A quick and easy, but interesting, case: why 3 different QRS morphologies?

One of my residents who is very smart sent me this ECG.Patient had "nonspecific chest symptoms worried she had pneumonia. She does not have pneumonia. "Here is the ECG:What do you think?The resident wrote:She has 3 different QRS morphologies:,1. One that looks narrow2. One that looks LBBB3. One that looks RBBB. Cardiology called them PVCs in their note, and I think the RBBB looking morphologies are PVCs with compensatory pause, but the LBBB ones clearly are not.Here is my interpretation:At a normal heart rate, the patient has left bundle branch block. But this left bundle has a long refractory period. Thus, whenever the next beat comes less shortly after the previous one, there is left bundle branch block. Whenever the next beat comes later after the previous, there is normal conduction.That ' s where the PVC is coming to play: every PVC is followed by a compensatory pause that is long, which gives the left bundle the time to repolarize.The PVC comes from the LV and has RBBB morphology.The LBBB comes from a long refractory period during normal sinus rhythm at a rate of 75 (RR interval 800 ms, less than 1 sec)The normal conduction comes after a long RR interval caused by the compensatory pause (longer than 1 second).===================================MY Comment by KEN GRAUER, MD (4/22/2020):===================================— An interesting example ofRate-RelatedBBB:I LOVE this case by Dr. Smith — because it highlights an important (albeit not ove...
Source: Dr. Smith's ECG Blog - Category: Cardiology Authors: Source Type: blogs