How smart watches, sports trackers and other gadgets measure the pulse? Part 1
Hello!
Very little is left before the start of our crowdfunding company of watches for measuring the level of stress EMVIO . There was a small respite and fingers asked for the keyboard.
')
At the very initial stage of development, we carried out a small analytical review of methods for measuring the pulse of a person and periodically updated it with new projects. I wanted to share this information with the community. We hope that it will be interesting to a wide circle of readers and will give an idea of the state of technology in this area.
In this review, the emphasis is precisely on the application of pulse measurement methods in “for fan” gadgets. Some methods have already been implemented in ready-made mass products, others are waiting in the wings. But first, a few words about what we actually measure and why this is important.
As you know, the heart is an autonomous muscular organ that performs the pumping function, providing a continuous flow of blood in blood vessels through rhythmic contractions. In the heart there is a section in which impulses are generated that are responsible for the contraction of muscle fibers, the so-called pacemaker. In the normal state, in the absence of pathologies, this area completely determines the heart rate. The result is a cardiac cycle - a sequence of contractions (systole) and relaxation (diastole) of the heart muscles, ranging from the atria and ending with the ventricles. In general, the pulse is understood as the frequency with which the cardiac cycle is repeated. However, there are nuances in which way we register this frequency.
In those days, when medicine did not have the technical means of diagnosis, the pulse was measured by all known methods - palpation, i.e. they applied a finger to a certain area of the body and listened to their tactile sensations, and counted the number of tremors of the artery wall through the skin for some time — usually 30 seconds or one minute. Hence the Latin name for this effect - pulsus, i.e. impact, respectively, measurement unit: beats per minute, beatsperminute (bpm). There are many methods of palpation, the most famous is probing the pulse on the wrist and neck, in the area of the carotid artery, which is so popular in the movies.
In electrocardiography, the pulse is calculated from the signal of the electrical activity of the heart - electrocardiogram (EX) by measuring the duration of the interval (in seconds) between adjacent R teeth EX and then converted into beats per minute using a simple formula: BPM = 60 / (RR interval) . Accordingly, you need to remember that this is the ventricular pulse, because the period of atrial contraction (PP interval) may differ slightly.
Attention !!! Just want to note an important point that makes confusion in the terminology and is often found in the comments to the articles about the gadgets with pulse measurement. In fact, the pulse, which is measured by contractions of the walls of blood vessels, and the pulse, which is measured by the electrical activity of the heart, have a different physiological nature, a different shape of the time curve, a different phase shift and, accordingly, require different registration methods and processing algorithms. Therefore, there can be no RR-intervals when measuring the pulse of the modulation of the volume of blood filling the arteries and capillaries and mechanical oscillations of their walls. And conversely, it is impossible to say that if you do not have RR-intervals, then you cannot measure intervals of a pulse wave that are similar in physiological significance.
So, here is our version of the review of the most common ways to measure the pulse and examples of gadgets that implement them.
After the discovery of the electrical activity of the heart at the end of the 19th century, it was technically possible to register it. The first one that really did was Willem Einthoven in 1902, with the help of his mega-device - a string galvanometer. By the way, he transmitted the ECG via a telephone cable from the hospital to the laboratory and, in fact, implemented the idea of remote access to medical data!
Three cans with “pickle” and an electrocardiograph weighing 270 kg! This is how the method was born, which today helps millions of people around the world.
For his work in 1924, he won the Nobel Prize. It was Einthoven who first received a real electrocardiogram (he came up with the name himself), developed a lead system - Einthoven's triangle and introduced the names of EX-segments. The most famous is the QRS complex - the moment of electrical excitation of the ventricles and, as the most pronounced in its temporal and frequency properties element of this complex, the R wave.
Painfully familiar signal and RR-interval!
In modern clinical practice, different lead systems are used for registering ECS: limb leads, chest leads in various configurations, orthogonal leads (according to Frank), etc. From the point of view of pulse measurement, you can use any lead, because in normal EX, the R tooth is present in one form or another on all leads.
When designing wearable gadgets and various sports simulators, the lead system was simplified to two point electrodes. The most well-known variant of the implementation of this approach is sports chest monitors in the form of a strap-heart monitor - HRM strap or HRM band. We think the readers, leading a sports lifestyle, such devices already exist.
