Slow Pathway Research Articles

DECENNIAL ANALYSIS OF AVNRT ABLATION IN A SINGLE CENTRE

Abstract Background Transcatheter ablation represents the first line therapy for atrio–ventricular nodal re–entrant tachycardia (AVNRT). Both radiofrequency (RF) and cryoablation (CRYO) have been shown to be effective, with a better safety profile for CRYO. Data on very long–term efficacy are scarces. Purpose To review our experience in AVNRT ablation performed in the last ten years and evaluate safety, efficacy and very long–term outcome of RF and CRYO. Methods We retrospectively analyzed all patients who underwent AVNRT ablation (RF or CRYO) between January 2013 and May 2023 at our centre. The majority of procedures were performed under electro–anatomical mapping (EAM) guidance to reduce radiation exposure. Acute efficacy endpoint was defined as slow pathway ablation, while long–term outcome was defined as the absence of AVNRT recurrence. Results A total of 387 consecutive patients (60,5% F; mean age 48 ± 19 years) were included; CRYO was performed in 241 patients (62%), RF in 131 patients (34%), hybrid approach (CRYO+RF) in 15 patients (4%). AVNRT induction was obtained during EPS in 295 cases (76%), in 25,4% after isoprotenerol administration. No significant differences for baseline characteristics were observed between the two groups except for age (CRYO: 42,2 ± 18 years vs RF 57,19 ± 16,5 years p <0.001). In pediatric population CRYO was performed in 93% of procedures. Figure 1 shows the distribution of RF/CRYO over 10 years at our centre, with a trend of significant increase in CRYO utilizations in the last years. Acute success was achieved in all 387 cases (100%; delta WP 28,5 ± 23,1 ms, delta AVN ERP 72,4 ± 44,7 ms). No permanent AV block or major periprocedural complications occurred, however transient AH prolongation was observed in 0.8% in CRYO pts vs 2,8% RF pts. The mean follow–up was 73±45 months, range 6–129 months. Overall long–term success rate was 93,3%, ranging from 91,3 % in CRYO and 96,6% in RF. Recurrence occurred in 26 cases (6,7%) after a median time of 20 months (4–40months). Recurrent AVNRT were treated with RF in the majority of cases, CRYO in 6 procedures, hybrid in only 1 case. Conclusions Transcatheter ablation is a safe and effective therapy for AVNRT. Our very long–term outcome data confirmed very high success rate even after 10 years follow–up. The increasing choice for CRYO may be due to the increasing number of pediatric and young patient referred to our centre.

Double Ventricular Response with Aberrant Conduction Leading to Ventricular Dysfunction.

Double ventricular response (DVR), where a single P wave results in two QRS complexes, is a rare presentation of dual AV node physiology. It has been associated with ventricular dysfunction in the setting of incessant tachycardia. We present the case of an otherwise healthy adolescent who had frequent DVR without tachycardia leading to left ventricular dysfunction. Slow pathway modification led to a significant reduction in ectopy and normalization of ventricular function. This highlights that DVR without tachycardia might lead to ventricular dysfunction in pediatric patients. Slow pathway modification with reduction of ectopy may be sufficient to restore ventricular function.

Peak frequency annotation algorithm–guided slow pathway ablation in typical atrioventricular nodal reentrant tachycardia

