The Critical Rate
Report:
Sinus tachycardia
VEBs
First degree AV block (PR about 0.24”)
Right bundle branch block, rate-dependent
Critical rate approx. 124/min
Comment:
The slight deceleration of the sinus rate from top to bottom reveals the critical rate, about 124/min, at which the right bundle branch conduction fails (fails, that is, to keep up with the left bundle branch conduction). In the top strip, even the VEB-engendered pause fails to restore normal conduction; in the middle strip, the VEBs are followed by a single normally conducted beat; in the bottom one, all the sinus complexes are normally conducted.
Note the typical QR configuration of the VEBs, with a tendency to a taller left rabbit ear. It must be said that on more than one occasion they have been (mis)taken for more complete expression of rate-dependent RBBB! Importantly, their initial vector is opposite to that of RBBB. They are associated with fully compensatory pauses.
If you have any suggestions for or feedback on this report, please let us know.
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