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Commonly used types of fingertip pulse oximeters were not consistent in their performance, with more frequent issues when tested in participants with darker skin tones, and sometimes fell short of regulations and manufacturer claims, according to a study from the Open Oximetry Project.

Among 11 fingertip pulse oximeters used during hypoxemia in healthy participants, five devices had a root mean square error (ARMS) >3%, which falls outside the acceptable FDA performance range, reported Isabella Auchus, MD, of the University of California San Francisco, and co-authors.

In addition, nine devices had worse performance in participants in the darkest skin pigmentation category versus those in the lightest category, they noted in

The reference device and eight other devices met International Organization for Standardization (ISO) criteria for ARMS of ≤4%, while 10 devices plus the reference device demonstrated higher ARMS in participants who were categorized at the lowest third of individual typology angle (ITA) values (having dark skin tones), compared with the highest third of ITA values (light skin tones) when measured at the area between the joint and the fingernail.

Seven devices and the reference device demonstrated a positive bias in participants who had darker pigmentation compared with those with lighter pigmentation. Most devices also demonstrated higher ARMS at lower arterial oxygen saturation (SaO2) levels, particularly among individuals with darker skin pigmentation.

"Our data support several steps that could be taken now to update regulatory guidance and improve performance standards," Auchus and team wrote. "The commonplace reliance on subjective, nonstandardized skin pigmentation assessment tools (e.g., pFP [perceived Fitzpatrick Scale]) is problematic and should be abandoned for several reasons, including the lack of standardized colors, inter-operator variability and bias, and the misappropriation of the initial purpose of the scales."

These findings come just over a month after an FDA advisory committee provided largely positive feedback to the agency's effort to improve the accuracy of pulse oximeters when used in darker-skinned patients. The FDA Medical Device Advisory Committee Anesthesiology and Respiratory Therapy Devices Panel did, however, raise some concerns, including the need for larger trials.

Studies dating back to the 1980s have uncovered the effects of dark skin pigmentation on currently used pulse oximeters. A study from last summer also showed that overestimations of Black patients' oxygen saturation by pulse oximetry may have led to delays in COVID-19 treatment and an unrecognized need for therapy among Black patients.

The 11 pulse oximetry devices tested included the Nellcor, serving as the reference device, the Nonin Onyx Vantage 9590, Masimo Mightysat, Walgreens MD300CN350R, Zacurate CMS 500DL, Walgreens OxyWatch C20, Choice MMed MD300CN340, Zacurate 500C, Bodymed BDMOXMTRBLK, Roscoe POX-ROS, Contec CMS50M, and the Biolight M70. The devices range in cost from below $60 to approximately $199.

While there was a wide range in oxygen saturation (SpO2)-SaO2 error between devices, the Nonin Vantage 9590 and Masimo Mightysat were ultimately deemed to have the best performance, while the Biolight and Roscoe performed the most poorly.

Among the devices tested, the Contec, Biolight, Masimo, and Nonin had all previously received FDA 510(k) premarket notification clearance and had met ISO standard 80601-2-61.

Participants were monitored using the respective devices, with researchers observing inspired oxygen, nitrogen, and carbon dioxide partial pressures in real time. These values were also adjusted through a partial rebreathing circuit as a way to achieve stable target SaO2 plateaus between 70% and 100% and partial pressure of carbon dioxide (PaCO2) values of 35-45 mm Hg.

The devices were tested among 34 participants who provided 4,360 blood samples. Among these participants, 27% were Asian, 24% were Caucasian, 21% were Black, 15% were Hispanic, and 15% were multi-ethnic. The participant population was evenly split on sex, and median age was 26.5. All were non-obese, non-smokers who did not have lung disease or cardiovascular comorbidities.

Patient skin pigmentation was assessed subjectively via the pFP, which ranges from one to six, with one being the lightest skin pigmentation, and six being the darkest, and the ITA via spectrophotometry for more objective measurements at nine different anatomical sites.

Thirty-five percent of participants were determined to have level III skin pigmentation on the pFP, 29% had level IV, 15% had level V, 12% had level VI, 6% had level II, and 3% had level I.

Determining whether a participant was categorized as "darkly-pigmented" was dependent on if their pigment was assessed by subjective or objective measurements. The number of participants categorized as having dark skin pigment was reduced when using the objective cutoff of an ITA less than 30◦ compared with the subjective pFP classification.

"The FDA guidance and ISO standards at the time of this study do not adequately account for the potential impact of skin pigmentation on POX [pulse oximeter] performance," Auchus and colleagues wrote. "We recommend that the pFP scale and other nonstandardized, subjective skin color scales no longer be used for defining diversity of skin pigmentation in POX validation studies."

All pulse oximeter devices were assigned a minimum of two participants with a darker-pigmented skin tone that exceeded 15% of the study population, in accordance with FDA guidance.

Auchus and team noted that the decision to focus on relatively young, healthy participants in the study could limit the generalizability of the findings. Other potential limitations included the small sample size of oximeters, the low level of recruitment of patients with darker skin tones, and a lack of accounting for effects of perfusion on pulse oximeter performance, among others.

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