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Ablation of Ventricular Outflow Tract Tachycardias
Key Points
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Ventricular outflow tract tachycardias include the right or left ventricular (LV) outflow tracts, aortic cusps, pulmonary artery, and the corresponding epicardium (LV summit region).
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The mechanism underlying outflow tract arrhythmias is usually triggered activity, and the site of origin is focal. Activation mapping during arrhythmia is optimal to identify the site of earliest activation as the target for ablation.
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Mapping of outflow tract tachycardias is often enhanced by the use of electroanatomic mapping systems. Computed tomography, intracardiac echocardiography, and coronary angiography are important adjuncts for mapping and ablation of coronary cusp and epicardial sites of origin.
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A comprehensive understanding of the anatomy of the outflow tract region is necessary to maximize safety and efficacy of ablation of outflow tract ventricular arrhythmias
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Challenges to ablating outflow tract tachycardias include mapping and ablation of noninducible tachycardias, and ablation of tachycardias originating from regions that are difficult to target because of anatomic constraints (including the LV summit region, parahissian, and/or intraseptal location).
Ventricular tachycardias (VTs) most frequently occur in patients with structural heart disease. However, approximately 10% of VTs occur in patients with a normal electrocardiogram (ECG), echocardiogram, and coronary angiography, and are hence considered to be idiopathic VTs (IVTs).
IVTs have been classified based on the ventricle of origin, ECG morphology (QRS configuration and axis), clinical pattern of the tachycardia (repetitive, nonsustained, or sustained), evidence of catecholamine dependency, and response to pharmacologic agents. In general, the clinical history, combined with the response of the arrhythmia to programmed stimulation, adenosine, verapamil, and propranolol may help differentiate among different types of IVTs.
This chapter will discuss briefly the mechanism and common clinical presentations of idiopathic outflow tract ventricular tachycardias (OTVTs) and will provide an in-depth description regarding the anatomic and electrocardiographic localization of these arrhythmias and current state of ablation strategies.
Mechanism
The most common origins of IVTs are the right and left ventricular outflow tracts (RVOT, LVOT). In earlier experiences, the majority of OTVTs originated from the RVOT (70%) and less frequently (20%) from the aortic cusps. However, in the more recent reports, OTVTs appear to be equally distributed between the RVOT and aortic cusps. In a minority of patients (10%), the site of origin can be the basal LV pulmonary artery or the LV epicardium.
Idiopathic OTVTs usually exhibit one of three clinical phenotypes: repetitive monomorphic premature ventricular complexes (PVCs; most common), nonsustained monomorphic VT, or exercise-induced sustained VT (least common). Considerable overlap may occur among the three phenotypes. For example, patients with only repetitive monomorphic PVCs may develop salvos of VT or sustained VT during exercise or isoproterenol infusion. The observation that ablating one phenotype at a discrete site eliminates the other two phenotypes of arrhythmia suggests that the three phenotypes are representative of the same focal cellular process.
The signature characteristic of idiopathic OTVTs is termination following administration of adenosine or verapamil. Lerman and colleagues elegantly demonstrated that the mechanism underlying most idiopathic OTVTs is triggered activity mediated by catecholamine-induced delayed afterdepolarizations. Catecholamine stimulation of the beta-adrenergic receptor results in an increase in intracellular cyclic adenosine monophosphate (cAMP) and I ca,L activities, resulting in spontaneous oscillatory release of Ca 2+ from the sarcoplasmic reticulum that activates a transient inward current ( I ti ) giving rise to delayed afterdepolarizations. Because activation of adenylyl cyclase and I ca,L is critical for the development of cAMP-mediated triggered activity, this arrhythmia is sensitive to a variety of electropharmacologic perturbations such as beta-blockers, calcium-channel blockers, vagal maneuvers, and adenosine. The effect of adenosine on VT (with rare exceptions) is mechanism specific, terminating only ventricular arrhythmias that are attributed to cAMP-mediated triggered activity.
