How to Read an ECG: Stanford Cardiologist Explains

How to Read an ECG
Dr. Marco Perez
Dr. Marco Perez is a Stanford Associate Professor of Medicine, Stanford Clinical Cardiac Electrophysiologist, Co-PI of the Apple Heart Study, and Co-Founder of Qaly.

Key Takeaways

Dr. Marco Perez is a Stanford Associate Professor of Medicine, Stanford Cardiac Electrophysiologist, Co-PI of the Apple Heart Study, and Co-Founder of Qaly.


If you asked any physician ten years ago whether a patient could learn how to read an ECG, I believe you'd have uniformly heard, "Go to medical school first." We now live in an era where anyone can go to the store and buy an ECG smartwatch that fits on their wrist, or in the palm of their hand. The technology that enables this is truly marvelous. More importantly, the opportunities that this unlocks are extraordinary. Patients and consumers are empowering themselves with an ECG watch, and it's reasonable to want to understand one's own physiology at a much deeper level. The consumer tools we have now are accelerating this future.

Check your ECG on these devices. An automated algorithm will try to determine if you have atrial fibrillation. You'll also, however, be able to look at the ECG tracing yourself, and I'm certain that many non-clinicians like yourself will naturally start to recognize patterns and start to wonder... "could I possibly decipher my own ECG?" The short answer is yes: I believe most people, with a bit of training, could learn how to read ECGs. It's not rocket science. And it's not a superpower you only get by going to medical school. It's a matter of pattern recognition, which begins to form once you understand what all those squiggly lines of an abnormal ECG represent.

Of course, there are plenty of caveats here. A full, clinical ECG has 12 leads, while the at-home consumer devices, for the most part, have one lead (although some have six). This narrows the field, so that we can focus on what things should look like on a single lead. If we try to learn what the different shapes and sizes of the ECG mean in the PQRST wave, it can get a little overwhelming. However, if we just focus on rhythm — in other words, the flow of electrical signal through the heart that keeps the heart beating — then I think things become more manageable. Of course, you could study the ECG for a few years and become a master of reading all aspects of the 12-lead ECG, but for what most of us want to do with our at-home ECG monitoring, focusing on rhythm from a single lead will get you pretty far when learning how to read an ECG strip like an Apple Watch ECG.

The other major caveat is that you should not be making your own clinical interpretations. For now, this new era of at-home monitoring is great for learning more about one's own body; however, if you need to know whether or not your ECG requires clinical intervention, particularly if you are feeling any symptoms, then I'd leave that to the pros with clinical training.

Traditional Training for Reading an ECG

A Doctor’s Journey

To master reading a full, 12-lead ECG takes many years. And most of us concede that it's a lifelong journey. When learning how to read ECG strips like an Inconclusive ECG, there are so many subtleties to the ECG that require you to see many examples of abnormalities to appreciate them well.

All doctors begin learning about ECGs — and how to read ECG strips — from their first year of medical school. We learn about heart anatomy, and about how electrical signals are transmitted at the cellular level. We then learn about the different diseases and how many of them can affect how the patterns of the ECG change from normal to the very abnormal. We also learn about normal variations. Like everything in biology, there's normal and natural variation, and we have to learn to distinguish these normal variations from truly abnormal patterns that represent disease. We also learn to deal with uncertainly. Not everything in medicine is black or white; in fact, when it comes to ECGs, we'll often show the ECGs to each other and debate (sometimes for longer than we want to admit) about what the “right” answer is.

In medical school, after learning the basics we're exposed to a few hundred patient ECGs. By the time we finish internship and residency training, this jumps to a few thousand. A few of us continue on to specialties where reading an ECG is an integral part of what we do, like cardiology or emergency medicine. There, we read many thousand more ECGs. While training, there's someone looking over your shoulder to make sure you aren’t missing anything critical. Of course, our patients are our best teachers — we quickly learn to connect abnormal ECGs like WPW ECGs and SVT ECGs to patient diseases and their outcomes.

