Ear type apparatus for measuring a bio signal and measuring method therefor

An apparatus for measuring a bio signal including a bio signal measurement unit, which is insertable into an ear to be in close contact with an internal surface of the ear, the bio signal measurement unit having a photo plethysmography (PPG) measurement module for radiating light of different wavelengths onto the internal surface of the ear, detecting light transmitted through the ear, and outputting a PPG signal including bio information, a control unit having a PPG signal processor for generating the bio information using the PPG signal measured by the PPG measurement module, and an output unit for displaying the bio information generated from the control unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ear type apparatus for measuring a bio signal and a measuring method therefor. More particularly, the present invention relates to an ear type apparatus for measuring a bio signal, such as temperature, respiration, pulse, and oxygen saturation, which can minimize a motion artifact caused by a subject's motion, and a measuring method therefor.

2. Description of the Related Art

When a human body is in an abnormal state, various changes may occur such as an increase in blood pressure, an increase in pulse rate, an increase in body temperature, or a change in an electric potential occurring during a heartbeat, which may be measured by an electrocardiogram. Among these changes, the increase in body temperature is the most representative sign of an abnormal state of a human body and is thus generally measured during a patient diagnosis in hospitals or general medical institutions. Conventionally, body temperature is measured using a mercury thermometer. Recently, various ear type thermometers for measuring a body temperature, i.e., inner body temperature without influence from external temperature, have been developed. In operation, such an ear type thermometer detects an amount of infrared rays emitted from an eardrum at an internal body temperature and converts the detected amount of infrared rays into a temperature value. The ear type thermometer is advantageous in that a measurement time is short and the body temperature can be conveniently measured by inserting the ear type thermometer into an ear.

A pulse indicates a dynamic extension of an artery that can be felt by a finger. Since the dynamic extension of an artery is due to a contraction of the heart, a heart rate, i.e., a heart's contraction rate, can be inferred from a pulse rate. When a human body is infected by a disease, the pulse rate, rhythm, or strength changes even when the human body is in a stable status. Accordingly, a person's state of health can be checked by measuring the pulse rate, rhythm, or strength.

Further, oxygen saturation indicates an amount of arterial blood (SpO2) in which oxygen is saturated. Oxygen saturation is measured to test a pulmonary function, estimate a concentration of oxygen in blood during oxygen therapy at home, or diagnose asthma and pulmonary emphysema. Human respiration is a process of discharging waste gas, i.e., carbonic acid gas, from a human body and providing oxygen to the human body. A human lung accommodates air coming from outside, emits carbonic acid gas, and absorbs oxygen. A pulmonary artery discharges carbonic acid gas collected throughout the human body through pulmonary alveoli using a difference in air pressure during exhalation. Conversely, blood in a pulmonary vein absorbs oxygen from inhaled air and then circulates to the heart. When respiration is unstable, a supply of oxygen is interrupted, which deteriorates the functions of a body's organs. In particular, oxygen saturation directly relates to an amount of oxygen supplied to the organs and thus provides very useful information regarding metabolism.

FIG. 1shows an example of a conventional ear type thermometer for measuring body temperature. The ear type thermometer shown inFIG. 1includes a housing150having a probe110through which infrared rays pass, a light receiver120that receives infrared rays emitted from at least one area from among a human eardrum and peripheral areas of the eardrum through the probe110, a signal processor130that calculates a temperature from an output of the light receiver120, and a display/sound unit140that displays the temperature.

The light receiver120includes a condenser device, which condenses infrared rays passing through the probe110, and an infrared receiver device, which is disposed to receive the infrared rays condensed by the condenser device to receive infrared rays emitted from at least one area from among the eardrum and the peripheral areas of the eardrum.

Disadvantageously, the conventional ear type thermometer shown inFIG. 1is a separate device that has to be additionally carried by a user. Moreover, a tip of the probe110of the thermometer needs to be in close contact with an internal surface of a subject's ear in order to accurately measure the subject's body temperature. However, when another person measures a subject's body temperature, the contact between the thermometer and the internal surface of the ear cannot be adjusted effectively. Although the subject can directly adjust the contact when measuring his own body temperature, the subject must remove the thermometer from the ear to view the display unit to check a measured value and verify whether the measurement has been accurately performed. Accordingly, this thermometer is not appropriate for self-diagnosis and is thus usually used when another person measures a subject's body temperature.

In order to apply such a conventional ear type thermometer to a remote medical treatment, since a measured value needs to be transmitted via a separate transmission apparatus, an interface is required. Accordingly, it is difficult to monitor results of the measurement frequently or for an extended period of time.

