ECG features of parasystole

What are the ECG features of parasystole?

ECG features of parasystole are:

• Varying coupling interval
• Inter ectopic interval has a common denominator other than 1
• Fusion beats

This means that there is a constantly firing focus which is protected from the dominant rhythm due to an entrance block to the focus, without an exit block.

Parasystole refers to an ectopic pacemaker that discharges with a constant rate competing with the natural pacemaker of the heart,  the sinoatrial node. Parasystolic pacemakers have been described to originate in the atrium, atrioventricular node, His bundle, and in the ventricle.

Fascicular parasystole and recurrent syncope has been reported by Steinfurt and colleagues [1]. That patient had recurrent presumably arrhythmic syncope due to parasystole originating from the left posterior fascicle. Patient underwent catheter ablation of the focus after exclusion of other causes and became asymptomatic. Although parasystole has a generally benign prognosis, the parasystolic beat can rarely induce ventricular tachycardia and fibrillation.

Even though fixed coupling intervals usually exclude parasystole, it can occur if the sinus rhythm is fixed to the parasystolic rhythm by retrograde P waves which reset the sinus node [2]. This can occur for short periods when parasystolic impulses will appear to be coupled to sinus impulses.

References

1. Johannes Steinfurt, Stefan Asbach, Katja E Odening, Thomas S Faber, Brigitte Stiller, Christoph Bode, Jürgen Biermann. Fascicular parasystole and recurrent syncope – a case report. Eur Heart J Case Rep. 2018 Mar 5;2(1):yty020. 
2. D Luke Glancy. Group Beating Resulting From Ventricular Parasystole. Proc (Bayl Univ Med Cent). 2003 Jul;16(3):352-3.