A comparison of electrocardiographic imaging based on two source types.

The aim of the study to compare the performance of two major source types involved in the imaging of the electric activity of the heart on the basis of potential differences observed on the thorax. The images depict either the timing of activation and repolarization of the myocardium or the potential field on a surface closely encompassing the myocardium. The depolarization and repolarization timing on a closed surface bounding the ventricular myocardium was derived from measured body surface potentials (BSPs), an MRI-based electric volume conductor model comprising the geometry of thorax, lungs, heart surface Sh, and cavities. The solution was constrained by using a template of the local transmembrane potentials (TMPs). The latter serve as the strength of the Equivalent Double Layer (EDL) source model (SM), which was used to compute the potential field on Sh (epicardium and endocardium). The second SM is the potential distribution on the epicardium Sp, referred to here as the Equivalent Potential Distribution (EPD). Its strength was estimated directly from the BSPs. The inflection points of the estimated electrograms (ELGs) were taken as markers of the timing of local depolarization and repolarization. The endocardial potential fields estimated using both sources exhibited qualitative similarity, as did the ELGs. With reference to the one generated by the EDL source, the magnitude of the estimated endocardial EPD field was smaller, the downslopes of the ELGs were lower. The timing of depolarization estimated from the EDL-based ELGs was highly correlated with those of the TMP templates, the EPD-based correlation was lower. For the repolarization timing the corresponding test values indicated an insufficient similarity. The EDL- and EPD-based source variants deserve to be studied alongside each other in the future development of electrocardiographic imaging.

The case of the QRS-T angles versus QRST integral maps.

This contribution discusses the QRS-T angle as well as the QRST integral map. Both of these topics have been tested in their application in extracting the major features of depolarization and repolarization: their spatio-temporal behaviour, and how much of their global or local nature might be deduced from signals that can be observed clinically. Recently, it is in particular the QRS-T angle that has received considerable attention, a method that stems directly from vectorcardiography, a subdomain of electrocardiography. The QRST integral map is a display of a map on the body surface of the integrals over time of the ECG signals observed at sets of electrodes. The common biophysical background of both techniques is highlighted. In particular it is explained why, in healthy myocardium, both provide a similar view on the global timing of the depolarization and repolarization of all cardiac myocytes, more specifically, on the dispersion of their action potential durations. In the presence of ischemia, the view obtained is of the integral over time of the transmembrane potentials, which comprises a 'mixture' of their timing and magnitude. The analysis of results of a simulation study emphasizes the large discrepancies that may be observed between the QRS-T angle in the frontal plane and its 3D variant. It is shown that the required vector representation of the signals may be derived from the 12-lead ECG by using the transfer matrix proposed in 1990 by Kors and colleagues.

Repolarization features as detectable from electrograms and electrocardiograms.

This contribution discusses the feasibility of extracting the major features of repolarization: its spatio-temporal behaviour, and how much of its global or local behaviour might be deduced from signals that can be observed experimentally. The analysis presented is based on source-volume-conductor configurations ranging from the classic cable theory, with sources derived from reaction diffusion computations, to a realistic thorax model comprising a whole heart model with electric sources represented by the equivalent electric double layer. The analysis focuses on the fact that the local activation recovery interval (ARI) at regions activated by expanding wave fronts is significantly longer than those activated by contracting ones. The consequences of this effect on observed magnitude and wave form of recorded signals are illustrated.

The inverse problem of bioelectricity: an evaluation.

This invited paper presents a personal view on the current status of the solution to the inverse problem of bioelectricity. Its focus lies on applications in the field of electrocardiography. The topic discussed is also relevant in other medical domains, such as electroencephalography, electroneurography and electromyography. In such domains the methodology involved rests on the same basic principles of physics and electrophysiology as well as on the applied techniques of signal analysis and numerical analysis.

Closed-form analytical expressions for the potential fields generated by triangular monolayers with linearly distributed source strength.

The solution of the mixed boundary value problem of potential theory involves the computation of the potential field generated by monolayer and double layer source distributions on surfaces at which boundary conditions are known. Closed-form analytical expressions have been described in the literature for the potential field generated by double layers having a linearly distributed strength over triangular source elements. This contribution presents the corresponding expression for the linearly distributed monolayer strength. The solution is shown to be valid for all observation points in space, including those on the interior, edges and vertices of the source triangle.

Potential applications of the new ECGSIM.

