Body-surface maps of heart potentials: tentative localization of pre-excited areas in forty-two Wolff-Parkinson-White patients

Body Surface Potential Mapping: A Perspective on High-Density Cutaneous Electrophysiology

The electrophysiological signals recorded by cutaneous electrodes, known as body surface potentials (BSPs), are widely employed biomarkers in medical diagnosis. Despite their widespread application and success in detecting various conditions, the poor spatial resolution of traditional BSP measurements poses a limit to their diagnostic potential. Advancements in the field of bioelectronics have facilitated the creation of compact, high-quality, high-density recording arrays for cutaneous electrophysiology, allowing detailed spatial information acquisition as BSP maps (BSPMs). Currently, the design of electrode arrays for BSP mapping lacks a standardized framework, leading to customizations for each clinical study, limiting comparability, reproducibility, and transferability. This perspective proposes preliminary design guidelines, drawn from existing literature, rooted solely in the physical properties of electrophysiological signals and mathematical principles of signal processing. These guidelines aim to simplify and generalize the optimization process for electrode array design, fostering more effective and applicable clinical research. Moreover, the increased spatial information obtained from BSPMs introduces interpretation challenges. To mitigate this, two strategies are outlined: observational transformations that reconstruct signal sources for intuitive comprehension, and machine learning-driven diagnostics. BSP mapping offers significant advantages in cutaneous electrophysiology with respect to classic electrophysiological recordings and is expected to expand into broader clinical domains in the future.

Electrocardiographic patterns of ventricular pre-excitation in dogs with right-sided accessory pathways

INTRODUCTION

The aim of the study was to describe the electrocardiographic features of ventricular pre-excitation (VPE) patterns characterized by the presence of delta (δ) wave, short P-δQRS interval, wide δQRS complexes in dogs with right-sided accessory pathways.

ANIMALS, MATERIALS AND METHODS

Twenty-six dogs with a confirmed accessory pathways (AP) via electrophysiological mapping were included. All dogs underwent a complete physical examination, 12-lead ECG, thoracic radiography, echocardiographic examination and electrophysiologic mapping. The AP were located in the following regions: right anterior, right posteroseptal, right posterior. The following parameters were determined: P-δQRS interval, δQRS duration, δQRS axis, δQRS morphology, δ-wave polarity, Q-wave, R-wave, R'-wave, S-wave amplitude, and R/S ratio.

RESULTS

In lead II, the median δQRS complex duration was 82.4 (IQR 7.2) and the median P-δQRS interval duration was 54.6 (IQR 4.2) msec. The median δQRS complex axis in the frontal plane was: + 68° (IQR 52.5) for right anterior APs, - 24 ° (IQR 24) for right postero-septal APs, - 43.5 ° (IQR 27.25) for right posterior APs (P = 0.007). In lead II, the polarity of the δ wave was positive in 5/5 right anterior APs and negative in 7/11 postero-septal APs and 8/10 in right posterior APs. In precordial leads of all dogs, R/S was ≤ 1 in V1 and > 1 in all leads from V2 to V6.

CONCLUSION

Surface electrocardiogram can be used to distinguish right anterior APs from right posterior and right postero-septal ahead of an invasive electrophysiological study.

Body Surface Potential Mapping: Contemporary Applications and Future Perspectives

Body surface potential mapping (BSPM) is a noninvasive modality to assess cardiac bioelectric activity with a rich history of practical applications for both research and clinical investigation. BSPM provides comprehensive acquisition of bioelectric signals across the entire thorax, allowing for more complex and extensive analysis than the standard electrocardiogram (ECG). Despite its advantages, BSPM is not a common clinical tool. BSPM does, however, serve as a valuable research tool and as an input for other modes of analysis such as electrocardiographic imaging and, more recently, machine learning and artificial intelligence. In this report, we examine contemporary uses of BSPM, and provide an assessment of its future prospects in both clinical and research environments. We assess the state of the art of BSPM implementations and explore modern applications of advanced modeling and statistical analysis of BSPM data. We predict that BSPM will continue to be a valuable research tool, and will find clinical utility at the intersection of computational modeling approaches and artificial intelligence.

Measuring defibrillator surface potentials: The validation of a predictive defibrillation computer model

Implantable cardioverter defibrillators (ICDs) are commonly used to reduce the risk in patients with life-threatening arrhythmias, however, clinicians have little systematic guidance to place the device, especially in cases of unusual anatomy. We have previously developed a computational model that evaluates the efficacy of a delivered shock as a clinical and research aid to guide ICD placement on a patient specific basis. We report here on progress to validate this model with measured ICD surface potential maps from patients undergoing ICD implantation and testing for defibrillation threshold (DFT). We obtained body surface potential maps of the defibrillation pulses by adapting a limited lead selection and potential estimation algorithm to deal with the limited space for recording electrodes. Comparison of the simulated and measured potential maps of the defibrillation shock yielded similar patterns, a typical correlation greater than 0.9, and a relative error less than 15%. Comparison of defibrillation thresholds also showed accurate prediction of the simulations. The high agreement of the potential maps and DFTs suggests that the predictive simulation generates realistic potential values and can accurately predict DFTs in patients. These validation results pave the way for use of this model in optimization studies prior to device implantation.

Absence of Rapid Propagation through the Purkinje Network as a Potential Cause of Line Block in the Human Heart with Left Bundle Branch Block

Background: Cardiac resynchronization therapy is an effective device therapy for heart failure patients with conduction block. However, a problem with this invasive technique is the nearly 30% of non-responders. A number of studies have reported a functional line of block of cardiac excitation propagation in responders. However, this can only be detected using non-contact endocardial mapping. Further, although the line of block is considered a sign of responders to therapy, the mechanism remains unclear.

Methods: Herein, we created two patient-specific heart models with conduction block and simulated the propagation of excitation based on a cellmodel of electrophysiology. In one model with a relatively narrow QRS width (176 ms), we modeled the Purkinje network using a thin endocardial layer with rapid conduction. To reproduce a wider QRS complex (200 ms) in the second model, we eliminated the Purkinje network, and we simulated the endocardial mapping by solving the inverse problem according to the actual mapping system.

Results: We successfully observed the line of block using non-contact mapping in the model without the rapid propagation of excitation through the Purkinje network, although the excitation in the wall propagated smoothly. This model of slow conduction also reproduced the characteristic properties of the line of block, including dense isochronal lines and fractionated local electrocardiograms. Further, simulation of ventricular pacing from the lateral wall shifted the location of the line of block. By contrast, in the model with the Purkinje network, propagation of excitation in the endocardial map faithfully followed the actual propagation in the wall, without showing the line of block. Finally, switching the mode of propagation between the two models completely reversed these findings.