FIG. 2shows an example of a conventional mobile apparatus that is capable of measuring a bio signal. The exemplary mobile apparatus shown inFIG. 2is a portable communication terminal, which allows a function of a heart to be diagnosed or obesity to be tested based on a heart rate and a body fat rate, which are detected from a user's body. This apparatus eliminates an inconvenience of carrying a separate apparatus solely for measuring bio information. Electrodes2a,2b,2c, and2dare attached to an outer surface of a mobile communication terminal in order to measure a user's bio information.

FIG. 3is a block diagram of the conventional mobile apparatus shown inFIG. 2. A portable communication terminal300includes a communication terminal module320and a bio-information measurement module310to provide dual functionality of voice communication and bio information measurement. The communication terminal module320includes a transceiver326as a user interface unit, a display unit321, such as a liquid crystal display (LCD), allowing communication of character information, and an input unit322such as a keypad. The input unit322is used by a user to operate or control the portable communication terminal300. Communication of information can be implemented by wireless transmission and reception of data via a wireless communication unit323. A memory unit324stores information regarding the user of the portable communication terminal300and data necessary for the operation of the central controller325.

The bio-information measurement module310includes a body fat measurer311and a heart rate measurer312. An interface unit313performs data interface between the portable communication terminal300and an external electronic apparatus, for example, a removable bio-information measurement module.

FIG. 4is a detailed block diagram of the heart rate measurer312. The heart rate measurer312includes a voltage generator401, electrodes402, an amplifier403, a pulse shaper404, a pulse counter405, and an interface unit406. When the electrodes402of the voltage generator401, which are attached to a main body of the portable communication terminal300, are in close contact with a part of a subject's body, for example, right and left hands, a voltage change signal due to the heart's beat is detected. The voltage change signal is amplified by the amplifier403, for example, a differential amplifier. The amplified voltage change signal is converted to a pulse signal by the pulse shaper404. The pulse signal is counted by the pulse counter405to obtain a heart rate. An output signal of the pulse counter405is a digital signal and is input to the interface unit406. The central controller (325ofFIG. 3) displays the heart rate on the display unit321and transmits it through the wireless communication unit323. Voltage measurement electrodes used to measure body fat in the body fat measurer311are also used as the electrodes402.

Disadvantageously, such a conventional portable communication terminal for measuring bio information using electrodes is influenced by a motion artifact caused by a force pressing the electrodes and is sensitive to contamination of the electrodes or the skin since the electrodes directly contact the skin. When the electrodes are exposed outside the communication terminal, they are easily damaged or contaminated.

To obtain bio information, such as oxygen saturation, a component in blood needs to be detected. Accordingly, a method of applying signals showing different characteristics according to concentrations of oxidized hemoglobin and reduced hemoglobin and obtaining the bio information using a difference between the signals is usually used. In conventional methods, however, since one electrode cannot apply different types of signals, bio information beyond a pulse rate cannot be appropriately measured.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for measuring a bio signal, which is convenient to carry, can be adjusted to be correctly positioned at a body part to be measured by a subject himself, and can transmit measured bio information without requiring a separate transmitter, thereby facilitating long-term monitoring. In addition, the apparatus can obtain pulse and respiration information and simultaneously measure oxygen saturation using changes in absorptance of light having at least two different wavelengths. The present invention further provides a method for measuring a bio signal.

According to a feature of an embodiment of the present invention, there is provided an apparatus for measuring a bio signal including a bio signal measurement unit, which is insertable into an ear to be in close contact with an internal surface of the ear, the bio signal measurement unit having a photo plethysmography (PPG) measurement module for radiating light of different wavelengths onto the internal surface of the ear, detecting light transmitted through the ear, and outputting a PPG signal including bio information, a control unit having a PPG signal processor for generating the bio information using the PPG signal measured by the PPG measurement module, and an output unit for displaying the bio information generated from the control unit.

According to another feature of an embodiment of the present invention, there is provided an apparatus for measuring a bio signal including a bio signal measurement unit, which is insertable into an ear to be in close contact with an internal surface of the ear, the bio signal measurement unit having a photo plethysmography (PPG) measurement module for radiating light of different wavelengths onto the internal surface of the ear, detecting light transmitted through the ear, and outputting a PPG signal including bio information, and further having a plurality of electrodes for outputting the PPG signal, an earphone having a speaker for outputting sound and a plurality of electrodes on an outer surface to be connected to the plurality of electrodes of the bio signal measurement unit to receive the PPG signal output from the bio signal measurement unit, a control unit having a PPG signal processor for receiving the PPG signal through the electrodes of the earphone and generating bio information using the PPG signal and a sound processor for outputting a sound signal to the earphone, and an output unit for displaying the bio information generated from the control unit.

