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BLOOD PULSE What they do & how they work Ever visited someone in hospital and noticed a small sensor clipped over one of their fingers? That’s actually the ‘business end’ of a blood pulse oximeter, used to monitor the oxygen level in a patient’s blood – an indication of how well their lungs and heart are working. Now that fully self-contained pulse oximeters are available on ebay for around $15 (including postage!), you can easily buy one for your personal use. Here’s a quick run-down on exactly what they do and how they work. E VEN BEFORE WE PRESENTED our new Arduino-based USB Electrocardiogram in the October 2015 issue of SILICON CHIP, we received a number of enquiries from readers regarding blood pulse oximeters. One reader even suggested that we might be able to describe one of these as a project in the magazine, as well. But let’s start by looking at what blood pulse oximeters actually do and how they work. When we breathe in, our lungs allow oxygen from the air to pass into our bloodstream. Most of the oxygen molecules become attached to haemoglobin, a protein located inside red blood cells. The blood in your arteries then carries the oxygenated haemoglobin around your body, so the oxygen can be transferred into the various tissues to provide them with ‘fuel’. The de-oxygenated blood then returns to the heart and lungs via your veins. So the main job of our lungs is to transfer oxygen from the air we breath into our blood and then the blood carries the oxygen to all of the tissues that need it. Anything that interferes with these functions – like a problem with our lungs or narrowing and/or block12  Silicon Chip ages in our arterial blood vessels – will have a significant effect on our overall health and well-being. This was realised by physicians many decades ago and various diagnostic procedures were developed to allow the health of our respiratory and circulatory systems to be assessed. Unfortunately many of these procedures were intrusive and/or painful. Then it was discovered in the 1930s that the oxygen level in arterial blood could be measured painlessly and non-intrusively using light at different wavelengths, either reflected from or passing through human tissue. By JIM ROWE The first “optical oximeter” is credited to G.A.Millikan in 1942, while in 1964 the first “absolute reading” ear oximeter was made by Shaw. It made use of eight different wavelengths of light and was commercialised by Hewlett-Packard. However because of its size and cost, it came to be used mainly in operating theatres and sleep laboratories. The pulse oximeter was developed in 1972 by bioengineers Takuo Aoyagi and Michio Kishi, at Nihon Kohden in Japan. This was in many ways the real breakthrough, using only two wavelengths of light (red and infrared or ‘IR’) but taking advantage of the differing absorption of these two wavelengths when passing through human tissue during the pulsing of arterial blood as a result of the heart’s pumping cycle. Since then, pulse oximeters have developed dramatically, shrinking in physical size to reach their current size of around 65 x 36 x 33mm – much the same size as a ‘clip on a fingertip’ sensor probe used with one of the earlier oximeters. The price has also fallen dramatically, to a point where they can now be bought on the internet for less than siliconchip.com.au E OXIMETERS $A10, plus a few dollars for postage. What they do There are two basic principles involved in the operation of a blood pulse oximeter. One is that the oxygenated haemoglobin (HbO2) in arterial ‘red’ blood and the de-oxygenated haemoglobin (Hb) in venous ‘red-blue’ RED LIGHT (660nm) blood differ quite markedly in terms of the way they absorb or pass light in particular, at the wavelengths of red light (around 660 nanometres) and IR light (around 915nm). This is shown by the graph of Fig.1, which shows the degree to which light at various wavelengths is absorbed by the HbO2 in arterial blood (red curve), INFRARED LIGHT (915nm) 10 ARTERIAL BLOOD HbO2 (Oxygenated Haemoglobin) ABSORBANCE VENOUS BLOOD Hb (Deoxygenated Haemoglobin) 1 0.1 600 700 800 900 1000 Fig.1: Oxygenated arterial blood absorbs more IR light, while de-oxygenated venous blood absorbs more red light. siliconchip.com.au WAVELENGTH (nanometres) compared with that absorbed by Hb in venous blood (blue curve). As you can see, arterial blood with its higher level of HbO2 absorbs very little red light, but somewhat more IR light. On the other hand, venous blood with its higher level of de-oxygenated Hb absorbs somewhat more red light, but less IR light. Next consider what happens if we pass light of these two different