Conclusions: Our simulation data suggest that the absence of rapid propagation of excitation through the Purkinje network is the major cause of the functional line of block recorded by non-contact endocardial mapping. The line of block can be used to identify responders as these patients loose rapid propagation through the Purkinje network.

Transmural and apicobasal gradients in repolarization contribute to T-wave genesis in human surface ECG

The cellular basis of the T-wave morphology of surface ECG remains controversial in clinical cardiology. We examined the effect of action potential duration (APD) distribution on T-wave morphology using a realistic model of the human ventricle and torso. We developed a finite-element model of the ventricle consisting of ∼26 million elements, including the conduction system, each implemented with the ion current model of cardiomyocytes. This model was embedded in a torso model with distinct organ structures to obtain the standard ECG leads. The APD distribution was changed in the transmural direction by locating the M cells in either the endocardial or epicardial region. We also introduced apicobasal gradients by modifying the ion channel parameters. Both the transmural gradient (with M cells on the endocardial side) and the apicobasal gradient produced positive T waves, although a very large gradient was required for the apicobasal gradient. By contrast, T waves obtained with the transmural gradient were highly symmetric and, therefore, did not represent the true physiological state. Only combination of the transmural and the moderate apicobasal gradients produced physiological T waves in surface ECG. Positive T waves in surface ECG mainly originated from the transmural distribution of APD with M cells on the endocardial side, although the apicobasal gradient was also required to attain the physiological waveform.

Corrected body surface potential mapping

In the method for body surface potential mapping described here, the influence of thorax shape on measured ECG values is corrected. The distances of the ECG electrodes from the electrical heart midpoint are determined using a special device for ECG recording. These distances are used to correct the ECG values as if they had been measured on the surface of a sphere with a radius of 10 cm with its midpoint localized at the electrical heart midpoint. The equipotential lines of the electrical heart field are represented on the virtual surface of such a sphere. It is demonstrated that the character of a dipole field is better represented if the influence of the thorax shape is reduced. The site of the virtual reference electrode is also important for the dipole character of the representation of the electrical heart field.

Effect of Radiofrequency Catheter Ablation on Doppler Echocardiographic Parameters in Patients With Wolff-Parkinson-White Syndrome

The aim of this study was to compare the conventional Doppler echocardiographic parameters before and after accessory pathway ablation in patients with Wolff-Parkinson-White (WPW) syndrome. Thirty patients (19 males, 11 females) aged 35.5 +/- 14.4 years were enrolled in the study. All patients underwent successful radiofrequency catheter ablation (RFCA). Echocardiograhic examination was performed before and after RFCA. Aortic and pulmonary flows, diastolic early (E) and late (A) transmitral filling velocities, their velocity time integrals (VTI), mitral diastolic filling time (DFT), deceleration time (DT), isovolumic relaxation time (IVRT), aortic ejection time, and aortic VTI were assessed before and after RFCA. We found that the pulmonary valve opened earlier than the aortic valve when the accessory pathway was located on the right ventricular side (P = 0.02). Otherwise, if the accessory pathway was located on the left ventricular side, the aortic valve opened earlier (P < 0.01). Intervals between the onsets of aortic and pulmonary flows were shortened after RFCA (P = 0.01). We also observed prolongation of DFT (P < 0.001), increases in A velocity (P < 0.05) and its VTI (P < 0.01), as well as a decrease in the E/A ratio (P < 0.01) and shortening of aortic ejection time (P = 0.01) with restoration of AV conduction. We conclude that Doppler echocardiographic examination can provide clues about accessory pathway location and RFCA causes some significant changes in Doppler echocardiographic time intervals. These changes confirm that cardiac synchrony is restored after RFCA.

How many ECG leads do we need?

The number of leads needed in clinical electrocardiography depends on the clinical problem to be solved. The standard 12-lead ECG is so well established that alternative lead systems must prove their advantage through well-conducted clinical studies to achieve clinical acceptance. Certain additional leads seem to add valuable information in specific patient groups. The use of a large number of leads (eg, in body surface potential mapping) may add clinically relevant information, but it is cumbersome and its clinical advantage is yet to be proven. Reduced lead sets emulate the 12-lead ECG reasonably well and are especially advantageous in emergency situations.

Noninvasive study of ventricular preexcitation using multichannel magnetocardiography

In clinical practice, noninvasive classification of ventricular preexcitation (VPX) is usually done with ECG algorithms, which provide only a qualitative localization of accessory pathways. Since 1984, single or multichannel magnetocardiography (MMCG) has been used for three-dimensional localization of VPX sites, but a systematic study comparing the results of ECG and MMCG methods was lacking. This study evaluated the reliability of MMCG in an unshielded electrophysiological catheterization laboratory, and compared VPX classification as achieved with the five most recent ECG algorithms with that obtained by MMCG mapping and imaging techniques. A nine-channel direct current superconducting quantum interference device (DC-SQUID) MMCG system (sensitivity is 20 fT/Hz0.5) was used for sequential MMCG from 36 points on the anterior chest wall, within an area 20 x 20 cm. Twenty-eight patients with Wolff-Parkinson-White syndrome were examined at least twice, on the same day or after several months to test the reproducibility of the measurements. In eight patients, the reproducibility of MMCG was also evaluated using different MCG instrumentation during maximal VPX and/or atrioventricular reentrant tachycardia induced by transesophageal atrial pacing via a nonmagnetic catheter. The results of VPX localization with ECG algorithms and MMCG were compared. Equivalent current dipole, effective magnetic dipole, and distributed currents imaging models were used for the inverse solution. MMCG classification of VPX was found to be more accurate than ECG methods, and also provided additional information for the identification of paraseptal pathways. Furthermore, in patients with complex activation patterns during the delta wave, distributed currents imaging revealed two different activation patterns, suggesting the existence of multiple accessory pathways.

Decreased amplitude of left ventricular posterior wall motion with notch movement to determine the left posterior septal accessory pathway in Wolff-Parkinson-White syndrome

OBJECTIVE

To determine preoperatively, by analysing asynchronous left ventricular wall motion, whether to approach through the right ventricle or the left ventricle when carrying out catheter ablation of the accessory pathway in Wolff-Parkinson-White syndrome, especially in patients with the pathway located on the septum.