Strap design example and Mr. gadget 80 lvl. The sensor pad is two ECG electrodes from different sides of the chest.
HRM straps from Garmin and Polar are popular in the market; there are also many Chinese clones. In such straps, the electrodes are made in the form of two strips of conductive material. The strap can be a part of the entire device or be fastened to it with clips. Pulse values are usually transmitted via Bluetooth via ANT + or Smart to a sports watch or smartphone. It is quite convenient for sports activities, but constant wear causes discomfort.
We experimented with such straps in terms of the possibility of estimating the variability of the pulse, considering them to be the standard, but the data coming from them turned out to be very smooth. A member of our team Kvanto25 published a post , how he understood the Polar strap protocol and connected it to a computer via Labview.
The next implementation of the two electrode system is the separation of the electrodes into two hands, but without permanent connection of one of them. In such devices, one electrode is fixed on the wrist in the form of the back wall of a watch or bracelet, and the other is placed on the front of the device. To measure the pulse, you need to touch the facial electrode with your free hand and wait a few seconds.
An example of a pulse meter with a front electrode (Beurer Pulsometer)
An interesting device using this technology is the Phyode W / Me bracelet, whose developers conducted a successful campaign on Kickstarter and their product is commercially available. On Habré about him was a post .
PhyodeW / Me Electrode System
The top electrode is combined with the button, so many people, viewing the device from the pictures and reading the reviews, thought that the measurement was just by pressing a button. Now you know that on such bracelets continuous registration with free hands in principle is not possible.
The advantage of this device is that pulse measurement is not the main goal. The bracelet is positioned as a means of conducting and controlling respiratory techniques, such as an individual trainer. We purchased Phyode and lost with it. Everything works as promised, a real ECG is recorded, corresponding to the classic first ECG lead. However, the device is very sensitive to finger movements on the front electrode, slightly shifted and the signal floated. Taking into account the fact that it takes about three minutes to collect statistics, the registration process looks stressful.
Here is another version of the use of the principle of two hands in the project FlyShark Smartwatch, which is laid out on Kickstarter .
Registration of the pulse in the project FlyShark Smartwatch. Be kind to hold your finger.
What else is new in this area? Be sure to mention the interesting implementation of the ECG electrode - a capacitive sensor of the electric field EPIC Ultra High Impedance ECG Sensor manufactured by Plessey Semiconductors.
Capacitive EPIC sensor for contactless ECG recording.
A primary amplifier is installed inside the sensor, so it can be considered active. The sensor is quite compact (10x10 mm), does not require direct electrical contact, and therefore has no polarization effects and should not be wetted. We think this solution is very promising for gadgets with the registration of the EX. We have not yet seen the finished devices on these sensors.
Truly the most common way to measure the pulse in the clinic and in everyday life! Hundreds of different devices from clothespins to rings. The method of plethysmography itself is based on the registration of changes in the volume of organ blood supply. The result of such registration will be a pulse wave. The clinical possibilities of plethysmography go far beyond the simple definition of the pulse, but in this case we are interested in it.
The determination of the pulse on the basis of plethysmography can be implemented in two main ways: impedance and optical. There is a third option - mechanical, but we will not consider it.
As the Medical Dictionary tells us, impedance plethysmography is a method of recording and studying the pulse oscillations of the blood supply to the vessels of various organs and tissues, based on recording changes in the total (ohmic and capacitive) electrical resistance to high frequency alternating current. In Russia, the term rheography is often used. This method of registration begins with the research of the scientist Mann (Mann, 30's) and the Russian researcher Kedrov A.A. (40s).
At present, the methodology of the method is based on a two or four-point scheme for measuring volume resistivity and consists of the following: a signal with a frequency from 20 to 150 kHz is passed through the organ under study using two electrodes (depending on the tissues studied).
Electrode impedance plethysmography system. Picture from here
The main condition imposed on the signal generator is the constancy of the current, its value is usually chosen not more than 10-15 μA. When a signal passes through a tissue, its amplitude is modulated by a change in the blood supply. The second system of electrodes removes the modulated signal; in fact, we have an impedance-voltage converter circuit. In the two-point scheme, the electrodes of the generator and receiver are combined. Further, the signal is amplified, the carrier frequency is removed from it, the constant component is eliminated and the delta we need remains.
