Pulse Oximetry

Pulse oximetry is a noninvasive technique of bedside transcutaneous monitoring of arterial oxygen saturation (SpO2) and pulse (the abbreviation SaO2 is used for the parameter measured with a blood gas test).

Technique Description

Pulse oximetry is based on transmission spectrophotometry that uses different optical properties of oxygenated and deoxygenated hemoglobin and applies an algorithm to estimate SaO2. Sensors applied to a finger, earlobe, forehead, or nose flap are used for the measurements.

Result Interpretation

A normal SpO2 level is from 95% to 98% (in patients aged >70 years, 94%-98%). In patients treated with oxygen, SpO2 may be as high as 99% or 100% (although targeting such high values is not recommended in most clinical situations [see Oxygen Therapy]). The unequivocally abnormal SpO2 values <90% correspond to partial pressure of oxygen in arterial blood (PaO2) <60 mm Hg.

The most important measurement limitations include motion artifacts and impaired peripheral perfusion. The SpO2 value may be overestimated by the presence of carboxyhemoglobin and, in concentrations of up to 20% to 30%, by methemoglobin. The results may be underestimated by up to 2% in patients with nail polish (particularly with older devices, and mostly with black nail polish) or onychomycosis. Application of the probe on other anatomical sites or even nail polish removal may be needed. The patient’s skin color may play a role, as those identifying as Black had an increased risk of occult hypoxia compared with White, Asian, and Hispanic, and this in turn was associated with an increase in organ dysfunction and mortality.Evidence 1High Quality of Evidence (high confidence that we know true effects of intervention). Sjoding MW, Dickson RP, Iwashyna TJ, Gay SE, Valley TS. Racial Bias in Pulse Oximetry Measurement. N Engl J Med. 2020 Dec 17;383(25):2477-2478. doi: 10.1056/NEJMc2029240. Erratum in: N Engl J Med. 2021 Dec 23;385(26):2496. PMID: 33326721; PMCID: PMC7808260. Wong AI, Charpignon M, Kim H, et al. Analysis of Discrepancies Between Pulse Oximetry and Arterial Oxygen Saturation Measurements by Race and Ethnicity and Association With Organ Dysfunction and Mortality. JAMA Netw Open. 2021 Nov 1;4(11):e2131674. doi: 10.1001/jamanetworkopen.2021.31674. PMID: 34730820. Until pulse oximetry algorithms are verified in more diverse patient populations, clinicians should consider the risk of occult hypoxia, especially in patients who identify as Black.