Second degree AV block type I interrupted by nonconducted PAC's in bigeminy


An elderly was being worked-up for GI bleed and routine monitoring captured this (rates dropping to 40's). 


Figure 1

What is the rhythm diagnosis? 

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Figure 2

The baseline rhythm is sinus rhythm at  a rate (initially) about 75 bpm. The first 4 QRS beats had a prolonged PRI (~ 360 ms) . The next subsequent beats (~ 40 bpm) with a PRI of ~ 280 ms. This behavior is seen in Wenckebach type of block.

What is the probable cause of the sudden onset of bradycardia? (red arrows - P waves)

There is a small nib at the terminal portion of the 4th QRS complex.The nibs are distorting the shape of the 4th QRS complex  compared to the first 3 QRS complexes (best seen in I, III, aVF, aVL). In the subsequent bradycardic QRS complexes, nibs can all be seen (best seen in V1 in the  ST segment). These are P waves. The differential here can be non-conducted PAC's or atrial echos (reentry). The RP interval of those P waves  vary (shorter in the 4th QRS complex than the rest). Thus this is more of a PAC than a reentry.

Interpretation: 

SR with a prolonged PRI with a Wenckebach periodicity interrupted with non-conducted PAC's, RBBB.

Patient was asymptomatic during these events.

#255

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