Premature atrial contraction

Definitions & Key takeaways

Premature atrial contraction is when the atria contract too early because of depolarization from an ectopic focus. While the sinoatrial node typically regulates the heartbeat during normal sinus rhythm, premature atrial contractions occur when another region of the atria depolarizes before the sinoatrial node and thus triggers a premature heartbeat. The ECG will show an early and abnormal P-wave, as well as one of three patterns, depending on when the PAC occurs: normal ventricular conduction, aberrant ventricular conduction, or a conduction block.

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Introduction0:00–1:21

The heart has two upper chambers, called the atria, so a premature atrial contraction, or PAC, is when the atria contract earlier than normal in the cardiac cycle.
So, if we simplify this heart a bit, normally, the sinoatrial (or SA) node sends an electrical signal that propagates out through the walls of the heart and contracts both upper chambers, then that signal moves to the atrioventricular or AV node, where the signal is delayed for a split second, and then goes down into the ventricles or lower chambers where it moves down the bundle of His into the left and right bundle branches and into each ventricle’s Purkinje fibers, causing them to contract as well.
So, in a healthy heart the upper chambers contract first, then shortly after, the lower chambers contract. On an electrocardiogram, or ECG, which measures the electrical activity of the heart via electrodes that are placed on the skin, the atrial depolarization, and therefore it’s contraction, is seen as a p-wave, the ventricular contraction is seen as a QRS complex, and the ventricular repolarization, and therefore it’s relaxation, is seen as a T-wave.

Pathology1:21–3:39

If an atrial cell outside of the SA node initiates a depolarization, it’s called an atrial ectopic focus. Now this can be a cell that’s part of the conduction system or it can just be another cardiac muscle cell.
This typically happens when atrial cells get irritated and stressed - the causes of that might include electrolyte imbalances, drugs like cocaine or methamphetamines, ischemic damage like a heart attack, or anything that increases sympathetic activity, like anxiety.
Another type of atrial ectopic focus is a reentrant loop, which is when there’s some tissue that doesn’t depolarize right, this could be like scar tissue after a heart attack - and as a result the wave of depolarization ends up circling around and around that tissue.
A reentrant loop basically starts sending out depolarization waves to the rest of the heart tissue each time the wave goes around.
So, in short, a particularly stressed out atrial cell or a reentrant loop can send out depolarization waves that contract the atria, and result in an early, or premature, atrial contraction.
One important effect of the premature depolarization is that the ectopic focus will depolarize the cells of the SA node causing the SA node to skip a cycle.
So let’s say the SA node is firing along as usual, but all the sudden an ectopic focus fires one off early. Now, if we measure out the interval between each SA node fire, we see that it should have fired here, but it doesn’t because it was depolarized by the ectopic focus.
This means that the SA node essentially skips one cycle of sending its own signal. So the sinus node waits the same interval but fires based on the ectopic beat.
So, what you end up with, is this interval with two sinus beats and an ectopic beat in the middle and this is less than that if it had been three sinus beat in row.
This is called a noncompensatory pause, where the sinus P wave to the ectopic P wave to the next sinus P wave is less than the sinus P wave to sinus P wave to sinus P wave.
Alright, so also the location of the ectopic focus in the atria determines how the heart contracts and therefore what the ECG looks like.

ECG changes3:39–7:14

If the ectopic focus is at the bottom of the atria, then the p-wave will be upside-down because the flow of electricity goes from the bottom of the atria to the top, which is in the opposite direction of how the depolarization normally spreads.
Also, the closer the ectopic focus is to the AV node, the shorter the PR interval, which is the time from the P wave to the QRS complex, this is because that wave has a shorter distance to travel and therefore gets there faster.
If the heart is beating fast enough, the p-wave from the premature atrial contraction might happen at the same time as the t-wave from the previous, normal contraction.
In this case the two waves will combine, making that t-wave look peaked or just a bit more lumpy than usual, kinda like a camel’s hump.
Also, though, the timing of the PAC will also determine how the heart contracts and therefore what the ECG looks like. When that depolarization wave from an atrial ectopic focus gets to the AV node, there are a couple options.
First, if the AV node is receptive, it gets conducted down to the ventricles, resulting in a normal QRS complex. Another scenario, though, is if a depolarization wave from an atrial ectopic focus gets through the AV node but get sent through the ventricles abnormally like when a bundle branch is in refractory.
This will lead to an aberrant signal. Most often, the aberrancy is due to a right bundle branch block because that’s the one that has a longer refractory period.
In this situation, the depolarization wave conducts down the left bundle branch and then through the ventricles aberrantly, resulting in an abnormally wide QRS complex.
Now - in both these situations, the QRS complex comes in early, which means an early ventricular contraction. This means that the heart didn’t have time to fill with as much blood as it normally would.
And when it doesn’t fill as much, it doesn’t stretch as much and therefore doesn’t pump as strongly as it normally would.
The last option is if the depolarization from an atrial ectopic focus gets to the AV node while it’s still in refractory, and this means that the signal doesn’t get conducted at all, meaning that there’s no QRS complex or ventricular contraction.
Even in that situation, though, the SA node still gets reset, and once again, one cycle later, the SA node fires a tiny bit early since it gets reset by the atrial ectopic focus, and it’s followed by a normal looking QRS complex.
In this scenario, since the ventricles are filling with blood the whole time, the ventricle ends up getting a bit overfilled and stretched more than it normally would.
Since it gets so stretched out this results in a really strong contraction that people can often feel called a palpitation.
Sometimes premature atrial contractions can keep happening rather than being isolated events. When a premature atrial contraction consistently comes after each normal cardiac cycle, it’s called atrial bigeminy and when one consistently comes after every two normal cardiac cycles, it’s called atrial trigeminy.
This so-called “group beating” can be differentiated from a type 1 second-degree atrioventricular heart block by identifying the abnormally shaped p-waves that are characteristic of an atrial ectopic focus.

Symptoms7:14–7:38

For symptoms, most people with premature atrial contractions don’t actually notice them, but if they keep occurring they can cause lightheadedness due to less blood being delivered to the brain and they might also feel palpitations.
In rare situations, an ectopic focus can trigger atrial fibrillation, which is a more serious arrhythmia where the atria essentially just quiver.

Diagnosis7:38–7:56

Premature atrial contractions are diagnosed with an ECG which shows an early and abnormal looking P-wave and a potential QRS that may go along with it.
Sometimes a Holter monitor - which is like a continuous ECG monitor used over a few days - is needed to capture the event.

Treatment7:56–8:29

Typically treatment for premature atrial contractions isn’t required. But if there is an obvious cause like a medication or use of a substance, then stopping that typically resolves the issue.
If the premature atrial contractions keep happening or cause palpitations, they can be treated with beta blockers or calcium channel blockers, both of which prevent the heart from beating too strongly.
If an ectopic focus is triggering atrial fibrillation, radiofrequency ablation can be done, which is where radiofrequency waves are used to destroy the tissue that’s causing the ectopic heartbeat.
Alright, as a quick recap - A premature atrial contraction is when the atria contract too early because of a depolarization from an ectopic focus.

Review8:29–9:49

The ECG will show an early and abnormal p-wave, as well as one of three patterns - normal ventricular conduction, aberrant ventricular conduction, or a conduction block -depending on when the PAC occurs.