Acute voltage, charge, and energy thresholds as functions of electrode size for electrical stimulation of the canine heart

Ultrasensitive amperometric determination of hand, foot and mouth disease based on gold nanoflower modified microelectrode

Given the widespread use of point-of-care testing for diagnosis of disease, micro-scale electrochemical deoxyribonucleic acid (DNA) biosensors have become a promising area of research owing to its fast mass transfer, high current density and rapid response. In this study, a gold nanoparticles modified gold microelectrode (AuNPs/Au-Me) was constructed to determine the hand, foot and mouth disease (HFMD)-related gene. The noble metal nanoparticles modification yielded ca. 7.4-fold increase in electroactive surface area of microelectrode, and the signal for HFMD-related gene was largely magnified. Under optimal conditions, the biosensor exhibited salient selectivity and sensitivity with a low detection limit of 0.3 fM (S/N = 3), which is sufficient for clinical diagnosis of HFMD. Additionally, the developed AuNPs/Au-Me was successfully applied to determining the polymerase chain reaction (PCR) amplified products of target gene. Thus, the electrochemical DNA biosensor possesses great potential in early-stage diagnosis and long-term monitoring of various disease.

Models of cardiac tissue electrophysiology: progress, challenges and open questions

Models of cardiac tissue electrophysiology are an important component of the Cardiac Physiome Project, which is an international effort to build biophysically based multi-scale mathematical models of the heart. Models of tissue electrophysiology can provide a bridge between electrophysiological cell models at smaller scales, and tissue mechanics, metabolism and blood flow at larger scales. This paper is a critical review of cardiac tissue electrophysiology models, focussing on the micro-structure of cardiac tissue, generic behaviours of action potential propagation, different models of cardiac tissue electrophysiology, the choice of parameter values and tissue geometry, emergent properties in tissue models, numerical techniques and computational issues. We propose a tentative list of information that could be included in published descriptions of tissue electrophysiology models, and used to support interpretation and evaluation of simulation results. We conclude with a discussion of challenges and open questions.

Current distribution under electrodes in relation to stimulation current and skin blood flow: are modern electrodes really providing the current distribution during stimulation we believe they are?

Carbonized rubber electrodes were tested extensively when they were first developed 30 years ago, but modern carbonized rubber electrodes have not received the type of scrutiny that the first electrodes received. Modern electrodes differ from the original electrodes in that they come with a self-adhesive electrode gel called hydrogel as part of their composition. The present study was undertaken to examine the current distribution and impedance characteristics of five brands of carbonized rubber electrodes and to examine the current distribution between electrodes during electrical stimulation in six subjects. Several different electrode sizes were tested between 3 and 10 cm. The current flow between the electrodes was determined by measuring the voltage across the skin on human subjects in 15 discrete locations between the electrodes. Blood flow was also measured between the electrodes with a laser Doppler flow meter to assess the physiological effect of current distribution on the skin at several skin temperatures. The results of these studies showed that at low currents, such as is used in TENS, very little current is actually applied through the skin due to the high impedance of the electrodes. At current levels normally used for electrical stimulation for functional movement, while current flow is better in most electrodes, it is very uneven, resulting in high current density in the centre of the electrodes and a fall off of at least 50% in current intensity at the edges of the electrode. There was very little difference in current density between small and large electrodes due to the high current density in the centre. Skin blood flow altered the movement of current between the electrodes and also may contribute to electrode performance. The implication of these studies is that electrode design needs to be altered for better current distribution, especially at low stimulation currents.

Measuring curvature and velocity vector fields for waves of cardiac excitation in 2-D media

Excitable media theory predicts the effect of electrical wavefront morphology on the dynamics of propagation in cardiac tissue. It specifies that a convex wavefront propagates slower and a concave wavefront propagates faster than a planar wavefront. Because of this, wavefront curvature is thought to be an important functional mechanism of cardiac arrhythmias. However, the curvature of wavefronts during an arrhythmia are generally unknown. We introduce a robust, automated method to measure the curvature vector field of discretely characterized, arbitrarily shaped, two-dimensional (2-D) wavefronts. The method relies on generating a smooth, continuous parameterization of the shape of a wave using cubic smoothing splines fitted to an isopotential at a specified level, which we choose to be -30 mV. Twice differentiating the parametric form provides local curvature vectors along the wavefront and waveback. Local conduction velocities are computed as the wave speed along lines normal to the parametric form. In this way, the curvature and velocity vector field for wavefronts and wavebacks can be measured. We applied the method to data sampled from a 2-D numerical model and several examples are provided to illustrate its usefulness for studying the dynamics of cardiac propagation in 2-D media.

