Spatial Methods of Epicardial Activation Time Determination in Normal Hearts

The purpose of this study was to demonstrate errors in activation time maps created using the time derivative method on fractionated unipolar electrograms, to characterize the epicardial distribution of those fractionated electrograms, and to investigate spatial methods of activation time determination. Electrograms (EGs) were recorded using uniform grids of electrodes (1 or 2 mm spacing) on the epicardial surface of six normal canine hearts. Activation times were estimated using the time of the minimum time derivative, maximum spatial gradient, and zero Laplacian and compared with the time of arrival of the activation wave front as assessed from a time series of potential maps as the standard. When comparing activation times from the time derivative for the case of epicardial pacing, spatial gradient and Laplacian methods with the standard for EGs without fractionation, correlations were high (R²=0.98, 0.98, 0.97, respectively). Similar comparisons using results from only fractionated EGs (R²=0.85,0.97,0.95) showed a lower correlation between times from the time derivative method and the standard. The results suggest an advantage of spatial methods over the time derivative method only for the case of epicardial pacing where large numbers of fractionated electrograms are found. © 2003 Biomedical Engineering Society. PAC2003: 8719Hh, 8719Nn, 8780Tq     

How Epicardial Electrodes Influence the Transmembrane Potential During a Strong Shock

This paper analyzes a possible artifact that may corrupt experiments studying defibrillation of the heart. Our hypothesis is that surface recording electrodes can influence the transmembrane potential during a shock. In the vicinity of an electrode, current leaves the intracellular space to take advantage of the low resistance of the extracellular path, thereby depolarizing the tissue. We calculate the transmembrane potential induced around a circular electrode when exposed to a uniform electric field. The bidomain model represents the electrical behavior of the cardiac tissue, and we account for electrode polarization impedance. Our results show that adjacent regions of depolarization and hyperpolarization exist around the electrode, and that the induced depolarization is greater than 100 mV for a 0.5 mm radius silver–silver chloride electrode in a 500 V/m electric field. We conclude that surface electrodes may produce artifacts during experiments designed to study defibrillation-strength electrical shocks. © 2001 Biomedical Engineering Society. PAC01: 8719Nn, 8719Hh, 8780-y     

Reproducibility of Image-Based Computational Fluid Dynamics Models of the Human Carotid Bifurcation

Recent studies have demonstrated the ability of magnetic resonance imaging (MRI) to provide anatomically realistic boundary conditions for computational fluid dynamics (CFD) simulations of arterial hemodynamics. To date, however, little is known about the overall reproducibility of such image-based CFD techniques. Towards this end we used serial black blood and cine phase contrast MRI to reconstruct CFD models of the carotid bifurcations of three subjects with early atherosclerosis, each imaged three times at weekly intervals. The lumen geometry was found to be precise on average to within 0.15 mm or 5%, while measured flow and heart rates varied by less than 10%. Spatial patterns of a variety of wall shear stress (WSS) indices were largely preserved among the three repeat models. Time-averaged WSS was reproduced best, on average to within 5 dyn/cm² or 37%, followed by WSS spatial gradients, angle gradients, and oscillatory shear index. The intrasubject flow rate variations were found to contribute little to the overall WSS variability. Instead, reproducibility was determined largely by the precision of the lumen boundary extraction from the individual MR images, itself shown to be a function of the image quality and proximity to the geometrically complex bifurcation region. © 2003 Biomedical Engineering Society. PAC2003: 8761Lh, 8757Nk, 8719Hh, 8719Rr, 8719Uv     

Relation between regional electrical activation time and subepicardial fiber strain in the canine left ventricle