BackgroundThe slow pathway potential is difficult to annotate because it is buried within the atrial potential. Omnipolar technology near field can automatically annotate the peak frequency potential associated with acquired intracardiac electrograms. ObjectivesThis study aimed to visualize the junction between the transitional cells and the slow pathway using a peak frequency map with omnipolar technology near field and evaluate whether the high-frequency site around the tricuspid annulus (TA) is an effective target for slow pathway ablation. MethodsThis prospective observational study enrolled 37 patients with typical atrioventricular nodal reentrant tachycardia. Patients underwent slow pathway ablation using a peak frequency map (n = 17) and the conventional approach based on anatomical and electrophysiological findings (n = 20). ResultsHigh-frequency sites were distributed at the TA side of the 4–5 o’clock position in all patients mapped using the peak frequency map of OTNF. The distance to the His bundle from the successful ablation site was farther (24.0 ± 4.8 mm vs 12.7 ± 4.0 mm; P < .0001), junctional rhythm was slower (88 ± 17 beats/min vs 115 ± 12 beats/min; P < .0001), the time to junctional rhythm after radiofrequency application was shorter (3.4 ± 1.4 seconds vs 8.2 ± 4.6 seconds; P < .0001), and the elimination rate of jump ups (71% vs 30%; P = .02) was higher in the peak frequency map–guided group. ConclusionThe high-frequency site of the TA at 4–5 o’clock in the peak frequency map could be a novel target of slow pathway ablation with high safety, efficiency, and efficacy.

Comparative utility of omnipolar and bipolar electroanatomic mapping methods to detect and localize dual nodal substrate in patients with atrioventricular nodal reentrant tachycardia.

Catheter-based slow pathway modification (SPM) for atrioventricular nodal reentrant tachycardia (AVNRT) is traditionally performed at empiric sites using anatomical landmarks and test ablation feedback within the triangle of Koch (TK). While studies have described more tailored techniques such as bipolar low voltage bridge (LVB) and wavefront collision identification, few have systematically compared the diagnostic yields of each and none have investigated whether omnipolar mapping technology provides incremental benefit. The objective of this study was to compare the utility of omnipolar andbipolar-derived qualitative and quantitative measurements in identifying and localizing dual AVN substrate in patients with versus without AVNRT. A retrospective case-control study of consecutive patients with paroxysmal supraventricular tachycardia undergoing electrophysiology study with both omnipolar and bipolar mapping from 2022-2023. Thirteen AVNRT cases (median age 16.1 years, 512 TK points) were compared to nine non-AVNRT controls (median age 15.7 years, 332 TK points). Among qualitative variables, an omnipolar activation vector pivot, defined as a ≥45 degree change in activation direction within the TK, had the highest positive (81%) and negative predictive values (100%) for identifying AVNRT cases and had a median distance of 1 mm from SPM sites. Among quantitative variables, the optimal discriminatory performance for successful SPM sites was observed using bipolar voltage restricted to a peak frequency >340 Hz (c statistic 0.75). Omnipolar vector pivot analysis represents an automated, annotation-independent qualitative technique that is sensitive and specific for AVNRT substrate and co-localizes with successful SPM sites. Bipolar voltage quantitatively describes SP anisotropy better than omnipolar voltage, and the addition of peak frequency signal analysis further optimizes the selection of SPM sites.

Video comprehension-based approach for seismic damage recognition of freestanding non-structural components

With the emergence of seismic resilience concepts, the objective of buildings’ seismic design shifts from life safety to earthquake-induced functional consequences. The seismic damage of nonstructural components (NSCs) is paramount in economical loss assessment and post-earthquake functional recovery. To rapidly assess the seismic damage of NSCs, this paper leverages the video-comprehension techniques to customize a novel deep learning model (which is called two-pathway vision transformer for the damage state recognition, TPViT-DMSR) that can recognize the freestanding NSCs’ damage states through the video footage. This model includes a slow and fast pathways to analyze the object’s movement at different speeds. The two pathways are fused through a unidirectional connection, which allows damage information to be better shared across two pathways. To improve the robustness of the model, the trajectory attention mechanism is integrated. To demonstrate the model’s applicability and efficacy, a comprehensive video dataset that describes various freestanding NSCs’ movement behavior is compiled via a series of shaking table tests. The application result reveals that the TPViT-DMSR model performs at a desired level with the mean average precision achieving at 74.87%. The model is also applied to the video footage collected from past earthquake events. The application results highlight that the model can deliver a reliable estimate of the damage state for NSCs.

