Idiopathic focal ventricular tachycardia

Mechanism of focal idiopathic ventricular tachycardia

Idiopathic VT comprises multiple discrete subtypes that are best differentiated by their mechanism, QRS morphology, and site of origin. Most forms of focal idiopathic VAs are adenosine-sensitive and are thought to be caused by catecholamine-induced, cyclic adenosine monophosphate (cAMP)–mediated delayed afterdepolarizations (DADs) and triggered activity. Several features of idiopathic VAs support triggered activity as the underlying mechanism (see Chapter 5 ). Heart rate acceleration facilitates VT initiation. This can be achieved by catecholamine infusion or rapid pacing from either the ventricle or the atrium. Additionally, termination of the VT is dependent on direct blockade of the dihydropyridine receptor by calcium channel blockers or by agents or maneuvers that lower cAMP levels (e.g., by activation of the M ₂ muscarinic receptor with edrophonium or vagal maneuvers, inhibition of the beta-adrenergic receptor with beta-blockers, or activation of the A ₁ adenosine receptor with adenosine). Furthermore, a direct relationship exists between the coupling interval of the initiating ventricular extrastimulus (VES) or ventricular pacing cycle length (CL) and the coupling interval of the first VT beat. Additionally, VT initiation is CL dependent; pacing cycle lengths (PCLs) longer or shorter than a critical CL window fail to induce VT. This critical window can shift with changing autonomic tone. Notably, a small proportion (11%) of VAs are insensitive to adenosine.

A subset of idiopathic focal VAs arises from the Purkinje system in either ventricle. Focal Purkinje VTs (classified as “propranolol-sensitive automatic VTs”) are typically provoked by exercise and catecholamines and are suppressed by beta-blockers (but not verapamil). Furthermore, programmed electrical stimulation fails to induce or terminate those arrhythmias. Purkinje VTs are transiently suppressed by adenosine and with overdrive pacing. These characteristics suggest abnormal automaticity as the underlying mechanism of focal Purkinje VTs, in contrast to the reentrant verapamil-sensitive fascicular VT. Purkinje VAs can manifest as monomorphic VT or PVCs or as an accelerated idioventricular rhythm that competes with and can be suppressed by NSR.

Other subtypes of idiopathic VAs include verapamil-sensitive, reentrant fascicular VT (see Chapter 28 ) and idiopathic polymorphic VT and ventricular fibrillation (VF) (see Chapter 35 ). Those arrhythmias are discussed elsewhere in this book. In this chapter, focal idiopathic monomorphic PVCs and VT are collectively referred to as “idiopathic ventricular arrhythmias (VAs).”

Mechanism of PVC-induced cardiomyopathy

Frequent idiopathic nonsustained VT or PVCs can precipitate a reversible form of dilated cardiomyopathy. The mechanisms of how frequently PVCs cause LV systolic dysfunction have yet to be elucidated. Potential mechanisms include: (1) dyssynchronous ventricular contraction caused by abnormal ventricular activation during PVCs (similar to LBBB or ventricular pacing); (2) alterations in intracellular calcium handling and membrane ionic currents (caused by postextrasystolic potentiation, which can be associated with cytosolic calcium overload and increased oxygen consumption); (3) abnormal ventricular filling (due to premature contraction followed by postextrasystolic pause); (4) myocardial and peripheral vascular autonomic dysregulation; and (5) concealed mechanical bradycardia (caused by inability of PVCs to generate sufficient ejection volume or pressure to allow opening of the aortic valve), which results in decreasing cardiac output and increasing LV diastolic pressures, leading to volume overload and LV dilation. Although PVC-induced cardiomyopathy was originally thought to be a type of tachycardia-induced cardiomyopathy, tachycardia is an unlikely mechanism since the overall heart rates in patients with frequent PVCs have remained within the normal range due to the usual compensatory pause following the PVC. Nonetheless, interpolated PVCs can increase the overall heart rate.

The most prominent predictor of cardiomyopathy in patients with frequent PVCs appears to be the daily burden of PVCs. However, the PVC burden necessary to induce LV dysfunction is not yet clearly defined. Although a PVC burden as low as 4% to 10% can be associated with cardiomyopathy, a PVC burden of more than 13% appears more likely to cause PVC-induced cardiomyopathy, and a cutoff PVC burden of 24% has been proposed as having the best sensitivity (79%) and specificity (78%) for the prediction of cardiomyopathy.

Considerable variability exists in susceptibility to PVC-related cardiomyopathy, even among patients with high PVC burden. Multiple other factors were linked to the development of PVC-induced cardiomyopathy ( Table 27.1 ). However, it is important to recognize that there is inconsistent evidence as to whether these factors represent independent predictors of PVC-induced cardiomyopathy. Furthermore, a significant overlap in these parameters usually exists between groups with and without PVC-induced cardiomyopathy, limiting their prognostic utility.

