Shedding Light on Pulse Oximetry

Column Author: Ron Palmen, MD | Internal Medicine-Pediatrics Resident

Column Editor: Kathleen Berg, MD, FAAP | Hospitalist - Pediatrics; Associate Professor of Pediatrics, University of Missouri-Kansas City School of Medicine; Clinical Assistant Professor of Pediatrics, University of Kansas School of Medicine

In an article in the February 2023 edition of The Link, “More Than a Number: Implications of Hypoxemia in the Outpatient Management of Bronchiolitis,” we reviewed evidence on the use of pulse oximetry in the setting of bronchiolitis. We addressed the difference between hypoxemia and hypoxia and discussed the use of pulse oximetry as a surrogate for evaluating hypoxemia. However, in addition to pulse oximetry’s inherent limitations, additional patient-level factors should be taken into account. In particular, growing evidence suggests that skin pigmentation may impact the accuracy of pulse oximetry. To explore this phenomenon, we must first dive into the intricacies of this frequently used medical technology. 

How is the percentage on the screen, which influences our triage ability and clinical judgment, generated? The glow exuded by the tape or clip we place on fingers, earlobes and foreheads is generated by photons emitting in red and near infrared light frequencies where hemoglobin primarily absorbs light. Oxyhemoglobin and deoxyhemoglobin have different absorptive capacities based on Beer’s Law, which takes into account path length and coefficients that account for scatter. Since calculating these absorptive capacities for individual patients in a given clinical scenario is not feasible, commercial pulse oximeters undergo calibrations with healthy, young adult volunteers.¹ 

If pulse oximetry depends on the absorption of light by hemoglobin, does the pigmentation of one’s skin make a difference? This question was explored in 2005 when findings by Bickler et al. suggested that SpO2 is overestimated in individuals with darker skin tone.² Since then, these results have been supported by several other studies of adult patients, including one by Sjoding et al., which found Black patients had nearly three times the frequency of occult hypoxemia that was not detected by pulse oximetry when compared to White patients.³ In another observational study of patients with SaO2 85%-89%, pulse oximeters overestimated SaO2 by an average of 5.8% in Asians, 3.9% in Blacks and 2.4% in Whites.⁴ 

Although oxyhemoglobin and deoxyhemoglobin absorb red and near infrared light frequencies in our tissues, they are not alone. Melanin and myoglobin also absorb light at these frequencies.¹ Therefore, in patients with more melanin content compared to the individuals used for commercial pulse oximeter calibration, additional absorption may overestimate the presence of oxyhemoglobin⁵ and may explain the findings of the studies above. This overestimation is accentuated when patients with more melanin are in hypoxic conditions,¹ adding to the inaccuracy of SpO2 in certain clinical situations. 

Beyond skin pigmentation, other factors play a role in pulse oximeter accuracy. Patient motion, irregular heart rhythms (including tachycardia), states of poor perfusion, and use of injectable dyes all artificially lower SpO2 readings. Ambient light, particularly infrared, also interacts with pulse oximetry sensors. Because calibrations are performed with healthy young adults, limitations of pulse oximetry are particularly important to consider in critically ill and neonatal patients.⁶  

Given the above evidence, it is reasonable to question if overestimation of SpO2 in children with darker complexion is contributing to inequities in care. Among children with bronchiolitis, are those with more skin pigmentation admitted less frequently? Interestingly, studies suggest the opposite. A study following healthy children over time found significant variance in the incidence of hospitalization due to bronchiolitis based on race and ethnicity: 8.6 for non-Hispanic White, 15.4 for non-Hispanic Black, 19.1 for Hispanic, and 6.5 for Asian infants per 1,000 births.⁷ Another study evaluated the overutilization of low-value interventions for children admitted for bronchiolitis. During hospitalization, White children were more likely to receive chest radiography and antibiotic treatment compared to non-White children, while non-White children were more likely to receive corticosteroids and bronchodilators.⁸ Zheng et al. reported similar results but found no statistically significant differences in terms of hospital length of stay, intensive care interventions, or follow-up care based on race and ethnicity.⁹ 

It is important to recognize the limitations of pulse oximetry, including the possibility of overestimating SpO2 in individuals with more skin pigmentation. Because this overestimation may contribute to health care inequities, pulse oximetry should augment, but not replace clinical judgment. Certainly, the question warrants additional study.

References: 

1. Nitzan M, Romem A, Koppel R. Pulse oximetry: fundamentals and technology update. Med Devices (Auckl). 2014;7:231-239. doi:10.2147/MDER.S47319
2. Bickler PE, Feiner JR, Severinghaus JW. Effects of skin pigmentation on pulse oximeter accuracy at low saturation. Anesthesiology. 2005;102(4):715-719. doi:10.1097/00000542-200504000-00004
3. Sjoding MW, Dickson RP, Iwashyna TJ, Gay SE, Valley TS. Racial bias in pulse oximetry measurement [published correction appears in N Engl J Med. 2021 Dec 23;385(26):2496]. N Engl J Med. 2020;383(25):2477-2478. doi:10.1056/NEJMc2029240
4. Crooks CJ, West J, Morling JR, et al. Pulse oximeter measurements vary across ethnic groups: an observational study in patients with COVID-19. Eur Respir J. 2022;59(4):2103246. doi:10.1183/13993003.03246-2021
5. Cabanas AM, Fuentes-Guajardo M, Latorre K, León D, Martín-Escudero P. Skin pigmentation influence on pulse oximetry accuracy: a systematic review and bibliometric analysis. Sensors (Basel). 2022;22(9):3402. doi:10.3390/s22093402
6. Fouzas S, Priftis KN, Anthracopoulos MB. Pulse oximetry in pediatric practice. Pediatrics. 2011;128(4):740-752. doi:10.1542/peds.2011-0271
7. Inagaki K, Blackshear C, Burns PA, Hobbs CV. Racial/ethnic disparities in the incidences of bronchiolitis requiring hospitalization. Clin Infect Dis. 2021;72(4):668-674. doi:10.1093/cid/ciaa113
8. Honcoop AC, Poitevien P, Kerns E, Alverson B, McCulloh RJ. Racial and ethnic disparities in bronchiolitis management in freestanding children’s hospitals. Acad Emerg Med. 2021;28(9):1043-1050. doi:10.1111/acem.14274
9. Zheng DX, Goel R, Mitri EJ, et al. Confirming racial/ethnic disparities in the management of severe bronchiolitis. Am J Emerg Med. 2022;58:333-335. doi:10.1016/j.ajem.2022.03.045