Cardiac-Related Wearable Features - An Overview

Why should you care about heart-related wearable notifications?

I have found that many medical providers aren’t aware of these basic features found in wearables that many of their patients are using. For example, did you know that there are three different numerous cardiac-related notifications, including three “AFib” features on the Apple Watch, each of which acts very differently? My goal in this article is for you to walk away with a basic understanding of these features. I will admit that when pulling together this information, I realize it isn’t as simple as I thought it was - I couldn’t find one reference with all the information together. Understanding the basics is useful when a patient comes to you saying they received a heart rate or Afib notification (or even more likely they don’t remember what the notification was, but it may have prompted a visit). I want you to be able to ask a few follow-up questions to understand what type of notification it was and where to see more details. This can enable you to obtain a more thorough HPI (history of present illness, in case any non-providers are reading this too). 

A colleague of mine, who is an adult cardiologist recently shared that typically 2x a month a patient will bring up an alert or something related to their wearable data. Her succinct thoughts on the topic resonated with me: “Patients are using it [wearables], so doctors need to know what they can do and how to look up the data.” As a pediatric cardiologist, it doesn’t occur as frequently in my practice, but it has happened with a few pediatric patients and I have had patient's parents show me data from their own wearable.

For simplicity, the below information is focused on the Apple Watch.  There are similar features in other wearables; for example, irregular rhythm notifications and ECG feature are available on some Fitbits and Samsung Galaxy Watches.

The basics of how it works:

The Apple Watch has had optical sensors that were used to measure heart rate since the initial version in 2015. The heart rate features (with the exception of ECG) are enabled based on a photoplethysmography (PPG), which is a non-invasive method to measure changes in blood volume that occurs through the cardiac cycle (i.e. every heartbeat). This method relies on the principles of absorption and scattering of specific light wavelengths. Red light is reflected by blood and green light is absorbed. With each heartbeat there is additional blood present in the small blood vessels and as a result more of the green light is absorbed. Typically the wearable devices that measure heart rate have red, green and infrared light emitting diodes (LEDs) and these are paired with light‑sensitive photodetector (PD), which can measure how much of each type of wavelength is reflected. 

Even if you didn’t know the name of the PPG waveform, you are likely familiar with it - it is the waveform shown on telemetry for pulse-oximetry. One key difference between a standard pulse-oximetry probe and a wrist-based wearable is the location of the photodetector in reference to the LED. Standard pulse-oximeter probes have the photodetector opposite the LED, so the mode is transmission. While a wearable on the wrist typically has the photodetector on the same side as the LED (all within the bottom side of the watch), so it relies on reflectance mode. There are additional technical challenges with reflectance mode, but those are beyond the scope of this article. The takeaway is that this method used in wearables to measure heart rate and beat-to-beat times is not novel and is the same principles used in pulse-oximetry.

For the ECG app, electrodes are used on the back of the watch and on the digital crown (button dial sticking out on the right of the watch face). When someone places their finger from their hand (opposite from the one wearing the watch) on the digital crown, a closed circuit is created between both arms, with the heart in the middle. This represents lead I on a standard ECG with lead on each arm. 

General reference about heart rate measurements on Apple Watch: https://support.apple.com/en-us/HT204666 

Detailed article on PPG and includes useful figures: https://www.mdpi.com/2079-9292/3/2/282 

It wasn’t until 2018 that the first regulated features were announced. There are 4 different types of notifications that can be received, all of which require set-up to turn on.  Many people won’t remember setting it as they may have done it years ago when they first set-up their watch. The ECG app is an active experience, not just running in the background. The newest feature is the AFib history, which provides a weekly report. See below for brief summaries of each feature and a table discussing the specific age requirements, FDA-clearance, and if relevant AFib history. 

For those features that are regulated by the FDA, they are considered Software as a Medical Device (SaMD). If you want to read more about what that means, here is a reference from the FDA:  https://www.fda.gov/medical-devices/digital-health-center-excellence/software-medical-device-samd  

What are the different heart features on the Apple Watch?

Notifications:

1. High Heart Rate - user set thresholds (options: 100, 110, 120, 130, 140, 150) and triggered when the user appears to be inactive for 10 minutes and threshold is passed

2. Low Heart Rate - user set thresholds (options: 40, 45, 50) and triggered when the heart rate falls below the threshold for 10 minutes

3. Irregular rhythm notification - informally known as "Afib alert" – this feature will intermittently “check for an irregular rhythm that might be suggestive of atrial fibrillation (AFib)”

4. Low cardio fitness notifications (will not discuss in this article)

ECG app - functionality for a user to manually take a 30-second ECG similar to a single-lead (or Lead I) and receive a visualization and one of the following classifications: Sinus rhythm, Atrial fibrillation, Low heart rate, High heart rate, Inconclusive, Poor reading

AFib History - for someone with Afib diagnosis, get a weekly report with estimated percentage of time heart showed signs of AFib

A quick summary of the features, complete with images is available on Apple’s website:  https://www.apple.com/healthcare/apple-watch/

Notification Messages:

• High Heart Rate - “High Heart Rate: Your heart rate rose above 120 BPM while you seemed to be inactive for 10 minutes starting at XX:XX AM/PM.” (Note, the “120” in this message is the user chosen value.)

