Frequency dependence of the cardiac threshold to alternating current between 10 Hz and 160 Hz

It is still unclear what fundamental criteria influence the ability of alternating current (AC) to induce ventricular fibrillation (VF) in vivo. As the VF threshold has a bowl-shaped relationship with frequency (showing a minimum threshold at some frequency), similar to the nervous system, one proposed model has assumed that the mechanisms underlying AC stimulation of nerves are at work for VF induction. More recent work has suggested a second approach, whereby a simple RC-like model is sufficient to understand the cardiac AC stimulation threshold\u27s frequency dependence, suggesting that some unarticulated mechanism is at work for VF. The paper directly tests these two models. In 12 intact dogs and 20 intact guinea pigs, DC pulses were used to stimulate AC square and AC sine waves at 10, 20, 40, 80 and 160 Hz. All electrodes were endocardial, with the return electrode being on a paw or thorax. It was found that, for square and sine wave stimulation in both dogs and guinea pigs, the stimulation threshold increased monotonically with frequency from 10 Hz up to 160 Hz (p\u3c0.01 for dogs and guinea pigs). Between 80 and 160 Hz, the AC stimulation threshold doubled, exactly as predicted by an RC model. It was concluded that the AC stimulation threshold is not bowl-shaped and is best understood with an RC model. As the VF threshold does exhibit a bowl-shape with frequency, as opposed to the stimulation threshold which does not, the VF induction frequency dependence must have different origins

Simultaneous double external DC shock techniques for atrial fibrillation: A simulation study

The success rate of direct current cardioversion (DCC), the most common method to convert atrial fibrillation (AF) to sinus rhythm (SR), depends on various factors including AF duration, prior anti-arrhythmic therapy, electrode position and size, transthoracic impedance, and the use of biphasic versus monophasic shocks. Recent, small clinical studies have reported using quadruple electrodes to deliver higher energy, in order to increase DCC success rates in refractory patients. This study aims to computationally model and compare double shock defibrillation with conventional single shock DCC, based on the two parameters, defibrillation threshold (DFT) and heterogeneity index (HI). DFT is the energy required to achieve a voltage gradient of 5 V/cm over 95% of the atrial myocardium. HI, calculated as the (95th-5th)/ 50th percentile of atrial electric field magnitudes, is a measure of non-uniformity. The electric field distributions in the myocardium were generated for over Ave thousand different conventional and quadruple electrode placements with electrodes of two different sizes. Results show that there is a significant decrease in DFT (p\u3c0.01) and HI (p\u3c0.01) with increase in electrode size and quantity. © 2008 IEEE

Esophageal electric fields are predictive of atrial cardioversion success-a finite element analysis

Background: Atrial fibrillation (AF) is a debilitating cardiac arrhythmia, one potential treatment of which is external cardioversion. Studies have shown external cardioversion success is affected by electrode placement and that esophageal electric fields (EEFs) during low strength shocks have the potential to be used in determining patient-specific optimal electrode placements during animal experiments. The objective of this study was to determine the relationship between EEFs and atrial defibrillation thresholds (ADFTs) during computer simulations using an anatomically realistic computer model of a human torso. Methods: Over 600 electrode placements were simulated during which EEFs were compared to ADFTs. Results: There was no single optimal electrode placement with multiple electrode placements resulting in similarly low ADFTs. There was over 40% difference in the ADFTs between the most and least optimal electrode configurations. There was no correlation between EEFs and ADFTs for all electrode placements, but a strong negative correlation when small shifts from clinically relevant electrode placements were performed. Conclusions: These results suggest a small shifts protocol from clinically relevant electrode placements has the potential to increase the probability of successful cardioversion on the first shock and reduce the cumulative number of shocks and energy to which patients are exposed

Electrode placement significantly affects transthoracic atrial defibrillation thresholds

Background: Placement of external electrodes can significantly affect the success rate of transthoracic atrial defibrillation, but clinical studies have not led to agreement on optimal electrode placements. This study uses an anatomically realistic, finite element model of the human torso for external atrial defibrillation to (1) investigate model parameters of skeletal muscle conductivity and anisotropy and the presence of subcutaneous fat and (2) investigate clinical defibrillation parameters of electrode size, shape, and location. Methods: The model computes electric fields in the atria given electrode location, applied voltage, and tissue conductivities. The model predicts atrial defibrillation threshold (ADFT) energy by requiring a voltage gradient of 5 V/cm over at least 95% of atrial myocardium. Results: The results compare favorably with a clinical study of 301 patients that reported an anterior-posterior (AP) electrode position required approximately 20% less energy than an anterior-anterior (AA) position. Results indicate that a change in electrode size has a different effect for AA compared to AP electrode placements. This study finds that variation in electrode placement by only a few centimeters can change ADFTs by up to 51%. Conclusion: This is the first computer model of transthoracic atrial defibrillation to our knowledge. The sensitivity of defibrillation thresholds to small shifts in electrode placement may account for the disagreement between clinical studies on optimal electrode placements

An esophageal probe for measuring three-dimensional electric fields during external cardiac defibrillation

External defibrillation is a common treatment for the cardiac arrhythmia atrial fibrillation. Electrode placement has been shown to affect defibrillation efficacy and required energy levels. We suggest investigating the relationship between esophageal electric fields (EEFs) and atrial defibrillation thresholds to determine the feasibility of creating patient-specific electrode placements using EEFs. This study presents the design and implementation of an esophageal probe (EP) that accurately measures three-dimensional electric fields. The root-mean-square error of the EP was 1.69% as determined by measurements performed in an electrolytic tank. The EP also performed well during in vivo testing in a pig. There was a strong positive relationship between EEF 2s and applied energy during defibrillation strength shocks. The EEF measurements were also repeatable, with less than 4.24% difference between repeated shocks. This is the first description of a probe designed specifically for measuring electric fields in the esophagus. © 2012 Institute of Physics and Engineering in Medicine

