Re: Why is a salt solution first applied to probes for an electrocardiogram?

Hi. Thank you for your question. You asked, “Why is a salt solution first applied to probes for an electrocardiogram? This question is relevant in part of the research paper that I am doing for Chemistry class. Throughout my research so far, I have found that when the electrodes are attached to the human body for an electrocardiogram, a salt solution is often applied to the probes, but I can't seem to find any information on why it is applied. Please help!”

An electrocardiogram (ECG) is a tool used for examining the electrical activity of the heart (to help diagnose its health). A doctor will place little sticky electrode pads containing conductivity gel onto the skin surface, then clip little wires to these pads and connect them to the ECG machine. When the heart beats, it releases extremely tiny electrical impulses (around 0.0001-0.003 volts, 0.05-100Hz) which are picked up by the ECG machine. An ink stylette on the machine traces the pattern of electrical charges onto a strip of paper which the doctor can then read and determine the health of the heart (certain ECG patterns are indicative of a heart attack, arrhythmias, high potassium…).

So why is there salt solution or some type of conductivity gel on those pads?

Again, those electrical impulses coming from the heart are really tiny. Since they have to make it through the surface of the skin to the electrode, an electrical pathway needs to be set up. The outside layer of the skin (the epidermis) is composed of dead cells, though --- dead cells don’t conduct electrical impulses very well. (This is actually one of the basic principles behind ECG --- if patches of the heart tissue are dead due to, say, a heart attack, the ECG reading will look different than if the heart was healthy, because those electrical impulses are slowed or blocked by the dead cells). So, we need to help get that electrical pathway through those dead cells in order to obtain a clear ECG signal.

Salts are great at conducting electrical signals. When they’re dissolved in water to make a solution, they produce two ions --- ions carry electrical currents. Each salt is a combination of a +ion and a –ion (such as table salt, which is NaCl, a sodium ion (Na+) combined with a chlorine ion (Cl-)). Adding conductivity gel or some sort of salt solution to the electrical pathway between the skin and the electrode pad reduces impedance of the electrical signal, allowing for a clearer signal to pass through and be read by the ECG machine. Other things doctors and nurses do to improve signals through the skin is to scuff the surface of the skin with sandpaper (to remove some of those dead surface skin cells) or to add alcohol to the skin (which reduces electrical impedance).

So, out of curiosity, what would happen if you didn’t add conductivity gel to the electrode pads? Well, you wouldn’t get a very clean signal from the heart. Instead, you'd probably get a wide, fuzzy ECG reading due to a type of electrical artifact we call “60 cycle interference”. Power to wall outlets is supplied by electrical currents of 60Hz, and often the ECG machine will pick up these signals “radiating” out of the wall instead of the heart’s signals! This happens a lot, especially when there’s a lot of other equipment running in the operating room or doctor’s office. Newer ECG machines are designed to reduce 60-cycle interference through various electrical filters.

Hope this helped to answer your question!

Sincerely,

G. Monreal