METHODS

73 patients with manifest Wolff-Parkinson-White syndrome who underwent successful catheter ablation were studied. Location of accessory pathway was classified as right ventricular side: right anterior paraseptum, right anterior, right lateral, right posterior, anterior septum, midseptum, right posterior septum; left ventricular side: left posterior septum, left posterior, left lateral, left anterior. Asynchronous systolic wall motion was analysed by cross sectional echocardiography.

RESULTS

Echocardiography showed that the amplitude of left ventricular posterior systolic wall motion was reduced when the pathway was located on the left ventricular side as opposed to the right ventricular side (mean (SD), 11.1 (1.7) v 12.9 (1.1) mm, p < 0.001), especially in patients with left posterior septal accessory pathway (9.7 (0.8) mm). There were no overlapping values between the left posterior septal accessory pathway and the right ventricular side accessory pathway. Posterior wall notch motion was observed in all patients with a left posterior septal accessory pathway (9/9), but not at all in patients with pathways located on the right ventricular side of the septum. In patients with a septal accessory pathway, an ECG algorithm provided poor information (relatively low sensitivity, specificity, and predictive value) for determining whether the subsite faced either the left (left posterior septum) or the right ventricle (anterior septum, midseptum, right posterior septum).

CONCLUSIONS

Decreased amplitude of left ventricular posterior wall motion with notch movement is an important finding for accessory pathways located on the left posterior septum. These findings provided clinically useful information for determining whether to approach catheter ablation from the right or the left ventricle.

Noninvasive localization of accessory pathways in patients with Wolff-Parkinson-White syndrome with the use of myocardial Doppler imaging

This study sought to examine the diagnostic accuracy of noninvasive prediction of accessory pathway localization in patients with manifest Wolff-Parkinson-White syndrome with the use of myocardial Doppler imaging as a new noninvasive mapping procedure. Myocardial Doppler imaging measures myocardial velocities and therefore can determine the site of earliest ventricular activation in patients with accessory bypass tracts. Twenty-five patients with manifest preexcitation were studied with the use of pulsed wave and M-mode myocardial Doppler imaging for the evaluation of the shortest electromechanical time interval in 9 basal myocardial segments. The new diagnostic test was compared with 3 electrocardiographic algorithms. An invasive mapping procedure served as reference standard. Abnormally short electromechanical time intervals were found in preexcited segments (27 +/- 12 ms vs 64 +/- 27 ms). Myocardial Doppler imaging correctly localized 84% of the accessory pathways and electrocardiographic algorithms only 48% to 60% of cases. Noninvasive prediction of accessory pathway localization by myocardial Doppler imaging is accurate and proved to be superior to prediction based on electrocardiographic algorithms.

Spatial resolution of body surface potential maps and magnetic field maps: a simulation study applied to the identification of ventricular pre-excitation sites

The spatial resolution of body surface potential maps (BSPMs) and magnetic field maps (MFMs) is investigated by means of an anatomically accurate computer model of the human ventricular myocardium. BSPMs and MFMs are calculated for the simulated activation sequences initiated at 35 pre-excitation sites located along the atrioventricular (AV) ring of the epicardium. Changes in the BSPMs and MFMs corresponding to different pre-excitation sites are quantified in terms of the correlation coefficient r. The spatial resolution (selectivity) for a given pre-excitation site is defined as the half-distance between those neighbouring locations at which morphological features of maps, in terms of r, become distinct (r < 0.95). It is found that, at 28 ms after the onset of pre-excitation and with no noise added, this distance +/- SD, for all sites along the AV ring for the 117-lead BSPMs, is 0.83 +/- 0.32 cm, and for the 64-lead and 128-lead MFMs it is 1.54 +/- 0.84 cm and 1.15 +/- 0.43 cm, respectively. The findings suggest that, when features of non-invasively recorded electrocardiographic and magnetocardiographic map patterns are used for identifying accessory pathways in patients suffering from WPW syndrome, BSPMs are likely to provide more detailed information for guiding the ablative treatment than MFMs. For some sites MFMs provide more information. Both modalities may provide additional assistance to the cardiologist in locating the site of the accessory pathway.

Cardiac memory in patients with intermittent Wolff-Parkinson-White syndrome

This study used body surface mapping to evaluate the ventricular repolarization process in the absence of delta waves in 13 patients with the intermittent Wolff-Parkinson-White (WPW) syndrome. The findings were compared with data from 30 normal individuals and 50 patients with the overt WPW syndrome. The QRST isointegral maps of patients with the overt WPW syndrome exhibited abnormal areas and the QRST departure maps showed a peculiar distribution to each accessory pathway site. The QRST isointegral map exhibited abnormal areas in 11 of the 13 cases (85%) of the intermittent WPW syndrome in the absence of delta waves. In 8 of these 11 cases (73%), the distribution of the departure map resembled that in the overt WPW syndrome. These findings suggest that abnormal ventricular repolarization due to cardiac memory is present in patients with the intermittent WPW syndrome even in the absence of delta waves.

The effects of inhomogeneities and anisotropies on electrocardiographic fields: a 3-D finite-element study

The aim of this study was to quantify the effects of selected inhomogeneities and anisotropies on computed electric potential fields associated with the electrocardiographic forward problem. The model construction was based on the Utah Torso model and included geometry for major anatomical structures such as subcutaneous fat, skeletal muscle, and lungs, as well as for epicardial fatpads, major arteries and veins, and the sternum, ribs, spine, and clavicles. Measured epicardial potentials served as the electrical source for solutions to the electrocardiographic forward problems computed using the finite element method (FEM). The geometry of the torso model for each simulation was constant, but different combinations of conductivities were assigned to individual organs or tissues. Comparisons of different conductivity combinations followed one of two basic schemes: 1) a homogeneous torso served as the reference against which we compared simulations with a single organ or tissue and assigned its nominal conductivity, or 2) a fully inhomogeneous torso served as the reference and we removed the effect of individual organs or tissues by assigning it the homogeneous conductivity value. When single inhomogeneities were added to an otherwise homogeneous isotropic model, anisotropic skeletal muscle (at a 15:1 anisotropy ratio) and the right and left lung had larger average effects (12.8, 12.7, and 12.1% relative error (RE), respectively) than the other inhomogeneities tested. Our results for removing single inhomogeneities show that the subcutaneous fat, the anisotropic skeletal muscle (with the degree of anisotropy equal to 7:1), and the lungs have larger average impacts on the body surface potential distributions than other elements of the model (with values of 14.9, 12.6, and 11.7% RE, respectively). The results also show that the size of the effect depended strongly on the distribution of epicardial potentials. The results of this study suggest that accurate representation of tissue inhomogeneity has a significant effect on the accuracy of the forward solution, with regions near the torso surface playing a larger role, in general, than those near the heart.