Electrocardiography

The surface ECG during NSR is usually normal. Up to 10% of patients have complete or incomplete RBBB. Other ECG abnormalities may be observed when idiopathic VAs coexist with structural heart disease.

Idiopathic focal VAs are characterized by frequent monomorphic PVCs, couplets, or salvos of nonsustained VT, interrupted by brief periods of NSR ( Fig. 27.1 ). Paroxysmal exercise-induced VT is characterized by sustained episodes of VT precipitated by exercise or emotional stress ( Fig. 27.2 ). The tachycardia rate is frequently rapid (CL <300 milliseconds) but can be highly variable. A single morphology for the VT or PVCs is characteristic. QRS morphology during VAs on the surface ECG is discussed later in this chapter.

Ambulatory cardiac monitoring

Holter monitors can assess the burden of VAs and their correlation to symptoms. Extended mobile cardiac telemetry is more appropriate when symptoms occur less frequently.

Several characteristics of idiopathic VAs can be observed on ambulatory cardiac monitoring. Idiopathic VAs typically occur at a critical range of heart rates (CL dependence). The coupling interval of the first PVC is relatively long (approximately 60% of the baseline sinus CL). A positive correlation exists between the sinus rate preceding the VT and the VT duration. Additionally, the VT occurs in clusters and is most prevalent on waking and during the morning and later afternoon hours. Catecholamines increase during waking hours and nocturnal decrease in catecholamine levels and predominance of vagal tone might account for the differences in the circadian PVC burden.

In general, there are three distinct PVC profiles based on the correlation of hourly PVC frequency distribution to baseline heart rate changes assessed on Holter monitoring; a positive correlation (i.e., PVCs burden is higher at faster heart rates) is observed in approximately 50% of patients. Such a diurnal profile predicts higher probability of PVC suppression by beta-blocker therapy as well as more likelihood of inducibility with isoproterenol infusion. A slow heart rate dependent-PVC profile is observed in a minority of patients, often younger, healthier, and, more frequently, females. Approximately one-third of patients demonstrate a PVC diurnal profile independent of heart rate changes. The lack of circadian fluctuation in PVCs frequency has been correlated with higher prevalence of heart disease as well as increased risk of PVC-induced cardiomyopathy.

Idiopathic focal VAs are very sensitive to autonomic influences, resulting in substantial day-to-day variability in the density of PVCs. Therefore, a single 24-hour recording may not reflect the true PVC burden. When frequent PVCs are strongly suspected, mobile cardiac telemetry extended over 7 days has been found to accurately reflect overall PVC burden.

Exercise electrocardiography

Exercise stress testing is recommended for evaluation of selected patients with exercise-related symptoms but may not be clinically helpful in those in whom the arrhythmias have already been documented on ambulatory recordings.

In patients presenting with paroxysmal VT, exercise testing can reproduce patients’ clinical VT 25% to 50% of the time. Exercise-provoked VT usually manifests as nonsustained or, less often, sustained VT. In those with isolated PVCs, the frequency of PVCs tends to increase during exercise. The VT can be initiated either during the exercise test or during the recovery period. Both scenarios likely represent examples of the dependence of VT on a critical window of heart rates for induction. This window can be narrow and only transiently present during exercise, resulting in induction of VT only during recovery. In some patients with repetitive monomorphic VT, the VT can become suppressed during exercise. The response of VAs to exercise can be helpful in planning the strategy for arrhythmia induction during an ablation procedure.

CPVT, also exercise dependent, can be distinguished from idiopathic focal VT by the alternating QRS axis with 180° rotation on a beat-to-beat basis (bidirectional VT) or polymorphic VT, which can degenerate into VF.

Echocardiography

Transthoracic echocardiography is necessary to exclude other causes of PVCs and for the assessment of LV impairment. Importantly, LVEF can be difficult to assess in patients with frequent ventricular ectopy and attempts should be made to assess the LVEF during cardiac cycles where no PVCs are observed. Nonetheless, LVEF is better determined by echocardiography over other non–real-time imaging modalities, including CMR, since the presence of frequent PVCs can lead to image corruption from an incorrect combination of data from single slices acquired over multiple cardiac cycles.

Cardiac magnetic resonance

CMR is of value when structural heart disease is suspected despite a normal echocardiogram. CMR can help exclude arrhythmogenic RV cardiomyopathy (ARVC), amyloidosis, and sarcoidosis, among other diseases, and can also evaluate the presence and extent of myocardial fibrosis in patients with LV dysfunction.

