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Special Problems in Ablation of Accessory Pathways
Key Points
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The approach to the difficult accessory pathway ablation is, first, to exclude cognitive ablation failure by confirming the tachycardia diagnosis and reevaluating the electrograms.
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Second, use a systematic approach to identify contributing technical factors, such as pathway-related factors (including location and atypical configuration) and associated cardiac structural abnormalities.
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Finally, devise an appropriate strategy. This may include optimizing pathway localization, adjusting the ablation approach to improve stability and tissue contact, and changing the ablation modality (conventional vs. saline-cooled radiofrequency ablation or cryoablation).
The atrioventricular (AV) groove is normally composed of fibrous tissue devoid of electrical conductive properties; this commits ventricular activation to proceed over the specialized AV conduction tissue, the His–Purkinje system. Accessory pathways (APs) are considered a remnant of incomplete separation of the atrial and ventricular myocardium by the annulus fibrosus during cardiogenesis. These resulting myocardial bridges are capable of electrical conduction that may facilitate early ventricular activation and provide the arrhythmogenic substrate for AV reentrant tachycardia.
Because of its low risk and high efficacy, catheter ablation is first-line therapy for symptomatic and some asymptomatic patients with APs. Ablation of APs was historically achieved first by surgical dissection and subsequently by direct current energy applied through transvenous catheters. The first successful catheter ablation of an AP using radiofrequency (RF) energy was performed in 1984.
Although successful pathway elimination is achieved in more than 95% of cases, primary success is occasionally elusive, resulting in lengthy procedures or multiple attempts. Furthermore, AP conduction may return after initial success. This chapter aims to outline problems that may be encountered during ablation of APs and to propose practical solutions.
General Considerations
The incidence of successful elimination and recurrence after RF catheter ablation of APs has been well documented. Initial ablation success is highest for left-sided pathways (97%) and is lower for right-sided pathways (88%) and septal connections (89%). Recurrence is also less frequent at the left free wall locations (5%) than at the right free wall (17%) and septum (11%).
Failed RF catheter ablation of APs is most frequently related to technical difficulties or misdiagnosis ( Box 27.1 ). Other factors are the coexistence of structural cardiac abnormalities, atypical pathway configuration, and high-risk AP locations, such as those adjacent to the AV node or within the coronary sinus (CS).
BOX 27.1
Causes of Failed Catheter Ablation of Accessory Pathways
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Misdiagnosis
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Misinterpretation of electrophysiologic data
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Previous ablation, low amplitude, or distorted electrogram recordings
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Incomplete electrophysiology study, inaccurate pathway localization, incomplete mapping
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Multiple tachycardia mechanisms (e.g., AP with AVNRT or ectopic tachycardia, pathway-to-pathway tachycardia)
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Inability to Heat
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Catheter instability, poor tissue contact, difficult access to target site
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Pathway location beyond range of RF lesion size (e.g., epicardial location)
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Associated Structural Cardiac Abnormalities
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Ebstein anomaly
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Persistent left-sided superior vena cava
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Atypical Pathway Configuration
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Multiple APs
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Oblique APs
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Epicardial APs
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Atypical AP connections
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High-risk AP Location
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Adjacent to the AV node: midseptal or anteroseptal pathway (risk of inadvertent AV block)
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Epicardial APs: Accessible within the coronary sinus or associated with diverticulum (risk of arterial stenosis–circumflex artery or distal right coronary branches)
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AP , Accessory pathway; AV, atrioventricular; AVNRT, atrioventricular nodal reentrant tachycardia; RF, radiofrequency energy.
Misdiagnosis
Misinterpretation of electrophysiologic data, by failure or inability to recognize atrial or ventricular activation sequence or AP potentials, prohibits accurate pathway localization. Appropriate interpretation of data may be precluded by factors attributable to the AP, such as overlapping electrograms, which are often seen with multiple and right free-wall pathways ( Fig. 27.1 ), or distorted low-amplitude electrograms at the site of RF lesions in patients with a previously unsuccessful ablation attempt.
Incomplete mapping may prevent accurate localization, a point of particular relevance to posteroseptal APs. If thorough mapping at the posteroseptal region on the right fails to identify a successful ablation site, careful mapping of the CS and the left posteroseptal region is essential. In the event that an epicardial pathway is suspected, CS angiography may identify a coexistent CS diverticulum or aneurysm.