Clinical Presentation
According to a recent population-based study from the Mayo Clinic, which analyzed patients from Olmstead County who were diagnosed with IVTs between 2005 and 2013, it appears that the incidence of IVTs in the general population has increased over time. These arrhythmias are most commonly observed in the third to sixth decade. Sustained IVT occurs with equal distribution between men and women although symptomatic PVCs are more frequently present in women. Interestingly, RVOT arrhythmias show a predilection for females, whereas those originating in the aortic cusps tend to be more common in men. In two large series (including one from the authors’ institution), 70% of patients with RVOT VT were women, whereas approximately 70% of patients with aortic cusp VTs were men.
The most common clinical phenotype of idiopathic OTVTs is repetitive monomorphic PVCs. Palpitations are the most common symptom (48% to 80%) and are usually experienced as isolated forceful beats resulting from the increased stroke volume associated with the normal beat following the PVC. Fatigue and low energy are also common symptoms seen in patients with at least a moderate burden of PVCs. This may be caused by ineffective ventricular contraction during PVCs resulting in a reduced cardiac output. Lightheadedness may be observed in 25% to 50% of patients, whereas true syncope is rare (10%). The latter usually occurs in patients with ventriculo atrial conduction and may be a result of baroreceptor reflex activity.
Exercise testing induces VT in approximately 70% of patients who present with sustained IVT, but only in 10% of patients whose clinical presentation is repetitive monomorphic PVCs. In the former group, sustained VT is typically observed either during exercise or in the recovery phase. This observation can be mimicked with pharmacologic agents as well. Thus in these patients the clinical arrhythmia can be induced during isoproterenol infusion as well as during an isoproterenol washout period. This highlights the evanescent nature of triggered activity-mediated arrhythmias.
Other scenarios that provoke these arrhythmias include physical or emotional stress, anxiety, and stimulants such as caffeine. In females, IVTs occur more often during the premenstrual and perimenopausal periods and with gestation, suggesting a role for hormonal influence.
Idiopathic OTVTs have a benign course in most patients, suggesting that the underlying pathophysiologic process is not progressive and that the tachycardia does not represent an early manifestation of an occult cardiomyopathy. This finding has been confirmed during long-term follow-up studies in patients with RVOT tachycardias. However, there are two exceptions to this general rule. First, patients with a high burden of ventricular arrhythmia can develop a reversible form of LV cardiomyopathy. Tanaka et al. recently demonstrated in a porcine model that prolonged ventricular bigeminy leads to diffuse interstitial fibrosis histologically and results in decreased unipolar voltage as seen with electroanatomic mapping. Consistent with this observation, Bogun et al. showed that a PVC burden greater than 24% was associated with development of PVC-induced cardiomyopathy (sensitivity and specificity of 80%). However, arrhythmia burden alone may be a poor predictor of future development of cardiac dysfunction because patients with a lower burden of arrhythmia (16% ± 4%) also have developed cardiomyopathy, whereas some patients with a higher burden (33% ± 13%) can maintain normal ventricular function over extended follow-up periods. Thus there may be other factors involved in the development of cardiac dysfunction in patients with outflow tract tachycardias. Nevertheless, in patients with OTVT who develop true tachycardia-mediated cardiomyopathy, suppression of the clinical arrhythmia, either pharmacologically or by ablation, is associated with improvement of LV function. It is good clinical practice to monitor cardiac function with periodic (for instance, annual) echocardiograms in asymptomatic patients with PVC burden exceeding 5%, and certainly if symptoms of LV dysfunction develop. Another clinical scenario that is not uncommon is the development of OTVTs in patients with structural heart disease. This is particularly challenging in patients with arrhythmogenic right ventricular cardiomyopathy (ARVC), where the appearance of OTVTs may be a manifestation of the underlying disease, and the prognosis of the arrhythmia in these patients is less benign than the typical OTVT population. Suspicion for ARVC should arise if the arrhythmias manifest multiple ECG morphologies. ECG algorithms have also been developed to improve differentiation between idiopathic OTVTs versus those occurring in the setting of ARVC. Signal-averaged ECG, cardiac magnetic resonance imaging (MRI), and electroanatomic voltage mapping may be helpful to diagnose ARVC in patients presenting with RVOT VT. In one study, voltage and local electrogram abnormalities on electroanatomic maps were helpful in differentiating idiopathic RVOT tachycardia from RVOT tachycardia caused by subclinical ARVC. In these patients the areas of voltage/electrogram abnormalities correlated with fibrofatty myocardial replacement seen on endomyocardial biopsies obtained from these locations.