It is important to recognize that CMR can reveal concealed myocardial structural abnormalities involving the RV and the LV in 15% to 20% of patients with apparently idiopathic VAs, including subepicardial or midmyocardial foci of fibrosis, acute inflammation, and focal fatty infiltration. Of note, the morphology of VAs appears to be related to the presence of such abnormalities on CMR. In contrast to the infrequent abnormalities noted among patients with VAs of LBBB pattern, CMR detected myocardial structural abnormalities (mainly involving the LV inferior and lateral wall) in a sizable proportion (41%) of patients with unexplained VAs with RBBB configurations consistent with an LV origin. Whether these abnormalities are causally related to the observed VAs is uncertain; however, the presence of myocardial structural abnormalities on CMR seems to predict spontaneous sustained arrhythmias among patients presenting with apparently idiopathic VAs of LV origin.

Arrhythmogenic right ventricular cardiomyopathy

RVOT VAs should, in particular, be distinguished from ARVC, a disorder with a more serious clinical outcome (see eTable 33.1 ). Although RVOT VAs are associated with a benign prognosis with no familial basis, it can be extremely difficult to distinguish from the concealed phase of ARVC, in which typical ECG and imaging abnormalities are absent. The VT in ARVC also affects young adults, is commonly catecholamine facilitated, and can originate from the RVOT. The distinction between the two entities has important prognostic and therapeutic implications, as discussed in detail in Chapter 33 .

Dilated cardiomyopathy

Idiopathic VAs can coexist with structural heart disease, including cardiomyopathy, in which setting the VAs may not be related to the cardiac disease. In a recent report, focal VTs, often originating from sites classic for otherwise “idiopathic” forms of VT, were inducible in 16% with structural heart disease (predominantly dilated cardiomyopathy) undergoing VT ablation. Those patients frequently presented with repetitive failure of VT termination by ICD antitachycardia pacing or shocks.

On the other hand, very frequent idiopathic nonsustained VT or PVCs can precipitate dilated cardiomyopathy that can improve or completely resolve after elimination of the VAs. Therefore, in patients who present with a dilated cardiomyopathy of unclear etiology and who have frequent PVCs or nonsustained VT, it is important to assess the contribution of VAs to LV systolic dysfunction. Failure to recognize a causal relationship between the arrhythmia and LV dysfunction can have important consequences. These patients can be misdiagnosed with “idiopathic” dilated cardiomyopathy and may not be offered therapeutic measures to reduce the PVC burden (e.g., catheter ablation), which can significantly impact LV systolic function and long-term outcome.

A reversible form of dilated cardiomyopathy precipitated by idiopathic VAs should be suspected when frequent PVCs or nonsustained VT are observed on a 24-hour Holter recording, especially when the QRS morphology is monomorphic and is suggestive of origin from typical locations of idiopathic VAs.

The causal relationship between the PVCs and the cardiomyopathy can be challenging to ascertain, and the diagnosis of PVC-induced cardiomyopathy is one of exclusion and often retrospective, verified only when elimination of the PVCs (typically by catheter ablation) results in recovery of LV function or based on the relative timing of PVCs and cardiomyopathy. Although pharmacological suppression of the PVCs, such as by amiodarone therapy, may help evaluate the relationship between the arrhythmia and cardiomyopathy, the usefulness of this approach and duration of therapy required have not been defined.

Predictors of reversibility of LV systolic dysfunction in patients with PVC-induced cardiomyopathy include the absence of myocardial scar on CMR, a large PVC burden at baseline, the absence of intraventricular conduction delay during sinus rhythm, and the absence of LV enlargement. Although no single cutoff value of PVC burden completely discriminates reversible from irreversible LV dysfunction, a burden of ≥13% at baseline was shown to predict recovery of LVEF following ablation of PVCs. Of note, early improvement in LVEF (at 1-week follow-up) post PVC ablation was shown to predict complete recovery of LV systolic function. One report suggested that patients with PVC QRS duration of 170 milliseconds or longer are unlikely to normalize their LV function after ablation of the PVCs, suggesting that PVC QRS duration can be a marker of the presence and severity of underlying structural heart disease.

Given the large magnitude of potential improvement in LV systolic function that could be achieved by elimination of frequent PVCs (regardless of whether the cardiomyopathy is in fact PVC-induced or PVC-worsened), evaluation for the presence of frequent PVCs (by ambulator cardiac monitoring) needs to be considered in all patients with depressed LVEF. This is particularly important in patients who meet criteria for prophylactic ICD implantation, as the recovery of LVEF following ablation can potentially remove the primary prevention indication for ICD implantation.

Idiopathic ventricular fibrillation

Idiopathic monomorphic focal VAs, although generally considered “benign,” can also trigger polymorphic VT and VF in some patients with idiopathic VF and no apparent structural heart disease (see Fig. 35.25 ). Hence, for patients presenting with idiopathic VF, it is imperative to pay special attention to recording potential monomorphic PVC “triggers” since catheter ablation of those foci can potentially prevent further episodes of VF or reduce the burden of arrhythmias. Also, patients with polymorphic VT and VF should be carefully evaluated for underlying channelopathies.

