Look at a list of recommended anesthesia monitoring tools. You’ll always see an ECG, usually near the top of the list. The 2011 AAHA Anesthesia Guidelines for Dogs and Cats even lists it first on their list, although I can’t tell if there is significance to the order of the list. But it’s safe to say that an electrocardiogram is highly recommended. At one time or another we’ve all watched an ECG wave form crawl across a screen. And we’ve all used little sayings to remember how to hook up the electrodes. Sayings like “white right, snow over grass, brown ground, and smoke over fire…”. Frankly I struggled to memorize all of those cute little ditties until I realized the placement location is written on each electrode. Then I promptly forgot them all. For the longest time, that’s all I really knew about an ECG: where to connect the electrodes.
It turns out, even that tidbit of knowledge is flawed.
Since I’m using an ECG monitor on my anesthetized patients, what should I really know about it? Let’s start with the basics of what the ECG tells me and what it doesn’t tell me (but I may think it does).
- ECG and EKG mean the same thing. The first ECG/EKG was manufactured in Germany where all things ‘cardiac’ begin with the letter “K”
- It tells you that there is electrical activity at the heart
- It graphs a tracing of the heart’s electrical activity
- It does not tell you that the heart is responding to the electrical activity
- The ECG does not tell you that the heart is beating
That last bullet – it doesn’t tell you that the heart is beating – was a bit of a wake up call for me. How can that be? The answer goes to the previous bullets: the ECG graphs the heart’s electrical activity, but it doesn’t tell you that the heart muscle fibers are responding to the electrical activity. Oh, it will in time, but not immediately. At least not at my level of skill interpreting ECG wave forms.
As a veterinary technician who does anesthesia, where should my level of skill interpreting ECG wave forms be? I find electrocardiography fascinating, and I’ve spent long hours with a cardiologist looking at wave forms and cardiac ultrasound images. But as an anesthetist, I only need to know one thing about an ECG wave form: what normal looks like. And if it looks anything other than “normal”, I draw the doctor’s attention to it.
A normal ECG wave form repeats a series of ‘blips’ in a row. Each normal blip has a designated letter identifier. The full complex contains the waves “P, Q, R, S, and T” with Q, R, and S usually combined and referred to as “QRS”. What each wave indicates with reference to the heart’s activity, is a conversation for another time. The important thing for us is that we see each of the lettered waves appear, and in order.
All of that said, sometimes the “P” wave is missing. Sometimes the “T” wave looks upside down. In other words, sometimes normal doesn’t look exactly normal, and it will take a little time, practice, and conversations with your DVM to recognize when a deviation from normal is significant.
But once we’ve established what a normal ECG is supposed to look like, it gets pretty easy to recognize what abnormal looks like. For instance, the image on the left is very obviously abnormal. You can easily see the normal order of the P, QRS, and T waves is interrupted by a very abnormal wave complex. This merits the attention of the doctor.
So the responsibility is on us to establish a readable ECG wave form to start with. If our initial ECG wave looks like the one on the right, we have no hope of identifying anything normal or abnormal. It’s not enough to clip the leads to the animal. We need an ECG tracing we can use.
Let’s talk about clipping the electrodes to the animal, because this sometimes requires some creativity. Earlier I mentioned that the one thing I knew about the ECG (where to clip the electrodes) is flawed. It helps to understand ‘flawed’ by realizing what the ECG actually does. The ECG detects and graphs electrical activity between two electrodes. That’s all. Most practices use an ECG with three electrodes, which reads the electrical activity between any two, and the third just has to be in contact with the body. The electrodes are labeled RA (Right Arm), LA (Left Arm) and LL (Left Leg).
Now, stay with me because this is where it gets a little fussy. The two electrodes between which the ECG reads electrical potential are determined by the “LEAD” you select on the ECG machine. The standard leads are described in the picture on the left: Lead I, Lead II, and Lead III. Most ECG machines default to read Lead II, so unless you actively change that setting, your ECG will default to read the electrical activity between the electrodes labeled RA and LL. That means the electrode labeled LA need only be in contact with the body.
In order for the ECG to read a Lead II, the heart must be between the RA and LL electrodes. Again, the LA electrode can be anywhere, just as long as it is in contact with the body. To illustrate, imagine you decided to clip the RA electrode near the paw of the right foreleg, and the LL electrode a little farther up the same leg (as shown). You would not get a readable tracing of a Lead II because the heart is not between the two electrodes. But that’s the only thing you have to remember about placing electrodes: the heart must be between the two electrodes that are reading electrical activity and the third electrode must be in contact with the body. Follow that rule and you’ll get an ECG tracing you can use every time. That leaves us a lot of opportunity to be creative about where we place the electrodes. And often that can be really helpful.
This patient requires that we be creative about where we place the ECG electrodes. The right forelimb is to be amputated. With your ECG set to a Lead II, where would you attach the RA, LL, and LA electrodes so they would not interfere with the surgery, but would still offer a useful ECG tracing? There are any number of correct answers to this question. My choice would be to clip the LL electrode to the cat’s left hind leg, and then clip the RA and LA electrodes together and slide them into the cat’s mouth. I would not clip them to the tongue or oral mucosa (ouch!). I would just slide them into the mouth. The moist oral cavity provides good contact to the electrodes, and having them clipped together assures that the LA electrode makes contact with the body. The heart is between the two electrodes that the ECG is reading (RA and LL) so I will get a useful tracing. And I will have access to all three electrodes throughout the surgery in case they need adjustment. Simple. Creative. Effective.
We are not cardiologists, so perfectly placed electrodes and carefully positioned patients are not necessary for us to get good information from an ECG tracing. That allows us to “hack” the placement of electrodes to suit awkward situations we sometimes find our patients in. As long as we learn to recognize abnormal wave forms and draw attention to them when we see them, and remember to keep the heart between the right two electrodes, the ECG is a simple and useful monitoring tool for the veterinary anesthetist.
Ken Crump AAS, AHT is a writer and animal anesthetist, and writes Making Anesthesia Easier for DarvallVet, a division of Advanced Anesthesia Specialists. He makes dozens of Continuing Education presentations on veterinary anesthesia and oncology across the United States and in Canada. Ken retired from the Veterinary Teaching Hospital at Colorado State University in 2008