SPOILER ALERT = My Answer to this Interesting Case: The KEY lies in the last 3 leads (V4,V5,V6) of the 12-lead tracing with simultaneously-obtained 3-channel recording. Earlier in the 12-lead ECG — the rhythm is regular sinus tachycardia at 110/minute with LBBB. The 2nd beat in leads V4,V5,V6 occurs early. Because there is underlying artifact and some baseline movement — it is impossible to determine if the P wave preceding this 2nd beat in V4,V5,V6 is preceded by a different-morphology P wave (ie, a PAC) — but it is my strong hunch that this is what happens. Then the 3rd beat occurs slightly later, being preceded by the longest R-R interval on this ECG. Note that this 3rd beat in V4,V5,V6 is narrow and preceded by a P wave. Presumably, this reflects resumption of sinus rhythm after the slight pause due to the PAC that occurred just before it. Following this 3rd beat in V4,V5,V6 — the rate increases again, and LBBB conduction resumes. So presumably the reason for the wide QRS complex on this tracing is rate-related LBBB.

There are a number of interesting points to make about rate-related BBB. #1) — Clinical significance is essentially the same as for persistent BBB. #2) — Rate-related BBB is often an extremely challenging diagnosis to make — since we cannot prove this diagnosis unless we clearly see onset of a conduction defect with increase in heart rate — and resolution of that conduction defect when the rate slows back down. #3) — Diagnosis of rate-related BBB is made even more difficult by the reality that the “rate of onset is often not the same as the rate of offset”. By this I mean that rate-related BBB may develop when the sinus rate increases from 60 to 90 or 100/minute — but after the BBB develops — the rate may need to drop down to far below the 90-100/minute range before the conduction defect will resolve. As a result, it becomes challenging to “prove” that the change in rate is the responsible causative factor. For example, in the 2-lead rhythm strip we see in this case (above the 12-lead) — the rate does not appreciably change, yet we see intermittent LBBB conduction. It would be impossible to diagnose “rate-related” BBB from this short rhythm strip alone. To be certain of my hunch — I’d actually need much more monitoring than what we are given in the 12-lead tracing below it, as I’d need to see over time that rate increase reliably leads to onset of a conduction defect that eventually goes away after the rate again slows down.