A complete initial electrophysiology study is necessary to exclude an unrecognized or unexpected tachycardia mechanism, because APs, AV nodal reentrant tachycardia (AVNRT), and ectopic tachycardias infrequently coexist. Furthermore, successful AP ablation may lead to the emergence of a latent AP, causing symptom recurrence and need for a repeat procedure. A repeat diagnostic study after presumed successful pathway ablation is strongly recommended to exclude this possibility.
A number of published series have identified factors associated with difficult or failed ablation and recurrence of AP conduction. In a retrospective analysis, Morady and associates identified six factors contributing to a failed or prolonged ablation session. In the failure group, the proportion of APs located at the right free wall was significantly greater than in the overall group (29% vs. 16%), with right anterolateral and right posterolateral locations specifically overrepresented. Of the six factors associated with initial ablation failure, problems related to catheter stability and inaccurate pathway localization accounted for most cases (48% and 26%, respectively). Other contributing factors were the presence of a putative epicardial pathway (5%), recurrent atrial fibrillation interfering with mapping (3%), unusual AP anatomy (1.5%), and procedure-related vascular complications preventing an ablation attempt (3%). Other series reproduced these findings, with the addition of time to conduction block as an added factor predicting recurrence
A more contemporary 14-year single center experience, as described by Belhassan in 2007, reconfirmed these findings. Recurrent accessory conduction was observed in 24.2% right free wall, 16.7% midseptal, 14.3% right anteroseptal, 13% posteroseptal, and 5% left free wall pathways. A contemporary analysis of 89 patients referred to three international centers for repeat ablation noted similar findings. Repeat procedures were successful in 81 patients (91%); factors associated with initial failure are very similar to those identified through the late 1990s. Interestingly, likely because of greater catheter stability using a transseptal approach and saline-irrigated catheters, left lateral pathways appeared to be underrepresented among ablation failures over time. Otherwise, catheter stability, inaccurate pathway localization, epicardial pathways, multiple or broad pathways, and proximity to the AV node continued to challenge even experienced operators. Thus the initial work by Morady continues to form a relevant framework for considering factors associated with AP ablation failure.
Inaccurate Pathway Localization
Inaccurate pathway localization, leading to RF energy application at inappropriate sites, continues to be a problem, predominantly under two circumstances, even in contemporary series. First, when the surface electrocardiogram (ECG) suggested right posteroseptal preexcitation, but subsequent successful ablation was achieved at the left posteroseptal region. In the second type, failure was observed during ablation of unrecognized oblique APs caused by disparate atrial and ventricular insertions at the AV junction. As a result, ventricular insertion sites did not correspond to the site of earliest retrograde atrial activation during ventricular pacing or orthodromic tachycardia. Similarly, atrial insertion sites were not identified by the earliest anterograde ventricular activation during preexcitation. Success was ultimately achieved by identifying the atrial or ventricular insertion sites on their corresponding side of the annulus, using appropriate pacing maneuvers, or by ablating a midportion of the pathway itself, as identified by AP potentials ( Fig. 27.2 ).
In an attempt to better resolve a number of ablation substrates, catheters have been developed with the distal ablation tip separated into discrete, insulated mini-electrodes. In at least one difficult repeat ablation, a left-sided pathway was successfully mapped and ablated using 0.8-mm mini-electrodes with 2.5-mm spacing. Such catheters are not irrigated leaving an obvious trade-off for operators using irrigation to mitigate thromboembolic risk on the left or facilitate ablation within the CS.
Nonfluoroscopic mapping systems have revolutionized atrial fibrillation and VT ablation. Application of these techniques for AP ablation has resulted in significant reduction in patient and operator fluoroscopic exposure. In the pediatric population, particularly sensitive to fluoroscopic exposure, nonfluoroscopic mapping has improved ablation outcomes as well. In a retrospective cohort of 651 cases, those who underwent 3-dimensional nonfluoroscopic mapping had higher success rates (97% vs 91%) compared with those with fluoroscopic approach, despite minimal differences in baseline characteristics. Recurrence rates were similar (5% vs. 9%). On multivariate analysis, only 3-dimensional mapping was associated with ablation success (odds ratio [OR] 3.1 [95% confidence interval [CI] 1.44–6.72; P < .01]). There is no reason to believe similar results would not be reproduced in adults. Certainly, the use of nonfluoroscopic mapping cannot replace careful mapping at the AV ring, but it should be considered as part of the approach to a difficult, repeat ablation.
