Noninvasive reconstruction of cardiac electrical activity: update on current methods, applications and challenges

Abstract

Electrical activity at the level of the heart muscle can be noninvasively reconstructed from body-surface electrocardiograms (ECGs) and patient-specific torso-heart geometry. This modality, coined electrocardiographic imaging, could fill the gap between the noninvasive (low-resolution) 12-lead ECG and invasive (high-resolution) electrophysiology studies. Much progress has been made to establish electrocardiographic imaging, and clinical studies appear with increasing frequency. However, many assumptions and model choices are involved in its execution, and only limited validation has been performed. In this article, we will discuss the technical details, clinical applications and current limitations of commonly used methods in electrocardiographic imaging. It is important for clinicians to realise the influence of certain assumptions and model choices for correct and careful interpretation of the results. This, in combination with more extensive validation, will allow for exploitation of the full potential of noninvasive electrocardiographic imaging as a powerful clinical tool to expedite diagnosis, guide therapy and improve risk stratification.

Figures

Fig. 1

Inverse reconstruction of epicardial potentials in the patient from the text. Body-surface potentials are measured with 256 electrodes on the patient’s torso (a) and the geometrical and conductivity relationship between heart and body surface is, in this case, determined by computed tomography (b). The patient-specific inverse model (c) is then used to reconstruct epicardial potentials (d). Panel e shows the patient’s ventricular tachycardia, which shared morphology with his frequent ventricular extrasystoles (VES) beats (panel f: first beat paced, second beat VES). (Person in panel a is not the patient)

Fig. 2

Schematic representation of forward/inverse models. A forward model describes the propagation of electromagnetic activity from the heart to the body surface; an inverse model reverses that relation, allowing for noninvasive reconstruction of electrical heart activity from measured body-surface potentials

Fig. 3

Reconstructed epicardial electrograms at one location of the left ventricle (indicated with an asterisk in Fig. 5) of a 63-year-old patient with native left bundle branch block (LBBB) during sinus rhythm and frequent ventricular extrasystoles (VES). In the reconstruction process, the same body-surface potentials (either sinus rhythm with LBBB or VES), but a different torso-heart geometry, were used: one with a diastolic cardiac geometry (resulting in the electrograms with a solid line), and one with a systolic cardiac geometry (dashed line). Using either a systolic or diastolic cardiac geometry results in significantly different electrograms on the same epicardial location, notably in the repolarisation phase. This indicates that cardiac contractile movement should be taken into account when reconstructing epicardial potentials. Voltage scales identical for both graphs

Fig. 4

Noninvasively reconstructed isochrones of a paced beat, on the right ventricle (RV) of the same patient as in Fig. 3, for different values of the regularisation parameter. Panel a shows an over-regularised setting, applying the constraints too heavily, resulting in a reconstruction that does not contain any relevant information. An optimally regularised solution provides the most adequate reconstruction (panel b), showing early activation (orange colour) on the location of pacing (indicated with an asterisk). An under-regularised setting results in a reconstruction that is dominated by the influence of noise (panel c)

Fig. 5

Reconstructed epicardial electrograms (first two columns, voltage scales identical over all graphs) and reconstructed activation isochrones (last two columns) for three different beats (row a: sinus beat with left bundle branch block pattern; b: biventricularly paced beat; c: extrasystolic beat). The reconstructed electrograms correspond to the epicardial location closest to the right and left pacing lead tips from the patient’s pacemaker. The insets show recorded electrograms from those leads at the same location for comparison, i.e. an endocardial recording for the right ventricle, and an epicardial recording for the left ventricle. The dashed line indicates the activation time (maximum − dV/dt). The ventricular activation isochrones, depicted in the last two columns, reflect the time of maximum − dV/dt per epicardial location. Locations of reconstructed earliest activation are indicated with symbol #. QRS duration (QRSd) is based on the 12-lead electrocardiography. Electrical synchrony (Esyn) is the difference between the mean activation times at the right ventricle (RV) and the mean activation times at the left ventricle (LV); a value close to zero usually results in more efficient contraction. For the paced beat (panel b), the pacing locations are indicated with symbol *