Ultra-high definition mapping to choose the optimal ablation strategy in atrial macro-re-entrant tachycardia

Macro-re-entrant atrial tachycardias (AT) often represent a challenging arrhythmia. They are generally related to an electroanatomical re-entrant circuit occurring as a consequence of atrial fibrillation (AF) ablation lesions, surgical scars in congenital repaired heart disease and, more rarely, to atrial fibrosis in structural heart disease. Atrial remodelling linked to a bad weight and risk factor management may play a role.1 Therapeutic approaches include antiarrhythmic drugs, which have limited effect, and transcatheter ablation the success of which depends on the appropriate circuit identification. The last one depends on the ability to detect and understand a complex electroanatomical substrate. Based on the above considerations, mapping resolution is one of the key factors in determining the success of the procedure and may be influenced by several factors. In fact the inter-electrode distance, the catheter orientation, the wave-front direction propagation, and the signals filtering may impact on EGM voltage amplitude producing unreliable mapping results as previously demonstrated.2 On the other hand, the use of catheters with an inter-electrode distance of 1 mm, when compared to a conventional 3.5-mm tip catheter, provides a more detailed low voltage area mapping having distinct EGMs. This allows an appropriate local activation time annotation compared with only 21.4% with 3.5-mm electrode tip catheters.1 In addition, finding low-voltage and/or long duration EGMs may be crucial because these are often associated with slow conduction critical sites.3 These advantages are emphasized especially when interpreting multi-fragmented EGMs, typical of low voltage areas that may be the site of critical isthmuses hidden inside an area of fibrosis and are often challenging to analyse in terms of time annotation. This is the reason why conventional approaches for AT mapping and ablation have unsatisfactory results. In fact the AT procedural success rate ranges from 73% to 100%, but a high recurrence rate (up to 53%) is often observed.4–7 To overcome these problems, an ultra-high density (UHD) mapping with a software able to create a reliable activation map is needed. However, collecting a really high number of points may be considered to be time-consuming. On the contrary, it has been demonstrated in experimental settings that this methodology may reduce the mapping time.8 The UHD Rhythmia mapping system (Boston Scientific, MA, USA) uses a 64-pole mini-basket mapping catheter (Orion; Boston Scientific, MA, USA), which incorporates small unidirectional electrodes (0.4 mmq; 2.5 mm spacing) to suppress far-field signals. The system is able to simultaneously elaborate different EGM parameters (voltage, activation, and duration) in order to identify areas with large local variations of these parameters (recognized as scar). This accuracy recognizes slow conducting uniform low voltage channel until it reaches the scar threshold of 0.03 mV which is 5–10 times lower than conventional mapping systems. Higher density and accuracy in EGM annotation forms a deeper understanding of the AT mechanisms.9 This new software is also able to identify errors in annotations of conventional mapping approach.10 The system has shown some benefits in mapping AT in patients with a severely diseased atrial wall or AT related to surgical incision or following AF radiofrequency ablation. Atrial tachycardia ablation with an UHD system has shown a lower proportion of undetermined arrhythmia mechanisms when compared to a conventional approach11 and a lower rate of recurrences during follow-up. Moreover, comparing conventional mapping with UHD mapping, interestingly, the Rhythmia arm showed a higher proportion of ATs linked to a macro-re-entry mechanism.11 This result may be explained by a superior detection of electroanatomical circuits in low voltage areas. Furthermore in a subgroup of patients who already underwent ineffective conventional ablation approach, mapping with the UHD allowed a successful ablation in the majority of cases.10 Importantly, the system was also able to better specify the arrhythmia mechanism: in fact most of previously defined ‘focal’ tachycardias were found to be macro-re-entrant arrhythmias. Entrainment manoeuvres has played a key role in circuit identification. On the other hand, pacing manoeuvres may produce variation in AT circuit activation. According to what reported in literature this risk is low, in particular when pacing is performed inside the critical isthmus.12 The highest mapping density of Rhythmia system leads to an easier and more reliable identification of sites with wavefront propagation narrowing or slowing. As demonstrated by Latcu et al.,9 the critical isthmus of macro-re-entrant AT can be identified by only looking at the propagation wavefront avoiding the entrainment manoeuvres and consequently the even low risk of modification of the AT. In our experience, we confirmed the value of UHD mapping in unveiling properly the circuit maintaining the atrial arrhythmia. However in very few cases the added value of entrainment manoeuvres allowed us a better discrimination between critical isthmus and bystander areas. This confirms the key role of a well-trained electrophysiologist when facing with different and complex contexts. The UHD mapping also identified and defined even more complex macro-re-entrant ATs. For example single-loop biatrial tachycardia, considered a rare form of atrial macro-re-entrant tachycardia, was correctly identified and successfully ablated in most of highly diseased patients.13 The benefit of an UHD approach is above all evident in patients who have undergone previous ablation to cure persistent AF ablation and/or incisional AT.14 Takigawa et al.14 have reported that around 20% of post AF ablation ATs are maintained by a multiple loop re-entry circuit and the activation mapping with the UHD system identified the correct circuit without manual annotation in ≈90% of cases. An appropriate RF delivery stopped the arrhythmia in the majority of cases. As explained above, the lower noise level recorded by the Rhythmia system formed a more accurate EGM analysis in atrial diseased tissues. Also the experience by Latcu et al.9 confirmed these results. Latcu et al. considered 33 cases of post-AF ablation or post-surgical repair AT in which the UHD mapping ablation approach interrupted the arrhythmia in 97% of cases. Latcu et al. found that the EGM recorded inside the critical isthmus showed much lower voltage amplitude with a significantly slower conduction velocity than the surrounding parts of the circuit. The combination of an extremely precise voltage mapping, with the ability to identify even the smallest scarred areas, together with a deep EGM analysis identified the ‘carousels’ and consequently increased the success rate of the ablation procedure.15 Another step forward will be the LUMIPOINT™ software. By classifying each EGM into predefined categories, it is apparently easier to identify target area of a re-entry circuit. Highlighting EGM within areas of interest, the map interpretation may be faster and easier. Moreover, with the Skyline function, temporal and spatial information of full chamber activation will be simultaneously available. An example of this software application is present in this supplement in the papers coming from Munkler et al.16 and Frontera et al.17 In conclusion, UHD mapping has led to a significant change in the mapping approach, allowing electrophysiologists to ‘perceive the imperceptible’, therefore forming a better understanding of arrhythmia mechanisms and consequently to design a more effective ablation strategy.18 This paper was published as part of a supplement supported by a grant by Boston Scientific Italy. Conflict of interest: none declared. The opinions expressed in this article are not necessarily those of the Editors of Europace or of the European Society of Cardiology.