Even in patients with no prior history of VF, it is also particularly important to distinguish the “malignant” from “benign” forms of idiopathic monomorphic VAs, because the malignant form often leads to unexpected SCD. However, identification of patients presenting with idiopathic focal VAs carrying some risk for VF remains challenging. Malignant VAs should be suspected in patients with known idiopathic focal VAs who present with syncope. Patients with idiopathic VF frequently (57%) experience syncopal episodes before presenting with cardiac arrest.

Several ECG parameters have been suggested as markers of malignant OT VAs. However, the diagnostic value of these ECG parameters is inconsistent, and their clinical utility is limited. Although idiopathic VF and polymorphic VT can arise from triggers located in the RVOT and LVOT, PVCs arising from the Purkinje network, RV moderator band, papillary muscles, and apical cardiac crux, due to unclear mechanism, appear to be particularly prone to induce sustained VT and VF, leading to syncope or cardiac arrest. Therefore, VAs with QRS morphology suggestive of those sites of origin should be carefully scrutinized.

The prognostic implications of the coupling intervals of clinical PVCs have been debated. Although no absolute coupling interval cutoff identifies potentially “malignant” PVCs, shorter coupling interval to the preceding QRS complex and a shorter CL during monomorphic VT were found to predict the coexistence of VF or polymorphic VT in patients with idiopathic RVOT VAs; however, significant overlap exists. In contrast, one report showed that the prematurity index (defined as the ratio of the coupling interval of the first VT beat or isolated PVC to the preceding R-R interval of the sinus cycle just before the VT or isolated PVC), but not the coupling interval, was the only independent predictor for polymorphic VT. A more recent report found that the second coupling interval of nonsustained VT was significantly shorter in malignant OT VT than in benign OT VT. A second coupling interval of nonsustained VT of less than 317 milliseconds could predict a malignant OT VT with modest diagnostic accuracy (sensitivity 58%, specificity 87.5%). However, prospective validation of these criteria is yet to be tested in a larger population.

Other arrhythmia mechanisms

Idiopathic OT VT should be differentiated from other forms of VT with an LBBB pattern, including bundle branch reentrant VT (see Chapter 30 ), reentrant VT following surgical repair of congenital heart disease (see Chapter 34 ), and postinfarction VT originating from the LV septum (see Chapter 26 ). In addition, antidromic AV reentrant tachycardia using an atriofascicular bypass tract also presents with wide complex tachycardia with an LBBB pattern (see Chapter 23 ). Often, the diagnosis of these VTs can be established without difficulty given the frequently associated structural heart disease. Nevertheless, idiopathic focal VT also can occur in patients with unrelated structural heart disease, and the distinction of those “ablatable” VTs has important prognostic and therapeutic implications.

Acute management

Acute termination of most forms of idiopathic VT can be achieved by vagal maneuvers or intravenous administration of adenosine. Intravenous verapamil is an alternative, provided the patient has adequate blood pressure and has a previously established diagnosis of a VT that is sensitive to verapamil. Hemodynamic instability warrants emergency cardioversion.

Chronic management

Conservative management

For patients with idiopathic VAs asymptomatic or only mildly symptomatic PVCs, normal LVEF, and no underlying disease on cardiac evaluation, reassurance and counseling, with no pharmacological therapy, are the preferred treatments given the generally benign long-term prognosis. Annual follow-up with ambulatory ECG monitoring and echocardiography is recommended in those with frequent PVCs (>10,000 PVCs per 24 hours) to monitor for possible development of cardiomyopathy, which should prompt initiation of therapy ( Fig. 27.3 ).

Not infrequently, patients with symptomatic PVCs are worried that their palpitations indicate serious cardiac disease, and once reassured as to the benign nature of their condition, many patients prefer no treatment or only trying lifestyle modifications over pharmacological or invasive options. Behavioral management (such as limiting caffeine and alcohol consumption, stress management, and physical conditioning) is a reasonable concomitant initial strategy, but efficacy is limited. Nevertheless, when there is a concern in patients about certain lifestyle factors contributing to frequent PVCs (such as alcohol and anxiety), ambulatory cardiac monitoring can help make the link between those factors and PVC clusters. In these cases, advocating for a trial avoiding the aggravating factor(s) may be useful. However, these interventions should be individualized, and currently there is insufficient evidence to recommend a broad set of lifestyle modifications in unselected patients.

On the other hand, many patients with frequent PVCs do not complain of palpitations and present primarily with occult cardiac symptoms, such as lack of energy and effort intolerance. Such complaints should not be dismissed as “unrelated,” and those patients should not be labeled “asymptomatic.” Treatment to reduce the burden of PVCs should be considered to alleviate symptoms. Similarly, treatment is recommended for patients who continue to have significant symptoms despite reassurance and those who develop cardiomyopathy. Additionally, in heart failure patients who are clinical nonresponders to cardiac resynchronization therapy in whom PVCs cause loss of effective biventricular pacing, treatment is recommended to reduce the burden of PVCs and optimize biventricular pacing.

