Impedances

Expert Advice:  Electrodes     EEG Signals

                             Disinfect Electrodes       Pediatric Leads

We have used the Piezo leg leads for some time and have been asked to change to the surface electrodes (2 per leg) per AASM recommendations. We have tried the ECG snap-on type and gold cups. We have 60Hz interference and are having trouble getting our impedences done. Do you have any advice on this?

Well, the good news is that most likely the two problems you are experiencing (impedance and 60 Hz) are related so if you fix one you will most likely fix both.

Impedance is the top priority to resolve because if you can get a handle on your impedances the 60Hz will resolve because low and balanced impedances allow your amplifier to eliminate the 60Hz artifact. To address your impedance problems, I suggest troubleshooting from a very basic perspective. First, verify the jack on the head box is working OK by prepping a spot on the scalp, apply an electrode, and verify a good impedance. Now, take that “known good” electrode and unplug it from the amplifier jack and plug it into your jack that you use for your leg leads. Check the impedance again and if the jack is setup properly and functioning the impedance should be the same as it was in the previous jack. If not, I suggest calling your equipment vendor to troubleshoot settings or hardware.

If the impedance is the same, your jack is fine and you have to focus on your skin prep as the next troubleshooting step. For this step, I suggest using a volunteer, if possible, and simply experimenting with different preps, different types of electrodes, and different application techniques. Leg leads and their impedances can be very challenging, but using these steps should assist in knowing where the problem lies which then will help you concentrate on the resolution.

Also, as mentioned, once the impedance is resolved your 60Hz should resolve as well. 60 Hz is seen when you have high or imbalanced impedances and the amplifier is unable to manage the “noise” and eliminate it for you. The key is balanced impedances between electrodes and as low a you can get them; and if this is accomplished your CMR feature (Common Mode Rejection) of your amplifier will “see” the 60Hz as a common artifact and will be able to eliminate it for you.