Influence of tissue-electrode distance on unipolar and bipolar voltage for large-tip, ring-, and mini-electrodes
Authors: Knecht S , Schlageter V , Badertscher P , Krisai P , Luca A , Katic J , Kueffer T , Spreen D , Schaer B , Osswald S , Kuhne M , Sticherling C 
Published: May 2024

Abstract

Background
The characteristics of intracardiac unipolar and bipolar voltage electrograms (EGM) acquired by electrophysiological catheters depend on the electrical source and on the electrode design and configuration.

Purpose
The aim of the study was to assess the impact of the electrode configuration and their distance from the myocardial tissue (electric source) on the unipolar and bipolar intracardiac electrograms recorded with a multi-electrode focal ablation catheter.

Methods
We retrospectively analyzed left atrial 3D electroanantomical maps (EAMs) of 25 patients performed in sinus rhythm using the IntellaNav Mifi OI catheter (Boston Scientific, USA) with a 4.5 mm tip-electrode, three mini-electrodes and three 2 mm ring electrodes. The unipolar and bipolar EGMs were characterized based on the peak-to-peak amplitude (amp), the signal duration (width), the maximal slope, and the relative high power spectrum (HF_rel) as ratio of the power spectrum of frequencies above and below 50Hz. Distances of the electrode from the tissue were calculated from the electroanatomic reconstruction. Except for the HF_rel with absolute values are reported, all characteristics are normalized using the tip-electrode for unipolar or tip-to-ring electrode values for bipolar measures. EGMs with bipolar voltage amplitude of &gt;0.5 mV were included to focus on healthy tissue.

Results
We analyzed the EGMs of 5183 catheter positions with horizontal to perpendicular orientation. From horizontal catheter orientation with a similar distance from the tissue for all electrodes, the unipolar EGM of the ring electrodes showed an increased amplitude (140%), slope (150%) and HF_rel (16% vs 11%) compared to the tip- and mini-electrodes. For bipolar EGM, however, the tip- ring pair showed the largest amplitude, width, and slope. The HF_rel was larger for the ring electrode pairs compared to the tip-ring electrode (58% to 39%).
The median amplitude (amp), slope, and HF_rel for the ring electrodes showed a strong power-law decay function with distance, whereas the width increases linearly with distance from the surface (Figure, blue dotted lines). The intersection (red circle) of the steep linear decline at close distance to the tissue (reflecting a strong influence on the EGM) with the flat line at larger distances (dash-dotted lines) was considered as nearfield boundary. A HF_rel cutoff of &gt;10% for unipolar (Figure, red arrow) and &gt;30% for bipolar EGMs might be used to identify this boundary.

Conclusion
We confirmed a higher amplitude for small ring-electrodes compared to larger tip electrodes in unipolar amplitude but not for bipolar electrograms. Furthermore, a strong decay of the amplitude, slope and HF_rel with distance could be observed. The decay functions are suggestive for a nearfield boundary distance below 4mm from the tissue. Further studies are warranted to confirm this observation.Summary of characteristics over distance