To determine the relation between regional electrical activation time and fiber strain, epicardial electrical activation and deformation were measured in six open-chest dogs at the left ventricular anterior free wall after 15 min of right atrial, left ventricular free wall, left ventricular apex, or right ventricular outflow tract pacing, when end-diastolic pressure was normal or elevated (volume-loading). Regional electrical activation was measured using a 192-electrode brush. Regional subepicardial fiber strain (e _f) was measured simultaneously in 16 regions, using optical markers which were attached to the epicardial surface and recorded on video. When relating regional e _f during the ejection phase to regional activation time, the best correlation was found when a hemodynamic time reference rather than an electrophysiological one is used. Using the moment of the maximum rate of change of left ventricular pressure as the time reference for electrical activation, regional electrical activation time (t _ea) and the degree of e _f during the ejection phase could be fitted by a linear regression equation e _f=a t _ea+b in which a=–3.46±0.73 s^–1 and b=–0.28±0.05. For electrical activation times ranging from -40 to -80 ms, fiber strain was estimated with an accuracy of ±0.026 (±SE) with this relation. During right atrial pacing, t _ea and e _f were on the average –48 ms and –0.10 respectively. On further investigation, the relation between e _f and t _ea appeared to be influenced by end-diastolic pressure. For normal (1.1 kPa) and elevated end-diastolic pressure (1.8 kPa), the slope of the linear regression line was –3.96 and –2.86 s^–1, respectively. Three conclusions may be drawn. Firstly, the time interval between the moment of regional electrical activation and the moment of the maximum rate of change of left ventricular pressure is an index of regional fiber strain. Secondly, it can be concluded from the above equations that electrical asynchrony of more than 30 ms causes non-uniformities in the degree of e _f of the order of mean e _f during pacing from the right atrium. Finally, differences in fiber strain during asynchronous electrical activation are less pronounced at larger filling pressures.     

心外膜临时起搏在心内直视手术后的应用

心外膜起搏电极更多情况下作为心内膜起搏电极的补充,主要在无法应用心内膜起搏电极或经静脉途径心内膜起搏电极植入失败以及心脏直视术中,成为治疗心脏手术后房室传导阻滞,心动过缓及暂时性节律紊乱的主要措施。本文概述了临时心外膜起搏器的各种适应证,及何时考虑过渡到永久性起搏,起搏器的种类的选择,起搏器导线移除等问题。     

Effective Epicardial Resistance of Rabbit Ventricles

This study evaluated effective resistances on the ventricular surfaces of arterially-perfused rabbit hearts. Effective resistances were determined with a four-electrode array that was parallel or perpendicular to epicardial fibers. Resistance along or across epicardial fibers was determined by applying current to the epicardium with two parallel line electrodes and measuring potentials in the region between the electrodes. Computer simulations were performed to gain insight into the distribution of current in the ventricular wall. The effective resistances were not different along versus across fibers. Simulations showed that transmural rotation of fibers causes current to be distributed differently when the electrode is oriented perpendicular versus parallel to epicardial fibers. When the array is oriented so that epicardial current is across fibers, the fraction of current that flows transmurally and along the deeper fibers increases while the fraction of current that flows epicardially decreases. This introduces isotropy of the effective resistance. Thus, in contrast to isolated cardiac fibers, the ventricular epicardium exhibits isotropic effective resistance due to transmural rotation of fibers. The rotation and isotropic resistance may be important for cardiac electrical behavior and effects of electrical current in the ventricles. © 1999 Biomedical Engineering Society. PAC99: 8719La, 8716Uv     

Novel Concept for a Noninvasive Cardiopulmonary Monitor for Infants: A Pair of Pajamas with an Integrated Sensor Module

We present a noninvasive cardiopulmonary monitor for use in infants in which sensors are incorporated into conventional pajamas. It consists of dry electrodes for picking up the electrocardiogram and of two capacitive based elastic strain gauges for the measurement of thoracic and abdominal respiratory movements. The quality of the signals was assessed by performing at home seven sleep recordings in parallel with a conventional system and 16 overnight sleep recordings. In the former tests, the acquisition of all mean R–R intervals agreed in both systems with an accuracy of 15.6 ms, determined by the sampling frequency of the commercial system. For the overnight recordings, more than 99% of the R peaks were correctly detected. In 1.5%, both respiratory traces were simultaneously out of range. Finally, it was observed that more saturated episodes and less errors in R-peak detection appeared in prone than in supine position. In summary, these results demonstrated that the dry electrodes can be a good alternative to the sticking electrodes, and that this simple system is reliable. In contrast with the existing monitors, skin irritations are avoided, redundancy of respiratory signals is provided and user-friendliness of the system is reached. © 2003 Biomedical Engineering Society. PAC2003: 8719Nn, 8780-y, 8719Uv     

