Apple Watch ECG Captures Pacemaker Activity

The hysteria surrounding Apple Watch 4’s single lead electrocardiogram (ECG) feature has simmered down over the last few months, but it was brought to my attention that pacemaker activity can also be detected by the application. Unlike 12-lead ECGs which provide significant amounts of data to localize pathologies to certain regions of the heart, 1-lead ECGs provide only basic heart monitoring information (heart rate, arrhythmias like atrial fibrillation, etc.) However, because pacemakers operate at the level of myocardial electrical conduction, it’s no surprise that we can see the characteristic “spikes” on the ECG app!

Now for a real life example – I had full consent from the individual to use their ECG strips, pacemaker information and history.

Let’s say a patient has a dual chamber pacemaker (leads in the right atrium and right ventricle) placed for a slow junctional rhythm in AAI mode at 60 beats per minute (bpm). In this mode, the right atrium is sensed. If an electrical impulse is sensed at least 60 times per minute (ie, from SA node depolarization), then the pacemaker does nothing. If no signal is sensed, then the atrium is paced at 60 bpm. This mode relies on normal conduction through the atrioventricular (AV) node to depolarize the ventricles thereby maintaining AV synchrony.

The top image represents normal sinus rhythm; however, in the same patient, the bottom image shows atrial pacing (at the pre-programmed rate of 60 bpm) with clearly visible atrial spikes.

Now what’s even cooler is this Medtronic pacemaker has a managed ventricular pacing (MVP) feature. This is essentially a ventricular pacing backup if normal AV conduction is lost. The device will switch from an AAI mode to a DDD mode, periodically assess if normal AV conduction returns, and then switch back to AAI pacing at that point. You can even see a short run of atrioventricular pacing (both atrial and ventricular spikes) here on the Apple Watch ECG app! How cool!

Atrial and ventricular pacing

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  1. Recently put in a Biotronik Edora dual chamber unit. Set at 60 to minimize slow condition sometimes, mostly very late at night. CLS is on, and think rate is at Medium. My IWatch 5 use to pick up my A-fib all the time,
    now I don’t. a reading at all, in my last visit with the Tech he showed me the Pacemaker singled an occurrence
    7% over 7 days. Would upgrading. to a newer I Watch model improve dedication ?

  2. I have had persistent Afib for 2 1/2 years which was reliably reported by the ECG on my Apple 5 watch. I recently received a biventricular pacemaker implant (Medtronics) and the ECG no longer works. When I try to check it the display shows 30 sec. 29 sec then loops back to 30 sec 29 sec, Is it possible that my watch is detecting the pacemaker pulse, currently set at 80 bpm? Heart rate is properly measured by my watch and other devices.

    Any thoughts?

  3. I have a single lead pacemaker and have been diagnosed with AFib. Can my Fitbit Sense Smartwatch ECG app accurately measure AFIB since I have the pacemaker.

    • In general, it’s going to depend on why you have the pacemaker in the first place and how it’s programmed. In the example presented in the post, the pacemaker was placed because of a symptomatic, slow heart rate. Therefore, whenever this junctional bradyarrhythmia occurred, the pacemaker would kick in and the Apple Watch showed the spikes on its ECG app. If the Fitbit works in a similar fashion, then for faster atrial fibrillation arrhythmias (where the native heart rate is faster than what the pacemaker’s rate is set to), I’d imagine the ECG app could still detect irregularly irregular intervals characteristic of AFib. Not medical advice, just my two cents with the major caveat that I’ve not actually used a Fitbit Sense device.

  4. Since my pacemaker was implanted 4 months ago my Apple watch 4 has shown a steady and significant decline in VO2 max. I have been sedentary the last few months but did walk at least a mile outdoors most days. Now starting back at the gym & my heart rate jumps to 140 or 150 when in the recent past it rarely went above 120. Deconditioned over a few months? Apple watch inaccurate with Medtronic Azure 2 lead unit? Got my second Covid shot 2 weeks ago. Not asking for medical advice per your disclaimer…..

    • Hey David! Honestly it could be any combination of what you mentioned, and big picture, the Apple Watch sensors aren’t a “gold standard” for any measurement (EKG, pulse oximetry, etc.) Thanks for sharing your experience!

  5. My husband has A-Fib and a pacemaker AV sync ST Judes When trying to use the watch ecg it will not capture a full reading. I don’t know if it is because of the pacemaker or if there is something else. I know he was in a regular rhythm because the Doctor was also monitoring pulse.
    Any advice would be appreciated.

