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How to Read an ECG

By Daniel Champigny - Jun 15, 2021


How to Read an ECG


The video above and article below share different information. Listen and read to both to get the most out of this post!



The ECG can be a formidable foe for the PA student. In my early days working on the ambulance, gaining hours to apply for PA school admission, it seemed very easy. We would transfer the patient to a STEMI center if we saw ST elevation on the paper, but beyond this, I did not know much more. During PA school, I struggled with how to read an ECG. Questions that students ask us commonly are what is the normal ECG pattern? As my ECG learning increased, I realized all of the nuances and abnormalities that little piece of paper can reveal. Today, we will review a systematic approach on how to read ECG strips, starting with the basics.


The normal physiology        

Like many of the other important topics in medicine, we have to learn the normal function and structure of organs prior to studying the diseases. For today’s article, we will focus entirely on the normal portions of an ECG, saving the pathology and complicated life-threatening conditions for later, as books have been written on specific ECG findings.


So, where do we start?

The first step to understanding an ECG is thinking about the why behind the ECG. This is the first step that helps us assure we are reading it properly and obtaining the correct diagnosis. For example, on the occasion that I order an ECG in primary care, I begin to ask myself:

  • What am I looking for?
  • What are the possible findings that may support my diagnosis?
  • What are the findings that would point away from my diagnosis?


ECG for pre-op and in the ED

If it is for a pre-operative clearance, what are the findings that would warrant cancellation of the surgery. If you are in the ER, what are the findings that you can’t afford to miss?


Narrowing the differential

By thinking about what we might expect and the symptoms the patient has, it allows us to help narrow down our findings, but this is not the end all be all. We must consider many different outcomes, options, or findings that might be present on our ECG.


First rules for ECG analysis

The first rules for our ECG analysis are, to always be considerate of what the patient tells you while maintaining an open mind and a broad differential. Be open to finding things that you might not have been looking for and may not support your preliminary diagnosis! Medicine will always surprise you.


Don’t let the machine do your job….


An additional pearl before even beginning to read the ECG is to avoid simply gravitating to whatever the machine read out tells us. This can be tempting and is certainly the easier option, especially when you have an urgent care waiting room full of coughing patients, but this is dangerous and wrong to do. The machine knows NOTHING about the patient. It’s nothing more than a machine and does not take into account the patients signs and symptoms. Their appearance, and what the patient is telling you.


Cover the “diagnosis” read by the machine

Any medical assistant or LPN who has worked with me at urgent care or family practice knows that I get upset if they put the ECG in front of me and say “Yep, looks like he has A-Fib”. That’s not what I want to hear. When an ECG is put in front of me, I immediately cover the machine’s report. It’s my responsibility to read the ECG.


Understanding what the ECG is telling us

Once we have an ECG in front of us, the first step is understanding what we are looking at. In many practices, we will be looking at a 12 lead ECG, which is a measure of the heart’s electrical conduction system, as energy flows through the heart. On a plane, the various leads correspond to different areas of the heart, each with a different outlook based on their location. For PANCE prep, the boards love to ask us what leads are associated with which part of the heart.

Lead Correspondence to Heart Areas

The heart rate - regular rhythm

Prior to taking a look at the different components of the ECG, we should probably try to figure out how fast the heart is beating. One of the most famous books of all time, “Rapid Interpretation of EKGs” by Dale Dubin M.D., tells us to look at the R wave first. The R wave is the tallest part of the ECG. The R wave typically falls on the darkened portion of a large box of the ECG. By seeing how often an R wave is present, we can easily determine how many beats are present, based on the time corresponding to each large box.

For example, you can calculate the heart rate by dividing the number of large boxes between two successive R waves into 300.  If the interval between two QRS complexes is two large boxes, then the rate is 150, because 300 ÷ 2 = 150 bpm.

The heart rate using the “300-150-100…” method

In this case, if we have an R wave above at the start and then immediately another R wave at the “300 mark”, the rate is roughly 300. Woops, too fast. With each corresponding next dark line, the rate gets slower and slower. This allows us to count the QRS complexes very quickly and get a good estimate of how fast the rate is.


Heart Rates using 300, 150, 100
     Heart rates associated with each of the large boxes in the following order are:
                                     300, 150, 100, 75, 60, 50, 43, 37, 33

The heart rate - irregular rhythm

If the heart rate is irregular, count the number of QRS complexes on the ECG and multiply by 6 to obtain the average heart rate. The ECG paper will show a period of 10 seconds. Therefore 6 x 10 = 60 seconds. 

