Pulse oximetry is a noninvasive method to assess arterial blood oxygenation. The percentage of hemoglobin that is oxygenated is measured by passing light of two different wavelengths (660 nm [for deoxyhemoglobin] and 940 nm [for oxyhemoglobin]) through a blood-carrying tissue (e.g., finger, earlobe, forehead), identifying the pulsatile component (which contains arterial blood and background tissue elements), and subtracting the nonpulsatile component to isolate the arterial component. The device can estimate the percent of oxygenated hemoglobin over the range of 100% to about 75%. Most clinicians regard the output of pulse oximeters to be inaccurate for percent saturation values of less than 70%, although the probability of a low saturation should not be discounted. Pulse oximetry measurements may help identify significant drops in Pao2 below 60 to 65 mm Hg but are relatively insensitive to changes in Pao2 from 90 to 65 mm Hg.