Today's ECG CHALLENGE offers a two-lead rhythm strip and a 12-lead ECG from an 87 year old man with a complaint of shortness of breath. We do not know any other clinical information. What is the underlying rhythm? How can we explain the changing QRS morphology? There are two distinctly different QRS complexes - one wide and one narrow. Comment below and let us know what you think. We will answer your questions as soon as we can, and will give a final diagnosis in one week.
ANSWER: Please see Dr. Grauer's comment below for the diagnosis. The intermittent left bundle branch block that is seen in this strip appears in the 12-lead ECG to be rate-related. That is, after a PAC, we see a pause, and then normal conduction for one beat. As the rate increases, the left bundle branch block resumes. The rhythm strip, which was recorded about 5 1/2 minutes before the 12-lead ECG, shows more intermittent normally-conducted beats, which don't seem to be rate-related. We would need a longer rhythm strip to definitively prove rate-related left bundle branch block, but it is a very common cause of intermittent LBBB.