What Is The Cause of Low Voltage (Unreadable or Unclassified) AliveCor/Kardia Mobile ECG Recordings?

The skeptical cardiologist has had several of his readers submit stories and tracings of AliveCor Mobile ECG recordings which yield unclassified or unreadable recordings. In some cases this is due to excess noise but a lot of these tracings suffer from low voltage: the height of the tracing is very small.
John, a skepcard reader, is typical.
Recently, he noted his heart was racing and made an AliveCor recording which came back interpreted by the app as normal

First tracing. Note the QRS complex (the large regular spikes) are 2 boxes high. Right in front of them is a little bump, the p wave indicating normal sinus rhythm

Three hours later he made a second recording which has drastically lower voltage: the only deflections visible are tiny QRS complexes, the p waves have disappeared. I think this is also normal sinus rhythm but because p waves can’t be seen this came back uninterpretable and if there were any irregularity AliveCor would have called it atrial fibrillation:
Second tracing. Note the QRS spikes now are less than half of a box tall. There are no consistent p waves visible (unless one has a good imagination). The bumps after QRS spikes are T waves.

John has a theory on the cause of some of his low voltage recordings which I shall reveal in a subsequent post after testing it.
In the meantime, if any readers have suggestions as to causes of low voltage recordings or have noted similar issues please comment below or send recordings and observations to DRP@theskepticalcardiologist.com.
Voltagophilistically Yours,


22 thoughts on “What Is The Cause of Low Voltage (Unreadable or Unclassified) AliveCor/Kardia Mobile ECG Recordings?”

  1. Just purchased the Kardia 6L from Amazon – received today. Using Samsung 8+ with Android 9. App seems to work fine, but I’ve taken about 20 ECG tests tonight. All reports so far look “scribbly” and highly irregular throughout all leads. Some reports are better than others as far as containing sections of stability in the patterns. I had educated myself via Youtube this evening re P, QRS, and T waves, but otherwise I am new to this. Kardia labeled all my tests tonight “normal”, and a recent ECG in the ER was fine, so I suspect it is the device itself or “user error” and not my heart. My only concern is that I did purchase this device due to much greater than normal palpitations. I read something about interference with this device. Would you have any samples of what interference looks like in Kardia reports? Thanks.

  2. I’ll add a bit more. I just read a bit more on this blog and did a 6 lead on my Kardia. I noticed lead #1 was low as usual, but lead #2 was quite strong (about 11 or 12 boxes in total) and looked similar to what the Holter monitor was showing.

    So, would it be reasonable to assume that I’m one of the people that gets a low lead 1 but the lead 2 is good and strong and showing as it should?


  3. This is interesting discussion. I’ve got the Kardia 6L and get very low voltage when doing my fingers (single lead test) or the 6 lead test using my knee. I wasn’t sure if there was something wrong with me as the QRS spike was very low (about 1/2 to 3/4) of one box vs my wife who seems to have a textbook looking higher QRS spike.

    The other day, I had a 24 hour Holter monitor attached to me. While at the hospital, the nurse had it hooked up to an Ipad and I could see these long QRS spikes on the ipad screen and she said everything looked normal on the screen. I asked if I could pull out my Kardia and do a test alongside the Holter while it was on her screen. My Kardia result came back as “Unclassified” and had a low spike (taking up 1/2 to 3/4) of the box whereas the Holter was showing , what the cardiology nurse suggested as, nice normal rhythm.

    So, the question is, is the Kardia getting low voltage, there is something wrong with my heart (I am getting extra beats and some fluttering) or is the Holter incorrect? Further testing will hopefully reveal this, but I’m actually hoping it’s the Kardia that’s incorrect as it seems to be the best case scenario here. 🙂


    • James,
      As you deduced after reading more on the blog, your Lead 1 is providing lower voltage recordings compared to lead II. Alivecor goes off Lead I.
      If you use single lead mode and put one of the electrodes on your left knee you can fool the Kardia into doing the analysis on lead II.
      Dr P

      • Thank you Dr. P.

