Biomedical subjects

Chad R Johnson

Publications and source records attributed to Chad R Johnson.

3 recordsLinked to original sources

PubMed2004

A novel implantable cardiac telemetry system for studying atrial fibrillation.

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice. Most in vivo experimental research on AF is performed in a surgical setting, on animals instrumented by external devices, or using commercial implantable pacemakers. This paper describes a novel implantable cardiac telemetry system, which allows the study of AF remotely in conscious and ambulatory animals over a few month period. To validate this concept, the system was built and implanted in a sheep for 3 months. During this period, the system was used to deliver chronic rapid atrial pacing for AF induction, and to record and measure atrial electrograms and atrial effective refractory period (AERP) daily. During the course of AF induction the AERP decreased, confirming the progression of the electrical remodeling process in the atria. Episodes of paroxysmal AF were successfully induced in the animal. Burst pacing therapy was delivered with the system, however, no AF termination was observed. Result shows that this telemetry-based pacing and monitoring system can be used to study AF in a conscious animal non-invasively for an extended period of time, making this system a unique research tool.

Animals↗

PubMed2003

Interactions between extracellular stimuli and excitation waves in an atrial reentrant loop.

UNLABELLED: Extracellular Stimuli in an Atrial Reentrant Loop. INTRODUCTION: The interactions between extracellular stimuli and excitation waves propagating in a reentrant loop are a complex function of stimulus parameters, structural properties, membrane state, and timing. Here the goal was a comprehensive understanding of the mechanisms and frequencies of the major interactions between the advancing excitation wave and a single extracellular stimulus, separated from issues of anatomic or geometric complexity. METHODS AND RESULTS: A modernized computer model of a thin ring of uniform tissue that included a pair of extracellular stimulus electrodes (anode/cathode) was used to model one-dimensional cardiac reentry. Questions and results included the following: (1) What are the major interactions between a stimulus and the reentrant propagation wave, and are they induced near the cathode or near the anode; and, for each interaction, what are the initiating amplitude range and timing interval? At the cathode, the well-known mechanism of retrograde excitation terminated reentry; changes in timing or amplitude produced double-wave reentry or phase reset. At the anode, termination occurred at different cells depending on stimulus amplitude. (2) Relatively how often did termination occur at the anode? For most stimulus amplitudes, termination occurred more often at the anode than at the cathode, although not always at the same cell. (3) With random timing, what is the probability of terminating reentry? Stimulation for 5 msec terminated reentry with a probability from 0% to approximately 10%, as a function of increasing stimulus amplitude. CONCLUSION: A single extracellular stimulus can initiate major changes in reentrant excitation via multiple mechanisms, even in a simple geometry. Termination of reentry, phase shifts, or double-wave reentry each occurs over well-defined ranges of stimulus amplitude and timing.

Action Potentials↗

PubMed2003

Membrane current from transmembrane potentials in complex core-conductor models.

Core-conductor models, used to integrate the behavior of the longitudinal currents with the distributed voltages of electrically active tissue, have evolved for over a century. A critical step in the use of such models is the computation of membrane current from the set of distributed transmembrane potential values that exist at a given moment, where the potentials are obtained either experimentally or computationally. Over time, interest has developed in a number of substantial extensions of the original model to include such features as nonuniform spatial resistances, loop instead of linear structure, and multiple sites of extracellular stimulation. This paper concisely restates and extends the equations for calculation of transmembrane currents with the systematic inclusion of alternative cases, noting how they reduce to the standard forms. An important issue is how complex the calculation of membrane current has to be. Thus, the paper goes on to show criteria (based on the uniformity of resistance and the presence of stimulation) for deciding when membrane currents can be obtained with a relatively simple calculation with a single equation involving local variables versus with a more complex calculation involving the simultaneous solution of a (possibly large) set of equations.

Electric Impedance↗