Early on in the product life cycle of the Apple Watch, Apple realized that it was on to a winner with its health tracking features. This is an aspect of the device that it has continued to refine with each iteration. It looks set to continue to this push by offering blood oxygen level sensing in a future model.
https://www.notebookcheck.net/Oxygen-level-sensing-coming-to-future-Apple-Watch-say-tipsters.456337.0.html