Isolated Rabbit Hearts Research Articles

Mechanisms underlying the spontaneous termination of torsades de pointes in an experimental model of long QT syndrome

BackgroundTorsades de pointes (TdP) represent a complex polymorphic ventricular tachycardia. While the triggering mechanisms of early afterdepolarization and increased dispersion of repolarization are well investigated, the sudden self-limiting termination remains poorly understood. ObjectiveThe purpose of this study was to perform analysis of TdP to investigate factors causing spontaneous termination. MethodsWe used a large data set from Langendorff experiments in isolated rabbit hearts in which drug-induced QT prolongation, bradycardia, and hypokalemia provoke TdP. We included 427 episodes with typical TdP characteristics of polymorphic self-terminating beats and twisting QRS complexes occurring in the presence of abnormal QT prolongation due to various different QT-prolonging drugs. The use of 8 monophasic action potential catheters allowed the characterization of action potential duration, configuration, and dispersion of repolarization beyond the capabilities of the surface electrocardiogram. To identify possible mechanisms of arrhythmia termination, the initial, midpoint, and terminal 3 ventricular complexes were analyzed for each episode. ResultsAn abrupt decrease in spatial dispersion over the course of a TdP episode was identified as a precursor for termination of TdP. Within the last 3 beats, a sudden significant decrease in the dispersion of repolarization was observed as a predictor of termination. In parallel, there was a decrease in action potential duration (action potential duration at 90% repolarization) before termination. Also, a change in action potential configuration (action potential duration at 90% repolarization/action potential duration at 50% repolarization ratio) in terms of the loss of action potential dome with a restitution of action potential triangulation was observed. ConclusionIn >400 TdP episodes, homogenization of myocardial repolarization with the recovery of an action potential configuration determines the termination of TdP episodes.

Electrophysiological Profile of Different Antiviral Therapies in a Rabbit Whole-Heart Model

Antiviral therapies for treatment of COVID-19 may be associated with significant proarrhythmic potential. In the present study, the potential cardiotoxic side effects of these therapies were evaluated using a Langendorff model of the isolated rabbit heart. 51 hearts of female rabbits were retrogradely perfused, employing a Langendorff-setup. Eight catheters were placed endo- and epicardially to perform an electrophysiology study, thus obtaining cycle length-dependent action potential duration at 90% of repolarization (APD90), QT intervals and dispersion of repolarization. After generating baseline data, the hearts were assigned to four groups: In group 1 (HXC), hearts were treated with 1 µM hydroxychloroquine. Thereafter, 3 µM hydroxychloroquine were infused additionally. Group 2 (HXC + AZI) was perfused with 3 µM hydroxychloroquine followed by 150 µM azithromycin. In group 3 (LOP) the hearts were perfused with 3 µM lopinavir followed by 5 µM and 10 µM lopinavir. Group 4 (REM) was perfused with 1 µM remdesivir followed by 5 µM and 10 µM remdesivir. Hydroxychloroquine- and azithromycin-based therapies have a significant proarrhythmic potential mediated by action potential prolongation and an increase in dispersion. Lopinavir and remdesivir showed overall significantly less pronounced changes in electrophysiology. In accordance with the reported bradycardic events under remdesivir, it significantly reduced the rate of the ventricular escape rhythm.

Importance of cardiac-synchronized vagus nerve stimulation parameters on the provoked chronotropic response for different levels of cardiac innervation.

