ECG Lead Isolation
-By Ivan Joyner
One of the required electrical safety tests that the biomed will do quite frequently is the lead isolation test. This article will describe in simple terms the what, where, and why of lead isolation.
Basic electronics tells us that in order to have a complete circuit or to allow current to flow we must have two points of contact. Now file that for just a moment and think back to what you know about ECG monitoring. In that procedure, ‘lead wires’ or signal acquisition cables from electrical instrumentation are attached to the skin of a patient. In addition, a nurse or technician applies conductive gel to enhance conductivity and make the patient a part of the circuit.
Now, suppose we had some malfunction in our system? We would want to make sure that our patient, who is now part of that electric circuit, would be protected from any harmful current that could be delivered to him through those lead wires. Go back to the electronic principle that we discussed earlier…two points of contact, right? Well it’s not uncommon to have 3, 5, 12 leads or more attached to the patient for some form of ECG analysis. Each of those lead wires is a conductor between our electrical equipment and our patient.
One final thought before we move on is this: the purpose of connecting our equipment to the patient is to provide those attending the patient with good diagnostic information at a critical time, possibly in the emergency room or during surgery. Of what benefit would all that equipment be if it burned out due to stray electric current being fed back into the amplifier? Imagine the lights going out while you’re walking through an unfamiliar room. So in addition to protecting the patient we want to protect the equipment.
How Lead Isolation Is Done
Lead isolation can be accomplished in a variety of ways:
• Photo electrically
We’re going to focus on the last method as that is the one that most biomeds seem to have trouble with. Op amps are often used for this form of isolation and a simple review of some key properties of operational amplifiers will clear this up for you.
One of the principle characteristics of the operational amplifier is its inherent high input impedance. What that means is that to voltage it essentially looks like a short however to current it looks like an open circuit. Since the input terminals have ‘infinite’ impedance (i.e., no current will flow through the input terminals), they are ideal for protection circuits. Patients are effectively insolated from undesirable leakage currents. So in practical terms, the patient lead wires would be connected to circuitry that effectively isolates the patient before sending or receiving any electric signals. I would love to make that a bit more complicated but for the biomed that’s essentially what we need to know.
Standards call for measurement of the current in a specified impedance connected between earth ground and each conductor that would connect a person to an electrical device. Remember our lead wires? If we are performing an electrical safety test on any piece of equipment with ECG monitoring capabilities we also need to perform lead leakage tests. The basics of the test are similar regardless of which brand of electrical safety analyzer is used; however, one thing must be noted. It would be difficult, if not impossible, to perform lead leakage tests if the electrical safety analyzer is not equipped to do so. Many hospital administrators seeking to address the bottom line skimp on this feature and attempt to save a few bucks by purchasing an analyzer without this function. If you don’t have this function I don’t know how the test could be completed. That being said, the lead wires from the device under test would simply be connected to the test posts on the electrical safety analyzer. The tests should be performed following the instructions of the test device and the results recorded.
This is a simple overview of the ECG isolation system. I hope that it will help you in your quest to become the Ultimate Biomed™.
About the Author: Ivan Joyner has been a biomed service technician for over 26 years. In addition to his duties as the manager of a hospital biomedical department he serves as an adjunct instructor of biomedical technology at a private biomedical training school on the west coast and is the publisher of the biomedical website www.bmetnow.com.
Mr. Joyner is an inaugural member of the Orange County chapter of the California Medical Instrumentation Association. He is currently working to complete his biomedical book series the Ultimate Biomed™. He may be reached at firstname.lastname@example.org