This contribution demonstrates some applications of the most recent release of ECGSIM, an interactive simulation program that enables the user to study the relationship between the electric current sources of the heart and the resulting electrocardiographic signals on the body surface as well as those on the surface of the heart. It aims to serve as an educational tool as well as a research tool. The examples are drawn from the topics discussed by the participants of the Magnetic Anatomic and eLectrical Technology meeting in Maastricht, the Netherlands (February 2011), reports of which are to be found in the current issue of the Journal of Electrocardiology. These examples include simulation of the atrial electrocardiogram, improved accessibility of endocardial source locations, and an explanation of ST elevations accompanying true TQ depressions.

ECG-based prediction of atrial fibrillation development following coronary artery bypass grafting.

In patients undergoing coronary artery bypass grafting (CABG) surgery, post-operative atrial fibrillation (AF) occurs with a prevalence of up to 40%. The highest incidence is seen between the second and third day after the operation. Following cardiac surgery AF may cause various complications such as hemodynamic instability, heart attack and cerebral or other thromboembolisms. AF increases morbidity, duration and expense of medical treatments. This study aims at identifying patients at high risk of post-operative AF. Early prediction of AF would provide timely prophylactic treatment and would reduce the incidence of arrhythmia. Patients at low risk of post-operative AF could be excluded on the basis of the contraindications of anti-arrhythmic drugs. The study included 50 patients in whom lead II electrocardiograms were continuously recorded for 48 h following CABG. Univariate statistical analysis was used in the search for signal features that could predict AF. The most promising ones identified were P wave duration, RR interval duration and PQ segment level. On the basis of these, a nonlinear multivariate prediction model was made by deploying a classification tree. The prediction accuracy was found to increase over time. At 48 h following CABG, the measured best smoothed sensitivity was 84.8% and the specificity 85.4%. The positive and negative predictive values were 72.7% and 92.8%, respectively, and the overall accuracy was 85.3%. With regard to the prediction accuracy, the risk assessment and prediction of post-operative AF is optimal in the period between 24 and 48 h following CABG.

Development of a toolbox for electrocardiogram-based interpretation of atrial fibrillation.

BACKGROUND: Atrial fibrillation (AF) develops as a consequence of an underlying heart disease such as fibrosis, inflammation, hyperthyroidism, elevated intra-atrial pressures, and/or atrial dilatation. The arrhythmia is initiated by, or depends on, ectopic focal activity. Autonomic dysfunction may also play a role. However, in most patients, the actual cause of AF is difficult to establish, which hampers the selection of the optimal mode of treatment. This study aims to develop tools for assisting the physician's decision-making process. METHODS: Signal analytical methods have been developed for optimizing the assessment of the complexity of AF in all of the standard 12-lead signals. The development involved an evaluation of methods for reducing the signal components stemming from the electric activity of the ventricles (QRST suppression). The methods were tested on simulated recordings, on clinical recordings on patients in AF, and on patients exhibiting atrial flutter (AFL) and atrial tachycardia. The results have been published previously. Subsequently, the implementation of the algorithms in a commercially available electrocardiogram (ECG) recorder, an implementation referred to as its AF-Toolbox, has been carried out. The performance of this implementation was tested against those observed during the development stage. In addition, an improved visualization of the specific ECG components was implemented. This was enabled by providing a separate view on ventricular and atrial activity, which resulted from the steps implied in the QRST suppression. Furthermore, a search was initiated for identifying meaningful features in the cleaned up atrial signals. RESULTS: When testing the implementation of the previously developed methods in the Toolbox on simulated and clinical data, the suppression of ventricular activity in the ECG produced residuals down to the level of physiologic background noise, in agreement with those reported on previously. The QRST suppression resulted in a better visualization of the atrial signals in AF, atrial AFL, sinus rhythm in the presence of atrioventricular blocks, or ectopic beats. Classifiers for AF and AFL that have been defined so far include the distinct spectral components (multiple basic frequencies), exhibiting distinct dominance in specific leads. The annotations of ventricular and atrial activities, ventricular and atrial trigger, as well as ratio between atrial and ventricular rates were greatly facilitated. The time diagram of ventricular and atrial triggers provides an additional view on rhythm disturbances. CONCLUSIONS: The AF-Toolbox that is currently developed for clinical applications has the potential of reliably detecting and classifying AF, as well as to correctly describe atrioventricular conduction, propagation blocks and/or ectopic beats. Based on the results obtained, a first industrial prototype has been built, which will be used to assess its performance in a routine clinical environment. The availability of this tool will facilitate the search for meaningful signal features for identifying the source of AF in individual patients.

