Pulse oximetry has been an essential tool in helping to save lives and improving the quality of life for well over 50 years. Pulse oximeters monitor the oxygen saturation and heart rate of the wearer noninvasively. Advanced units are able to display the pulsatile nature of the arterial blood vessels. The pulse oximeter is an important medical tool that aids in the diagnosis of cardiac and vascular anomalies in neonatal, pediatric, and adult patients. Analysis of the pulsatile signal gives trained professionals a wealth of information about the respiratory, circulatory, and cardiovascular systems -- all without taking a blood sample.
Pulse oximeters are also being used in the home. They are routinely used to determine home oxygen liter-flow and to monitor respiration and heart rate during exercise. Serious athletes and professional trainers monitor oxygen levels to optimize efficiency during strenuous workouts.
This year’s Consumer Electronics Show (CES) in Las Vegas witnessed many new heart rate monitor devices that are being made available to consumers. Many of these devices measure heart rate by acquiring the electrical activity of the heart, predominantly with a chest strap or wristwatch. Chest strap heart monitors have been around a long time, and they work well -- but tend to be uncomfortable. The typical wrist-based heart rate monitors is bio-potential-based, and requires contact with both hands to take a measurement. The ideal device would be a passive, but continuously measuring vitals, including heart rate. The device needs to be comfortable to wear and positioned on the body in a location that does not interfere with daily activities. Ideal locations include the wrist, ears, forehead, forearm, calf, neck or ankle. Principle of operation The operating principle for a pulse-oximeter being used to measure the blood oxygen saturation level is to project light (normally LED) sources of different wavelengths through the body tissue to a photo-detector. Typically, the LED wavelengths being used for pulse-oximeters are red (~660 nm) and infrared (~905 nm). These two wavelengths of light, are absorbed by the blood at different rates, dependent upon the blood oxygen level.
When the blood is oxygenated red light is absorbed to a lesser extent than when using the infrared (IR) light. When the blood is deoxygenated, red light is absorbed to a greater extent than the IR light. This can be seen in Figure 1,which shows light absorption by wavelength and whether the blood is oxygenated (HbO2) or de-oxygenated (Hb).
Figure 1: Oxygenated versus deoxygenated blood light absorption of IR and RED
Common body locations for taking pulse-oximeter measurements are through the tissue at the fingertip, earlobe, or foot (normally on infants). LEDs intensity, tissue thickness, skin color, sensor placement, and the oxygenated and deoxygenated blood light absorption all need to be considered during measurement.
The oximeter calculates oxygen saturation by taking the ratio of the absorption of red and infrared light, separating the time invariant parameters (intensity of light, skin color, tissue type, and deoxygenated blood) from the time varying parameters (oxygenated blood). Normal oxygen saturation values for a healthy individual range between 95 and 100 percent. The measured signal is pulsatile in nature due to the arterial blood vessels which expand and contract corresponding to each heartbeat. There are two types of pulse-oximeter measurements: transmissive and reflective. Reflective is normally used on the chest or forehead locations. Transmissive, which is more common, is normally used for finger, earlobe, or infant foot locations.
Placement of the LED light source and the photo-diode, along with separation of the light and detector, differentiate these two device types. The transmissive method projects a light through the tissue and a photodiode measures the light, which makes it through to the opposite side. In the reflective method, the light source and photodiode are the same surface. The signal levels for reflective are much smaller, so the user needs to be mindful of where it’s placed and how the circuit is designed.