Premature complexes: atrial vs junctional vs ventricular
How to tell the difference between the three different types of premature complexes: atrial, junctional and ventricular.
Introduction
SA node: the normal pacemaker
The pacemaker, a part of the heart tissue, is responsible for initiating and spreading myocardial depolarisation throughout the heart.
Normally, myocardial depolarisation starts from the sinoatrial (SA) node (i.e. the SA node is normally the pacemaker). This has the following two consequences on the 12-lead ECG:
atrial depolarisation occurs from top to bottom, shown with a positive P wave in the inferior leads (II, III, aVF) and a negative P wave in aVR.
atrial depolarisation precedes ventricular depolarisation, shown with a P wave preceding the QRS complex.
Ectopics: when the pacemaker is different
If myocardial activation for a single beat didn’t begin at the SA node, it’s an ectopic beat. If the ectopic focus is responsible for many or most beats, it’s called an ectopic rhythm.
There are two types of single ectopic beats, depending on when they occur:
Premature complexes: ectopics occuring before the next expected beat
Escape complexes: ectopics occuring after the next expected beat
Depending on the location of the ectopic, you can get:
Premature/escape atrial complexes (PACs) (from the atria)
Premature/escape junctional complexes (PJCs) (from the AV junction)
Premature/escape ventricular complexes (PVCs) (from the ventricles)
We’ll focus on premature ectopics in this post.
Disclaimer: Below you’ll find the typical characteristics of PACs, PJCs, and PVCs. However, if you delve deeper into the literature, you’ll find that there are many conditions where some of the characteristics mentioned below are not valid. While these are the most typical characteristics, they are not 100% unique to each type of premature complex.
Premature atrial complexes (PACs)
Focus of the ectopic: somewhere in the atria
There is a P wave before the QRS complex.
The P wave has different morphology than the normal sinus P wave (unless it originates from an area very close to the SA node, in which case it may be indistinguishable from a sinus P wave on a 12-lead transthoracic ECG).
The PR interval is probably different as well, depending on the distance of the ectopic focus from the AV node.
The QRS complex is usually narrow (or the same as the normal QRS complexes). If it’s not narrow, then there are three possibilities:
There’s an underlying intraventricular conduction delay (e.g. a bundle branch block).
The PAC occurs very early and encounters one of the two bundle branches in refractory state (whichever has the longest refractory period - usually the right bundle branch), and conducts with a bundle branch block. For example, if it finds the right bundle branch in refractory state, then the subsequent QRS will have a right bundle branch block morphology.
The PAC occurs too early, and both the right and left bundle branches are in a refractory state, therefore not conducting to the ventricles and not producing a subsequent QRS complex altogether. In this case, you will only see a P wave without conducting to the ventricles (blocked beats)
Usually, the depolarisation triggered by the ectopic atrial focus during a PAC will depolarise and “reset” the SA node. Resetting the SA node essentially means that once depolarised, it will enter a refractory state, preventing it from automatically depolarising again until the next expected time. This causes a pause that is roughly the same as the normal RR interval, known as a non-compensatory pause.
The way to determine if the postextrasystolic pause is non-compensatory is as follows: as seen in the image below, if you add RR(1)1 and RR(2), the sum will be less than 2 normal RR intervals.
As mentioned above, a non-compensatory pause is characterised by the following relationship with the normal RR intervals:
Premature junctional complexes (PJCs)
Focus of the ectopic: somewhere in AV junction (AV node or His bundle)
Less frequent than PACs or PVCs
Usually there’s no P wave seen, or there might be a P wave just before or after the QRS, resulting from retrograde depolarisation of the atria. If retrograde atrial depolarisation occurs during ventricular depolarisation, the retrograde P wave will be hidden inside the QRS.
If a P wave is seen, as it is retrograde (i.e. it travels in the opposite direction compared to normal - from the AV junction to the atria), it’s usually negative in the inferior leads (as it the electrical signals travel superiorly, away from the inferior leads).
Similarly to PACs, the QRS complex is usually narrow but it can also be wide if there’s an underlying bundle branch block or if the PJC occurs early enough to find one of the bundle branches in refractory state.
PJCs usually have a non-compensatory pause, due to retrograde activation and reset of the SA node.
Premature ventricular complexes (PVCs)
Focus of the ectopic: somewhere in the ventricles
There is no P wave before the QRS complex (and so no PR interval either).
The QRS complex is wide because PVCs originate from the ventricles and don’t follow the normal pathway for depolarising them. Conduction occurs via cell-to-cell transmission outside the conduction system, which has a much slower conduction velocity.
Since the QRS is broad and abnormal, you should expect to see abnormal ST segment and T wave as well. This is because the depolarisation of the ventricles affects their repolarisation.
In contrast to PACs and PJCs, PVCs usually have a compensatory pause. This occurs because PVCs don’t typically depolarise the atria and SA node, preventing the SA node from resetting. SA nodal depolarisation still happens as scheduled, potentially causing atrial depolarisation, but the P wave is likely hidden within the wide QRS of the PVC. Consequently, the SA node waits to depolarise again, as if the PVC never occurred.
The way to determine if the postextrasystolic pause is compensatory is as follows: as seen in the image below, if you add RR(1) and RR(2), the sum will be equal to 2 normal RR intervals.
As mentioned above, a compensatory pause is characterised by the following relationship with the normal RR intervals:
Examples
See the correct answer in the second footnote.2
See the correct answer in the third footnote.3
Summary
These are the typical characteristics of PACs, PJCs and PVCs at a glance:
Let me know if you have any questions.
Further reading
Heaton J, Yandrapalli S. Premature Atrial Contractions. [Updated 2023 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK559204/
Sattar Y, Hashmi MF. Premature Ventricular Complex. [Updated 2025 Feb 16]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK547713/
Surawicz, B., Knilans, T.K. (2008). Chou’s Electrocardiography in Clinical Practice (6th ed.). Saunders Elsevier.
This interval is called coupling interval: it’s the time interval from the normal beat preceding the extrasystolic beat, until the extrasystolic beat.
In this case, you can see several PACs that come very early, therefore producing a wide QRS complex with an RBBB morphology. You can see the premature P waves buried inside the previous T waves (tip: compare the T waves with each other). So, despite that the QRS is wide, these are not PVCs but PACs.
The third and fifth complexes are PVCs. You can see this because they lack a preceding P wave and have a wide QRS complex. Additionally, they have a compensatory pause. You’ll also notice that the two QRS complexes, although wide and different from the normal ones, appear dissimilar to each other. This is because they are multifocal PVCs, meaning they originate from different foci inside the ventricles.