Computer simulation of epicardial potentials using a heart-torso model with realistic geometry

Previous cardiac simulation studies have focused on simulating the activation isochrones and subsequently the body surface potentials. Epicardial potentials, which are important for clinical application as well as for electrocardiographic inverse problems studies, however, have usually been neglected. This paper describes a procedure of simulating epicardial potentials using a microcomputer-based heart-torso model with realistic geometry. Our heart model developed earlier is composed of approximately 65,000 cell units which are arranged in a cubic close-packed structure. An action potential waveform with variable in duration is assigned to each unit. The heart model, together with the epicardial surface model constructed recently, are mounted in an inhomogeneous human torso model. Electric dipoles, which are proportional to the spatial gradient of the action potential, are generated in all the cell units. These dipoles give rise to a potential distribution on the epicardial surface, which is calculated by means of the boundary element method. The simulated epicardial potential maps during a normal heart beat and in a preexcited beat to mimic Wolff-Parkinson-White (WPW) syndrome are in close agreement with those reported in the literature.

Magnetocardiographically-guided catheter ablation

After more than 30 years since the first magnetocardiographic (MCG) recording was carried out with induction coils, MCG is now approaching the threshold of clinical use. During the last 5 years, in fact, there has been a growing interest of clinicians in this new method which provides an unrivalled accuracy for noninvasive, three-dimensional localization of intracardiac source. An increasing number of laboratories are reporting data validating the use of MCG as an effective method for preoperative localization of arrhythmogenic substrates and for planning the best catheter ablation approach for different arrhythmogenic substrates. In this article, available data from literature have been reviewed. We consider the clinical use of MCG to localize arrhythmogenic substrates in patients with Wolff-Parkinson-White syndrome and in patients with ventricular tachycardia in order to assess the state-of-the-art of the method on a large number of patients. This article also addresses some suggestions for industrial development of more compact, medically oriented MCG equipments at reasonable cost.

Pace mapping using body surface potential maps to guide catheter ablation of accessory pathways in patients with Wolff-Parkinson-White syndrome

BACKGROUND. A pace mapping technique using body surface potential maps (BSPMs) was developed to guide the positioning of an ablation catheter at the ventricular insertion point of accessory pathways (AP) in patients with the Wolff-Parkinson-White syndrome (WPW). METHODS AND RESULTS. The study was performed on 30 WPW patients. BSPMs were recorded with 63 leads distributed over the entire torso surface. The catheter used for radiofrequency ablation was first placed in the vicinity of the ventricular preexcitation site predicted by BSPMs recorded during the delta wave. BSPMs were then recorded during pacing with this catheter, the comparison between the preexcited and paced BSPMs indicated whether the pacing site was too anterior or posterior with respect to the preexcitation site, and the catheter was moved accordingly. This process was repeated until the preexcited and paced BSPMs were highly correlated (r > or = 0.8), and ablation then was attempted. It was possible to successfully ablate the AP in 28 patients after an investigation that lasted 54 +/- 44 minutes between the recording of the first paced BSPM and that of the BSPM paced at the successful ablation site. Patients with left free wall pathways needed less investigation time compared with patients with pathways of other locations (46 +/- 9 versus 100 +/- 25 minutes, p = 0.031). The sensitivity of BSPM pace mapping was assessed using pacing with a multipolar catheter, and significant changes were observed on the BSPMs for beats with pacing sites that were only 5 mm apart. CONCLUSIONS, BSPM pace mapping allowed us to achieve a 93% success rate with short investigation durations, provides significant information that cannot be obtained with the standard 12-lead ECG, is a self-correcting procedure that reduces the importance of BSPM alterations due to individual differences in the shape of the torso or heart, and is applicable only to patients with AP showing antegrade conduction.

Relation between recovery sequence estimated from body surface potentials and T wave shape in patients with negative T waves and normal subjects

BACKGROUND

Advances in analytical methods of the epicardial electrical potentials allowed us to demonstrate spatial distributions of local recovery. Because local recovery will be reflected in events on body surface ECG mapping, abnormalities in recovery sequence that may be responsible for the origin of negative T waves can be detected from body surface potentials.

METHODS AND RESULTS

Eighty-seven unipolar ECGs were recorded simultaneously from the entire thorax in patients having negative T waves on left anterior precordial leads and in normal subjects. These included 40 patients with anterior myocardial infarction (MI), 21 patients with left ventricular hypertrophy (LVH), and 44 male volunteers. We measured Tmax time, defined as the instant of maximal first derivative of the T wave as the index of local recovery (Wyatt's method). Parameters related to T wave potentials were positive T wave amplitude, negative T wave amplitude, and T integral. Significant correlations were observed between the Tmax time and each of the T wave potentials. The T wave potentials were dependent on Tmax times. In the anterior MI, the late Tmax times were located on the upper left anterior chest and early Tmax times on the lower right lateral chest. In the LVH, the area showing a delayed recovery was displaced in a left downward direction compared with anterior MI.

CONCLUSIONS

Body surface Tmax time distributions clearly separate two negative T wave groups, i.e., anterior MI and LVH. Appearance of the negative T waves correlates well with the presence of the area with delayed Tmax time on the spatial distribution.

Localization of accessory pathways in Wolff-Parkinson-White syndrome by high-resolution magnetocardiographic mapping

Fifteen patients with Wolff-Parkinson-White syndrome were studied with standard 12-lead electrocardiogram, invasive electrophysiologic study, and high-resolution magnetocardiographic (MCG) mapping. In addition, intraoperative epicardial mapping was performed in seven surgically treated patients. The MCG characteristics of ventricular preexcitation for different locations of the atrioventricular accessory pathways were described in terms of morphology and field patterns. Three mathematical source models in semi-infinite conducting space were used for localization computations: the current dipole model, the truncated current multipole model and the magnetic dipole model. Finally, the localization results of MCG and invasive mappings and electrocardiograms were compared. The mean three-dimensional distance between the localization results obtained from MCG maps and electrophysiologic study was 3.9 cm for the magnetic dipole model, 4.8 cm for the truncated current multipole model, and 7.3 cm for the current dipole model. The corresponding distances in the seven intraoperatively mapped cases were 2.3 cm for the magnetic dipole model, 5.2 cm for the truncated current multipole model, and 6.3 cm for the current dipole model. In conclusion, noninvasive MCG mapping may significantly contribute to the invasive catheter mapping for optimal preoperative localization of preexcitation site and atrioventricular accessory pathways in Wolff-Parkinson-White syndrome.

