Clinical Implications of Unmasking Dormant Conduction After Circumferential Pulmonary Vein Isolation in Atrial Fibrillation Using Adenosine: A Systematic Review and Meta-Analysis

Purpose: Circumferential pulmonary vein isolation (CPVI) is a routine ablation strategy of atrial fibrillation (AF). The adenosine test can be used to unmask dormant conduction (DC) of pulmonary veins after CPVI, thereby demonstrating possible pulmonary vein re-connection and the need for further ablation. However, whether adenosine test could help improve the long term successful rate of CPVI is still controversial. This systemic review and meta-analysis was to determine the clinical utility of the adenosine test. Methods: PubMed, EMBASE, Web of Science and Cochrane Library database were searched through July 2016 to identify relevant studies using the keywords “dormant pulmonary vein conduction,” “adenosine test,” “circumferential pulmonary vein isolation,” and “atrial fibrillation.” A random-effects model was used to compare pooled outcomes and tested for heterogeneity. Results: A total of 17 studies including 5,169 participants were included in the final meta-analysis. Two groups of comparisons were classified: (1) Long-term successful rate in those AF patients underwent CPVI with and without adenosine test [Group A (+) and Group A (−)]; (2) Long-term successful rate in those patients who had adenosine test with and without dormant conduction [Group DC (+) and Group DC (−)]. The overall meta-analysis showed that no significant difference can be observed between Group A (+) and Group A (−) (RR 1.08; 95% CI 0.97–1.19; P = 0.16; I2 = 66%) and between Group DC (+) and Group DC (−) (RR 1.01; 95% CI 0.91–1.12; P = 0.88; I2 = 60%). Conclusion: Pooled meta-analysis suggested adenosine test may not improve long-term successful rate in AF patients underwent CPVI. Furthermore, AF recurrence may not be decreased by eliminating DC provoked by adenosine, even though adenosine test was applied after CPVI.


INTRODUCTION
Atrial fibrillation (AF) is a common cardiac arrhythmia, placing a significant burden on healthcare systems worldwide. It has been estimated that 33.5 million people suffering from AF with an increasing prevalence partly attributable to an aging population (Thacker et al., 2013;Chugh et al., 2014). Because pulmonary veins (PVs) are often the triggering sites for initiating and maintaining AF, circumferential PV isolation (CPVI) has been cornerstone of catheter ablation strategy to restore sinus rhythm for AF (Haïssaguerre et al., 2000;Jaïs et al., 2008;Kirchhof et al., 2017). Although feasibility and visibility of the three-dimensional electroanatomic mapping system have been improved, AF recurrence remains a problem due to PV reconnection after CPVI ablation (Ouyang et al., 2005). A study suggested that 20% of AF patients required repeat procedures after a median follow-up of 13 months (Hocini et al., 2005). Previous studies have suggested that PV re-connection can be identified by unmasking dormant conductions (DCs) induced by adenosine (Arentz et al., 2004;Theis et al., 2015;Ghanbari et al., 2016). The adenosine test has been used extensively to identify DCs (Arentz et al., 2004). The mechanism is thought to involve hyperpolarization of the membrane potential of dormant PVs by activating the I KAdo inward rectifier current, which would transiently establish PV reconnection (Datino et al., 2010).
A recent systematic review and meta-analysis has demonstrated a positive outcome on assessment and ablation of dormant conduction (McLellan et al., 2013). However, some of the enrolled studies were based on segmental ablation strategy. Moreover, many studies suggested that whether DCs are associated with high rate of AF recurrence or adenosine test can improve clinical outcome of PVI remains controversial (Elayi et al., 2013;Kobori et al., 2015;Theis et al., 2015;Ghanbari et al., 2016;Kim et al., 2016). Several investigators have attempted to use the appearance of DCs as indication of further ablation using adenosine test after PVI for AF ablation, while results were restricted by low number of participants (McLellan et al., 2013). Therefore, if adenosine test will help to improve ablation success rates after CPVI remains controversial. We conducted this systematic review and meta-analysis to determine the clinical significance of unmasking DCs after CPVI based on long-term follow up using adenosine test as the guidance of extra ablation for AF patients.

Search Strategy
The databases Pubmed, EMBASE, Web of Science and Cochrane library were searched using searching terms and related items including keywords "dormant pulmonary vein conduction, " "adenosine test, " "circumferential pulmonary vein isolation, " and "atrial fibrillation."

Inclusion and Exclusion Criteria
The inclusion criteria were limited to articles published in English, involving human subjects of adult age, and published between 2010 and 2016. The exclusion criteria were: (1) ablation for non-AF patients; (2) no adenosine test used; (3) studies including fewer than 90 participants; (4) follow-up period <12 months; (5) CPVI was not used for AF ablation; (6) articles that were case reports, reviews and meta-analyses.

