Case Study: Local Impedance Guided Cavotricuspid Isthmus (CTI) Dependent Atrial Flutter Ablation

Typical atrial flutter (AFL), or cavotricuspid isthmus (CTI) dependent flutter, is a macro-reentrant atrial tachycardia that follows along the right atrial septum, across the roof, down the lateral wall of the right atrium with the CTI as the common isthmus. The overall incidence is approximately 0.9%.¹ CTI ablation is an effective treatment for typical AFL with procedural success rates greater than 90% when bi-directional block across the CTI is achieved.² At times, it may be difficult to achieve this end point due to variable anatomy, including conduction bundles, pouches, and muscular ridges. Workflows have evolved from conventional 4 mm catheters to irrigated 8 mm ablation catheters.³ Catheter positioning and radiofrequency ablation (RFA) delivery can be guided by a combination of electrograms, intracardiac echo (ICE), fluoroscopic imaging, inter-lesion distance (ILD), and generator impedance.

Local impedance (LI) monitoring allows one to measure changes in the local electric field around the catheter tip and may be an additional useful tool in CTI ablation (DIRECTSENSE Technology, INTELLANAV MIFI OI; Boston Scientific). In a bench study, Garrott et al found that monitoring LI drops during RF ablation reflected the temperature increase needed below the tissue surface for adequate tissue heating (>50^°C) for durable lesions.⁴ LI allows physicians more control over lesion delivery, giving continuous feedback on volumetric tissue heating, which in turn improves the quality and safety of RF lesions.⁴

A recent study by Saraf et al compared conventional fluoroscopy, contact force, and LI-based strategies to guide CTI-dependent AFL ablation. They found that the total ablation time was significantly shorter when guided by LI.⁵ Additionally, time from the start of ablation to CTI bi-directional block was shorter when guided by LI versus contact force or conventional fluoroscopy. This case study further highlights the procedural efficiency and added potential safety benefits of using DIRECTSENSE to guide CTI ablation for AFL.

Case Description

A 76-year-old male with a history of CTI-dependent flutter presented for catheter ablation. RHYTHMIA HDx and ICE (ViewFlex, Abbott) were used to guide ablation. CTI linear ablation was performed from the tricuspid annulus to the inferior vena cava by point-by-point ablation with a 4-mm-tip catheter equipped with microelectrodes in the tip to allow for LI monitoring (INTELLANAV MIFI OI^™; Boston Scientific). Lesions were delivered at 40W with a 30 ml/min irrigation flow rate. LI measurements were used to guide the duration of the lesion. Lesions were considered complete when there was a plateau in the local impedance versus time trace and MIFI signals attenuated. Attention was paid to the rate of the LI drop. If a rapid drop of >-20Ω occurred, ablation was stopped to reduce the risk of a steam pop.^4,6 Bi-directional block was confirmed by pacing from the ablation catheter positioned on the lateral-medial wall and then pacing from the proximal CS.

The CTI line was created with a total of 10 discrete RF applications with 10 AutoTags (Figure 1). Lesions with LI drops of ≥-15Ω were tagged red, while an ILD of ≤6 mm was used to guide the spacing between lesions. The blood pool LI was 90Ω, and the average baseline tissue LI was 102Ω (Table 1). Monitoring for changes of local impedance measurements between blood pool and tissue assisted in determining catheter-tissue coupling and catheter stability. The average LI drop was -20Ω with an average ablation time of 14 seconds per lesion. The total ablation time was 2 minutes and 18 seconds. The longest ILD was 5.45 mm. Bi-directional block was achieved after first pass with a trans-isthmus conduction time of 185 ms and 200 ms from lateral-medial and medial-lateral walls, respectively.