If the device is calibrated (for the clinic this is a prerequisite), then the values in ohms can be plotted along the Y axis. The result is such a signal.
Examples of ECG time curves, impedance plethysmogram (rheogram) and its derivative with simultaneous recording. ( from here )
Very indicative picture. Pay attention to where the RR-interval is located on the EKS, and where the distance between the vertices corresponds to the duration of the cardiac cycle on the rheogram. Also note the sharp front of the R wave and the gentle front of the systolic phase of the rheogram.
Quite a lot of information can be obtained from the pulse curve on the circulatory state of the test organ, especially synchronously with the ECG, but we only need a pulse. It is not difficult to determine it - you need to find two local maxima corresponding to the maximum amplitude of the systolic wave, calculate the delta in seconds ∆T and then BMP = 60 / ∆T .
We have not yet found any examples of gadgets that use this method. But there is an example of the concept of an implantable sensor for monitoring blood circulation in an artery. Here is an article about him. The active sensor is planted directly on the artery, communicates with the host device by inductive coupling. We believe that this is a very interesting and promising approach. The principle of operation is clear from the picture. The match is shown to understand the size :) A 4-point registration scheme and a flexible printed circuit board are used. I think, if you wish, you can finish the idea for a wearable micro gadget. The advantage of this solution is that the consumption of such a sensor disappears a little.
Implantable blood flow and pulse sensor. Looks like an accessory to Johnny Mnemonic.
At the end of this section, we make a remark. At one time, we believed that pulse was measured in this way in the well-known startup HealBeGo, because in this device the basic functionality is implemented by impedance spectroscopy, which, in fact, is rheography, only with a variable frequency of the probing signal. In general, everything is already on board. However, according to the description of the characteristics of the device, pulse in HealBe is measured mechanically using a piezo sensor (about this method in the second part of the review).
Optical is the most common way to measure heart rate in terms of mass use. The narrowing and expansion of the vessel under the action of arterial pulsation of the blood flow causes a corresponding change in the amplitude of the signal obtained from the output of the photodetector. The very first devices were used in the clinic and measured the pulse from a finger in the lumen or reflection mode. The shape of the pulse curve repeats the rheogram.
Illustration of the principle of photoplethysmography
The method has found wide use in the clinic and soon the technology was applied in home appliances. For example, in compact pulse oximeters that record the pulse and oxygen saturation of the blood in the capillaries of the finger. There are hundreds of modifications in the world. For the home, for the family will go well, but not suitable for constant wear.
Pulsoksimetr ordinary and clip for the ear. Thousands of them!
There are options with ear clips and headphones with built-in sensors. For example, this option from Jabra or the new project Glow Headphones . The functionality is similar to HRM straps, but more stylish design, familiar device, free hands. Constantly wearing plugs in the ears will not, but for jogging in the fresh air to the music at the time.
Jabra Sport Pulse ™ Wireless and Glow Headphones. The pulse is registered intra-ear (in-ear sensor) method.
Breakthrough
The most tempting was the measurement of the pulse from the wrist, because it is such a familiar and comfortable place. The first was the Mio Alpha watch with a successful Kickstarter company.
Product creator Liz Dickinson pathetically proclaimed this device the Holy Grail of pulse measurement. The sensor module was developed by the guys from Philips. To date, this is the highest quality device for continuous measurement of the pulse from the wrist using photoplethysmography.
Further, the world has become such worthy things as Basis B1, Samsung Galaxy Gear and Gear Fit, Moto 360, and of course, expected by all fans of Apple's AppleWatch brands.
You give smart watches a lot and different!
Now we can say that the technology has been developed and introduced into mass production. In all such devices, pulse measurement is performed on the reflected signal.
Emitter wavelength selection
Now a few words how to choose the wavelength of the radiator. It all depends on the problem to be solved. The rationale for the choice is well illustrated by the light absorption schedule of hydroxy and deoxyhemoglobin with the spectral characteristics of the emitters superimposed on it.