Anatomic Substrate
Most patients with idiopathic OTVT have demonstrated lack of gross structural heart disease as evidenced by ECG, echocardiography, radionuclide imaging, and cardiac angiography. However, in patients with RVOT VTs that were initially thought to be idiopathic in nature, studies using MRI have shown structural abnormalities including the presence of focal wall thinning, abnormal systolic wall motion, and focal fatty infiltration, findings not appreciated using conventional imaging. The most common site of abnormality in most cases was the RV free wall. However, the abnormal substrate in these studies did not consistently correspond with the site of arrhythmia origin and subsequent studies have shown a considerably lower prevalence of such abnormalities (approximately 10%). Investigators have also conducted right ventricular biopsies in patients with RVOT tachycardias that have shown a wide spectrum of findings ranging from normal tissue to myocardial atrophy and fibrofatty replacement consistent with ARVC. Importantly, during electroanatomic mapping, identification of areas manifesting fractionated electrograms and voltage abnormalities consistent with scar can serve as target locations for obtaining endomyocardial biopsy to increase the diagnostic yield in patients with suspected ARVC, myocarditis, and/or cardiac sarcoidosis who present with OTVTs.
Idiopathic OTVT may also occur in patients with other unrelated cardiac diseases, including patients with a history of coronary disease. In such patients, the presence of an old infarction that is anatomically distinct from the outflow tract signifies the coexistence of two unrelated processes. Ellis et al. reported the presence of unrelated structural heart disease in 43% of patients (42/97) who underwent ablation of an idiopathic OTVT. In that series, patients with idiopathic OTVT and unrelated structural heart disease were more likely to have the source of the arrhythmia ablated from the coronary cusps or basal LV region, when compared with patients lacking structural heart disease in whom the arrhythmia most commonly arose from the RVOT.
Anatomy of the Outflow Tracts
A comprehensive understanding of the anatomic relationship between the structures in the outflow tract region is crucial to successful localization and ablation of these arrhythmias. In the thoracic cavity the cardiac orientation is such that the RVOT, and particularly its free wall, is the most anterior structure. The superior aspect of the RVOT crosses the anterior aspect of the LVOT such that its anterior-most extension together with the pulmonic valve often lies leftwards of the aortic valve ( Fig. 28.1 A ). The pulmonic valve annulus lies superior to the aortic annulus, and the aortic valve is tilted rightward in the horizontal plane such that the left coronary cusp (LCC) typically lies superior (cranial) to the right coronary cusp (RCC) and noncoronary cusp (NCC). The RCC is in close proximity to the RVOT region, and it is because of this anatomic relationship that successful ablation of OTVT originating from the superior and septal RVOT (its leftward aspect) can sometimes be accomplished from the RCC and vice versa. In fact, the structure immediately anterior to the aortic valve is the posterior muscular infundibular portion of the RVOT (see Fig. 28.1A and B ). Importantly, the penetrating bundle of His resides in the membranous interventricular septum directly underneath the NCC.
The classic understanding of outflow tract and great artery junction is that there is an abrupt termination of ventricular myocardium at the level of the semilunar valves. However, it has now been established that myocardial sleeves extend into the aorta and pulmonary artery for variable distances in some patients. The outflow tract–great artery junction is complex both in terms of its development and histology with multiple tissue types interfacing in this region (arterial smooth muscle, cardiac muscle, collagen, and valve tissue). In the pulmonic valve region, the myocardial sleeves can extend symmetrically across the valve cusps from few millimeters to more than 2 cm into the pulmonary artery. The myocardial extensions above the aortic valve are significantly different from those observed in the pulmonic valve. At the base of the RCC, the LV myocardium is closely apposed to the aortic wall with myocardial sleeves running in parallel to the base of the cusp. By contrast, the LCC is separated from the LV myocardium by a thick band of fibrous tissue interspersed with strands of ventricular myocardium.
