , if a pulse wave signal has intensity equal to or higher than a certain threshold value, which allows the pulse wave signal to be used as a pulse wave, the pulse wave measuring apparatus  10  stops the search for an optimum combination at that time. Note that steps  23  to  32  are the same as the processing in steps  2  to  11 . 
   FOURTH EMBODIMENT 
   A fourth embodiment of the invention will be explained with reference to  FIG. 13 . 
   Apart from a combination of the light-emitting element L and the light-receiving element P with a high autocorrelation value calculated according to autocorrelation, intensity of a pulse wave signal may be equal to or lower than a predetermined threshold value. 
   In this embodiment, as shown in  FIG. 13 , when an amplitude of the pulse wave signal Y(t) falls to be lower than the predetermined threshold value, the pulse wave measuring apparatus  10  detects a combination of the light-emitting element Land the light-receiving element P, which has the pulse wave signal Y(t) with signal intensity equal to or higher than the threshold value, again. 
   Consequently, it is possible to realize robust pulse wave measurement. 
   FIFTH EMBODIMENT 
   A fifth embodiment of the invention will be explained with reference to  FIG. 14 . 
   In the first embodiment, the sensor module  11  is attached to a site near a radial artery or an ulnar artery of the wrist  19 . However, in this embodiment, as shown in  FIG. 14 , the same effect as the first embodiment is obtained by attaching the sensor module  11  to a carotid artery of a neck  21 . 
   SIXTH EMBODIMENT 
   A sixth embodiment of the invention will be explained with reference to  FIGS. 15 and 16 . 
   In this embodiment, unlike the first and the sixth embodiments, the sensor module  11  is attached to a planta artery near an ankle  22  as shown in  FIG. 15 . 
   However, since a surface of the skin near the ankle  22  is extremely rough because of a melleolus, a pulse wave is detected by curving the sensor module  11  to stick firmly to the skin as shown in  FIG. 16 . 
   MODIFICATIONS 
   The invention is not limited to the embodiments described above and can be modified in various ways as long as modifications do not depart from the spirit of the invention. 
   The invention is suitably applied to measurement of a pulse wave for various purposes such as autonomous nervous system measurement, prevention of life-style related diseases, and sleep state measurement.