Gas station without pumps

2012 August 20

Better electrode placement for EKG blinky

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 17:00
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After talking with an EKG expert, I changed the placement of the electrodes to correspond more to the conventional limb electrodes (LA=left arm, RA=right arm, LL=left leg).  I didn’t want to use limb electrodes, because there are then more muscles that can cause EMG interference, so I put the electrodes on the torso about where the limbs connect.  He suggested that Lead II (LL-RA) would give the most canonical signal for recognizing the parts of the EKG waveform.

Connection of electrodes to get “Lead I” which is LA-RA.  The top two electrodes should probably be placed more symmetrically—I think that the right-arm (–) electrode may be better placed than the left-arm (+) electrode.
A portion of the Lead I signal. The PQRST are all quite clear. The voltage scale is approximate, based on a computed gain for the EKG blinky amplifier. The zero-line is set by averaging the readings for about 10 seconds of recording.  The sampling period was 3 msec.
The Lead II electrodes (LL-RA). This is the conformation that the EKG expert said would most likely give the canonical waveform for a normal, healthy individual.
The waveform for Lead II is indeed quite strong, with very clear P and T waves. Note: despite the same time scale, the Lead I, Lead II, and Lead III recordings were all done separately, with the wires moved between recordings.  They do not correspond to the same heartbeats.
The arrangement of wires for Lead III (LL-LA).
The R wave is very hard to see in Lead III, probably because the dipole is oriented almost horizontally, so the vertical lead placement sees little difference in the field.  The R wave was not strong enough to flash the LED.

I wonder if I should add a ‘calibrate’ button to the blinky EKG to put out a 1mV pulse for a fixed duration so that the gain does not have to be estimated. That may be overkill for the blinky, but would make sense if I decide to make a 2-channel EKG. Note that 2 channels (say Lead II and Lead III) are really all the data in the first 6 “leads” of a 12-lead EKG. All the rest (Lead I, aVR, aVL, aVF) can be computed from those two.

It looks like a gain of about 2000 is needed to turn the 0.7mV R waves into 1.4V pulses to light up the LED.  Gains over 3500 would clip the tops of the R waves.


  1. […] Better electrode placement for EKG blinky […]

    Pingback by Order and topics for labs « Gas station without pumps — 2012 August 20 @ 17:05 | Reply

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  3. […] about electrode placement below the collar bones to avoid interference from other muscles (see Better electrode placement for EKG blinky). After a lot of questioning about how to keep the LA and RA signals between the power rails of the […]

    Pingback by Twenty-fifth day of circuits class | Gas station without pumps — 2013 March 8 @ 21:35 | Reply

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