Study Selection
Data from the different studies were entered in pre-specified spreadsheet in Microsoft Excel. All potentially relevant reports were retrieved as complete manuscripts and assessed for compliance with the inclusion criteria. Two reviewers (C.C. and D.L.) independently reviewed each included study and disagreements were resolved by adjudication with input from a third reviewer (Y.X.). Records matching searching goal were enrolled.

Data Analysis
The meta-analysis was performed using Review Manager (RevMan 5. 3, Cochrane Collaboration, Oxford, UK). Relative risk (RR) values with 95% confidence intervals (CI) were calculated. Categorical variables were pooled using the Mantel-Hanseal method. The I 2 statistic from the standard chi-square test (χ 2 ), which describes the percentage of the variability in effect estimates resulting from heterogeneity. A fixed effect model was used if I 2 ≤ 0.25, otherwise the random effect model was used (Higgins and Green, 2011). P-value < 0. 05 (two-tailed) was considered statistical significant.

Quality Assessment
We used the modified Newcastle-Ottawa scale for quality assessment of non-randomized trials and the methodological quality of RCTs was assessed by the components recommended by the Cochrane Collaboration (Higgins and Green, 2011). The quality of each trial except RCTs was quantified by a score of 0-9.
These studies used selective venography or 3-dimensional Electroanatomical Mapping System (including CARTO, Ensite NavX) to identify the PV antrum and subsequently performed CPVI. In four studies, PVI was guided by cryoballoon (second generation cryoballoon, CB-2G) (Van Belle et al., 2012;Compier et al., 2015;Kumar et al., 2015;Tebbenjohanns et al., 2016). The endpoint of electrophysiological study was the presence of entrance block defined by the circular mapping catheter (Lasso, Biosense Webster) or the elimination of all PV potentials or establishment of a bidirectional conduction block between left atrium (LA) and PVs. All participants underwent further ablation if DCs was induced. Two studies described the additional use of superior vena cava isolation (Compier et al., 2015;Kumar et al., 2015).
In this meta-analysis, we supposed to determine: (1) if adenosine test would help to increase the success rate of PVI; and (2) furthermore, if DCs induced by adenosine play an important role in AF recurrence after CPVI. Hence, in the first part, Group A (+) and Group A (−) were divided according to whether adenosine was administrated or not. And in the second part, Group DC (+) and Group DC (−) were divided according to whether the DCs appeared or not after adenosine administration. All of DCs induced by adenosine test in Group A (+) and Group DC (+) patients were eliminated after CPVI. The baseline characteristics of these studies are listed in Table 1, and those of procedure parameter are shown in Table 2. Quality assessment of the included studies was made using the Newcastle-Ottawa Scale for non-randomized case-control studies and the Cochrane Collaboration's tool for randomized trials ( Table 3).

Long-term Success Rate of PVI Between Group A (+) and Group A (−)
The pooled meta-analysis demonstrated that there was no significant difference in freedom from recurrent AF between Group A (+) and Group A (−) (RR = 1.08, 95% CI: 0.97-1.19, P = 0.16, I 2 = 66%; Figure 2). A funnel plot plotting standard errors against the logarithms of the RR are shown in Figure 3, demonstrating no significant publication bias.

Long-term Success Rate of PVI Between Group DC (+) and Group DC (−)
No significant difference was observed between Group DC (+) and Group DC (−) with a pooled RR of 1.01 (95% CI: 0. 91-1.12; P = 0. 88; I 2 = 60%; Figure 4). A funnel plot plotting standard errors against the logarithms of the RR are shown in Figure 5, demonstrating no significant publication bias.

