Fascicular ventricular tachycardia

ECG: tachycardia of 140 bpm, regular, monomorphic, moderately broad QRS (130 ms), with right delay pattern, left axis; atrioventricular dissociation with atrial rate slower than ventricular rate (positive sinus activity in leads I, II, negative in aVR identifiable between certain QRS complexes);

Comments: the recorded electrocardiogram corresponds to a typical pattern of fascicular ventricular tachycardia in a healthy heart. This type of tachycardia occurs preferentially in the young adult in absence of underlying cardiac disease with a male predominance. The tachycardia attacks can be sustained and sometimes prolonged (several hours), occurring most often at rest, although exercise and stress can sometimes appear as a triggering factor, and are mainly accompanied by symptoms such as palpitations rather than very rare syncope or evolution toward heart rhythm disease. These tachycardias, considered as benign since they do not degenerate into ventricular fibrillation, represent approximately 10% of so-called idiopathic tachycardias in a healthy heart.

The mechanism underlying these tachycardias has long been debated, although it is now established that tachycardia is related to reentry, the tachycardia being inducible and quite easily terminated by atrial or ventricular pacing. The tachycardia is said to be fascicular even if it is unclear whether, in terms of substrate, the circuit includes only fascicular tissue or also the surrounding myocardium. In its most common form, the anterograde pathway travels in the septum from the base to the apex, and subsequently penetrates the Purkinje fibers in the lower third of the septum, with retrograde activation occurring mostly along the posterior fascicle, and more rarely along the anterior fascicle

A fascicular tachycardia therefore presents certain electrocardiographic characteristics:

• a monomorphic tachycardia generally between 140 and 200 bpm
• often minimally widened QRS complexes (less wide than for most other types of ventricular tachycardia) between 100 and 140 ms; the RS interval (delay between the start of the QRS and the peak of the S wave) is often short (60 to 80 ms), reflecting rapid electrical forces at the beginning of the QRS in conjunction with the rapid activation by the Purkinje fibers, whereas it can exceed 100 ms for other types of ventricular tachycardia
• right delay pattern
• the axis deviation is dependent on the location of the reentry site; in the most common form (90 to 95% of cases), the axis is left or extreme-left reflecting an exit point of the circuit in proximity to the posterior hemibranch; in the less frequent form (5 to 10% of cases), the axis is right reflecting an exit point of the circuit in proximity to the anterior hemibranch; there is a third exceptional form where the exit point is located in the upper aspect of the septum with a more atypical electrocardiographic pattern of narrow QRS with right or left delay;
• the typical pattern of right bundle branch block with left axis can lead to a misdiagnosis of supraventricular tachycardia with conduction aberration; certain electrocardiographic signs, when present,  can make it possible to affirm that it is indeed a ventricular tachycardia: atrioventricular dissociation, capture and/or fusion complexes;
• the baseline electrocardiogram is usually normal even if aspecific repolarization disorders can be observed after an episode of tachycardia (negative T waves).

The proposed treatment in this setting depends on the frequency and severity of the symptoms. Historically, these tachycardias have been shown to be responsive to Verapamil (this is a diagnostic criterion) while rather resistant to amiodarone. In clinical practice, preventive beta-blocker therapy is generally prescribed as first-line treatment, although some patients with highly interspaced and well-supported episodes do not require treatment. When medical treatment is minimally effective or poorly supported, radiofrequency ablation can be proposed with a high success rate and a limited recurrence rate. Some complications such as temporary or permanent bundle branch blocks have been described.

Take-home message: a fascicular ventricular tachycardia occurs preferentially in a young man without heart disease with a characteristic electrocardiographic pattern: monomorphic tachycardia, slightly widened QRS, right delay, left axis, atrioventricular dissociation