A jump in the atrioventricular conduction curve is not caused by a switch from fast pathway to slow pathway conduction.

Background: A jump in the atrioventricular (AV) conduction curve is the current clinical criterion of dual-pathway electrophysiology. However, the assumption that a jump indicates a switch from fast pathway (FP) to slow pathway (SP) conduction remains unconfirmed. This study was carried out to investigate whether a jump indeed indicates a transition from FP to SP conduction, and if not, what the potential cause is. Methods: Eighty-one experimental records from rabbit AV nodal preparations containing the following data were analyzed: 1) had at least one AV conduction curve and 2) had recording of His electrogram alternans (a validated new index of dual-pathway conduction). Most cases also had intracellular action potential recordings from the AV nodal fibers. Results: Of the 81 preparations, 11 (13%) showed a jump in the AV conduction curve. The jumps always occurred after the FP to SP transition. The FP-SP transition occurred at prematurity at 196 ± 39ms versus the jump at 114 ± 13ms (p < 0.001). The beat with a jump showed an SP-FP pattern in seven and an SP-SP pattern in four preparations. The jumps were always associated with and most likely caused by the formation of intranodal/nodal-atrial reentry and its subsequent conduction, rather than a switch from FP to SP conduction. Conclusion: Contrary to what has been assumed, a transition from FP to SP conduction does not produce a jump in the AV conduction curve. A jump in the AV conduction curve is most likely caused by the formation of intranodal/nodal-atrial reentry and its subsequent conduction.

The Upper Common Pathway in Atrioventricular Nodal Reentrant Tachycardia: A Comprehensive Review of Evidence and Current Perspectives.

Atrioventricular nodal reentrant tachycardia (AVNRT) is the most common form of paroxysmal supraventricular tachycardia, and its diagnostic and therapeutic approaches have been well-established. Traditionally, AVNRT is understood to be an intranodal reentry having two bystander pathways; the upper common pathway (UCP) which connects to the atrium and the lower common pathway which connects to the ventricle. However, the existence of the UCP remains a subject of ongoing debate. The assertion of the UCP's presence is supported by electrophysiological evidence suggesting that the atrium is not essential for the perpetuation of AVNRT. Nonetheless, numerous anatomical studies have failed to identify any structure that could be conclusively designated as the UCP. The histological and electrophysiological characteristics of the slow and fast pathways, which are the core components of AVNRT, suggest the inclusion of atrial myocardium in the reentry circuit. While clear interpretation of these discrepancies remains elusive, potential explanations may be derived from existing evidence and recent research findings concerning the actual AVNRT circuit.

Voltage and propagation mapping: New tools to improve successful ablation of atrioventricular nodal reentry tachycardia.

Mapping system is useful in ablation of atrioventricular nodal reentry tachycardia (AVNRT) and localization of anatomic variances. Voltage mapping identifies a low voltage area in the Koch trianglecalled low-voltage-bridge (LVB); propagation mapping identifies the collision point (CP) of atrial wavefront convergence. We conducted a prospective study to evaluate the relationship between LVB and CP with successful site of ablation and identify standard value for LVB. Three-dimensional (3D) maps of the right atria were constructed from intracardiac recordings using the ablation catheter. Cut-off values on voltage map were adjusted until LVB was observed. On propagation map, atrial wavefronts during sinus rhythm collide in the site representing CP, indicating the area of slow pathway conduction. Ablation site was selected targeting LVB and CP site, confirmed by anatomic position on fluoroscopy and atrioventricular ratio. Twenty-seven consecutive patients were included. LVB and CP were present in all patients. Postprocedural evaluation identified standard cut-off of 0.3-1 mV useful for LVB identification. An overlap between LVB and CP was observed in 23 (85%) patients. Procedure success was achieved in all patient with effective site at first application in 22 (81%) patients. There was a significant correlation between LVB, CP, and the site of effective ablation (p = .001). We found correlation between LVB and CP with the site of effective ablation, identifying a voltage range useful for standardized LVB identification. These techniques could be useful to identify ablation site and minimize radiation exposure.

