Entrainment of marginally stable excitation waves by spatially extended sub-threshold periodic forcing

We analyze the effects of spatially extended periodic forcing on the dynamics of one-dimensional excitation waves. Entrainment of unstable primary waves has been studied numerically for different amplitudes and frequencies of additional sub-threshold stimuli. We determined entrainment regimes under which excitation blocks were transformed into consistent 1:1 responses. These responses were spatially homogeneous and synchronized in the entire excitable medium. Compared to primary pulses, pulses entrained by secondary stimulations were stable at considerably shorter periods which decreased at higher amplitudes and greater number of secondary stimuli. Our results suggest a practical methodology for stabilization of excitation in reaction-diffusion media with regions of reduced excitability.Comment: 6 pages, 6 figure

Entrainment of marginally stable excitation waves by spatially extended sub-threshold periodic forcing

We introduce a novel approach of stabilizing the dynamics of excitation waves by spatially extended sub-threshold periodic forcing. Entrainment of unstable primary waves has been studied numerically for different amplitudes and frequencies of additional sub-threshold stimuli. We determined entrainment regimes under which excitation blocks were transformed into consistent 1:1 responses. These responses were spatially homogeneous and synchronized in the entire excitable medium. Compared to primary pulses, pulses entrained by secondary stimulations were stable at considerably shorter periods which decreased at higher amplitudes and greater number of secondary stimuli. Our results suggest a practical methodology for stabilization of excitation in reaction-diffusion media such as nerve tissue with regions of reduced excitability

Exercise-induced QT/R-R–interval hysteresis as a predictor of myocardial ischemia

Objectives: Exercise-induced QT/RR hysteresis exists when, for a given R-R interval, the QT interval duration is shorter during recovery after exercise than during exercise. We sought to assess the association between QT/RR hysteresis and imaging evidence of myocardial ischemia.Background: Because ischemia induces cellular disturbances known to decrease membrane action potential duration, we hypothesized a correlation between QT/RR and myocardial ischemia.Methods: We digitally analyzed 4-second samples of QT duration and R-R–interval duration in 260 patients referred for treadmill exercise stress and rest single photon emission computed tomography myocardial perfusion imaging; a cool-down period was used after exercise. None of the patients were in atrial fibrillation or used digoxin, and none had marked baseline electrocardiographic abnormalities. Stress and rest myocardial perfusion images were analyzed visually and quantitatively to define the extent and severity of stress-induced ischemia. QT/RR hysteresis was calculated using a computerized algorithm.Results: There were 82 patients (32%) who manifested myocardial ischemia by single photon emission computed tomography myocardial perfusion imaging. The likelihood of ischemia increased with increasing QT/RR hysteresis, with prevalence according to quartiles of 20%, 30%, 26%, and 49% (P = .003 for trend). In analyses adjusting for ST-segment changes, exercise capacity, heart rate recovery, and other confounders, QT/RR hysteresis was independently predictive of presence of myocardial ischemia (adjusted odds ratio for 100-point increase of QT/RR hysteresis, 1.61; 95% confidence interval, 1.22-2.12; P = .0008). QT/RR hysteresis was also predictive of severe ischemia.Conclusion: Exercise-induced QT/RR hysteresis is a strong and independent predictor of myocardial ischemia and provides additional information beyond that afforded by standard ST-segment measures

Assessing QT-RR interval hysteresis in 12-lead electrocardiograms

The amount of QT-RR interval hysteresis accumulated during the load and recovery phases of exercise stress test reflects the degree of exercise induced myocardial ischemia. Therefore the evaluation of hysteresis from 12-lead ECG (12-SL) is an important practical modality for the assessment of severity of coronary artery disease. Commercial QT and RR waveform analyzers are regularly subjected to interval detection artifacts which prevent reliable estimation for the magnitude of QT-RR hysteresis in one or multiple leads. We present a new signal processing technique which overcomes this deficiency. It quantifies the level of faulty interval measurements and using a threshold eliminates the need to process potentially corrupt leads

Feasibility of non-invasive determination of the stability of propagation reserve in patients

