Deep Dive #1 - High/Low Heart Rate notifications:

This first deep dive into Cardiac-Related Wearable Features, is focusing on High and Low Heart Rate notifications. If you didn’t read the overview article, I would start there: https://www.linkedin.com/pulse/cardiac-related-wearable-features-overview-bronwyn-harris-ngxwc/ It not only talks about other cardiac-related features, but also discusses how heart rate is measured.

The High Heart Rate and Low Heart Rate Notifications can be turned on by anyone 13 years or older and the specific threshold is chosen during set-up from a limited set of options. These notifications work on any Apple Watch Series 1 or later (including SE) and it does not matter whether someone has a history of AFib or not (unlike some of the other features). 

For High Heart Rate the user set thresholds (options: 100, 110, 120, 130, 140, 150) and is triggered when the user appears to be inactive for 10 minutes and HR threshold is passed. I have not seen any published specifics about what qualifies as “passing the HR threshold”, for example does it require 100% of the HR readings in that 10 minutes to be above the threshold, or the majority or something else? Is there a minimum number of readings? From a high heart rate notification that I received (see screenshot below), I don’t think it requires all of the readings during that time period to be above 100%.  

For Low Heart Rate the user set thresholds (options: 40, 45, 50) and triggered when the heart rate falls below the threshold for 10 minutes. Similarly to High Heart Rate, I haven’t seen published specifics on what qualifies as passing the threshold, but from alerts that I have received, it doesn’t require all of the readings in the 10 minutes to be below the threshold. 

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

Clinical thoughts on a High Heart Rate Notifications:

Receiving this notification is very non-specific, as there are a lot of situations that can result in heart rate increasing at rest. These include feelings of fear/anxiety or significant caffeine intake, but can also include serious medical problems such as hyperthyroidism, arrhythmias, pulmonary embolism, serious infections, and placental abruption.  Several stories of life-saving notifications have been posted in the news. In these stories the individual sometimes ignores the first notification but typically describes realizing something is off when they reflect on how they are feeling. As a result, the notification serves the purpose of prompting self-reflection and validating and potentially magnifying (in a positive way for these scenarios) symptoms that they may have been feeling but ignoring. Example news story: https://abc7.com/costa-mesa-orange-county-pregnant-woman-saved-by-apple-watch/12722403/ 

The notification message provides the simple facts: “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.)

Image below shows a High Heart Rate Notification that I received. This was during a stressful time (job interview after late travel and also combined with significant coffee intake). For me this alert did not cause me any alarm, it actually helped me to pause and take some deep breaths and take some time to relax in between my interviews. However as a pediatric cardiologist who routinely works with wearables, I realize that I am not the standard user. Even the simple messaging with the facts about the heart rates being above a threshold can feel alarming without context. A reddit post confirms the range of reactions from assuming it is stress to thinking it is a heart attack.. https://www.reddit.com/r/AppleWatch/comments/y43gqq/has_anyone_gotten_a_high_heart_rate_alert_on/ 

Even though heart rate readings are now readily available to patients, I don’t think that we do a great job as a medical community educating them on what are normal heart rate values, understanding what can influence it and also red flags that should alert them to seek emergency care. There are lots of patient-facing resources for blood pressure readings, which have a similar nuance with many different ranges of abnormal (SBP of 135 is very different from 185, just like a heart rate at rest of 105 is very different from 150) and also specific symptoms that trigger a need for emergency care. Example of patient facing materials related to blood pressure: https://www.heart.org/en/health-topics/high-blood-pressure/understanding-blood-pressure-readings/hypertensive-crisis-when-you-should-call-911-for-high-blood-pressure 

Clinical thoughts on a Low Heart Rate Notifications:

I think that a low heart rate notification is much less likely than high heart rate alerts to raise awareness of serious medical conditions, as the incidence of third-degree heart block in the general population is approximately 0.03%. Furthermore, normal bradycardia is common during sleep and is generally not a cause for concern, nor does it necessitate notifying a physician. https://www.ncbi.nlm.nih.gov/books/NBK545199/

I have occasionally received these notifications and even got one the night before I wrote this (refer to the screenshot below from my Health App). I know family members and patients who have received them. A family member and separately a patient went to the ER due to a notification and both were found to not have any underlying issue. Although the notifications function as intended—alerting when the heart rate drops below a user-defined threshold for 10 minutes—they frequently capture instances of non-pathological bradycardia, especially during sleep. The notification message provides the simple facts: “Low Heart Rate: Your heart rate fell below 40 BPM for 10 minutes starting at XX:XX AM/PM.” (The “40” in this message is the customizable value.)

