Biomedical subjects

Andrew E Pollard

Publications and source records attributed to Andrew E Pollard.

12 recordsLinked to original sources

PubMed2006

Cell-to-cell electrical interactions during early and late repolarization.

Cardiac electrical activity is significantly affected by variations in the conductance of gap junctions that connect myocytes to one another. To better understand how intrinsic (single cell) electrical activity is modulated by junctional conductance, we used a two-myocyte coupling system in which physically separate cells were electrically coupled via a variable resistance set by the investigator. This brief review summarizes our findings regarding: (1) the effect of the early phase of action potential repolarization (phase 1) and transient outward current (I(to)) on action potential conduction, and (2) the effect of coupling on the action potential plateau (late repolarization). We found that inhibition of I(to) markedly increased the ability of action potentials to propagate from cell-to-cell when junctional conductance was low. Electrically coupling two myocytes together also suppressed their beat-to-beat variability in action potential duration and contraction. Similarly, early afterdepolarizations (EADS) were readily suppressed by connecting a normal myocyte to one generating EADs. This high sensitivity of the plateau to variations in junctional interactions arises from the large increase in membrane resistance that occurs during this phase of the action potential.

Action Potentials↗

PubMed2006

Interactions between paced wavefronts and monomorphic ventricular tachycardia: implications for antitachycardia pacing.

OBJECTIVES: Interactions between paced wavefronts and monomorphic ventricular tachycardia (VT) dictate antitachycardia pacing outcomes. We used optical mapping to assess those interactions during single and dual site pacing of rabbit ventricular epicardium. METHODS AND RESULTS: Monomorphic VTs were initiated in six isolated rabbit hearts that were endocardially cryoablated to limit viable tissue to visible epicardium and establish apical tissue as the anatomic anchor. Preparations were optically mapped during single (n = 39) and dual (n = 43) site pacing at 50%-90% of VT cycle length (CL) with eight pulses per trial. Overall, we found six pulses that abruptly terminated VT. This occurred because the VT wavefront collided with the antidromic portion of the paced wavefront and the orthodromic portion of paced wavefront blocked in the VT's refractory region. When effective, dual site pacing that captured tissue at both leads simultaneously terminated the VT immediately, while single site pacing or dual site pacing that captured tissue at only one lead terminated the VT after resetting advanced the orthodromic wavefront. We found 12 pulses that induced polymorphic VT, with 11 of those pulses occurring during capture at only one lead. Expansion of the combined antidromic-VT wavefront around one or both ends of the arc of conduction block formed by the interaction of the orthodromic wavefront with the VT's refractory region initiated functional reentry. Six of these polymorphic VTs were nonsustained because the underlying wavefronts self-terminated. The wavefronts did persist for 4.2 +/- 3.5 cycles before self-terminating in these trials, and the post-pacing cycles presented a 146% increase in CL variability, compared with the variability prior to pacing. These temporal characteristics are similar to those of delayed termination in patients with ICDs. CONCLUSIONS: The main difference between pulses that terminated abruptly and pulses that induced polymorphic VT was the effective separation of the antidromic and orthodromic portions of the paced wavefront from one another.

Animals↗

PubMed2005

Cardiac microimpedance measurement in two-dimensional models using multisite interstitial stimulation.

We analyzed central interstitial potential differences during multisite stimulation to assess the feasibility of using those recordings to measure cardiac microimpedances in multidimensional preparations. Because interstitial current injected and removed using electrodes with different proximities allows modulation of the portion of current crossing the membrane, we hypothesized that multisite interstitial stimulation would give rise to central interstitial potential differences that depend on intracellular and interstitial microimpedances, allowing measurement of those microimpedances. Simulations of multisite stimulation with fine and wide spacing in two-dimensional models that included dynamic membrane equations for guinea pig ventricular myocytes were performed to generate test data ( partial differentialphio). Isotropic interstitial and intracellular microimpedances were prescribed for one set of simulations, and anisotropic microimpedances with unequal ratios (intracellular to interstitial) along and across fibers were prescribed for another set of simulations. Microimpedance measurements were then obtained by making statistical comparisons between partial differentialphio values and interstitial potential differences from passive bidomain simulations (Deltaphio) in which a wide range of possible microimpedances were considered. Possible microimpedances were selected at 25% increments. After demonstrating the effectiveness of the overall method with microimpedance measurements using one-dimensional test data, we showed microimpedance measurements within 25% of prescribed values in isotropic and anisotropic models. Our findings suggest that development of microfabricated devices to implement the procedure would facilitate routine measurement as a component of cardiac electrophysiological study.

Animals↗

PubMed2005

Measuring surface potential components necessary for transmembrane current computation using microfabricated arrays.

