Idiopathic ventricular arrhythmias (VAs), encompassing ventricular tachycardia (VT) and premature ventricular contractions (PVCs), originating from the right or left ventricular outflow tract (RVOT and LVOT, respectively), are commonly perceived as benign in patients without evident structural heart disease.¹² However, cardiologists face two daunting challenges:³

1- Idiopathic ventricular fibrillation (IVF) and/or polymorphic ventricular tachycardia (PVT) can occasionally stem from seemingly "benign" PVCs.⁴⁵⁶

2- Malignant VAs due to cardiac channelopathies or concealed/early cardiomyopathies [such as Brugada syndrome (BS), catecholaminergic polymorphic VT (CPVT), cardiac sarcoidosis (CS), arrhythmogenic right ventricle dysplasia (ARVD)] may be misclassified as benign idiopathic VAs.⁷⁸⁹

The term "idiopathic" implies the exclusion of organic heart disease; however, differentiating truly idiopathic RVOT VA from RVOT VA related to subclinical forms of ARVD or CS is challenging.¹⁰ ARVD may exhibit extreme phenotypic variability, even within patients from the same family with the same genetic mutation.¹¹ The disease's natural history often begins with a concealed phase progressing to biventricular involvement, making early detection elusive.

Normal electrocardiographic, echocardiographic, and even cardiac magnetic resonance (CMR) imaging studies may not exclude the presence of early "concealed" ARVD manifestations, such as isolated epicardial scar involvement.¹²

Recent observations suggest that epicardial voltage abnormalities may be more extensive than endocardial, particularly in patients with less overt forms of ARVD, adding another layer of uncertainty.¹³ Structural heart disease may become apparent during follow-up,¹⁴ with arrhythmias triggering other serious events.¹⁵¹⁶ Additionally, CS may overlap in clinical presentation with ARVD, complicating differentiation.¹⁷

Reassuring patients and their families about the benign nature of arrhythmias can be challenging, especially given the risk of sudden death.³ Recommending aggressive therapy for all RVOT-PVC patients would be imprudent, considering the rarity of malignant RVOT and the risks associated with medical or ablative therapies. Discharging RVOT-VA patients without implanting an ICD, even after successful ablation therapy, poses another dilemma. Thus, distinguishing between benign RVOT-VT and malignant arrhythmias originating from the RVOT is crucial but remains a clinical challenge.³

When a patient presents with apparently idiopathic premature ventricular contractions (PVCs), conducting a thorough initial assessment is essential. This includes obtaining a detailed history for episodes of syncope or presyncope, as well as any family history of sudden cardiac death (SCD), which may help identify patients at greater risk. Key questions to guide therapy and identify high-risk patients include:¹⁸

Is there evidence of underlying structural heart disease?

Does the patient have a family history of SCD?

Are there any electrocardiographic findings suggestive of a malignant form of ventricular arrhythmias (VAs) rather than a benign form?

Is the patient experiencing symptoms related to the PVCs?

It's important to note that a history of syncope with malignant characteristics may serve as a predictor of the presence of malignant VAs in patients with idiopathic ventricular tachycardia (VT) originating from the right ventricular outflow tract (RVOT). However, the absence of symptoms may not necessarily be reassuring, as many patients who experience ventricular fibrillation (VF) arrest had only benign palpitations previously.

Left ventricular function and dimensions should be assessed echocardiographically; however, cardiac magnetic resonance imaging (CMR) should be considered, particularly in patients with a family history of sudden cardiac death (SCD) or those with suspected malignant electrocardiographic findings.¹⁸ CMR with late gadolinium enhancement (LGE) can identify small regions of myocardial scar from previous myocardial infarction, fibrosis from non-ischemic cardiomyopathy, or edema/fibrosis from inflammatory disorders, aiding in target definition for ablation. LGE, in conjunction with structural assessment, can distinguish true idiopathic outflow ventricular tachycardia (VT)/premature ventricular contractions (PVCs) from those due to early-stage diseases such as arrhythmogenic right ventricular dysplasia (ARVD) or cardiac sarcoidosis (CS).¹⁹

The results suggest a potential role for electrocardiographic imaging and late gadolinium enhancement in the early diagnosis and noninvasive follow-up of ARVD patients. Voltage mapping is effective in distinguishing early or concealed ARVD/C from idiopathic VT,²⁰ with nonuniform conduction and fractionated electrograms present in the early concealed phase of ARVD. The slowing and slurring of QS unipolar electrograms, along with the longer distance from the earliest activation site to the breakout site in RVOT endocardium, suggest that the triggers of ARVD may originate from the mid- or sub-epicardial myocardium.²¹

