Use of cardiac electric near-field measurements to determine activation times

High-Resolution Measurement of Local Activation Time Differences From Bipolar Electrogram Amplitude

Localized changes in myocardial conduction velocity (CV) are pro-arrhythmic, but high-resolution mapping of local CV is not yet possible during clinical electrophysiology procedures. This is in part because measurement of local CV at small spatial scales (1 mm) requires accurate annotation of local activation time (LAT) differences with very high temporal resolution (≤1 ms), beyond that of standard clinical methods. We sought to develop a method for high-resolution measurement of LAT differences and validate against existing techniques. First, we use a simplified theoretical model to identify a quantitative relationship between the LAT difference of a pair of electrodes and the peak amplitude of the bipolar EGM measured between them. This allows LAT differences to be calculated from bipolar EGM peak amplitude, by a novel "Determination of EGM Latencies by Transformation of Amplitude" (DELTA) method. Next, we use simulated EGMs from a computational model to validate this method. With 1 kHz sampling, LAT differences less than 4 ms were more accurately measured with DELTA than by standard LAT annotation (mean error 3.8% vs. 22.9%). In a 1-dimensional and a 2-dimension model, CV calculations were more accurate using LAT differences found by the DELTA method than by standard LAT annotation (by unipolar dV/dt timing). DELTA-derived LAT differences were more accurate than standard LAT annotation in simulated complex fractionated EGMs from a model incorporating fibrosis. Finally, we validated the DELTA method in vivo using 18,740 bipolar EGMs recorded from the left atrium of 10 atrial fibrillation patients undergoing catheter ablation. Using clinical EGMs, there was agreement in LAT differences found by DELTA, standard LAT annotation, and unipolar waveform cross-correlation. These results demonstrate an underlying relationship between a bipolar EGM's peak amplitude and the activation time difference between its two electrodes. Our computational modeling and clinical results suggest this relationship can be leveraged clinically to improve measurement accuracy for small LAT differences, which may improve CV measurement at small spatial scales.

Initial Changes in the Heart Conduction System Illustrated by Difference Map Patterns in Adolescent Patients After Kidney Transplantation: A Pilot Study

BACKGROUND

Cardiovascular complications (CVCs) in patients with end-stage renal disease (ESRD) often require hospitalization and are associated with an increased risk of fatality. Although kidney transplantation (KTx) improves a patient's status, CVCs are still a serious risk factor, so early identification is very important for final therapeutic outcome.

METHODS

This study included 5 post-KTx patients (age, 20.8 ± 1.16 years), dialyzed before KTx, and followed up for 6.7 ± 1.71 years. Body surface potential mapping (BSPM) was performed 4 times: twice before and twice after KTx. Electrocardiographic data were processed into map plotting to illustrate differences in ventricular activation times (VATs).

RESULTS

A comparative analysis of difference maps, both of dialyzed patients and normal subjects, highlighted certain specificities in the distribution of VAT changes for the left anterior fascicle block (LAFB). The maps clearly showed a significant correlation between the intensity of changes and duration of dialysis before KTx. After KTx, VATs seemed to be similar to those in normal subjects; however, this was true only for patients dialyzed for <1 year. The patients dialyzed for >1 year showed persistent conduction abnormalities on their VAT maps.

CONCLUSION

Summary differences in VAT maps can enable diagnostics of initial activation propagation abnormalities in the heart. Short-term dialysis therapy before KTx imposes positive effects with regression of heart conduction changes. These observations need to be verified in a larger study population.

Electroanatomical characterization of atrial microfibrosis in a histologically detailed computer model

Fibrosis is thought to play an important role in the formation and maintenance of atrial fibrillation (AF). The propensity of fibrosis to increase AF vulnerability depends not only on its amount, its texture plays a crucial role as well. While the detection of fibrotic tissue patches in the atria with extracellular recordings is feasible based on the analysis of electrogram fractionation, as used in clinical practice to identify ablation targets, the classification of fibrotic texture is a more challenging problem. This study seeks to establish a method for the electroanatomical characterization of the fibrotic textures based on the analysis of electrogram fractionation. The proposed method exploits the dependence of fractionation patterns on the incidence direction of wavefronts which differs significantly as a function of texture. A histologically detailed computer model of the right atrial isthmus was developed for testing the method. A stimulation protocol was conceived which generated various incidence directions for any given recording site where electrograms were computed. A classification method is derived then for discriminating three types of fibrosis, no fibrosis (control), diffuse, and patchy fibrosis. Simulation results showed that electrogram fractionation and amplitudes and their dependence upon incidence direction allow a robust discrimination between different classes of fibrosis. Finally, to minimize the technical effort, sensitivity analysis was performed to identify a minimum number of incidence directions required for robust classification.

