Monday, October 3, 2011

Fusion Beat During Supraventricular Tachycardia: No criterion is absolutely accurate in differentiating wide complex tachycardia

Yesterday, I posted this case of wide complex tachycardia, and some steps to help in differentiating VT from SVT with aberrancy.

One step was this:  
"Do a quick look for obvious fusion beats and AV dissociation.  If found, then VT."  It is generally thought that fusion beats are diagnostic of VT.

This prompted our excellent electrophysiologist, Dr. Rehan Karim, to make the point that no criterion or algorithm can make a certain diagnosis of wide complex tachycardia.  And so he provided me with the interesting tracing below (sorry, no 12-lead because this just happened to occur in the EP lab while they were recording):

There is a wide complex tachycardia; rate was 171.   It was proven at EP study to be SVT.  You can see the RBBB morphology, with rSR' in V1 (green arrow) and a wide S-wave in lateral leads (see lead I, red arrow).   There is a fusion beat (black arrow) which was proven in the EP lab to be due to a PVC occurring in the midst of the SVT!!

Learning point: whatever system or rule you are applying, it is not perfect.  Dr. Karim also wanted to make the point that Brugada's algorithm had very good sensitivity and specificity in the derivation group, but attempts to validate it were not nearly as successful.  This may be just as true for Sasaki's rule. 

Nevertheless, while appreciating the limitations of all algorithms, I like Sasaki's rule because it depends on many of same principles as the other algorithms (principles which, though fallible, are pretty reliable), but is quite a bit simpler to apply.


  1. That's very interesting Dr Smith, I remember being taught that the presence of fusion beats in wide complex tachycardias is virtually diagnostic of VT.

    Stupid question though, how would you differentiate this from a capture beat? Would you just expect a capture beat to come earlier, or is the morphology different?

  2. I don't think you can differentiate it. The only way we know that this is SVT with a PVC is that there were internal cardiac recordings in the EP lab when it happened.