Significant p values (<0.05) are highlighted in bold.
Despite a trend towards lower peak AI, use of a long sheath at the Mid CTI was associated with a greater and more rapid drop in impedance.
Discussion
Our study made use of Ablation Index (AI) to examine ablation lesion characteristics in the treatment of typical atrial flutter. We found that using a target AI of 600Wgs across the CTI was safe, but there were significant inconsistencies in energy delivery according to anatomical site, with particularly notable variance seen in the Mid CTI.
Whilst mean contact force and power were similar across all CTI sites, higher peak AI values were seen Mid CTI, which appears to be mediated through compensatory increases in lesion duration. However, at this site, increased peak AI values did not necessarily correlate with greater impedance drop, and both the rate of impedance drop and the mean lesion temperature were significantly lower at this site despite similar catheter tip power, mean contact force and irrigation techniques, suggestive of inferior lesion efficacy.
We propose this occurs because of significant temporal fluctuations in contact force and catheter tip angle which are most exaggerated at the Mid CTI (figures 3 & 4). It is accepted that changes in catheter angle of incidence affect lesion size , and that consistent tissue contact produces larger lesions than intermittent contact (20). The fluctuations seen in our study are likely to indicate catheter tip instability encountered due to established anatomical anomalies Mid CTI (13). Consequently, whilst the relationship between AI and ID is mostly linear (figures 1 & 2) – and hence peak AI is a reasonable surrogate for lesion efficacy in atrial flutter – we suggest that operators should exercise caution at the Mid CTI, where higher peak AI values do not necessarily equate to more effective ablation.
The finding that fluctuations in contact force Mid CTI become more pronounced at higher AI values is unexpected (figure 3); we suggest this may relate to the accumulation of local tissue oedema as a result of more prolonged radiofrequency applications, which may in turn further retard impedance drop - this hypothesis could not be assessed in our study. Importantly, the use of a long sheath appears to confer additional stability to the catheter tip and overcome the majority of inconsistencies in force and angle, although a significant increase in axial plane fluctuation was noted at the ventricular margin (figure 4). This finding may represent catheter and sheath overreach at the point of the valve annulus into the ventricular inflow tract; a long sheath remained effective in stabilising contact force at this site.
The optimal AI for safe and effective CTI ablation is not well established; our target AI of 600Wgs was based on retrospective analysis performed at our centre (18), and is in excess of that which is generally recommended for left atrial procedures. Importantly, we did not record any acute or long-term complications with our protocol for CTI ablation. Our mechanistic study was not designed to assess long-term procedural efficacy, however it is encouraging that 97.4% of patients were in sinus rhythm after 6 months’ follow-up. Longer term clinical review is in progress.
To our knowledge, we are the second group to analyse AI in the ablation of atrial flutter. Zhang et al. (2019) compared AI-guided ablation versus contact-force guided ablation of the CTI, and found higher rates of first-pass conduction block in their AI-guided group (21). These authors delivered ablation with AI targets of 500Wgs to the anterior two thirds of the CTI and 400Wgs to the posterior third; these target values were derived from studies of PVI and adjusted according to accepted variations in CTI thickness. They found that acute reconnection of the CTI was more common at the ventricular aspect of the CTI with AI values of <450Wgs. In contrast to our study, the authors’ protocol delivered higher AI values at the anterior segments and found that this was associated with a relatively larger drop in impedance. Temporal changes in contact force or catheter angle, and the correlation of impedance drop with different peak AI values, were not examined. Our study is the first to make use of precise AI-associated lesion delivery characteristics to define the heterogeneity encountered when ablating the CTI.
Whilst our findings may have been more sharply delineated with the addition of peri- or -intraprocedural imaging studies, in clinical practice outcomes for CTI ablation without these techniques are already reasonable . Accordingly, our aim was to explore a novel utility of an existing technology as a vehicle for incremental improvements in procedural success and safety, which could be adopted clinically without considerable additional resources. We have examined the correlation of pre-specified AI targets with post hoc values of mean ID at the CTI; a multi-centre trial (LOCALIZE) assessing the efficacy of PVI guided by live measurements of local impedance is ongoing, and may further contribute to our understanding of predicting lesion transmurality from intraprocedural variables (22).
Our study has important limitations. Whilst our patient sample is multicentre, patient numbers are small; the number of extracted lesion data points was sufficient to permit key analyses, however meaningful comparisons of additional potential confounders – for example catheter curve, patient co-morbidities, or the use of general anaesthetic or of class III anti-arrhythmic drugs – could not be performed. AI itself is validated only for ablation catheters made by Biosense Webster; our results may not be generalisable to equipment from other manufacturers. Likewise, our operators did not use surround flow (STSF) catheters, which are known to have different biophysical efficacy than their ST equivalent and hence our findings cannot be extrapolated to this technology . We used a measure of impedance drop as marker of ablation efficacy; this is a widely accepted surrogate in the literature however histological analysis is the gold standard for the assessment of lesion quality and this was not available in our study.
Conclusions
In the ablation of CTI-dependent atrial flutter, the use of ablation index (AI) appears safe when delivering lesions up to 600Wgs. Whilst AI is generally a reliable marker of ablation efficacy by impedance drop, we suggest that operators exercise caution when interpreting AI values at the Mid CTI, where anatomical anomalies may introduce catheter instability and hence impede energy delivery. Higher AI values are required Mid CTI to deliver lesions comparable to the CTI margins, and this phenomenon seems to be mediated by temporal fluctuations in contact force and catheter angle. It appears that the use of a long sheath may mitigate these fluctuations and restore a more linear relationship between AI and ID. In the absence of a long sheath, an ablation strategy which prescribes higher AI targets to the Mid CTI may improve acute lesion efficacy and hence long-term outcomes, however this requires prospective validation.
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FIGURE LEGENDS
Figure 1: Relationship between peak ablation index (Wgs) and mean impedance drop (Ohms; mean ± SD) for all CTI lesions. Pearson’s R2=0.89, p<0.0001.
Figure 2: Relationship between peak ablation index (Wgs) and mean impedance drop (Ohms) per lesion according to CTI anatomical site (V CTI R2=0.95, p<0.0001, Mid CTI R2=0.15, p=0.21, IVC CTI R2=0.88, p<0.0001).
Figure 3: Average temporal fluctuation in contact force (mg/s) per ablation lesion according to peak ablation index, stratified according to CTI anatomical site and the use of a long sheath.
Figure 4: Average temporal fluctuation in catheter angle of incidence (degrees/s) in the lateral and axial planes for each ablation lesion, stratified according to CTI anatomical site and the use of a long sheath. Significant p values highlighted; other relationships non-significant.