Occult Hypoxemia Is More Common in Black Patients than in White Patients About to Undergo ECMO for Respiratory Failure
By J. Brady Scott, PhD, RRT, RRT-ACCS, AE-C, FAARC, FCCP
Associate Professor, Department of Cardiopulmonary Sciences, Division of Respiratory Care, College of Health Sciences, Rush University, Chicago
SYNOPSIS: In this retrospective analysis of de-identified Extracorporeal Life Support Organization registry data, the prevalence of occult hypoxemia was higher in Black patients than in white patients. Hispanic and Asian patients had a similar prevalence of occult hypoxemia compared to white patients.
SOURCE: Valbuena VSM, Barbaro RP, Claar D, et al. Racial bias in pulse oximetry measurement among patients about to undergo extracorporeal membrane oxygenation in 2019-2020: A retrospective cohort study. Chest 2022;161:971-978.
Valbuena and colleagues used de-identified data from the Extracorporeal Life Support Organization (ELSO) registry to determine if pulse oximetry is less accurate in non-white adult patients undergoing extracorporeal membrane oxygenation (ECMO) for respiratory failure.1 Data used for the study was collected retrospectively from 324 different centers between January 2019 and July 2020. The research team hypothesized that pulse oximetry would be less accurate for Black, Hispanic, and Asian patients compared to white patients.
Arterial oxygen saturations (SaO2) were obtained from arterial blood gas results drawn no more than six hours before ECMO initiation. Those SaO2 values were compared to peripheral oxygen saturation (SpO2) values. The investigators attempted to collect SaO2 values that corresponded with SpO2 values but admitted the exact synchrony of the two values was not validated. SpO2 values of 92% to 96% were used in this study since the investigators felt the bedside physicians were less likely to make oxygen adjustments based on those percentages. The outcome of interest was the prevalence of occult hypoxemia (defined as SaO2 ≤ 88% despite SpO2 between 92% and 96%). Patient race and ethnicity were obtained from what was entered by the ECMO center; objective measures of skin tone were not available for analysis.
Chi-square tests were used to compare pre-ECMO occult hypoxemia in white patients to that in patients of the other races and ethnicities. Logistic regression, adjusted for sex and measured SpO2, was performed to compare each race and ethnicity group to white patients. The study team also conducted a secondary analysis to examine occult hypoxemia for SpO2 values that were > 96% across patients of different races and ethnicities.
Overall, 372 SpO2-SaO2 pairs were analyzed for occult hypoxemia when SpO2 was between 92% and 96%, which included 186 white, 51 Black, 70 Hispanic, and 65 Asian patients. For white patients, 19 of 186 (10.2%; 95% confidence interval [CI], 6.2% to 15.3%) had SaO2 ≤ 88%. Eleven of 51 Black patients (21.5%; 95% CI, 11.3% to 35.3%), six of 70 Hispanic patients (8.6%; 95% CI, 3.2% to 17.7%), and six of 65 Asian patients (9.2%; 95% CI, 3.5% to 19.0%) had SaO2 ≤ 88%. When the groups were compared, there was a statistically significant difference between white and Black patients (P = 0.031), but not between white and Hispanic or Asian patients. When compared to white patients, Black patients had a higher risk of occult hypoxemia (odds ratio [OR], 2.57; 95% CI, 1.12-5.92; P = 0.026), whereas Hispanic and Asian patients were at no additional risk. For the secondary analysis, evaluating pre-ECMO SpO2 values > 96%, seven of 101 white patients (6.9%; 95% CI, 2.8% to 13.8%), seven of 34 Black patients (20.6%; 95% CI, 8.7% to 37.9%), three of 37 Hispanic patients (8.1%; 95% CI, 0.6% to 18.2%), and one of 24 Asian patients (4.2%; 95% CI, 0.1% to 21.1%) had SaO2 ≤ 88%. When compared to white patients, Black patients with a SpO2 > 96% had a higher risk of occult hypoxemia (OR, 3.52; 95% CI, 1.12-11.10), whereas Hispanic and Asian patients were at no additional risk.
The study authors concluded that occult hypoxemia is more prevalent in Black patients when compared to white patients about to undergo ECMO for respiratory failure. They also noted that pulse oximetry has limitations and racially differential usefulness in evaluating hypoxemia for patients in acute respiratory failure.
This paper adds to the collective body of literature suggesting that pulse oximetry accuracy is affected by skin color. As stated by Okunlola et al,2 there has been a renewed interest in pulse oximetry because of the COVID-19 pandemic and a retrospective study published by Sjoding et al3 highlighting significant errors in pulse oximeter performance. In fact, Senators Elizabeth Warren, Cory Booker, and Ron Wyden sent a letter to the United States Food and Drug Administration in 2021 urging them to review the interaction between a patient’s skin color and the accuracy of pulse oximeters.4
The results of this study are interesting, but several significant limitations need to be considered. Race was used to define patients since there was a lack of objective data on skin tone available for analysis. Okunlola et al2 suggested that using race to define patients with dark skin pigmentation is problematic. They proposed that standardized, less subjective measures be used to understand the impact of skin tone on pulse oximeter performance. Additionally, they suggested that objective measures be done at the site of pulse oximeter measurements (e.g., both sides of the fingertip).2 The retrospective nature of the analysis contributed to additional study limitations. It was unknown if SaO2 and SpO2 were collected simultaneously, and since oxygen levels can drop over minutes, the difference between the two values may not be accurately represented. Also, the investigators were unable to adjust for severity of illness and did not have pertinent comorbidity data, such as smoking status or diabetes, that might affect pulse oximetry readings. Finally, the type of pulse oximeters used was not recorded, which might have been helpful in understanding how device brands might differ.
Well-designed prospective studies are needed to address pulse oximetry performance in patients with dark skin pigment. The study by Valbuena and colleagues further highlights the need to advance this research to promote healthcare equity and improve patient safety.1
- Valbuena VSM, Barbaro RP, Claar D, et al. Racial bias in pulse oximetry measurement among patients about to undergo extracorporeal membrane oxygenation in 2019-2020: A retrospective cohort study. Chest 2022;161:971-978.
- Okunlola OE, Lipnick MS, Batchelder PB, et al. Pulse oximeter performance, racial inequity, and the work ahead. Respir Care 2022;67:252-257.
- Sjoding MW, Dickson RP, Iwashyna TJ, et al. Racial bias in pulse oximetry measurement. N Engl J Med 2020;383:2477-2478.
- Elizabeth Warren, United States Senator for Massachusetts. Senators Warren, Booker and Wyden urge FDA to address concerns about dangerous pulse oximeter inaccuracies for patients of color.
In this retrospective analysis of de-identified Extracorporeal Life Support Organization registry data, the prevalence of occult hypoxemia was higher in Black patients than in white patients. Hispanic and Asian patients had a similar prevalence of occult hypoxemia compared to white patients.
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