Insufficient Energy Delivery and Heating
Catheter Stability:
Technical considerations such as catheter instability or difficult access may give rise to poor endocardial contact, leading to insufficient energy delivery and local heat production at the ablation target. This problem is more frequently encountered with right-sided APs because of a less clearly defined anatomic groove delineating the tricuspid annulus. Poor energy delivery, indicated by a low power output of the RF generator during temperature-controlled RF ablation, may achieve the desired tip–tissue interface temperature, but does not provide adequate depth of energy penetration for elimination of AP conduction. Tissue contact and catheter access to the target site are often improved by the use of long, preformed intravascular sheaths, or deflectable ablation catheters of varying reach, curve, and tip size, which are designed in a variety of configurations to allow access to all locations on either the tricuspid or mitral annulus.
Catheter stability may also be compromised if ablation is performed during orthodromic AV reentrant tachycardia. Ablation during tachycardia is sometimes necessary if retrograde fusion during ventricular pacing obscures the pathway location in patients with concealed APs. Abrupt slowing of heart rate on RF-induced termination of tachycardia frequently results in catheter dislodgment, preventing full-duration RF current delivery at the successful site. Entrainment of the tachycardia by ventricular pacing during ablation overcomes this potential problem. While maintaining retrograde activation over the AP, entrainment prevents an abrupt change in ventricular rate, allowing a stable catheter position during pathway ablation for continued RF energy delivery despite tachycardia termination.
Even in contemporary ablation series, inability to guide the ablation catheter to the endocardial target, catheter instability, or inadequate tissue contact, or a combination of these, has contributed to failed or prolonged ablation in up to 48% of patients. Successful pathway elimination was achieved in some cases by a change in the ablation approach. Specifically, for left-sided pathways, a retrograde aortic approach was switched to a transseptal approach; for pathways located on the right, an inferior vena cava approach was switched to a superior vena cava approach. In other cases, the use of a long guiding sheath, multiple operators, or ablation catheters of varying distal configurations during tip deflection proved successful. The addition of saline-irrigated ablation, in combination with these steps, has produced mixed results at initial ablation attempts, but improved results for initial ablation failures, especially for posteroseptal or right free wall pathways.
With the advent of robotic systems, APs have been successfully ablated using magnetically guided catheters, as well as robotically manipulated sheaths (Amigo; Hansen). Reduced operator fluoroscopic exposure was universally reported, but no obvious increased ablation success was reported, likely as a result of the low pathway ablation failure rate and because catheter stability can be improved manually by experienced operators who are willing to modify their standard techniques using different sheaths, catheter configurations, and approaches.
Recently, force sensing at the tip of the catheter has facilitated pulmonary vein isolation for treatment of atrial fibrillation with demonstration of improved outcomes. The same force sensing technology has been used in AP ablation in case reports only. Given the high success rates with traditional catheters, it may be very difficult to demonstrate improvement in outcomes, although in difficult cases, optimizing force contact will undoubtedly facilitate ablation energy delivery and lesion development in individual cases. Routine use is likely limited by cost at this time in some health care systems.
Saline-Irrigated Ablation
The common use of saline-irrigated ablation has made inability to heat much less a problem. Saline irrigation reduces interfacial heating and shifts the point of maximal heating into the tissue rather than focusing it at the tissue surface. As a result, deeper conductive heating occurs and produces deeper tissue lesions, an attribute essential for targets beyond the range of conventional RF lesions, such as epicardial APs. Before determining that a deeper lesion using saline irrigation is required, care must be taken to optimize mapping and pathway localization as well as maximize catheter stability. Saline-irrigated ablation may be able to overcome some, but not all factors associated with insufficient energy delivery and heating.
The routine use of irrigated RF, for left-sided APs may reduce rare thromboembolic complications. Although intuitively based on the ablation experience with atrial fibrillation, this has never been proven. A recent small randomized study demonstrated superiority of irrigated catheters for successful ablation of posteroseptal and right free wall APs. With time, irrigated RF may become first-line for ablation of most, if not all APs. Further advances in catheter technology now allow force contact measurements from the tip of the ablation catheter. This has been demonstrated to impact outcomes in ablation of atrial fibrillation. It is likely that the selective use of this new technology for difficult pathways will be explored in coming studies. The ability to make deeper lesions with saline irrigation should not supplant high-quality mapping. Edema produced by a large number of failed applications can make further mapping and successful ablation very difficult. In addition, with deeper lesions there is an increased risk of collateral damage-related injury, commonly to the coronary arteries and its branches.