Body surface isopotential mapping of the entire QRST complex in the Wolff-Parkinson-White syndrome. Correlation with the location of the accessory pathway

Body surface potential maps were recorded during sinus rhythm and during atrial pacing at the time of electrophysiologic studies in 42 patients with Wolff-Parkinson-White syndrome. The locations of the accessory pathways were determined by epicardial mapping during surgery in 34 patients and by multicatheter endocavitary electrophysiologic studies in eight additional patients. During delta wave inscription, the shape and extension of areas of the negative and positive potentials on the thorax correlated better with the preexcitation site (69% of patients) than with the localization of the minimum potential alone (45.2% of patients). Typical potential distributions were present from the beginning of the delta wave and remained stationary during the first half of the QRS complex. During marked preexcitation, the superposition of atrial activity on the delta wave produced a mixed pattern in the earliest maps. However, these alterations of early delta thoracic potential distribution did not persist longer than 30 msec. The spread of the negative potentials during the last half of the QRS complex also characterized each localization: right-sided preexcitation reproduced the depolarization sequence of left bundle branch block, left-sided preexcitation reproduced that of right bundle branch block, and posterior pathways resembled left anterior fascicular block. Anterior left ventricular and more anterior left lateral ventricular preexcitations mimicked a right bundle branch block-left posterior fascicular block pattern. There was good correlation between the body surface potential map obtained during the ST segment and the site of the right-sided preexcitation. However, in left-sided preexcitations, ST patterns concordant with delta wave patterns were found less frequently than in right-sided preexcitations.(ABSTRACT TRUNCATED AT 250 WORDS)

Sample size and dimensionality in multivariate classification: implications for body surface potential mapping

This paper presents empirically determined guidelines for specifying the number of features appropriate for multivariate classification studies for given sample sizes. Sample size was considered adequate if the mean distance between two sample sets, taken from the same continuous multivariate distribution and projected onto the best separating direction, remained below a prescribed level. To quantitate the sample size requirement, homogeneity of sample set pairs of equal size. N, taken from the same continuous multivariate distribution was studied as a function of dimensionality. M. Homogeneity was characterized by the maximum absolute distances (Dmax) between the corresponding pairs of empirical cumulative probability distributions on the best separating projection. Computer generated data sets were used to estimate the cumulative probability distribution, P(D)M.N, for sample sizes, N, ranging from 5 to 100 and the dimensionality, M, ranging from 1 to 4. An empirical relationship between the estimated step-polygons and the Kolmogorov type one dimensional limiting distribution L(z) has been established. Based on the sample size data of 34 key papers on clinical body surface potential mapping (BSPM) it is noted that in 30% of the cases only one, and in 6% of the cases only two parameters could be used for statistical group representation to ensure a reasonable reliability (Dmax less than 0.2). In 56% of the published cases the sample sizes could not guarantee this reliability even for one feature or parameter.

Electrocardiographic body surface potential mapping in the Wolff-Parkinson-White syndrome. Noninvasive determination of the ventricular insertion sites of accessory atrioventricular connections

BACKGROUND

A reliable, noninvasive procedure to determine the location of accessory atrioventricular connections in patients with Wolff-Parkinson-White syndrome would add an important diagnostic tool to the clinical armamentarium.

METHODS AND RESULTS

Body surface potential mapping (BSPM) using 180 electrodes in various-sized vests and displayed as a calibrated color map was used to determine the ventricular insertion site of the accessory atrioventricular (AV) connections in 34 patients with Wolff-Parkinson-White syndrome. Attempts were made to determine the 17 ventricular insertion sites described by Guiraudon et al. All 34 patients had an electrophysiologic study (EPS) at cardiac catheterization, and 18 had surgery so the ventricular insertion sites could be accurately located using EPS at surgery. A number of physiologic observations were also made with BSPM.

CONCLUSIONS

The following conclusions were drawn: 1) BSPM using QRS analysis accurately predicts the ventricular insertion site of accessory AV connections in the presence of a delta wave in the electrocardiogram; 2) the ventricular insertion sites of accessory AV connections determined by BSPM and by EPS at surgery were identical or within one mapping site (1.5 cm or less) in all but four of 18 cases; three of the four exceptions had more than one accessory AV connection, and the other had a very broad ventricular insertion; 3) BSPM and EPS locations of the accessory AV connections correlated very well in the 34 cases despite the fact that BSPM determines the ventricular insertion site and EPS determines the atrial insertion site of the accessory AV connection; 4) as suggested by the three cases of multiple accessory AV connections, EPS and BSPM may be complementary since BSPM identified one pathway and EPS identified the other (in the case with a broad ventricular insertion, BSPM and EPS demonstrated different proportions of that insertion); 5) BSPM using ST-T analysis is very much less accurate in predicting the ventricular insertion site of accessory AV connections unless there is marked preexcitation; 6) standard electrocardiography using the Gallagher grid methodology (but with no attempt at stimulating maximal preexcitation) was not as accurate as QRS analysis of BSPM in predicting the ventricular insertion site of the accessory AV connection; however, exact comparison is hampered by the different number and size of the Gallagher and Guiraudon insertion sites; 7) BSPM using QRS analysis appears to be very accurate in predicting right ventricular versus left ventricular posteroseptal accessory AV connections; 8) typical epicardial right ventricular breakthrough, indicative of conduction via the specialized AV conduction system, occurs in all patients with left ventricular free wall accessory AV connections; 9) epicardial right ventricular breakthrough was not observed in cases with right ventricular free wall or anteroseptal accessory AV connections; 10) epicardial right ventricular breakthrough can occur in the presence of posteroseptal accessory AV connections, whether right or left ventricular; and 11) the delay in epicardial right ventricular breakthrough in cases with left ventricular insertion may provide a marker to estimate the degree of ventricular preexcitation.