The optimal slow pathway ablation site in atrioventricular nodal reentrant tachycardia cases with an inferiorly located His bundle.

The optimal slow pathway (SP) ablation site in cases with an inferiorly located His bundle (HIS) remains unclear. In 45 patients with atrioventricular nodal reentrant tachycardia, the relationship between the HIS location and successful SP ablation site was assessed in electroanatomical maps. We assessed the location of the SP ablation site relative to the bottom of the coronary sinus ostium in the superior-to-inferior (SPSI), anterior-to-posterior (SPAP), and right-to-left (SPRL) directions. The HIS location was assessed in the same manner. The HIS location in the superior-to-inferior direction (HISSI), SPSI, SPAP, and SPRL were 17.7±6.4, 1.7±6.4, 13.6±12.3, and -1.0±13.0mm, respectively. The HISSI was positively correlated with SPSI (R2=0.62; P<.01) and SPAP (R2=0.22; P<.01), whereas it was not correlated with SPRL (R2=0.01; P=.65). The distance between the HIS and SP ablation site was 17.7±6.4mm and was not affected by the location of HIS. The ratio of the amplitudes of atrial and ventricular potential recorded at the SP ablation site did not differ between the high HIS group (HISSI≥13mm) and low HIS group (HISSI<13mm) (0.10±0.06vs. 0.10±0.06; P=.38). In cases with an inferiorly located HIS, SP ablation should be performed at a lower and more posterior site than in typical cases.

Novel near-field detection algorithm–guided slow pathway ablation in atrioventricular nodal reentrant tachycardia

Novel near-field detection algorithm–guided slow pathway ablation in atrioventricular nodal reentrant tachycardia

Seasonal and interannual dissolved organic carbon transport process dynamics in a subarctic headwater catchment revealed by high-resolution measurements

Abstract. Dissolved organic carbon (DOC) dynamics are evolving in the rapidly changing Arctic and a comprehensive understanding of the controlling processes is urgently required. For example, the transport processes governing DOC dynamics are prone to climate-driven alteration given their strong seasonal nature. Hence, high-resolution and long-term studies are required to assess potential seasonal and interannual changes in DOC transport processes. In this study, we monitored DOC at a 30 min resolution from September 2018 to December 2022 in a headwater peatland-influenced stream in northern Finland (Pallas catchment, 68° N). Temporal variability in transport processes was assessed using multiple methods: concentration–discharge (C–Q) slope for seasonal analysis, a modified hysteresis index for event analysis, yield analysis, and random forest regression models to determine the hydroclimatic controls on transport. The findings revealed the following distinct patterns: (a) the slope of the C–Q relationship displayed a strong seasonal trend, indicating increasing transport limitation each month after snowmelt began; (b) the hysteresis index decreased post-snowmelt, signifying the influence of distal sources and DOC mobilization through slower pathways; and (c) interannual variations in these metrics were generally low, often smaller than month-to-month fluctuations. These results highlight the importance of long-term and detailed monitoring to enable separation of inter- and intra-annual variability to better understand the complexities of DOC transport. This study contributes to a broader comprehension of DOC transport dynamics in the Arctic, specifically quantifying seasonal variability and associated mechanistic drivers, which is vital for predicting how the carbon cycle is likely to change in Arctic ecosystems.

Photodissociation of the CH2Br radical: A theoretical study.