Non-invasive imaging of cardiac activation and recovery.

The sequences of activation and recovery of the heart have physiological and clinical relevance. We report on progress made over the last years in the method that images these timings based on an equivalent double layer on the myocardial surface serving as the equivalent source of cardiac activity, with local transmembrane potentials (TMP) acting as their strength. The TMP wave forms were described analytically by timing parameters, found by minimizing the difference between observed body surface potentials and those based on the source description. The parameter estimation procedure involved is non-linear, and consequently requires the specification of initial estimates of its solution. Those of the timing of depolarization were based on the fastest route algorithm, taking into account properties of anisotropic propagation inside the myocardium. Those of recovery were based on electrotonic effects. Body surface potentials and individual geometry were recorded on: a healthy subject, a WPW patient and a Brugada patient during an Ajmaline provocation test. In all three cases, the inversely estimated timing agreed entirely with available physiological knowledge. The improvements to the inverse procedure made are attributed to our use of initial estimates based on the general electrophysiology of propagation. The quality of the results and the required computation time permit the application of this inverse procedure in a clinical setting.

Application of the fastest route algorithm in the interactive simulation of the effect of local ischemia on the ECG.

A method is described to determine the effect on the ECG of a reduced propagation velocity within an ischemic zone. The method was designed to change the activation sequence throughout the ventricles interactively, i.e. with a response time in the order of a second. The timing of ventricular ischemic activation was computed by using the fastest route algorithm, based on locally reduced values of the propagation velocities derived from a standard, normal activation sequence. The effect of these local reductions of the velocities on the total activation sequence, as well as the changes in the electrocardiogram that these produce, are presented.

Analysing the potential of Reveal for monitoring cardiac potentials.

AIMS: The implantable loop recorder (ILR) continuously monitors the heart's electric activity by means of a subcutaneous bipolar electrogram (Elg). Currently, the relationship between the Elg and the surface electrocardiogram (ECG) has been poorly documented. This model-based study aimed at investigating the differences between the bipolar surface and subcutaneous signals, as well as the effect of the insulating boundary of the ILR on these signals. Additionally, the model is used for determining the optimal implant location of the device. METHODS AND RESULTS: Sinus rhythm ECG of a complete heart cycle was simulated by means of a biophysical model. Different volume conductors were created to investigate the effect of the insulating boundary of the ILR. The Elg closely matched the nearby bipolar ECG, both in morphology and in amplitude. The optimal localization and orientation of the ILR was found to depend on the Elg signal feature of interest, e.g. PQ, QRS, or STT waveforms. CONCLUSION: The differences between the bipolar ECG on the surface and the subcutaneous electrogram are negligible. The optimal implant location may be based on nearby surface recordings. The simulation model is an eligible tool for determining the optimal implant location for the ILR, for all signal features of interest.

Spatial dynamics of atrial activity assessed by the vectorcardiogram: from sinus rhythm to atrial fibrillation.

AIM: This study aims at developing methods for extracting spatiotemporal information about the electric activity of the atria from electrocardiographic signals, in particular during atrial fibrillation. METHODS: A biophysical model of the atria and a volume conductor model of the thorax were used to simulate the atrial electrical activity as expressed on the atrial surface as well as on the thorax surface. In all, 22 different types of atrial electric activity were generated, 20 of which related to atrial fibrillation (AF). The spatiotemporal behaviour of the 'true' equivalent dipole expression of these activities was documented as well as those of their estimation based on body surface potentials, the vectorcardiogram. Measures were developed for describing the spatial complexity of atrial signals as observed in the 'atrial' vectorcardiogram. RESULTS: Coherence between time course of the vectorcardiogram and the electrical atrial activity of the simulated sinus rhythm and typical atrial flutter has been observed. Identification of the local extremes of the distribution of instantaneous vector orientations revealed the location of stable and single atrial activity sources. Moreover, the spatial complexity of the vectorcardiogram can be quantified in a very natural way by the proposed features and their visualization. CONCLUSIONS: The proposed analysis extracts spatial information that has hitherto remained unnoticed in non-invasive studies on atrial fibrillation (AF).

