54yo man with known a-fib presents to the ED with abdominal pain. You are on CCU overnight and get a call from J pod asking for help interpreting this ECG.
ECG credit Life in the fast lane
Answer Ashman phenomenon with underlying a-fib
Ashman phenomenon is an aberrantly conducted beat occurring in the setting varying R-R intervals. The duration of myocardial repolarization is proportional to the preceding R-R interval, therefore a longer R-R will have a longer repolarization period. If a short R-R interval follows a long R-R interval and some of the myocardium has not repolarized, an aberrant beat will be conducted. The aberrant beat often is a RBBB morphology in the Ashman phenomenon as the right bundle typically takes longer to repolarize than the left leading to conduction down the left bundle, and myocyte to myocyte conduction in the right bundles territory.
This phenomenon is most often seen in atrial fibrillation, but can be seen in other variable R-R interval settings as well.
Reviewing the prior EKG one can see the long R-R interval followed by the aberrant beat.
Notably this can be confused with a PVC but has an origin above the AV node. This is in general a benign phenomenon without clinical significance.