Marcella C. Woods

Effects of elevated extracellular potassium on the stimulation mechanism of diastolic cardiac tissue.

During cardiac disturbances such as ischemia and hyperkalemia, the extracellular potassium ion concentration is elevated. This in turn changes the resting transmembrane potential and affects the excitability of cardiac tissue. To test the hypothesis that extracellular potassium elevation also alters the stimulation mechanism, we used optical fluorescence imaging to examine the mechanism of diastolic anodal unipolar stimulation of cardiac tissue under 4 mM (normal) and 8 mM (elevated) extracellular potassium. We present several visualization methods that are useful for distinguishing between anodal-make and anodal-break excitation. In the 4-mM situation, stimulation occurred by the make, or stimulus-onset, mechanism that involved propagation out of the virtual cathodes. For 8-mM extracellular potassium, the break or stimulus termination mechanism occurred with propagation out of the virtual anode. We conclude that elevated potassium, as might occur in myocardial ischemia, alters not only stimulation threshold but also the excitation mechanism for anodal stimulation.

Polarity reversal lowers activation time during diastolic field stimulation of the rabbit ventricles: insights into mechanisms

To fully characterize the mechanisms of defibrillation, it is necessary to understand the response, within the three-dimensional (3D) volume of the ventricles, to shocks given in diastole. Studies that have examined diastolic responses conducted measurements on the epicardium or on a transmural surface of the left ventricular (LV) wall only. The goal of this study was to use optical imaging experiments and 3D bidomain simulations, including a model of optical mapping, to ascertain the shock-induced virtual electrode and activation patterns throughout the rabbit ventricles following diastolic shocks. We tested the hypothesis that the locations of shock-induced regions of hyperpolarization govern the different diastolic activation patterns for shocks of reversed polarity. In model and experiment, uniform-field monophasic shocks of reversed polarities (cathode over the right ventricle is RV-, reverse polarity is LV-) were applied to the ventricles in diastole. Experiments and simulations revealed that RV- shocks resulted in longer activation times compared with LV- shocks of the same strength. 3D simulations demonstrated that RV- shocks induced a greater volume of hyperpolarization at shock end compared with LV- shocks; most of these hyperpolarized regions were located in the LV. The results of this study indicate that ventricular geometry plays an important role in both the location and size of the shock-induced virtual anodes that determine activation delay during the shock and subsequently affect shock-induced propagation. If regions of hyperpolarization that develop during the shock are sufficiently large, activation delay may persist until shock end.

The Potential of Dual Camera Systems for Multimodal Imaging of Cardiac Electrophysiology and Metabolism

Fluorescence imaging has become a common modality in cardiac electrodynamics. A single fluorescent parameter is typically measured. Given the growing emphasis on simultaneous imaging of more than one cardiac variable, we present an analysis of the potential of dual camera imaging, using as an example our straightforward dual camera system that allows simultaneous measurement of two dynamic quantities from the same region of the heart. The advantages of our system over others include an optional software camera calibration routine that eliminates the need for precise camera alignment. The system allows for rapid setup, dichroic image separation, dual-rate imaging, and high spatial resolution, and it is generally applicable to any two-camera measurement. This type of imaging system offers the potential for recording simultaneously not only transmembrane potential and intracellular calcium, two frequently measured quantities, but also other signals more directly related to myocardial metabolism, such as [K+]e, NADH, and reactive oxygen species, leading to the possibility of correlative multimodal cardiac imaging. We provide a compilation of dye and camera information critical to the design of dual camera systems and experiments.

The magnetic field associated with a plane wave front propagating through cardiac tissue

An action potential propagating through a two-dimensional sheet of cardiac tissue produces a magnetic field. In the direction of propagation, the intracellular and extracellular current densities are equal and opposite, so the net current is zero. However, because of the unequal anisotropy ratios in the intracellular and extracellular spaces, the component of the current density perpendicular to the direction of propagation does not, in general, vanish. This line of current produces the magnetic field. The amplitude of the magnetic field is zero only if the action potential propagates parallel to or perpendicular to the fiber direction, or if the tissue has equal anisotropy ratios.

High-Resolution High-Speed Panoramic Cardiac Imaging System

A panoramic cardiac imaging system consisting of three high-speed CCD cameras has been developed to image the surface electrophysiology of a rabbit heart via fluorescence imaging using a voltage-sensitive fluorescent dye. A robust, unique mechanical system was designed to accommodate the three cameras and to adapt to the requirements of future experiments. A unified computer interface was created for this application-a single workstation controls all three CCD cameras, illumination, stimulation, and a stepping motor that rotates the heart. The geometric reconstruction algorithms were adapted from a previous cardiac imaging system. We demonstrate the system by imaging a polymorphic cardiac tachycardia.