The article "Ask the doctor" from Harvard Health provides effective reassurance about low heart rates on smartwatches, while also highlighting what to watch out for and when to seek medical attention. https://www.health.harvard.edu/heart-health/low-heart-rate-warnings-via-smart-watch As mentioned above, I think this type of patient education about wearable data and notifications is important and can help empower patients while reducing unnecessary visits (or messages) to providers. 

In think it is important to also note that there are cases, often involving older individuals or significantly lower heart rates while awake, where serious issues have been identified following a low heart rate notification, as noted in this CBS News article: https://www.cbsnews.com/philadelphia/news/apple-watch-saves-mans-life-frank-haggerty-lawrenceville-new-jersey/ and this blog post: https://law.stanford.edu/2022/12/21/fda-pathway-for-wearable-medical-devices/ 

Personally, I set my threshold at 50 BPM out of curiosity, but this is not a broadly applicable recommendation. Patients not at high risk of heart block should consider setting their threshold at 40 or 45 BPM and should ignore alerts received during sleep. If these notifications cause undue stress, they can opt to disable them entirely.

Turning on/off the notifications:

There are multiple different ways to turn on (or off) these notifications and change the thresholds. One way is from your iPhone to open the Apple Watch App >> tap Heart (make sure it is “My Watch” tab, not a family member’s) >> scroll down and you will see the High Heart Rate with the current threshold chosen or “Off” if the feature is turned off.

Other Wearable Devices:

While this article focused on the Apple Watch, many other wearables also have this same feature. All of the ones that I have seen also use the 10 minute period and for the high heart rate also require inactivity and they have user set thresholds.

Information about the feature on Fitbit can be found here (scroll down to the question about heart-rate notifications): https://support.google.com/fitbit/answer/14237938?hl=en#zippy=%2Cwhat-are-heart-rate-notifications

Information about Garmin Abnormal Heart Rate Alert: https://support.garmin.com/en-US/?faq=GMKI3wym8p3TlgLhEw5toA#

Heart rate accuracy:

As a clinician thinking about a patient who received one of these notifications, it is natural to want to know about the heart rate reading accuracy. First, I would ask you to consider what accuracy is actually needed in this setting. If a patient messaged you saying they got a notification that their heart rate is above 120 for 10 minutes, is it essential that the real heart rate is within +/- 5 bpm of 120 or would you take the same action whether the true value was 100, 110, 120, 130, or 140?

As these are not FDA regulated readings, there isn’t official documentation and testing results published, however there are peer reviewed studies that have looked into this question. Many of these studies look into accuracy during exercise, as a common use case is identifying exercise intensity and estimating caloric expenditure. It is important to note that for the high heart rate alert, by definition, the user will be inactive and presumable that is likely also true for the low heart rate (since heart rate is expected to increase during exercise). One useful study to review by Bartholomew et al., had 65 participants and evaluated 3 different devices: Apple Watch, Fitbit Charge HR, and Garmin Forerunner 225. The difference expressed in mean absolute percentage error (MAPE) for the Apple Watch was between 1.14% and 6.70% with the higher error seen with increased exercise intensity. The Fitbit and Garmin had MAPEs of 4% and 11% respectively at baseline. 

Assuming a MAPE of ~10%, someone with heart rate readings consistently in the 120s, is expected to have heart rates over 100 bpm, which isn’t expected at rest for a healthy adult. Knowing that the values come while at rest, that the accuracy is high enough that as a provider, you should pay attention when a patient tells you that they received a notification and make sure to ask follow-up questions and if possible verify the heart rate reading.  

Study reference: https://pubmed.ncbi.nlm.nih.gov/28302596/ 

Systematic review on the topic: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8817215/

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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 and my own data.