# Performance of Wearable Pulse Oximetry During Controlled Hypoxia Induction: Instrument Validation Study

## TL;DR

This study compared the accuracy of Apple Watch Series 7 and a clinical pulse oximeter during low blood oxygen levels, finding both overestimated oxygen saturation, especially at critical thresholds.

## Contribution

The study evaluates wearable pulse oximetry accuracy during controlled hypoxia and identifies limitations in consumer and clinical devices under low oxygen saturation.

## Key findings

- Both devices overestimated oxygen saturation, with the Apple Watch being less accurate than the clinical oximeter.
- Accuracy declined notably at oxygen saturation levels below 88%, with both devices exceeding FDA thresholds.
- No significant relationship was found between oxygen desaturation rate and measurement error for either device.

## Abstract

Oxygen saturation is a crucial metric used for monitoring patients with lung disease or respiratory illness who are at risk of hypoxemia (low blood oxygen saturation). Early and accurate identification of abnormal oxygen saturation is important for these patients, who may develop significant desaturation and hypoxemia symptoms during their daily activities.

This study aimed to evaluate the accuracy of Apple Watch Series 7 and a clinical-grade pulse oximeter, Masimo MightySat Rx, under hypoxemia and to assess whether measurement error is influenced by the oxygen desaturation rate (ODR).

We calculated the ODR of each measurement and conducted a comparative analysis of the displayed oxygen saturation readings from both the Masimo MightySat Rx finger pulse oximeter and Apple Watch Series 7 with arterial blood oxygen saturation (SaO2) readings obtained from a blood gas analyzer.

Both the Masimo MightySat Rx pulse oximeter and the Apple Watch Series 7 tended to overestimate oxygen saturation. The pulse oximeter readings were more likely to fall within 2% of the acceptable (as specified by Masimo) peripheral oxygen saturation (SpO₂) error range than the Apple Watch (49.03% vs 32.14%). Notably, both devices had limitations under low oxygen saturation levels (<88%), with an accuracy root mean square difference (Arms) of 3.52% (95% CI 3.18%‐3.86%) and 5.82% (95% CI 5.32%‐6.31%) for the Masimo MightySat Rx and Apple Watch Series 7, respectively. Among the blood oxygen measurements taken during a high ODR (ie, ≥2% SpO2 per minute), which is a rate clinically correlated with sleep apnea, the Arms increased slightly by 0.75% for the Masimo MightySat Rx and decreased by 0.28% for the Apple Watch Series 7.

Both devices consistently overestimated SpO2, with accuracy declining notably during hypoxemia. The Apple Watch Series 7 mean bias suggests a likelihood of missing instances of hypoxemia, particularly at arterial oxygen saturation values below but close to 88%. Both the Apple Watch Series 7 and Masimo MightySat Rx exhibited Arms values exceeding the US Food and Drug Administration threshold under conditions of hypoxemia. While past studies have implicated high ODRs in increasing measurement error, we found no statistically significant relationship between ODR and measurement error for either device. Overall, our findings of SpO2 overestimation and high Arms values underscore the need for caution when interpreting oxygen saturation values from these devices. The small sample size and limited diversity in skin tone and age restrict the generalizability of our findings. Future studies should include larger and more diverse populations to evaluate the performance of wearable-based pulse oximetry.