ECG criteria
- Regular rhythm, ventricular rate usually around 120–200 bpm (most commonly 150–200 bpm), sudden onset and termination
- QRS usually around 100–140 ms, frequently less than 120 ms, but may be wider; a relatively narrow QRS is a characteristic feature, but not an exclusion criterion for VT
- QRS morphology in V1 resembles right bundle branch block (variable: qR, R, rsR', and other forms); the shape itself does not differentiate VT from SVT with aberrancy
- In the posterior fascicular form (more common) — marked left axis deviation: QRS negative in II, III, aVF, positive in I and aVL
- In the anterior fascicular form (less common) — right axis deviation: QRS negative in I and aVL, positive in II, III, aVF
- AV dissociation, capture beats, or fusion beats upon careful rhythm analysis confirm the ventricular origin
- Terminated or slowed by the administration of verapamil, which serves as an additional (but not primary) diagnostic sign
What it looks like on ECG
On the surface ECG, fascicular VT appears as a regular tachycardia with a rate typically around 120–200 bpm (most commonly 150–200 bpm) and a QRS complex that is noticeably narrower than many other VTs — often around 100–140 ms, frequently less than 120 ms. This creates a trap: such tachycardia is easily mistaken for supraventricular tachycardia with aberrant conduction.
Lead V1 usually demonstrates an RBBB-like QRS morphology; the specific shape (qR, R, rsR', and other variants) is variable and by itself does not allow differentiation between fascicular VT and SVT with aberrancy.
The direction of the QRS axis is determined by the location of the re-entry circuit and the exit zone of excitation, rather than being a direct sign of anatomical fascicular damage:
| Variant | Circuit localization | Frontal plane axis | QRS in II, III, aVF |
|---|---|---|---|
| More common (~90%) | area of the left posterior fascicle | marked left axis deviation | negative |
| Less common | area of the left anterior fascicle | right axis deviation | positive |
| Rarely | upper septal fascicular VT | usually normal or close to normal | Variable |
The ST segment and T wave are difficult to evaluate during tachycardia due to the heart rate. The sinus ECG after termination of the episode often lacks specific changes, although a normal ECG does not exclude structural heart disease; transient ST-T changes following tachycardia are also possible, so clinical evaluation and imaging are indicated as appropriate. With careful analysis of a long rhythm strip, P wave dissociation from the QRS complexes, ventricular capture beats, or fusion beats can be observed — classic signs of a ventricular origin.
Mechanism
The mechanism involves re-entry utilizing the fascicular-Purkinje system and adjacent myocardium, most frequently in the region of the left posterior (less commonly anterior) fascicle of the left bundle branch. An area of abnormal, slowly conducting Purkinje tissue with pathological conduction properties is postulated; the exact anatomical basis of the circuit is variable and is not restricted solely to a fibrous strand. Because the impulse rapidly engages the distal His-Purkinje system, ventricular depolarization occurs via an almost physiological pathway, which is why the QRS remains relatively narrow, unlike VT originating from the working myocardium. Verapamil blocks calcium-dependent slow conduction in the critical zone of the re-entry circuit — hence the name "verapamil-sensitive VT".
Causes
- Idiopathic form — occurs in a structurally normal heart, most commonly in young people (adolescents, adults under 40), predominantly in males
- Re-entry involving the fascicular and Purkinje system and abnormal conducting fibers is hypothesized; the exact anatomical substrate is variable and not associated with ischemia, cardiomyopathy, or scar.
- Extremely rarely described in combination with structural heart disease — in such cases, it requires more careful evaluation and differentiation from scar-related VT.
Differential diagnosis
- SVT with aberration (functional or preexisting bundle branch block): marked axis deviation increases the probability of VT, but SVT with aberration in the presence of a preexisting fascicular block may also present with marked axis deviation; comparison with baseline ECG, signs of AV dissociation, capture beats, and validated algorithms (Brugada, Vereckei) are helpful, though they have limitations in relatively narrow QRS.
- SVT with pre-excitation / antidromic AV re-entrant tachycardia: conduction via an accessory pathway represents pre-excitation rather than bundle branch block-type aberration; the QRS morphology typically differs from a typical RBBB pattern.
- Right ventricular outflow tract tachycardia: in this condition, the QRS has a left bundle branch block morphology with an inferior axis, rather than an RBBB-like morphology.
- VT in structural heart disease (post-infarction, scar-related): structural VT is suspected based on history (prior myocardial infarction, cardiomyopathy), echocardiography/MRI, baseline sinus ECG (scar, pathological Q wave, ST-T changes), and a combination of features rather than QRS width alone.
- A relatively narrow QRS in VT in general is rare; this exact resemblance to SVT is the main diagnostic pitfall.
Clinical significance and management
The prognosis is generally favorable — the heart is structurally normal, sudden death is uncharacteristic, but the tachycardia can be symptomatic (palpitations, presyncope, rarely syncope) and recurrent. The physician should:
- Record a 12-lead ECG during the episode and after its termination to compare morphology, assess the QT interval, and check for signs of structural pathology.