Electrophysiological abnormalities colocalized with LGE scar indicate a relationship with structural disease.²² Ventricular arrhythmogenicity during isoproterenol testing has been found to be highly sensitive for the diagnosis of ARVD, especially in its early stages.²³ However, most electrophysiologists rely on structural assessment of the right ventricle or electrocardiogram findings to exclude a diagnosis of ARVD, reserving invasive evaluation for symptomatic patients justifying an ablation procedure. In daily clinical practice, the 12-lead electrocardiogram (ECG) remains a frequently used bedside tool for evaluating patients with palpitations. These ventricular arrhythmias typically exhibit a characteristic electrocardiographic morphology, with a left bundle branch block (LBBB) configuration and an inferior axis.²⁴ While no electrocardiographic parameters have been identified to distinguish patients with a grim prognosis from those with benign RVOT-VAs,²⁵ certain ECG clues may warrant further evaluation of VAs,²⁶²⁷²⁸ especially in cases where PVCs appear idiopathic in patients with previously normal hearts.²⁹

a- The coupling interval (CI) of PVCs

PVCs with short or ultra-short coupling intervals (CIs), known as "R-on-T" extrasystoles falling on the peak or the descending limb of the T-wave of the preceding sinus beat,⁷⁹¹⁶ are considered ominous signs in patients with idiopathic ventricular fibrillation (IVF). This phenomenon is particularly observed in patients prone to develop phase-II reentry,³⁰ such as those with hyperacute myocardial infarction³¹ or Brugada syndrome³². The presence of ventricular fibrillation (VF) in the absence of identifiable structural heart disease or known genetic arrhythmia syndromes is often attributed to short coupled PVCs arising from the Purkinje system or ventricular myocardium. Viskin et al. termed this phenomenon a “short-coupled variant of outflow tract VT”.⁷

The CI of the first PVC beat that initiates ventricular arrhythmias is often⁷ but not always shorter⁴¹⁴ than isolated benign PVCs originating from the right ventricular outflow tract (RVOT). This shorter CI may decrease on the vulnerable portion of the T-wave, inducing fast malignant ventricular arrhythmias.²⁶ Studies have shown correlations between the shorter CI of initiating PVCs and the more malignant form of RVOT ventricular tachycardia (VT).¹⁴ Knecht et al. reported that patients with triggering PVCs originating from the RVOT showed a CI of 355±23 msec, whereas those originating from the Purkinje system exhibited a CI of 276±22 msec (p<.001).³³ However, the CI may vary among patients, and it may not always distinguish between malignant and benign RVOT-VT.⁴

Some researchers have introduced indices like the prematurity index (PI) and the QT index to assess the risk of ventricular arrhythmias. These indexes, calculated based on the CI of the first VT beat or isolated PVCs relative to the preceding R-R interval or QT interval, were found to be shorter prior to the onset of polymorphic VT despite similar CIs.³⁴ Recent studies have reported similar CIs between idiopathic RVOT ventricular arrhythmias and early arrhythmogenic right ventricular dysplasia (ARVD) patients, suggesting a focal rather than reentrant mechanism.²⁵ When the coupling interval (CI) of the first beat was comparable, Kim et al. reported that the CI of the second beat of NSVT was significantly shorter in the malignant group compared to the benign group (313 ± 58 msec vs. 385 ± 83 msec, P<.01).³⁵

In summary, while shorter coupled PVCs are considered a risk factor for triggering malignant ventricular arrhythmias, a definite cut-off between benign and malignant remains elusive.²⁶ Additionally, both short and long CIs may coexist in the same patients who have both benign and malignant PVCs.²⁶

b- Specific or disorganized morphologies reflecting abnormal conduction and electrical substrate

In 2010, Marcus et al. published a proposed modification of the Task Force Criteria for the diagnosis of ARVD,³⁶ introducing electrocardiographic criteria such as right precordial T-wave inversion (TWI) beyond V2 and epsilon waves. However, these well-known ECG criteria are validated for the manifest phase of ARVD with definite ARVD and overt structural cardiomyopathic changes.^3738394041 Therefore, the results of these studies had limited clinical value because they mostly referred to overt ARVD patients, whose ECG and echocardiographic findings are easily distinguishable from those of patients with idiopathic RVOT arrhythmia without the need for analyzing the morphology of the ectopic beats. Hence, the second important topic is to differentiate electrocardiographically the early/concealed phase of ARVD from benign outflow tract VAs.

