Electrocardiograms (ECGs) recorded simultaneously at many body surface sites can be used to construct a sequence of body surface potential distributions that correspond to cardiac electrical activity; this technique is referred to as body surface potential mapping (BSPM).1,2 The noninvasively acquired BSPM data can be used, in turn, along with a mathematical model that accounts for geometry and electrical properties of the thorax, to reconstruct electrical potentials on the epicardial surface. The problem of calculating epicardial potentials from recorded BSPM data constitutes one of the formulations of the inverse problem of ECG.3,4
The calculation of epicardial potentials from noninvasively acquired electrical and anatomic data is also referred to, quite aptly, as electrocardiographic imaging.5,6 The procedure requires BSPM data, accurate representation of the patient's thoracic geometry, routines for calculating transfer coefficients relating epicardial and body surface potentials,7 and reliable inverse-solution techniques for estimating epicardial potentials from BSPM data.3,4 Patient-specific anatomic data can be obtained by computed tomography (CT) or magnetic resonance imaging (MRI). ECG imaging has shown its potential in theoretical and experimental studies,5,6 and it promises to find its way into routine clinical applications.8,9
The aim of this chapter is to illustrate the use of ECG imaging in clinical cardiac electrophysiology, in particular as an aid to radiofrequency catheter ablation of ventricular arrhythmias. In this chapter, we introduce our methodology of BSPM, with particular attention to applications in clinical cardiac electrophysiology; we deal with the mathematical formulation of the relationship between potentials on the epicardial surface and body surface and show how this relationship is used to solve the inverse problem of ECG in terms of epicardial potentials; we describe the application of ECG imaging during catheter-ablation procedures in the clinical cardiac electrophysiology laboratory; and we discusses results reported in this chapter in the context of current clinical practice.
To obtain the BSPM data required for ECG imaging, 120 disposable radiolucent Ag/AgCl electrodes (Ref. 31.8778.26; Covidien, Dublin, Ireland) were placed on the patient's torso in 18 strips according to the standard Dalhousie configuration (Fig. 12–1). The electrode array was connected to the 128-channel Mark-6 acquisition system (BioSemi Inc., Amsterdam, the Netherlands), which was coupled via a fiber optics cable with a laptop computer. The ECG signals were displayed and stored with MAPPER software (Dalhousie University, Halifax, Nova Scotia, Canada).
Dalhousie standard array of 120 electrodes on the torso for body surface potential mapping (BSPM). The left half of the grid represents the anterior chest, and the right half represents the posterior chest. Transverse levels (labeled 1′, 2, …, 10′) and equiangular planes (labeled A, A′, …, P, P′) are marked after Frank,10 with levels 1-inch apart. Potentials at 352 nodes (solid squares) are interpolated from those recorded at electrode ...