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Learning how to read a 12-lead ECG is a never-ending process. We still rely on each other to learn the subtleties of ECG reading, and we do our best to pick up rare findings that can impact the care of our patients (like ones that might appear on a borderline ECG). Most importantly, it takes time to appreciate that interpreting the ECG in a clinical setting requires not just pattern recognition, but an understanding of your patient and their background, in order to come up with the best possible interpretation of what their ECGs are trying to tell you. More than anything, that's what takes us clinicians so long to get comfortable reading ECGs.

Despite the fact that the ECG has been around for so long, there's still an active area of research expanding our knowledge and understanding of how to interpret the ECG. I find this journey to be incredibly exciting.

A Technician’s Journey

Most of the technicians who train to read ECGs are primarily trained on how to read heart rhythms from monitors. Few technicians are trained on how to interpret all aspects of a full, 12-lead ECG. There are actually computer algorithms that usually do the “first pass” of a 12-lead ECG interpretation. These algorithms are OK at recognizing certain patterns (like a completely normal ECG), but not great at reading others. Technicians generally don't do a technical over-read of a 12-lead ECG, primarily because the clinicians will use the automated algorithms as a starting point and then can quickly provide their own interpretation.

That being said, technicians are very well trained in learning how to recognize abnormal rhythms, largely focusing on rhythm interpretation from single or 3-lead ECGs. Experienced technicians can even make rhythm determinations more easily than less experienced clinicians. Technicians will typically take courses that last several dozen to several hundred hours, understanding the basic anatomy and physiology, and then recognizing normal and abnormal patterns like an abnormal or normal PR interval, or an abnormal or normal QRS interval. They, too, will practice reading hundreds, and often thousands of ECGs before ultimately achieving certification.

The primary difference between a technical over-read and a clinical interpretation is that the technician does not meet the patient. They don’t know if the patient is having chest pain or heart palpitations like PVCs or PACs, or if the patient has a family history of heart disease. Meanwhile, a clinician takes all of this clinical information into account and makes a decision on management of the patient based not just on their own technical read, but also on the clinical factors. This is why the technical read of your ECG is never a substitute for a clinical interpretation.

Can You Learn to Read Your Own ECG?

The short answer: yes, I believe that with some training, you could learn to make technical reads of your own ECGs — particularly if you focus on rhythm interpretation from a single ECG lead. You can learn the anatomy and physiology required to understand the tracings. You can learn what a normal tracing should look like, and what some common rhythm abnormalities look like. After looking at several examples, you'll start to notice the patterns and what to look for.

Perhaps more importantly, I think most people could learn what their normal ECG looks like, and then recognize when their ECG has changed. Take your own ECG, and really aim at understanding what each wave looks like. You can learn to recognize common abnormalities like premature beats and atrial fibrillation. You may not get to the point where you can recognize more unusual heart rhythms, but at least you can learn to recognize that something doesn't look normal. Much like when a dermatologist asks you to “keep an eye” on a mole and let them know if you see any changes, I think you could learn to do something similar with your ECG. If there's a big change, you may not know immediately what it is, but you'll know enough to ask someone more experienced to look at it.

Again, I believe we are living in a new era of patient empowerment. This comes with great opportunity, some responsibility and inevitably, a bit of risk as well. For those of you who want to understand your physiology better, I encourage you to learn more about your measurements, but always know when to ask for help. What I'd caution on: not to start making your own clinical interpretations. If you have new symptoms, don’t rely on your own technical read of the ECG. This is what your health care provider is there for. 

Nevertheless, I believe that people will find new power in seeing and pointing out unusual changes in their ECGs to their clinicians, prompting faster and more appropriate clinical care for their hearts. Stay heart healthy.

- Dr. Perez

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Have trouble interpreting your ECG? On the Qaly app, human experts will interpret your watch ECGs for arrhythmias and irregular heartbeats within minutes. Get started for free today.

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