Bromine atom (Br) reactions lead to ozone depletion in the troposphere and stratosphere. Photodegradation of bromocarbons is one of the main sources of bromine atoms in the atmosphere. Here, we use high-level abinitio methods, including spin-orbit effects, to study the photodissociation of the CH2Br radical. All possible fragmentation pathways, namely CH2Br + hν → CH2 + Br, HCBr + H, and CBr + H2, have been analyzed. Potential-energy curves of the ground and several excited electronic states along the corresponding dissociating bond distance of each pathway have been calculated. Considering the actinic fluxes of solar irradiation in the troposphere and in the stratosphere in the relevant range of frequencies, it is found that the first five excited states of CH2Br can be accessed from the ground state. Analysis of the potential curves shows that the pathways producing CH2 + Br and HCBr + H can proceed through a fast direct dissociation mechanism, while the pathway leading to CBr + H2 involves much slower dissociation mechanisms like internal conversion between electronic states, predissociation, or tunneling through exit barriers. The main implications are that the two faster channels are predicted to be dominant, and the slower pathway is expected to be less relevant. The tropospheric and stratospheric solar actinic fluxes also allow for further dissociation of the HCBr and CBr fragments, generating additional Br atoms, provided that they survive possible collisions with other atmospheric reagents. Finally, we discuss the possible effect of each of the three CH2Br dissociation pathways on the depletion of atmospheric ozone.

A conserved CENP-E region mediates BubR1-independent recruitment to the outer corona at mitotic onset

The outer corona plays an essential role at the onset of mitosis by expanding to maximize microtubule attachment to kinetochores.1,2 The low-density structure of the corona forms through the expansion of unattached kinetochores. It comprises the RZZ complex, the dynein adaptor Spindly, the plus-end directed microtubule motor centromere protein E (CENP-E), and the Mad1/Mad2 spindle-assembly checkpoint proteins.3,4,5,6,7,8,9,10 CENP-E specifically associates with unattached kinetochores to facilitate chromosome congression,11,12,13,14,15,16 interacting with BubR1 at the kinetochore through its C-terminal region (2091-2358).17,18,19,20,21 We recently showed that CENP-E recruitment to BubR1 at the kinetochores is both rapid and essential for correct chromosome alignment. However, CENP-E is also recruited to the outer corona by a second, slower pathway that is currently undefined.19 Here, we show that BubR1-independent localization of CENP-E is mediated by a conserved loop that is essential for outer-corona targeting. We provide a structural model of the entire CENP-E kinetochore-targeting domain combining X-ray crystallography and Alphafold2. We reveal that maximal recruitment of CENP-E to unattached kinetochores critically depends on BubR1 and the outer corona, including dynein. Ectopic expression of the CENP-E C-terminal domain recruits the RZZ complex, Mad1, and Spindly, and prevents kinetochore biorientation in cells. We propose that BubR1-recruited CENP-E, in addition to its essential role in chromosome alignment to the metaphase plate, contributes to the recruitment of outer corona proteins through interactions with the CENP-E corona-targeting domain to facilitate the rapid capture of microtubules for efficient chromosome alignment and mitotic progression.

Relationship between the atrial-activation pattern around the triangle of Koch and successful ablation sites in slow-fast atrioventricular nodal reentrant tachycardia.

The precise details of atrial activation around the triangle of Koch (ToK) remain unknown. We evaluated the relationship between the atrial-activation pattern around the ToK and success sites for slow-pathway (SP) modification ablation in slow-fast atrioventricular reentrant tachycardia (AVNRT). Thirty patients with slow-fast AVNRT who underwent successful ablation were enrolled. Atrial activation around the ToK during sinus rhythm was investigated using ultra-high-density mapping pre-ablation. The relationships among features of atrial-activation pattern and success sites were examined. Of 30 patients (22 cryoablation; 8 radiofrequency ablation), 26 patients had a collision site of two wavefronts of delayed atrial activation within ToK, indicating a success site. The activation-search function of Lumipoint software, which highlights only atrial activation with a spatiotemporal consistency, showed non-highlighted area on the tricuspid-annulus side of ToK. In 23 of the patients, a spiky potential was recorded at that collision site outside the Lumipoint-highlighted area. Fifteen cryoablation patients with a success site coincident with a collision site outside the Lumipoint-highlighted area had significantly more frequent disappearances of SP after initial cryoablation (46.7% vs. 0%, p = .029), fewer cryoablations (3.7 ± 1.8 vs. 5.3 ± 1.3, p = .045), and shorter procedure times (170 ± 57 vs. 228 ± 91 min, p = .082) compared to the seven cryoablation patients without such sites. Four patients had transient AV block by ablation inside the Lumipoint-highlighted area with fractionated signals, but no patient developed permanent AV block or recurrence post-procedure (median follow-up: 375 days). SP modification ablation at the collision site of atrial activation of the tricuspid-annulus side along with a spiky potential could provide a better outcome.

