Refinement of voltage-based atrial scar assessment using pacing thresholds and local impedance
Authors: Bates A , Paisey J R , Yue A , Banks P , Roberts P R , Ullah W 
Published: May 2023

Abstract

Funding Acknowledgements
Type of funding sources: Private company. Main funding source(s): Boston Scientific

Background
Common cut-offs for dense scar and ‘low-voltage areas’ on atrial 3D electroanatomical mapping are 0.05mV and 0.50mV respectively. These are not physiologically validated and may not fully represent underlying scar compared with unipolar readings or the novel parameter local impedance (LI).

Objectives
Investigate the relationship of bipolar voltage, unipolar voltage and LI with pacing threshold (PT). Establish cut-offs for electrically inert (EI), partially active (PA) and electrically active (EA) atrial tissue based upon pacing capture.

Methods
Patients undergoing AF ablation were recruited. 3D electroanatomical maps were created. PTs were assessed using the MiFi and Stablepoint (SP) ablation catheters between the tip and first ring electrodes. PTs were assessed with a 2ms pulse duration commencing at 5mA and moved up or down accordingly. LI was referenced against a blood pool reading (LIr). EI sites were defined as no pacing capture, PA capture between 5-20mA and EA less than 5mA.

Results
292 PTs and LI readings were recorded in 40 patients (Male: 21/40; Age: 66.7 ± 8.8; Paroxysmal AF: 10, Persistent AF: 30, De novo 21, Redo 19). A weak correlation exists between SR voltages and PT (Spearman's Rho - Bipolar: -0.35, Unipolar: -0.26, both p &lt;0.0005). Weak correlations exist between LI and PT, (Spearman's Rho - MiFi = -0.35, SP = - 0.33, both p &lt; 0.0005), but moderate-strong correlations exists between LIr and PT (Rho - MiFi = -0.52, SP = -0.57, both p &lt;0.0005, [Figure 1]).
For the EI-PA cut-off, the ability to distinguish capture / no capture for bipolar voltages was fair (AUROC = 0.74), but poor for unipolar, (AUROC = 0.68). The lowest tissue voltage values with capture were 0.03mV and 0.31mV respectively. In contrast, LIr had excellent discriminatory ability (AUROC – MiFi = 0.87, SP = 0.90, lowest value with tissue capture = 1Ω for both MiFi and SP).
For the PA-EA cut-off, the ability to distinguish PT above/below 5mA for bipolar voltages was fair (AUROC = 0.69, optimal cut-off 1.29mV), and poor for unipolar voltages, (AUROC = 0.63). In contrast, LIr had good discrimination, (AUROC - MiFi = 0.82, optimal cut-off 12.5Ω, SP = 0.89, optimal cut-off 10.5Ω). Using these cut-offs, there was a significant difference between PTs between EI, PA and EA, for bipolar voltage, (EI: 20mV, PA 6.0 ± 6.3mV, EA: 2.6 ± 2.6mV, p &lt; 0.0005) and LIr, (MiFi: EI: 20mV, PA 7.8 ± 7.5mV, EA: 2.8 ± 2.8mV. SP: EI 20mV, PA: 4.0 ± 2.5mV, EA: 2.2 ± 0.9mV, both p &lt; 0.0005, [Figure 2]).

Conclusions
PTs suggest conventional voltage cut-offs could be improved. LIr shows potential as a novel parameter in assessing tissue health complimentary to bipolar voltages. Based on pacing capture data, what has previously been labelled dense scar is not electrically inert and should be ablated if clinically indicated. Values of 0.03mV and 1.31mV, and for potential future LIr mapping, 0.5Ω and 10.5Ω are suggested as dense scar and low voltage area thresholds.