- Perform a structural evaluation of the heart (echocardiography, MRI if necessary) to rule out an organic cause of VT masquerading as fascicular; a normal sinus ECG by itself does not rule out structural pathology.
- Any regular wide QRS tachycardia should initially be managed as VT. In case of instability, perform emergency electrical cardioversion. Intravenous verapamil may be used by an experienced physician only in reliably diagnosed, hemodynamically stable verapamil-sensitive fascicular VT; do not use it for tachycardia of undetermined broad complexes.
- For recurrent symptomatic episodes, refer for catheter ablation, which is highly effective due to the well-localized anatomical substrate.
- For undetermined wide tachycardia, the priority is management as VT. Adenosine may be cautiously considered only in strictly selected stable cases of regular monomorphic tachycardia with readiness for immediate cardioversion; do not use beta-blockers as a diagnostic test.
Common interpretation errors
- Diagnosing "SVT with aberration" solely due to a relatively narrow QRS — a narrow or relatively narrow QRS does not rule out a ventricular origin; the diagnosis is based on a combination of morphology, characteristic axis deviation, episode data, and exclusion of structural disease.
- Ignoring signs of AV dissociation, capture beats, and fusion beats, which require careful review of the long rhythm strip.
- Applying Brugada/Vereckei criteria without considering that these algorithms are designed for differentiating regular wide complex tachycardia and have limitations, especially with a relatively narrow QRS.
- Using verapamil as a diagnostic test in undetermined wide QRS tachycardia — erroneous administration in ventricular tachycardia of another origin, reduced LV function, or pre-excited tachycardia can cause hemodynamic collapse.
- Omitting structural evaluation of the heart when the clinical presentation is atypical for a purely idiopathic form, or erroneously concluding that none is present based solely on a normal post-episode ECG.
1. What QRS morphology in lead V1 is most characteristic of fascicular VT?
Fascicular VT is associated with re-entry within the left bundle branch fascicle, so the activation direction creates an RBBB-like QRS morphology in V1, although the specific shape is variable.
2. What QRS axis is typical for the most common (posterior fascicular) variant of fascicular VT?
With a re-entry circuit in the posterior fascicle of the left bundle branch (the most common variant), the QRS is negative in II, III, aVF and positive in I, aVL — marked left axis deviation.
3. Which drug is traditionally used to terminate a reliably diagnosed, hemodynamically stable fascicular VT?
Verapamil blocks calcium-dependent slow conduction in the critical zone of the re-entry circuit of fascicular VT, which is why it often terminates or slows down the tachycardia. It is used only when fascicular VT is reliably diagnosed, not for undetermined wide-complex tachycardia.
Frequently asked questions
How to differentiate fascicular VT from SVT with aberrant conduction?
Key signs of ventricular origin include AV dissociation, ventricular capture beats, and fusion beats on a long rhythm strip. RBBB morphology with a marked left axis deviation (in the posterior fascicular variant) is more typical of VT than of aberrant SVT, but the specific morphology of the QRS complex in V1 is not a reliable differential diagnostic feature. Comparison with a baseline ECG and validated algorithms (Brugada, Vereckei) are more important than QRS duration alone. Termination with verapamil is not a reliable differential criterion.
Is fascicular ventricular tachycardia dangerous?
In most cases, it occurs in structurally normal hearts and is not associated with a high risk of sudden cardiac death, unlike VT associated with scar or cardiomyopathy pathology. Nevertheless, episodes can cause prominent symptoms and recur, thus requiring verification of the diagnosis and, in cases of frequent episodes, definitive treatment.
Why is fascicular VT called verapamil-sensitive?
The re-entry circuit underlying this tachycardia involves a segment of slow, likely calcium-dependent conduction within abnormal Purkinje tissue. Verapamil blocks calcium-dependent conduction in the critical zone of the circuit, which is why it frequently terminates or slows the episode, giving the tachycardia its name.
Why is the QRS narrower in fascicular VT than in typical ventricular tachycardia?
The re-entry impulse rapidly engages the distal His-Purkinje system, so ventricular depolarization occurs in an almost physiological manner, unlike VT propagating slowly through ordinary myocardium. This accounts for the relatively narrow QRS complex, typically around 100–140 ms, and frequently less than 120 ms.
Does a normal ECG after an episode rule out structural heart disease?
No. A normal or near-normal sinus ECG after tachycardia termination does not rule out structural heart disease. Idiopathic fascicular VT often occurs with a structurally normal heart, but this is established through clinical evaluation and imaging (echocardiography, MRI if necessary), rather than solely by the appearance of the post-tachycardia ECG.
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Related terms
More in the section "Ventricular Arrhythmias"
Sources
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- Hampton J. The ECG in Practice
- ESC Guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death, 2022
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.