Specific Challenges:
Coronary Sinus and Epicardial (Intravenous) Accessory Pathways
At the outset, it is important to clarify that virtually all free wall and posteroseptal APs, regardless of location, are epicardial in anatomic location, based on available histopathologic data and surgical experience where virtually all pathways were ablated from the epicardium. Most course close to the annulus and can be ablated from the endocardium. On occasion, they connect atrium to ventricle farther from the annulus, deeper in the fat pad, making endocardial ablation more challenging and emphasizing their epicardial location. This can be particularly true for pathways in close approximation to the CS and its branches.
The CS originates at its ostium within the right atrium and extends distally to the valve of Vieussens, where it receives the great cardiac vein. Other major tributaries are the left obtuse marginal vein, the posterior left ventricular vein (posterior coronary vein or PCV), the middle cardiac vein (MCV), and the right coronary vein, also known as the small cardiac vein ( Fig. 27.3 ).
The CS provides a conduit for catheter access, permitting mapping of left-sided APs adjacent to the mitral annulus and of posteroseptal (paraseptal) APs traversing the inferior pyramidal space. Moreover, it provides a means of access to epicardial areas of the myocardium for potential ablation of epicardial pathways.
A myocardial coat around the CS is present in all individuals. It is composed of bands of muscle arising from the right and left atrial walls and extends in most cases to, and occasionally beyond, the CS junction with the great cardiac vein. Electrical continuity therefore exists between both atria and this muscular sleeve. The tributaries of the CS are usually devoid of a myocardial coat. Nonetheless, sleeve-like muscular extensions covering the proximal portions of the MCV and PCV are present in 3% and 2% of hearts, respectively, potentially serving as connections between the ventricle and the CS and completing a CS–AP connection ( Fig. 27.4 ).
The association of CS diverticula with posteroseptal and left posterolateral APs is well documented. Myocardial fibers found within diverticula frequently connect the ventricle with the CS musculature. Other anatomic anomalies, such as fusiform or bulbous enlargement of the CS tributaries, have also been reported to be associated with such connections.
Sun and associates identified a CS AP in 36% of 480 patients with posteroseptal or left posterior APs. During anterograde AP conduction, the presence of a CS AP was established by the recording of ventricular activation at the MCV, PCV, or neck of a CS diverticulum earlier than endocardial ventricular activation. At the same site, a high-frequency potential was recorded before the earliest recorded far-field ventricular potential. This high-frequency potential, analogous to an AP potential, was generated by the muscular extension of the CS myocardial coat, which formed a connection to the epicardial surface of the ventricle.
Retrograde angiography in patients with CS AP demonstrated a CS diverticulum in 21% to 31% of cases, most frequently extending from the CS and the MCV. Fusiform or bulbous venous enlargement was identified in 9% of patients, but CS anatomy was normal in the remaining 70%, suggesting that most CS APs occur without a diverticulum or other venous anomaly.
Because of their location, successful ablation of these APs is accomplished only by RF current delivered within the CS or by direct percutaneous catheter access to the pericardial space. Specific ECG features have been described to identify manifest pathways requiring such an approach. A negative delta wave in lead II predicts a successful ablation site within the CS or MCV with a sensitivity of 87%, but with a relatively low specificity (79%) and positive predictive value (50%). However, a steep positive delta wave in lead aV R and a deep S wave in lead V 6 (R < S) yield high specificity and positive predictive values of 99% and 91%, respectively, for a successful ablation site within the CS. These ECG findings, along with a difficult or previously failed ablation attempt, suggest that definition of the coronary venous anatomy, using CT scanning or retrograde CS injection, and subsequent detailed mapping may prove helpful in identification of a successful epicardial ablation target. Regardless of these ECG criteria, which are fallible, the practical approach is careful endocardial mapping in the region of the interest by an experienced operator, with reversion to an alternate plan if mapping along the usual endocardial annulus is not productive.
