ExperimentalCombined local impedance and contact force for radiofrequency ablation assessment
Graphical abstract
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.
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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.