Entrainment of Intestinal Slow Waves with Electrical Stimulation Using Intraluminal Electrodes

The aim of this study was to investigate whether the intestinal stimulation would be feasible using a less invasive method: intraluminal electrodes. The study was performed in nine healthy hound dogs (15–26 kg). Four pairs of electrodes were implanted on the serosa of the jejunum at an interval of 5 cm with the most proximal pair 35 cm beyond the pylorus. An intestinal fistula was made 20 cm beyond the pylorus. Simultaneous recordings of intestinal myoelectrical activity were made for 2 h in the fasting state from both intraluminal and serosal electrodes. Various pacing parameters were tested. The frequency of the intestinal slow wave recorded from the intraluminal electrodes was identical to that from the serosal electrodes , p < 0.001), and so was the percentage of normal 17–22 cycles/min waves (95.8±33.9% vs 98.16±1.33%, r=0.96, p<0.01).p < 0.01). A complete entrainment of the intestinal slow wave was achieved in every dog with electrical stimulation using intraluminal ring electrodes. The effective pacing parameters were pulse width of 70 ms, amplitude of 4 mA and frequency of 1.1 IF (intrinsic frequency). The time required for the entrainment of the intestinal slow wave with intraluminal pacing was 25.0±2.1s. The maximum driven frequency was found to be 1.43±0.01 IF. The results reveal that intraluminal pacing is an effective and efficient method for the entrainment of intestinal slow waves. It may become a potential approach for the treatment of intestinal motor disorders associated with myoelectrical abnormalities. © 2000 Biomedical Engineering Society. PAC00: 8754Dt, 8719Ff, 8717Nn     

Model-Based Analysis of Optically Mapped Epicardial Activation Patterns and Conduction Velocity

A novel parametric model-based method was developed to quantify epicardial conduction patterns and velocity in an isolated Langendorff-perfused rabbit heart. The method incorporated geometric and anatomical features of the left and right ventricles into the analysis. Optical images of propagation were obtained using the voltage-sensitive dye DI-4-ANEPPS, and a high-speed digital camera. Activation maps were extracted from these images and interpolated onto a three-dimensional finite-element model of epicardial geometry and fiber structure. Activation time was expressed as a function of local parametric coordinates, and a conduction velocity vector field was computed from the gradient of the scalar field. Activation times measured using bipolar electrodes did not differ significantly from times measured using the optical mapping technique. The method was able to detect a 34% decrease in average fiber velocity and a 28% decrease in average cross-fiber velocity following the addition of 0.5 mM heptanol into the perfusate. The combination of optical mapping with a three-dimensional geometric model of the ventricles provides a new tool to quantify wave-front propagation relative to anatomy at a relatively high spatial resolution. © 2000 Biomedical Engineering Society. PAC00: 8719Nn, 8719Hh, 8719Ff, 8762+n, 8710+e     

Mechanoelectric Feedback in a Model of the Passively Inflated Left Ventricle

Mechanoelectric feedback has been described in isolated cells and intact ventricular myocardium, but the mechanical stimulus that governs mechanosensitive channel activity in intact tissue is unknown. To study the interaction of myocardial mechanics and electrophysiology in multiple dimensions, we used a finite element model of the rabbit ventricles to simulate electrical propagation through passively loaded myocardium. Electrical propagation was simulated using the collocation-Galerkin finite element method. A stretch-dependent current was added in parallel to the ionic currents in the Beeler–Reuter ventricular action potential model. We investigated different mechanical coupling parameters to simulate stretch-dependent conductance modulated by either fiber strain, cross-fiber strain, or a combination of the two. In response to pressure loading, the conductance model governed by fiber strain alone reproduced the epicardial decrease in action potential amplitude as observed in experimental preparations of the passively loaded rabbit heart. The model governed by only cross-fiber strain reproduced the transmural gradient in action potential amplitude as observed in working canine heart experiments, but failed to predict a sufficient decrease in amplitude at the epicardium. Only the model governed by both fiber and cross-fiber strain reproduced the epicardial and transmural changes in action potential amplitude similar to experimental observations. In addition, dispersion of action potential duration nearly doubled with the same model. These results suggest that changes in action potential characteristics may be due not only to length changes along the long axis direction of the myofiber, but also due to deformation in the plane transverse to the fiber axis. The model provides a framework for investigating how cellular biophysics affect the function of the intact ventricles. © 2001 Biomedical Engineering Society. PAC01: 8716Uv, 8719Nn, 8719Hh, 8719Rr, 8718Bb, 8710+e, 0270Dh, 8717Aa     

