Monomorphic ventricular tachycardia

Articolo redatto e revisionato del Dott. Luca Moretti

Monomorphic ventricular tachycardia is a rapid rhythm of ventricular origin in which the QRS complexes maintain a substantially uniform configuration during the episode. Monomorphism indicates a repetitive activation sequence, not a single etiology: it may reflect the stable exit of a scar-related circuit, reentry involving the conduction system, or a focal source. The form may be non-sustained or sustained, well tolerated or associated with collapse. Duration, morphology and hemodynamic consequences are complementary descriptions.

In patients with structural heart disease, especially after myocardial infarction, scar-related reentry is a common and clinically relevant mechanism. In people without identifiable disease, idiopathic forms exist and are often amenable to targeted treatment. Epidemiology therefore depends on context: a series of patients with defibrillators does not describe the same population as an outpatient clinic for palpitations in young adults. The presence of a typical morphology guides the investigation, but does not replace verification of the substrate.

The monomorphic nature allows relatively detailed anatomical and electrophysiological analysis. This monograph explores the relationship among QRS morphology, circuit and therapeutic target; the operational pathway for a persistent episode is also addressed in sustained ventricular tachycardia. Distinction from polymorphic forms is essential, because a stable activation sequence allows mapping and pacing strategies that do not apply in the same way to a continuously variable arrhythmia.

Circuit architecture and focal mechanisms

In scar-related reentry, fibrosis separates bundles of still-viable myocardium, creating tortuous pathways, regions of slow conduction and anatomical or functional blocks. A circuit may include a critical isthmus, entry and exit regions and return loops. During part of the cycle, activity remains confined within slow-conducting tissue; the surface QRS appears when the impulse reaches myocardium capable of activating a sufficient ventricular mass. Morphology therefore mainly reflects the exit and subsequent propagation, not the complete geometry of the circuit.

A single scar may support several tachycardias with different rates or morphologies. Partly shared circuits may use different exits and the same exit may be reached through different pathways. Comparison among clinical morphologies is useful for planning ablation, but does not allow precise counting of circuits. Drugs, refractoriness and direction of propagation may also alter the QRS or cycle length without creating a new disease. Monomorphism of the episode therefore does not imply anatomical uniqueness of the target.

An ischemic scar is often subendocardial or transmural and follows a coronary distribution. In non-ischemic cardiomyopathies the substrate may be intramural, subepicardial, septal or distributed less predictably. This difference affects accessibility of the circuit from the endocardium and the possibility that an apparently adequate ablation leaves critical tissue at depth. Magnetic resonance helps identify these distributions, although it does not replace electrical mapping of the conduction actually present.

The His-Purkinje system may be an essential part of reentry. In bundle-branch reentrant ventricular tachycardia, activation travels in opposite directions along components of the conduction system and uses ventricular connections to complete the circuit. In idiopathic fascicular forms, circuits and tissues with different properties are involved. Recognition of the mechanism changes the target and the risk of atrioventricular block; not all tachycardias with bundle-branch block morphology are simply myocardial propagation from a peripheral focus.

Focal forms due to triggered activity may be facilitated by adrenergic stimulation and delayed afterdepolarizations. In some outflow-tract tachycardias, adenosine may terminate the arrhythmia by interfering with the responsible signaling pathway. Abnormal automaticity may produce gradual acceleration and deceleration, but these features are not always evident in the brief recorded segment. Response to pacing, mode of induction and pharmacology are therefore interpreted together, avoiding assignment of a definitive mechanism from clinical observation alone.

Stable morphology does not mean hemodynamic stability. Cycle length may shorten, conduction may change and the arrhythmia may degenerate into a disorganized form. Perfusion also depends on function, atrioventricular synchrony and the presence of ischemia. A relatively slow incessant tachycardia may instead cause or worsen arrhythmia-induced cardiomyopathy without causing immediate collapse. The relationship between mechanism and consequences should be assessed over the entire history, not only during one observed episode.

Clinical phenotypes and recognition of heart disease

In a patient with previous myocardial infarction, a regular wide-QRS tachycardia has a high probability of being ventricular. The scar substrate may manifest long after the coronary event, and absence of pain does not exclude the mechanism. Active ischemia should be investigated when the context suggests it, but recurrent monomorphic tachycardia over scar is not necessarily eliminated by revascularization. Treatment of coronary disease and treatment of the arrhythmic circuit may require separate and complementary interventions.

