The Basic Guide to ACLS ECG Interpretation

ECG interpretation is a critical component in emergency life saving procedures. It has become an art form of sorts over the years where many medical professionals have invested thousands of hours memorizing and decoding the heart’s rhythm and its meaning.

The heart is a complex muscular organ and to understand the basics of how to interpret the clues it may give out at any time can better equip any medical professional and increase their odds of saving a life. So if you’re studying for your ACLS or PALS certification or maybe you just want to start learning about ECG strips in general, this guide will help you get started and lay the foundation for interpreting the most common heart indicators you can utilize in real world situations.

What Is An ECG?

ECG stands for Electrocardiography (sometimes referred as EKG). Basically ECG is a tool that allows you to see the waves of electrical activity occurring in the heart. The heart has a normal electrical rhythm and when this rhythm is disrupted, cardiac issues can result. By understanding a normal electrical rhythm in comparison to an abnormal rhythm, you may be able to quickly diagnose the issue and act quickly to reverse it.

How Is It Helpful For ACLS Or PALS?

ECG Interpretation is very helpful for taking the ACLS or PALS course because understanding an electrical rhythm and recognizing abnormal electrical activity can help diagnose issues quickly and dictate what responsive procedures are necessary.

A Peak Behind The ECG

Here’s what a normal ECG strip may look like:


Think of it like this, the above image is basically a snapshot of what’s happening in the heart. All these dips and waves represent something. They each represent a piece of the heart. To understand this further, let’s explore the basic anatomy of a heart.


The heart has 4 chambers. The top 2 chambers are referred to as the atriums and the bottom 2 are the ventricles. Blood comes into the heart through the top atriums and then is pumped into the ventricles which is then pumped to the rest of the body. The electrical activity of the heart, controlling this pumping action, is absolutely vital in ensuring blood is pumped adequately throughout the body.

Now let’s imagine a beating heart:


In order for blood to enter and exit each chamber of the heart, it has to empty and fill over and over again. This form of blood movement requires a “squeezing” and “relaxing” effect in each chamber. This is controlled by the heart's electrical system.

The electrical conduction system in the heart is made up of:

  • The SA Node. This node is located in the right atrium of the heart and sets a normal heart rate of 60 to 100 beats per minute.
  • The AV Node. This is located in the interatrial septum and sets a rate of 40 to 60 BPM.
  • The HIS Bundle. This is an important part of the electrical conduction system of the heart, as it transmits impulses from the AV node to the ventricles of the heart.


The heart nodes control this pumping action and allow for the heart chambers to relax (fill) and contract (pump) blood illustrated below:


You can actually see the above behavior in an ECG strip.

ECG Wave Interpretation

Now if we go back to the ECG tracing, each wave and dip represents a piece of the heart. To better distinguish specific waves and dips in the ECG strip, we assign each wave and dip a letter.

ECG Waveform - ProMedCert

We decipher basic observations by reading the following components of the ECG strip:

  • The P wave: This represents a contraction of the atria. This is referred to as depolarization or the squeezing effect. This electrical wave represents blood being pumped out of the atria and into the ventricles.
  • The QRS Complex: This represents a contraction of the ventricles. This is the depolarization or squeezing of the ventricles. This entire wave complex represents blood being pumped out of the ventricles and into the rest of the body.
  • The T Wave: This represents a relaxation of the ventricles. This is referred to as repolarization or filling of the ventricles. It represents blood filling the ventricles from the atria.
  • The U Wave: This wave really has no known reason for being there so we don’t have any normal interpretation. However, a prominent U-Wave could indicate a serious cardiac issue.

Obviously in looking at the ECG strip, you can see the QRS complex stands out the most. This is because the ventricles in the heart are much bigger than the atria therefore indicate larger electrical activity.

Now, you may ask, why is there no wave indicating the atria repolarization? Well, you can’t see it on an ECG because the QRS complex is such a prominent electrical activity, it literally overtakes the relaxation wave of the Atria.

So now that we are equipped with knowledge of what each part of the ECG wave represents, we can utilize this information to identify arrhythmias.