Voltage-Sensitive Dye Mapping of Activation and Conduction in Adult Mouse Hearts

A custom-made apparatus based on a charge-coupled-device camera has been used to monitor changes in fluorescence from Langendorff-perfused adult mouse hearts stained with a voltage-sensitive dye, di-4-ANEPPS. With this approach it is possible to monitor activation of the ventricles at high temporal (375 s/frame) and spatial resolution 72 × 78pixels,100 ×100 m/pixel. In sinus rhythm, activation occurred with a complicated breakthrough pattern on both ventricles, and a total activation time of 3.51 ± 0.16ms (32 °C). A stimulus applied near the apex of the left ventricle resulted in a single activation wave front with a total activation time of 8.18 ± 0.25 ms. Pacing from a site near the middle of the left ventricular epicardial surface revealed anisotropic conduction, indicating that conduction occurs preferentially in the direction of the predominant fiber orientation. The total activation time in this configuration was 5.44 ± 0.24 ms. The difference in total activation time between sinus rhythm and epicardial stimulation suggests an important role for transmural conduction (the Purkinje system) in the mouse heart. These findings provide much of the necessary background needed for studying conduction abnormalities in genetically altered mice and suggest that the comparison of sinus rhythm and epicardial pacing can be used to reveal transmural conduction abnormalities. © 2000 Biomedical Engineering Society. PAC00: 8719Nn, 8719Hh, 8716Uv, 8764Ni     

Criteria for the Selection of Materials for Implanted Electrodes

There are four criteria that must be considered when choosing material for an implanted electrode: (1) tissue response, (2) allergic response, (3) electrode-tissue impedance, and (4) radiographic visibility. This paper discusses these four criteria and identifies the materials that are the best candidates for such electrodes. For electrodes that make ohmic contact with tissues: gold, platinum, platinum–iridium, tungsten, and tantalum are good candidates. The preferred insulating materials are polyimide and glass. The characteristics of stimulator output circuits and the importance of the bidirectional wave- form in relation to electrode decomposition are discussed. The paper concludes with an analysis, the design criteria, and the special properties and materials for capacitive recording and stimulating electrodes. © 2003 Biomedical Engineering Society. PAC2003: 8754Dt, 8780Fe, 8768+z, 8719Nn     

The effect of nontransmural necroses on epicardial potential maps during paced activation: a simulation study

A number of studies have indicated that epicardial potentials provide detailed spatiotemporal information about the spread of electrical activation within the ventricular wall. Here, we used a computer model to simulate activation sequences and corresponding epicardial potential maps in the ventricles damaged by localized necroses. Our findings agreed with those of experimental studies performed for epicardial pacing locus in a complete transient loss of one of the positive areas when the necrosis was located subepicardially, and in a transient gap in the expanding positive areas when the necrosis was located intramurally and subendocardially. This study--by systematically comparing simulated epicardial potential maps with those recorded on the exposed canine hearts--constitutes an important step in validation of our model.     