Preferably, the PPG measurement module includes a light source unit for radiating light onto the internal surface of the ear and a photodetector for detecting light radiated onto the internal surface of the ear and then transmitted through the ear. The light source unit may include a first light source for radiating light of a first wavelength onto the internal surface of the ear, and a second light source for radiating light of a second wavelength onto the internal surface of the ear, wherein the first and second wavelengths are different.

Preferably, the PPG signal processor includes a peak detector for detecting peaks of the PPG signal and a signal processor for generating the bio information using values of the peaks. The signal processor may include a pulse detector for calculating a time interval between the peaks to measure a pulse rate. The signal processor may include a respiration detector for band-pass filtering the PPG signal to measure a respiration frequency. The signal processor may include a reflection coefficient detector for detecting an AC component and a DC component from each of PPG signals detected at different wavelengths and measuring reflection coefficients and an oxygen saturation detector for detecting oxygen saturation in blood using a ratio between the reflection coefficients of the different wavelengths.

The PPG signal processor may further include an amplifier for amplifying the PPG signal and a filter for removing noise components from the PPG signal amplified by the amplifier and then outputting the PPG signal to the peak detector.

Preferably, the bio signal measurement unit further includes a temperature measurement module for sensing infrared rays radiated from a body and outputting an electrical signal corresponding to the sensed infrared rays, and wherein the control unit further includes a temperature processor for calculating a body temperature using the electrical signal output from the temperature measurement module. The temperature measurement module may include a waveguide installed near an eardrum for guiding infrared rays radiated from the eardrum and a light receiver sensing the infrared rays guided by the waveguide and converting the infrared rays to the electrical signal. The waveguide may be made of a material that can reflect infrared rays. The temperature processor may include an amplifier for amplifying the electrical signal received from the temperature measurement module, a filter for removing noise from the amplified electrical signal, and an analog-to-digital converter for converting the electrical signal to a digital signal.

The output unit may be a liquid crystal display apparatus. Further, the output unit may be a liquid crystal display apparatus of a mobile communication terminal or a compact disc player.

The bio information unit may further include a mobile communication terminal through which the bio information generated from the control unit is wirelessly transmitted to a predetermined medical institution.

According to still another feature of an embodiment of the present invention, there is provided a method of measuring a bio signal using an ear type bio signal measurement apparatus including a bio signal measurement unit, which is insertable into an ear to measure a bio signal, a control unit for generating bio information using the measured bio signal, and an output unit for outputting the bio information, the method including (a) receiving infrared rays radiated from an eardrum and measuring a body temperature using the bio signal measurement unit, (b) radiating light having different wavelengths onto an internal surface of an ear, which is in close contact with the bio signal measurement unit, to measure a photo plethysmography (PPG) signal including bio information and measuring at least one bio signal from among the group consisting of oxygen saturation, a pulse rate, and a respiration frequency, using the PPG signal, and (c) outputting the at least one bio signal measured in (a) and (b), wherein (a) and (b) are simultaneously performed.

Preferably, (b) includes (b1) radiating the light having the different wavelengths onto the internal surface of the ear, receiving the light transmitted through the ear, and outputting the PPG signal, using a PPG measurement module included in the bio signal measurement unit having a side thereof in close contact with the internal surface of the ear; (b2) detecting peaks of the PPG signal; and (b3) generating bio information using the detected peaks.

Preferably, (b3) includes detecting an AC component and a DC component from each of PPG signals detected at the different wavelengths and measuring reflection coefficients of the different wavelengths, and calculating oxygen saturation in blood using a ratio between the reflection coefficients of the different wavelengths. Preferably, (b3) includes band-pass filtering the PPG signal to detect a respiration frequency. In addition, (b2) may include band-pass filtering the PPG signal collected for a predetermined period of time, detecting an inflection point by differentiating the filtered PPG signal, and storing the inflection point as a peak when the inflection point has a value exceeding a predetermined threshold value.

Preferably, (b3) may include measuring a pulse rate using a time interval between peaks of the PPG signal. The output unit may be a liquid crystal display apparatus of a mobile communication terminal, and (c) may further include wirelessly transmitting the bio signals measured in (a) and (b) to a predetermined medical institution through the mobile communication terminal.

According to yet another feature of an embodiment of the present invention, there is provided a recording medium having recorded therein a program for executing the above-described method in a computer.

DETAILED DESCRIPTION OF THE INVENTION

Korean Patent Application No. 2003-29365, filed on May 9, 2003, and entitled: “Ear Type Apparatus for Measuring a Bio Signal and Measuring Method Therefor,” is incorporated by reference herein in its entirety.

FIG. 5Ais block diagram of an apparatus for measuring a bio signal according to a first embodiment of the present invention.FIG. 5Bshows an example in which an apparatus for measuring a bio signal according to the first embodiment of the present invention is applied to a mobile apparatus.

