Ashman phenomenon

The Ashman phenomenon causes intermittent aberrant ventricular conduction in irregularly irregular rhythms. It is most commonly seen in atrial fibrillation, though can occur with other irregularly irregular supraventricular rhythms (ie MAT, atrial flutter with variable conduction). Conducted premature atrial contractions are another common time that this is seen (this is why many conducted PAC's have a bundle branch block appearance). It almost always occurs with rapid ventricular responses. It occurs as a ventricular refractory period is influenced by the prior RR interval. In long RR intervals, the refractory period lengthens; in short RR intervals, the refractory period is shorter. In regular rhythms, this is not noted as the RR intervals are consistent, so intermittent aberrant conduction is not seen. However, because irregularly irregular rhythms have changing RR intervals, aberrant conduction can occur when a beat occurs with a shorter RR interval than the prior RR interval.

Usually, the aberrant conduction is in the pattern of a RBBB as the right bundle typically has a somewhat longer refractory period than the left bundle; however, it can also produce a LBBB pattern. This does not require treatment and is clinically unimportant except providers are often fooled into believing there are frequent PVC's or even nonsustained VT when multiple aberrant beats occur sequentially.

Examples:

Further Reading:

https://litfl.com/ashman-phenomenon/