Electrocardiographic Imaging for Cardiac Arrhythmias and Resynchronization Therapy

Helder Pereira; Steven Niederer; Christopher A. Rinaldi

Disclosures

Europace. 2020;22(10):1447-1462. 

In This Article

Electrocardiographic Imaging

Electrocardiographic imaging provides a mapping system that stores and displays electrophysiological data for analysis. The procedure consists of the patient wearing multiple electrodes (from 50 to close to 300, depending on the manufacturer), which can be contained within a vest or strips, connected to a mapping system, which records the electrical signals (Figure 2).

Figure 2.

The ECGi procedure. Body surface potentials are recorded from several electrodes. The patient-specific heart-torso geometry is obtained from thoracic CT or MRI scan. The data are combined using mathematical algorithms to reconstruct epicardial potentials and unipolar electrograms on the heart surface. Maps of epicardial activation and recovery can be further derived from the electrograms. Adapted from Rudy et al.10 CT, computed tomography; ECGi, electrocardiographic imaging; MRI, magnetic resonance imaging. Author's permission granted and RightsLink License number 4851570792908.

A computed tomography (CT) scan or magnetic resonance imaging (MRI) is used to display a 3D image of the heart (Figure 2B). During excitation, the heart creates an electrical potential field between the body surface and the epicardium. The potential field on the torso can be measured through electrodes on the torso. One of the functions of ECGi is to provide images of the instantaneous potential fields over time allowing a more complete view on the spatio-temporal[11] epicardium data from measurements on the torso; an inverse problem of electrocardiography—which aims to recover noninvasively regional information about intracardiac electrical events from electrical measurements on the body surface.[12] Apart from epicardial potentials, experimental validation studies[13] have developed inverse electrocardiographic solutions through equivalent sources as single fixed-location dipole, a multipole series, moving dipoles, multiple fixed-location dipoles,[14,15] homogeneous volume conductor,[16] and activation wavefronts.[17]

Mathematically, it can be expressed through the following equation:[18] ϕT = E , where ϕT is the electric field on the torso surface and ϕE is that on the epicardium. The information regarding the relation of the two surfaces (epicardium and torso) is expressed by the transfer matrix 'A'.

Even small errors in the measurement of ϕT (often resulting from electrode positioning) can be automatically reflected as a large error in the calculation of ϕE . The most common mechanism used to control this inverse problem is Tikhonov regularization, a method that imposes constraints on ϕE .[19] Mathematically, this can be expressed as the vector that minimizes ϕE by: minϕE [|ϕT−A ϕE |[2] + t|E |[2]], here t is a parameter of regularization and L is a regularization operator.

With these algorithms, the ECGi system provides a 3D reconstructed image of the heart, from the information acquired from CT/MRI along with the location and electrical data of the electrodes (Figure 3C), as well as a beat-by-beat activation map,[6] which gives an accurate onset of electrical activation. Tikhonov regularization and the generalized minimal residual algorithm are utilized throughout several steps such as heart and body-surface segmentation (image segmentation), creating the heart and torso geometric surface representation (generation of mesh), numerical algorithms and visualization.[20]

Figure 3.

ECGi systems usually consist of a vest embedded with 50 to approximately 300 electrodes (depending on the manufacturer) that is fitted to the patient's torso, which allows the acquisition of electrograms (A). Patients with the vest still in position will need to undergo a thoracic CT scan to determine the precise anatomic relation between the cardiac geometry and the torso electrodes (B). The ECGi system combines each data set obtained from the vest and CT scan, and an activation waveform map of both ventricles' epicardial surface is generated and combined to construct 3D epicardial isochrone maps (C). CT, computed tomography; ECGi, electrocardiographic imaging.

The ability to extract data non-invasively significantly enhances diagnosis. Based on a mathematical approach, ECGi is able to identify the electrical source for a given body-surface potential distribution. Identical body-surface potentials can appear due to various cardiac electrical activity patterns, most of which are unlikely to occur. In a clinical sense, ECGi surpasses the limitations of the current 12-lead ECG techniques because it provides a cardiac geometry where the onset of electric activity can be observed through activation maps of arrhythmias of the heart surface. Furthermore, standard ECG requires deciphering of body-surface data using cardiac activity under the assumption of a 'standard' heart shape and size in a 'standard' torso. In contrast, ECGi uses the precise heart-torso geometry of the patient to pinpoint the site of arrhythmia and locate the sequence on the heart.[21]

Research has shown that the ECGi mapping system can effectively deliver better information than conventional techniques, even for normal cardiac physiology.[18] This is achieved because ECGi merges body-surface electrical data and the anatomical information from medical imaging to compute and process epicardial unipolar electrograms that are displayed through the epicardial surface of the heart.[22] The method currently used for non-invasive diagnosis and detection of cardiac electrical activity involves a 12-lead ECG and is a routine part of medical care. However, ECG only measures cardiac electrical activity on the surface of the body and not on the heart itself. Thus, ECG is constrained by a spatial resolution that can determine regional electrical activity but cannot locate the regions where arrhythmic activities occur in the heart.[21,23] The following text describes the development of ECGi over time, along with research that addresses different complex types of arrhythmias and illustrates how ECGi can deliver accurate information that can better inform an appropriate therapeutic approach.

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