      • I have a very extensive collection of Apple Watch and Loop recorder outputs that I would be happy to upload for educational and general discussion purposes.
        I am on my fourth ventricular lead since 3/ 7/2019.
        I have extensive ventricular pacing.
        Atrial and vpacing
        Loop recorder 10 sec asystole with one escape beat a 5 seconds. Failure to sense and failure to capture. Pacemaker testing with failure to capture at 7 volts and 1.5 ms width. Etc etc All except loop recorder on Apple Watch 4 and 6. All on me personally. I write this from my hospital bed 5 hours after a surgical team remove and replace my third failed lead ( heart muscle issue not equipment related) there was a backup thoracic surgical team there as well because the v lead had been in place 14 months.
        I also have 16 sec of new lead vpacing.
        67 year old Australian male Sydney Australian

        • Thank you so much for the comment, Doug! I hope you’ve been doing well after your procedure, and I’m sorry to hear you’ve been through the process so many times! If you’re interested in sharing, please feel free to send your outputs to [email protected]. 🙂

  6. Hi. my husband has a St. Jude dual lead pacer. He concerned about whether or not its ok to wear the Apple Watch 5 because of the magnets. What are your thoughts on it?

    • I’m sorry, but it’s not appropriate for me to give medical recommendations (however benign) over the Internet. This is a good question for your electrophysiologist/cardiologist.

      • A question for Rishi:
        I have afib and a pacemaker, and most frequently my Apple 5 watch says “inconclusive reading” when I do an afib reading. I read somewhere that “inconclusive reading” can come from pacemaker interference – other times “sinus rhythm”. Do you know if “inconclusive reading” means there may be an afib event too small for watch to acknowledge as afib, or possibly due to pacemaker interference at that point? “Inconclusive reading” is so vague.

        • Hey Ruth! Yes, it’s possibly related to interference from the pacemaker. Taken from Apple’s website:

          “If you consistently receive an inconclusive result, even after trying the steps above, it could be due to one of the following situations:

          – Your heart rate is between 100 and 120 BPM.
          – You have a pacemaker or implantable cardioverter defibrillator (ICD).
          – The recording may show signs of other arrhythmias or heart conditions that the app is not designed to recognize.
          – Certain physiological conditions may prevent a small percentage of users from creating enough signal to produce a good recording.”

          • Before I had my Medtronic Azure two lead pacemaker implanted, I was able to monitor my ECG on my Apple 5 watch and my KardiaMobile. After the PM implant, they will no longer monitor ECG. The only report I get from them is the transitions from the PM, no heart ECG. The Watch and the Kardia report the same thing.

    • Hey Holly,
      I am actually trying to research the same answer. My dad also has a St Jude pacemaker and he just suffered a stroke a few weeks ago. I bought this watch bc of the fall risk protection and other great features but I also read that it might interfere with medical devices so I havent let him wear it just yet. Have you found any further info on this?

  7. My Apple Watch ecg catch’s mode switching as it switches from my setting DDD ( I have rate turned off as well as CLS off) to DDIR. It also shows pacing spikes in ventricular pacing. I have not seen atrial spikes at all thought I am atrial pacing 45% of the time.
    I have a Biotronik Edora duel chamber unit.
    The first time I saw mode switching spikes ( they were confirmed by Biotroniks) they looked like pacing spikes on the ST segment ( bit scary). I have Sick Sinus Syndrome ( Brady Tachy) and previously arrested prior to pacemaker placement.

  8. Does anyone have information about the Apple Watch being unable to detect paced beats properly? I heard about someone that got a pacemaker after having an Apple Watch and they are now getting alerts for low heart rates on their watch. They had their pacemaker checked and it appears to be functioning normally. It has been difficult to get recordings that overlap to prove what the true rhythm is when the watch is alerting.

    • That iwatch reads what’s technically called “lead I”.
      When lead I (which reads the voltage difference between right and left arm) has a low level, the iwatch may be unable to detect it.
      In most people, lead I has a pretty strong voltage differential, therefore the iwatch can easily give a reading. Occasionally, a conduction disturbance (like a bundle branch block or an axis deviation) cause lead I to have a low signal, therefore the iwatch no longer detects it. A pacemaker may also cause lead I to have a low signal. It all depends on the person, where the PM leads end up inside the heart etc.
      If you, as an experiment, try wearing the iwatch around an ankle, and touching the button with your left (lead II) or right (lead III) arm, you may get a better reading. And gain some flexibility in the process…😉

  9. Mine does not show pacer spikes. I think it depends on where your leads attach in your heart. Mine are in my septal wall which is more natural than apex. So this isn’t true for all pacemaker people

    • Thanks for sharing! I know a few people who are A-paced and/or AV-paced who this has worked for, but with a single vector EKG, it probably can’t capture everything. The lead location shouldn’t make a difference, but again, the Watch 4 wasn’t even designed for this, so who knows? 😆


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