Narrow VEBs & Trifascicular Block

Report:

Sinus rhythm 85/min

Second degree AV block, unspecified

VEBs in bigeminy

Right axis deviation +120o

Left posterior hemiblock

Right bundle branch block

Comment:

It’s best to observe the bigeminal VEBs in the rhythm strip: no two are the same. Not only that – they look more normal than the conducted sinus beats, the latter widened by the RBBB + LPHB. Are they fusion beats?

It is a general rule that a fusion beat’s PR interval must not be shorter than the basic sinus PR interval by more than 0.06” – it only takes this long to completely capture the ventricles. This case is an exception – the RBBB + LPHB make ventricular capture much more prolonged. There is less objection here to PR interval shrinking from 0.28” for conducted sinus beats to 0.14” for fusion beats – but it’s still too much. The fusion here also means that the first degree block is not in the AV node – otherwise the VEB would be complete before the sinus impulse could even begin to depolarise the ventricles. It would also be very interesting to speculate where the VEBs, precisely, come from. Most likely, they come from somewhere in the septal region, activating the two ventricles almost synchronously, with variable differential conduction in the two fascicles of the left bundle branch. This, rather than fusion, is the real explanation for their different shapes. Do they arise beyond the level of the blocks? Yes, most likely they do: the blocks could in fact reside in the bundle of His itself.

The nature of the previous 2:1 AV block is now difficult to define, beyond the general designation of second degree AV block. The VEBs themselves can induce advanced AV block67, but that variety is beyond standard classifications. Even then, is this block advanced (less than 50% conduction)? That depends on whether there is fusion or not. If there is, there is overall 3:2 conduction!

Below (Fig 404a) is a similar trace, but with uniform, also narrow, VEBs.

Fig 404a.

Fig 405. 62 year old lady following reversal of muscle relaxant by 5mg neostigmine and 0.3 mg atropine.