Light absorption curve by hemoglobin and the main emission spectra of pulse photoplethism graphic sensors.
The choice of wavelength depends on the fact that we want to measure the pulse and / or saturation of the blood oxygenation of SO2.
Just a pulse. For this case, the important region where the absorption is maximum is the range from 500 to 600 nm, not counting the maximum in the ultraviolet part. Typically, the value is 525 nm (green) or with a small offset of 535 nm (applied in OSRAM SFH 7050 - Photoplethysmography Sensor).
Green LED pulse sensor - the most popular option in smart watches and bracelets. A red LED is used in the sensor of the Samsung Galaxy S5.
Oximetry In this mode, it is necessary to measure the pulse and assess the oxygen saturation of the blood. The method is based on the difference in the absorption of hemoglobin bound (oxy) and non-oxygen-related (desoka). The maximum absorption of deoxygenated hemoglobin (Hb) is in the “red” (660 nm) range, the maximum absorption of oxygenated (Hb02) hemoglobin in the infrabasic (940 nm). To calculate the pulse, a channel with a wavelength of 660 nm is used.
Yellow for EMVIO. For our EMVIO device, we chose from two ranges: 525 nm and 590 nm (yellow color). In this case, we took into account the maximum spectral sensitivity of our optical sensor. Experiments have shown that there is practically no difference between them (within our design and the selected sensor). Artifacts of motion, individual properties of the skin, thickness of the subcutaneous layer of the wrist, and degree of pressure of the sensor against the skin interrupt any difference. We wanted to somehow stand out from the general “green” list and for the time being we stopped at the yellow color.
Of course, measurements can be carried out not only from the wrist. There are non-standard options for choosing a pulse registration point on the market. For example, from the forehead. This approach is used in the project of a smart helmet for cyclists Life beam Smart helmet developed by the Israeli company Lifebeam. In the proposals of this company there are also baseball caps and sun visors for girls. If you constantly wear a baseball cap, then this is your option.
The cyclist is pleased that he does not need to wear a HRM strap.
In general, the choice of registration points is quite large: the wrist, finger, earlobe, forehead, hand bitz, ankle and foot for babies. Full expanse for developers.
A big plus of the optical method is the ease of implementation on modern smartphones, where a standard video camera is used as a sensor, and a flash LED is used as a transmitter. In the new smartphone Samsung Galaxy S5 on the back of the case, for the convenience of the user, there is already a regular pulse sensor module, perhaps other manufacturers will implement similar solutions. This can be crucial for devices in which there is no continuous registration, smartphones will incorporate their functionality.
New horizons photoplethysmography
Further development of this method is associated with the rethinking of the functionality of the optical sensor and the technological capabilities of modern wearable devices in terms of video processing in real time. As a result, we have the idea of measuring the pulse on the video image of the face. The backlight is natural light.
The original decision, taking into account the fact that the video camera is a standard attribute of any laptop, smartphone and even smart watches. The idea of the method is disclosed in this work .
Subject N3 is obviously tense - the pulse is under 100 beats / min, probably handing in the work to its leader Subject N2. Subject N1 was just passing by.
First, fragments of the face are selected on frames, then the image is decomposed into three color channels and unfolds along the timeline (RGB trace). The selection of the pulse wave is based on the decomposition of the image by the method of analysis of independent components (ICA) and the selection of the frequency component associated with the brightness modulation of pixels under the action of blood pulsation.
The Philips Innovation Lab implemented a similar approach in the form of the Vital Signs Camera program for the iPhone. Very interesting thing. The averaging of values is of course large, but in principle the method works. A similar project is developing Fujitsu Laboratories .
Types of screens Vital Signs Camera.
So in the future, video surveillance systems will be able to remotely measure your heart rate. The NSA office will rejoice.
The end of the review in the next post “ How smart watches, sports trackers and other gadgets measure the pulse? Part 2 ". In this part, we will talk about more exotic ways to register the pulse, which are used in modern gadgets.
Good luck! And once again we invite you to the site of our EMVIO project.
Very little is left before the start of our crowdfunding company of watches for measuring the level of stress EMVIO . There was a small respite and fingers asked for the keyboard.
