Transcript

You are called overnight to review the telemetry for a patient on your service, and you see the following. What do you see? That’s right, this is a regular, wide complex tachycardia.

Before you’ve had the chance to think further, a nurse comes over to you and asks you to check out this EKG on another patient.

What do you see? That’s right, it’s another regular, wide complex tachycardia. What’s on the differential for these types of rhythms?

The differential diagnosis for this includes ventricular tachycardia or supraventricular tachycardia, also known as SVT with aberrancy, with the most common aberrancy being a bundle branch block. Most people think of SVT as a narrow, complex tachycardia, but if you have a concurrent bundle branch block, it can look a lot like vt.

Let’s figure out how we can distinguish SVT with aberrancy versus vt like a cardiologist. First, let’s start with a review of some anatomy. The conduction system starts with the SA node, which is the pacemaker of the heart. It then relays conduction over the AV node, which serves as a relay station that helps the top chambers of the heart and the bottom chambers communicate.

From the AV node, it then goes into the His Purkinje system and then down into the left and right bundle branches that stimulate the ventricles in a coordinated fashion, which ultimately results in a narrow QRS complex.

Let’s review that again. So in a normal conduction system that goes from the SA node to the AV node, the His Purkinje system, and then the bundle branches, you should get a narrow QRS complex. If there’s any disruption to this system, such as in a bundle branch block, or if the electrical signals of the heart originate outside of the SA node, such as in ventricular myocytes, this disrupts normal conduction and you get a wide QRS complex.

This is why both SV2 of the Barentsy and VT can look like this, meaning they have a wide QRS complex. But what causes the wide QRS complex? The wide QRS complex is due to myocyte to myocyte propagation of electrical signal, instead of going through a normal conduction pathway. Let’s think about this a little more with our two differential diagnoses, starting with ventricular tachycardia.

VT starts below the level of the AV node and is typically initiated in ventricular myocytes. This results in myocyte to myocyte propagation of electrical signals, which inherently results in a wide QRS complex. For supraventricular tachycardias, these originate above the level of the AV node, and they’re usually narrow complex because they still use the normal conduction system.

However, if there’s an aberrancy, such as a bundle branch block, and the normal conduction system is disrupted, this will ultimately result in myocyte to myocyte propagation, leading to a wide QRS complex. Okay, let’s move on. What are some EKG features that can help us confirm the diagnosis of ventricular tachycardia and don’t usually show up with supraventricular tachycardia with aberrancy?

One of these is a capture beat. A capture beat is a normal supraventricular beat that is able to be conducted normally through the normal conduction system and occurs between the wide complex beats of ventricular tachycardia. One way we can remember that a capture beat won’t be seen in SVT with aberrancy is because an SVT with aberrancy The bundle branch block will prohibit the normal conduction of beats through the His Purkinje and bundle branch system.

Thus, you will only see wide QRS complexes and SVT with aberrancy. Another is a fusion beat. A fusion beat occurs when a normal capture beat and then a wide complex beat from ventricular tachycardia coincide at the same time to form this intermediate hybrid morphology that then ultimately results as this fusion beat.

Let’s talk about some of the other EKG clues that may point us more towards VT rather than SVT with aberrancy. These may include, One An extremely wide QRS complex over 140 milliseconds in duration. Two Extreme axis deviation, meaning that instead of the axis being down here, the axis is all the way up here.

And finally, number three, concordance of the precordial leads, meaning that the QRS complex points in the same direction, either positive or negative, and leads V1 through V6.  However, the pathognomonic EKG finding for VT is AV dissociation, meaning that the atria and the ventricles are beating at different rates, with the atria beating slower than the ventricles.

If you see this finding on EKG, you don’t have to look for other clues to differentiate VT from SVT with aberrancy Take a look at this strip from our first case to explore this further. As you can see here, the atria are the red arrows, and they’re beating at a different rate from the ventricles, or the blue arrows.

In this case, the red arrows are beating slower than the blue arrows. This confirms that these ventricular signals are originating outside of the atria, and this person is in VT. On the other hand, if you see P waves conducted in a one to one fashion with the QRS complexes, there is no AV dissociation, and this is supraventricular tachycardia.

Let’s talk a little bit more about SVT with aberrancy, pulling in our EKG from case number 2. In patients who have SVT with abherrancy it can be helpful to look at their baseline EKG and see if they have a resting bundle branch block at baseline. So for example, here in case number two, you see their baseline EKG has a wide QRS complex that looks like a left bundle morphology.

And when they’re in SVT, they have a similar wide complex that also looks like a left bundle morphology. So as a pro tip for someone who has wide complex tachycardia, you can look at their baseline EKG and see if they have an existing bundle branch block. If they do and if that bundle branch block looks similar when they’re in SVT, this is most likely to be SVT with aberrancy.

Also, looking at this EKG, this person has a normal QRS axis, since the QRS complexes are upright and leads 1 and 2, and they do not have precordial concordance, since as you can see here, the QRS complexes are negative or downsloping in V1 through V3, but they’re positive or upsloping in V4 through V6.

Remember in VT, people are more likely to have extreme axis deviation and predominantly precordial concordance. Thus, this person likely has SVT with aberrancy rather than VT. If you’re still stuck differentiating between the two, there’s another pro tip that we can use looking at lead AVR.   Typically in SVT with aberrancy in AVR, we should see a quick, down sloping QRS complex like we see here in case 2.

If we see anything other than this,  such as here in case 1,  this is more likely to be VT.  Okay, let’s review what we’ve learned. First, the differential diagnosis for wide complex tachycardia includes One Ventricular tachycardias and two Superventricular tachycardias with aberrancy. Second, AV dissociation is pathognomonic for ventricular tachycardia, but other EKG clues we should consider include capture beats, fusion beats, extremely wide QRS complexes, precordial concordance, and extreme axis deviation.

Third, remember to look at their baseline EKGs to find clues for aberrancy like a bundle branch block. In SVT with aberrancy, the QRS morphology will likely look similar to their baseline EKG morphology. And finally, don’t forget to look at AVR. In SVT with aberrancy, the QRS morphology is more likely to be negative with a sharp downstroke and any other QRS morphology in AVR is more suggestive ventricular tachycardia.

Full Byte

You’re called overnight to review the tele alarms alerting you to this rhythm. What is your ddx?