Referring toFIG. 5A, the apparatus according to the first embodiment of the present invention includes a bio signal measurement unit500, which is insertable into an ear to measure a bio signal; a control unit550, which calculates bio information using the bio signal measured by the bio signal measurement unit500; and a display unit590, which displays the bio information on a screen for a user. The bio signal measurement unit500includes a temperature measurement module510, which measures body temperature using infrared rays radiated from an internal surface of an ear, and a photo plethysmography (PPG) measurement module520, which is installed on an outer surface of the bio signal measurement unit500to closely contact the internal surface of the ear and measure a PPG signal.

Referring toFIGS. 5A and 5B, the bio signal measurement unit500can be easily inserted into the ear due to its shape. The PPG measurement module520is installed at the outer surface of the bio signal measurement unit500to closely contact the ear surface. The temperature measurement module510is installed in the bio signal measurement unit500at a position that will be in relatively close proximity to an eardrum. The bio signal measurement unit500may have a same shape as an earphone530, as shown inFIG. 5B. However, since it is preferable that the temperature measurement module510is positioned near the eardrum so that it can effectively sense infrared rays radiated from the eardrum, it is preferable to shape the bio signal measurement unit500as a conical frustum and to dispose the temperature measurement module510at a top of the conical frustum-shaped bio signal measurement unit500. The temperature measurement module510includes a waveguide511guiding infrared rays near the eardrum to the bio signal measurement unit500and a light receiver513implemented by an infrared sensor to sense the infrared rays input through the waveguide511.

For the display unit590, a separate display apparatus or a display apparatus included in an existing apparatus can be used. In the example shown inFIG. 5B, a mobile apparatus is used as the display unit590. The display unit590may be implemented by a liquid crystal display (LCD) of a mobile communication terminal (as shown inFIG. 5B), a personal digital assistant (PDA), a compact disc player, or the like. When a mobile communication terminal is used, bio information can be transmitted to a predetermined medical institution, so that remote examination can be performed. Hereinafter, it is assumed that a mobile apparatus is used for the display unit590.

InFIG. 5B, the control unit550is shown separate from the bio signal measurement unit500. The control unit550calculates bio information using a signal received from the bio signal measurement unit500and outputs the bio information to the display unit590. When a mobile apparatus is used for the display unit590, the control unit550can be installed within the mobile apparatus. When the control unit550is separately installed outside the mobile apparatus, it can be provided with a jack, which can be connected to the earphone530, as shown inFIG. 5B, so that the control unit550controls a sound signal output from the mobile apparatus and outputs the sound signal to the earphone530.

FIG. 6is a block diagram showing the control unit550shown inFIG. 5A. The control unit550includes a temperature processor570, which converts a signal detected by the infrared sensor of the temperature measurement module510to a temperature value; a PPG signal processor580, which generates measurement values of a pulse rate, a respiration frequency, and oxygen saturation using the PPG signal measured by the PPG measurement module520; and a transmitter565, which selectively transmits an output signal from the temperature processor570and an output signal from the PPG signal processor580to the mobile apparatus according to a selection signal of the mobile apparatus used for the display unit590. When the earphone530, which can output a sound signal from the mobile apparatus, is connected to the control unit550, the control unit550further includes a sound processor560, which receives a voice signal through a microphone535, outputs a voice signal from the mobile apparatus through a speaker537, and adjusts the volume of the output voice signal. Meanwhile, it will be apparent that the signals of the sound processor560, the temperature processor570, and the PPG signal processor may be directly input to the mobile apparatus, and a control unit (not shown) included in the mobile apparatus including the display unit590may selectively output the signals.

FIG. 7is a detailed block diagram showing the temperature processor570shown inFIG. 6. The temperature processor570includes an amplifier571, which amplifiers a signal output from the temperature measurement module510; a filter572, which removes noise components from the amplified signal; and an analog-to-digital (A/D) converter573, which converts the filtered signal to a digital signal and transmits the digital signal to the transmitter565.

FIG. 8is a detailed block diagram showing the PPG measurement module520and the PPG signal processor580shown inFIG. 6. The PPG measurement module520includes a first light source, which radiates light onto a body part, i.e., an internal surface of an ear closely contacting the bio signal measurement unit500, at which a bio signal is to be measured; a second light source, which radiates light having a different wavelength than the light of the first light source onto the body part; and a photodetector, which detects light that has been radiated from the first and second light sources and then transmitted through and reflected from the body part with bio information. The PPG signal processor580includes an amplifier581, which amplifies a signal output from the photodetector; a filter583, which removes noise components from an output signal of the amplifier581; a peak detector585, which detects a peak from an output signal from the filter583; and a signal processor587, which calculates bio information using a peak value of a signal detected from the light from the first light source and a peak value of a signal detected from the light from the second light source and outputs the bio information to the display unit590.

