Commercial smartwatch detects low blood oxygen saturation

Compared with a medical-grade pulse oximeter, the Apple Watch Series 6 can detect reduced levels of blood oxygen saturation (SpO₂) as accurately as a medical-grade pulse oximeter does, according to a recent study.

“The SpO₂ monitoring technology used in this smartwatch is sufficiently advanced for the indicative measurement of SpO₂ outside the clinic and can detect states of reduced blood oxygen saturation,” the researchers said.

Twenty-four healthy participants (mean age 24 years, 19 men) participated in the present analysis, each of whom was given the smartwatch device, to be worn on the left wrist. As a reference standard, a pulse oximeter sensor was worn on the left middle finger.

To induce changes in SpO₂, participants underwent a breathing circuit using a three-way nonrebreathing valve. First, participants inhaled ambient air through the breathing circuit for 2 minutes, followed by a 5-minute desaturating phase, where they were exposed to a hypoxic gas mixture from a Douglas bag.

In the final phase of the experiment, participants were again stabilized by inhaling ambient air. SpO₂ measurements were collected using the smartwatch and oximeter devices at different time points throughout the circuit.

In total, researchers obtained 1,284 valid paired readings for analysis, after excluding data points that were faulty. SpO₂ measurements ranged from 76 percent to 100 percent, with most (75 percent) falling between 90 percent and 100 percent. Only 1 percent of SpO₂ measurements fell below 80 percent. [Digit Health 2022;doi:10.1177/20552076221132127]

Bland-Altman plots were constructed out of 642 individual data points derived from the complete and valid SpO₂ readings. Results showed a 0.0-percent bias between the smartwatch and oximeter for all data points, with a 95 percent confidence interval of –0.2 to 0.3.

Moreover, the 95 percent limits of agreement between the two devices ranged from –5.8 percent to 5.9 percent, indicating no significant difference between the two. The widest differences in measurement between the smartwatch and oximeter were –9 percent and 17 percent.

Overall, using pooled SpO₂ readings, researchers calculated a root mean square difference between the two devices of 3.0 percent.

Of note, dividing SpO₂ readings into different ranges showed that the smartwatch was more accurate, with respect to the medical-grade oximeter, at higher values. For instance, when SpO₂ was between 90 and 100 percent, the bias between devices was –0.3 percent, with lower and upper limits of agreement of –5.8 percent and 5.1 percent, respectively.

Meanwhile, when SpO₂ measurements were <90 percent, the bias was 1.2 percent with wider limits of agreement, ranging from –5.3 percent to 7.6 percent.

“The clinical standard so far has been isolated measurements under the supervision of health professionals, which are taken with a relatively large time lag and then compared with the prevalence of the clinically relevant events in the population,” the researchers said. “The results of our study are intended to help fill this gap.”

Current findings suggest that smartwatch technology has advanced enough to be able to provide accurate and real-time measurements of blood oxygen level for patient care. This could have important implications for the management of conditions such as sleep apnoea or chronic obstructive pulmonary disease.

“We further suggest that the exact requirement of each of these potential healthcare applications need to be articulated and wearable devices evaluated against those requirements,” they added.