Computer model of excitation and recovery in the anisotropic myocardium. II. Excitation in the simplified left ventricle

A computer model of propagated excitation and recovery in anisotropic cardiac tissue was described in the first report of this series. The model consists of a large number of excitable elements whose subthreshold interactions are governed by the anisotropic bidomain theory but whose suprathreshold behavior (action potential) is largely preassigned. As described in the first report, the model's performance was tested in rectangular and cubic arrays of excitable elements. This second report deals with three-dimensional simulations in a simplified left ventricle with anisotropy; specifically, the activation process in the "normal" ventricle is described (exemplified by the activation sequences started from various endocardial, intramural, and epicardial sites). To further substantiate our model's validity, we compare simulated epicardial and body-surface potential distributions with experimental findings in isolated canine hearts and with clinical evidence provided by electrocardiographic body-surface mapping.

Body surface mapping and nontraditional ECG leads in patients with Wolff-Parkinson-White syndrome

A method of ECG mapping from 90 points on the chest surface is described in 41 male and 17 female patients, aged 6 to 59 years. All also underwent invasive electrophysiological investigation and intraoperative epicardial mapping. Fifty-two patients had one, three patients two, and one patient had three anomalous accessory pathways. Two patients had nodoventricular tracts (Mahaim fibers). We distinguished seven zones along the atrioventricular groove (AVG) to compare the data derived from epicardial, endocardial, and body surface mapping. A microcomputer was used for the analysis of all ECGs to construct and analyze the isopotential maps. The criterion for localization of the anomalous accessory pathways was determined after analysis of the data from all 58 patients. The localization criterion was the appearance of a minimal deflection (-0.09 +/- 0.03 mV) on the surface isopotential maps within the first 0.28 msec of the QRS complex. This criterion for localization of anomalous accessory pathways from the chest surface was proposed on the basis of comparison of data from selective coronary angiography, the ventriculogram, and the chest X ray i.e., radiographic-topographic-anatomical data. In 20 patients, 10-20 nontraditional ECG leads were recorded from the chest to reflect the atrioventricular groove. The number of nontraditional ECG leads depended on patient age, weight, and height. Localization of the accessory pathway in one of the seven zones was established by the earliest delta wave and its maximum deviation. It was possible to localize the anomalous accessory pathway and to suspect multiple pathways in 95% of cases using nontraditional ECG leads and the listed criteria.(ABSTRACT TRUNCATED AT 250 WORDS)

Noninvasive recovery of epicardial potentials in a realistic heart-torso geometry. Normal sinus rhythm

The inverse problem in electrocardiography implies the reconstruction of electrical events within the heart from information measured noninvasively on the body surface. Deduction of these electrical events is possible from measured epicardial potentials, and, thus, a noninvasive method of recovering epicardial potentials from body surface data is useful in experimental and clinical studies. In the present study, an inverse method that uses Tikhonov regularization was shown to reconstruct, with good accuracy, important events in cardiac excitation. The inverse procedure was employed on data obtained from a human-torso tank in which a beating canine heart was placed in the correct anatomical position. Comparison with the actual, measured epicardial potentials indicates that positions and shapes of potential features (maxima, minima, zero potential line, saddles, etc.) are recovered with good accuracy throughout the QRS. An error in position of up to 1 cm is typical, while amplitudes are slightly diminished. In addition, application was extended from the above setting, in which the geometry was precisely known and potentials at a large number of leads were measured accurately, to a situation that is more representative of clinical and experimental settings. Effects of inaccuracy in location of the position of the heart were examined. A stylized torso that approximates the actual geometry was designed, and its performance in the inverse computations was evaluated. A systematic method of reduction of the number of leads on the body surface was proposed, and the resulting lead configurations were evaluated in terms of the accuracy of inverse solutions. The results indicate that the inverse problem can be stabilized with respect to different types of uncertainties in measured data and offer promise in the use of the inverse procedure in clinical and experimental situations.

Diagnostic value of body surface potential mapping in old anterior non-Q myocardial infarction

Body surface potential maps (BSM) were recorded from 140 chest leads in 30 healthy control subjects (C) and in 20 patients who had had an acute non-Q wave myocardial infarction (MI) 1-82 months before the study, to identify reliable indices of necrosis. In 12 MI patients the QRS complex was within normal limits on standard 12-lead ECG (group A), and in 8 patients no pathologic Q waves were present but the R waves were small and did not normally increase from V1 to V4 (group B). In each subject instantaneous potential distributions throughout the QRS interval were examined. Moreover, the potential--time integrals relating to three intervals (first 40 msec, mid-third, and last third of QRS) were calculated at each lead point and displayed as integral (I) maps. For each time interval, deviation index maps (DI), indicating the standardized differences from normal values, were calculated. An area where the integral values differed at least 2 SD from normal mean was considered abnormal. In most group A patients the inspection of instantaneous potential maps did not reveal definitively abnormal patterns. In group B patients a greater variety of patterns was found and in four cases the characteristic features of the anterior Q wave MI were observed. The DI maps of the first 40 msec of QRS provided the best diagnostic accuracy: areas of negative values 2 SD lower than normal were present in all group B patients (100%), in 8 group A patients (67%), and in 4 group C subjects (13%).(ABSTRACT TRUNCATED AT 250 WORDS)

Body surface maps and the conventional 12-lead ECG compared by studying their performances in classification of old myocardial infarction

The performance of body surface potential maps and the 12-lead ECG in the detection of old myocardial infarction has been compared in a two-group (54 normals; 52 infarctions) classification procedure (linear discriminant analysis). Three methods for data reduction of body surface maps were compared: 1) time integration, 2) one-step reduction in eigenvectors and 3) two-step reduction in spatial and temporal eigenvectors. Features were taken from the reduction variables by a stepwise selection procedure. From 90% to 93% correct classifications could be obtained using three features from the map data over the initial 30 ms (Q interval) of the QRS wave for all three methods considered. Using the 100 ms (QRS) interval 86% correct classifications were obtained using method 1, and up to 90% and 87% for methods 2 and 3, respectively. In a further analysis the classification based on body surface maps was compared to the one based on the 12-lead ECG. The 12-lead ECG was treated as a restricted set of the body surface mapping leads, so the same methods of data reduction, feature extraction and classification could be applied to both sets of data. Applying method 1 (time integration) 89% correct classifications were obtained using data taken from the 30 ms interval of the 12-lead ECG and a subsequent reduction to three features. When using the 100 ms interval the result was 79% also using three features. The results of method 2 applied to the 12-lead ECG were 89% (30 ms interval, three features) and 78% (100 ms interval, three features).

