Measuring Einthoven's triangle using OpenBCI

KZurnKZurn
edited April 2015 in Research
Brooks and I were trying to implement the ECG Einthoven triangle with the OpenBCI unit. The largest signal of the Einthoven triangle is right arm to left leg, so we chose to implement this as the differential signal. We used the right leg as the reference signal. 

The negative of the differential should be on the right arm, and the positive should be connected to the left leg. This is what’s called “Lead II”.  

We found that the signal was inverted when we connected the right arm to the positive (upper pin) of the differential input, so in this experiment we switched the right arm connection to the negative input, and the left leg to the positive input. 

----> Anybody have any ideas why this is inverted? 



Here’s what we did:


Materials: 

3 snap electrodes & cables (TDE-205)

3 cloth electrode pads (TD-141C)

note: we have also implemented this with the EEG wet electrodes with similar results.

Connect the reference electrode just above the right ankle, on the inside of the leg.

Connect the differential signal leads by placing one on the inside of the right wrist, with the other end on the “4” positive pin (the pin further from the PCB board). 

Connect the other end of the differential signal, the left leg, by attaching the electrode just above the ankle on the inside of the left leg. Connect this to the negative input for the “4” pin on the openBCI board (the pin closer to the PCB board). 

image

Unplug unused leads from the openBCI board, this is a good habit to help prevent interference. Extraneous wires can act as antennas. 

Manually turn off all channels except for Channel 4, by clicking on the colored number boxes to the left of the strip chart. 

Make sure the amplitude is set to 24x for Channel 4. 

On the white settings boxes on the upper right, make sure it’s set to a notch at 60Hz, and Max Frequency is set to 120 Hz. 


Set the openBCI unit on your lap. If you set it on a table, it’s likely to pick up interference such as 60hz signals. Holding it on your body will keep it as isolated as possible. 


What we saw on the software display

We saw an “upward deflecting R-wave” on the strip chart display for Channel 4. The head showed a blob of activity around Channel 4, as expected. The chart for EEG Data (As received) showed a peak around the heart rate frequency, with a notch at 60hz. 

image







Comments

  • wjcroftwjcroft Mount Shasta, CA
    Karl @KZurn and Brooks @jbzurn , hi.

    'Cool' demo, and I assume from your shorts there it's balmy in Florida(!)  :-)

    Your description did not mention adjusting the channel settings on the "Chan Set" tab for the Channel 4. Unless you do this the channel will not operate in differential mode. But instead is relative to the SRB2 reference lead. See the ECG tutorial section,


    See the Channel Set tab photo in that section. You want to mimic what Conor did for the ECG channel 4 in that photo. Turn off the SRB2; this enables + - differential. You may also want to adjust your gain. In your text above you say "We used the right leg as the reference signal.". However in differential mode, SRB2 is not used. But you would still want Ground (Bias) connected (also called DRL Driven Right Leg.)

    In Conor's tutorial, the Bias was set to "Don't Include", since otherwise the ECG swamped out the simultaneous EEG channels. But in your case since you are doing ECG only, You can leave the Bias as Include.

    Cheers,  William

  • KZurnKZurn
    Thanks for the feedback!

    We applied the changes you suggested (turning off SRB2 and connecting the BIAS to the left leg.

    We also switched the negative back to the right arm, where it's supposed to be.

    Interesting result: Karl's ECG was exactly the same, still had a downward deflection. So, for the heck of it, Brooks connected herself to the ECG. Surprisingly, now the reading has a normal, upward deflection. We put it back on Karl and again, downward deflection.

    So I guess our conclusion is, Karl should see his GP. Or maybe Brooks. LOL. 


    System settings: 

    image





    Karl ECG

    image



    Brooks ECG

    image

  • wjcroftwjcroft Mount Shasta, CA
    Karl, fascinating.


    I'm no doc, obviously. But it looks like you both have the P and T waves, and they are oriented correctly in both ECGs. (Karl's P is just a slight fuzz vs Brooks.) But the real difference is in the R and S waves. And their relative sizes. Have no idea whatsoever what that implies.


  • KZurnKZurn
    William,
    I think I will adjust my test procedures.  I first put in a sine wave then an actual physiological signal.  I am going to add a step-put in a signal from a simulator.  I had forgot my Lead II was a little abnormal.  I looked over an old 12 Lead ECG and noticed it.  There are no problems as the ECG was evaluated and found within the allowable limits.
    Karl
  • biomurphbiomurph Brooklyn, NY
    Karl,

    Try swapping the electrodes from your right and left arm.
    I think what you are seeing is merely an artifact of the P and N input polarity.

  • wjcroftwjcroft Mount Shasta, CA
    Joel, the hard-to-overlook evidence is that he kept the same montage for himself and Brooks. (And I think alternated a few times.) Brooks ECG looked fine, but his was inverted.

    The other item is that he is using the Einthoven Triangle (right arm to left leg; with Bias on right leg. Called Lead II montage.) So, as you say, it may be instructive to see what shows up on other Lead arrangements.

  • biomurphbiomurph Brooklyn, NY
    Perhaps you could stand on your head?
    Or spin around real fast?
  • KZurnKZurn
    What was a little confusing was I am used to seeing my ECG with a high cutoff of 150 Hz.  The OpenBCI board was set at 50 Hz this would attenuate the higher frequencies of the R wave.  Brooks was running the computer so I was looking at a distance and mistake the S wave for the R wave.
    Karl
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