The influence of vagus nerve stimulation (VNS) parameters on provoked cardiac effects in different levels of cardiac innervation is not well understood yet. This study examines the effects of VNS on heart rate (HR) modulation across a spectrum of cardiac innervation states, providing data for the potential optimization of VNS in cardiac therapies. Utilizing previously published data from VNS experiments on six sheep with intact innervation, and data of additional experiments in five rabbits post bilateral rostral vagotomy, and four isolated rabbit hearts with additionally removed sympathetic influences, the study explored the impact of diverse VNS parameters on HR. Significant differences in physiological threshold charges were identified across groups: 0.09 ± 0.06μC for intact, 0.20 ± 0.04μC for vagotomized, and 9.00 ± 0.75μC for isolated hearts. Charge was a key determinant of HR reduction across all innervation states, with diminishing correlations from intact (r = 0.7) to isolated hearts (r = 0.44). An inverse relationship was observed for the number of pulses, with its influence growing in conditions of reduced innervation (intact r = 0.11, isolated r = 0.37). Frequency and stimulation delay showed minimal correlations (r < 0.17) in all conditions. Our study highlights for the first time that VNS parameters, including stimulation intensity, pulse width, and pulse number, crucially modulate heart rate across different cardiac innervation states. Intensity and pulse width significantly influence heart rate in innervated states, while pulse number is key in denervated states. Frequency and delay have less impact impact across all innervation states. These findings suggest the importance of customizing VNS therapy based on innervation status, offering insights for optimizing cardiac neuromodulation.

1,4-Disubstituted Piperazin-2-Ones as Selective Late Sodium Current Inhibitors with QT Interval Shortening Properties in Isolated Rabbit Hearts.

Late sodium current (INa) inhibitors are a new subclass of antiarrhythmic agents. To overcome the drawbacks, e.g., low efficacy and inhibition effect on K+ current, of the FDA-approved late INa inhibitor ranolazine, chain amide 6a-6q, 1,4-disubstituted piperazin-2-ones 7a-7s, and their derivatives 8a-8n were successively designed, synthesized, and evaluated in vitro on the NaV1.5-transfected HEK293T cells by the whole-cell patch clamp recording assay at the concentration of 40 μM. Among the new skeleton compounds, 7d showed the highest efficacy (IC50 = 2.7 μM) and good selectivity (peak/late ratio >30 folds), as well as excellent pharmacokinetics properties in mice (T1/2 of 3.5 h, F = 90%, 3 mg/kg, po). It exhibited low hERG inhibition and was able to reverse the ATX-II-induced augmentation of late INa phenotype of LQT3 model in isolated rabbit hearts. These results suggest the application potentials of 7d in the treatments of arrhythmias related to the enhancement of late INa.

Effect of the mitochondrial membrane potential on the absorbance of the reduced form of cytochrome c oxidase

The effect of mitochondrial membrane potential (ΔΨm) on the absorbance of the reduced cytochrome c oxidase (COX) was evaluated in isolated rabbit heart mitochondria using integrating sphere optical spectroscopy. Maximal reduction of the mitochondrial cytochromes was achieved by either blowing nitrogen to remove oxygen, or by adding cyanide. Gradual depolarization of ΔΨm by adding increasing concentrations of uncoupler resulted in an increase of up to 50 % in the absorbance of cytochrome aa3 under nitrogen saturation, and of 25 % with cyanide. Cytochrome aa3 absorbance increases were also observed in the presence of cyanide with apyrase (20 %) or oligomycin (12 %). The bL heme absorbance also decreased as expected from ΔΨm depolarization. A ~ 1 nm red shift in the peak wavelength of cytochrome aa3 was observed under anoxic conditions as ΔΨm was depolarized. Importantly, cytochrome c and c1 absorbances remained constant at levels corresponding to full reduction under all experimental manipulations of ΔΨm, especially with cyanide. These data suggest that ΔΨm-dependent changes in the absorbance of reduced COX were due to a variable extinction coefficient of heme a and/or a3 as a function of ΔΨm. A similar increase in the reduced cytochrome aa3 absorbance without changes in cytochrome c and c1 was observed in the perfused rabbit heart when decreasing ΔΨm with uncoupler. Our results imply that COX absorbance in its fully reduced state does not simply reflect the oxygen tension but also the ΔΨm. This may prove useful in monitoring ΔΨm under anoxic or ischemic conditions in intact tissue.

