Ashman phenomenon is a form of aberrancy which was described by Gouaux and Ashman in atrial fibrillation [Gouaux JL, Ashman R. Auricular fibrillation with aberration stimulating ventricular paroxysmal tachycardia. Am Heart J. 1947;34:366]. Ashman phenomenon is aberrancy due to the change in refractory period with varying cycle length. The refractory period increases with the preceding cycle length and vice versa. Hence if a short cycle follows a long cycle, aberrancy can result. The aberrancy is usually of the right bundle branch (RRBB) type because the refractory period of the right bundle is more than that of the left bundle branch. The aberrancy can be maintained in subsequent beats due to a concealed trans septal conduction which keeps the right bundle refractory. This rather fast sequence of wide QRS complexes in the setting of atrial fibrillation mimic a run of ventricular tachycardia. But the differentiating features are the initiating long short sequence and the irregularity in the RR intervals during the tachycardia.
Recognition of Ashman phenomenon in a strip with atrial fibrillation can be clinically important. If the person is on digoxin, occurrence of Ashman phenomenon does not call for dose reduction, but if it is ventricular ectopy, dose reduction is needed. Same applies in a more sinister way to a run of wide QRS complex tachycardia.
Fish C has proposed a few criteria for the diagnosis of Ashman phenomenon [Fisch C. Electrocardiography of arrhythmias: from deductive analysis to laboratory confirmation--twenty-five years of progress. J Am Coll Cardiol. 983;1:306-16]:
1. Long-short sequence terminating in the wide QRS. Occasionally a reverse sequence has also been reported. Though RBBB aberrancy is more common, LBBB aberrancy can also occur.
2. RBBB aberrancy has a normal initial QRS vector.
3. Varying coupling intervals of the aberrant QRS complexes
4. Full compensatory pause is not seen as the underlying rhythm is atrial fibrillation.
Richard Ashman was a physiologist at Louisiana State University School of Medicine in New Orleans [Kennedy LB, Leefe W, Leslie BR. The Ashman phenomenon. J La State Med Soc. 2004;156:159-62].