RF ablation within the coronary venous system has been shown to be successful and safe for the elimination of epicardial APs ( Fig. 27.5 ). A number of series have demonstrated epicardial AP potentials within the CS in cases when endocardial ablation has failed, suggesting a marker for those that might be best approached this way. Reports of CS injury after RF catheter ablation are infrequent, possibly because of the lack of clinical sequelae and symptoms of CS stenosis. Nevertheless, RF current delivery within the vein has been associated with endoluminal thrombosis, stenosis, and acute occlusion , perforation leading to cardiac tamponade, and damage to adjacent structures. The proximity of the right coronary artery and its AV nodal branch with the proximal MCV, and crossover points of the left anterior descending and left circumflex arteries with the great cardiac vein, represent potential sites of susceptibility for coronary artery spasm or myocardial infarction during RF ablation. Risk of arterial injury has been shown to be inversely related to distance from the artery, with injury rates of 50% within 2 mm, 7% within 3 to 5 mm, and 0% greater than 5 mm from the artery. Selective coronary angiography to delineate the relation of a prospective ablation site to the coronary arteries is prudent before RF current application within the CS. Luminal patency may also be reassessed after ablation. Irrigated RF ablation results in a lower incidence of impedance rises and coagulum formation, but care must be used to prevent arterial injury given the propensity for deeper lesions. Thorough mapping and the use of high standards in accepting suitable electrograms for ablation, including identification of AP potentials, can minimize complications by reducing the number of required RF current pulses. Failure to eliminate AP conduction with conventional RF has prompted more routine substitution with a saline-irrigated catheter. Ablation failure may be related to AP anatomy, such as a broad pathway insertion or an epicardial location. Conversely, suboptimal energy delivery may be contributory, as can occur with catheter instability or poor tissue contact. In these circumstances, successful ablation is often accomplished by enhanced delivery of RF energy to the tissue, which in turn produces larger and deeper endomyocardial lesions. However, the benefits of saline-irrigated RF ablation are not without risk. Higher energy delivery may permit subendocardial tissue temperature to rise above 100°C. Plasma boiling and tissue desiccation at these temperatures, frequently accompanied by an audible pop, may result in crater formation and wall rupture. Accordingly, saline-irrigated RF ablation should be used judiciously, with power limited to 30 W and at temperatures not exceeding 45°C, to reduce the risk of steam pop and perforation.
Cryoablation is the modality of choice for cases in close proximity to arterial branches. Cryothermal ablation within the CS has been successfully used to eliminate epicardial posterolateral APs. Cryolesions are associated with less endothelial disruption and thrombus formation than RF lesions. Furthermore, the safety of cryothermal ablation adjacent to the coronary arteries has been demonstrated by extensive surgical experience and by recent catheter-based studies using animal models.
Pericardial Access for Epicardial Mapping and Ablation
Percutaneous pericardial access for epicardial mapping and ablation has been used for a number of arrhythmia substrates when endocardial ablation has failed, including APs. In an initial series of 10 cases, five had earliest activation recorded epicardially, although three of these were right atrial appendage to right ventricular pathways. These were the only three cases successfully ablated from the epicardium. In a more recent series of 21 patients, all of whom had failed a median two prior ablations, Scanavacca et al. used simultaneous epicardial (pericardial, subxiphoid) and endocardial access to more completely map earliest activation. AP location was posteroseptal in 12 (57%), left free wall and left posterior in four (19%), right posterior or right lateral in three (14.2%), and anteroseptal in two (9.5%). The authors contend that epicardial access contributed to successful ablation depending on the relative timing of the earliest endocardial and epicardial mapping sites. In six cases, where epicardial activation was earlier, epicardial ablation was successful. This included two of three right lateral pathways, three of 12 posteroseptal pathways, and one of four left lateral pathways.
At sites where endocardial and epicardial activation were equally early ( n = 3), epicardial mapping was used to direct endocardial ablation with successful ablation in two of three. In a further nine cases, epicardial sites were later directing the operator back to the endocardium for more thorough mapping resulting is successful ablation in five. In three cases neither endocardial or epicardial activation was early, resulting in ablation failure in all three. Four of 12 in the latter two groups underwent surgical access, cardiopulmonary bypass with successful cryoablation; one had a large CS diverticulum resected as part of the procedure.
Septal Pathways
The location of an AP may dictate the need for special consideration of the ablation approach to reduce potential complications. By definition, septal APs have an atrial insertion located within the triangle of Koch. Anteroseptal and midseptal pathways course near the septum in close anatomic relationship to the His bundle and AV node. As a result, surgical division or RF catheter ablation is associated with an increased risk of AV block, reported to occur in up to 36% of patients. In light of this, ablation of such pathways is often deferred in the absence of drug-refractory symptoms, a short anterograde effective refractory period, or rapidly conducted atrial fibrillation.
