Clinical

Electrodes/Sensors Q&A

Published on

Leah Hansen, R.EEG/EP T, responds to Sleep Review readers’ questions about electrodes and sensors.

2012-04 10-01

Leah Hanson, R.EEG/EP T

Leah Hanson, R.EEG/EP T, director of sales at Rhythmlink International LLC and featured expert on Sleep Review’s Expert Insight, offers readers advice on everything from how to clean electrodes without degrading the wire’s conductivity to the best electrode length. In addition, she helps readers make sense of the metal offerings available and whether disposable electrodes are on par with reusable options.

Q: Is it okay to soak the entire electrode wire in hot water for paste removal before disinfecting? It seems like paste residue could find its way inside the box connection end and eventually degrade the wire’s conductivity.

Hanson: You are correct that paste residue can get inside the connector (the jack box end of the electrode) and degrade the connector, affecting its ability to perform. But soaking the entire lead—meaning you are submerging the connector end—can degrade the connector as well. The reality of the situation is that the connection points (crimp junctions) can, and most likely will, degrade over time even if proper cleaning and disinfecting steps are taken.

If you have a specific issue with paste all over the leads, be very careful with soaking the entire lead. If water gets inside the connector, it can corrode the connection points. Clean and disinfect thoroughly but with as much caution as possible. And, if you have frequent breakages, or frequent situations where paste or other materials are getting on the connection points, you should consider a disposable single-patient electrode as an alternative.

Q: What types of disposable electrodes do you recommend? And what are the pluses and minuses of using disposable leads?

Hanson: Really, any type of disposable should be considered as the quality has gotten very good. Disposable single-patient EEG cups are available through a variety of sources (research “disposable EEG cup electrodes” online to get a wide variety of options), and they function very well.

There are a number of benefits to disposables; the most obvious is the lowered risk to the patient of cross contamination. Despite all of your efforts to clean and disinfect thoroughly, you cannot dispute that a risk of cross contamination is always present when using reusable electrodes (with the exception of sterilization). The next benefit is cost. That’s right: cost. I know many of you won’t believe me, but I encourage you to do a full cost analysis of the use of reusables versus the use of disposables. There are suppliers that offer such a comparison at no charge; and you will be shocked how equal or less expensive it is to use disposables versus reusables in your service. Contact your favorite supplier and get a quote for disposable EEG cups for your service and spend time running through an analysis exercise. But analyze the costs completely, including every minute of labor (including salary and benefits), cost for chemicals, cost of unused inventory in your storage room, etc. The direct purchase price of disposable electrodes will be much higher per year than reusables, but that is only a portion of the actual cost of maintenance of electrodes.

As far as disadvantages to disposables, the absence of a gold disposable electrode would be one. The truth is gold plating—that is what is on the outside of the electrode—is expensive and the cost of using a “gold” disposable would be difficult to justify in the budget.

Notice I did not list quality anywhere in either the pro or con section. This is because you need to determine quality using your staff, your application techniques, your patient demographics, etc. I strongly encourage you to contact disposable EEG cup electrode suppliers and ask for a sample of disposable EEG cup electrodes and test them on one of your patients and see how they perform compared to your reusable. I predict you will find the quality is equal to a reusable. But test this for yourself.

Q: When and why do you change high and low frequency filters? When do you use notch filters: 60 Hz, 50 Hz?

Hanson: High and low frequency filters, as well as the 60/50 Hz, are used to help us see the data that we care about and get rid of the data we don’t care about. Think of your filters as the bass knob on your stereo. You may like more bass than someone else, so you change the setting to hear more bass in your songs. This doesn’t change the music coming into your stereo, just how you hear it. The filters on your machine are exactly the same. Low frequency filters allow fast frequencies to pass but limit the slow frequencies we see on our display. High frequency filters allow the slower frequencies to pass but limit the fast frequencies we see on our display. A 60 Hz filter completely blocks anything that is being recorded at a frequency of 60 Hz (and frequencies around 60). 60 Hz of data is common environmental artifact/noise from other machines, TVs, monitors, dimmer switches, etc; 60 Hz is also seen due to imbalanced impedances between electrodes, so be sure your impedances are balanced and low—and you should not need your 60 Hz filter (50 Hz is the same as 60 Hz, but is mainly seen outside of the United States).

You always want to understand your filters so you can understand what you are NOT seeing. And remember you are not changing the data, just what you are seeing. And to ensure this is how your system is set up, contact your equipment manufacturer and they can clarify your system’s setup configuration. I recommend using a previously collected PSG or EEG and changing the filters; you will quickly see the differences that filters make. For example, if you use a high filter of 15 Hz, you will see very clean data because you are not displaying faster frequencies like EMG; and this can help, but it can also risk missing data you want to see like snoring. The recommended guidelines help each application have a starting point for filter settings, but I encourage you to understand your filter options beyond the default settings. It could make your scoring tasks a lot easier if you understand filters and can lead to good decisions to help enhance your patient’s data. If you want guidance for filter settings, contact your equipment manufacturer and they can help you understand how your data are saved in each channel and which channels.

Q: Is there a recommended maximum length of electrodes used for sleep recordings—excluding legs, of course?

Hanson: The recommendation is to use a lead that is as short as possible to get the job done. The electrodes act like antennas for noise and artifact, and the longer the electrode, the better the antenna. It’s just like an antenna on a TV—to get a better signal, you raise the antenna so you can get more signal. Our leads really function in the same way. So whether you are using short or longer electrodes, always be sure to “bundle” the electrodes together so each electrode receives the noise and unwanted signals in a similar manner. If you have 10 electrodes that you have connected to your patient and nine electrodes are bundled together and one lead is hanging by itself, the bundle will receive the noise in a similar way and this allows the Common Mode feature of your amplifier to eliminate the noise better because the noise is “common.” But that dangling electrode will receive noise completely different than the rest, thus making the Common Mode feature ineffective and will allow the noise into your recording. So, use whatever length you need, but do your best to keep the length as short as possible to assist your data collection efforts and ensure clean data signals, and bundle any extra length and leads together.

Q: What metals are used in EEG electrodes? I’ve seen gold with silver, or gold with gold, and gold with other metals.

Hanson: Yes to all. Reusable metal electrodes can be a single pure metal or a combination of a base metal covered by a coating of another metal type. For instance, you can purchase pure silver electrodes, which are called silver EEG electrodes. Or you can buy gold EEG electrodes, which are really silver-based electrodes that are plated with a thin layer of gold. This is probably one of the most common electrodes commercially sold. Unless you have a specific metal requirement, the “base” of the electrode can be different than the outer layer without causing any problems. This can keep costs down and allow the electrodes to be more affordable. And even disposable electrodes are typically an ABS plastic base covered by a thin layer of silver/silver chloride coating. To know the details of your specific electrode, contact your supplier for the details and they can tell you the base and any outer metal material. But I think you will be hard pressed to see many differences in the data between the most common metal types and combinations.

Rhythmlink International LLC

PO Box 2045, Columbia, SC