Basic mechanisms of cardiac impulse propagation and associated arrhythmias

Propagation of excitation in the heart involves action potential (AP) generation by cardiac cells and its propagation in the multicellular tissue. AP conduction is the outcome of complex interactions between cellular electrical activity, electrical cell-to-cell communication, and the cardiac tissue structure. As shown in this review, strong interactions occur among these determinants of electrical impulse propagation. A special form of conduction that underlies many cardiac arrhythmias involves circulating excitation. In this situation, the curvature of the propagating excitation wavefront and the interaction of the wavefront with the repolarization tail of the preceding wave are additional important determinants of impulse propagation. This review attempts to synthesize results from computer simulations and experimental preparations to define mechanisms and biophysical principles that govern normal and abnormal conduction in the heart.

Assessment of Radiofrequency Ablation Effect From Unipolar Pacing Threshold

Methods for determining if an ablation lesion has been created by RF current application are limited, but needed. This study sought to determine if a change in pacing threshold at the ablation site might be used to assess creation of an ablation lesion. Peak-to-peak amplitude of the bipolar electrogram (EGM) and the unipolar pacing threshold were determined before and after creation of RF lesions using irrigated tip (63 lesions in 11 patients) or conventional ablation catheters (33 lesions in 9 patients) in infarct scars for ablation of ventricular tachycardia. The threshold was measured during continuous pacing at a cycle length of 600 ms by a decrementing output current at a pulse width of 2 ms. The unipolar pacing threshold increased by 254 +/- 248% (from 5.7 +/- 3.5 to 15.1 +/- 6.7 mA, P<0.001) after irrigated tip ablation and by 155 +/- 144% (from 5.9 +/- 3.4 to 12.3 +/- 5.7 mA, P<0.001) after conventional ablation (P<0.05 for irrigated tip vs conventional). EGM amplitude decreased by 17 +/- 27% (from 0.39 +/- 0.32 to 0.30 +/- 0.21 mV) after irrigated tip ablation and by 16 +/- 24%(from 0.48 +/- 0.27 to 0.41 +/- 0.20 mV) after conventional ablation (irrigated tip vs conventional, P=NS). There was no correlation between the change in bipolar EGM amplitude and the pacing threshold. An increase in unipolar pacing threshold is a marker of lesion creation. In regions of infarction, the relative change in threshold produced by ablation is substantially larger than the change in bipolar electrogram amplitude. The greater increase in pacing threshold after irrigated tip ablation compared to conventional ablation suggests that the magnitude of change reflects lesion size.

How electrode size affects the electric potential distribution in cardiac tissue

We investigate the effect of electrode size on the transmembrane potential distribution in the heart during electrical stimulation. The bidomain model is used to calculate the transmembrane potential in a three-dimensional slab of cardiac tissue. Depolarization is strongest under the electrode edge. Regions of depolarization are adjacent to regions of hyperpolarization. The average ratio of peak depolarization to peak hyperpolarization is a function of electrode radius, but over a broad range is close to three.

Effects of electroporation on the transmembrane potential distribution in a two-dimensional bidomain model of cardiac tissue

INTRODUCTION

Defibrillation shocks, when delivered through internal electrodes, establish transmembrane potentials (Vm) large enough to electroporate the membrane of cardiac cells. The effects of such shocks on the transmembrane potential distribution are investigated in a two-dimensional rectangular sheet of cardiac muscle modeled as a bidomain with unequal anisotropy ratios.

