Bigeminy or Quadrigeminy?

Report:

Sinus rhythm approx. 54/min

First degree AV block, variable

VEBs, bigeminal

Concealed retrograde conduction

Pacemaker escape beats

Comment:

This is an allorhythmia, a repetitive sequence of (in this case) four beats. The VEBs themselves are, however, bigeminal: they depend on the preceding beat to occur, with characteristic fixed coupling interval.

This extrasystolic coupling is fixed regardless of whether the preceding beat is paced or sinus.

The first VEB in each quartet is interpolated (intercalated). It does not block the following sinus P wave completely, but prolongs its PR interval by concealed conduction to the AV node above. Concealed, because it is not manifest per se but only through its effect on the subsequent beat (in this case, the next P wave, causing or increasing first degree AV block).

What unimpeded AV conduction would be like is seen below (Fig 223a), in straight bigeminy from the same source: alternate P waves are either completely blocked or conducted at just over 0.24”. Again, the bigeminal VEBs are caused by paced (first one), fusion (second), pseudofusion (third and fourth) or sinus (last) beats.