Ashman Phenomenon

Ashman phenomenon is an aberrant ventricular conduction due to a change in QRS cycle length, and it can be seen in any supraventricular arrhythmia. ^[1]  It is gnerally described as a wide QRS complex that follows a short R-R interval preceded by a long R-R interval. ^[2]

In 1947, Gouaux and Ashman ^[2] reported that in atrial fibrillation, when a relatively long cycle was followed by a relatively short cycle, the beat with a short cycle often has right bundle-branch block (RBBB) morphology, ^{[3, 4]}  although left BBB (LBBB) morphology can also occur. ^{[1, 2]} This causes diagnostic confusion with premature ventricular complexes (PVCs) or, rarely, ventricular tachycardia. ^[1] If a sudden lengthening of the QRS cycle occurs, the subsequent impulse with a normal or shorter cycle length may be conducted with aberrancy.

No geographic variations occur. ^[2] Ashman phenomenon is related to the underlying pathology of the cardiac conduction system and is a common electrocardiographic (ECG) finding in clinical practice.

No treatment is needed for isolated complexes. ^[2] Treat the underlying cardiac condition as appropriate.

As Ashman phenomenon is simply an ECG manifestation of the underlying condition, not a disease process itself, morbidity and mortality is related to the underlying condition (often, atrial fibrillation). ^[2]

Ashman phenomenon is an intraventricular conduction abnormality caused by a change in the heart rate. This is dependent on the effects of rate on the electrophysiological properties of the heart and can be modulated by metabolic and electrolyte abnormalities and the effects of drugs.

The aberrant conduction depends on the relative refractory period of the components of the conduction system distal to the atrioventricular node. The refractory period depends on the heart rate. Action potential duration (ie, refractory period) changes with the R-R interval of the preceding cycle; shorter duration of action potential is associated with a short R-R interval and prolonged duration of action potential is associated with a long R-R interval. A longer cycle lengthens the ensuing refractory period, and, if a shorter cycle follows, the beat ending it is likely to be conducted with aberrancy.

Aberrant conduction results when a supraventricular impulse reaches the His-Purkinje system while one of its branches is still in the relative or absolute refractory period. This results in slow or blocked conduction through this bundle branch and delayed depolarization through the ventricular muscles, causing a bundle-branch block configuration (ie, wide QRS complex) on the surface ECG, in the absence of bundle-branch pathology. A RBBB pattern is more common than a left bundle-branch block (LBBB) pattern because of the longer refractory period of the right bundle branch.

Several studies have questioned the sensitivity and specificity of the long-short cycle sequence. Aberrant conduction with a short-long cycle sequence has also been documented.

Conditions causing an altered duration of the refractory period of the bundle branch or the ventricular tissue cause Ashman phenomenon. These conditions are commonly observed in atrial fibrillation, atrial tachycardia, and atrial ectopy.

A study by Sardar et al indicated that dofetilide, a delayed rectifier potassium current (I_Kr) blocker used to treat atrial fibrillation, can promote the development of Ashman phenomenon, possibly through a reverse use-dependence effect associated with prolongation of the ventricular refractory period. ^[5] The study involved 10 patients with atrial fibrillation who underwent dofetilide loading, receiving 250-500 micrograms of the drug every 12 hours. The investigators found that the total number of Ashman beats rose from 42±24 prior to the administration dofetilide to 93±79 after the first dose of the drug and 133±101 after the second dose. ^[5]