')
At the very initial stage of development, we carried out a small analytical review of methods for measuring the pulse of a person and periodically updated it with new projects. I wanted to share this information with the community. We hope that it will be interesting to a wide circle of readers and will give an idea of the state of technology in this area.
In this review, the emphasis is precisely on the application of pulse measurement methods in “for fan” gadgets. Some methods have already been implemented in ready-made mass products, others are waiting in the wings. But first, a few words about what we actually measure and why this is important.
Little about our heart
As you know, the heart is an autonomous muscular organ that performs the pumping function, providing a continuous flow of blood in blood vessels through rhythmic contractions. In the heart there is a section in which impulses are generated that are responsible for the contraction of muscle fibers, the so-called pacemaker. In the normal state, in the absence of pathologies, this area completely determines the heart rate. The result is a cardiac cycle - a sequence of contractions (systole) and relaxation (diastole) of the heart muscles, ranging from the atria and ending with the ventricles. In general, the pulse is understood as the frequency with which the cardiac cycle is repeated. However, there are nuances in which way we register this frequency.
What do we think is the pulse
In those days, when medicine did not have the technical means of diagnosis, the pulse was measured by all known methods - palpation, i.e. they applied a finger to a certain area of the body and listened to their tactile sensations, and counted the number of tremors of the artery wall through the skin for some time — usually 30 seconds or one minute. Hence the Latin name for this effect - pulsus, i.e. impact, respectively, measurement unit: beats per minute, beatsperminute (bpm). There are many methods of palpation, the most famous is probing the pulse on the wrist and neck, in the area of the carotid artery, which is so popular in the movies.
In electrocardiography, the pulse is calculated from the signal of the electrical activity of the heart - electrocardiogram (EX) by measuring the duration of the interval (in seconds) between adjacent R teeth EX and then converted into beats per minute using a simple formula: BPM = 60 / (RR interval) . Accordingly, you need to remember that this is the ventricular pulse, because the period of atrial contraction (PP interval) may differ slightly.
Attention !!! Just want to note an important point that makes confusion in the terminology and is often found in the comments to the articles about the gadgets with pulse measurement. In fact, the pulse, which is measured by contractions of the walls of blood vessels, and the pulse, which is measured by the electrical activity of the heart, have a different physiological nature, a different shape of the time curve, a different phase shift and, accordingly, require different registration methods and processing algorithms. Therefore, there can be no RR-intervals when measuring the pulse of the modulation of the volume of blood filling the arteries and capillaries and mechanical oscillations of their walls. And conversely, it is impossible to say that if you do not have RR-intervals, then you cannot measure intervals of a pulse wave that are similar in physiological significance.
How do gadgets measure pulse?
So, here is our version of the review of the most common ways to measure the pulse and examples of gadgets that implement them.
1. Pulse measurement by electrocardiogram
After the discovery of the electrical activity of the heart at the end of the 19th century, it was technically possible to register it. The first one that really did was Willem Einthoven in 1902, with the help of his mega-device - a string galvanometer. By the way, he transmitted the ECG via a telephone cable from the hospital to the laboratory and, in fact, implemented the idea of remote access to medical data!
Three cans with “pickle” and an electrocardiograph weighing 270 kg! This is how the method was born, which today helps millions of people around the world.
For his work in 1924, he won the Nobel Prize. It was Einthoven who first received a real electrocardiogram (he came up with the name himself), developed a lead system - Einthoven's triangle and introduced the names of EX-segments. The most famous is the QRS complex - the moment of electrical excitation of the ventricles and, as the most pronounced in its temporal and frequency properties element of this complex, the R wave.
Painfully familiar signal and RR-interval!
In modern clinical practice, different lead systems are used for registering ECS: limb leads, chest leads in various configurations, orthogonal leads (according to Frank), etc. From the point of view of pulse measurement, you can use any lead, because in normal EX, the R tooth is present in one form or another on all leads.
Sports chest pulse sensors
When designing wearable gadgets and various sports simulators, the lead system was simplified to two point electrodes. The most well-known variant of the implementation of this approach is sports chest monitors in the form of a strap-heart monitor - HRM strap or HRM band. We think the readers, leading a sports lifestyle, such devices already exist.