• Low Heart Rate - “Low Heart Rate: Your heart rate fell below 40 BPM for 10 minutes starting at XX:XX AM/PM.” (Note, the “40” in this message is the user chosen value.)

• Irregular rhythm notification - “Your heart has shown signs of an irregular rhythm suggestive of atrial fibrillation. If you have not been diagnosed with AFib by a physician, you should talk to your doctor.”

ECG Messages (after manually taking an ECG):

• Sinus Rhythm: “This ECG does not show signs of atrial fibrillation. Apple Watch cannot check for signs of heart attack. If you believe you are having a medical emergency, call emergency services.”

• Atrial Fibrillation: “This ECG shows signs of AFib. If this is an unexpected result, you should talk to your doctor.”

AFib History:

• Weekly notification on watch - “AFib History: Over the last week, your heart rhythm showed signs of AFib [4]% of the time. The preview week it was also [4]%.” Note: The percentages and comparison to the prior week customized. It will never show 0%, it will show as 2% or less.

• Additional information shown on the connected iPhone: Health app >> Browse >> Heart >> AFib History. It will show a graph of the AFib History, shown as a percentage.

A few additional takeaways:

Irregular rhythm notification:

• A key thing to know about this feature, it is NOT constantly checking for AFib, hours pass between checks and multiple checks must be positive before an alert is triggered. 

(Deep dive article coming up will discuss limitations and study results, but in general if someone receives this notification, something atypical and either sustained or frequently occurring, is happening.)

•  Since it is not checking constantly, a lack of a notification does NOT mean that someone hasn’t had episodes of AFib. 

ECG Messages (after manually taking an ECG):

• The user can export a PDF that shows the lead in the standard format and sizing that you typically see on a 12-lead ECG.

• Does NOT detect heart attacks or ST changes and this is specified directly to patients at the time of the reading, but still important to emphasize.

AFib History:

• It does NOT notify a user when they are in AFib.

• It is not constantly checking for AFib, so can miss some instances, although it does check more frequently than the irregular rhythm notifications.

• IRN is automatically turned off when AFib history is set-up, as it isn’t supposed to be used when there is a history of AFib.

Prior notifications:

• To see what notifications or features someone has turned on and what values are chosen (if relevant), the Health Checklist is a great place to see this all. Health App >> Top right corner - click on your picture or initials >> Under Features selected “Health Checklist” >> View with all the features and what is active. From this same you view you click and turn on/off a feature and adjust thresholds. See below image from my Health App.

• To view specifics about an alert received (and can see the time of day and if there were multiple alerts, the frequency): Health App >> Browse tab >> “Heart” Category >> Summary of today’s data, Past 7 days, Past 30 days, Past 12 months, Older. If there is a notification it will show up there and when you click on one High Heart Rate Notification, it will then list the last three ones and if you hit “Show All Data” you can view all the prior notifications with the heart rate ranges and the date and time of day.

Other:

• Notifications will not occur if someone is not wearing their watch (duh), if it is in low power mode or if there is not a good signal such as when it isn’t worn properly (if very loose or too far down the wrist). 

If you read the above summary and want to know more, stay tuned for the deep dive articles!

References:

https://support.apple.com/guide/watch/heart-health-apde39f5426c/watchos - heart health

https://support.apple.com/en-us/HT208931 - heart health notifications

https://support.apple.com/en-us/HT212214 - track your AFib history

https://support.apple.com/en-us/HT204666 - how heart rate is measured

https://support.apple.com/en-us/HT208955 - taking an ECG

Fitbit: https://www.fitbit.com/global/us/technology/irregular-rhythm 

Samsung Galaxy Watch: https://news.samsung.com/us/fda-cleared-irregular-heart-rhythm-notification-for-galaxy-watch/ 

About the Author: Bronwyn Harris is a practicing physician who enjoys building digital clinical tools that utilize patient-generated health data.

Intended Audience: Any health professional curious about PGHD, whether it is because they come across it in clinical care, are conducting related research, building digital tools or just want to better understand it. If you have any specific questions or topics that you would like to see in the future, please message me!

What is PGHD? There is not a singular definition, but I like this one "Patient-generated health data (PGHD) are health-related data created, recorded, or gathered by or from patients (or family members or other caregivers) to help address a health concern.” https://www.healthit.gov/topic/scientific-initiatives/pcor/patient-generated-health-data-pghd

Examples of PGHD include: symptoms tracked by a patient, heart rate data, step count, BP measurements and treatment history

Disclaimer: This post is meant to be informative, but is not a complete review of the topic. Specifics about wearable data and smartphones often change with new software updates, so information above may become out of date. This post is not providing medical advice or input on how to practice medicine. While I worked at Apple in the past, I wasn’t directly involved in any of these features and all the information provided here is from publicly available sources.