What we refer to nowadays as the “Ashman Phenomenon” is really the “Gouaux-Ashman Phenomenon.” It was first elaborated by Drs. Gouaux and Ashman of New Orleans, Louisiana. However, English speakers struggled with pronouncing Gouaux (it’s pronounced “GOO-OH”), but the issue was finally resolved and today we refer to it as the Ashman Phenomenon… with apologies to Dr. Gouaux.
At the top of this post is a snippet from a rhythm strip demonstrating the Ashman Phenomenon. The QRS complexes are labelled R1, R2, R3 and R4 for discussion purposes.
The Ashman Phenomenon established that – in the His-Purkinje system – the length of an R – R interval (the R1 – R2 interval) determines the length of the refractory period following “R2”. There is a direct proportionality: if the R1 – R2 interval is long, the refractory period following R2 will be long; if the R1 – R2 interval is short, the refractory period following R2 will be short. The dotted arrows in the snippet indicate the refractory periods – blue for the long R1 – R2 interval and red for the short R2 – R3 interval.
There is no problem with aberrant conduction as long as the rate and rhythm remain stable. Each QRS will still conduct normally even when the rate is very slow and the R – R intervals are long. If, however, a premature supraventricular complex appears (R3), it will likely fall within the refractory period of one of the bundle branches (almost always the right bundle branch) and result, typically, in a right bundle branch block morphology.
Bear in mind that the Ashman Phenomenon occurs only in the His-Purkinje system and not in the AV node. (The AV node reacts quite differently to premature beats and accelerated rhythms.) If a second early beat immediately follows (in our snippet, that would be R4) it may or may not conduct aberrantly (the one in our snippet above does not). The Ashman Phenomenon requires a long-short R – R interval (R1 – R2 – R3) whereas a second premature beat (R4) would be ending a short-short interval (R2 – R3 – R4) in which R3 is followed by a short refractory period. This is shown in the ECG snippet above (the red dotted arrow). If R4 had been aberrantly conducted, it would be for a different reason and not due to the Ashman Phenomenon. Also, the Ashman Phenomenon refers only to supraventricular complexes entering the ventricles via the His-Purkinje system. PVCs and ventricular pre-excitation are, by definition, aberrantly-conducted.
The refractory period of the right bundle branch is normally longer than the refractory period of the left bundle branch. This makes it even more likely that the prolongation of the refractory period following a longer R – R interval will result in right bundle branch block morphology of the beat that terminates the short R – R interval.
A corollary to the Ashman Phenomenon is that as the ventricular rate increases (i.e., as the R – R intervals decrease, or shorten) the refractory periods of the bundle branches are getting shorter. This is why most SVTs are narrow-complex tachycardias. Occasionally the first beat of an SVT may be slightly or even completely aberrant, but the recurring short-short intervals assure that the rhythm remains a narrow-complex rhythm.
The Ashman Phenomenon is generally a physiologic response to a premature beat. It does not imply a pathological process. It was originally described in atrial fibrillation (what better rhythm to find lots of long-short R – R intervals?). However, one must be careful in diagnosing the Ashman Phenomenon during atrial fibrillation because ventricular ectopy often ends long-short cycles also. The Ashman Phenomenon can also appear during sinus rhythm. So, if you should note the sudden appearance of a right bundle branch block in a single, isolated QRS – check to see if that beat ends a long-short R – R cycle.