REFLECTANCE PULSE OXIMETRY AT THE FOREHEAD IMPROVES BY PRESSURE ON THE PROBE

In this study, we investigated the possibility of improving reflectanc e (back-scatter) pulse oximetry measurements by pressure applied to th e probe. Optimal signal detection, with the probe applied to an easily accessible location, is important to prevent erroneous oxygen saturat ion readouts. At the foreheads of 10 healthy adult volunteers, the eff ects of pressure applied onto the reflectance pulse oximeter probe wer e studied. Distances between the LEDs (660 nm and 940 nm) and the thre e photodiodes in the sensor were 4 mm, 7 mm, and 10 mm. For each detec tor, recordings were evaluated regarding red-to-infrared (R/IR) ratios and pulse sizes in relation to the stepwise increased pressure applie d to the probe. R/IR variability decreased with applied pressures betw een 60 and 120 mm Hg. These findings are partly attributed to a corres ponding increase in red and infrared pulse sizes at the detectors, whi ch results in an improved signal-to-noise ratio. It is thought that pr essure onto the oximeter sensor forces venous blood out of the tissues underneath the sensor. Consequently, the disturbing influence of puls ating and non-pulsating venous blood is reduced. Moreover, the increas ed difference in vessel diameter between diastole and systole and the corresponding difference in light absorption and an increase in flow v elocities, causes an increase in pulse size with increasing pressure o n the probe. Pressure applied to the probe may be useful in increasing the accuracy of reflectance pulse oximetry.