The risk notwithstanding, several studies have demonstrated effective RF interruption of these pathways with preservation of AV nodal conduction. Conventional RF energy with step-up power titration has been successful with a low incidence of AV block. Placement of a His bundle recording catheter as a reference point allows estimation of the distance between the ablation target and the AV node. Once a suitable target without a His bundle recording is identified, RF energy may be delivered, commencing at a power setting of 10 W. If pathway interruption has not occurred and AV conduction is unchanged after 10 to 15 seconds, the power is increased by 5 W every 10 to 15 seconds, with continuous attention given to the earliest recognition of AV nodal impairment (development of atrium-to-His interval prolongation or AV block), accelerated nodal rhythm, or catheter displacement. Ablation commencing with a low-power setting has been recommended by several authors to achieve the desired result, while producing the smallest possible lesion. Catheter stability can be facilitated in patients under general anesthetic with judicious use of apnea. The success of this approach and the frequent observation of temporary pathway conduction block as a result of catheter-induced mechanical trauma attest to a superficial subendocardial location of these pathways.
An alternative and arguably safer approach for elimination of septal pathways is cryoablation. The well-established properties of cryothermy, including reversibility, small discrete lesion size, and cryoadherence, make this ablation modality particularly suitable if lesions are required adjacent to the AV node. Stability secures accurate lesion placement, and reversibility allows functional testing of the target before ablation, a combination that makes inadvertent permanent AV block an unlikely complication ( Fig. 27.6 ). This technology intuitively offers significant advantages over RF energy for septal pathway ablation, especially in children and young adults.
Some anteroseptal APs are in close proximity to the noncoronary cusp (NCC) of the aortic valve. Ablation from within the NCC using a retrograde aortic approach can be an option for these pathways. Because of the risks involved with high septal ablation from the right atrium, it is reasonable to map the NCC to assess activation time in any septal pathway that is anterior to the midseptal region. The safety and efficacy of an NCC approach to ablation was first documented in several case reports, some of which had initial failed ablation from the septum and subsequently were found to have earlier atrial activation in the NCC with successful RF ablation at this site. Suleiman et al initially reported successful cryoablation from the NCC and then a subsequent case in which cryoablation failed but RF ablation then eliminated pathway conduction. Balasundaram et al. used this approach to successfully map and ablate an anteroseptal pathway in a 4-month-old infant, and presented an excellent image illustrating the relationship of the NCC to the anteroseptal region ( Fig. 27.7 ). The His bundle enters the ventricle traversing between the RCC and the NCC. The NCC lies at a juncture between the upper boundary of the atrial septum and the posterior right ventricular outflow tract, permitting an alternate approach to high septal APs, which may be closer to the pathway and further from the AV node than the high septal approach from the right atrium. Contrast angiography of the aortic root will show the catheter in the posterior-most aspect of the aorta in the right anterior oblique (RAO) projection, and to the left of the His in the left anterior oblique (LAO) projection. Intracardiac echocardiography (ICE) imaging has also proven helpful in guiding the ablation catheter to the NCC and then rightward and posterior toward the AP. Xie et al. presented a case series that compared seven ablations from the NCC as the initial approach to a septal AP and 10 conventional ablations from the right atrial septum. Ablation from the NCC was successful in 11 of 12 patients, and from the septum in five of 12. Transient AV block occurred in three patients with septal ablation, all of whom had a subsequent successful NCC ablation, and late AV block occurred in 1 septal patient. Suleiman examined the ECG findings in 3 patients with an AP ablated from the NCC and found that these patients had a small positive deflection to the delta wave in lead V1, in contrast to the usual isoelectric or negative delta wave in patients with a right free wall AP. They also observed that the delta wave in lead III was relatively isoelectric or much less positive than the delta wave in lead II, possibly reflecting the rightward location of the aortic root, and the fact that lead II measures leftward conduction while lead III measures rightward conduction.
With four possible variables in the approach to the anteroseptal pathway–atrial, NCC, RF, and cryothermy–a reasonable strategy would be initial examination of the ECG, and careful mapping of the annulus along the septum. If the earliest atrial activity during tachycardia or ventricular pacing is above the midseptum, then the NCC could also be mapped. If the NCC has the earliest retrograde atrial activity and/or AP potential, then the evidence suggests that cryoablation or RF ablation at this site is effective and relatively safe. If the NCC signal is not clearly early, and if no His potential is evident, then initial cryoablation at the atrial septal site would avoid the small risks involved in arterial access and aortic root cannulation. If cryoablation at the atrial septum is ineffective then an attempt at NCC ablation would be warranted.
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