Evaluation of design parameters for a reflection based long-term pulse oximetry sensor

Pulse oximetry is a standard parameter for many years in clinical patient monitoring. There are also sensors that can be used at home. But all these sensors use the transmission based method to measure the oxygen saturation which require finger or ear clips that are uncomfortable and confining and therefore not fit for long-term monitoring. Sleep-related breathing disorders or breathing thin air in high altitudes can lead to insufficient oxygen intake. Insufficient oxygen supply can cause permanent damage to the tissue and in some cases even death. For early ambulatory diagnosis, a reflection based longterm pulse oximetry sensor would be the best solution. There are no available devices on the market, even so many studies showed promising results. That why, we evaluated design parameters for a reflection based long-term pulse oximetry sensor, located at the wrist. The prototype we developed consists of two LEDs (one red and one infrared), a photodiode and the evaluation board SLAU480B from Texas Instruments. We tested three sensor layouts and discovered that the distance between the LEDs and the photodiode, the contact pressure and motion artefacts were the most important signal influencing parameters. After the Signal processing we obtained a signal to noise ratio of 22 dB (red) and 30 dB (infrared) and a AC/DC ratio of 1-3 %, which should be more than enough to calculate the SpO2 value. Also motion artifacts were tested and seem to affect the measurement at the lower wrist strongly. In conclusion we found some reasons why there is no such device on the market yet. Keywords—Pulse oximetry; long-term monitoring; reflexion; hypoxia; sensor; SpO2; PPG