The new watch faces, like this fire one, are really neat.

In September, Apple announced an electrocardiogram, or EKG, for its upcoming Apple Watch, to help users at risk for a medical condition known as atrial fibrillation. That's potentially useful for at-risk seniors, if Apple can prove to the medical community that its sensor is sufficiently reliable and accurate.

But Apple's endgame might be much bigger: Turning the Apple Watch into a blood pressure monitor, potentially helping tens of millions of people who suffer from high blood pressure.

"I think Apple is going after the biggest measurement in health care and they're going to disrupt it," said Graeme Moffat, chief scientist at brain sensing company Interaxon, who closely follows the biomedical space.

Moffat noticed that in October 2017, Apple was granted a patent for a system to approximate blood pressure using data that could be obtained with sensors, such as "pulse transit time." (That patent was also noticed by Patently Apple.)

Pulse transit time means the time delay for the pressure wave to travel between two sites in an artery. It can potentially be measured by analyzing data from a pulse sensor on the wrist, and an EKG sensor that measures the waves as they leave the heart. The new Apple Watch Series 4 contains both.

More than 100 million Americans suffer from high blood pressure, which can increase the risk of heart attacks, strokes and other serious health prooblems. By contrast, only 2.7 million people have atrial fibrillation.

But many people with high blood pressure don't check it frequently enough. Home kits with cuffs are cumbersome and time-consuming, and many patients don't stick with their measurements.

Although the idea of using wearables to measure pulse transit time is still experimental, many in the medical community view it as a more convenient potential solution. Wearables also provide a more continuous monitoring system, which could provide feedback to patients and their doctors in near real-time instead of only when patients remember to measure their blood pressure.

To bring this solution to life, Apple would have to overcome many of the technical and scientific challenges — many start-ups have tried and failed in their quest to bring cuffless blood pressure monitors to market. Then Apple would need to invest in the necessary clinical studies.

Any new type of measurement would need to be calibrated with a traditional cuff to ensure accuracy. Moffat suggests Apple might ask users to agree to share their data from third-party blood pressure cuffs. Apple also benefits from access to millions of users with Apple Watches that it can enroll in medical research (it has already shown its willingness to do that with the Apple Heart study).

"Apple could create a very powerful system for tracking cardiac health," said Moffat. "It could provide a new way for users to manage high blood pressure, which has some very big implications."