wavelengths through human tissue which normally has a good blood circulation – like that in a human fingertip. This is what happens in an oximeter, as shown in Fig.2. As you can see, two LEDs just above the fingertip are used to provide the Fig.2: Taking advantage of RED IR LED LED the behaviour illustrated at left, blood pulse oximeters use this simple measuring setup (FINGER) to measure the ratio of red and IR light passing through a fingertip. PIN PHOTODIODE January 2016  13 LIGHT PASSING THROUGH FINGERTIP AND REACHING THE PIN PHOTODIODE IR LIGHT REACHING PHOTODIODE RED LIGHT REACHING PHOTODIODE TIME PULSES OF ARTERIAL (HbO2-RICH) BLOOD REACHING CAPILLARIES IN FINGERTIP Fig.3: A graph showing the pulsing nature of both red and IR light passing through a fingertip to reach the photodiode underneath. The pulses correspond to pulses of arterial blood passing through the fingertip capillaries. light, while a PIN photodiode underneath responds to the light which passes through the fingertip without being absorbed. Now we come to the second principle involved in the pulse oximeter’s operation and the reason why they’re called “pulse” oximeters. This can be understood as follows. After a pulse of oxygen-carrying blood has been pumped out by the heart’s left ventricle and circulated via the arteries, the oxygen is rapidly transferred out into the tissues via the tiny capillaries linking the arterial and venous blood vessels. As a result, inside a region like a fingertip (or an ear lobe), the level of HbO2 in the capillaries has dropped significantly, while the level of de-oxygenated Hb in them has risen to a relatively high level. This means that overall and as shown in Fig.1, the capillaries and tissues in the fingertip absorb a relatively high proportion of the red light but a somewhat smaller proportion of IR light. In other words, the ratio of red light to IR light passing through the fingertip to reach the PIN photodiode is relatively low. But as soon as the next pulse of arterial blood arrives from the heart, with its higher level of HbO2, this situation changes markedly. Now and for a brief time the capillaries have a considerably higher level of HbO2 and as a result, the absorption of red light drops significantly, while that of the IR light rises. So the ratio of red light to IR light reaching the PIN photodiode swings high – at least until the oxygen passes out into the tissues. The end result is that the levels of red and IR light passing through the fingertip swing up and down cyclically in time with the ‘heartbeat’ pulses of arterial blood reaching it. This is shown in Fig.3, where the transmitted 14  Silicon Chip red light level is represented by the red graph while that for the IR light is represented by the blue-purple graph. As you can see, the ratio between the two swings back and forth in time with the pulses of HbO2-rich blood reaching the fingertip capillaries. In essence, it’s the peaks in the red/ IR light ratio which are the main indicator of the person’s ‘circulatory health’, because they’re an indicator of the degree of HbO2 ‘saturation’ in their arterial blood. So it’s the job of the pulse oximeter as a whole to measure the amplitude of these peaks in the red/IR light ratio, and work out the corresponding ‘saturation pulse oxygenation level’ (usually shortened to SpO2). How they work At this stage you’re probably wondering how, if the oximeter uses the simple sensing set-up shown in Fig.2, it can work out the ratio of red light to IR light reaching the single PIN photodiode under the fingertip. The answer to this is quite straightforward: it does so by switching the red and IR LEDs on and off in sequence, so they’re never on at the same time. This allows the transmitted light at each wavelength to be measured separately. In fact there’s also a step in the Inside a ContecOximeter – there’s not much to it and similar oximeters are available online for less than $15.00! switching sequence where neither LED is turned on. This allows the oximeter circuitry to measure the amount of external ‘ambient’ light which may be able to reach the PIN photodiode (around or through the fingertip), allowing it to be subtracted from the transmitted red and IR light levels to get a more accurate reading of both. So the oximeter is repeatedly switching through a ‘red LED only/IR LED only/neither’ sampling sequence, at a rate of about 50 times per second. This speed is high enough to ensure that the red/IR ratio peaks can be captured faithfully, as a normal human heart pulse rate varies between about once and twice per second (60 – 120 bpm but much higher rates can be sustained during heavy exercise). From this you won’t be surprised