Common Heart Arrhythmias

Atrial Fibrillation (A-Fib)

Atrial fibrillation refers to a quiver meaning an inadequate contraction. So whenever, you hear A-Fib - you think Atrial quiver. A-Fib occurs when the SA node is firing rapidly, it’s creating an unusually high heart rate for the atria. In basic terms, what’s happening is that the heart rate is so fast, the atria doesn’t have enough time or thrust to pump all the blood out of the atria and into the ventricles. This causes some blood to stick around in the Atria and continues to circulate around in the atria for each heart beat. This is referred to as stagnant blood and we all know stagnant blood can eventually clot. Now imagine the repercussions of blood clotting in the heart! If a blood clot travels from the atria and makes it way to the ventricles and then on to the lungs or anywhere else in the body, it could cause a pulmonary embolism or even a stroke if it travels to the brain. So this is very important to recognize early to limit blood clots.

The animation below illustrates the atria giving off a faster than normal rate exhibited in A-Fib:


Here’s what the ECG rhythm looks like:
Atrial Fibrillation (A-Fib) ECG - ProMedCert

The characteristics of this rhythm that makes it unique are you can clearly see the QRS complex (representing the ventricles), but you can’t easily identify the P-Wave. It looks like a flat quiver. A-Fib is a very fast rate between 300-650 BPM. Patients exhibiting A-Fib symptoms are usually prescribed with blood thinners to prevent blood clots from developing.

Atrial Flutter (A-Flutter)

A-Flutter is very similar to A-Fib. It has the same rapid firing of the SA node however usually not as fast. It usually is between 250-350 BPM. The difference you will see in the ECG strip is instead of the flat quiver for the P-wave , you see something that is referred to as Saw Tooth. The P-wave looks like the teeth of a saw, jagged lines. However, as you can see, the QRS complex is still prominent and visible.

Here’s what an Atrial Flutter ECG rhythm may look like:
Atrial Flutter (A-Flutter) - ProMedCert

Supra Ventricle Tachycardia (SVT)

SVT is also a very fast rhythm and is originated in the SA node. It’s usually exercise-induced and looks very similar to ventricular tachycardia but has an even faster heart beat. You can identify all the normal waves but they are happening so fast, the waves seem to be compacted into each other. The rate is usually around 100-300 BPM and can usually be reversed by performing vagal maneuvers (bearing down or ice packs, blowing through straw, etc). This can help trigger the SA node to kick back into normal rhythm. If not, you will have to move on to various medical therapies.

Here’s what a Supra Ventricle Tachycardia ECG rhythm may look like:
Supraventricular Tachycardia (SVT) - ProMedCert

Ventricular Fibrillation (V-Fib)

V-Fib, just like A-Fib, refers to a quiver or inadequate contraction. However, this time the quivering is occurring in the ventricles. This is a very serious arrhythmia and can be deadly (It's also a shockable rhythm). If the ventricles are quivering then the blood isn't fully emptying and as a result, blood is not adequately sent to the rest of the body as it should. This decreased amount of blood perfusion could basically suffocate the body with lack of oxygen delivery.


Since the ventricles are contracting so quickly with no regards to the atrium, you are going to see quivering of the QRS complex. Since the QRS is such a prominent wave, it's going to create an erratic ECG wave that can look something like this:
Ventricular Fibrillation ECG - ProMedCert

Ventricular Tachycardia (V-Tach)

V-Tach is an abnormally fast rate within the ventricles. The ventricles are contracting too quickly with no regards to the Atria. In this ECG rhythm, you will see a wide QRS complex and very fast rate. The Q-Waves have defined high peaks. It's not as erratic as Ventricular Fibrillation but if not treated soon, can lead to to V-Fib.

Here's an animation illustrating V-Tach in the heart:

Here's what a V-Tach ECG may look like:
Ventricular Tachycardia ECG - ProMedCert


There are many more types of arrhythmias to study; however, this basic guide should lay the framework for ECG analysis and allow you to have a solid understanding before taking your ACLS or PALS certification courses.