Although frequent PVCs can precipitate LV systolic dysfunction in some patients who are otherwise asymptomatic, there is currently no risk stratification model to reliably identify patients at risk. Therefore, prophylactic PVC elimination for the sole purpose of preventing PVC-induced cardiomyopathy is not recommended.

Lead V ₁ polarity

Lead V ₁ is a unipolar lead positioned at the right anterior chest wall. Therefore, VAs originating from anterior and rightward structures are expected to produce a predominantly negative deflection (i.e., LBBB pattern), whereas more posterior sites of origin produce more positive deflections (RBBB pattern). Since the RVOT and pulmonary artery are the most anterior cardiac structures, all VAs originating from these regions have an LBBB configuration, with rare exceptions. Similarly, VAs originating from the anterior aspect of the LVOT (e.g., right aortic sinus of Valsalva [ASV]) typically exhibit an LBBB pattern.

Precordial transition

In PVCs with an LBBB pattern, a precordial transition from negative to positive (i.e., the first precordial lead in which the R wave amplitude exceeds the S wave amplitude) occurs earlier as the site of the PVC origin shifts from the RV free wall toward the interventricular septum. In PVCs with an RBBB pattern, a precordial transition from positive to negative occurs earlier as the site of the PVC origin shifts from the LV base to the apex.

As the site of VT origin shifts progressively more posteriorly and leftward from the RVOT free wall toward the lateral mitral annulus (sequentially passing through the medial aspect of the RVOT, right ASV, left ASV, and LV summit), the precordial transition becomes sequentially earlier, thereby transforming the precordial QRS morphology from a late transition LBBB pattern to a positively concordant RBBB pattern.

Lead V ₆ polarity

Lead V ₆ is located near the apex of the heart. A predominantly negative lead V ₆ (QS pattern) suggests a PVC origin near the apex (e.g., apical cardiac crux), whereas a predominantly positive lead V ₆ polarity suggests origin in the base of the ventricle (e.g., ventricular OT). An R/S wave amplitude ratio <1 in lead V ₆ suggests an origin in the middle of the ventricle (e.g., LV papillary muscles, left fascicles).

Frontal plane horizontal axis

Lead I primarily reflects the horizontal axis. Structures closer to the left axilla will produce a deeply negative complex in lead I (i.e., rightward axis); conversely structures closer to the right axilla are strongly positive in lead I (i.e., leftward axis). Furthermore, leads II/aVL and III/aVR also have net leftward and rightward vectors, respectively. Hence, as the site of origin of the VT moves progressively to the left (progressively from most rightward to leftward aspects of the RVOT, to the left ASV, LV summit, and lateral mitral annulus) the QRS assumes progressively less positive/more negative deflection in lead I, a taller R wave in lead III than in lead II, and a larger S wave in lead aVL compared to lead aVR.

Frontal plane vertical axis

The inferior leads (II, III, and aVF) reflect the vertical axis. Given the high anatomical location of the left and right OTs in relation to the rest of the ventricles, all OT VAs exhibit positive deflections in the inferior leads (vertical axis). However, the magnitude of the inferiorly directed vector diminishes as the site of origin shifts from superior to inferior regions of the OT (such as from the subpulmonic region to the para-Hisian region). Additionally, leads aVL and aVR exhibit QS complexes in the vast majority of OT VAs, as these two leads are not only leftward and rightward but also superior leads. Therefore, VAs originating from the superior aspect of the ventricles produce negative deflection in leads aVL and aVR, whereas VAs arising from more apical locations exhibit positive deflections.

In contrast, VAs arising from portions of the heart that are attitudinally inferior in the chest (such as the mid- to apical aspect of the RV, inferior interventricular septum, cardiac crux, and LV inferior septum and posterior papillary muscle) exhibit a superior frontal axis.

Notably, the presence of inferior lead discordance (predominantly positive QRS complex in lead II with negative QRS complex in lead III, or vice versa) suggests an exit from a midcavitary structure and is highly specific for a limited number of anatomical locations, namely, the para-Hisian region and RV papillary muscle/moderator band for positive/negative discordance (frontal axis of −30° to +30°) and the LV anterolateral papillary muscle for negative/positive discordance (frontal axis of +150° to +210°).

QRS duration and notching

Relatively narrow QRS complexes without notching often originate in the ventricular septum or in proximity to the His-Purkinje system (HPS; due to parallel activation of both ventricles), whereas VAs originating from the free ventricular walls often exhibit wider QRS complexes with the presence of a middle or late notching in the QRS complex (because of sequential ventricular activation).

