Ashman phenomenon was first described in 1947 by Dr. Gouaux and Dr. Ashman. In their initial description, they showed that the earlier in the cycle a premature atrial contraction (PAC) occurs, and the longer the preceding cycle is, the more likely it is that the PAC will be conducted with aberration, and the conduction deviates from the normal pathway. However, this is an electrocardiogram (EKG) finding and has no impact on mortality or morbidity.

Ashman phenomenon is often misinterpreted as a premature ventricular contraction (PVC) because of its appearance on the EKG as a single wide QRS complex, and in patients with atrial fibrillation as narrow QRS complexes. It has also been seen in other supraventricular tachyarrhythmias. Single, wide QRS aberrant complexes are almost entirely caused by the Ashman phenomenon. Because the Ashman phenomenon is an EKG finding and not a disease process, there are no symptoms associated with this. Instead, any symptoms the patient would feel would be due to the underlying cardiac condition, which in many cases, is atrial fibrillation.[1]

The term Ashman phenomenon or Ashman’s beat is used when describing a particular occurrence on an EKG that has been described as a wide QRS complex that follows a short RR interval preceded by a long RR interval. The QRS complex originates above the AV node and not at the right or left ventricle, and due to this, presents as a right bundle branch block (RBBB) or left bundle branch block (LBBB).[2] This occurs because the refractory period of the myocardium is proportional to the length of the prior RR interval. A shorter RR is associated with a shorter duration of action potential and vice versa. With a longer RR cycle, there will be an increase in the refractory period, and if a short RR follows, the beat that terminates the cycle is likely to be conducted aberrantly.[3] Due to the fact that the refractory period of the right bundle is longer than that of the left bundle, the right bundle will still remain in the refractory period at the time the supraventricular beat gets to the His-Purkinje system. This results in the blocked conduction through one bundle, leading to an electrocardiogram with bundle branch block morphology. Since the refractory period of the right bundle branch is longer than the left bundle branch block, a right bundle branch block pattern is more common, but a left bundle branch pattern can still be seen just less frequently. The RBBB pattern can be associated with left fascicular blocks.[4]

A practitioner can easily diagnose Ashman phenomenon by using the Fisch criteria as first described by Dr. Charles Fisch:

• A cycle that is relatively long, immediately ahead of the cycle terminated by the aberrant QRS complex: A short-long-short interval is even more likely to initiate aberration. Aberration could be either LBBB or RBBB or both, and both patterns may be noticed even in the same patient.
• RBBB from aberrancy with a normal orientation of the initial QRS vector: The concealed propagation of aberration is possible, such that there is a likelihood of a series of wide QRS supraventricular beats.
• Irregular coupling of aberrant QRS complexes
• Absence of a full compensatory pause[4][5]

The degree of aberrant conduction may vary from beat to beat and from patient to patient. While a right bundle branch block pattern is more common, a left bundle block patter pattern or combined block pattern may be observed sometimes even in the same patient. In such situations where both LBBB and RBBB are noted, the patient likely has an underlying heart block, and an Ashman beat is not being seen.[6]

Ashman phenomenon is often confused with premature ventricular complexes due to the similar appearance and subtle nature on EKG.[7] It is important to differentiate between premature ventricular complexes and Ashman phenomenon as they are often confused for one another. While both Ashman phenomenon and premature ventricular complexes can have a widened QRS, subtle differences can be noted between the two. A regular coupling of a widened QRS complex would support the diagnosis of premature ventricular complexes versus Ashman phenomenon. Also, if the practitioner were to note a full compensatory pause, this would favor a ventricular origin of the noted beat, while the lack of the compensatory pause favors aberrancy.[6]

In summation, QRS morphology is the most crucial clue when it comes to distinguishing between a supraventricular or ventricular origin of wide QRS complexes.

There have been no reported data on the geographic differences noted with Ashman phenomenon. Ashman phenomenon is linked to the underlying pathology of the conduction system of the heart and thus is nearly always linked to a conduction irregularity. Ashman beat is a common EKG finding in clinical practice; however, it is often misinterpreted.[5]

Ashman phenomenon has most widely been associated with atrial fibrillation; wherein, it has been noted to occur secondary to the chaotic impulses from the atrium. These chaotic impulses cause variability in the RR cycle length leading to the opportunity for aberrant conduction.[6]

Ashman's beat is caused by R-R interval length inconsistency. Longer R-R intervals, such as those seen in bradycardia, have a longer refractory period, whereas shorter R-R intervals, such as those seen in tachycardia, have a shorter refractory period.[6] 

In Ashman phenomenon, an elongated R-R internal followed by a successive shorter R-R interval is seen, and during this time, the cardiac myocytes are still in the refractory period. The cardiac myocytes are still in the refractory period during this time due to the initial extended/prolonged R-R interval. This electrical abnormality often causes a block to be noted on EKG, often a right bundle branch block pattern is noted, but left bundle branch block patterns can also be seen as discussed above.[8]