to hear that there’s a microcontroller at the heart of virtually all pulse oximeters. The basic configuration is shown in Fig.4, and the micro controls the LED switching sequence, measures the output from the PIN photodiode via a current-to-voltage converter and its internal ADC (analog to digital converter), crunches this data to work out the SpO level, and displays the result on a small LCD readout. With most of the latest pulse oximeters the micro also measures the time between arterial blood pulses and displays the corresponding heart beat rate in beats per minute. It often displays the varying red/IR light ratio as a ‘bouncing bar chart’ as well. You can see from Fig.4 that there’s not a lot inside a modern pulse oximeter. Which explains how, thanks to surface-mount technology, it can all be squeezed (along with a couple of AAA cells) into a tiny fingertip enclosing probe like the ones shown in the photos. It also explains how the latest devices can be sold for such a low price. So that’s what blood pulse oximesiliconchip.com.au What is a “normal” blood oxygen level? In order to function properly, you body needs a certain amount of oxygen in the bloodstream. When the level falls below a certain amount, “hypoxia” (or hypoxemia) occurs. But what is this amount? Blood oxygen levels vary slightly from person to person; however in a healthy person, a level of between 95 and 100% is considered normal – in other words, at least 95% of the body’s ability to transport oxygen via the bloodstream is happening. (In truth, 100% can never really be achieved – 99% is about the maximum). Between 90 and 95%, a conscious person may experience a “shortness of breath”. Below 90% is cause for concern and, indeed, may require administration of pure oxygen to make up the shortfall. Hypoxia has a number of causes, mostly to do with illness or disease (especially of the lungs). Another reason is drowning or near-drowning, where first-aid (CPR) has brought a person back from near death. Pure oxygen is always administered once breathing has been re-established, because hypoxia is almost certainly ters do and how they do it. Now for the question that seems to have occurred to at least a few of the SILICON CHIP readers: Why not do an oximeter project, perhaps as an add-on to the ECG project in the October 2015 issue? Since there’s apparently so little inside a pulse oximeter, as shown in Fig.4, this is a fair question. In fact, we recently built a prototype Arduino-based oximeter, designed to hook up to a PC via a USB cable (like the ECG project). But there were significant problems: 1. Although the SpO2 level can be worked out from the transmitted peak red light/IR light ratio, the relationship between them isn’t a linear one. Because of this the micro in commer- present (the depth depending on length of immersion). One cause getting increasing attention these days is sleep apnoea, where the person effectively “forgets” to breathe for a period during sleep, lasting from a few seconds to a few minutes. This results in no fresh oxygen getting into the lungs and, therefore, into the bloodstream. Blood oxygen levels drop quite quickly – in a medical situation this would almost certainly set off a patient alarm so appropriate attention can be given. While the body should have an “automatic” response to severe sleep apnoea, waking the person, before this occurs hypoxia will occur at some level, along with hypercapnia, an excess of C02 in the bloodstream. In sleep apnoea, saturation HbO2 levels of 85-90% are relatively common, while levels below 80% are considered severe/extreme. Prolonged levels below 80% risk organ and tissue damage, including irreversible brain damage and in the worst cases, death. cial pulse oximeters uses a ‘lookup table’ to find the SpO2 level corresponding to the peak red/IR ratio – and the data stored in the lookup table must be prepared by testing a reasonable number of human subjects. This is fine if you’re building a mass-produced commercial oximeter, but it isn’t really feasible when it comes to a ‘one off’ DIY project. 