Strap design example and Mr. gadget 80 lvl. The sensor pad is two ECG electrodes from different sides of the chest.
HRM straps from Garmin and Polar are popular in the market; there are also many Chinese clones. In such straps, the electrodes are made in the form of two strips of conductive material. The strap can be a part of the entire device or be fastened to it with clips. Pulse values are usually transmitted via Bluetooth via ANT + or Smart to a sports watch or smartphone. It is quite convenient for sports activities, but constant wear causes discomfort.
We experimented with such straps in terms of the possibility of estimating the variability of the pulse, considering them to be the standard, but the data coming from them turned out to be very smooth. A member of our team Kvanto25 published a post , how he understood the Polar strap protocol and connected it to a computer via Labview.
With two hands
The next implementation of the two electrode system is the separation of the electrodes into two hands, but without permanent connection of one of them. In such devices, one electrode is fixed on the wrist in the form of the back wall of a watch or bracelet, and the other is placed on the front of the device. To measure the pulse, you need to touch the facial electrode with your free hand and wait a few seconds.
An example of a pulse meter with a front electrode (Beurer Pulsometer)
An interesting device using this technology is the Phyode W / Me bracelet, whose developers conducted a successful campaign on Kickstarter and their product is commercially available. On Habré about him was a post .
PhyodeW / Me Electrode System
The top electrode is combined with the button, so many people, viewing the device from the pictures and reading the reviews, thought that the measurement was just by pressing a button. Now you know that on such bracelets continuous registration with free hands in principle is not possible.
The advantage of this device is that pulse measurement is not the main goal. The bracelet is positioned as a means of conducting and controlling respiratory techniques, such as an individual trainer. We purchased Phyode and lost with it. Everything works as promised, a real ECG is recorded, corresponding to the classic first ECG lead. However, the device is very sensitive to finger movements on the front electrode, slightly shifted and the signal floated. Taking into account the fact that it takes about three minutes to collect statistics, the registration process looks stressful.
Here is another version of the use of the principle of two hands in the project FlyShark Smartwatch, which is laid out on Kickstarter .
Registration of the pulse in the project FlyShark Smartwatch. Be kind to hold your finger.
What else is new in this area? Be sure to mention the interesting implementation of the ECG electrode - a capacitive sensor of the electric field EPIC Ultra High Impedance ECG Sensor manufactured by Plessey Semiconductors.
Capacitive EPIC sensor for contactless ECG recording.
A primary amplifier is installed inside the sensor, so it can be considered active. The sensor is quite compact (10x10 mm), does not require direct electrical contact, and therefore has no polarization effects and should not be wetted. We think this solution is very promising for gadgets with the registration of the EX. We have not yet seen the finished devices on these sensors.
2. Measurement of pulse based on plethysmography
Truly the most common way to measure the pulse in the clinic and in everyday life! Hundreds of different devices from clothespins to rings. The method of plethysmography itself is based on the registration of changes in the volume of organ blood supply. The result of such registration will be a pulse wave. The clinical possibilities of plethysmography go far beyond the simple definition of the pulse, but in this case we are interested in it.
The determination of the pulse on the basis of plethysmography can be implemented in two main ways: impedance and optical. There is a third option - mechanical, but we will not consider it.
Impedance plethysmography
As the Medical Dictionary tells us, impedance plethysmography is a method of recording and studying the pulse oscillations of the blood supply to the vessels of various organs and tissues, based on recording changes in the total (ohmic and capacitive) electrical resistance to high frequency alternating current. In Russia, the term rheography is often used. This method of registration begins with the research of the scientist Mann (Mann, 30's) and the Russian researcher Kedrov A.A. (40s).
At present, the methodology of the method is based on a two or four-point scheme for measuring volume resistivity and consists of the following: a signal with a frequency from 20 to 150 kHz is passed through the organ under study using two electrodes (depending on the tissues studied).