METHODS AND RESULTS

The membrane is represented by a capacitance Cm, a leakage conductance g(l) and a variable electroporation conductance G, whose rate of growth depends exponentially on the square of Vm. The stimulating current Io, 0.05-20 A/m, is delivered through a pair of electrodes placed 2 cm apart for stimulation along fibers and 1 cm apart for stimulation across fibers. Computer simulations reveal three categories of response to Io: (1) Weak Io, below 0.2 A/m, cause essentially no electroporation, and Vm increases proportionally to Io. (2) Strong Io, between 0.2 and 2.5 A/m, electroporate tissue under the physical electrode. Vm is no longer proportional to Io; in the electroporated region, the growth of Vm is halted and in the region of reversed polarity (virtual electrode), the growth of Vm is accelerated. (3) Very strong Io, above 2.5 A/m, electroporate tissue under the physical and the virtual electrodes. The growth of Vm in all electroporated regions is halted, and a further increase of Io increases both the extent of the electroporated regions and the electroporation conductance G.

CONCLUSION

These results indicate that electroporation of the cardiac membrane plays an important role in the distribution of Vm induced by defibrillation strength shocks.

A spatial scale factor for electrophysiological models of myocardium

In the United States among males 20-64 years old about 1/3 of deaths are classified as 'sudden cardiac death', and 1/4 had no forewarning, nor did autopsy turn up any visible cause. The heart just switches from its normally periodic pumping to an alternative mode called 'fibrillation' more resembling electrical turbulence. In normal tissue its mechanism is a geometrically re-entrant mode of normal propagation. Everything about this spatial pattern depends upon the magnitude of 'D', the one term with dimension involving space in the pertinent biophysical equations. Explicit or implicit estimates in current literature span orders of magnitude. In this article I argue from a diversity of recent experiments for a narrower range of realistic values. It has an important role in the spatio-temporal evolution of fibrillation and in defibrillation.

Clinical performance of steroid-eluting pacing leads with 1.2-mm2 electrodes

To raise pacing impedance and reduce battery current drain, new tined steroid-eluting leads were developed with 1.2-mm2 hemispherical electrodes, instead of conventional 5-8 mm2. Twenty-two unipolar J-shaped atrial leads and 25 unipolar ventricular leads (models 4533 and 4033, respectively) were implanted in 33 consecutive patients and followed for a mean of 25 months (range 18-29). Handling characteristics of atrial leads were found favorable. The leads slipped easily into the right atrial appendage and were easy to position. Handling characteristics of ventricular leads were satisfying, but more efforts had to be applied to cross the tricuspid valve. Special care was taken to avoid perforation of the myocardium due to the small lead tip. Following implantation, four ventricular and one atrial lead exhibited instability of pacing thresholds that resolved spontaneously within 1-3 days of implantation. Except for this, no lead malfunctioned. The reoperation rate was zero. The mean electrogram amplitudes of 15 mV (ventricle) and 4 mV (atrium), and the mean chronic pacing threshold of 0.085 ms at 1.6 V (app. 0.43 V at 0.5 ms) were comparable with the best values seen in the literature on passive fixation leads. The rest of the electrophysiological parameters were enhanced: mean pacing impedances were 984 omega (acute) and 900 Q (chronic), mean slew rates 3.26 V/s (ventricle) and 1.75 V/s (atrium), mean acute voltage threshold at 0.5 ms was 0.25 V, mean current and energy thresholds calculated at 0.5 ms were 260 microA and 32 nJ (acute) and 478 microA and 103 nJ (chronic). The electrical characteristics of these leads provide for increased pacemaker longevity in combination with substantial safety margins for pacing and sensing.