Fine detail in body surface potential maps: accuracy of maps using a limited lead array and spatial and temporal data representation

In order to evaluate the accuracy with which a limited lead array can be used to estimate fine details of the thoracic distribution of cardiac potentials, we compared 192-lead body surface maps and those constructed using a subset of 32 leads. We also evaluated preservation of detail in body surface maps reconstructed following spatial and temporal data representation, a method proposed for quantitative comparison of maps. Maps were analyzed with respect to four previously reported normal map features recorded with extensive lead arrays. The maps constructed from 32 leads accurately reproduced all map features with 92% or greater accuracy. Maps constructed after spatial and temporal data representation had a reproduction accuracy of 93% and 98% respectively for two map features more than 100 microV in amplitude but accuracy with respect to the two map features less than 100 microV in amplitude was 86% and 59% respectively. The study demonstrates that a selected limited lead array permits accurate estimation of the body surface distribution of cardiac potentials even when potentials are low level or occur in regions not directly sampled by a recording electrode. To represent potentials of less than 100 microV, more coefficients would be required to permit accurate spatial and temporal representation.

Mapping of body surface potentials in patients with the idiopathic long QT syndrome

Body surface potential maps were recorded from 140 chest leads in 25 patients affected by the idiopathic long QT syndrome (LQTS) and in 25 healthy control subjects matched for age and sex. Potential time integrals of the QRST and ST-T intervals were calculated at each lead point and displayed as isointegral (ISOI) maps. The main abnormalities noted on the QRST and ST-T ISOI maps were one area of negative values larger than normal in the right anterior and inferior thorax and a complex multipeak distribution of the integral values. At least one abnormality was present in 19 (76%) of the patients with LQTS and four (16%) of the control subjects (p less than .001). Each ISOI map was also represented as a weighted sum of nine fundamental components (eigenvectors) to detect and quantitate the nondipolar content. The percent contribution of the nondipolar eigenvectors (all eigenvectors beyond the third) was significantly higher in the LQTS group than in the control group (p less than .005). Specifically, an abnormally high nondipolar content on the QRST ISOI maps was observed much more frequently for patients with LQTS than for control subjects (nine or 36% vs one or 4%), and this was also true on the ST-T ISOI maps (14 or 56% vs one or 4%). No correlation was found between the major abnormalities on body surface maps and syncopal episodes. However, the high prevalence (76%) of these alterations among the patients with LQTS and their infrequent occurrence in the control population strongly suggests that they may be useful markers for the diagnosis of atypical cases. The prominent electronegative area on the anterior thorax can be related to delayed repolarization of a portion of the anterior wall of the heart. This finding is in agreement with the hypothesis that lower than normal right cardiac sympathetic activity is the main pathogenetic mechanism of LQTS. Multipeak distribution and high nondipolar content suggest regional electrical disparities in the ventricular recovery process. This may in part account for the high susceptibility of patients with LQTS to malignant arrhythmias.

Identification of best electrocardiographic leads for diagnosing anterior and inferior myocardial infarction by statistical analysis of body surface potential maps

In view of the increasing interest in quantifying and modifying the size of myocardial infarction (MI), it is important to look for clinically practical subsets of electrocardiographic leads that allow the earliest and most accurate diagnosis of the presence and electrocardiographic type of MI. A practical approach is described, taking advantage of the increased information content of body surface potential maps over standard electrocardiographic techniques for facilitating clinical use of body surface potential maps for such a purpose. Multivariate analysis was performed on 120-lead electrocardiographic data, simultaneously recorded in 236 normal subjects, 114 patients with anterior MI and 144 patients with inferior MI, using as features instantaneous voltages on time-normalized QRS and ST-T waveforms. Leads and features for optimal separation of normal subjects from, respectively, anterior MI and inferior MI patients were selected. Features measured on leads originating from the upper left precordial area, lower midthoracic region and the back correctly identified 97% of anterior MI patients, with a specificity of 95%; in patients with inferior MI, features obtained from leads located in the lower left back, left leg, right subclavicular area, upper dorsal region and lower right chest correctly classified 94% of the group, with specificity kept at 95%. Most features were measured in early and mid-QRS, although very potent discriminators were found in the late portion of the T wave.(ABSTRACT TRUNCATED AT 250 WORDS)

Body surface potential maps in old inferior myocardial infarction. Assessment of diagnostic criteria

We assessed the accuracy of criteria for diagnosing an inferior myocardial infarction from body potential maps. Body surface potential maps were recorded from 140 lead points on the entire chest surface in three groups of subjects: group A consisted of 15 patients with an old inferior myocardial infarction and typical electrocardiographic signs of necrosis; group B consisted of 15 patients with an old inferior myocardial infarction, but without electrocardiographic signs of necrosis (inferior myocardial infarction was documented during the acute phase); group C consisted of 30 healthy controls. In each subject body surface potential distributions were examined every 2 msec of the QRS complex. Moreover, the potential-time integrals relating to three intervals (QRS, the first 20 and the first 40 msec of the QRS complex) were calculated at each lead point and transferred to diagrams representing the thoracic surface explored (isointegral maps). For each time interval, the mean isointegral map obtained from group C subjects was subtracted from the isointegral map of each patient. The value obtained at each lead point was then divided by the standard deviation of the normal values for that point; the resulting values indicating the standardized differences from normal values were transferred to another map (deviation index isointegral map, DI map). We considered a reliable index of inferior myocardial infarction an area where the time-integral values were at least 2 SD lower than normal, in the inferior half of the thorax. A number of variables relative to instantaneous potential distribution and to isointegral maps were considered. The DI maps of the first 40 msec of QRS gave the most accurate criteria; in fact, an area of negative values 2 SD lower than normal was found in all group A patients and in 11 out of 15 group B patients (sensitivity 100% in group A, 73% in group B and specificity, 83%). Thus our results indicate that body surface potential maps have greater diagnostic information content than the 12 standard electrocardiographic leads and demonstrate the usefulness of the time integral analysis of body surface potentials for diagnostic interpretation.

The role of initial minimum potentials on body surface maps in predicting the site of accessory pathways in patients with Wolff-Parkinson-White syndrome

Forty-one patients (23 men and 18 women, ages 20 to 66 years) with Wolff-Parkinson-White syndrome were studied with isopotential body surface maps during sinus rhythm to find the most reliable index for predicting the sites of single accessory pathways. The sites predicted by surface maps were compared with those confirmed by multicatheter electrophysiologic study or in the course of surgical operation. Location of the initial minimum by a time criterion, 40 msec after onset of the QRS complex, was not reliable enough for prediction in patients with the small delta wave on their electrocardiograms, because ventricular activation via the normal conduction pathway significantly influenced the location of the minimum. Location of the minimum by an amplitude criterion, -0.15 mV or slightly deeper, was influenced minimally by fusion of ventricular activation, the patient's body size, or age and corresponded well to the site of the accessory pathway in 36 of 41 patients. Those minima appeared on circumscribed areas of the map in accordance with the anatomic subdivisions of the atrioventricular ring. Thus location of the minimum by the amplitude criterion was an excellent index for predicting the site of the accessory pathway, regardless of the degree of ventricular fusion. These amplitude-based map features suggest that nonstandard electrocardiograms recorded from selected positions on the body surface can be used as accurate predictors of the sites of accessory pathways.

