Help reading an EKG?

I am doing this shadowing deal in the ER and there are a few things I need to be able to talk about on a basic level. But for some reason I cannot for the life of me figure out the meaning of an EKG. I am missing some piece of it. My question is, what electrical events do the waves on the EKG represent? Everywhere I’ve looked says, for example, that the T wave represents ventricular repolarization. But how can a “wave” represent “re”-polarization? The voltage at the beginning of the T wave is the same as the voltage at the end of the T wave. I don’t see how the polarity of ions across the cell membrane has changed in that case. On the other hand, if they were saying that the first half of the wave was showing depolarization and the second half was showing repolarization, that would make sense to me.
Am I wrong to be thinking that the voltage being measured on the EKG is the same as the voltage changes that occur across the cell membranes as they send the electrical impulse through the heart? (And is that how the message gets sent, in a way that is similar to nerve cells when they are transmitting action potentials?) Also, where the EKG shows a straight line, is that supposed to be showing the resting potential of the cells that transmit this signal? If so, then where does the zero voltage line go through the graph?
I’m kind of suspecting that I’m misunderstanding something big here, because the y-axis is not even labeled on any of the EKGs I’ve seen, and that part seems important if you’re directly looking at cell membrane potentials.
Thanks in advance–I’m super confused right now!

Try getting copies of the following books. They are great in helping understand ECGs.
–Dale Dubin, MD, “Rapid Interpretation of EKGs”. This one is especially good for the depolarization-repolarization questions you had.
–Vernon R. Stanley, MD, PhD, “12-Lead ECG Interpretation for Medical Personnel” and the workbook that goes with it, “12-Lead Workbook for the Medical Practitioner”. Then if you can get his heart ruler, things really start falling into place!
The more EKGs you look at, the more you’ll begin to understand.

The EKG leads can only read changes in the electrical state of the cells and that is the case for every single lead. Don’t worry about ‘waves’ as the ‘waves’ are electromagnetic waves produced by the ion flow. I think it is easier if you just think about depolarization (cells becoming more positive) and repolarization (cells becoming more negative).

An increase in depolarizing cells heading toward a lead would yield a positive deflection (above the baseline).

The up and down part of the upward deflection is like the rate of the cells becoming depolarized. As more and more cells are depolarizing the deflection is upward moving. As fewer and fewer cells are depolarizing the deflection moves back towards baseline.

The opposite is true for repolarization (repolarization of cells towards the lead will yield a negative deflection since the cells are becoming more negative in charge).

Dubin’s book explains this well.


Am I wrong to be thinking that the voltage being measured on the EKG is the same as the voltage changes that occur across the cell membranes as they send the electrical impulse through the heart? (And is that how the message gets sent, in a way that is similar to nerve cells when they are transmitting action potentials?) Also, where the EKG shows a straight line, is that supposed to be showing the resting potential of the cells that transmit this signal? If so, then where does the zero voltage line go through the graph?

I think you are trying to read too much into the EKG. The lead is reading the change in the state of the cells. At rest the cells can be thought of as negatively charged (not true in reality but it’s the way the model works). As the cells depolarize they become more positive and this depolarization begins at the SA node and travels through the heart cells in a particular manner. The EKG leads are seeing this wave of change in electrical state cells. The height of the deflection is really the rate at which the cells are depolarizing.

Ok, thanks! I’ll get those books. And I won’t try to take the “waves” too literally. It sounds like what’s on the y-axis isn’t voltage per se but rate of depolarization?
Hopefully I can get the books today!

The waves on an EKG are the summation of vectors. The tracing does not denote ionic conductance in the sense that the tracing of a patch-clamped ionic channel does. Deviation from baseline in an EKG show direction of movement of the wavefront of repolarization or depolarization of the myocardial muscle mass. That directionality being assigned more by convention & by the designation of + vs - on the EKG patches on the chestwall.
Dubins, mentioned above, says it much more clearly & is the book most folks cut their teeth upon leanring to read EKGs. Just as an aside, Dubins himself is in prison…I am loathe to mention for what as I am not sufficiently confident in my memory & would hate to publicly malign him in error.

I also recommend Dr Dubin’s book.

There was a great controversy among my fellow med students about what exactly he had done and whether we could buy his book. My understanding from being the designated googler of our group is that he was a plastic surgeon who got himself in trouble for doing cocaine, having relations with a teenage girl, and videotaping the result. He lost his medical license and went to jail. No longer in prison, he wrote the EKG book. Frankly, if you believe at all in redemption or rehabilitation I think you can buy the book in good conscience since he had to do something after he’d paid his debt to society, and teaching medical folks how to understand EKGs is a pretty good thing to do. Beats making license plates. On the other hand, if you want to take a stand against people who do bad things to teenage girls, even if they did them a while ago, you might want to avoid the book. I submit for your consideration:

I personally bought the book with very mixed feelings, but, it’s a pretty helpful resource if you can overcome the mixed feelings.