Initial experience with a co-radial bipolar pacing lead

A new type of endocardial bipolar pacing lead has been designed to overcome the potential drawbacks of the conventional coaxial bipolar pacing lead. We prospectively evaluated the new co-radial bipolar pacing leads (Intermedics Thin-Line), which are thinner (5 Fr vs 6-8 Fr) than standard coaxial bipolar leads. X-ray visibility and lead handling were subjectively assessed (excellent, good, adequate, or poor) at implant; lead impedance, sensitivity threshold, and pacing threshold were measured at implant, then at 1, 3, 6, 12, and 18 months. The results were as follows: 103 patients (51 M; age 63.8 +/- 17.4 years) received 71 atrial (A) and 89 ventricular (V) leads. X-ray visibility was excellent in 59/103; good in 23/103; adequate in 11/103; and poor in 10/103. Overall handling was excellent in 56/71 A and 69/89 V; good in 11/71 A and 18/89 V; adequate in 3/71 A and 1/89 V; poor in 1/71 A and 1/89 V. There were two perioperative complications. At implant: impedance in A and V were 370.1 +/- 74.7 and 501.5 +/- 124.4 omega, sensing thresholds in A and V were 3.0 +/- 1.5 and 9.9 +/- 5.0 mV, pacing thresholds at 0.45 ms in A and V were 0.59 +/- 0.21 and 0.41 +/- 0.15 volt, respectively. At 1, 3, 6, 12, and 18 months of follow-up: no pacing lead related complications were reported; pacing lead characteristics remained outstanding and stable. This new lead appears to have significant clinical advantages over the conventional coaxial bipolar pacing lead. Long-term follow-up is required to confirm its reliability and chronic performance characteristics.

A mathematical model of make and break electrical stimulation of cardiac tissue by a unipolar anode or cathode

Numerical simulations of electrical stimulation of cardiac tissue using a unipolar extracellular electrode were performed. The bidomain model with unequal anisotropy ratios represented the tissue, and the Beeler-Reuter model represented the active membrane properties. Four types of excitation were considered: cathode make (CM), anode make (AM), cathode break (CB), and anode break (AB). The mechanisms of excitation were: for CM, tissue under the cathode was depolarized to threshold; for AM, tissue at a virtual cathode was depolarized to threshold; for CB, a long cathodal pulse produced a steady-state depolarization under the cathode and hyperpolarization at a virtual anode. At the end (break) of the pulse, the depolarization diffused into the hyperpolarized tissue, resulting in excitation. For AB, a long anodal pulse produced a steady-state hyperpolarization under the anode and depolarization at a virtual cathode. At the end (break) of the pulse, the depolarization diffused into the hyperpolarized tissue, resulting in excitation. For AB stimulation, decay of the hyperpolarization faster than that of the depolarization was necessary. The thresholds for rheobase and diastolic CM, AM, CB, and AB stimulation were 0.038, 0.41, 0.49, and 5.3 mA, respectively, for an electrode length of 1 mm and a surface area of 1.5 mm2. Threshold increased as the size of the electrode increased. The strength-duration curves for CM and AM were similar except when the duration was shorter than 0.2 ms, in which case the AM threshold rose more quickly with decreasing duration than did the CM threshold. CM and AM resulted in similar strength-frequency curves. The model agrees qualitatively, but (in some cases) not quantitatively, with experiments.

Effects of bipolar point and line stimulation in anisotropic rabbit epicardium: assessment of the critical radius of curvature for longitudinal block

Excitation front shape and velocity were studied in anisotropic perfused rabbit epicardium stained with potentiometric fluorescent dye. In the combined results from all experiments, convex excitation fronts produced by stimulation with a single electrode propagated longitudinally 13.3% slower than flat excitation fronts produced by stimulation with a line of electrodes. For transverse propagation, the two stimulation methods produced similar flat excitation fronts and velocities. The critical excitation front radius of curvature for longitudinal block (Rcr), calculated from excitable media theory, was 92 microns in control hearts. In hearts exposed to diacetyl monoxime (20 mmol/L), which decreases inward sodium current, Rcr was 175 microns. The slower longitudinal propagation velocity of convex fronts versus flat fronts and the theoretically predicted critical radius of curvature may be important for propagation and block of ectopic depolarizations in the heart.