A quantitative method for the localization of the ventricular pre-excitation area in the Wolff-Parkinson-White syndrome using singular value decomposition of body surface potentials

In order to localize quantitatively the site of ventricular pre-excitation, singular value decomposition (SVD) was applied to the body surface potential distributions of patients with the Wolff-Parkinson-White syndrome. The body surface potentials of sixty-two patients were recorded during sinus rhythm and pre-excitation by means of thirty electrodes placed on the anterior thoracic wall. The sites of the anomalous bundles had been determined beforehand by multicatheter electrophysiologic study. Considerable data reduction was obtained by using the SVD technique and displaying the potential distribution during the delta wave on two isofunction maps of the first two positional vectors and their corresponding two singular values (SV). A distinction was made between two types of isofunction maps. A type-S (single extreme) and a type-D (double extremes). A quantitative analysis was performed with the orientation of the zero line on the isofunction map being represented by the angle alpha or beta, and the singular values quotient (SVQ) of the two first singular values. The angle beta belonging to type D was used to subdivide this group of pre-excitation areas. The parameters SVQ and alpha belonging to type-S were illustrated in a graph on which a distinction between the various locations of the pre-excitation areas can be seen.

Spatial variation of QT intervals in normal persons and patients with acute myocardial infarction

The QT interval is a clinically important electrocardiographic measurement. This study attempted to determine 1) whether this interval was spatially distributed in a physiologically meaningful way on the torso of normal subjects, and 2) if these spatial patterns were altered in patients with acute myocardial infarction. To do so, 30 patients were studied within 72 hours of the onset of acute myocardial infarction (15 with an anterior and 15 with a posterior lesion) along with 50 normal control subjects. Electrocardiographic signals were registered from 150 torso electrodes; the QT interval in each lead was determined by a combined automated-manual method, and the durations displayed as "isointerval maps." In the normal subjects, the difference between the longest and shortest interval in each case was 59.4 +/- 12.9 ms. Long QT intervals were spatially located over the left lateral torso and short QT intervals were found over the right inferior chest. Acute infarction modified this distribution in relation to lesion location; the longest QT intervals were centrally positioned in anterior infarction and caudally located in inferior infarction. Thus, QT intervals in normal and abnormal states have distinctive spatial distributions that are consistent with known regional myocardial electrophysiology.

Effects of age, sex, and body habitus on QRS and ST-T potential maps of 1100 normal subjects

Body surface potential maps provide more detailed regional cardiac electrophysiologic information than the standard electrocardiogram. We performed a large-scale study of a normal population to form a comparison base for evaluation of the clinical utility of this technique. We analyzed body surface maps from 1113 normal subjects from 10 to 80 years old to detail map features as a function of age, sex, and body habitus. Maps were analyzed by visual inspection and by a spatial and temporal data reduction technique that allows statistical comparison of map features. On average, both QRS and ST-T potentials decreased with increasing age. Potential pattern distributions remained constant from 10 to 40 years. Beyond age 40, larger numbers of maps from normal subjects showed depolarization patterns consistent with delayed activation of the left anterior fasicle, despite normal 12-lead electrocardiograms. Only minor QRS potential amplitude and distribution differences were noted when male and female subjects were compared within groups of similar age and body habitus. Male subjects consistently showed greater average T potential amplitudes. Slender body habitus was associated with a more horizontal "zero" potential line. In female subjects over age 40 there were more extensive low-level negative potentials recorded over the precordium during the ST segment than in men. This study defines the range of normal body surface potential maps in a large clinically normal population and provides a basis for qualitative and statistical comparison with map features of patients with disease.

Ability of standard ECG parameters to detect the body surface isopotential abnormalities of pacing induced myocardial ischemia in the dog

The ability of parameters derived from standard, scalar ECG leads to predict abnormal body surface isopotential distributions was tested in 25 dogs. Atrial pacing to rates of 170 beats per minute or greater, two weeks after implantation of an ameroid constrictor on the left circumflex coronary artery, resulted in abnormal isopotential patterns due to subendocardial ischemia. A total of 96 scalar ECG variables (potential and slope measurements at 20, 40, 60 and 80 msec into the ST-segment in each of 12 leads) were computed. Ability of each variable and potential-slope pairs to distinguish normal from ischemic map forms was tested using discriminant function analysis and simple classification procedures. Results demonstrated that accuracy of prediction was lead-dependent, and time-dependent, with different sensitivities, specificities and boundary values in different leads and at different instants during the ST-segment. These new concepts may have direct relevance to the selection of clinical exercise ECG diagnostic criteria.

Application of the single moving dipole inverse solution to the study of the Wolff-Parkinson-White syndrome in man

The single moving dipole (SMD) inverse solution was performed in 28 patients with the Wolff-Parkinson-White preexcitation syndrome to see if the calculated position of the SMD during the initial delta wave could indicate the site of the underlying accessory pathway. This site was first estimated to be at one of eight locations around the atrioventricular ring, from the patient's QRS and ST segment body surface potential maps, as has been described by others. Next, SMD parameters were calculated during the delta wave so as to approximate, on a numerical torso model, the patient's body surface potential map. Visualization of the calculated position of the SMD around the atrioventricular ring was done by projecting it on a plane parallel to this ring. This plane corresponded to the most basal transverse section of a heart model present in the torso model. One limitation was the use of non-varying heart and torso models for all patients. As a result, the SMD technique lacked the precision to separate accessory pathway sites into eight atrioventricular locations. However it was capable of distinguishing between patients belonging to the larger classes of right-sided, posterior, and left-sided preexcitation, formed by combining adjacent atrioventricular accessory pathway locations. With more accurate heart and torso models, it may be possible to increase SMD resolution so as to locate accessory pathway sites deep within the heart. This would represent an advantage over the surface potential map approach which only identifies the site of earliest epicardial breakthrough associated with the accessory pathway.