FIG. 9is a detailed block diagram showing the signal processor587shown inFIG. 8. In order to measure a subject's pulse, the signal processor587includes a pulse detector910, which calculates a time interval between peaks detected by the peak detector585and measures a pulse based on the time interval.

In order to measure a subject's oxygen saturation, the signal processor587includes an alternating current (AC) detector920, which detects changes between maximum values and minimum values of a waveform output from the peak detector585to detect a light intensity variation due to pulsatile components of an artery; a direct current (DC) detector922, which detects the minimum values of the waveform output from the peak detector585to detect a light intensity due to non-pulsatile components; a reflection coefficient detector924, which calculates a reflection coefficient using a DC component and an AC component of a pulse wave; and a oxygen saturation detector926, which calculates oxygen saturation using the reflection coefficient.

In order to detect a subject's respiration frequency (rate), the signal processor587includes a band pass filter (BPF)930, which band pass filters a pulse signal received from the peak detector585, and a respiration detector935, which detects a respiration frequency using the band pass filtered pulse signal.

Hereinafter, a method of measuring a bio signal according to an embodiment of the present invention will be described with reference toFIGS. 10 through 15B.

Initially, a method of measuring temperature using the bio signal measurement unit500will be described. Since a temperature of human skin tissue varies at different body parts and rapidly changes depending on external temperature, it is important to select an appropriate body part for temperature measurement. Generally, a contact type thermometer is used at an armpit or the rectum, and a non-contact type thermometer is used in an ear canal near an eardrum. The temperature of the eardrum is medically known as being very close to internal body temperature and barely influenced by external temperature. The internal body temperature and radiant electromagnetic energy or infrared energy are related as follows.

A total amount of electromagnetic energy radiated from a black body is proportional to the fourth power of the temperature of the black body according to Stefan-Boltzmann's Law, as shown in Formula (1).
Q=σT4(1)

Here, Q represents a total amount of electromagnetic energy radiated from the black body, T denotes the temperature of the black body, and σ denotes a constant called the Stefan-Boltzmann constant. An amount of electromagnetic energy radiated from a body, such as a human body, that is not completely black is influenced by radiant components of the body. Such a body is referred to as a gray body. When the emissivity of the gray body is ω, Formula (1) is modified into Formula (2).
Q=ωσT4(2)

Here, the emissivity ω has a value between zero (0) and one (1). The emissivity ω of a human body in a far infrared band is almost one (1), exhibiting characteristics near to those of a black body. Accordingly, an absolute internal body temperature can be calculated using the total amount of infrared energy radiated from the internal body. In addition, a change in the infrared energy is proportional to the fourth power of a change in the internal body temperature.

FIG. 10is a graph of an intensity of radiant energy versus wavelength at different temperatures of a black body. Energy radiated from the black body at a constant temperature gradually increases as the wavelength increases and reaches a peak. Thereafter, when the wavelength further increases, the radiant energy decreases. A peak of such a characteristic curve changes as temperature changes, and a wavelength at which the peak occurs also varies with the temperature. As shown inFIG. 10, when the temperature is 1100 K, a peak occurs at a wavelength of about 2.5 μm. When the temperature decreases to 800 K, a peak occurs at a wavelength of about 3.8 μm and the intensity of radiant energy decreases. A wavelength λ giving maximum radiant energy at a particular temperature T is defined by Formula (3).
λ(max)=0.29/T(3)

Since targets of a non-contact type infrared thermometer generally have a temperature of about 30–40° C., the targets radiate far infrared rays in which a wavelength of about 8–12 μm provides maximum radiant energy. Accordingly, a photodetector for detecting the far infrared rays is required to have a satisfactory response characteristic in a band of about 8–12 μm.

A filter of a general sensor used in infrared thermometers needs to have a frequency response characteristic as shown inFIG. 11. More specifically, it is preferable that a response is great at a wavelength of about 6–16 μm, but almost constant transmission appears at the band of about 8–12 μm. Though a total amount of electromagnetic energy radiated from the black body is proportional to the fourth power of the temperature of the black body, when a measurement range is very narrow, for example, 30–40° C., as in a thermometer, the total amount of electromagnetic energy can be considered as being linear within the range of 30–40° C.FIG. 12is a graph of output voltage of the sensor used inFIG. 11versus temperature. As described above, a linear characteristic appears in a temperature range of 30–40° C.