Epicardial sites of origin

Idiopathic VAs can originate from epicardial ventricular locations, namely the LV summit and cardiac crux. ECG characteristics that can predict those epicardial sites of origin are mainly based on the concept that when ventricular activation starts at the epicardial level, the initial part of the wavefront progresses slowly through the myocardial wall until reaching the Purkinje system, which is located subendocardially. This slow transmural activation is reflected as slurred onset of the QRS on the surface ECG. Furthermore, propagation of ventricular activation from the epicardial surface results in a QS pattern in the overlying ECG leads. This contrasts with endocardial VT exit sites, which produce an initial “r” wave in the corresponding ECG leads, reflecting transmural ventricular activation in an endocardial-to-epicardial direction.

Several ECG characteristics can help distinguish epicardial idiopathic VTs from endocardial arrhythmias ( Table 27.2 ), including: (1) pseudodelta wave (so-called because of its similarity to the slurred upstroke delta wave observed during ventricular preexcitation) ≥34 milliseconds (measured from the earliest ventricular activation to the earliest fast deflection in any precordial lead); (2) long R-wave peak time in lead V ₂ (i.e., interval from the beginning of the QRS complex to the time of initial downstroke of the R wave after it has peaked [previously known as the intrinsicoid deflection ]) >85 milliseconds; (3) shortest RS complex duration (measured from the earliest ventricular activation to the nadir of the first S wave in any precordial lead) >120 milliseconds; and (4) QRS duration >200 milliseconds.

Furthermore, the degree of initial QRS slurring, as measured by the maximal deflection index (defined as the shortest time from QRS onset to maximal deflection [i.e., the largest positive or negative amplitude deflection] in any of the precordial leads divided by the total QRS duration), can help identify epicardial sites of origin. A delayed shortest precordial maximal deflection index (≥0.55) favors an epicardial origin and discriminates LV summit VTs from those originating within the aortic sinuses of Valsalva. Similarly, a peak deflection index (determined in the inferior lead presenting the tallest R wave by dividing the time from the QRS onset to the peak QRS deflection by a total QRS duration) of >0.6 predicts an epicardial site of origin.

Electrocardiographic algorithms

A recently proposed ECG algorithm summarizes the major specific ECG features, taking into consideration several ECG principles discussed above ( Fig. 27.4 ). This ECG algorithm, however, does not differentiate the detailed sites of origins in some regions. Several other ECG-based algorithms were developed to distinguish between sites of origin of VAs arising from a predefined region of the heart (e.g., algorithms for localization of VAs origination from the ventricular OT region). Those latter algorithms are discussed later in the chapter ( Table 27.3 ).

Induction of tachycardia

Not infrequently, idiopathic VAs foci can become inactive in the EP laboratory environment, likely due to sedative medications or deviation from daily routine activities (e.g., exercise or caffeine intake) that can affect VT activity. Thus, in preparation for the ablation procedure, antiarrhythmic drugs usually are withheld for at least five half-lives before the EP study and, if possible, minimal sedation is used throughout the procedure. Additionally, it may be appropriate to monitor the patient in the EP laboratory initially without sedation. If no spontaneous tachycardia is observed, isoproterenol is administered. If no VAs could be induced during isoproterenol administration or during the washout phase, a single quadripolar catheter may be placed in the RV, and programmed electrical stimulation is performed. If VT focus remains quiescent, the procedure may be aborted and reattempted at a future date. If the clinical VAs are inducible at any step, the full EP catheter arrangement and EP study are undertaken.

Several pharmacological agents may be used to enhance inducibility of idiopathic VAs. Isoproterenol infusion (targeting a 20%–30% increment in heart rate) is the first choice. Importantly, VAs can develop (with and without programmed electrical stimulation) only during the washout phase after discontinuation of isoproterenol (analogous to VAs occurring during the recovery phase postexercise). Additionally, epinephrine infusion following administration of isoproterenol can potentially improve inducibility of PVCs. Of note, the variable circadian distribution of PVCs can predict the likelihood of drug response in the EP laboratory. Likelihood of inducibility with isoproterenol infusion is higher when the PVC diurnal profile on Holter monitoring shows larger PVC burden at faster baseline heart rates.

If needed, intravenous atropine and aminophylline may be sequentially administered (with and without programmed electrical stimulation) to attenuate the potential antiarrhythmic effects of endogenous acetylcholine and adenosine, which inhibit cAMP. Calcium infusion can also facilitate arrhythmia induction.