        Since I have the newer 6Lead Kardia device, would it be reasonable to assume that I may as well take my readings in 6 lead mode which requires me to have the back of the device on my knee vs using it in single lead mode and having one side on my knee?


        • If you do it in 6 lead mode, the algorithm will work off lead I even though it has lead II available. As far as I know, only by utilizing the single lead with left knee on one electrode can you get the algorithm to process lead II.

  4. Was wondering where the second article on the Kardia low amplitude
    would be published or where it might be… I know this is an old thread but Ididn’t see it
    Also – thought maybe this might be of interest to The skeptical Biomedical Engineer – here’s some links to the the Kardia audio output spectral plots. Recal most cellphones no longer use things like electret mics – most mfg’s have switched to MEMS devices like those offered from Knowles https://www.digikey.com/products/en/audio-products/microphones/158?k=knowles%20mems%20audio
    So here’s a link to a JPG of the spectral content of the Kardia:
    And a vid as I place my hands on the touchpads:
    On youtube if you’re weird about going to links:


  5. I have very low voltage readings when on my left side and often get afib readings which I think are PACs. AliveCor Kardia gave me a great tip: Use lead 2 connections: left knee and right hand. Now readings are GREAT and I can see p waves very clearly. I wish I had learned this before. Would be curious as to why my lead 1 readings are so low voltage, but my heart is in good condition, so am not overly concerned!

    • Hello Vickie,
      Not sure what you mean with your tip. How do you use lead 2 connections? How do I use the Kardia pad besides my thumbs or fingers.

      • Tom,
        You can obtain a lead 2 recording by placing one of the electrodes on your knee and placing two fingers of your opposite hand on the other electrode.
        I’ve noted higher voltage recordings with this approach in general but it is difficult to obtain and maintain stability with this approach. It also requires removal of pants and is less convenient.
        Dr. P

  6. I take my readings every single day. I have a low voltage reading every day, sometimes almost flatlined. I too have been researching for months to find out why this is. I’m thinking it could be Amyloidosis or some other form of myocardiopathy.

  7. I have tested myself, and hundreds of other people. I have a low-voltage heart every single test that I have taken. Almost flat line. My pulse rate is generally between 50 and 60. I would like to find out the reason for this. If anyone could learn some invites it would be greatly appreciated!

    • I’m in the process of compiling my thoughts on this issue after receiving feedback and doing some experimentation.
      Have you tried putting the device directly on your left chest or using the hand/knee approach? It is a bit cumbersome but the chest recordings are typically better with larger voltage than the standard arm/arm recordings.
      Although one reader felt position (lying/sitting) varied the voltage this doesn’t seem consistent.
      On reader commented that time of day effected recordings, another that hand dryness did.
      I haven’t found that to be a consistent cause of altered voltage.

      • I have tested myself in various positions. With my hands on my knees, on a bench, etc. I’ve done the same for many of my clients in my personal training studio. My test always comes back almost flatlined. My ECG voltage is very very low. I am currently 5’5″ at 111 lbs. so I don’t think it is related to being overweight. I really appreciate any help! Thanks!

      • You can try putting the watch on your leg and touching the crown. This will yield lead II which is more useful in some individuals (typically thin, female)
        I’m curious about the left hand backward or body to the left. Can you send me examples of the changes you see? (drp@theskepticalcardiologist.com)
        Let me know if the leg method improves the tracing

  8. I have the same issue: frequently my Kardia readings have extremely low voltage; it seems to be mostly (but not always) in the morning. I can’t tell if it’s a real (physical) issue or just measurement error. It’s very frustrating.

  9. I have had readings that had low amplitude, albeit not as pronounced as this. I believe a contributing factor is whether or not the hands (and electrodes) are clean, as well as how wet or dry your hands are. I try to thoroughly wash my hands and wipe the electrodes (preferably with an alcohol wipe) prior to use. I have seen increases in amplitude after doing this.

  10. Hele dr. P.
    ever tried to use two Alivecor devices nearby each other … guess what !
    Specific high frequency noise might also interfere, whilst an obstructed microphone and damaged electrodes could have an effect on signal quality …
    The skeptical Biomedical Engineer


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