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

Abstract

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 SpO 2 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.

Evaluation of key design parameters for mitigating motion artefact in the mobile reflectance PPG signal to improve estimation of arterial oxygenation

“…Dresher and Mendelson (2006) concluded that SpO 2 and PR errors during motion artefact (induced by walking) were minimized at a contact pressure of 12 KPa. We used a sensor with a sensor area of 0.42 cm 2 , so a contact force of 0.5 N corresponds to a contact pressure of 12 KPa which agrees with the findings of Dresher and Mendelson (2006), while Bideaux et al (2015) concluded that a contact force of 15 mmHg (2 KPa or 0.2 N cm −2 ) and motion artefacts were hard to handle in a wristbased commercial product. Our results showed that a contact force of 0.5 N used with our sensor would be ideal for a forehead sensor.…”

“…Due to the interdependencies of one or several of these variables with each other, these approaches may have resulted in suboptimal solutions to minimize the effects of motion artefact during signal acquisition. Bideaux et al (2015) investigated the AC/DC ratio and signaltonoise ratio of reflectiontype pulse oximeter sensors on the wrist using different design parameters such as contact pressure and the distance between the pho todiode and the red and infrared LEDs. Bideaux et al concluded that constraints related to a minimum contact pressure of 15 mmHg (or 0.2 N cm −2 ) and motion artefacts were major impediments in the creation of a com mercial product.…”

“…Pulse oximetry measure ments from the forehead offer a potential advantage in mobile settings that require unimpeded use of the hands that can introduce excessive motion artefacts, such as while monitoring soldiers on a battlefield. In applications such as sports or during sleep, Bideaux et al (2015) found that reflectance sensors did not interfere with the subject's activity, while transmission mode sensors were found to be uncomfortable and confining. Bideaux et al also conducted market research which showed that reflectance mode sensors were predominantly used over transmissionbased sensors in applications where the sensor was used to monitor blood oxygen level over a long period of time such as several hours or days.…”

Evaluating Quality of Photoplethymographic Signal on Wearable Forehead Pulse Oximeter With Supervised Classification Approaches

“…The reflective sensor can be applied to other parts of body, such as forehead or wrist. Therefore, the reflective PPG sensor is more suitable for the long-term and wearable measurement of physiological signals [3].…”