Subgroup Analyses
Additional subgroup analyses were performed for radiofrequency catheter ablation (RFCA) and CB-2G catheter ablation for PVI in Group A (+) and Group A (−). For RFCA, no difference in success rate was observed in Group A (+) and Group A (−) for patients with a RR of 1.02 (95% CI: 0.89-1.17; P = 0.80; Figure 6), which was accompanied by significant   Kobori et al., 2015 Unclear risk of selection bias (insufficient information about the sequence generation and allocation concealment); Unclear risk of performance bias (insufficient information about blinding of participants and personnel); Unclear risk of detection bias (insufficient information about blinding of outcome assessment); low risk of attrition bias (complete outcome for all the patients enrolled); Unclear risk of reporting bias (insufficient information about selective reporting); Unclear risk of other bias (insufficient information about other sources of bias).
- Theis et al., 2015 Unclear risk of selection bias (insufficient information about the sequence generation and allocation concealment); Unclear risk of performance bias (insufficient information about blinding of participants and personnel); Unclear risk of detection bias (insufficient information about blinding of outcome assessment); low risk of attrition bias (complete outcome for all the patients enrolled); Unclear risk of reporting bias (insufficient information about selective reporting); Unclear risk of other bias (insufficient information about other sources of bias).
- Elayi et al., 2013 Adequate case definition; consecutive series of cases; hospital controls; adequate information concerning the selection and definition of controls; groups controlled for all the baseline characteristics; ascertainment of outpatient exposure to adenosine text based on medical records for experiment groups; patients not blinded to case-control status.; same non-Response rate for both groups.
6 Ghanbari et al., 2016 Low risk of selection bias (treatment assignment was concealed in numbered, sealed envelopes, the research staff opened the envelope and revealed the randomization assignment in the electrophysiology laboratory and insufficient information about the sequence generation); Unclear risk of performance bias (insufficient information about blinding of participants and personnel); Unclear risk of detection bias (insufficient information about blinding of outcome assessment); low risk of attrition bias (complete outcome for all the patients enrolled); Unclear risk of reporting bias (insufficient information about selective reporting); Unclear risk of other bias (insufficient information about other sources of bias).
- Kumagai et al., 2010 Adequate case definition; consecutive series of cases; hospital controls; adequate information concerning the selection and definition of controls; groups controlled for all the baseline characteristics; ascertainment of outpatient exposure to adenosine text based on medical records for experiment groups; patients not blinded to case-control status.; same non-Response rate for both groups.
6 Compier et al., 2015 Adequate case definition; consecutive series of cases; hospital controls; adequate information concerning the selection and definition of controls; groups controlled for all the baseline characteristics; ascertainment of outpatient exposure to adenosine text based on medical records for experiment groups; patients not blinded to case-control status.; same non-Response rate for both groups.
6 Kumar et al., 2015 Adequate case definition; consecutive series of cases; hospital controls; adequate information concerning the selection and definition of controls; groups controlled for all the baseline characteristics; ascertainment of outpatient exposure to adenosine text based on medical records for experiment groups; patients not blinded to case-control status.; same non-Response rate for both groups.

6
Van Belle et al., 2012 Adequate case definition; consecutive series of cases; hospital controls; adequate information concerning the selection and definition of controls; groups controlled for all the baseline characteristics except the LA* diameter; ascertainment of outpatient exposure to adenosine text based on medical records for experiment groups; patients not blinded to case-control status.; same non-Response rate for both groups.
5 Tebbenjohanns et al., 2016 Adequate case definition; consecutive series of cases; hospital controls; adequate information concerning the selection and definition of controls; groups controlled for all the baseline characteristics except the age and history with AF*,; ascertainment of outpatient exposure to adenosine text based on medical records for experiment groups; patients not blinded to case-control status.; same non-Response rate for both groups. heterogeneity (I 2 = 73%). Similarly, for CB-2G, success rates for those who underwent adenosine testing (n = 134) were not significantly different from those who did not have such a test (n = 212), with a pooled RR of 1.18 (95% CI = 0. 99-1.42; P = 0.07; Figure 7) with significant heterogeneity (I 2 = 62%).

Sensitivity Analysis
Sensitivity analysis included study design and adenosine test, and none of them showed significant interference with study outcomes. Results are shown in Table 4.

DISCUSSION
Adenosine testing after AF ablation procedures has been widely adopted for demonstrating DCs, which are further ablated to reduce AF recurrence rates (Hocini et al., 2005). However, in our study, the result of pooled meta-analysis suggested that adenosine test may not help to reduce the long-term AF recurrence after CPVI, and further subgroup analysis also confirmed the result. Some recent studies also suggested negative result of adenosine test based on CPVI (Theis et al., 2015;Ghanbari et al., 2016). The reason might be explained by the mechanism of PVI reconnection after CPVI ablation dose not totally attributed by DCs

