The Critical Rate
Report:
Sinus tachycardia
VEBs
First degree AV block (PR about 0.24â)
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.
Fig 2. Postoperative ECG in a 73 year old man in renal failure following a transurethral prostatic resection (TUR).
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