Overestimation of Oxygen Saturation Measured by Pulse Oximetry in Hypoxemia. Part 1: Effect of Optical Pathlengths-Ratio Increase.

On average, arterial oxygen saturation measured by pulse oximetry (SpO) is higher in hypoxemia than the true oxygen saturation measured invasively (SaO), thereby increasing the risk of occult hypoxemia. In the current article, measurements of SpO on 17 cyanotic newborns were performed by means of a Nellcor pulse oximeter (POx), based on light with two wavelengths in the red and infrared regions (660 and 900 nm), and by means of a novel POx, based on two wavelengths in the infrared region (761 and 820 nm). The SpO readings from the two POxs showed higher values than the invasive SaO readings, and the disparity increased with decreasing SaO. SpO measured using the two infrared wavelengths showed better correlation with SaO than SpO measured using the red and infrared wavelengths. After appropriate calibration, the standard deviation of the individual SpO-SaO differences for the two-infrared POx was smaller (3.6%) than that for the red and infrared POx (6.5%, < 0.05). The overestimation of SpO readings in hypoxemia was explained by the increase in hypoxemia of the optical pathlengths-ratio between the two wavelengths. The two-infrared POx can reduce the overestimation of SpO measurement in hypoxemia and the consequent risk of occult hypoxemia, owing to its smaller increase in pathlengths-ratio in hypoxemia.