Local Electrical Impedance Mapping of the Atria: Conclusions on Substrate Properties and Confounding Factors

Abstract

The treatment of atrial fibrillation and other cardiac arrhythmias as a major cause of cardiovascular hospitalization has remained a challenge predominantly for patients with severely remodeled substrate. Individualized ablation strategies are extremely important both for pulmonary vein isolation and subsequent ablations. Current approaches to identifying arrhythmogenic regions rely on electrogram-based features such as activation time and voltage. Novel technologies now enable clinical assessment of the local impedance as tissue property. Previous studies demonstrated its use for ablation monitoring and indicated its potential to differentiate healthy substrate, scar, and pathological tissue. This study investigates the potential of local electrical impedance-based substrate mapping of the atria for human in-vivo data. The presented pipeline for impedance mapping particularly contains options for dealing with undesirable effects originating from cardiac motion, catheter motion, or proximity to other intracardiac devices. Bloodpool impedance was automatically determined as a patient-specific reference. Full-chamber, left atrial impedance maps were drawn up from interpolating the measured impedances to the atrial endocardium. Finally, the origin and magnitude of oscillations of the raw impedance recording were probed into. The most dominant reason for exclusion of impedance samples was the loss of endocardial contact. With median elevations above the bloodpool impedance between 29 and 46 Ω, the impedance within the pulmonary veins significantly exceeded the remaining atrial walls presenting median elevations above the bloodpool impedance between 16 and 20 Ω. Previous ablation lesions were distinguished from their surroundings by a significant drop in local impedance while the corresponding regions did not differ for the control group. The raw impedance was found to oscillate with median amplitudes between 6 and 17 Ω depending on the patient. Oscillations were traced back to an interplay of atrial, ventricular, and respiratory motion. In summary, local impedance measurements demonstrated their capability to distinguish pathological atrial tissue from physiological substrate. Methods to limit the influence of confounding factors that still hinder impedance mapping were presented. Measurements at different frequencies or the combination of multiple electrodes could lead to further improvement. The presented examples indicate that electrogram- and impedance-based substrate mapping have the potential to complement each other toward better patient outcomes in future.

In Silico Study of Local Electrical Impedance Measurements in the Atria - Towards Understanding and Quantifying Dependencies in Human

“…The elevation of LI in tissue contact above the bloodpool LI ranged from 14 Ω for 0 mm distance to the tissue, i.e. 0 g so-called "contact force", to 33 Ω for −2 mm distance to the tissue and compared well to clinically observed mean ranges between 16 Ω and 20 Ω [27] for the IntellaNav MiFi™ OI. The simulated upper bound for an immersion depth of 2 mm thus likely overestimates the LI for clinical mean contact force applications due to the disregard of realistic tissue deformation.…”

“…In clinical setups, the inflation and deflation of the lungs is an additional confounding factor with evident impact on the LI measurement [27] due to the close proximity of the lungs to selected parts of the cardiac chambers. While the conductivity of inflated lungs is reported to be 0.0954 S m at 14.5 kHz, the conductivity increases to 0.247 S m in deflated state [21].…”

Effect of Contact Force on Local Electrical Impedance in Atrial Tissue - an In Silico Evaluation

“…However, healthy clinical data shows a drop at 3 g and 5 g, which was not observed in silico but could be explained due to the extremely difficulty getting exact CF values below 10 g measured by human. However, trends of clinical data followed the expected be- havior as scar tissue usually presents lower LI values than healthy tissue [10]. In silico healthy values showed more similar results when comparing with clinical data than scar tissue.…”

Combined contact force and local impedance dynamics during repeat atrial fibrillation catheter ablation

“…Combination of CF and LI monitoring may allow to improve ablation procedures in regions of low voltage: 1) LI mapping may complement electrogram-based substrate maps potentially facilitating selection of optimal ablation sites within previously ablated areas ( Unger et al, 2021 ). This is supported by our findings confirming baseline LI as a better predictor for a ∆LI > 20 Ω compared to local voltage.…”