NonInvasive Imaging of Cardiac Activation and Rec

Authors: Peter M van Dam Thom F Oostendorp André C Linnenbank Adriaan van Oosterom

Publish Date: 2009/06/27

Volume: 37, Issue: 9, Pages: 1739-1756

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Abstract

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 nonlinear 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 settingThe sequence of activation and recovery of the heart atria and ventricles has physiological significance and clinical relevance The standard 12lead electrocardiogram ECG commonly used in diagnostic procedures provides insufficient information for obtaining an accurate estimate of these sequences in all types of abnormality or diseaseSince the inception of electrocardiography1249 several methods have been developed aimed at providing more information about cardiac electric activity on the basis of potentials observed on the body surface The differences between these methods relate to the implied physical description of the equivalent generator representing the observed potential field The earliest of these are the electric current dipole a key element in vectorcardiography314 and the multipole expansion16 Neither of these source models offer a direct view on the timing of myocardial activation and recovery or other electrophysiologically tinted featuresFrom the 1970s onwards the potential of two other types of source descriptions have been explored2122 This development stemmed from increased insight in cardiac electrophysiology and advances in numerical methods and their implementation in ever more powerful computer systems The results of both methods are scalar functions on a surface The solving of the implied inverse problem may accordingly be viewed as a type of functional imaging which has led to their characterization as eg “Noninvasive Electrocardiographic Imaging ECGI”42 or “Myocardial Activation Imaging”29 A brief characterization of both methods is as followsThe first surface source model is that of the potential distribution on a closed surface closely surrounding the heart somewhat like the pericardium referred to here as the pericardial potential source PPS model The model is based on the fact that barring all modeling and instrumentation errors a unique relation exists between the potentials on either of two nested surfaces one being the body surface the other the pericardium provided that there are no active electric sources in the region in between It was first proposed at Duke by Martin and Pilkington37 its potential has subsequently been developed by several other groups45202346The second type of surface source model evolved from the classic model of the double layer as an equivalent source of the currents generated at the cellular membrane during depolarization described by Wilson et al 69 Initially this current dipole layer model was used to describe the activity at the front of a depolarization wave propagating through the myocardium1150 Later Salu47 expressed the equivalence between the double layer at the wave front and a uniform double layer at the depolarized part of the surface bounding the myocardium based on solid angle theory62 This source description has been explored by others826293845This paper reports on recent progress made in inverse procedures based on the second type of surface source model the equivalent double layer on the heart’s surface as a model of the electric sources throughout the myocardium we refer to it as the EDL model In contrast with the PPS model the EDL source model relates to the entire surface bounding the atrial or ventricular myocardium epicardium endocardium and their connection at the base As mentioned in the previous paragraph the EDL was initially used for modeling the currents at the depolarizing wave front only Based on the theory proposed by Geselowitz1718 it was found to be also highly effective for describing the cardiac generator during recovery the repolarization phase of the myocytes The transmembrane potentials TMPs of myocytes close to the heart’s surface act as the local source strength of the double layer Several examples of its effectiveness in forward simulations have appeared in the literature28315163 A striking example of the model’s potential in inverse procedures is seen in the paper by Modre et al39 an application dedicated to the atrial activation sequenceIn our application the TMPs’ wave forms were specified by an analytical function involving just two parameters markers for the timing of local depolarization and repolarization These parameters were found by using a standard parameter estimation method minimizing the difference between observed body surface potentials BSPs and those based on the source description Since the body surface potentials depend nonlinearly on these parameters a nonlinear parameter estimation technique is required which demands the specification of initial estimates It is here that some novel elements are reported on and a major part of the paper is devoted to their handling The initial parameters for the timing of local depolarization were based on the fastest route algorithm taking into account global properties of anisotropic propagation inside the myocardium Those pertaining to recovery were based on electrotonic interaction as being the driving force for the spatial differences in the local activationrecovery interval With the other elements of the method being similar to those used in previous work the emphasis of the paper is on the description of the novel elements

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