Based on the above-described response of an infrared sensor, the operating principle of the temperature measurement module510of the present invention will be described with reference toFIGS. 5B and 7. As described above, the temperature measurement module510includes the waveguide511for collecting light and the light receiver513implemented by an infrared sensor. The waveguide511is disposed near an eardrum to collect radiant infrared rays. The waveguide511includes a material reflecting infrared rays therewithin to guide the collected infrared rays to the light receiver513. Then, the light receiver513installed within the bio signal measurement unit500detects the infrared rays and generates an electrical detection signal according to an amount of the infrared rays.

Since the electrical detection signal is too weak to be transmitted or digitized, the signal is amplified by the amplifier571. The amplified detection signal includes a plurality of noise components, but a signal component required for measurement of body temperature is a DC component appearing in a peak wavelength rather than an AC component changing over time. Accordingly, the amplified detection signal is filtered by the filter572to remove the noise and AC components. The filtered detection signal is converted to a digital value by the A/D converter573. The A/D converter573also converts the digital value to a temperature value to be displayed to a user.

However, when the display unit590is implemented by an LCD included in a mobile apparatus, such as a mobile communication terminal as shown inFIG. 5B, the A/D converter573simply converts an analog signal to a digital signal. The digital signal can be converted to a temperature value by an operation unit included in the mobile apparatus and then output to the user through the display unit590. In addition, it is obvious to those skilled in the art that the control unit550including the temperature processor570can be installed within the mobile apparatus including the display unit590so that the temperature measurement module510may be directly connected to the mobile apparatus.

When a user measures his own body temperature using a thermometer provided in the ear type bio signal measurement unit500, the user inserts the bio signal measurement unit500into his ear and monitors the display unit590. Thus, the user himself can take a measurement and check the results of the measurement. In addition, when re-measurement is required since the bio signal measurement unit500is not appropriately inserted into the ear, the user himself can adjust the insertion of the bio signal measurement unit500.

A method of measuring oxygen saturation according to the first embodiment of the present invention will now be described with reference toFIGS. 8,9and13. Oxygen saturation is a percentage of a concentration of oxidized hemoglobin in a concentration of the total hemoglobin, i.e., a quantification of an amount of oxygen with which blood is saturated in order to maintain the normal functions of human cells. Many methods of detecting oxygen saturation using light having at least two different wavelengths have been researched and developed. In a representative method of measuring oxygen saturation among them, red light and infrared light are radiated onto vital tissue, absorbance of pulsatile components in the arterial blood is obtained at each wavelength, and oxygen saturation is calculated using a ratio between absorbances at different wavelengths. Most light radiated onto a human body is absorbed by non-pulsatile components such as bones and tissue having a constant transmission path, and about 1–2% of the light is absorbed by the pulsatile components in the arterial blood. The amount of light absorbed by the pulsatile components and the amount of light absorbed by the non-pulsatile components can be obtained at each wavelength using the intensities of the light transmitted through the human body. A ratio between an amount of light absorbed by the non-pulsatile components and an amount of light absorbed by the pulsatile components at each of the two different wavelengths of the red light and the infrared light, respectively, indicates a light absorptance of hemoglobin in the arterial blood. The oxygen saturation is calculated from a ratio between the amounts of light absorbed by hemoglobin at the two different wavelengths. InFIG. 13, “Ip” and “Iv” denote a maximum point and a minimum point, respectively, in a pulsatile component (alternating current).

Referring toFIG. 8, which shows the PPG measurement module520and the PPG signal processor580, incident light from the first light source is transmitted through the body part. When the incident light passes through a path “a”, it encounters a blood vessel, in this case, an artery, and is modulated by pulsation. When the incident light passes through a path “b”, it is not influenced by pulsation. When a radius of the artery is “ra” and a radius of the body part is “rb”, the entire time-invariant component DC of light detected by the photodetector is composed of a time-invariant component DCaof the light passing through the path “a” and a time-invariant component DCbof the light passing through the path “b”, as shown inFIG. 13, and is expressed by Formula (4).
DC=DCa+DCb(4)

DCacan be expressed by Formula (5).
DCa=f(ra,rb,λ)DC(5)

Here, f(ra,rb,λ) is a constant denoting a factor changing according to the structure of the body part including an artery, and λ denotes a wavelength of the incident light. An intensity of light transmitted through the body part is modulated by as much as a variation ΔODtotof light attenuation ODtotby a change in an amount of blood due to pulsation of the artery. Here, the variation ΔODtotis for the light passing through the path “a” and can be expressed by Formula (6).
ΔODtot=AC/DCa=f−1(ra,rb,λ)AC/DC(6)

Since it is very difficult to accurately measure f(ra,rb,λ), reflection coefficients R1and R2for two wavelengths λ1and λ2are measured, and then a ratio R12=R1/R2is obtained, as shown in Formula (7), in order to calculate oxygen saturation without having to accurately measure f(ra,rb,λ).