Ultrafast electronic relaxation pathways of the molecular photoswitch quadricyclane

The light-induced ultrafast switching between molecular isomers norbornadiene and quadricyclane can reversibly store and release a substantial amount of chemical energy. Prior work observed signatures of ultrafast molecular dynamics in both isomers upon ultraviolet excitation but could not follow the electronic relaxation all the way back to the ground state experimentally. Here we study the electronic relaxation of quadricyclane after exciting in the ultraviolet (201 nanometres) using time-resolved gas-phase extreme ultraviolet photoelectron spectroscopy combined with non-adiabatic molecular dynamics simulations. We identify two competing pathways by which electronically excited quadricyclane molecules relax to the electronic ground state. The fast pathway (<100 femtoseconds) is distinguished by effective coupling to valence electronic states, while the slow pathway involves initial motions across Rydberg states and takes several hundred femtoseconds. Both pathways facilitate interconversion between the two isomers, albeit on different timescales, and we predict that the branching ratio of norbornadiene/quadricyclane products immediately after returning to the electronic ground state is approximately 3:2.

Orthodromic Reciprocating Tachycardia Relying on Aberrant Conduction: The Need for a Larger Circuit.

Aberrant conduction during orthodromic reciprocating tachycardia (ORT) prolongs the ventriculoatrial conduction time, which can be essential for the maintenance of tachycardia in specific cases. We searched for ORT relying on aberrancy among 220 cases in our center. Three patients showed the phenomenon of aberrancy-dependent ORT. All accessory pathways were located at the lateral regions of the atrioventricular annulus. None of them had a baseline bundle branch block (BBB). Creating a functional BBB was necessary to induce the tachycardias. In two cases, termination of tachycardias was directly associated with resolution of the aberration. In the other case, re-entry required both BBB and slow pathway conduction. We conclude that extra transseptal time caused by aberrancy can be an integral part of the ORT circuit, which explains the infrequent and unsustainable episodes of ORT in certain patients and is useful in understanding the circuit and localizing the pathway.

Membrane Epilipidome-Lipid Modifications, Their Dynamics, and Functional Significance.

Lipids are characterized by extremely high structural diversity translated into a wide range of physicochemical properties. As such, lipids are vital for many different functions including organization of cellular and organelle membranes, control of cellular and organismal energy metabolism, as well as mediating multiple signaling pathways. To maintain the lipid chemical diversity and to achieve rapid lipid remodeling required for the responsiveness and adaptability of cellular membranes, living systems make use of a network of chemical modifications of already existing lipids that complement the rather slow biosynthetic pathways. Similarly to biopolymers, which can be modified epigenetically and posttranscriptionally (for nucleic acids) or posttranslationally (for proteins), lipids can also undergo chemical alterations through oxygenation, nitration, phosphorylation, glycosylation, etc. In this way, an expanded collective of modified lipids that we term the "epilipidome," provides the ultimate level of complexity to biological membranes and delivers a battery of active small-molecule compounds for numerous regulatory processes. As many lipid modifications are tightly controlled and often occur in response to extra- and intracellular stimuli at defined locations, the emergence of the epilipidome greatly contributes to the spatial and temporal compartmentalization of diverse cellular processes. Accordingly, epilipid modifications are observed in all living organisms and are among the most consistent prerequisites for complex life.

