Mitral annular electrogram morphology - a balanced signal does not identify the anatomical annulus
Authors: Williams S , Buytaert D , Bodagh N , Kotadia I , Gharaviri A , Baptiste T , Corrado C , Solis-Lemus J , Niederer S , Whitaker J , Sim I , De Potter T , O'neill M 
Published: May 2023

Abstract

Funding Acknowledgements
Type of funding sources: Foundation. Main funding source(s): British Heart Foundation

Background
Electrogram morphology is used to identify the atrio-ventricular annuli during catheter ablation procedures with a "balanced" signal considered to represent the annuli. However, anatomical studies highlight a significantly greater mass of ventricular myocardium compared to atrial myocardium at the valve plane suggesting the correct electrical signature of the anatomical annulus may not be a balanced signal.

Purpose
To identify the relationship between atrial and ventricular electrogram components at the anatomically defined mitral valve annulus.

Methods
Mapping of the mitral annulus was performed in patients undergoing ablation for paroxysmal atrial fibrillation. Following femoral venous access and trans-septal puncture, left atrial anatomy was defined using rotational angiography performed using a 100ml contrast injection 4 seconds prior to X-ray acquisition, during rapid pacing and apnoea while covering 200 degrees at 40 degrees/s. A SmartTouch SF ablation catheter (Biosense Webster) was advanced into the left atrium and used to record intracardiac electrograms at four positions around the mitral annulus and at five predetermined levels ranging from ventricular to atrial (Figure, panel A). Data analysis was performed using OpenEP (https://openep.io). For each electrogram, the distance to the mitral valve plane and the amplitudes of atrial and ventricular electrogram components were calculated (Figure, panel B and C). Mean electrogram component amplitudes were calculated including all points recorded within ±2.5mm of the mitral valve plane. For each patient, a linear regression model was fitted and used to identify the atrial/ventricular ratio at the anatomically defined mitral valve annulus (Figure, panel D).

Results
The protocol was completed in 9 patients, including analysis of 240 atrial or ventricular electrogram components. Within ±2.5mm of the anatomically defined mitral valve annulus, the mean recorded amplitudes of atrial and ventricular electrogram components was 0.15±0.17mV and 1.35±1.27mV, respectively (A/V ratio 1:9). Using the linear regression model, the ventricular electrogram component was calculated to be 16.5±11.6 times greater in amplitude than the atrial electrogram component at the mitral valve plane (A/V ratio 1:16). The use of a balanced signal to identify the mitral valve annulus resulted in a positional error of -7.9±9.5mm (i.e. identifying a level which is approximately ~8mm too "atrial" compared to the anatomically defined annulus).

Conclusion
The conventional use of a "balanced" electrogram signal does not identify the anatomical mitral valve annulus. An atrial/ventricular electrogram ratio between 1:9 and 1:16 best approximates the mitral valve annular plane determined by left atrial rotational angiography. These findings have relevance in target site selection for an accessory pathway and the ventricular extent of linear ablation at the mitral isthmus.