Influence of ischemic core muscle fibers on surface depolarization potentials in superfused cardiac tissue preparations: a simulation study

Thin-walled cardiac tissue samples superfused with oxygenated solutions are widely used in experimental studies. However, due to decreased oxygen supply and insufficient wash out of waste products in the inner layers of such preparations, electrophysiological functions could be compromised. Although the cascade of events triggered by cutting off perfusion is well known, it remains unclear as to which degree electrophysiological function in viable surface layers is affected by pathological processes occurring in adjacent tissue. Using a 3D numerical bidomain model, we aim to quantify the impact of superfusion-induced heterogeneities occurring in the depth of the tissue on impulse propagation in superficial layers. Simulations demonstrated that both the pattern of activation as well as the distribution of extracellular potentials close to the surface remain essentially unchanged. This was true also for the electrophysiological properties of cells in the surface layer, where most relevant depolarization parameters varied by less than 5.5 %. The main observed effect on the surface was related to action potential duration that shortened noticeably by 53 % as hypoxia deteriorated. Despite the known limitations of such experimental methods, we conclude that superfusion is adequate for studying impulse propagation and depolarization whereas repolarization studies should consider the influence of pathological processes taking place at the core of tissue sample.

Effect of kidney transplantation on heart conduction disturbances in children treated with chronic hemodialysis--a pilot study

Aim of the study was to assess the effect of KT on heart conduction in HD children. Non-invasive electrocardiographic method of BSPM was used. Isochrone maps, presenting a VAT distribution, were taken from eight HD patients and 26 normal subjects. Patients were divided into two groups: I--three children were HD <12 months prior to KT; II--five children were HD >12 months prior to KT. After KT, the groups were marked as IP and IIP. Serum iPTH and phosphate levels were significantly higher in both HD groups than in controls, with a considerable normalization after transplantation. HD patients demonstrated neither conduction abnormalities on ECG nor left ventricular hypertrophy. Group-mean VAT maps revealed: I and II--similar patterns of complete LBBB; IP--partial normalization to a pattern of anterior fascicle block; IIP--preserved pattern of LBBB. Intraventricular conduction disturbances found in HD children using BSPM were alleviated by KT. Short HD therapy increases a chance of conduction disturbances regression after KT, contrary to the longer HD treatment. BSPM is more sensitive than standard ECG in detecting heart conduction impairments in the HD patients.

Oblique propagation of activation allows the detection of uncoupling microstructures from cardiac near field behavior

Wave fronts of cardiac activation, when propagating oblique to the fiber axis, reveal small fractionations and distortions in extracellular potential waveforms Phi(e) as well as in parameters derived from Phi(e) like dPhi(e)/dt and the gradient of Phi(e), the cardiac near field E. dPhi(e)/dt shows multiple deflections and E changes its morphology forming abnormal or even two or multiple subsequent loops. We analyze segments of such irregular loops of E obtained from in-vitro experiments and from computer simulation of a 2D-tissue sheet with a longitudinal oriented obstacle. In computer simulations we found that the individual sections of E reflect fairly well individual pathways of activation separated and delayed by the presence of a structural obstacle similar to connective tissue embedded longitudinally in heart tissue. Electrophysiological experiments with papillary muscles confirm this near field behavior.

Accuracy of local conduction velocity determination from non-fractionated cardiac activation signals

Local velocity and direction of cardiac excitation spread can be estimated from four extra-cellular electric potential waveforms recorded simultaneously by closely-spaced electrodes. In this work, three methods for the determination of these parameters in case of non-fractionated electrograms were compared. Waveforms recorded during in-vitro experiments were analyzed in terms of their noise components. Noise according to the experimental environment was generated and added to noise-free waveforms obtained from computer simulations. For each of the three investigated methods the accuracy of the determined parameters velocity and direction was evaluated for three different noise levels and three types of propagation.

A new floating sensor array to detect electric near fields of beating heart preparations

A new flexible sensor for in vitro experiments was developed to measure the surface potential, Phi, and its gradient, E (electric near field), at given sites of the heart. During depolarisation, E describes a vector loop from which direction and magnitude of local conduction velocity theta can be computed. Four recording silver electrodes (14 microm x 14 microm) separated by 50 microm, conducting leads, and solderable pads were patterned on a 50 microm thick polyimide film. The conductive structures, except the electrodes, were isolated with polyimide, and electrodes were chlorided. Spacer pillars mounted on the tip fulfil two functions: they keep the electrodes 70 microm from the tissue allowing non-contact recording of Phi and prevent lateral slipping. The low mass (9.1 mg) and flexibility (6.33 N/m) of the sensor let it easily follow the movement of the beating heart without notable displacement. We examined the electrodes on criteria like rms-noise of Phi, signal-to-noise ratio of Phi and E, maximum peak-slope recording dPhi/dt, and deviation of local activation time (LAT) from a common signal and obtained values of 24-28 microV, 46 and 41 dB, 497-561 V/s and no differences, respectively. With appropriate data acquisition (sampling rate 100 kHz, 24-bit), we were able to record Phi and to monitor E and theta on-line from beat-to-beat even at heart rates of 600 beats/min. Moreover, this technique can discriminate between uncoupled cardiac activations (as occur in fibrotic tissue) separated by less than 1 mm and 1 ms.