Apple Watch ECG Cannot Diagnose Atrial Fibrillation Faster Than 120 BPM

The Apple Heart Study received great fanfare at least year’s AHA meetings  and was subsequently published in the NEJM.  Many Apple Watch (AW) wearers having heard of this study may have concluded the device will reliably identify atrial fibrillation (AF).

In my commentary on the Apple Heart Study I pointed out several issues with relying on Apple Watch for AF diagnosis, most significantly false positive notifications. Recent patient experiences have, in addition, made me concerned about false negative notifications and a lack of sensitivity.

AW ECG is inherently limited in diagnosing AF above 120 BPM. This guarantees a substantial number (possibly the majority) of AF episodes will not be recognized. Such false negative notifications may falsely reassure patients that they don’t have AF and delay them seeking medical attention.

Recently, I saw a patient who was referred to me for an abnormal 12-lead ECG. While reviewing his symptoms we discovered that his AW had registered high heart rates, sometimes up to 150 beats per minute, which lasted for several hours. 

Although the AW had recorded this high heart rate it had not notified him of the possibility that he had atrial fibrillation or even that he had a high heart rate.

He had made the ECG recording below using the AW and the results came back inconclusive.


The AW ECG recording clearly shows atrial fibrillation going at a rapid rate-over 150 beats per minute-but the accompanying interpretation gives no hint that the patient had AF.

Based on the combination of an absence of any irregular heart rate/AF warnings from his AW and the absence of a diagnosis of AF when he made AW ECG recordings of the fast rates the patient assumed that he did not have atrial fibrillation.

Why is this? Apparently Apple has decided not to check for AF if  the heart rate is over 120 BPM.

Given that most patients with new-onset AF will have heart rates over 120 BPM (assuming they are not on a rate slowing drug like a beta-blocker) it appears likely that Apple Watch ECG will fail to diagnose most cases of AF.

I asked my patient to record an ECG with his watch every time he felt his heart racing after our office visit. A few days later he was sitting in an easy chair after Thanksgiving watching TV and had another spell of racing heart. This time the heart rate was less than 120 BPM and the AW was able to analyze and make the diagnosis.


The inability of AW ECG to diagnose AF when the rate is >120 BPM further adds to my concerns about widespread unsupervised use of the device. When we combine inconclusive high heart rate analyses with the unknown sensitivity of the irregular heartbeat notification algorithm the AW may be providing many patients who have atrial fibrillation with a false sense of security.

Skeptically Yours,



7 thoughts on “Apple Watch ECG Cannot Diagnose Atrial Fibrillation Faster Than 120 BPM”

  1. Jeff,

    I can count to ten and multiply by six or use any other pulse counting method but it will not tell me if I’m in afib versus aflutter versus SVT, versus just a lot of PACs, etc.
    Both the Kardia and Apple Watch will generate a high quality ecg that will tell you.

    Equally important, the internet is full of anecdotes of people running to their doctors with reports of “fast heart beats” only to be told after a normal ECG that they probably had a panic attack and not to worry about it.I But when you walk into your doctor’s office with a tracing of afib, SVT, aflutter, etc, from the Apple Watch or the Kardia, it’s a totally different story. A good cardiogist or EP will accept it and act accordingly.


  2. Most people can count their number of heartbeats that occur in 6 seconds. A $10.00 watch will do it. Normal average resting rate will be about 60 BPM or 6 beats in 6 seconds. No need to count for a full minute.
    120 BPM is twice that, or 12 beats in 6 seconds. Easy enough, even counting the irregular rate that AF produces. Upwards of 120 BPM sitting relaxed in a chair is a matter of deep concern, AF or not.
    Are there really people out there who can’t feel their own pulse? Radial? Carotid?

    As one who can detect my own pulse, I would much rather present my pulse-counting concerns directly to my electrophysiologist. It’s my experience that even ER cardiologists can read a 12 lead EKG incorrectly, much less me myself with no medical training.

    So, would I depend on a not inexpensive gadget or two to tell me what’s what, no matter how sophisticated the AI’s algorithms?

    Beyond all that, it seems to me that the psychological/emotional stress of essentially taking on an electrophysiologist’s responsibility for determining your own cardiac welfare might well be rhythmically counterproductive.

    • Jeff,
      it’s remarkably difficult to get patients to take their pulse regularly.
      In addition, accurate heart rate estimation requires that the heart generate enough pressure for the beat to be felt at the periphery. With short RR intervals noted at high heart rates, the heart does not have time to fill properly resulting in very little generation of pressure and missed beats at the periphery. Thus, we frequently see patients in rapid afib with a pulse ox pulse in the 70s or a Fitbit/Garmin device recording normal heart rates.

  3. Just want to add that the newer Kardia 6L generates a six lead ecg tracing, while the older Kardia’s and the Apple Watch generate a single lead tracing. Personally, I don’t find that much more useful information in the six lead, but your doctor might with certain conditions.


  4. I use both the Apple Watch and the Kardia for monitoring afib and other arrhythmias. Both have pluses and minuses and both use algorithms that are prone to false positives and false negatives.

    That said, the actual ECG tracings themselves are high quality single lead, and can offer the basis of an accurate diagnosis by a trained professional and in some instances an informed patient. This holds true at all heart rates, including those above 120 on the Apple Watch.

    Hopefully, someone with a resting heart rate of over 120 would inform their doctor, as your patient did, but I do think Apple could emphasize that a little more with their messaging and/or change their algorithms to recognize afib at all rates similar to the Kardia.

    I don’t think the answer is yet in whether the benefits of these devices exceed the risks caused by the stated limitations of these devices, but personally both the Apple Watch and Kardia have been very beneficial to me personally in both becoming an informed patient and managing my arrhythmias.


  5. Your recommended blogger Dr. John Mandrola, who now mostly tweets, champions the practice of “conservative medicine”. In line with that philosophy, he does not believe that screening the walking well to be beneficial.
    These new watches and such are likely to produce plenty of false positives as well as false negatives – creating fear, confusion, and unneeded expense.
    At what point of an individual’s calculated risk would an implantable monitor be cost effective while eliminating fear and confusion? Are THEY reliable enough?


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