This study was designed to test the feasibility of using microfabricated electrodes to record surface potentials with sufficiently fine spatial resolution to measure the potential gradients necessary for improved computation of transmembrane current density. To assess that feasibility, we recorded unipolar electrograms from perfused rabbit right ventricular free wall epicardium (n = 6) using electrode arrays that included 25-microm sensors fabricated onto a flexible substrate with 75-microm interelectrode spacing. Electrode spacing was therefore on the size scale of an individual myocyte. Signal conditioning adjacent to the sensors to control lead noise was achieved by routing traces from the electrodes to the back side of the substrate where buffer amplifiers were located. For comparison, recordings were also made using arrays built from chloridized silver wire electrodes of either 50-microm (fine wire) or 250-microm (coarse wire) diameters. Electrode separations were necessarily wider than with microfabricated arrays. Comparable signal-to-noise ratios (SNRs) of 21.2 +/- 2.2, 32.5 +/- 4.1, and 22.9 +/- 0.7 for electrograms recorded using microfabricated sensors (n = 78), fine wires (n = 78), and coarse wires (n = 78), respectively, were found. High SNRs were maintained in bipolar electrograms assembled using spatial combinations of the unipolar electrograms necessary for the potential gradient measurements and in second-difference electrograms assembled using spatial combinations of the bipolar electrograms necessary for surface Laplacian (SL) measurements. Simulations incorporating a bidomain representation of tissue structure and a two-dimensional network of guinea pig myocytes prescribed following the Luo and Rudy dynamic membrane equations were completed using 12.5-microm spatial resolution to assess contributions of electrode spacing to the potential gradient and SL measurements. In those simulations, increases in electrode separation from 12.5 to 75.0, 237.5, and 875.0 microm, which were separations comparable to the finest available with our microfabricated, fine wire, and coarse wire arrays, led to 10%, 42%, and 81% reductions in maximum potential gradients and 33%, 76%, and 96% reductions in peak-to-peak SLs. Maintenance of comparable SNRs for source electrograms was therefore important because microfabrication provides a highly attractive methods to achieve spatial resolutions necessary for improved computation of transmembrane current density.

Algorithms↗

PubMed2005

Use of translucent indium tin oxide to measure stimulatory effects of a passive conductor during field stimulation of rabbit hearts.

Biomathematical models and experiments have indicated that passive extracellular conductors influence field stimulation. Because metallic conductors prevent optical mapping under the conductor, we have evaluated a passive translucent indium tin oxide (ITO) thin-film conductor to allow mapping of transmembrane potential (V(m)) and stimulatory current under the conductor. A 1-cm ITO disk was patterned photolithographically and positioned between 0.3-cm(2) mesh shock electrodes on the ventricular epicardium of isolated perfused rabbit hearts stained with 4-{2-[6-(dibutylamino)-2-naphthylenal]ethenyl}-1-(3-sulfopropyl)-, hydroxide, inner salt (di-4-ANEPPS). For a 1-A, 10-ms shock during the action potential plateau, optical maps from fluorescence collected using emission ratiometry (excitation at 488 nm and emissions at 510-570 and >590 nm) indicated that the disk altered V(m) by as much as the height of an action potential. DeltaV(m) became more positive near the edge of the disk, where the ITO conductance gradient was parallel to applied current, and more negative near the opposite edge, where the gradient was not parallel to current. For diastolic shocks, the disk expedited membrane excitation at the sites of positive DeltaV(m) in the heart and in a cardiac model with realistic ITO disk surface and interfacial conductances. Optical maps of ITO transmittance and the model indicated that the disk introduced anodal and cathodal stimulatory current at opposite edges of the disk. Thus ITO allows study of the stimulatory effects of a passive conductor in an electric field.

Animals↗

PubMed2005

Contributions of Purkinje-myocardial coupling to suppression and facilitation of early afterdepolarization-induced triggered activity.

Electrical loading by ventricular myocardium modulates conduction system repolarization near Purkinje-ventricular junctions (PVJs). We investigated how that loading suppresses and facilitates early afterdepolarizations (EADs) under conditions where there is a high degree of functional coupling between tissue types, which is consistent with the anatomic arrangement at the peripheral conduction system-myocardial interface. Experiments were completed in eight rabbit right ventricular (RV) free wall preparations. Free-running Purkinje strands were locally superfused, and action potentials were recorded from strands. RV free walls were bathed in normal solution. Surface electrograms were recorded near strand insertions into downstream free wall myocardium. Detailed histology was performed to assemble a computer model with interspersed Purkinje and ventricular myocytes weakly coupled throughout the region. Delays from Purkinje upstrokes to downstream peripheral conduction system and myocardial activation were comparable between experiments and simulations, supporting model node-to-node electrical coupling, i.e., the functional coupling. Purkinje action potential duration (APD) prolongation with localized isoproterenol in experiments and calcium current enhancement in simulations failed to establish EADs. With myocardial APD prolongation by delayed rectifier potassium current inhibition or L-type calcium current enhancement accompanying Purkinje APD prolongation in simulations, however, EAD-induced triggered activity developed. Collectively, our findings suggest competing contributions of the myocardial sink when there is a high degree of functional coupling between tissue types, with the transition from suppression to facilitation of EAD-induced triggered activity depending critically upon myocardial APD prolongation.

