Episode 4: VT or Not…?

Audio Version:

Video Version:

Cardiac Rhythm Management (CRM) devices are widely used to address heart rhythm disorders thanks to sophisticated algorithms. But can we always trust them?

Let’s have a look at an interesting case study from our clinic.

This is an event that happened to a 52-years old male patient with a Boston Scientific Implanted Cardiac Defibrillator (ICD). What type of tachycardia could it be?

Was it:

  • A ventricle tachycardia (VT)?
  • An atrial tachycardia (AT)?
  • An accessory pathway (AVRT)?
  • An AV node re-entry tachycardia (AVNRT)?

Patient’s CRM device algorithm recognized it as a VT event.

CRM device recognition:

  • A ventricle tachycardia (VT)
  • An atrial tachycardia (AT)
  • An accessory pathway (AVRT)
  • An AV node re-entry tachycardia (AVNRT)

But can we trust this result?

A closer look at the case

As we can see in the case report, an average Atrial rate was faster than average Ventricle rate. However, the criteria for VT event is the opposite, isn’t it?

And what about EGM?

There are equal atrial and ventricle rates occurring in the event and the device recognizes it as VT in the marker channel.

In consequence on this recognition, the device provides anti-tachycardia pacing (ATP) therapy in the ventricle to override the VT and stops it without having to give a shock to our patient.

It helped, but was it really VT?

How it started

After an A sensed and V sensed beat we can see a premature ventricular complex (PVC) that is correctly labeled in the marker channel as well.

This premature ventricular beat is followed by an A paced and V paced beat, so obviously the premature ventricular beat had no connection/effect to the tachycardia.

Nevertheless, the next A sensed beat is conducted down to the ventricle and THAT seems to be the start of the tachycardia.

This is followed by a second  A beat that goes down to the ventricle.

And what is more interesting – AV delay changed!

But VT doesn’t start with an atrial event! That means it was probably not VT and the CRM device was not right.

Was it:

  • A ventricle tachycardia (VT)?
  • An atrial tachycardia (AT)?
  • An accessory pathway (AVRT)?
  • An AV node re-entry tachycardia (AVNRT)?

Tachycardia morphology

What’s more, if we have a look at the shock EGM, we can see a wide complex QRS representing a contraction of the ventricle.

A premature V beat has a very different shape from the tachycardia beat. That brings another point against VT because of the QRS morphology of the tachycardia looks like a normal sinus rhythm beat.

VA interval

Not only did the tachycardia morphology prove that it’s probably not VT. A short VA interval could direct us to an AVNRT.

A typical AVNRT generally conducts from the atrium to the ventricle down the slow pathway and back up the fast pathway. In other words, we can’t confirm it’s AVNRT, but it’s a likely option.


The event is terminating through ventricular pacing (ATP).

The ventricles are being paced at a faster rate than the tachycardia and the tachycardia is still running in the atrium at its own rate, at the same time. That confirms it’s not an accessory pathway (AVRT) because otherwise, it would be running at the V pacing rate, as  AVRT by definition involves both the A and V.

It has already been proven that it’s not a VT and it’s not an accessory pathway.

Was it:

  • A ventricle tachycardia (VT)?
  • An atrial tachycardia (AT)?
  • An accessory pathway (AVRT)?
  • An AV node re-entry tachycardia (AVNRT)?

The tachycardia terminated by itself. And it didn’t terminate in the ventricle but in the atrium. As you can see in the following strip from a similar event in the same patient (also recognized as VT).

It’s very unlikely it was an atrial premature beat and it’s also unlikely that it is an atrial tachycardia because it wouldn’t just stop on an atrial beat if the atrium was running.

Was it:

  • A ventricle tachycardia (VT)?
  • An atrial tachycardia (AT)?
  • An accessory pathway (AVRT)?
  • An AV node re-entry tachycardia (AVNRT)?

Don’t trust solely in devices

Boston Scientific devices know how to detect the difference between SVT and VT. How does the devices work?

The device reading is based on rhythm ID algorithm that correlates to the morphology. However, since the device is counting only what’s happening on the ventricular channel and not on the atrial channel, as it's marked in the Ventricle blanking period (the sensed atrial beats are recorded in brackets), that’s why it’s detecting the event as VT and not SVT.

Thanks to the device our patient received a VT treatment and it helped him, even though it wasn’t a VT. We found out that the event was an SVT, most likely an AVNRT. Therapy treatment for this diagnosis is a simple ablation. Whereas the type of treatment for VT or AVNRT would be totally different.

In this case, the patient has undergone a successful SVT ablation in the EP lab, because he was symptomatic with this SVT, and we wanted to avoid more of these events being wrongly recognized as VT by mistake. In the future, his defibrillator might not just give him anti-tachycardia pacing, but an inappropriate electric shock. And that’s what we certainly didn't want.


In order to recognize heart rhythm disorders correctly, it’s important to constantly train our electrophysiology skills. Even though we work with modern devices, the algorithms are sometimes not reliable enough.

With this in mind, there is a final key point to remember–

when the device recognizes a VT event, always ask: is it really a VT or not?

Copy Editor: Pavlina Cickova

 Here at EPme.me we love feedback; our aim is to help you learn so please do get in touch with ideas for sessions you’d like to see. Whether you work in electrophysiology, devices, both or neither, we want to hear from you. Follow our YouTube channel to keep up to date with leading cardiac electrophysiology from around the world. Sign up for our newsletter to receive our free ECG cheatsheet  – you’ll wonder how you did without it.

COMING UP: Next few weeks, we're gonna be talking about using devices in arrhythmia diagnosis. Some really interesting case studies from devices, to explore further the effects and use of devices in EP.

Thank you so much!