Fundamental to the understanding of IVTs arising near the aortic and mitral valve is the concept of the LV ostium (see Fig. 28.1C ). The LV ostium is best described as an elliptical opening at the base of LV and is covered by a tough fibrous structure, which is perforated by the aorta anteriorly and the mitral valve posteriorly. The aorta is joined to the LV ostium at an angle of approximately 30° above the horizontal plane with the NCC being most inferior, the LCC most superior, and the RCC most anterior and in direct contact with the LV ostium. The aortic valve itself has a central location at the base of the heart and LV ostium with relationships to all other valves and cardiac chambers. The RCC is positioned immediately posterior to the infundibular aspect of the RVOT. Caudally, the RCC is continuous with the anterior LVOT, and at the level of valve insertion, there is either a true physical continuity or very close proximity to myocardial tissue from either the LVOT or the posterior RVOT. The NCC lies posterior and to the right of the RCC and as such it has no anatomic relationship with any ventricular myocardium. Instead, the NCC lies adjacent to the interatrial septum and has relations to both the right and left atria. This may explain why ventricular arrhythmias rarely originate from the NCC. The commissure between the RCC and NCC forms part of the membranous interventricular septum and is the location of the penetrating bundle of His. The LCC lies superior, posterior, and to the left of the RCC. The commissure between the LCC/RCC is immediately posterior and caudal to the distal RVOT (see Figs. 28.1A and B ). Myocardial tissue is present in the LCC but not to the same extent as in the RCC. In addition, the LCC is usually separated from the LVOT by connective tissue.
The LV summit is an interesting entity, and this region is being increasingly recognized as a source of epicardial IVTs. The LV summit is the triangular epicardial aspect of the most superior portion of the LV extending cephalad to the aortic portion of the LV ostium and is bounded by the origin of the left anterior descending (LAD) artery and left circumflex coronary artery (LCx). The base of this triangle is defined by the first septal perforator of the LAD, and this triangular region is bisected by the anterior interventricular vein (AIV). As will be discussed later, the close proximity of the epicardial coronary vessels make it particularly challenging to target IVT originating from the LV summit region.
Within the OTVT region, the sources of these arrhythmias show a predilection for certain locations. Thus in one study, approximately 80% of RVOT tachycardias were localized to the superior anteroseptal aspect just below the pulmonic valve. Similarly, LVOT tachycardias have been found to originate from a narrow location that includes the LCC and RCC, the bordering aspects of LV ostium (septal parahissian, aorto-mitral continuity [AMC], and the superior mitral annulus), and the adjoining LV summit region. In the entire outflow tract region almost 90% of idiopathic tachycardias seem to originate from a fairly narrow anatomic zone. This observation raises an interesting question about why this location is predisposed to triggered activity. It is conceivable that this may be related to of its embryologic origin (from neural crest cells). However, it remains unclear whether this tissue is naturally present in all individuals and manifests arrhythmias in a select few, or if it is only seen in those patients that subsequently develop OTVTs. Alternatively, predisposition to OTVTs may be the result of an acquired inflammatory process, such as subclinical myocarditis resulting in fibrosis that can disrupt the perivalvular tissue, thus isolating myocardial cell groups, which in turn may develop abnormal resting or threshold membrane potentials thereby promoting abnormalities of impulse formation (triggered activity and/or abnormal automaticity).
Diagnosis
Electrocardiographic Patterns and Anatomic Localization
Because OTVTs have a focal source and occur in patients with structurally normal hearts, their site of origin can usually be predicted by the standard 12-lead ECG. However, given the narrow anatomic zone from which OTVTs originate, coupled with anatomic variations (especially with regard to the heart position within the chest cavity), electrocardiographic overlap between adjacent sites exists and accurate localization requires invasive mapping. Nevertheless, understanding ECG patterns of OTVTs is a useful tool for arrhythmia localization and procedure planning.