So glad I didn’t know any of that when I bought Dubin’s book
The book that a lot of us liked at GWU was Thaler’s The Only EKG Book You’ll Ever Need which I bought in desperation after things just didn’t “click” with Dubin’s book. I liked Thaler MUCH better. Dubin was a little hokey for my tastes.
Here’s the listing on Amazon.

Well, I seem to be back to the drawing board. I bought the Dubin book, and I’ve looked into the others mentioned here, but I still am stuck with some of my original questions. I’m not really trying to interpret EKGs so much as I’m trying to figure out what kind of circuitry is involved in the setup and what’s being measured. If anyone wants to help me out with any part of this, I would totally appreciate it!
Anyway, so all these leads are placed on the person, and then the EKG machine records a current. So if I was drawing a circuit diagram, would the person’s body be drawn as the source of electric potential? I am confused because in one picture in Dubin’s book, the EKG machine looks like IT’S supposed to be the source of potential. It looks just like a car battery, with positive and negative electrodes and wires going into the person. This just inherently does not make sense to me because the machine should be measuring voltage (and recording a current), not applying a voltage to the patient!
Another thing I don’t get in Dubin is why you would place positive and negative electrodes ON a person. Are the electrodes actually charged up BEFORE they go on the person, or is it the potential inside the person’s heart that CAUSES one electrode to be positive whereas another one is negative?? I’m so confused.
Does anyone know where I can find a circuit diagram along with a description of how the current is measured? I can only find diagrams myself that focus on the internal wiring of the EKG! Is this subject covered in medical school so that everyone understands it better than I seem to right now? Maybe the reason I am having such a hard time with this is that the circuitry involved (12 leads, vectors, changing polarity, etc.) is way more complicated than anything I saw in my physics class.
Sorry if this question is totally dense or I am missing something really basic. I know I am also having a hard time phrasing my questions since I’m confused on such a basic level.

Ok, now someone has told me that in fact a voltage could be applied to the person, so as to raise the overall voltage far enough above zero to have a working voltage to measure any subsequent changes from. They also said the electrodes could be charged ala a capacitor, and that what is being measured is the changing electric field in between the sides of the capacitor.

Every day I get a different description! If anyone has another book recommendation, please let me know. Otherwise I’ll just have to bite the bullet and go to the electronics library, like a regular geek.

I think that you are way overthinking this problem. The EKG machine measures the inherent electrical activity in the heart itself. The pos & neg designations for the leads are purely convention and not anything to do with an applied current. It has to do with whether or the deflection made on the paper is up or down (vector toward a + electrode is an upward deflection).
The reason I am using the term ‘vector’ is that the heart is a 3-D organ in 3-D space & each myocardial cell’s membrane changes its potential (depolarize -> repolarize -> depolarize) - so, the net result is a vector (a summation of the millions of tiny voltage changes assoc with each myocardial cell going through its voltage cycle). The fact that this electrical activity is organized & synchronized (neccessary for the myocardium to contract in the manner of an efficient pump) is why those alterations are effective summed as vectors.
Does this help? It is a simplification…when you start “reading” EKGs you must take all of this info coupled with a ton more data AND (most importantly) the pt’s clinical picture to make a “read” of what is going on. But this is the basics.

Thanks! That does help. It especially answers my question about the electrodes being designated positive or negative.
I also broke down and checked out some textbooks on the subject. I was pretty glad to see that there was a whole wall of books devoted to EKGs, including PhD dissertations and mathematical treatises–so I’m not the only one who thinks this is a formidable subject. I got the kind of books that go right down to the triple integrals and other ugly stuff that I know I’m not going to want to deal with. This means that I can solve the problem of overanalyzing it just by avoiding it all together, meanwhile being pleased that at least I KNOW it’s not as simple as some of the sources imply! Sometimes a mean-looking stack of books on my floor is all I need.
Thanks again!

Dave did a great service to you by pointing you in the direction of vectors. Thinking in terms of individual cell membrane potentials in an ECG is, even for an electrical engineer, just asking for trouble.
The other thing that might help you simplify this is to remember that a 12 lead ECG is just a printing convention for displaying 12 unique views of the same event. Each lead is one positive and one negative lead (even though in all but three the negative lead is a ‘virtual’ lead). The printout you get is just the agreed upon way of displaying the same event as captured from 12 different viewpoints.
Good luck!
Take care,

You do not need to apply voltage.
I think that one way to think about it–hope I’m not wrong to think about it this way–is that you are measuring fluctuations in current (created by ion fluxes of waves of depolarization) which then create fluctuations in voltage–V=IR. Mostly R does not change in the ECG setup (although see below for one example where it does).
For more explanations see these googled results for which I can not safely totally vouch:
A particularly to-your-point explanation can be found at:
An example of an instance where the R in the V=IR does drop, and the V therefore drops, can be found in this interesting article:
good luck

Thank you so much! Joe, those links are really helpful. I am just starting to look at them and they seem to really address my questions. So thanks again.
And thanks to everyone for all the responses!
Just wait til I start medical school and we have to understand MRIs, CT scans, ultrasound, X-rays, and every other kind of test under the sun. I can only imagine I’ll have 800 additional questions so everyone, get ready.