Localized right precordial QRS prolongation,⁴² terminal activation delay (TAD),³⁹ prolonged S-wave upstroke,⁴³ QRS fragmentation,⁴⁴ or notching,²⁸ epsilon waves,³⁶ and T-wave inversion (TWI) in right precordial leads⁴⁵ reflect localized depolarization and repolarization abnormalities, showing the delay in activation. Fontaine et al. proposed the concept that the conduction abnormalities observed in patients with ARVD result from parietal block without definite alteration of conduction in the bundle branches.⁴⁶ They suggested that a QRS duration in leads V1–V3 that exceeds the QRS duration in V6 by 25 msec or greater in the presence of a right bundle branch block (RBBB) pattern is a marker of parietal block and therefore can be used as a diagnostic criterion for ARVD patients demonstrating a complete RBBB pattern.⁴⁶

Later, the r’/s ratio <1 in V1 in the setting of a complete RBBB has been found to be the best diagnostic marker of ARVD.⁴⁷ Peters et al. first proposed localized right precordial QRS prolongation (defined as the ratio of the sum of the QRS duration in leads V1–V3 versus V4–V6 of ≥ 1.2) as an ECG marker for ARVD.⁴⁸ Parietal block, defined as intra-right ventricular conduction slowing, is a major diagnostic criterion for ARVD; and Tandri et al. reported that the total RV endocardial activation duration (EAD) >65 msec accurately differentiates ARVD from idiopathic VT⁴⁹ as a sensitive marker of intra-RV conduction delay in ARVD. Cox et al. suggested a proposed additional criterion of prolonged TAD in V1–V3 as an indicator of activation delay.³⁸³⁹

The terminal S wave length, area, and duration in the right precordial leads are diagnostically useful and suitable for automatic analysis in ARVD.⁴¹ Nasir et al. reported that prolonged S-wave upstroke in V1 through V3 ≥55 msec was the most prevalent ECG feature (95%) and correlated with ARVD severity among those without RBBB.⁴³ Novak et al. reported that the intrinsicoid deflection time (>80 msec) was an independent predictor of early ARVD.²⁵

Noninvasive cardiac electrophysiology mapping systems have also revealed electrophysiological substrate properties such as nonuniform conduction and fractionated electrograms that differ in the early concealed phase of ARVD patients compared with healthy controls.²²

c- The QRS duration of PVC

The PVC QRS duration and axis have been evaluated in discriminating malignant versus benign forms. A longer QRS duration in VT (and/or sinus rhythm) may indicate the presence of intrinsic electrical disease and slowed conduction, which could facilitate the conversion of benign PVCs to PVT or IVF. QRS complexes in VT can often be broad, reflecting various factors that contribute to QRS prolongation (such as far lateral/nonseptal and often epicardial exits that are distant from the His-Purkinje system, and extensive scar tissue that slows conduction). Longer QRS duration, delayed precordial R/S transition, and QRS notching in lateral leads (leads I and aVL) have been found to be useful in distinguishing ARVD from idiopathic RVOT-VA.²⁸ Bastiaenen et al. reported that maximal QRS duration, number of PVC morphologies, and maximal PVC fragmentation distinguished ARVD patients, including those with incomplete disease expression, from healthy controls and patients with RVOT ectopy.⁵⁰ Ainsworth et al. reported that a QRS duration >120 msec in lead I, combined with a QRS axis <30°, was a sensitive and specific marker for the diagnosis of ARVD.⁵¹ Hoffmayer reported that the most sensitive characteristic for the detection of ARVD was a QRS duration in lead I of ≥120 msec (88% sensitivity, 91% negative predictive value).⁵²