Ventricular stimulation can initiate the VAs in less than 65% of patients and, in contrast to reentrant VT, rapid ventricular pacing is usually more effective than VES. Induction of sustained VT is less common in patients with repetitive monomorphic VT than in those with paroxysmal sustained VT. In one report, sustained VT was inducible during EP testing in 78% of patients who presented clinically with sustained VT, in 48% of patients who presented with nonsustained VT, and in 4% of those with PVCs only. Most episodes of triggered activity VT induced by ventricular stimulation are usually nonsustained. Reproducibility of VT induction using all methods is less than 50%, whereas reproducibility of induction with single or double VESs is approximately 25%. Induction with atrial pacing is not uncommon.

Typically, the initial cycle of the VT bears a direct relationship to the PCL (whether or not VESs are delivered following the pacing drive). Thus, the shorter the initiating PCL, the shorter the interval to the first VT beat and the shorter the initial VT CL. Similarly, the initial cycle of the VT bears a direct relationship to the coupling interval of the VES initiating the VT. Occasionally, with the addition of very early VESs or with very rapid ventricular pacing (PCL <300 milliseconds), a sudden jump in the interval to the first VT complex may be observed, such that it is approximately twice the interval to the onset of the VT initiated by later coupled VESs. This can be caused by failure of the initial delayed afterdepolarization (DAD) to reach threshold, whereas the second DAD reaches threshold. Thus, in triggered activity VAs caused by DADs, the coupling interval of the initial VT complex either shortens or suddenly increases in response to progressively premature VES; it usually does not demonstrate an inverse or gradually increasing relationship (in contrast to reentrant VT).

Of note, successful induction of VAs can be linked to a critical window of heart rates. Ventricular PCLs longer or shorter than the critical CL window may fail to induce VT. This critical window can shift with changing autonomic tone. Therefore, it is important to perform ventricular pacing at a wide spectrum of CLs to define the range of CLs most successful in inducing the VAs. The site of ventricular stimulation has no effect on the initiation of triggered activity VT as long as the paced impulse reaches the focus of the VT (in contrast to reentrant VT).

VT induction can be inconsistent. Induction is exquisitely sensitive to the immediate autonomic status of the patient. Therefore, noninducibility during a single EP study is not enough evidence to attribute the arrhythmia to a nontriggered activity mechanism.

Tachycardia features

Idiopathic focal VAs can manifest in the form of sustained VT or, more commonly, frequent monomorphic PVCs, couplets, and salvos of nonsustained VT. When sustained, the VT rate is frequently rapid (CL <300 milliseconds) but can be highly variable.

During VT, the His potential follows the onset of the QRS (i.e., negative His-ventricular [HV] interval) and is usually buried inside the local ventricular electrogram, but it can precede the QRS in the setting of VAs originating from the HPS. Ventriculoatrial conduction may or may not be present. The VT is very sensitive to adenosine, Valsalva maneuvers, carotid sinus massage, edrophonium, verapamil, and beta-blockers.

Diagnostic maneuvers during tachycardia

Idiopathic focal VT cannot be entrained by overdrive ventricular pacing (i.e., no QRS fusion demonstrable). Notably, rapid ventricular pacing during the VT can result in acceleration of the VT. Additionally, a direct relationship exists between the overdrive PCL and the coupling interval and CL of the VT resuming after cessation of pacing, characteristic of DAD-related triggered activity. VES results in a decreasing resetting response curve characteristic of DAD-related triggered activity.

Activation mapping

Activation mapping can be performed by point-by-point mapping with a standard mapping-ablation catheter, a high-density multielectrode mapping catheter, or multielectrode arrays. In idiopathic focal VAs, activation mapping is typically performed in conjunction with electroanatomical mapping.

The goal of activation mapping is to identify the site of origin (focus) of VT/PVCs, defined as the site with the earliest bipolar recording preceding the surface QRs (typically more than 20–25 milliseconds presystolic) with a QS unipolar electrogram configuration.

Initially, one should seek the general region of the origin of VAs as indicated by the surface ECG. All 12 leads should be inspected and the lead showing the earliest and most discernible onset of the QRS during VT or PVCs should be selected as the reference point for subsequent mapping. It is important to record examples of VT or PVCs prior to inserting catheters, because the catheters can cause ectopic complexes that resemble the target VT or PVCs ( eFig. 27.1 ).

Activation times are generally measured from the onset or the first rapid deflection of the bipolar electrogram to the earliest onset of the QRS on the surface ECG during VT or PVCs. Once an area of relatively early local activation is found, small movements of the catheter tip in that region are undertaken until the site is identified with the earliest possible local activation relative to the tachycardia complex. Bipolar electrograms at the site of origin are modestly early (preceding the surface QRS by 20–45 milliseconds) and have high-amplitude and rapid slew rates. Fractionated complex electrograms and middiastolic potentials are not typically observed at sites of origin of idiopathic VAs.

