Friday, 6 May 2016

What is the Ashman phenomenon?

Take a look at the ECG rhythm strip below (Fig. 1). How would you describe the broad QRS complex near the middle of the strip? Is this a ventricular ectopic beat? Or something else?

Fig. 1 Rhythm strip

In fact, this broad QRS complex is an aberrantly-conducted beat, and this is an example of the Ashman phenomenon. This phenomenon occurs when you get two beats with a long RR interval followed by a beat with a short RR interval (Fig. 2). In this situation, the beat with the short RR interval is aberrantly conducted - typically with a right bundle branch block morphology.

Fig. 2 Details of the Ashman phenomenon

The Ashman phenomenon occurs because the refractory period of conduction system is proportional to the RR interval of the preceding beat. When you have two beats separated by a long RR interval, the following refractory period will be relatively long. However if the next beat arrives after a short RR interval, the conduction system won't have fully recovered from its refractory period, and so its conduction will be abnormal, giving rise to a broad QRS.

The right bundle has a slightly longer refractory period than the left bundle, so the aberrantly-conducted beat is typically conducted with an RBBB morphology.

The Ashman phenomenon is typically seen in the setting of atrial fibrillation, where the RR interval varies from beat to beat. The phenomenon was first described in 1947.

The significance of the Ashman phenomenon is that the broad QRS complexes are commonly mistaken for ventricular ectopics (also known as premature ventricular complexes or PVCs). However, they're not ectopic beats. Instead, these broad QRS complexes represent the aberrant conduction of a supraventricular beat. The phenomenon itself is harmless, but it's important not to misdiagnose the aberrantly-conducted beats for PVCs.

To learn more about ECG interpretation, check out the ECG Black Belt Workshop at Medmastery. In addition, the following links will take you to some key references:

Gouaux JL, Ashman R. Auricular fibrillation with aberration simulating ventricular paroxysmal tachycardia. Am Heart J 1947; 34: 366-73.

Ashman Phenomenon at the Medscape website

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