2. While the hardware, firmware and software side of the project’s electronics was fairly straightforward, the physical side of the fingertip sensor was tricky – involving a couple of small PCBs in this part alone, linked by ribbon cable and mounted inside the two parts of the smallest ‘jiffy box’ enclosure fitted with a small hinge and lined with adhesive black felt. This sensor assembly by itself was larger than one of the low cost commercial oximeters and not as effective or attractive. And yes we also looked at the possibility of using a cheap oximeter as the head-end, and interfacing its signals to an Arduino. Trouble is, these units are not necessarily based on a standard micro and even if we settled on one particular unit, there would be no guarantee of continuing supply. So that is where it stands for the moment. In the meantime, if you’d like your own pulse oximeter you are best advised to buy one of the surprisingly low-cost units available via the internet. Want to try some smartphone apps out? See our list of heart rate monitors overleaf: V+ IR LED K  RED LED A  A K (FINGER) K TURN ON RED LED SET RED LED CURRENT  MICRO CONTROLLER A TURN ON IR LED PIN PHOTODIODE LCD READOUT MODULE SET IR LED CURRENT ADC INPUT CURRENT TO VOLTAGE CONVERTER siliconchip.com.au Fig.4: In a basic pulse oximeter the micro switches the two LEDs on and off, measures the light levels reaching the photodiode, works out the corresponding SpO2 level and displays this on the LCD readout. January 2016  15 Heart monitoring apps for smartphones Runtastic Heart Rate Monitor (iOS and Android) www.runtastic.com Pulse Phone (iOS) www.antimodular.com Instant Heart Rate (Android, iOS and Windows) Free. instantheartrate.com ADT Pulse (iOS) Free (Says Android but URL not found) www.adt.com MotionX 247 (iOS) – sleep tracker AND heart rate monitor http://24-7. motionx.com/ Runtastic Heart Rate Monitor is available for both Android and iOS smartphones and not only measures heartbeat but stores and graphs a great deal of heart-related data as well. There’s a simple free version and a paid version. If you have a reasonably modern smartphone, there are quite a large number of apps which use the camera in your phone to read heart rate (in some cases, among other things). Note that none of these apps can measure blood oxygen levels but knowing your heart rate while resting, during mild activity and during intense activity is essential information, something your medico would find really helpful. In fact, if your health care professional suspects any of a variety of cardio-related problems, he or she is likely to send you off for a “Stress ECG” test. While this looks at a lot more than heartbeat (eg, it also graphs your heart activity), the fundamental tests of resting, mild activity and intense (or stressful!) activity form the basis of a Stress ECG test. How do these apps work? Most work in one of two ways (and in some cases both ways) – they usually use the smartphone’s inbuilt white LED flash and camera to examine the blood flow (usually in your finger, just like the pulse oximeter) and compute the differences between pulses. In some (fewer) cases, they simply use the phone’s inbuilt camera to focus on a face and look at the almost invisible movement in facial features with each pulse of the blood vessels. Some phone apps offer both types, so you can look at your own heartrate 16  Silicon Chip or someone elses! Because there is only one light source in the flash/camera method, the app is not capable of determining venous or arterial blood flow so cannot determine oxygen levels. Similarly, in the facial recognition method, this is not possible. There are some drawback in using the apps: most of the flash/camera tests require intimate contact with both flash and lens, without movement. While this is not particularly difficult, it does run the risk of oiling or smudging the camera lens. And the facial recognition app requires the subject (and camera!) to stay perfectly still and in focus for a time. But apart from those, we didn’t have too much trouble. There are also some apps which use the phone’s inbuilt microphone to actually listen for the heartbeat. Heart Rate (iOS) – Also has facial recognition to measure heartbeat www.azumio.com Cardioo (iOS) ditto heart rate but not designed to measure from finger www.cardiio.com And if you’re really keen . . . check out www.iphoneness.com/iphoneapps/best-heart-rate-monitors-foriphone/ for 23 of the top heart rate monitors for iPhone. There are similar sites for Android smartphones. SC Where from, how much: Some of the apps listed below are free, others have a small charge (the highest we found was $US1.99). The old adage that you get what you pay for really doesn’t apply here because some of the best features are in the free apps! We’re not going to go out on a limb and recommend any particular app – do your own research and decide which one is right for you. First stop could be the iTunes/App Store or Google Play (of course, it also depends on which type of smartphone you have! In no particular order, here are some to look at (there are many more – Dr Google is your friend . . .) Instant Heart Rate is a simple free app for Android, iOS and Windows and can link to other health applications from the same company. siliconchip.com.au