This article will interface you with the step by step things required when you are about to make your own Pulse oximeter.
Principles of Pulse Oximetry Technology:
The principle of pulse oximetry is based on the red and infrared light absorption characteristics of oxygenated and deoxygenated hemoglobin. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated (or reduced) hemoglobin absorbs more red light and allows more infrared light to pass through. Red light is in the 600-750 nm wavelength light band. Infrared light is in the 850-1000 nm wavelength light band.
Pulse oximetry uses a light emitter with red and infrared LEDs that shines through a reasonably translucent site with good blood flow. Typical adult/pediatric sites are the finger, toe, pinna (top) or lobe of the ear. Infant sites are the foot or palm of the hand and the big toe or thumb. Opposite the emitter is a photodetector that receives the light that passes through the measuring site.
There are two methods of sending light through the measuring site: transmission and reflectance. In the transmission method, as shown in the figure on the previous page, the emitter and photodetector are opposite of each other with the measuring site in-between. The light can then pass through the site. In the reflectance method, the emitter and photodetector are next to each other on top the measuring site. The light bounces from the emitter to the detector across the site. The transmission method is the most common type used and for this discussion the transmission method will be implied.
After the transmitted red (R) and infrared (IR) signals pass through the measuring site and are received at the photodetector, the R/IR ratio is calculated. The R/IR is compared to a “look-up” table (made up of empirical formulas) that convert the ratio to an SpO2 value. Most manufacturers have their own look-up tables based on calibration curves derived from healthy subjects at various SpO2 levels. Typically a R/IR ratio of 0.5 equates to approximately 100% SpO2, a ratio of 1.0 to approximately 82% SpO2, while a ratio of 2.0 equates to 0% SpO2.
The major change that occurred from the 8-wavelength Hewlett Packard oximeters of the ’70s to the oximeters of today was the inclusion of arterial pulsation to differentiate the light absorption in the measuring site due to skin, tissue and venous blood from that of arterial blood.
At the measuring site there are constant light absorbers that are always present. They are skin, tissue, venous blood, and the arterial blood. However, with each heart beat the heart contracts and there is a surge of arterial blood, which momentarily increases arterial blood volume across the measuring site. This results in more light absorption during the surge. If light signals received at the photodetector are looked at ‘as a waveform’, there should be peaks with each heartbeat and troughs between heartbeats. If the light absorption at the trough (which should include all the constant absorbers) is subtracted from the light absorption at the peak then, in theory, the resultants are the absorption characteristics due to added volume of blood only; which is arterial. Since peaks occur with each heartbeat or pulse, the term “pulse oximetry” was coined. This solved many problems inherent to oximetry measurements in the past and is the method used today in conventional pulse oximetry.
Still, conventional pulse oximetry accuracy suffered greatly during motion and low perfusion and made it difficult to depend on when making medical decisions. Arterial blood gas tests have been and continue to be commonly used to supplement or validate pulse oximeter readings. The advent of “Next Generation” pulse oximetry technology has demonstrated significant improvement in the ability to read through motion and low perfusion; thus making pulse oximetry more dependable to base medical decisions on.
The main things to remember is that you are looking at two different output voltages. One from a red LED, and another from an infrared LED. The basic principle of the whole thing is sort of like what you probably did as a kid, taking a flashlight, holding it to your hand at night and and seeing your bones.
Now getting to the LabVIEW portion of everything
As you can see in the above figure all you are really looking at is two voltages. That means you will simply need to have two separate analog input channels measuring the voltages coming from the different LEDs.
- The Pulse will be really easy to see because the voltages will be going up and down easily showing the systole and diastole of the heart. You should be able to simply plot one of the two lines on a waveform graph and see the heartbeat (keep in mind that pulse oximetry is VERY sensitive to noise artifact).
- The Oxygen Saturation is a little more difficult to measure and may require some custom scaling. However, even this should not be too difficult as it is just a matter of comparing the two different voltages and figuring out what the difference is and how exactly that relates to a percentage. If you get a percentage of less than about 90%, then you may want to head straight to the nearest hospital or stop smoking.
That is the basics of how everything works. And from the LabVIEW end of things all you need is a Data Acquisition device and a simple program to continuously read the voltages from your sensors. If you have LabVIEW 7.0 or higher, then open LabVIEW and go to Help > Find Examples > Browse > Hardware Input and Output > DAQmx > Analog Measurements > Cont Acq&Graph Voltage-Int Clk.vi. That example should allow you to easily read some voltages. You can then modify the example in whatever way you deem necessary to compare the two measurements and extract the pulse oximetry information that you need.