Electrode impedance plethysmography system. Picture from here
The main condition imposed on the signal generator is the constancy of the current, its value is usually chosen not more than 10-15 μA. When a signal passes through a tissue, its amplitude is modulated by a change in the blood supply. The second system of electrodes removes the modulated signal; in fact, we have an impedance-voltage converter circuit. In the two-point scheme, the electrodes of the generator and receiver are combined. Further, the signal is amplified, the carrier frequency is removed from it, the constant component is eliminated and the delta we need remains.
If the device is calibrated (for the clinic this is a prerequisite), then the values in ohms can be plotted along the Y axis. The result is such a signal.
Examples of ECG time curves, impedance plethysmogram (rheogram) and its derivative with simultaneous recording. ( from here )
Very indicative picture. Pay attention to where the RR-interval is located on the EKS, and where the distance between the vertices corresponds to the duration of the cardiac cycle on the rheogram. Also note the sharp front of the R wave and the gentle front of the systolic phase of the rheogram.
Quite a lot of information can be obtained from the pulse curve on the circulatory state of the test organ, especially synchronously with the ECG, but we only need a pulse. It is not difficult to determine it - you need to find two local maxima corresponding to the maximum amplitude of the systolic wave, calculate the delta in seconds ∆T and then BMP = 60 / ∆T .
We have not yet found any examples of gadgets that use this method. But there is an example of the concept of an implantable sensor for monitoring blood circulation in an artery. Here is an article about him. The active sensor is planted directly on the artery, communicates with the host device by inductive coupling. We believe that this is a very interesting and promising approach. The principle of operation is clear from the picture. The match is shown to understand the size :) A 4-point registration scheme and a flexible printed circuit board are used. I think, if you wish, you can finish the idea for a wearable micro gadget. The advantage of this solution is that the consumption of such a sensor disappears a little.
Implantable blood flow and pulse sensor. Looks like an accessory to Johnny Mnemonic.
At the end of this section, we make a remark. At one time, we believed that pulse was measured in this way in the well-known startup HealBeGo, because in this device the basic functionality is implemented by impedance spectroscopy, which, in fact, is rheography, only with a variable frequency of the probing signal. In general, everything is already on board. However, according to the description of the characteristics of the device, pulse in HealBe is measured mechanically using a piezo sensor (about this method in the second part of the review).
Optical plethysmography or photoplethysmography
Optical is the most common way to measure heart rate in terms of mass use. The narrowing and expansion of the vessel under the action of arterial pulsation of the blood flow causes a corresponding change in the amplitude of the signal obtained from the output of the photodetector. The very first devices were used in the clinic and measured the pulse from a finger in the lumen or reflection mode. The shape of the pulse curve repeats the rheogram.
Illustration of the principle of photoplethysmography
The method has found wide use in the clinic and soon the technology was applied in home appliances. For example, in compact pulse oximeters that record the pulse and oxygen saturation of the blood in the capillaries of the finger. There are hundreds of modifications in the world. For the home, for the family will go well, but not suitable for constant wear.
Pulsoksimetr ordinary and clip for the ear. Thousands of them!
There are options with ear clips and headphones with built-in sensors. For example, this option from Jabra or the new project Glow Headphones . The functionality is similar to HRM straps, but more stylish design, familiar device, free hands. Constantly wearing plugs in the ears will not, but for jogging in the fresh air to the music at the time.
Jabra Sport Pulse ™ Wireless and Glow Headphones. The pulse is registered intra-ear (in-ear sensor) method.
Breakthrough
The most tempting was the measurement of the pulse from the wrist, because it is such a familiar and comfortable place. The first was the Mio Alpha watch with a successful Kickstarter company.
Product creator Liz Dickinson pathetically proclaimed this device the Holy Grail of pulse measurement. The sensor module was developed by the guys from Philips. To date, this is the highest quality device for continuous measurement of the pulse from the wrist using photoplethysmography.
Further, the world has become such worthy things as Basis B1, Samsung Galaxy Gear and Gear Fit, Moto 360, and of course, expected by all fans of Apple's AppleWatch brands.
You give smart watches a lot and different!
Now we can say that the technology has been developed and introduced into mass production. In all such devices, pulse measurement is performed on the reflected signal.
Emitter wavelength selection
Now a few words how to choose the wavelength of the radiator. It all depends on the problem to be solved. The rationale for the choice is well illustrated by the light absorption schedule of hydroxy and deoxyhemoglobin with the spectral characteristics of the emitters superimposed on it.
