Vignette: A 67 yo with history of Diabetes Mellitus, Coronary Artery Disease, S/P CABG, Heart Failure with reduced Ejection Fraction (HFrEF), peripheral artery disease, s/p stenting is admitted due to sepsis. In telemetry, the several premature ventricular complexes (PVC's), couplet and nonsustained ventricular tachycardia (VT's) were noted. Aside from that the ventricular rate would jump from 60's to 100's (Image 1).
Image 1 - Heart Rate Trend or Heart Rate Histogram
When the patient is in the 100's, the telemetry strip would look like this (Image 2):
Image 2 - ECG case
Q: This is junctional tachycardia (True or False)
Dissecting the Case
It is tempting to label Image 2 as junctional tachycardia (JT) considering this is a regular narrow QRS complex rhythm with no discernible P waves or the P waves could be inverted in lead II right after the QRS complexes with a rate of about 107 bpm. However, every time the rhythm changes it is preceded by either a PVC or a couplet (Image 3 and 4). Are these coincidental or not?
Image 3 - 2 Lead Strip during conversion
Image 4 - Full Disclosure during conversion
The Supraventricular Tachycardia (SVT) Called AV Nodal Reentry Tachycardia (AVNRT)
There are several SVT's and here are the diagram (Image 5):
Image 5 - The SVT's
The SVT called AVNRT is one of the SVT's and it is the most common SVT referred for ablation. As the name implies it is a reentry tachycardia. This SVT depends on 2 AV nodal pathways namely a fast AV nodal pathway (FP) and a slow AV nodal pathway (SP) as seen in Image 5.
It needs to be initiated by something before the reentry tachycardia happens. After the initiation, it will look like a regular narrow QRS complex tachycardia (NCT) with no retrograde P waves seen or inverted P waves seen right after the QRS especially in Typical AVNRT as seen in Image 5. So, once initiated it will look like an accelerated junctional rhythm or junctional tachycardia.
What triggers an AVNRT?
Initiation by a PAC
The typical trigger or initiating event for AVNRT is a properly-timed premature atrial complex (PAC) which is also done in the laboratory during studies. However, we can catch it in cardiac telemetry. In patients or people with dual AV nodal pathways, during the sinus rhythm, the impulse coming from the sinoatrial node will be conducted in both the fast and slow AV nodal pathways. However, the impulse conducted in the slow pathway is blocked in the common AV nodal tract because the impulse from the pathway arrived early. So, what is seen in the surface ECG is a short PRI as seen in Image 6. After the sinus impulse, the fast AV nodal pathway will be in the recovery period longer (FP has long refractory period) than the slow AV nodal pathway (SP has short refractory period). A PAC that happens during this time will be conducted in the slow AV nodal pathway but is blocked in the fast AV nodal pathway. What is seen in the surface ECG is a PAC with a long PRI (Image 6).
Image 6 - AVNRT triggered or initiated by a PAC
Also, this same impulse will find the fast AV nodal pathway fully recovered and can now conduct "retrograde" and depolarized the atria (Image 7). This will be seen as a retrograde P wave right after the QRS in leads II, III and aVF or a small r in V1. This is termed "pseudo-s" and "pseudo-r". This completes the reentry cycle until it terminates spontaneously or by medications.
Image 7 - AVNRT Initiated by a PAC
Initiation by Atrial Couplet or Atrial Tachycardia
AVNRT can also be initiated or triggered by atrial couplet or atrial tachycardia. In Image 8, after 5 sinus complexes, a short run of atrial arrhythmia (atrial tachycardia) initiated an AVNRT. On the 3rd AT, a long PRI can be seen which means this impulse was conducted in the slow pathway. After that it reentered the fast pathway. This generated a retrograde P wave right after a QRS and the reentry continues.
Image 8 - AVNRT Initiated by Atrial Tachycardia
AVNRT Initiated by PVC or Couplet
Back to the ECG case (Image 9), the presence of the couplet IS NOT coincidental but a necessary event for the rhythm change or conversion. The 2nd ventricular ectopic was conducted retrograde that depolarized the atria. This same impulse is conducted on one of the AV nodal pathways (slow pathway) and depolarized the ventricles. This impulse also is conducted in the fast AV nodal pathway which then depolarized the atria and created the retrograde P waves right after the QRS. This completes the reentry circuit.
Image 9 - AVNRT Initiated by Couplet
What about the rate?
I know you will be confused with the rate. The rate of AVNRT is about 100-280 bpm according to Das and Zipes. Also, FORGET SOME THINGS WHAT YOU LEARN IN BASIC ECG CLASS. One of them is using rate in differentiating SVT's. The rates of SVT's overlap. So, it is not a useful tool for rhythm interpretation.
Take Home Message
This case reminds everyone to broaden the differential of a regular narrow QRS complex tachycardia. It is not junctional tachycardia by default. Several basic ECG books seem by reflex jump into interpreting these kinds of strips as junctional tachycardia. Once you see these kinds of rhythms, look at the initiating event and understand what is happening. It could be AVNRT.
To the case, this patient had a prior ablation for AVNRT (which I intentionally omitted in the vignette). As for the NSVT, the patient is on beta-blocker.