Cancellation of ventricular activity in the ECG: evaluation of novel and existing methods.

Due to the much higher amplitude of the electrical activity of the ventricles in the surface electrocardiogram (ECG), its cancellation is crucial for the analysis and characterization of atrial fibrillation. In this paper, two different methods are proposed for this cancellation. The first one is an average beat subtraction type of method. Two sets of templates are created: one set for the ventricular depolarization waves and one for the ventricular repolarization waves. Next, spatial optimization (rotation and amplitude scaling) is applied to the QRS templates. The second method is a single beat method that cancels the ventricular involvement in each cardiac cycle in an independent manner. The estimation and cancellation of the ventricular repolarization is based on the concept of dominant T and U waves. Subsequently, the atrial activities during the ventricular depolarization intervals are estimated by a weighted sum of sinusoids observed in the cleaned up segments. ECG signals generated by a biophysical model as well as clinical ECG signals are used to evaluate the performance of the proposed methods in comparison to two standard ABS-based methods.

Vectorcardiographic lead systems for the characterization of atrial fibrillation.

OBJECTIVE: The aim of the study was to design a vectorcardiographic lead system dedicated to the analysis of atrial fibrillation (AF). METHODS: Body surface potentials during AF were simulated by using a biophysical model of the human atria and thorax. The XYZ components of the equivalent dipole were derived from the Gabor-Nelson equations. These served as the gold standard while searching for an optimal orthogonal lead system for the estimation of the heart vector while using a limited number of electrode positions. Six electrode configurations and their dedicated transfer matrices were tested by using 10 different episodes of simulated AF and 25 different thorax geometries. RESULTS: Root-mean-square-based relative estimation error of the vectorcardiogram using the Frank electrodes was 0.39. An adaptation of 4 of the 9 electrode locations of the standard electrocardiogram, with 1 electrode moved to the back, reduced the error to 0.24. CONCLUSION: The Frank lead system is suboptimal for estimating the equivalent dipole components (VCG) during AF. Alternative electrode configurations should include at least 1 electrode on the back.

Adaptation of the standard 12-lead electrocardiogram system dedicated to the analysis of atrial fibrillation.

OBJECTIVE: The objective of the study was to design a lead system aimed at studying atrial fibrillation (AF), while being anchored to the standard 12-lead system. METHODS: The location of 4 of the 6 precordial electrodes was optimized while leaving the remaining 5 of the 9 electrodes of the standard 12-lead system in place. The analysis was based on episodes of 11 different variants of AF simulated by a biophysical model of the atria positioned inside an inhomogeneous thorax. The optimization criterion used was derived from the singular value decomposition of the data matrices. RESULTS: While maintaining VR, VL, VF, V1 and V4, the 4 new electrode positions increased the ratio of the eighth and the first singular values of the data matrices of the new configuration about 5-fold compared with that of the conventional electrode positions. CONCLUSION: The adapted lead system produces a more complete view on AF compared with that of the standard 12-lead system.

The surface Laplacian operator of the potentials on a bounded volume conductor has a unique inverse.

In the discussion on the use of the surface Laplacian (SL) of the distribution of bioelectric potentials on the body surface, the question remained open whether a complete specification of the SL of the potential over the surface bounding a volume conductor would uniquely specify the potential on that surface up to a constant. This paper reports that this is indeed the case. In addition, it is shown that the integral of the SL over a closed surface is zero, a property that may serve as a check on the accuracy of any numerical approximation of the SL.

Atrial repolarization as observable during the PQ interval.

OBJECTIVE: We aimed to study the involvement of atrial repolarization in body surface potentials. METHODS: Electrocardiograms of healthy subjects were recorded using a 64-lead system. The data analysis focused on the PQ intervals while devoting special attention to the low-amplitude signals during the PQ segment: the segment from the end of the P wave until onset QRS. The data were analyzed by inspecting body surface potential maps and the XYZ signals of the vectorcardiogram. RESULTS: Standard P-wave features exhibited normal values. The local potential extremes were found at positions not sampled by the standard leads. The PQ segment was found to be not isoelectric, the time course of the potential distribution being very similar to that during the P wave but for a reversed polarity and about 3-fold lower magnitudes. CONCLUSION: The results demonstrate a significant involvement of atrial repolarization during the PQ interval and essentially discordant "atrial T waves," suggesting a small dispersion of atrial action potential durations.