Light absorption curve by hemoglobin and the main emission spectra of pulse photoplethism graphic sensors.
The choice of wavelength depends on the fact that we want to measure the pulse and / or saturation of the blood oxygenation of SO2.
Just a pulse. For this case, the important region where the absorption is maximum is the range from 500 to 600 nm, not counting the maximum in the ultraviolet part. Typically, the value is 525 nm (green) or with a small offset of 535 nm (applied in OSRAM SFH 7050 - Photoplethysmography Sensor).
Green LED pulse sensor - the most popular option in smart watches and bracelets. A red LED is used in the sensor of the Samsung Galaxy S5.
Oximetry In this mode, it is necessary to measure the pulse and assess the oxygen saturation of the blood. The method is based on the difference in the absorption of hemoglobin bound (oxy) and non-oxygen-related (desoka). The maximum absorption of deoxygenated hemoglobin (Hb) is in the “red” (660 nm) range, the maximum absorption of oxygenated (Hb02) hemoglobin in the infrabasic (940 nm). To calculate the pulse, a channel with a wavelength of 660 nm is used.
Yellow for EMVIO. For our EMVIO device, we chose from two ranges: 525 nm and 590 nm (yellow color). In this case, we took into account the maximum spectral sensitivity of our optical sensor. Experiments have shown that there is practically no difference between them (within our design and the selected sensor). Artifacts of motion, individual properties of the skin, thickness of the subcutaneous layer of the wrist, and degree of pressure of the sensor against the skin interrupt any difference. We wanted to somehow stand out from the general “green” list and for the time being we stopped at the yellow color.
Of course, measurements can be carried out not only from the wrist. There are non-standard options for choosing a pulse registration point on the market. For example, from the forehead. This approach is used in the project of a smart helmet for cyclists Life beam Smart helmet developed by the Israeli company Lifebeam. In the proposals of this company there are also baseball caps and sun visors for girls. If you constantly wear a baseball cap, then this is your option.
The cyclist is pleased that he does not need to wear a HRM strap.
In general, the choice of registration points is quite large: the wrist, finger, earlobe, forehead, hand bitz, ankle and foot for babies. Full expanse for developers.
A big plus of the optical method is the ease of implementation on modern smartphones, where a standard video camera is used as a sensor, and a flash LED is used as a transmitter. In the new smartphone Samsung Galaxy S5 on the back of the case, for the convenience of the user, there is already a regular pulse sensor module, perhaps other manufacturers will implement similar solutions. This can be crucial for devices in which there is no continuous registration, smartphones will incorporate their functionality.
New horizons photoplethysmography
Further development of this method is associated with the rethinking of the functionality of the optical sensor and the technological capabilities of modern wearable devices in terms of video processing in real time. As a result, we have the idea of measuring the pulse on the video image of the face. The backlight is natural light.
The original decision, taking into account the fact that the video camera is a standard attribute of any laptop, smartphone and even smart watches. The idea of the method is disclosed in this work .
Subject N3 is obviously tense - the pulse is under 100 beats / min, probably handing in the work to its leader Subject N2. Subject N1 was just passing by.
First, fragments of the face are selected on frames, then the image is decomposed into three color channels and unfolds along the timeline (RGB trace). The selection of the pulse wave is based on the decomposition of the image by the method of analysis of independent components (ICA) and the selection of the frequency component associated with the brightness modulation of pixels under the action of blood pulsation.
The Philips Innovation Lab implemented a similar approach in the form of the Vital Signs Camera program for the iPhone. Very interesting thing. The averaging of values is of course large, but in principle the method works. A similar project is developing Fujitsu Laboratories .
Types of screens Vital Signs Camera.
So in the future, video surveillance systems will be able to remotely measure your heart rate. The NSA office will rejoice.
The end of the review in the next post “ How smart watches, sports trackers and other gadgets measure the pulse? Part 2 ". In this part, we will talk about more exotic ways to register the pulse, which are used in modern gadgets.
Good luck! And once again we invite you to the site of our EMVIO project.
Source: https://habr.com/ru/post/377197/
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