Action Potentials↗

PubMed2004

Feasibility of cardiac microimpedance measurement using multisite interstitial stimulation.

This study was designed to test the hypothesis that analyses of central interstitial potential differences recorded during multisite stimulation with a set of interstitial electrodes provide sufficient data for accurate measurement of cardiac microimpedances. On theoretical grounds, interstitial current injected and removed using electrodes in close proximity does not cross the membrane, whereas equilibration of intracellular and interstitial potentials occurs distant from electrodes widely separated. Multisite interstitial stimulation should therefore give rise to interstitial potential differences recorded centrally that depend on intracellular and interstitial microimpedances, allowing independent measurement. Simulations of multisite stimulation with fine (25 microm) and wide (400 microm) spacing in one-dimensional models that included Luo-Rudy dynamic membrane equations were performed. Constant interstitial and intracellular microimpedances were prescribed for initial analyses. Discrete myoplasmic and gap-junctional components were prescribed intracellularly in later simulations. With constant microimpedances, multisite stimulation using 29 total electrode combinations allowed interstitial and intracellular microimpedance measurements at errors of 0.30% and 0.34%, respectively, with errors of 0.05% and 0.40% achieved using 6 combinations and 10 total electrodes. With discrete myoplasmic and junctional components, comparable accuracy was maintained following adjustments to the junctions to reflect uncoupling. This allowed uncoupling to be quantified as relative increases in total junctional resistance. Our findings suggest development of microfabricated devices to implement the procedure would facilitate routine measurement as a component of cardiac electrophysiological study.

Artifacts↗

PubMed2004

Effects of electrical shocks on Cai2+ and Vm in myocyte cultures.

Changes in intracellular calcium concentration (DeltaCa(i)2+) induced by electrical shocks may play an important role in defibrillation, but high-resolution DeltaCa(i)2+ measurements in a multicellular cardiac tissue and their relationship to corresponding Vm changes (DeltaVm) are lacking. Here, we measured shock-induced DeltaCa(i)2+ and DeltaV(m) in geometrically defined myocyte cultures. Cell strands (width=0.8 mm) were double-stained with Vm-sensitive dye RH-237 and a low-affinity Ca(i)2+-sensitive dye Fluo-4FF. Shocks (E approximately 5 to 40 V/cm) were applied during the action potential plateau. Shocks caused transient Ca(i)2+ decrease at sites of both negative and positive DeltaV(m). Similar Ca(i)2+ changes were observed in an ionic model of adult rat myocytes. Simulations showed that the Ca(i)2+ decrease at sites of DeltaV+m was caused by the outward flow of I(CaL) and troponin binding; at sites of DeltaV-m it was caused by inactivation of I(CaL) combined with extrusion by Na-Ca exchanger and troponin binding. The important role of I(CaL) was supported by experiments in which application of nifedipine eliminated Ca(i)2+ decrease at DeltaV+m sites. Largest DeltaCa(i)2+ were observed during shocks of approximately 10 V/cm causing simple monophasic DeltaV(m). Shocks stronger than approximately 20 V/cm caused smaller DeltaCa(i)2+ and postshock elevation of diastolic Ca(i)2+. This was paralleled with occurrence of biphasic negative DeltaVm that indicated membrane electroporation. Thus, these data indicate that shocks transiently decrease Ca(i)2+ at sites of both DeltaV-m and DeltaV+m. Outward flow of I(CaL) plays an important role in Ca(i)2+ decrease in the DeltaV+m areas. Very strong shocks caused smaller negative DeltaCa(i)2+ and postshock elevation of diastolic Ca(i)2+, likely caused by membrane electroporation.

Aniline Compounds↗

PubMed2004

Comparison of conventional and biventricular antitachycardia pacing in a geometrically realistic model of the rabbit ventricle.

