Reflectance pulse oximetry at the forehead improves by pressure on the probe

In this study, we investigated the possibility of improving reflectance (back-scatter) pulse oximetry measurements by pressure applied to the probe. Optimal signal detection, with the probe applied to an easily accessible location, is important to prevent erroneous oxygen saturation readouts. At the foreheads of 10 healthy adult volunteers, the effects of pressure applied onto the reflectance pulse oximeter probe were studied. Distances between the LEDs (660 nm and 940 nm) and the three photodiodes in the sensor were 4 mm, 7 mm, and 10 mm. For each detector, recordings were evaluated regarding red-to-infrared (R/IR) ratios and pulse sizes in relation to the stepwise increased pressure applied to the probe. R/IR variability decreased with applied pressures between 60 and 120 mm Hg. These findings are partly attributed to a corresponding increase in red and infrared pulse sizes at the detectors, which results in an improved signal-to-noise ratio. It is thought that pressure onto the oximeter sensor forces venous blood out of the tissues underneath the sensor. Consequently, the disturbing influence of pulsating and non-pulsating venous blood is reduced. Moreover, the increased difference in vessel diameter between diastole and systole and the corresponding difference in light absorption and an increase in flow velocities, causes an increase in pulse size with increasing pressure on the probe. Pressure applied to the probe may be useful in increasing the accuracy of reflectance pulse oximetry.