In dilated and arrhythmogenic cardiomyopathies, sarcoidosis and sequelae of myocarditis, the arrhythmia may precede a severe reduction in function. A monomorphic presentation does not demonstrate that the pathological process is inactive: inflammation and fibrosis may coexist. The history includes the course of heart failure, conduction disturbances, extracardiac manifestations and family history. Identifying the disease is important even when ablation succeeds, because the substrate may evolve and generate new circuits.

In repaired congenital heart disease, patches, incisions and valvular structures may delimit anatomical isthmuses capable of supporting reentry. Reconstruction of surgical anatomy guides mapping and allows interpretation of a tachycardia in relation to possible conduction corridors. An echocardiogram that describes only the ejection fraction is insufficient for this purpose. Operative records, imaging and assessment in centers experienced in the specific heart disease may be decisive.

Idiopathic outflow-tract tachycardias often show a morphology consistent with the anatomical origin and a relationship with adrenergic activity. Fascicular forms may have a relatively narrow QRS and characteristic pharmacological sensitivity. Origins from papillary muscles, annuli and other regions also occur. The term idiopathic requires concordance among phenotype, history, ECG and investigations: a typical appearance must not obscure syncope, family history or signs of heart disease.

Symptoms include rapid palpitations, fatigue, dyspnea, chest discomfort and presyncope; syncope or cardiac arrest indicates severe hemodynamic consequences. A relatively slow tachycardia may be perceived as worsening of heart failure or reduced functional capacity. In patients with devices, episodes slowed by drugs may not exceed the detection threshold. Clinical suspicion should therefore be compared with ECG and programming, without assuming that absence of recorded therapies excludes the arrhythmia.

Structural assessment begins with echocardiography and may require magnetic resonance, coronary evaluation or investigations for inflammation and infiltration. Distribution of late enhancement on magnetic resonance provides information on fibrosis, but not every fibrotic area necessarily participates in the clinical tachycardia. Genetic testing is considered when phenotype and family history suggest an inherited cardiomyopathy. Etiologic diagnosis builds the context for prognosis, family screening and choice of protection, in addition to the possibility of ablation.

Electrocardiographic diagnosis of monomorphic tachycardia

A twelve-lead ECG should be acquired during the arrhythmia when conditions allow. Assessment begins with regularity, QRS duration, axis and the relationship between atrial and ventricular activity. Atrioventricular dissociation strongly supports a ventricular origin; capture and fusion beats may make it recognizable. However, one-to-one retrograde conduction is possible and atrial activity may be hidden within the complexes. Failure to see independent P waves does not equal proof of ventricular dependence on the atrium.

Precordial concordance, an extreme axis and morphologies incompatible with usual bundle-branch blocks increase the probability of ventricular tachycardia. Brugada algorithms and those based on lead aVR use features of depolarization to distinguish primary ventricular activation from aberrant supraventricular conduction. Performance decreases with incomplete recordings, drugs, pre-excitation and complex baseline abnormalities. The result of an algorithm should be one component of reasoning, not a verdict independent of context.

Brugada method criteria include absence of RS complexes in the precordial leads and, when an RS is present, an interval from the onset of R to the nadir of S greater than 100 milliseconds, as well as dissociation and morphological criteria. These signs should not be applied to an inadequate-quality tracing or used in isolation to justify a potentially dangerous drug. The differential diagnosis also includes pre-excited tachycardias, in which abnormal activation may meet apparently ventricular criteria.

A left bundle-branch block morphology suggests predominantly right-sided or septal initial activation; a right bundle-branch block configuration often points toward a left-sided origin. An inferior axis may suggest a superior site, such as an outflow tract, whereas a superior axis points toward inferior regions. These are useful anatomical relationships, but circuit exit, scar extent and preferential conduction may modify the correspondence. Surface localization prepares mapping without replacing it.

