Some pundits were surprised to hear that the blood oxygen measurement feature on the new Apple Watch Series 6 was available immediately when the watch went on sale - unlike the Series 4's ECG, which had to wait several months for approval by the FDA. Now we know why.

Apple was able to skip the approval process by presenting the pulse oximeter as a wellness gadget rather than a medical device. This means Apple didn't have to submit any data that proves the accuracy of the sensors and algorithms - although the firm is currently running three medical studies into the benefits of the pulse oximeter related to asthma, heart failure and breathing problems caused by COVID-19.

Matt Grennan, Assistant Professor of Health Care Management at the Wharton School of the University of Pennsylvania, told The Verge that the Apple Watch Series 6 was confusing because it contained two applications, only one of which was medical-grade. Some reviews have found that the oximeter does not deliver consistent results.

This mixture of medical and fitness features in one device is possible thanks to the distinction made by the FDA. While the US authorities believe that pulse oximetry belongs to class II, which simply provides information about the state of the body, the situation with the ECG is somewhat different: the electrocardiogram or its results can affect the medical care that the user receives.

An electrocardiogram is designed to detect abnormal heart rhythms that can be potentially life-threatening. Precisely for this reason, the devices with such a measurement are subject to stricter guidelines than, for example, pulse oximetry. Apple has not had the heart rate measurement approved by the FDA.

If you'd like to read more about this feature, read How to measure blood oxygen with Apple Watch.

This article originally appeared on Macwelt. Translation by David Price.