INTRODUCTION: ICDs often are programmed with antitachycardia pacing (ATP) as the first response to ventricular tachycardia (VT). Many ICDs have an additional lead available for ventricular pacing. We hypothesized that using the additional lead for ATP would improve therapy by advancing the orthodromic wavefront, thereby reducing the size of the excitable gap and inducing block of all reentrant activity. METHODS AND RESULTS: Monomorphic VT was initiated in a thin-walled model of rabbit ventricular myocardium that included an apical infarct and anatomically realistic dimensions. ATP with up to eight pulses was delivered at 90% of VT cycle length to one (conventional) or two (biventricular) stimulation areas. Stimulation areas were adjusted from 0.017 cm2 to 0.169 cm2 to modulate interactions between the antidromic and VT wavefronts, and between the orthodromic wavefront and the VT's refractory region. During conventional ATP, we found that larger stimulation areas terminated the VT in three pulses. Continued pacing after termination caused VT reinitiation in the reversed direction in some instances. With smaller stimulation areas, conventional ATP simply reset the circuit. During biventricular ATP, larger stimulation areas terminated VT in one pulse. There were no instances of reinitiation with reversal. However, with smaller stimulation areas, prolongation of refractoriness near the additional stimulation area facilitated induction of functional reentry with pathways modified by continued pacing. CONCLUSION: Our modeling suggests that biventricular ATP is superior to conventional ATP under conditions where the additional ventricular lead effectively advances the orthodromic wavefront. Failure to achieve this advancement poses a risk of VT acceleration.

Animals↗

PubMed2003

Spontaneous activity induced in rabbit Purkinje myocytes during coupling to a depolarized model cell.

OBJECTIVE: The development of an "injury current" secondary to heterogeneous ion accumulation and cellular uncoupling across the ischemic border zone has been implicated as a trigger for arrhythmias arising during acute ischemia. The purpose of the present study was to determine the effects of injury current across the Purkinje-ventricular interface in the development of abnormal automaticity. METHODS: Patch clamp and electronic cell coupling techniques were used to record action potentials from and to apply injury current to isolated rabbit Purkinje myocytes. Injury current was dependent upon: (1) a coupling resistance, which was varied to simulate different degrees of cellular uncoupling, and (2) the difference in Purkinje membrane potential and depolarized ischemic myocardium, which was represented by a passive resistor-capacitor circuit with initial voltages of -70, -60, or -50 mV. RESULTS: During coupling to the moderately depolarized cell (-60 or -50 mV), Purkinje myocytes developed repetitive, spontaneous activity within a window of coupling resistances. This abnormal automaticity was dependent upon L-type calcium current, as cadmium or nifedipine completely suppressed coupling-induced spontaneous activity. CONCLUSIONS: Our results demonstrate that injury current alone can induce spontaneous activity in normal Purkinje myocytes. The level of myocardial depolarization and the degree of cellular uncoupling required to induce this activity suggest spontaneous Purkinje activity induced by injury current as a potent trigger for acute ischemic arrhythmias.

Action Potentials↗

PubMed2003

From myocardial cell models to action potential propagation.

Membrane equations that describe sarcolemmal currents and ion transfer processes are important building blocks for theoretical studies of action potential propagation in cardiac tissue. Introduction of such ionic models into cellular and tissue networks allows analyses of passive contributions associated with tissue structure to be considered alongside active contributions from myocytes themselves in studies involving arrhythmia initiation, maintenance and termination. Maturation of contemporary membrane equations that attempt to replicate voltage clamp experiments from different species and tissue types with specific examples of modifications to extend those equations for simulations under conditions of rapid pacing, myocardial ischemia and remodeling following myocardial infarction are considered. Additionally, the integrating of membrane equations into models where coupling to represent current flow paths associated with the anisotropic tissue structure is described.

Action Potentials↗

PubMed2002

Modulation of triggered activity by uncoupling in the ischemic border. A model study with phase 1b-like conditions.

OBJECTIVE: Triggered beats during regional ischemia may depend upon the electrical source and sink charge interactions between adjacent regions of normal and ischemic cardiac tissue that are partly controlled by electrical coupling. METHODS: To study these relationships, we modified parameters in the Luo-Rudy dynamic membrane equations to reflect physiologic conditions associated with phase 1b arrhythmias. Superthreshold delayed afterdepolarizations (DADs) formed after pacing. Coupling contributions were then examined using: (i) a single phase 1b myocyte connected via a variable resistance to a single normal myocyte, and (ii) a multicellular fiber with a 1-cm segment of phase 1b myocytes connected to a 1-cm normal segment having resistance changes that were confined to the ischemic segment. Integration of ionic, capacitive and coupling currents during DAD initiation allowed charge quantification. RESULTS: In cell pairs, phase 1b myocyte DADs were suppressed at resistances where normal myocyte pacing resulted in phase 1b myocyte excitation. Coupling charge requirements limited capacitive charging in the phase 1b myocyte, which occurred in combination with diastolic hyperpolarization that shifted transmembrane potential from threshold. In multicellular fiber simulations, DADs were suppressed with strong coupling in the phase 1b segment. Moderate uncoupling of that segment allowed superthreshold DAD formation away from the border that initiated action potential propagation in the normal segment. With severe uncoupling, propagation failed at the border. CONCLUSIONS: These findings support the clinical and experimental observation that intermediate uncoupling is an important contributor to phase 1b arrhythmogenesis.

Calcium↗