Performance of Wearable Pulse Oximetry During Controlled Oxygen Desaturation

Introduction: Oxygen saturation is a critical measurement for monitoring patients with lung disease or other conditions who are at risk for hypoxemia. Recently, consumer wearable devices began incorporating oxygen saturation measurement tools, widening their use among individuals with lung disease. However, their performance has not been well characterized, particularly during desaturation due to limitations in collecting comparison arterial blood during exercise induced hypoxemia. To address this knowledge gap, we performed a controlled oxygen desaturation study in nine healthy participants to mimic exercise induced hypoxia, reducing the blood oxygen saturation levels from 100% to 60% using a dedicated gas delivery system. We conducted a comparative analysis of the non-invasive oxygen saturation measured on the Masimo MightySat Rx and Apple Watch Series 7 with the ‘gold standard’ invasive arterial oxygen saturation.

Materials and

Methods: This study involved two sequential, stepwise oxygen desaturation sequences. In the first sequence the oxygen saturation (SaO2) was reduced in stepwise fashion from ~100% to ~60%, and then back to ~100% over the course of approximately 60 minutes. The second sequence involved decreasing participants’ SaO2 from ~100% to ~80%, and then back to ~100% over the course of 48 minutes. During the desaturation sequences, the displayed pulse oximeter saturation (SpO2) of two devices (the Apple Watch Series 7 and the Masimo MightySat Rx) were recorded at the time of arterial blood draws. The RespirAct RA-MR is a sequential gas delivery system used to control oxygen saturation and provide steady-state plateaus prior the blood gas sampling1. The blood gas samples were then analyzed by the Radiometer ABL90 Flex co-oximeter to yield the arterial oxygen saturation (SaO2), considered to be the ‘gold standard’ measurement.

Results, Conclusions, and Discussions: Results
Accuracy of all measurements
Three hundred and five out of 308 (99%) and 284 out of 308 (92%) possible SpO2 measurements were taken successfully from Masimo MightySat Rx and Apple Watch Series 7, respectively. A Bland–Altman analysis demonstrated the difference between the SpO2 measurements from the Apple Watch and Masimo MightySat Rx and reference SaO2 measurements (Fig 1A, Fig 1B). The Mean Directional Error (MDE) of SpO2 was 1.80% and 3.26%, respectively, for the Masimo MightySat Rx and the Apple Watch.

Accuracy of measurements under hypoxemia
Among the 308 measurements in the ABG analysis, 165 SaO2 measurements (54%) are below 88 percent, which is the clinical threshold for hypoxemia2. Among these 165 valid SaO2 measurements, 164 (99%) and 140 (85%) SpO2 measurements were collected simultaneously from the Masimo MightySat Rx and Apple Watch Series 7, respectively.

A Bland–Altman analysis demonstrated the difference between the SpO2 measurements from each the Apple Watch and the Masimo MightySat Rx against ABG SaO2 measurements under hypoxemia (Fig 1C, Fig 1D). The MDE of SpO2 were 1.96% and 4.99% for the Masimo MightySat Rx and the Apple Watch, respectively.

Discussion
This study highlights the limitations of noninvasive oxygen saturation monitoring. Our analysis demonstrates the trend of overestimation for both devices and that the severity of overestimation by the Apple Watch Series 7 was higher than the overestimation by the Masimo MightySat Rx. The severity of the overestimation increases under hypoxemia. The MDE of the Masimo MightySat Rx and the Apple Watch Series 7 increases from 1.80% to 1.96% and from 1.96% to 4.99%, respectively.

Conclusion
In this study, we measured the blood oxygen saturation of 9 participants using the Apple Watch Series 7 and one clinical-grade oximeter during a controlled desaturation process. We calculated the MDE for both devices compared to the gold standard, arterial blood gas analysis. Compared to the gold standard, both devices demonstrated limitations under low oxygen saturation levels ( < 88%). However, while the Masimo MightySat Rx experienced an error of 1.96% which is within the FDA expected error of 2-3%, the Apple Watch Series 7 experienced an error of 4.99%.

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