Efficacy and safety between radiofrequency ablation and types of cryoablation catheters for atrioventricular nodal reentrant tachycardia: A Network Meta-analysis and Systematic Review.

Atrioventricular nodal reentrant tachycardia (AVNRT) is the most common supraventricular tachycardia referred for ablation. Periprocedural conduction system damage was a primary concern during AVNRT ablation. This study aimed to assess the incidence of permanent atrioventricular (AV) block and the success rate associated with different types of catheters in slow pathway ablation. A literature search was performed to identify studies that compared various techniques, including types of radiofrequency ablation (irrigated and nonirrigated) and different sizes of catheter tip cryoablation (4, 6, and 8-mm), in terms of their outcomes related to permanent atrioventricular block and success rate. To assess and rank the treatments for the different outcomes, a random-effects model of network meta-analysis, along with p-scores, was employed. A total of 27 studies with 5110 patients were included in the analysis. Overall success rates ranged from 89.78% to 100%. Point estimation showed 4-mm cryoablation exhibited an odds ratio of 0.649 (95%CI: 0.202-2.087) when compared to nonirrigated RFA. Similarly, 6-mm cryoablation had an odds ratio of 0.944 (95%CI: 0.307-2.905), 8-mm cryoablation had an odds ratio of 0.848 (95%CI: 0.089-8.107), and irrigated RFA had an odds ratio of 0.424 (95%CI: 0.058-3.121) compared to nonirrigated RFA. Our study found no significant difference in the incidence of permanent AV block between the types of catheters. The success rates were consistently high across all groups. These findings emphasize the potential of both RF ablation (irrigated and nonirrigated catheter) and cryoablation as viable options for the treatment of AVNRT, with similar safety and efficacy profile.

Empirical slow pathway ablation in patients with paroxysmal palpitations and pounding in the neck: A 15-year follow-up

Empirical slow pathway ablation in patients with paroxysmal palpitations and pounding in the neck: A 15-year follow-up

Incidence, clinical characteristics, electrophysiological characteristics and outcomes of patients with baseline PR prolongation undergoing radiofrequency ablation for Atrioventricular nodal reentrant tachycardia

Aims and objectivesAtrioventricular nodal reentrant tachycardia (AVNRT) is the most common supraventricular tachycardia (SVT). Prolonged PR interval(>200 ms) on baseline electrocardiogram (ECG) is uncommon in such patients. The aim of the current study was to evaluate the incidence, clinical, electrophysiological characteristics, and outcomes of patients with baseline prolongation of PR interval undergoing radio-frequency ablation (RFA) for AVNRT. MethodsOver 10 years, out of the total number of 1435 patients with diagnosed AVNRT, 16 patients had prolonged PR intervals at baseline. All underwent elective RFA. A retrospective analysis of clinical, and electrophysiological characteristics and outcomes was done. The PR interval and atria-ventricular block cycle length values were compared with those patients with a normal interval at baseline and had undergone a successful slow pathway modification for AVNRT. ResultsOut of 1435 patients with AVNRT, 16 (0.9 %) patients had baseline PR prolongation on ECG. The mean(+SD) age of the study population was 62.9 + 15.9 years. 10 (62.5 %) were males. The average PR interval was 264.2 + 24.1 ms. Slow fast AVNRT was seen in all. The anatomical site of success for ablation was the lower part of Koch's triangle in all patients. During ablation, a good sustained junctional rhythm was noted in all, with no AV (Atrioventricular) block or PR prolongation noted during ablation in any of the patients. PR interval decreased by more than 20 ms in 10 (62.5 %) patients. AVBCL (AV node block cycle length) increased on an average of 58.7 ms post-ablation. Only one patient developed AV block on follow-up. ConclusionA prolonged PR interval on baseline ECG is uncommon in patients with AVNRT. In these patients, slow pathway modification can be done safely and effectively. AVBCL (AV node block cycle length) increases immediately post-ablation. The risk of AV block though low persists on follow-up.