Comparison with the sinus-rhythm ECG and any premature ventricular contractions clarifies whether a reproducible triggering morphology exists. A tachycardia with a QRS identical to the baseline bundle-branch block may still be ventricular, especially in reentry involving the conduction system. A fusion sequence may instead generate slight variations and make monomorphism less obvious. In implanted devices, electrograms and atrioventricular relationships provide additional information, but automated classification must be verified on the signals.

Hemodynamic tolerance and response to adenosine or vagal maneuvers are not absolute proof of a supraventricular origin. Some focal ventricular tachycardias are adenosine-sensitive, and a supraventricular tachycardia may be poorly tolerated in a compromised heart. When uncertainty persists in a wide-complex tachycardia, initial management should retain a safety margin for the ventricular hypothesis. Definitive interpretation may require complete documentation of the episode or an electrophysiological study.

Electrophysiological study and target mapping

The electrophysiological study analyzes induction, activation sequence and response to pacing. A scar-related tachycardia may be initiated or terminated by programmed impulses, whereas a focal mechanism may require adrenergic stimulation or show a different rate dependence. These responses should not be interpreted schematically: multiple mechanisms may coexist and the induced arrhythmia must be compared with the clinical one. Preprocedural documentation reduces the risk of treating only an easily inducible tachycardia that was not responsible for the events.

During a tolerated tachycardia, activation mapping reconstructs the temporal sequence of signals. In reentry, diastolic potentials may identify active tissue between two QRS complexes; not every diastolic signal, however, belongs to an indispensable pathway. Nearby regions or pathways without a critical role may be activated passively. Temporal data are integrated with pacing, anatomy and response to energy, seeking to demonstrate that the site actually participates in maintenance of the circuit.

Entrainment consists of driving a reentrant tachycardia with pacing slightly faster than the tachycardia. The relationship between the post-pacing interval and tachycardia cycle length, fusion and stimulus-to-QRS time help determine the catheter's position relative to the circuit. A return close to the tachycardia cycle and concealed fusion may support participation in a protected isthmus, but the conclusion requires confirmation of capture and interpretation of conduction. Pacing may also accelerate or terminate the arrhythmia, limiting the amount of information obtainable.

When tachycardia is not tolerated, substrate mapping during sinus or paced rhythm becomes central. Low-voltage areas, fragmented signals, late potentials and local abnormal ventricular activity may identify tissue capable of supporting reentry. Voltage depends on contact, orientation, thickness and direction of the activation wavefront; a map is not a histological photograph of fibrosis. Pacing from different sites may reveal abnormalities not evident with a single activation sequence.

Pace mapping compares the paced QRS with the clinical QRS and may approximate the site of exit or a focus. A high match does not by itself demonstrate that the site is indispensable, because different regions may access the same exit pathway. In focal mechanisms, sufficiently early local activation relative to QRS onset is also sought. Integration with imaging and three-dimensional anatomy helps distinguish contiguous origins, including regions of the outflow tracts and aortic cusps.

An epicardial or intramural substrate may require additional strategies. An epicardial approach carries specific risks, including coronary or phrenic-nerve injury, bleeding and difficulties related to adhesions. In deep septal circuits, access from a single surface may be insufficient. Techniques for deeper lesions or difficult targets belong in experienced centers and selected indications. Anatomical complexity should be discussed before the procedure because it affects likelihood of success, duration and the potential need for further interventions.

Drug therapy, ablation and antiarrhythmic protection

Termination of the episode depends first on stability. Monomorphic tachycardia with a pulse and instability requires synchronized cardioversion; in the absence of a pulse, the resuscitation algorithm for a shockable rhythm is followed. In a stable patient, drugs and cardioversion are selected according to function, heart disease, QT and diagnostic probability. The availability of a specific treatment for an already demonstrated form does not justify its empirical use in any wide-complex tachycardia.

Chronic therapy distinguishes symptom control, reduction of recurrences and prevention of arrhythmic death. Beta-blockers have a role in many patients with heart disease and in some adrenergic forms. Amiodarone or sotalol may reduce episodes and defibrillator therapies, but require selection and surveillance for adverse effects. Class Ic drugs may be used in selected idiopathic forms, whereas relevant ischemic or structural heart disease radically changes the benefit-risk balance. Treatment of the underlying disease remains essential even when the arrhythmia is suppressed.

