Incongruous BBBB
Report:
Sinus rhythm 84 – 88/min
Advanced second degree AV block
3:1 & 4:1 AV block
Right bundle branch block
PR interval 0.32”
Left bundle branch block
PR interval 0.20”
Comment:
This is quite rare. The two bundle branch blocks have different but fixed PR intervals - an incongruous bilateral bundle branch block (BBBB). This is because each conducting bundle has a different conduction time.
The alternative explanation, that only RBBB or only LBBB complexes are conducted, with contralateral BBB escapes, is also possible. The PR intervals are constant because the escape interval is from a LBBB or RBBB complex with its own fixed PR interval.
However, there are periods of sustained LBBB and RBBB conduction, each block retaining its fixed PR interval. This swings the probability in favour of BBBB. It is unlikely that the constancy of PR interval would persist so far.
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