Here, ACλ1and ACλ2denote time-variant components with respect to the first and second wavelengths λ1and λ2, and DCλ1and DCλ2denote time-invariant components with respect to the first and second wavelengths λ1and λ2. For example, Formula (7) can be obtained using a pulse oximeter.

Consequently, as shown in Formula (7), the reflection coefficient detector (924ofFIG. 9) divides the time-variant component ACλ1or ACλ2, which has been input from the photodetector through the peak detector585and detected by the AC detector920, by the time-invariant component DCλ1or DCλ2detected by the DC detector922to obtain a variation ΔODtot,λ1or ΔODtot,λ2of light attenuation at each wavelength, and divides the variation ΔODtot,λ1of attenuation of light from the first light source by the variation ΔODtot,λ2of attenuation of light from the second light source to obtain a ratio of the reflection coefficient of the first light source to the reflection coefficient of the second light source.

The oxygen saturation detector926calculates a concentration CHbof hemoglobin in blood using at least one ratio R12received from the reflection coefficient detector924. According to an embodiment of the present invention, when the first and second wavelengths λ1and λ2are selected, the hemoglobin concentration CHbis calculated using the ratio R12, as shown in Formula (8).

Here, ε1denotes an absorption coefficient with respect to the first wavelength λ1; ε2denotes an absorption coefficient with respect to the second wavelength λ2; k1and k2denote constants determined by the first and second wavelengths λ1and λ2and characteristics of scattering and absorbing incident light at a predetermined body part; and a1and a2denote constants determined by a size of a scattered particle, a refractive index of hemoglobin, a refractive index of serum, and the first and second wavelengths λ1and λ2.

The oxygen saturation detector926calculates oxygen saturation S using the measured hemoglobin concentration CHb, as shown in Formula (9), and outputs the oxygen saturation S to the display unit590.

Hereinafter, a procedure in which the oxygen saturation detector926detects oxygen saturation will be described. One wavelength λx is selected from among at least two wavelengths, and another wavelength λo having a maximum difference in an absorption coefficient according to a type of hemoglobin is selected.

The wavelengths λx and λo are derived based on bio spectroscopy. While some wavelengths can or cannot be well absorbed according to an amount of hemoglobin (Hb) and oxy-hemoglobin (HbO2) in blood, other wavelengths are well absorbed regardless of the amount of Hb and HbO2. In the present invention, the reference wavelength λx is barely influenced by the amount of Hb and HbO2, and the wavelength λo readily changes according to the amount of Hb and HbO2. For example, the wavelength λo may be a wavelength of 660 nm giving a maximum difference between an absorption coefficient for Hb and an absorption coefficient for HbO2, and the wavelength λx may be a wavelength of 805 nm selected from a near infrared band of 800 through 950 nm. A discussion of these characteristics of wavelengths may be found in a book by J. G. Webster entitled “Design of Pulse Oximeters,” at pages 40–55, published in 1997.

The oxygen saturation detector926obtains a variation ΔODtot,λoof light attenuation at the selected wavelength λo and a variation ΔODtot,λxof light attenuation at the selected wavelength λx and obtains a ratio Roxof the variation ΔODtot,λoto the variation ΔODtot,λx.

Thereafter, the oxygen saturation detector926calculates oxygen saturation S in blood using the ratio Roxand the hemoglobin concentration CHbaccording to Formula (9).

Here, εHbO2,odenotes an absorption coefficient for HbO2with respect to the wavelength λo; εHb,odenotes an absorption coefficient for Hb with respect to the wavelength λo; εHb,xdenotes an absorption coefficient for Hb with respect to the wavelength λx; kxand kodenote constants determined by the wavelengths λx and λo and characteristics of scattering and absorbing incident light at a predetermined body part; and axand aodenote constants determined by a size of a scattered particle, a refractive index of hemoglobin, a refractive index of serum, and the wavelengths λx and λo.

FIG. 14is a flowchart of a method of measuring a pulse rate according to the first embodiment of the present invention. A method of measuring a pulse rate according to the first embodiment of the present invention will be described with further reference toFIGS. 9 and 14.

When a pulse wave necessary for performing a measurement of oxygen saturation is measured, a change in a blood flow rate in an artery is caused by a heart beat. A pulse rate is measured to measure the heart rate. As shown inFIG. 8, light transmitted through a predetermined body part is received and converted into an electrical signal by the photodetector. The electrical signal is amplified by the amplifier581and collected for a predetermined period of time, thereby forming PPG data. In step S1410, the PPG data is band pass filtered by the filter583. In step S1420, the peak detector585differentiates the band-pass filtered signal and finds an inflection point at which a slope changes from positive to negative. In step S1430, an inflection point value is compared with a threshold value set initially, and the inflection point is detected as a peak when the inflection point value exceeds the threshold value, as shown inFIG. 15A. In step S1440, the pulse detector910calculates an average of time differences between peaks and, in step S1450, calculates a pulse rate per minute by dividing 60 seconds by the average time difference.

