A person uses an Apple Watch Series 3 at the Apple Omotesando store on September 22, 2017 in Tokyo, Japan. Tomohiro Ohsumi | Getty Images

When Apple Watch was first released in 2015, it came with a built-in basic heart rate monitor. That pushed Apple into the health care sector, as users started reaching out to the company with stories about how the device had saved their life. As the company's COO Jeff Williams told CNBC in 2017, "We've been really surprised with the deluge of letters and emails we've gotten where just the simple heart rate monitor was able to have such an impact on people's lives." Now, the company is expected to take these health ambitions a step further by introducing an electrocardiogram or "ECG" sensor that measures the heart's rhythm — and not just the heart rate. That's according to Apple analyst Ming-Chi Kuo, who issued a research note seen by CNBC on Monday. The note said that the ECG "will attract more users." Kuo is known for having a particularly strong track record for predicting updates for Apple products. Assuming Kuo is correct, Apple releasing an ECG is a big deal for people with certain diseases. But it's also complicated because the company would need to figure out how to communicate sensitive medical information to consumers without freaking them out. The last thing Apple would want to do with its device is send tens of thousands of anxious users into the emergency room thinking they're having a life-threatening medical problem when they're not. So after talking to a series of health experts, including cardiologists and technologists, here are some questions we're asking on the eve of the event:

Will Apple need approval from federal regulators?

It depends. If Apple shows the ECG reading to a consumer, then yes. That would make the Apple Watch a regulated medical device. But Vic Gundotra, CEO of AliveCor, a start-up making big waves in the space, sees another path. He suggests that the company could use the ECG to get more accurate heart rate data, which wouldn't necessarily require an approval process. That's because Apple might not want to take on the risk of providing erroneous information back to a user.

"Is Apple ready to take on that kind of liability? I doubt it," he said. If Apple decides to go down the regulatory route, the company faces another decision. It might need to the green light for its ECG sensor as well as the algorithms that sit on top of it that provide feedback to users ("abnormal" or "normal", for instance). AliveCor did that, so we know it's possible. As Gundotra recalls, the FDA approved both the algorithms and the hardware at the same time.

What would the ECG look like?

It's not clear how Apple would design an ECG — there are a variety of different options, including a watch band sensor that could be similar in style to the AliveCor product, the KardiaBand. The company could also build it into the watch itself using the button on the side and sensors on the bottom of the device.

Who would this help?

Potentially millions of people. If Apple gets the watch approved as a medical device, one of the first uses would be to screen for Atrial Fibrillation ("AFib"), a heart rhythm disorder and a common cause of stroke. We already know that Apple wants to help people at risk for the condition, which kills more some 130,000 people in the United States alone. It developed a Heart Study app in 2017 to pickup on potential problems in a general population. Those who enrolled for the study and were deemed at risk were sent a patch from a third-party medical device company to perform an EGG, which is the gold standard test for diagnosing the condition. So for Apple, building its own makes sense. Why not just point a user to the ECG sensor on the Apple Watch instead? That move could bring its Apple Watch to an older demographic of users. People over the age of 65 have an elevated risk of AFib, and they might opt to get a smartwatch with an ECG for peace of mind alone.

Will physicians be involved? And how?

If Apple wants to use an ECG to screen for and diagnose disease, it will need to convince doctors first. Clinicians are a naturally skeptical bunch, and they might be concerned about swarms of healthy people bringing Apple Watch data to their office without much context. Apple will need to prove to this community in particular that it won't have a high rate of false positives (people who think they have a condition when they don't) and false negatives (people who think they're fine when they're not). Doctors might also be concerned about the potential liability if users blame them for failing to see signal in the noise. Another question for those that get a positive result: What will be the next step for a user who gets an abnormal result, but doesn't have a doctor nearby? And will Apple be responsible for these users?

Can it work without a dramatic improvement in battery life?