Complete Heart Block (CHB) ? Think Again

No clinical hx. Is this complete heart block (CHB)?

Image 1

This is sinus rhythm (SR) at about 88 bpm, first degree AV block, second degree advanced heart block or high-degree AV block, right bundle branch block.

It is very tempting to call this CHB. However, the clue that a sinus P wave is conducted is the irregularity of RR interval. There is sudden shortening of the RR interval (R4R5) vs the rest of the RR interval. If this is CHB the RR interval would be the same in all because in CHB the ventricles will be under the control of an ectopic junctional or ventricular focus. So, it would beat like clockwork. If there was shortening or difference in the RR interval, this means that a sinus impulse was conducted.

A simple trick to differentiate CHB from advanced heart block is the regularity of the RR interval.