Elsevier

Heart Rhythm

Volume 17, Issue 8, August 2020, Pages 1371-1380
Heart Rhythm

Experimental
Combined local impedance and contact force for radiofrequency ablation assessment

https://doi.org/10.1016/j.hrthm.2020.03.016Get rights and content

Background

The combination of contact force (CF) and local impedance (LI) may improve tissue characterization and lesion prediction during radiofrequency (RF) ablation.

Objective

The purpose of this study was to evaluate the utility of LI combined with CF in assessing RF ablation efficacy.

Methods

An LI catheter with CF sensing was evaluated in swine (n = 11) and in vitro (n = 14). The relationship between LI and CF in different tissue types was evaluated in vivo. Discrete lesions were created in vitro and in vivo at a range of forces, powers, and durations. Finally, an intercaval line was created in 3 groups at 30 W: 30s, Δ20Ω, and Δ30Ω. In the Δ20Ω and Δ30Ω groups, the user ablated until a 20 or 30 Ω LI drop. In the 30s group, the user was blinded to LI.

Results

In vivo, distinction in LI was found between the blood pool and the myocardium (blood pool: 122 ± 7.02 Ω; perpendicular contact: 220 ± 29 Ω; parallel contact: 207 ± 31 Ω). LI drop correlated with lesion depth both in vitro (R = 0.84) and in vivo (R = 0.79), informing sufficient lesion creation (LI drop >20 Ω) and warning of excessive heating (LI drop >65 Ω). When creating an intercaval line, the total RF time was significantly reduced when using LI guidance (6.4 ± 2 minutes in Δ20Ω and 8.1 ± 1 minutes in Δ30Ω) compared with a standard 30-second workflow (18 ± 7 minutes). Acute conduction block was achieved in all Δ30Ω and 30s lines.

Conclusion

The addition of LI to CF provides feedback on both electrical and mechanical loads. This provides information on tissue type and catheter-tissue coupling; provides feedback on whether volumetric tissue heating is inadequate, sufficient, or excessive; and reduces ablation time.

Introduction

Catheter-tissue coupling is crucial for the effective delivery of radiofrequency (RF) energy during ablation. Force, surface area of contact, and tissue type influence the effective delivery of RF energy. For example, high force on scarred myocardium will not produce the same resistive heating as high force on healthy myocardium. Likewise, low surface area of contact on a ridge will not produce the same heating as ablation in a pocket. Historically, parameters including generator impedance (GI), contact force (CF), and surface temperature have been used to assess catheter-tissue coupling and resistive heating, each with varying success.1,2 GI drops of >10 Ω have been traditionally associated with effective energy delivery to healthy tissue.3, 4, 5, 6, 7, 8 However, GI utility is limited because it is measured from the catheter tip to an indifferent electrode on the patient back and is thus subject to variability based on internal fluid levels, thoracic mass, and patch placement.2

Recently, CF has been used to assess tissue contact and catheter stability.1 While CF can confirm catheter-tissue contact, it does not provide information on tissue heating, account for variable tissue properties, or surface area of coverage.1 Furthermore, while force-time integral (FTI) and other lesion indexing algorithms have been used as surrogates for RF dose, adequacy for assessing lesion size is variable across studies, with some suggesting that GI may be a stronger predictor of lesion formation.8, 9, 10

More recently, local impedance (LI) measured between minielectrodes on a catheter tip and the proximal ring has suggested a means of assessing catheter-tissue coupling. A correlation between LI drop and durable lesions was demonstrated on the bench, in vivo swine hearts, and clinically demonstrating the sensitivity of LI to local volumetric heating of tissue.11, 12, 13 We hypothesized that the use of a similar LI measurement (on a different catheter without minielectrodes) combined with CF may improve the assessment of tissue coupling and RF lesion formation.

Section snippets

Force and impedance measurements

A mapping system with investigational software (Rhythmia Mapping System, Boston Scientific, Maple Grove, MN) and open-irrigated ablation catheter (IntellaNav StablePoint) with LI (DIRECTSENSE) and CF sensing capabilities were used.

In vitro lesion formation

Lesions (n = 137) were created in explanted swine tissue across a range of power (20–40 W), duration (10–60 seconds), and CF (0–40 g). Figure 1A shows representative LI drops for 3 RF applications. Lesions were excluded from measurement for the occurrence of a steam pop (n = 30) or undefined lesion boundaries (n = 25). Models using LI drop as a predictor variable for lesion size performed well for lesion depth (R = 0.84) and width (R = 0.87) (Figure 1C). Models using FTI as a predictor variable

Discussion

To our knowledge, this is the first evaluation of adding LI technology to CF during ablation in vitro and in vivo. Overall, the data demonstrate

  • 1.

    the value of LI plus CF in discerning both mechanical contact and electrical coupling;

  • 2.

    strong correlation between LI drop and lesion dimensions in vitro and in vivo, providing real-time LI feedback of sufficient lesion creation (LI drop >20 Ω) and a warning of excessive heating (LI drop >65 Ω); and

  • 3.

    significant reduction in RF time at standard power in a

Conclusion

The addition of LI to CF improves tissue characterization and lesion assessment. LI demonstrates a superior correlation to lesion dimensions in vitro and in vivo, with an operating range of 20–65 Ω consistently resulting in effective, yet safe, lesions. Finally, when LI was used in a point-by-point workflow with consistent CF, the visualization of LI significantly reduced RF time.

References (21)

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Cited by (32)

  • Role of catheter location on local impedance measurements and clinical outcome with the new direct sense technology in cardiac ablation procedures

    2022, IJC Heart and Vasculature
    Citation Excerpt :

    We are not able to provide information on local wall contact during the LI measurements as the used catheter was not capable of simultaneous real-time CF assessment. Nevertheless, pre-clinical experimental data suggest that local CF and LI show a good correlation pointing at the impact of tissue contact on dynamic baseline LI. [11] Preclinical findings that tissue thickness is not correlated to baseline LI go along with our data of the highest LA values in the (thin) LA and RV outflow tract. [11]

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This work was supported by Boston Scientific.

Dr Garrott, Dr Laughner, Dr Gutbrod, Dr Shuros, Dr Sulkin, Mr Bush, and Mr Pottinger are employees of Boston Scientific. Dr Kapa received research funding from Boston Scientific. Dr Meyers has received consulting fees from Boston Scientific. The rest of the authors report no conflicts of interest.

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