Prior to taking a look at the different components of the ECG, we should probably try to figure out how fast the heart is beating. One of the most famous books of all time, “Rapid Interpretation of EKGs” by Dale Dubin M.D., tells us to look at the R wave first. The R wave is the tallest part of the ECG. The R wave typically falls on the darkened portion of a large box of the ECG. By seeing how often an R wave is present, we can easily determine how many beats are present, based on the time corresponding to each large box.


The P-Wave

It is very helpful for us to know immediately what each component of the ECG is representing, as it can help us think through the different pathologies that might be contributing. The first portion of the ECG is the P wave, this is the portion when the atria depolarize/trigger, once the Sino-atrial (SA) node triggers, also known as “the pacemaker of the heart”.

Normal ECG Waveforms


Abnormal P waves

The P wave is important because it can help clue us in to what might be going on with the heart. In atrial fibrillation, P waves are absent. In certain conditions, such as an enlarged atrium, the P wave may look different than normal, showcasing abnormal heart intrinsic ability.

ECG of the Heart Cycle


The QRS Complex

As the process continues to move along, we have what is termed the “QRS complex”. The QRS complex is the portion of the ECG where the ventricles begin to depolarize. The QRS is seen as a classic short downstroke (Q wave), large upstroke (R wave) and then corresponding downstroke (S wave). It is usually fairly narrow and quick, but a lengthened or widened QRS can indicate difficulty with the heart’s electrical system.


The T-Wave

Once the QRS complex is completed, the next portion of the ECG is the T wave. The T wave is the portion of the ECG where the ventricles start to repolarize. This is at the close of the cycle, with the whole portion from the start of the P wave through the T wave, being about 1 second in duration.


Average heart rate – bradycardia & tachycardia

The average human has about 60 to 100 cycles of this per minute, with anything less than 60 being termed sinus bradycardia and anything over 100 being termed sinus tachycardia.


PR Segment & QT Interval

Portions of the time taken for these things to happen can be measured, including the PR segment, the length of the QRS complex, the QT interval.


Importance of interval duration

As with anything in medicine, normal values for the duration it should take for these items to happen have been documented and it is important for students to have a rough idea of these. Abnormally long or shortened intervals can be indicative of serious pathology.


Normal Characteristics

Here is a chart that will help you commit to memory what is “normal” for the heart’s electrical conduction system, courtesy of Research Gate. These become particularly important when we talk about heart block or shortening of the QT interval due to electrolyte abnormality.


Normal Heart Characteristics


What is the PR interval

For the above, it’s helpful to clearly solidify what this means. The PR interval is the beginning of the P wave to the beginning of the QRS.


What is the QRS Segment?

The QRS is the start of the Q wave through the end of the S wave.


What is the ST interval?

The ST interval is the beginning of the S wave to the end of the T wave. The QT interval, on average is typically about 0.4 seconds.


How to measure the length of intervals   

When measuring all of these, it is helpful to look at the EKG boxes themselves. Quick hint here: this is also measured out for you!

  • 1 small square = 0.04 seconds
  • 5 small squares = 1 Large Square.
  • 1 Large Square = 0.2 seconds
  • 5 Large squares = 1 second

The ECG can definitely be a daunting task for students and new graduates, but the best thing to do is practice. When I first began practicing, I was overwhelmed by ECGs. I was used to taking exams, where there was bound to only be one correct answer. In medicine, ECGs can be a mixed bag, which can be scary.


The different components of the ECG

The goal of today’s article is to simply introduce the different components that make up an ECG. It is important for the student not to try to master everything at once. The ECG is a stepwise process that takes a lot of time to master, even for the most experienced clinician. Start with mastering normal and then only when you can always determine what is normal, then move on to the pathology.


When to use the machines “diagnosis”

My recommendation is to keep looking at normal ECGs. Any time an ECG is ordered, on your patient, whether in the ER, primary care, or for a preoperative evaluation; look at it and gain some confidence as to the interpretation. Only after you have a systemized approach to how you read the ECG are you able to come up with ALL THE INFORMATION on your own. Only after you have mastered this, should you start to take a look at the machine’s interpretation as well as your own.


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