d- The QRS axis of PVC

The PVC QRS axis has also been evaluated in discriminating between malignant and benign forms. Idiopathic RVOT PVCs typically arise from the endocardial and septal portions of the RVOT, while ARVD-related PVCs typically originate from the epicardial layer of the anterior wall.²⁰²⁴ RVOT-VAs with a positive QRS wave in lead I usually arise from the posterior attachment side of the septum or free wall, whereas those with a negative QRS wave in lead I usually arise from the anterior attachment side of the septum or free wall.¹ Kurosaki et al. reported that malignant arrhythmias originating from the RVOT, although rare, should be considered when the patient experiences a syncopal episode and RVOT PVCs with a positive QRS complex in lead I.⁵³ In contrast, a predominantly negative QRS complex in lead I has been found to predict early ARVD as a malignant form.²⁵ Novak et al. reported that an intrinsicoid deflection time >80 msec, QS pattern in lead V1, and QRS axis >90° were independent predictors of early ARVD; however, the coupling interval did not differ between the two groups.²⁵ Since peritricuspid ventricular reentry is a frequent mechanism of VT in patients with ARVD, which can be identified by detailed 3D electroanatomical mapping,⁵⁴ VTs typically exit from the inferior or inferolateral basal region near the tricuspid valve (VT with LBBB morphology and superior axis), or from the outflow tract region (with an inferior axis).³⁹ Hoffmayer et al. reported that a QRS duration in lead I of ≥120 msec, earliest onset QRS in lead V1, QRS notching, and a transition at V5 or later were independent predictors of the presence of early ARVD.⁵² Ainsworth et al. reported that a QRS duration >120 msec in lead I, coupled with a QRS axis <30°, was a sensitive and specific marker for the diagnosis of ARVD.⁵¹ Novak et al. also reported that an intrinsicoid deflection time >80 msec and predominantly negative QRS morphology in lead I had the highest specificity (76%, 95% CI 58–89).²⁵

e- The QRS morphology of PVC

The PVC QRS morphology also assists in the differential diagnosis between idiopathic RVOT arrhythmias and early ARVD.²⁵ The presence of QS morphology in lead V1²⁵ or aVR⁵⁵ derivations, as well as QRS notching, should raise a high clinical suspicion of an underlying cardiomyopathy when evaluating patients with apparently idiopathic RVOT VAs, prompting further in-depth clinical evaluation⁵² (Figure 1). Hoffmayer et al. proposed a scoring system that assigns 3 points for sinus rhythm anterior TWI in leads V1–V3 and during VA; 2 points for QRS duration ≥120 msec in lead I, 2 points for QRS notching, and 1 point for precordial transition at lead V5 or later⁵⁶ (Figure 2 and Figure 3). A score greater than 5 predicts ARVD with a sensitivity of 84% and a specificity of 100%.⁵⁶ Novak et al. reported that an intrinsicoid deflection time >80 msec, QS pattern in lead V1, and predominantly negative QRS complex in lead I were independent predictors of early ARVD. While no single feature was accurate enough to enable differential diagnosis, the combination of these three criteria demonstrated high specificity (91%) for early ARVD. Furthermore, Novak et al. reported that an intrinsicoid deflection time >80 msec, QS morphology in lead V1, and an axis >90° were VEB features independently associated with the diagnosis of ARVD. Conversely, QS morphology in lead V1 and negative QRS polarity in lead I are typical features of arrhythmias arising from the anterior RVOT wall near the pulmonary valve, rather than the septal/postero-septal RVOT region.²⁰ The combination of QS morphology in lead V1, predominantly negative QRS complex in lead I, and an intrinsicoid deflection time >80 msec showed high specificity for an underlying early cardiomyopathy.²⁵ Additionally, the presence of multiple PVC morphologies (≥2 distinct PVC patterns) or a pleomorphic pattern may also be related²⁹ (Figure 3 and Figure 4).

Malignant polymorphic RVOT VAs are rare, and the primary challenge lies in distinguishing the small minority of patients with this malignant disease from the majority of patients with benign idiopathic RVOT VAs.10 Although the PVC triggers for malignant arrhythmias can be successfully ablated with a high success rate, long-term outcomes following ablation are unknown.²⁶ Therefore, another concern arises when discharging RVOT-VA patients without implanting an ICD, even after successful ablation therapy. Structural changes may be absent or subtle and confined to a localized region of the RV in the early stages of the disease, but they commonly progress to more diffuse right ventricular disease and left ventricular involvement. Consequently, the initial benign presentation may not reassure against a malignant outcome later.⁵⁷ Patients should be evaluated for the future requirement of ICDs in specific conditions.¹¹¹⁵

In the era of personalized medicine, next-generation genomic sequencing technologies58 and three-dimensional noninvasive cardiac activation imaging techniques are likely to become even more prevalent in aiding the clinical diagnosis and management of VAs. However, until these technologies become part of daily practice, it is essential to consider the "bear track hypothesis"⁵⁹⁶⁰ before undertaking idiopathic "benign" PVC ablation procedures: "If you see bear tracks in the forest, there is a risk of attack by the bear. However, erasing these tracks might not eliminate this risk."