Sunday, July 4, 2021

ECG artifact strips from Fatmaxxer - a guide

 Part 2 -
ECG arrhythmia and artifact visualization tips 

The latest version of Fatmaxxer has an incredibly useful feature for those of us interested in heart rate variability quality and interpretation - namely the ability to inspect the ECG details surrounding the alleged artifact.  As previously reviewed here, there are many reasons for a "beat" not to occur where it was supposed to be.  Having software tell us that there are artifacts does not "diagnose" the particular cause.  Although one can infer some causation such as missed beats (HR becomes 1/2 normal), arrhythmia can't easily be diagnosed this way.  Yes, some new algorithms have been developed to detect atrial fibrillation from HRV, but they are not "gold standard" as of yet.  As one engages in higher exercise intensity, artifacts rise and the importance of understanding the cause increases.  Are they serious or not?  In this first post on the matter, we will go through a brief guide on how to setup, record and graph the ECG extraction from Fatmaxxer.  Although Kubios premium can easily display this ECG, we will not be using that so users with simple spreadsheet software can take advantage of this feature.

Requirements - Fatmaxxer with developer settings on, "preferred device" set for the Polar H10 currently in use, Excel or similar

Use case - you have recorded a session with Fatmaxxer, there were artifacts present.  Each time Fatmaxxer detects an artifact, that series of electrical data will be written to a file with a suffix of .ecg. In addition, the app will take a baseline recording at about 10s elapsed time

Note - the following was not a true artifact, but serves as an example.  It was created by having the artifact correction threshold at the aggressive 5% during resting, where HRV is high and "false" artifacts are detected.

Step one - find and export the ECG log either through the menu system or straight out of the Android/data/online Fatmaxxer/files/logs folder

Next step - open the file with Excel:

The 4 columns represent the 

  • A - timestamp of the artifact (Unix time), 
  • B - segment (the "number" designation of the artifact - if you have 10 artifacts, you will see 0 to 9 for segment number). 
    If artifacts continue frequently enough, the segment may contain multiple instances and be longer than 10s.  About 5 s of no artifact beats are needed to "end" a particular segment.
  • C - sampleNr (the granular time of the ECG signal - since the same rate is 130Hz, each unit is 1/130 of a second) and 
  • D - yV (the ECG signal strength - in other words the mV of the signal at that particular time)

Next step is to simply graph a given segment numbers worth of data:

As an example, I chose the sampleNr and yV of segment "0" as x and y to plot, but did not include past the green line where segment 1 begins

The familiar Excel default "scatter" graph of the third (sampleNr) and forth (yV) column yields the following:

The next step is to change point size, color, add line connections, adjust axis and add some fixed time markers (arrows) to get an idea of regularity.

  • This is the entire, approximate 10s strip from Fatmaxxer.


Here is the same type of data in Kubios premium:


Zoomed Fatmaxxer ECG in Excel to explore waveform morphology:

  • Good looking narrow QRS waveform, defined P and T waves are present.
  • To do this, just change the x axis scale.

A further example of a "segment" containing an artifact.  The artifact (atrial premature complex) is not that apparent until the x axis (time) is zoomed to a smaller scale.  Then we see the early RR interval of the APC, then the delay in the next normal beat, then resumption of normal timing:


The APC occurs at about the 450 timing mark which is more obvious now that we know where it is (red circle):


  • ECG strips of artifact related segments are now possible with Fatmaxxer.
  • This is an incredibly helpful feature for HRV, sports researchers and consumer users alike.
  • Potential capture of atrial, ventricular arrhythmia's, fibrillation and differentiation of artifact type is possible.
  • Although a great tool, avoidance of Kubios premium makes this affordable for all.
  • For the recording of accurate, finely granular time varying DFA a1 as well as pertinent ECG tracings, Fatmaxxer is a fantastic tool to have. Many thanks and compliments to the developer for this new feature.

Heart rate variability during dynamic exercise

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