This is another example of Ashman’s phenomenon occurring during atrial fibrillation with rapid ventricular response. Ashman’s requires an irregular pattern of QRS complexes. Repolarization of any QRS complex is proportional to the previous R-R interval. Therefore when a relatively long R-R is followed by short R-R the QRS ending the short interval may find a portion of the conduction system still refractory (not yet fully repolarized). The RBBB is predictably the portion of the conducting system that is slowest to repolarize, therefore the beat following the short interval is aberrant with a RBBB morphology. If subsequent beats occur rapidly they will also find the RBBB still refractory and continue with RBBB morphology.

V1 lead from this EKG show the RBBB morphology with wide QRS and tall R wave in V1.

Also see “Ashman’s Phenomenon” in EKG Discussion for more examples of this common rhythm finding.