Efficiency

The total ablation time for this case study was 2 minutes, 18 seconds with a maximum RF application time per lesion of 22 seconds. This is consistent with a recent 30-patient CTI ablation study that showed the average LI-guided CTI ablation time was 3.2±1.3 min. LI-guided CTI reduced ablation time by 43% vs conventional fluoroscopy (ie, use of electrograms, fluoroscopic imaging, and RF generator impedance) and a 44% reduction vs contact force with electroanatomical mapping with no differences in acute success.⁵ The number of lesions required to achieve block was not significantly different between the three groups and was similar to the number of lesions in this case study. This suggests that LI-guided ablation shortens the time needed for each lesion versus waiting for a pre-specified time as with conventional fluoroscopic imaging and RF generator impedance and contact force with electroanatomically guided CTI ablation. An adjustment in workflow from a drag lesion set to point-by-point has created efficiency and reduced the number of lesions and overall ablation time since adopting the DIRECTSENSE Technology.

Safety

In this case study, the LI drops were ≥-16. Lesion 9 (Table 1) had a rapid LI drop (-32Ω LI drop in <10 seconds), so the lesion was terminated to avoid a potential steam pop. Sasaki et al studied the optimal LI drop for effective RF ablation during CTI ablation in 50 patients.⁷ The authors demonstrated that the absolute and percentage LI drops were significantly greater at effective ablation sites versus ineffective ablation sites. The optimal LI drops for an effective RF ablation during CTI was ≥-12Ω. Understanding the minimum LI targets for creation of RF transmural lesions for CTI ablation is important to preventing excessive ablation which may result in steam pops, thrombus/char formation, and extra-cardiac damage. The ability to measure a biophysical parameter directly from the tissue differentiates the LI monitoring technology from other ablation metrics that combine ablation inputs.⁸

Conclusion

This case study highlights the efficiency and potential safety benefits of using LI to guide CTI ablation for AFL. The total ablation time was 2 minutes, 18 seconds with LI drops ≥-16Ω with a maximum RF application time per lesion of 22 seconds. One lesion was terminated early due to a rapid drop in LI to avoid a potential steam pop. Monitoring of LI during CTI ablation may allow for tailoring of ablation by monitoring a biological parameter directly from the tissue, improving the efficiency and potentially the safety of the procedure.

Disclosure: Dr. Miske reports consulting fees as a mapping student proctor for Boston Scientific; he has no other conflicts of interest to report regarding the content herein.

This article is published with support from Boston Scientific.

References

1. Kircher S, Rolf S, Hindricks G, Sommer P. Ablation of typical atrial flutter using a novel non-fluoroscopic electromagnetic catheter tracking system. Interv Cardiol. 2014;6(2):1-10.

2. Pérez FJ, Schubert CM, Parvez B, Pathak V, Ellenbogen KA, Wood MA. Long-term outcomes after catheter ablation of cavo-tricuspid isthmus dependent atrial flutter: a meta-analysis. Circ Arrhythm Electrophysiol. 2009;2(4):393-401.

3. Jais P, Haissaguerre M, Shah DC, et al. Successful irrigated-tip catheter ablation of atrial flutter resistant to conventional radiofrequency ablation. Circulation. 1998;98:835-838.

4. Garrott K, Laughner J, Gutbrod S, et al. Combined local impedance and contact force for radiofrequency ablation assessment. Heart Rhythm. 2020;17(8):1371-1380.

5. Saraf K, Black N, Garratt C, Muhyaldeen S, Morris G. Local impedance and ultra-high density 3-dimensional mapping results in improved ablation metrics for cavotricuspid isthmus dependent atrial flutter compared with conventional ablation and contact force-guided ablation with 3-dimensional mapping. Authorea. January 14, 2021.

6. Sulkin MS, Laughner JI, Hilbert S, et al. Novel measure of local impedance predicts catheter-tissue contact and lesion formation. Circ Arrhythm Electrophysiol. 2018;11(4):e005831.

7. Sasaki T, Nakamura K, Inoue M, et al. Optimal local impedance drops for an effective radiofrequency ablation during cavo-tricuspid isthmus ablation. J Arrhythm. 2020;36(5):905-911.

8. Das M, Luik A, Shepherd E, et al. Local catheter impedance drop during pulmonary vein isolation predicts acute conduction block in patients with paroxysmal atrial fibrillation: initial results of the LOCALIZE clinical trial. Europace. 2021;23(7):1042-1051.