Once the site with the earliest bipolar signal is identified, the unipolar signal from the distal ablation electrode should be used to supplement conventional bipolar mapping ( Fig. 27.5 ). The unfiltered (0.05 to >300 Hz) unipolar electrogram morphology should show a monophasic QS complex with a rapid negative deflection. Although this electrogram configuration is very sensitive for successful ablation sites, it is not specific (70% of unsuccessful ablation sites also manifest a QS complex). In fact, a QS configuration can be observed with unipolar recordings from a relatively large area (exceeding 10 mm in diameter and significantly larger than the VT focus), since the entirety of the heart is directed away from most locations in the ventricular OT region. The timing of the unipolar electrograms recorded at those sites distant from the VT focus, however, is later than that at the site of origin of the VT. Thus, a QS complex should not be the only mapping finding used to guide ablation ( Fig. 27.5 ). Nonetheless, successful ablation is unusual at sites with an RS complex, because these are generally distant from the VT focus.

The timing of the unipolar electrograms is also important. Concordance of the timing of the onset of the bipolar electrogram with that of the filtered or unfiltered unipolar electrogram, with the rapid downslope of the S wave of the unipolar QS complex coinciding with the initial peak of the bipolar signal, helps ensure that the tip electrode, which is the ablation electrode, is responsible for the early component of the bipolar electrogram ( Fig. 27.5 ). Additionally, the presence of slight ST elevation on the unipolar recording and the ability to capture the site with unipolar pacing are used to indicate good electrode contact.

In addition to the electrogram temporal relationship assessment, concordant negativity in the initial forces (first 20 milliseconds) of both unipolar and bipolar electrograms further improves the accuracy of conventional mapping to localize the site of origin of PVCs.

Careful catheter manipulation during mapping should seek to avoid mechanical trauma that can transiently abolish the arrhythmia. Importantly, catheter manipulation frequently induces PVCs that can closely mimic target PVCs. The findings on the mapping catheter during these catheter-induced PVCs are invariably excellent (e.g., substantial presystolic activation time, sharp QS on the unipolar recording). These complexes must be analyzed and carefully compared with prerecorded VT or PVC complexes to avoid delivery of RF energy at sites with no relevance to actual VT.

It is important to recognize that some myocardial fibers in the RVOT can potentially be in continuity with those in the LVOT. Consequently, when mapping is confined to the RVOT, the endocardial site with the earliest presystolic activation time does not necessarily indicate the focus of the tachycardia. This can be observed especially in tachycardias originating in the supravalvular ASVs, whereby the activation wavefront after leaving the focus exits both to the infra-valvular LVOT myocardium and to the posterolateral subpulmonic RVOT. Then, mapping in the RVOT merely localized the breakthrough site, where ablation would not eliminate the arrhythmia.

When mapping above the aortic or pulmonary valve, near- and far-field potentials are typically recorded. The exact nature and cause of these spikes are unknown but are thought to be analogous to pulmonary vein potentials; that is, they represent electrical activation of myocardial sleeves distal to valve attachment. Because of the overlapping nature of the OT and supravalvular region, when two potentials are seen, only the near-field potential should be used for activation timing. In addition to noting the actual timing of activation, the timing of the near-field electrogram relative to the far-field electrogram should be evaluated. Typically, in sinus rhythm, the near-field potential (representing local supravalvular myocardial activation) is recorded after the far-field ventricular electrogram (likely representing OT activation) separated by an isoelectric period (presumably from conduction delay across the site of valve insertion). If the sequence of activation is reversed during VT or PVC, that is, the near-field electrogram precedes the far-field electrogram by a similar or greater isoelectric period duration, an etiological role for the supravalvular myocardium can be inferred. This finding alone, however, does not suggest that ablation at this particular site will be successful because other supravalvular locations may show earlier activation than the site being mapped. On the other hand, if the near-field electrogram still succeeds the far-field ventricular electrogram during VT or PVC, then the supravalvular tissue, although present, is a bystander being passively activated during arrhythmia originating in the OT myocardium below the valve. In some cases, the near-field electrogram is fused with the far-field electrogram during tachycardia. This suggests an origin of arrhythmia exactly at the ASV or passive activation from a true distant site of origin to both the supravalvular and infravalvular myocardium.

Patience is required during mapping of idiopathic arrhythmias, because of the rapidity of conduction from one site to another, the activation time of an “early” site that is >5mm distant from the actual source (i.e., greater than the radius of an RF application) may be a mere 5 to 10 milliseconds different from the source activation time. Diligent searching and dense mapping in earliest regions are necessary to identify the optimal ablation site.

Pace mapping

Given the focal nature of idiopathic VAs, comparing the paced QRS configuration with that of VT/PVC is particularly useful for locating the arrhythmia focus in a structurally normal heart. Pace mapping is used to confirm the results of activation mapping, but it also can be of great value as the primary mapping technique when the VAs are scarcely inducible. Although there are some limitations to this technique, several studies have reported relatively high success rates (80%) for ablation guided primarily by pace-mapping techniques.