Fascicular beats , rhythm and tachycardia

On rare occasions , ectopic impulse arise from one of the fascicles of the left bundle branch system . Fascicular tachycardia is uncommon and not usually associated with underlying structural heart disease. thus , the ectopic impulse originating form one of the fascicle may produce isolated premature beats , slow escape rhythm , or tachycardia. The fascicular beats typically exhibit an incomplete right bundle branch block pattern with marked left or right axis deviation .

In a broad sense , fascicular beats , rhythms and tachycardia belongs to ventricular arrhythmias . clinically , the fascicular beats , rhythm, and tachycardia seems to be relatively benign arrhythmias . it has been shown that a fascicular tachycardia is observed not uncommonly during the first 24-72hours of acute MI ,but the arrhythmia is usually transient and self-limited . fascicular tachycardia or rhythm should be considered when dealing with a relatively slow tachycardia with an incomplete RBBB pattern with a marked left or right axis deviation , providing that bizzare QRS complex are not preceded by premature P waves .

It originates from the region of the posterior fascicle (or occasionally the anterior fascicle) of the left bundle branch and is partly propagated by the His-Purkinje network. It therefore produces QRS complexes of relatively short duration (0.11-0.14 s). Consequently, this arrhythmia is commonly misdiagnosed as a supraventricular tachycardia.
The QRS complexes have a right bundle branch block pattern, often with a small Q wave rather than primary R wave in lead V1 and a deep S wave in lead V6. When the tachycardia originates from the posterior fascicle the frontal plane axis of the QRS complex is deviated to the left; when it originates from the anterior fascicle, right axis deviation is seen.

References 
1- ECG Diagnosis , second edition , Edward K,.Chung.K,M.D.
2- Broad complex tachycardia—Part II
BMJ 2002; 324 doi: http://dx.doi.org/10.1136/bmj.324.7340.776 (Published 30 March 2002)

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