The classic ECG profile of RVOT tachycardia is a left bundle branch block (LBBB) pattern with inferior axis evidenced by tall R waves in leads II, III, and aVF, and QS complexes in aVR and aVL. Fig. 28.2 shows common sites of origin of OTVTs in relation to the precordial leads V 1 -V 3 and limb lead I. Because lead V 1 is a unipolar right-sided lead, tachycardias originating from the anterior (free wall) RVOT propagate posteriorly and away from V 1 , creating a predominantly negative or LBBB pattern. Tachycardias originating further posteriorly and leftward (septal aspect) in the RVOT region while still having an LBBB pattern usually manifest a small R wave preceding the large S wave in lead V 1 . Because of the continuity between the posterior RVOT and the anterior LVOT, a similar small R wave (preceding the S wave) in lead V 1 can be seen with tachycardias originating in the RCC region. Further posterior or leftward from the RCC is the AMC and tachycardias originating from this region show more prominent R waves, which are preceded by a small q wave. In the authors’ experience, this qR pattern in lead V 1 together with inferiorly directed axis is pathognomonic of tachycardia origin from an AMC location. Further posterior or leftward to the AMC sits the LCC, and tachycardias originating from this location usually manifest prominent R waves in V 1 . Beyond the LCC is the mitral annulus, and tachycardias originating from this region usually manifest large monophasic R waves in lead V 1 .
Other leads can also be helpful in localizing OTVTs. The ratio of QRS complexes in leads aVR and aVL was found to be useful in predicting the relatively superior or inferior location of the tachycardia source within the outflow tract region. In general, leads aV R and aV L being located at the respective shoulders, usually manifest prominent negative (QS) complexes for OTVTs. However, for tachycardias originating at a lower level in the RVOT (near the His-bundle region), complexes in lead aV L become progressively less negative or even isoelectric to positive, whereas complexes in lead aV R remain negative. Conversely, when the source of the tachycardia is leftward and in the superior most aspect of the outflow tract (such as above the pulmonic valve), complexes in lead aV L may be more negative than complexes in lead aV R . Similarly, the ratio of R waves in leads II and III can also help differentiate a more superior from an inferior location of tachycardia sources within the outflow tract region. Interestingly, for OTVTs, QRS complexes in leads II and III are usually mirror images of those in leads aV R and aV L, respectively.
Although the 12-lead ECG is extremely helpful in localizing the site of origin of OTVTs, it is important to verify accurate ECG lead position because even minor changes in the location of certain leads can substantially alter the morphology of QRS complexes recorded by those leads. This is particularly true for precordial leads V 1 and V 2 as well as limb lead I. It is not uncommon for precordial leads V 1 and V 2 to be misplaced an intercostal space higher or lower than their standard location (fourth intercostal space). Similarly, the electrodes for limb lead I can sometimes be displaced from the shoulder to the nearby upper chest area. These lead displacements are particularly common during exercise test protocols (done to evoke outflow tract ventricular arrhythmias) or during electrophysiology study (to accommodate the precordial defibrillation pads). Such minor changes in lead placement have been shown to alter the morphology of QRS complexes significantly, which in turn can affect the accuracy of the ECG to localize the site of origin of the clinical arrhythmia. Thus superior displacement of precordial leads V 1 and V 2 can result in reduced R wave amplitude and R/S ratio, whereas inferior displacement of these leads can increase R wave amplitude and the R/S ratio ( Figs. 28.3A and B ). It is thus conceivable that in a case in which VT is originating from the cusp region, if leads V 1 and V 2 are misplaced superiorly resulting in reduced R wave amplitude and lower R/S ratio of QRS complexes, then the tachycardia origin may be erroneously localized to the RVOT region. Similarly, for tachycardias originating in RVOT, if leads V 1 and V 2 are misplaced inferiorly, then because of increased R wave amplitude and R/S ratio, the source may be erroneously localized to the cusp region. It was also found that anterior displacement of the upper limb leads from shoulders to the adjacent upper chest/torso can significantly reduce the R wave amplitude and/or even reverse the QRS polarity in limb lead I ( Fig. 28.3C ). This alteration in QRS morphology can influence one’s ability to distinguish anterior/leftward accurately from posterior/rightward location of the tachycardia source in the superior RVOT region.
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