Finite element computer modeling of transthoracic atrial defibrillation

Placement of external electrodes can significantly affect the success rate of transthoracic atrial defibrillation, but studies have not led to agreement on optimal electrode placements. This study aims to (1) develop an anatomically realistic, finite element model of the human torso for external atrial defibrillation, (2) investigate model parameters of skeletal muscle conductivity and anisotropy and the presence of subcutaneous fat, and (3) investigate clinical defibrillation parameters of electrode size, shape, and location. The model predicts atrial defibrillation threshold (ADFT) energy by requiring a voltage gradient of 5 V/cm over at least 95% of atrial myocardium. The model compares favorably with a clinical study of 301 patients that reported an anterior-posterior electrode position required approximately 20% less energy than an anterior-anterior position. Results indicate that a change in electrode size has a different effect for different electrode placements. This study finds that variation in electrode placement by only a few centimeters can change ADFTs by up to 51%. This is the first computer model of transthoracic atrial defibrillation to our knowledge. Our computer model is not limited to a few empirically selected electrode placements as in clinical studies and can test any location, size, and number of electrode placements

An algorithm for EMG noise detection in large ECG data

Large collections of electrocardiogram recordings (ECG) are valuable for researchers. However, some sections of the recorded ECG may be corrupted by electromyogram (EMG) noise from muscle. Therefore, EMG noise needs to be detected and filtered before performing data processing. In this study, an automated algorithm for detecting EMG noise in large ECG data is presented. The algorithm extracts EMG artifact from the ECG by using a morphological filter. EMG is identified by setting a threshold for the moving variance of extracted EMG. The algorithm achieved 100% detection rate on the training data. The algorithm was tested on 150 test signals from three sets of test signals (50 signals in each set). Set 1 was created by adding EMG noise to EMG-free ECG signals, set 2 was manually selected ECG sections which contain EMG noise, and set 3 contained randomly selected ECG signals. Sensitivity was 100%, 94%, and 100% on sets 1, 2, and 3, respectively. All sets had 100% specificity. The algorithm has computational complexity of O(N). © 2004 IEEE

Esophageal electric fields are correlated to atrial defibrillation thresholds: Towards patient-specific optimization of external atrial defibrillation

Studies have investigated the effect of defibrillator paddle position on the efficacy of external electrocardioversion of atrial fibrillation, without agreeing upon an optimal placement. We wish to investigate using esophageal electric fields (EEFs) to predict atrial defibrillation thresholds (ADFTs) on a patient-specific basis. We propose to (1) investigate the relationship between EEFs and ADFTs using computer simulations, (2) develop an esophageal probe that can accurately measure three-dimensional electric fields and (3) investigate the relationship between EEFs and ADFTs values in-vivo. Sixteen anterior-anterior and eleven anterior-posterior placements were simulated yielding a negative relationship between EEFs and ADFTs (R2=0.91 and 0.93, respectively). An esophageal probe was developed that accurately measures EEFs. Animal studies showed a negative relationship between EEFs and ADFTs. This data suggests using EEFs to predict ADFTs on a patient-specific basis is plausible. © 2006 IEEE

Esophageal electric fields are predictive of atrial defibrillation thresholds

Background: Atrial fibrillation (AF) is a common cardiac arrhythmia characterized by disorganized cardiac electrical activity. Defibrillation electrode placement has been shown to affect the amount of energy and number of shocks required to defibrillate. The objective of this study was to investigate the relationship between esophageal electric fields (EEFs) and atrial defibrillation thresholds (ADFTs) to determine the feasibility of using EEFs during a low-strength shock to predict patient-specific defibrillation electrode placements. Methods: AF was induced and defibrillated according to a Bayesian four-shock protocol for 12-electrode placements in six pigs. EEFs were measured during each of the four shocks of the protocol and during a 1-J shock for each electrode placement. Squared EEFs (EEF 2s) during all shocks were compared to the ADFTs using a linear regression. Results: There was a negative relationship between EEF 2s during the 1-J shocks and ADFTs, with median R 2 values of 0.863 and 0.840 for anterior-anterior (AA) and anterior-posterior (AP) electrode placements, respectively. There was a strong, positive relationship between applied energy and EEF 2s, with median R 2 values of at least 0.866 for all animals. The placement with the highest EEF 2 resulted in the lowest ADFT for both AA and AP placements in four of six pigs. In the other two animals, this held for one electrode set but not both. Conclusions: There was a strong negative relationship between EEF 2s during 1-J shocks and ADFTs for both AA and AP electrode placements. These preliminary results suggest that using EEF 2s to predict patient-specific electrode placements is feasible. © 2011 Wiley Periodicals, Inc

Heart Rate Variability in Rats with Aldosterone-Induced Chronic Heart Failure

The renin-angiotensin-aldosterone system contributes to the pathophysiology of chronic heart failure. In this study, chronic heart failure is gradually induced in uninephrectomized rats (n = 4) by aldosterone infusion (0.75 μg/h) and high Na and low Mg diet for four weeks followed up by high Na and low Mg diet alone for an additional six weeks. A control rat (n = 1) is untreated and unoperated. Electrocardiograms were recorded continuously during the entire experiment. Time and frequency domain heart rate variability (HRV) parameters are computed to study the long-term trends of heart rate and HRV. The results show that aldosterone infusion decreases heart rate and circadian variability in the treatment rats. After stopping aldosterone infusion, heart rate increases with less circadian variability. ANOVA indicates change of heart rate (p \u3c 0.001) during and after aldosterone infusion periods (within each treatment rat)