This study investigates the feasibility of using surface ECG recordings to assess stability of cardiac propagation. Our novel method customizes a reaction diffusion model of cardiac excitation using measurements of patient’s repolarization dynamics. The customized model determines the stability-of-propagation reserve (SoPR) that measures the proximity of the patient’s minimum level of refractoriness to the critical level associated with conduction instability. Using measurements from 15 patients, we compared SoPRs determined from the unipolar intracardiac electrograms and from the surface ECG leads. SoPRs computed from intracardiac and surface measurements correlated, and a formula was developed that estimated the intracardiac SoPR from surface measurements with 5% accuracy. Thus, this study may lead to new noninvasive tests for the risk of sudden cardiac death

Action potential restitution and hysteresis in a reaction-diffusion system with pacing rate dependent excitation threshold

We have demonstrated that rate dependent restitution and action potential duration-refractory period hysteresis can be reproduced in a one-dimensional two-variable Chernyak-Starobin-Cohen reaction-diffusion medium with variable excitation threshold. We show that restitution and hysteresis depend on the relationship between pacing period and steady state excitation threshold and also on the rate of excitation threshold adaptation after an abrupt change in pacing period. It was also observed that the onset of action potential duration alternans is determined by the minimal stable wavefront speed, which could be approximated by the analytical critical speed of a stable solitary pulse. This approximation was suitably accurate regardless of the adaptation constant of excitation threshold, its dependence on pacing interval, or magnitude of the slopes of restitution curves

Critical scale of propagation influences dynamics of waves in a model of excitable medium

Background: Duration and speed of propagation of the pulse are essential factors for stability of excitation waves. We explore the propagation of excitation waves resulting from periodic stimulation of an excitable cable to determine the minimal stable pulse duration in a rate-dependent modification of a Chernyak-Starobin-Cohen reaction-diffusion model. Results: Various pacing rate dependent features of wave propagation were studied computationally and analytically. We demonstrated that the complexity of responses to stimulation and evolution of these responses from stable propagation to propagation block and alternans was determined by the proximity between the minimal level of the recovery variable and the critical excitation threshold for a stable solitary pulse. Conclusion: These results suggest that critical propagation of excitation waves determines conditions for transition to unstable rhythms in a way similar to unstable cardiac rhythms.Established conditions were suitably accurate regardless of rate dependent features and the magnitude of the slopes of restitution curves

Exercise-induced QT/R-R-interval hysteresis as a predictor of myocardial ischemia

Abstract: Objectives: Exercise-induced QT/RR hysteresis exists when, for a given R-R interval, the QT interval duration is shorter during recovery after exercise than during exercise. We sought to assess the association between QT/RR hysteresis and imaging evidence of myocardial ischemia. Background: Because ischemia induces cellular disturbances known to decrease membrane action potential duration, we hypothesized a correlation between QT/RR and myocardial ischemia

Reaction-diffusion memory unit: Modeling of sensitization, habituation and dishabituation in the brain.

We propose a novel approach to investigate the effects of sensitization, habituation and dishabituation in the brain using the analysis of the reaction-diffusion memory unit (RDMU). This unit consists of Morris-Lecar-type sensory, motor, interneuron and two input excitable cables, linked by four synapses with adjustable strength defined by Hebbian rules. Stimulation of the sensory neuron through the first input cable causes sensitization by activating two excitatory synapses, C1 and C2, connected to the interneuron and motor neuron, respectively. In turn, the stimulation of the interneuron causes habituation through the activation of inhibitory synapse C3. Likewise, dishabituation is caused through the activation of another inhibitory synapse C4. We have determined sensitization-habituation (BSH) and habituation-dishabituation (BHDH) boundaries as functions between synaptic strengths C2 and C3 at various strengths of C1 and C4. When BSH and BHDH curves shift towards larger values of C2, the RDMU can be easily inhibited. On the contrary, the RDMU can be easily sensitized or dishabituated if BSH and BHDH curves shift towards smaller values of C2. Our numerical simulations readily demonstrate that higher values of the Morris-Lecar relaxation parameter, greater leakage and potassium conductances, reduced length of the interneuron, and higher values of C1 all result in easier habituation of the RDMU. In contrast, we found that at higher values of C4 the RDMU becomes significantly more prone to dishabituation. Based on these simulations one can quantify BSH and BHDH curve shifts and relate them to particular neural outcomes