FIG. 15Billustrates a method of detecting a respiration frequency according to an embodiment of the present invention. Referring toFIGS. 9 and 15B, an AC component of a PPG is synchronized with a respiration signal as well as a heart beat. A PPG signal and respiration are related as follows. According to a mechanism based on maintenance of homeostasis of a human body, during inhalation, an intra-thoracic pressure decreases, the amount of blood returning to the heart increases, a blood pressure increases due to an increase in a cardiac output, and a depressor center is excited to expand peripheral arteries. In contrast, during exhalation, the peripheral arteries are contracted. A change in an optical path length due to expansion and contraction of the peripheral arteries is reflected on the PPG. Synchronization between the AC component and the respiration signal occurs because a change in a blood flow rate is caused by respiration and reflected on the PPG.

In an embodiment of the present invention, frequency components in a respiration signal band are classified using a digital filter in order to extract a respiration signal from a PPG signal. A PPG signal output from the peak detector585is filtered by the BPF930having a cut-off frequency of about 0.13–0.48 Hz including a frequency band of a normal respiration signal. The respiration detector935detects a respiration signal from the filtered PPG signal, calculates an average respiration frequency by dividing 60 seconds by an average period of the respiration signal, and outputs the average respiration frequency to the display unit590.

FIGS. 16A and 16Bshow a PPG signal and a respiration signal, respectively, measured to verify the present invention.FIG. 17Ashows a PPG waveform obtained by band-pass filtering the PPG signal shown inFIG. 16A.FIG. 17Bshows a waveform obtained by removing low-frequency components from the respiration signal shown inFIG. 16B. It may be seen from a comparison ofFIGS. 17A and 17Bthat the band-pass filtered PPG signal closely correlates with the respiration signal.

FIG. 18is a block diagram of an apparatus for measuring a bio signal according to a second embodiment of the present invention. Referring toFIG. 18, the second embodiment is different from the first embodiment in that a bio signal measurement unit1800has a cap shape so that it may be mounted on an earphone1830reproducing voice from an existing portable apparatus when a bio signal is measured. The bio signal measurement unit1800, a controller1850, and a display unit1890have the same structures as those described in connection with the first embodiment, and thus only the difference between the first and second embodiments will be described.

In the second embodiment, the earphone1830supplies driving power to a temperature measurement module including a waveguide1811and an infrared sensor1813and to a PPG measurement module1820and has a plurality of electrodes1835on an outer surface for receiving a measured signal. The bio signal measurement unit1800has a recess into which the earphone1830is inserted. A plurality of electrodes1815and1825are disposed in the recess so that they are connected to the electrodes1835of the earphone1830when the earphone1830is inserted into the recess. The waveguide1811collecting infrared rays and the infrared sensor1813converting the collected infrared rays to an electrical signal are installed within the cap shape of the bio signal measurement unit1800.

When measuring a bio signal, a user mounts the cap-shaped bio signal measurement unit1800on the earphone1830such that the electrodes1815and1825are connected to the electrodes1835and then inserts the bio signal measurement unit1800combined with the earphone1830into his ear.

The present invention may be realized as a code that is recorded on a computer readable recording medium and can be read by a computer. The computer readable recording medium may be any type of medium on which data that can be read by a computer system can be recorded, for example, a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disc, or an optical data storage device. The present invention may also be realized as carrier waves (for example, transmitted through Internet). Alternatively, computer readable recording media may be distributed among computer systems connected through a network so that the present invention can be realized as a code that is stored in the recording media and can be read and executed in the computers.

As described above, an apparatus for measuring a bio signal according to an embodiment of the present invention includes a module measuring various types of bio information and is structured to be insertable into the ear so that various types of bio information can be simultaneously measured and an influence of a motion artifact can be minimized. In addition, an error occurring due to contamination or damage of a sensor can be reduced.

Moreover, since an apparatus for measuring a bio signal according to an embodiment of the present invention can be connected to a mobile apparatus such as an earphone, it is convenient to carry. Further, a user is able to reposition the mount of the apparatus based on feel while observing a measured value displayed on a mobile apparatus. Thus, the user is bale to perform measurements on himself and self-diagnose a condition.

When an apparatus for measuring a bio signal according to an embodiment of the present invention is combined with a mobile communication terminal, a measured bio signal can be displayed to a user through a display apparatus included in the mobile communication terminal and easily transmitted to a remote medical institution through the mobile communication terminal. As a result, remote medical treatment is possible.