By definition, a wide complex tachycardia is a heart rhythm with a QRS duration ≥ 120 ms (0.12 s) and a ventricular rate ≥ 100.
When this criteria is met, and the the rhythm is regular (no variability in the R-R interval) then it’s a regular wide complex tachycardia.
This is a very broad and inclusive definition of regular wide complex tachycardia! But in my experience, it’s the best way to approach the problem. It keeps you out of trouble!
The differential diagnosis for regular wide complex tachycardia includes:
- SVT with BBB or aberrancy (includes sinus tachycardia and atrial flutter)
- Ventricular tachycardia
- Paced rhythms
- Atrioventricular reentrant tachyardia (AVRT) with antidromic conduction (WPW)
- Electrolyte derangement or drug toxicity
Take a look at the following ECG which was recorded from a postictal seizure patient.
Using the large block method, we know the ventricular rate is > 100 and the QRS duration is > 3 small blocks, or 120 ms (0.12 s).
The rhythm is regular, so this is a regular wide complex tachycardia.
The next question we need to ask is, “Could this be VT?”
For this ECG, the computer measured the heart rate at 119, which makes VT unlikely, but still a possibility.
This tachycardia also shows LBBB morphology with a left axis deviation. This is the expected pattern for a paced rhythm with the pacing lead in the apex of the right ventricle, so that also needs to be considered.
There are two things you can do to rule this out. The first is to simply expose the patient’s chest and look for a pacemaker pocket. The second is to look for the telltale “blips” in front of the QRS complexes.
Sometimes this is only visible in one lead, so look carefully!
Next, we should look for sinus P waves. A ventricular rate of 119 suggests sinus tachycardia. Sinus tachycardia must be part of the differential diagnosis for regular wide complex tachycardias!
Do you see any P waves? Look at the downslope of the T waves in the inferior leads (I, II and III) and lead V2. There’s some type of atrial complex there, and it could be a sinus P wave. Or, this could be an atypical 2:1 atrial flutter (atrial flutter with an abnormally slow flutter rate).
Either way, there appears to be a clear relationship between P waves and QRS complexes, suggesting a supraventricular origin.
Let’s look at another case.
This ECG was recorded on an interfacility transport with a patient experiencing an intracranial hemmorhage.
Here we have a regular wide complex tachycardia with a left axis deviation. Whenever I see a rhythm strip showing a wide complex tachycardia with a right or left axis deviation, I try to guess whether or not the QRS complex will be positive or negative in lead V1.
If it’s positive, then it will be a bifascicular pattern. If it’s negative, then it will be a LBBB with left axis deviation, which is the expected pattern for a paced rhythm when the pacing lead is in the apex of the right ventricle.
Does this patient have a pacemaker? Yes!
If you look at the bottom of this ECG, the block arrows are the LP12′s pacing detector. They’re not always accurate, but it increases the possibility that this is a paced rhythm.
The paramedic in charge of the transport elected to perform a 12 lead ECG.
This 12 lead ECG shows LBBB morphology (rS complex in lead V1 and a monophasic R wave in lead I).
If you look carefully at this 12 lead ECG, you can see little “blips” in front of the QRS complex in leads V3-V6. They also line up with the block arrows from the pacing detector.
This is a paced rhythm.
Is the pacemaker functioning properly? Who knows! You’d have to understand how the pacemaker is programmed to answer that question. In the meantime, a paced rhythm at 125 ppm probably isn’t hurting the patient, and required no intervention during the transport.
The other thing you might notice about this ECG is that the GE-Marquette 12SL interpretive algorithm is giving the ***ACUTE MI SUSPECTED*** message.
Probably because of the discordant ST segment elevation > 5 mm in several leads and the concordant ST segment depression in lead V2.
Just remember, neurological insult can create ST changes similar to STEMI on the 12 lead ECG! Patients who present with an abnormal neuro exam and an ECG suggestive of STEMI generally get a CT scan before they are sent to the cath lab.
Is this a STEMI? Probably not.
Next time, we’ll look at another unusual presentation of a regular wide complex tachycardia with an unexpected outcome!
Differential diagnosis of wide complex tachycardias – Part II