A high-voltage cardiac stimulator for field shocks of a whole heart in a bath

Defibrillators are a critical tool for treating heart disease; however, the mechanisms by which they halt fibrillation are still not fully understood and are the subject of ongoing research. Clinical defibrillators do not provide the precise control of shock timing, duration, and voltage or other features needed for detailed scientific inquiry, and there are few, if any, commercially available units designed for research applications. For this reason, we have developed a high-voltage, programmable, capacitive-discharge stimulator optimized to deliver defibrillation shocks with precise timing and voltage control to an isolated animal heart, either in air or in a bath. This stimulator is capable of delivering voltages of up to 500 V and energies of nearly 100 J with timing accuracy of a few microseconds and with rise and fall times of 5 micros or less and is controlled only by two external timing pulses and a control computer that sets the stimulation parameters via a LABVIEW interface. Most importantly, the stimulator has circuits to protect the high-voltage circuitry and the operator from programming and input-output errors. This device has been tested and used successfully in field shock experiments on rabbit hearts as well as other protocols requiring high voltage.

Elevated potassium concentration converts excitation mechanism from make to break

We used optical fluorescence imaging to study the mechanism of unipolar stimulation of cardiac tissue under elevated potassium (K/sup +/) conditions. We examined K/sup +/ concentrations of 4, 8, 10, and 12 mM for a /spl plusmn/6 mA, 10 ms stimulus, approximately 20/spl times/ the cathodal threshold with 4 mM K/sup +/. For 4 and 8 mM K/sup +/, make stimulation was observed. Raising the concentration to 10 mM, however, resulted in break stimulation. When the K/sup +/ concentration was increased to 12 mM, the tissue failed to respond to stimuli as large as 9 mA. We conclude that elevated potassium, as might occur in myocardial ischemia or fibrillation, alters not only stimulation threshold but also the excitation mechanism for point stimulation.

Diastolic Field Stimulation: the Role of Shock Duration in Epicardial Activation and Propagation

Detailed knowledge of tissue response to both systolic and diastolic shock is critical for understanding defibrillation. Diastolic field stimulation has been much less studied than systolic stimulation, particularly regarding transient virtual anodes. Here we investigated high-voltage-induced polarization and activation patterns in response to strong diastolic shocks of various durations and of both polarities, and tested the hypothesis that the activation versus shock duration curve contains a local minimum for moderate shock durations, and it grows for short and long durations. We found that 0.1-0.2-ms shocks produced slow and heterogeneous activation. During 0.8-1 ms shocks, the activation was very fast and homogeneous. Further shock extension to 8 ms delayed activation from 1.55 ± 0.27 ms and 1.63 ± 0.21 ms at 0.8 ms shock to 2.32 ± 0.41 ms and 2.37 ± 0.3 ms (N = 7) for normal and opposite polarities, respectively. The traces from hyperpolarized regions during 3-8 ms shocks exhibited four different phases: beginning negative polarization, fast depolarization, slow depolarization, and after-shock increase in upstroke velocity. Thus, the shocks of >3 ms in duration created strong hyperpolarization associated with significant delay (P < 0.05) in activation compared with moderate shocks of 0.8 and 1 ms. This effect appears as a dip in the activation-versus-shock-duration curve.

Differences in cardiac activation times for endocardium and epicardium in response to external electric shock

Externally applied electric shock E/sub a/, which are used to terminate cardiac defibrillation, operate by altering the cardiac transmembrane potential through a mechanism that is not yet understood. The effect of E/sub a/ on endocardium and epicardium was studied on Langendorff-perfused, di-4-ANEPPS stained isolated rabbit right ventricles (RV) to which a 10-40 V/cm E/sub a/ was applied from plate electrodes inside a Tyrodes bath. Field stimulation of the flat RV preparation caused immediate distribution depolarization followed by a rapid, non-propagative activation of the entire RV wall. For every E/sub a/ strength tested, the activation time T/sub a/, of the epicardium was shorter than epicardial T/sub a/. This difference in T/sub a/ may be attributed to a higher degree of anatomical heterogeneities on the endocardial surface.

Phase space: a new perspective for studying cardiac reentry

Phase plane analysis is a standard analytical technique in nonlinear dynamics that has only recently been applied to unresolved issues concerning the initiation and maintenance of cardiac fibrillation, as well as the tissue response to strong stimuli. Using this method, we can localize phase singularities and filaments and quantify spatial and temporal heterogeneities associated with the cardiac cycle.