Electromechanical wave imaging (EWI) validation in all four cardiac chambers with 3D electroanatomic mapping in canines in vivo

Abstract

Characterization and mapping of arrhythmias is currently performed through invasive insertion and manipulation of cardiac catheters. Electromechanical wave imaging (EWI) is a non-invasive ultrasound-based imaging technique, which tracks the electromechanical activation that immediately follows electrical activation. Electrical and electromechanical activations were previously found to be linearly correlated in the left ventricle, but the relationship has not yet been investigated in the three other chambers of the heart. The objective of this study was to investigate the relationship between electrical and electromechanical activations and validate EWI in all four chambers of the heart with conventional 3D electroanatomical mapping. Six (n = 6) normal adult canines were used in this study. The electrical activation sequence was mapped in all four chambers of the heart, both endocardially and epicardially using the St Jude's EnSite 3D mapping system (St. Jude Medical, Secaucus, NJ). EWI acquisitions were performed in all four chambers during normal sinus rhythm, and during pacing in the left ventricle. Isochrones of the electromechanical activation were generated from standard echocardiographic imaging views. Electrical and electromechanical activation maps were co-registered and compared, and electrical and electromechanical activation times were plotted against each other and linear regression was performed for each pair of activation maps. Electromechanical and electrical activations were found to be directly correlated with slopes of the correlation ranging from 0.77 to 1.83, electromechanical delays between 9 and 58 ms and R ² values from 0.71 to 0.92. The linear correlation between electrical and electromechanical activations and the agreement between the activation maps indicate that the electromechanical activation follows the pattern of propagation of the electrical activation. This suggests that EWI may be used as a novel non-invasive method to accurately characterize and localize sources of arrhythmias.

Figure 1. EWI acquisition and motion and strain estimation flowchart

(1) 2 seconds high frame rate acquisition (2000 Hz) in standard apical views with an unfocused transmit sequence. (2) RF image formation using channel data. (3) Segmentation and 1D axial displacement estimation using 1D cross-correlation. Motion maps are generated. (4) Axial incremental strain estimated using a least-square estimator. (5) EWI isochrones are obtained semi-manually by mapping zero-crossings within the mask for each apical view. Pseudo 3D isochrones are then generated. LA/RA = left/right atrium, LV/RV = left/right ventricle, RF = radio-frequency.

Figure 2. Echocardiographic views

(A) Position of the probe when acquiring a 4-chamber view (grey). The yellow plane represents the short-axis plane of view. (B) Apical 4-chamber view superimposed on a diagram of the heart. (C) Short axis views detailing all four EWI apical views used in this study. LA/RA = left/right atrium, LV/RV = left/right ventricle. Heart diagram in (A) courtesy of www.echocardiographer.org

Figure 3. Block diagram of the processing for EWI validation with 3D electroanatomical mapping

Both electromechanical and electrical activation maps are generated and co-registered using anatomical landmarks. Plots of electromechanical vs. electrical activation times are then generated and linear regression is performed to obtain slope, intercept and R² values.

Figure 4. LV endocardial EWI validation

in two canines during normal sinus rhythm. Both electrical activation maps were acquired using a 64-electrode basket catheter. The origins of both activation maps (t = 0 ms) correspond to the beginning of the QRS on the ECG. LV/RV = left/right ventricle.

Figure 5. LV epicardial EWI validation

in one canine during normal sinus rhythm and during LV lateral wall focal pacing. The electrical activation maps were acquired using a bipolar catheter. The origins of both activation maps (t = 0 ms) correspond to the beginning of the QRS on the ECG. LV/RV = left/right ventricle.

Figure 6. RV EWI validation

in one canine during normal sinus rhythm. Epicardial map of the anterior, lateral, and some posterior regions (top) and endocardial activation map of the septum (bottom) were acquired using a bipolar catheter. 4-chamber, 3-chamber, and 5-chamber views were used to generate the pseudo 3D EWI isochrones. Both endocardial and epicardial points are used to generate the linear regression. The origins of both activation maps (t = 0 ms) correspond to the beginning of the QRS on the ECG. LV/RV = left/right ventricle.

Figure 7. Atrial EWI validation in two canines during normal sinus rhythm

(A) Endocardial RA validation, (B) LA and RA epicardial validation. Both endocardial and epicardial maps were acquired using a bipolar catheter. The origins of both activation maps (t = 0 ms) correspond to the beginning of the P-wave on the ECG. IVC/SVC = inferior/superior vena cava, LA/RA = left/right atrium, LAD = Left anterior descending artery, LV/RV = left/right ventricle.