Study
Assessment Classification (attributable stars) Zhang et al., 2014 Adequate case definition; consecutive series of cases; hospital controls; adequate information concerning the selection and definition of controls; groups controlled for all the baseline characteristics; ascertainment of outpatient exposure to adenosine text based on medical records for experiment groups and controls; patients not blinded to case-control status.; same non-Response rate for both groups.
7 Kim et al., 2016 Adequate case definition; consecutive series of cases; hospital controls; adequate information concerning the selection and definition of controls; groups controlled for all the baseline characteristics; ascertainment of outpatient exposure to adenosine text based on medical records for experiment groups and controls; patients not blinded to case-control status.; same non-Response rate for both groups.
7 Kaitani et al., 2014 Adequate case definition; consecutive series of cases; hospital controls; adequate information concerning the selection and definition of controls; groups controlled for all the baseline characteristics; ascertainment of outpatient exposure to adenosine text based on medical records for experiment groups and controls; patients not blinded to case-control status.; same non-Response rate for both groups.
7 Macle et al., 2015 Low risk of selection bias (randomization was done with permuted blocks of eight and the allocation sequence was computer-generated by an independent organization); low risk of performance bias (Patients were enrolled by study personnel and masked to their randomization assignment for the duration of the trial and study staff doing catheter ablations could not be masked to treatment allocation); low risk of detection bias (All efficacy and adverse outcomes were assessed by an independent adjudicating committee masked to treatment allocation); low risk of attrition bias (complete outcome for all the patients enrolled); low risk of reporting bias (we can find the research plan with "Adenosine following pulmonary vein isolation to target dormant conduction elimination (ADVICE): methods and rationale" though Pubmed); Unclear risk of other bias (insufficient information about other sources of bias). FIGURE 2 | Forest plot comparing long-term success rates of PVI between Group A (+) and Group A (−).
Frontiers in Physiology | www.frontiersin.org (Linz et al., 2018). Potential mechanisms of AF recurrence after CPVI may due to failure of trans-mural injury of PVAs (Rostock et al., 2006), heterogeneity of myocardial sleeves extending into the pulmonary veins (Ho et al., 2001) or so on. As a consequence, the necessity and applicability of adenosine test diminished in the context of CPVI adoption ablation strategy and Whether other techniques, such as pacing along the PVI line by the distal tip of the ablation catheter to identify viable myocardium or potential gaps (Schaeffer et al., 2015) improves PVI outcome should be investigated in the future. However, a recent meta-analysis has shown that long-term success rates of PVI were improved by further eliminating DCs that have been identified by adenosine test for patients underwent segmental ablation for AF (McLellan et al., 2017). The discrepancy results with the results of the previous meta-analysis (McLellan et al., 2017) may due to improved ablation strategies (Ouyang et al., 2004). The 3-dimensional Electroanatomical Mapping System for RFA provides better visualization and reduce the need for excessive ablation (Ouyang et al., 2004). Ablation strategies based on CPVI ablation, instead of segmental ablation, FIGURE 3 | Funnel plot of standard errors against logarithms of odds ratios for studies comparing long-term success rates of PVI between Group A (+) and Group A (−).
were comprehensively adopted for AF patients either paroxysmal AF or persistent AF, leading to better AF control in the longterm (Gepstein et al., 1997). Previous studies had shown that segmental ablation was inferior to long term treatment compared with CPVI, and leads to more complications, such as pulmonary stenosis (Oral et al., 2003). Additionally, cryo-application offers spherical contact with the PV autrum (PVA), guided by annular Achieve catheter and vasography, provided CPVI by the singleshot technique (Nakagawa et al., 2007). Consequently, modifying skills and appliances, meaningful of adenosine administration may have diminished the need for AF re-ablation.
Complications arising from ablating DCs could further contribute to the lack of efficacy. For example, excessive ablation creates scarring of the atrial myocardium, which can serve as substrates for re-entry (Pappone et al., 2004;Tse et al., 2016). Indeed, a previous study compared anatomically guided CPVI with wide atrial ablation, demonstrating that the latter approach significantly increased the likelihood of micro-reentry ablation by producing areas of conduction slowing and block (Hocini et al., 2005). Moreover, we found that fluoroscopic FIGURE 5 | Funnel plot of standard errors against logarithms of odds ratios of studies comparing long-term PVI success rate between Group DC (+) and Group DC (−).
FIGURE 4 | Forest plot comparing long-term PVI success rate between Group DC (+) and Group DC (−).
Frontiers in Physiology | www.frontiersin.org   time and procedure time were prolonged due to adenosine administration.

LIMITATIONS
This systematic review and meta-analysis has several potential limitations. There was moderate heterogeneity across the included studies, which may be due to the following factors. Firstly, differences in study participants between each study especially the types of AF, and in detection criteria were observed. Secondly, several studies have used additional methods during adenosine testing for provoking DCs, such as isoproterenol administration during adenosine test. Thirdly, the dose of adenosine, administration method and procedure (such as waiting period after adenosine) used to unmask dormant conduction was not uniform, this may affect the clinical outcomes. Fourthly, the successful rate of PVI may vary across medical centers due to variation in technical competencies, skills, and outcome measures. As such, the readers are advised to interpret the findings carefully. Nevertheless, funnel plot analysis revealed no significant publication bias. RCTs on CB-2G did not include a high number of participants and additional clinical trials are needed to confirm these findings.

CONCLUSIONS
In conclusion, regular adoption of adenosine test could not further improve PVI success rate basing on long-term observation and elimination of DCs provoked by adenosine after CPVI did not significantly reduce AF recurrence after catheter ablation.