In symptomatic idiopathic forms from the right ventricular outflow tract and in fascicular tachycardias, ablation may be an appropriate initial choice. At other sites, the balance depends on accessibility, proximity to vulnerable structures and patient preferences. Incessant tachycardia associated with dysfunction makes effective control particularly important. The decision should not be based solely on duration of the individual episode, but on total exposure and functional consequences.

In scar-related tachycardias, ablation may reduce recurrences, electrical storms and device therapies. The VANISH2 trial, conducted in 416 patients with prior myocardial infarction, clinically significant tachycardia and a defibrillator, compared initial ablation with sotalol or amiodarone. The composite outcome of death, electrical storm, appropriate shock or clinically treated sustained tachycardia occurred in 50.7% of the ablation group and 60.6% of the drug-therapy group during a median follow-up of 4.3 years. The result supports discussion of an early ablation strategy in that population; by itself it does not demonstrate a reduction in mortality considered separately and does not automatically extend to non-ischemic cardiomyopathies.

Procedural objectives may include elimination of the clinical tachycardia, non-inducibility and modification of pathological signals or channels. No endpoint guarantees absence of recurrence: induction may be limited by safety and the substrate may evolve. Outcome assessment also considers reduction in clinical burden and shocks. An intervention that does not eliminate every inducible arrhythmia may still improve the course, whereas a favorable acute outcome does not automatically justify discontinuation of indicated protection.

The implantable cardioverter-defibrillator is selected according to diagnosis, event history and risk. Ablation does not usually replace it in structural heart disease with an established indication, although selected situations exist in which a tolerated tachycardia and preserved or mildly reduced function allow different strategies. In successfully treated idiopathic forms, the context is different. The decision should make residual risk and the quality of evidence applicable to the individual case explicit, avoiding equivalence between disappearance of tachycardia and disappearance of the disease.

Outcomes, recurrences and long-term surveillance

Prognosis is heterogeneous. A well-characterized idiopathic form may be eliminated with a circumscribed intervention, whereas a circuit in progressive cardiomyopathy may represent only one manifestation of the disease. Function, extent and distribution of scar, heart-failure status and comorbidities influence outcomes. Success rates of idiopathic tachycardias cannot be used to describe ablations in patients with a diffuse intramural substrate or severe hemodynamic compromise.

Recurrence may have the same morphology as the first episode or a different configuration. In the first case, recovery of conduction in a treated region is possible; in the second, another exit, an uneliminated circuit or progression of the substrate may be involved. Comparison requires the original tracings and not just the name of the diagnosis. Preserving the clinical ECG and electrograms is therefore a practical component of continuity of care.

Follow-up includes symptoms, ventricular function, therapies and device data when present. Antitachycardia pacing can terminate many monomorphic tachycardias through programmed impulses and reduce shocks, but may be ineffective or accelerate the arrhythmia. Programming should consider clinical rate, tolerance and previous response. Drugs that slow tachycardia may require review of detection zones; otherwise a persistent episode may not be recognized by the device.

In patients with suspected arrhythmia-induced cardiomyopathy, reassessment documents reduction of burden and functional recovery. Improvement after control of the arrhythmia supports its causal contribution, but does not exclude concomitant heart disease. Persistent dysfunction requires assessment of residual burden, duration of injury and alternative diagnoses. In inherited or inflammatory cardiomyopathies, disease monitoring continues even when recordings show no recurrences.

Drug safety is reassessed with ECG and tests relevant to the active agent, especially after clinical changes or new prescriptions. Reduction of episodes must be balanced against hypotension, bradycardia, QT prolongation and extracardiac toxicity. A patient without shocks but limited by adverse effects has not necessarily achieved the best possible result. Comparison among drug therapy, programming review and repeat ablation should also consider quality of life and preferences.

A cluster of recurrences, syncope, worsening heart failure or repeated shocks requires prompt reassessment. In complex cases, coordination among electrophysiology, imaging, genetics and heart-failure care is useful. Monomorphism provides valuable information on the pathway of the impulse, but effective management requires linking that pathway to the anatomy and evolution of the heart disease. Control of the circuit is one part of care, to be integrated with protection and treatment of the disease.