Delayed Activation and Retrograde Propagation in Cardiac Muscle: Implication of Virtual Electrode Effects

Point cathodal stimulation of cardiac tissue was shown previously to produce both a dog-bone shaped virtual cathode transverse to the muscle fibers and two longitudinal virtual anodes. We hypothesize that virtual anodes can cause a region of delayed activation, separating two regions of early activation caused by the virtual cathode. Using a high-density electrode array in 42 superfused epicardial slices from 14 canine left ventricles, we observed regions of early and delayed activation and different pathways of retrograde propagation corresponding to the earlier patterns. Retrograde propagation was seen from the transversely located early activation area through areas of delayed activation toward the cathode, and from the early activation area toward the cathode directly. These pathways caused a wide dispersion in the direction of retrograde propagation (2° ± 31°, n = 179, relative to the fast axis of threshold activation; radial velocity: 0.5 ± 0.2m/s, n = 95, in 12 slices from 8 hearts with stimuli of 330 s, 0.8–30 mA). Delayed activations were observed 0° ± 6° (n = 32) from the axis in 23 maps (at differing stimulation strengths) recorded in 13 slices from 10 hearts. We conclude that point cathodal stimulation induce delayed activation along the fiber axis and retrograde propagation both along and transverse to the axis. © 2000 Biomedical Engineering Society. PAC00: 8719Ff, 8719Hh, 8716Uv, 8754Dt, 8719Nn     

The quantitative assessment of epicardial fat distribution on human hearts: Implications for epicardial electrophysiology

Epicardial electrophysiological procedures rely on dependable interfacing with the myocardial tissue. For example, epicardial pacing systems must generate sustainable chronic pacing capture, while epicardial ablations must effectively deliver energy to the target hyper‐excitable myocytes. The human heart has a significant adipose layer which may impede epicardial procedures. The objective of this study was to quantitatively assess the relative location of epicardial adipose on the human heart, to define locations where epicardial therapies might be performed successfully. We studied perfusion‐fixed human hearts (n = 105) in multiple isolated planes including: left ventricular margin, diaphragmatic surface, and anterior right ventricle. Relative adipose distribution was quantitatively assessed via planar images, using a custom‐generated image analysis algorithm. In these specimens, 76.7 ± 13.8% of the left ventricular margin, 72.7 ± 11.3% of the diaphragmatic surface, and 92.1 ± 8.7% of the anterior right margin were covered with superficial epicardial adipose layers. Percent adipose coverage significantly increased with age (P < 0.001) and history of coronary artery disease (P < 0.05). No significant relationships were identified between relative percent adipose coverage and gender, body weight or height, BMI, history of hypertension, and/or history of congestive heart failure. Additionally, we describe two‐dimensional probability distributions of epicardial adipose coverage for each of the three analysis planes. In this study, we detail the quantitative assessment and probabilistic mapping of the distribution of superficial epicardial adipose on the adult human heart. These findings have implications relative to performing epicardial procedures and/or designing procedures or tools to successfully perform such treatments. Clin. Anat. 31:661–666, 2018. © 2018 Wiley Periodicals, Inc.     

Direct Inference of the Spectra of Pericardial Potentials Using the Boundary-Element Method

New methods, based on Tikhonov regularization, were developed to infer the magnitude and phase of pericardial potentials directly. These methods were tested in an adult-male torso model using measured human epicardial potentials. With 1% noise added to body-surface potentials, regularization with an optimal parameter at each frequency from 1 to 100 Hz gave an average relative error (RE) in inferred spectral magnitudes of 0.44. Regularization with the composite–residual–smoothing–operator (CRESO) parameter increased the RE slightly to 0.47. With 10% additive noise, 10 mm overestimation of heart radius, and a 10 mm error in heart position, the average CRESO parameter from 1 to 100 Hz gave an average RE of 0.71. Performance was frequency dependent. The smallest REs occurred at low frequencies. With 1% noise, optimal regularization gave average REs of 0.20, 0.40, and 0.53 in the 1–15, 15–46, and 46–100 Hz bands, respectively. Direct inference of spectral magnitudes was more accurate than Fourier transformation of inferred time-domain waveforms. Results suggest that when heart size and location are not known, minimum REs in spectral estimates are found using an overestimated heart size and a regularization parameter which is the average value over the frequency band of interest. © 1999 Biomedical Engineering Society. PAC99: 8719Hh, 8719Nn, 0260Lj     