Acute and long-term ventricular stimulation thresholds with a new, iridium oxide-coated electrode

Efforts have been made to design electrodes that significantly reduce not only the acute and chronic stimulation thresholds, but also attenuate the early peaking phenomenon and polarization. At two voltage levels (2.7 V and 5.4 V, respectively), we evaluated the right ventricular stimulation thresholds obtained with a new, iridium oxide-coated electrode in ten patients who received a VVI pacemaker. Measurements were made at implant and at multiple intervals for 1 year. Pulse width stimulation thresholds at implant were as follow: 0.04 +/- 0.008 msec at 2.7 V, 0.03 +/- 0.004 msec at 5.4 V; values at 2 weeks were 0.14 +/- 0.06 msec at 2.7 V, 0.07 +/- 0.025 msec at 5.4 V; values at 3 months were 0.09 +/- 0.03 msec at 2.7 V, 0.05 +/- 0.01 msec at 5.4 V; values at 1 year were 0.08 +/- 0.02 msec at 2.7 V, 0.04 +/- 0.01 msec at 5.4 V. The maximal increase of 0.11 +/- 0.05 msec occurred at 2.7 V, 2 weeks after implant. Our results indicate that this new electrode provides low acute and long-term stimulation thresholds, as well as an attenuated early peaking phenomenon, being able to stimulate safely at 2.7 V even early after implant.

Chronic animal testing of new cardiac pacing electrodes

To evaluate the electrical performance of new electrode technologies, 24 leads containing either carbon coated porous titanium (BIOPORE, (Intermedics, Inc., Freeport, TX], iridium oxide (IROX), or iridium oxide coated with polyethylene glycol (IROX-PEG) electrodes (eight of each) were implanted into the ventricles of 12 canines. Stimulation threshold data was measured at regular intervals for 24 weeks. Low acute values were observed for all leads (0.32 +/- .13 V at 0.6 msec pulse width), but the IROX-PEG electrode demonstrated lower subchronic, peak, and chronic values. Compared to implant, the IROX-PEG electrodes' stimulation thresholds rose only 0.23 V when chronic conditions occurred. There were no significant differences between the electrodes in pacing impedance or R wave amplitude measurements. We conclude that both IROX and IROX-PEG technologies represent a promising approach to the design of more efficient cardiac pacing leads.

A new efficient NanoTip lead

The ideal lead has low, stable acute and chronic thresholds, high pacing impedance, and good sensing. Leads with low, stable thresholds have been developed, but pacing impedance has been in the 600 omega region. One way to increase pacing impedance is to decrease the electrode's surface area. The threshold performance and sensing ability of less than 5 mm2 electrodes have been considered questionable, up to now. We have developed a 1.5 mm2 porous, platinized, steroid-eluting electrode and have demonstrated in canine studies that it has excellent performance. Chronic thresholds are low at about 0.65 +/- 0.28 V (ventricular) and 0.42 +/- 0.12 V (atrial) at 0.5 msec. Chronic pacing impedance is in the 1200-1300 omega region, but mean chronic R and P wave source impedance is less than or equal to 1500 omega. Sensing is excellent, with almost double the P wave amplitudes usually measured in the canine.

Initiation of sustained ventricular tachyarrhythmias in a canine model of chronic myocardial infarction: importance of the site of stimulation

The importance of the site of stimulation to the initiation of sustained ventricular tachyarrhythmias was determined in 24 adult mongrel dogs. Studies were performed 3-30 days after two-stage occlusion of the mid- or distal left anterior descending coronary artery, modified by a reperfusion stage. Unipolar cathodal stimuli of twice-threshold intensity and 2 msec duration were introduced at five to 24 sites in each dog in the distribution of occluded and nonoccluded vessels. Strength-interval curves were constructed from 232 measurements at these sites and local properties of excitability and refractoriness were correlated with the ability to initiate arrhythmias. All dogs had sustained ventricular tachyarrhythmias inducible from at least one site. Intramyocardial sites with normal excitability and refractoriness within 2 cm of an area of infarction were most often successful (27 of 44, 61%) in the initiation of sustained arrhythmias. Less successful sites included normal left ventricular plunge electrode sites less than 2 cm from an area of infarction (eight of 32, 25%) (p = 0.002), left ventricular plunge electrode sites within an area of infarction (20 of 103, 19%) (p less than 0.001), normal right ventricular sites (five of 24, 21%) (p less than 0.001), and endocardial catheter sites (six of 29, 21%), (p less than 0.001). These findings suggest that local properties of excitability and refractoriness at the site of stimulation, as well as anatomic and geometric factors, may be critical in the initiation of sustained ventricular tachyarrhythmias using the technique of programmed electrical stimulation.