[Biomedical engineering] Question about ECG signal electrodes

[Lajka]

Hiyo,

Could someone please explain to me how exactly, or why, does the ground electrode in ECG minimize the interference?

And, also, why is it said that the patient could be at risk because of the ground electrode?

I really don't get it, so if there's anyone here who could clarify this for me, that would be awesome!

Much appreciated,
Lajka

 

[enc08]

Humans act as antennas, picking up interference from the mains at 50/60Hz depending on the country you're in. ECG signals are drowned out in the presence of this interference, so what's usually done is that the interference is measured by the circuit, flipped, and fed back in a small amount through the ground lead. Superposition of the two opposite signals means the net amount of interference is reduced.

Simply connecting someone to ground is dangerous as it invites current to flow through the body along a relatively low resistance path.

 

[Lajka]

Hi, man, thank you for your answer!

Would you happen to know some litterature/website where I could learn more about this? It sounds very interesting, and I am pretty new in all this.

Thanks!

 

[enc08]

Hi,

This gives fairly detailed coverage of reducing interference: http://www.robots.ox.ac.uk/~neil/teaching/lectures/med_elec/notes2.pdf

 

[physics baws]

(this is my another account, can't remember the password at the moment :D)

Whoa, that looks awesome! Thank you very much for that!

I'll have a closer look when I come to the office. In the meantime, I tried to make a small drawing (before I saw this post) to see if I can graphically explain to myself what you have written in your first post.

You can see it here:
http://ge.tt/4WQvuTe/v/0
(sorry for the smallish text, but it should be readable if you zoom in a little)

Maybe you can have a look and tell me if that is/isn't what you meant?

Again, thank you very much for your help!

P.S. Okay, just one question, I threw a glance at the paper before I get out. It says:

The ECG voltage V is not the only signal found at the input of the amplifier; one major
source of interference is the electrical power system. Capacitance between power lines in the
wall, floor and ceiling and nearby equipment couples current into the patient, wires and
machine. This current flows through the skin−electrode impedances on the way to ground.
The capacitance to these power line sources varies with proximity but is of the order of 50pF
which corresponds to an impedance of 64M Ω at 50Hz. If the right leg is connected to the
common ground of the amplifier through an electrode with contact impedance of, say, 5k Ω ,
the mains potential of 240V will appear as a 20mV noise input. This value is well in excess
of the ECG signal itself.

Well, if we connect the right leg to the ground, shouldn't noise disappear because we short-circuited that capacitance between the human and the ground (SCENARIO 1 in my sketch above)?