Estimation of Cardiac Conduction Velocities Using Small Data Sets

Cardiac surface conduction velocities could provide valuable information about the speed and angle of a propagating electrical wave front. It would be advantageous to develop catheter-based velocity mapping devices to improve the visualization of cardiac arrhythmias. However, catheter tip size is limited to 2.5 mm in diameter, restricting the number and size of electrodes that can be placed on a catheter tip. We address the feasibility of estimating conduction speed and angle from small data sets suitable for recording from a catheter device in a standard clinical environment. We estimated cardiac conduction velocities from data subsets of 4–7 closely spaced electrograms, and then compared these estimates to velocities estimated from a larger reference grid. We studied 137 ventricular beats and 17,756 velocity vectors from six swine hearts. Average differences in angle between the two estimates were 0.4° ± 16° while average differences in speed were 5% ± 33%. These angle and speed differences provide an initial quantitative assessment of velocity accuracy for the purposes of catheter-based vector mapping. © 2003 Biomedical Engineering Society. PAC2003: 8719Hh, 8719Nn, 8780-y     

Selective Microstimulation of Central Nervous System Neurons

The goal of this study was to identify stimulus parameters and electrode geometries that were effective in selectively stimulating targeted neuronal populations within the central nervous system (CNS). Cable models of neurons that included an axon, initial segment, soma, and branching dendritic tree, with geometries and membrane dynamics derived from mammalian motoneurons, were used to study excitation with extracellular electrodes. The models reproduced a wide range of experimentally documented excitation patterns including current-distance and strength-duration relationships. Evaluation of different stimulus paradigms was performed using populations of fifty cells and fifty fibers of passage randomly positioned about an extracellular electrode(s). Monophasic cathodic or anodic stimuli enabled selective stimulation of fibers over cells or cells over fibers, respectively. However, when a symmetrical charge-balancing stimulus phase was incorporated, selectivity was greatly diminished. An anodic first, cathodic second asymmetrical biphasic stimulus enabled selective stimulation of fibers, while a cathodic first, anodic second asymmetrical biphasic stimulus enabled selective stimulation of cells. These novel waveforms provided enhanced selectivity while preserving charge balancing as is required to minimize the risk of electrode corrosion and tissue injury. Furthermore, the models developed in this study can predict the effectiveness of electrode geometries and stimulus parameters for selective activation of specific neuronal populations, and in turn represent useful tools for the design of electrodes and stimulus waveforms for use in CNS neural prosthetic devices. © 2000 Biomedical Engineering Society. PAC00: 8717Nn, 8719La, 8719Nn, 8717Aa     

Accuracy of Quadratic Versus Linear Interpolation in Noninvasive Electrocardiographic Imaging (ECGI)

Electrocardiographic Imaging (ECGI) is a cardiac functional imaging modality, noninvasively reconstructing epicardial potentials, electrograms and isochrones (activation maps) from multi-channel body surface potential recordings. The procedure involves solving Laplace’s equation in the source-free volume conductor between torso and epicardial surfaces, using Boundary Element Method (BEM). Previously, linear interpolation (LI) on three-noded triangular surface elements was used in the BEM formulation. Here, we use quadratic interpolation (QI) for potentials over six-noded linear triangles. The performance of LI and QI in ECGI is evaluated through direct comparison with measured data from an isolated canine heart suspended in a human-torso-shaped electrolyte tank. QI enhances the accuracy and resolution of ECGI reconstructions for two different inverse methods, Tikhonov regularization and Generalized Minimal Residual (GMRes) method, with the QI-GMRes combination providing the highest accuracy and resolution. QI reduces the average relative error (RE) between reconstructed and measured epicardial potentials by 25%. It preserves the amplitude (average RE reduced by 48%) and morphology of electrograms better (average correlation coefficient for QI ∼ 0.97, LI ∼ 0.92). We also applied QI to ECGI reconstructions in human subjects during cardiac pacing, where QI locates ventricular pacing sites with higher accuracy (≤ 10 mm) than LI (≤ 18 mm).