Divergent electrophysiologic action of dapagliflozin and empagliflozin on ventricular and atrial tachyarrhythmias in isolated rabbit hearts.

The use of SGLT-2 inhibitors has revolutionized heart failure therapy. Evidence suggests a reduced incidence of ventricular and atrial arrhythmias in patients with dapagliflozin or empagliflozin treatment. It is unclear to what extent the reduced arrhythmia burden is due to direct effects of the SGLT2 inhibitors or is solely a marker of improved cardiac function. One hundred five rabbit hearts were allocated to eight groups and retrogradely perfused, employing a Langendorff setup. Action potential duration at 90% of repolarization (APD90), QT intervals, effective refractory periods, conduction velocity, and dispersion of repolarization were obtained with monophasic action potential catheters. A model for tachyarrhythmias was established with the IKr blocker erythromycin for QT prolongation associated proarrhythmia as well as the potassium channel opener pinacidil for a short-QT model. An atrial fibrillation (AF) model was created with isoproterenol and acetylcholine. With increasing concentrations of both SGLT2 inhibitors, reductions in QT intervals and APD90 were observed, accompanied by a slight increase in ventricular arrhythmia episodes. During drug-induced proarrhythmia, empagliflozin succeeded in decreasing QT intervals, APD90, and VT burden whereas dapagliflozin demonstrated no significant effects. In the presence of pinacidil induced arrhythmogenicity, neither SGLT2 inhibitor had a significant impact on cardiac electrophysiology. In the AF setting, perfusion with dapagliflozin showed significant suppression of AF in the course of restitution of electrophysiological parameters whereas empagliflozin showed no significant effect on atrial fibrillation incidence. In this model, empagliflozin and dapagliflozin demonstrated opposite antiarrhythmic properties. Empagliflozin reduced ventricular tachyarrhythmias whereas dapagliflozin showed effective suppression of atrial arrhythmias.

Image-Decomposition-Enhanced Deep Learning for Detection of Rotor Cores in Cardiac Fibrillation.

Rotors, regions of spiral wave reentry in cardiac tissues, are considered as the drivers of atrial fibrillation (AF), the most common arrhythmia. Whereas physics-based approaches have been widely deployed to detect the rotors, in-depth knowledge in cardiac physiology and electrogram interpretation skills are typically needed. The recent leap forward in smart sensing, data acquisition, and Artificial Intelligence (AI) has offered an unprecedented opportunity to transform diagnosis and treatment in cardiac ailment, including AF. This study aims to develop an image-decomposition-enhanced deep learning framework for automatic identification of rotor cores on both simulation and optical mapping data. We adopt the Ensemble Empirical Mode Decomposition algorithm (EEMD) to decompose the original image, and the most representative component is then fed into a You-Only-Look-Once (YOLO) object-detection architecture for rotor detection. Simulation data from a bi-domain simulation model and optical mapping acquired from isolated rabbit hearts are used for training and validation. This integrated EEMD-YOLO model achieves high accuracy on both simulation and optical mapping data (precision: 97.2%, 96.8%, recall: 93.8%, 92.2%, and F1 score: 95.5%, 94.4%, respectively). The proposed EEMD-YOLO yields comparable accuracy in rotor detection with the gold standard in literature.

Improving omnipolar electrogram reconstruction: an animal model study

Abstract Background Cardiac arrhythmias are a significant cause of morbidity and mortality worldwide. The characterization of cardiac tissue's electrophysiological substrate is crucial for the diagnosis and treatment of these conditions. High-density (HD) electrode arrays, such as the Advisor™ HD Grid Mapping Catheter and Sensor Enabled™ (SE), are commonly used to examine cardiac tissue locally and create activation maps. However, the current method for reconstructing omnipolar EGMs, a novel technique claimed to be orientation-independent, has limitations. The enhancement of this reconstruction methodology has the potential to improve the accuracy and reliability of electrophysiological mapping. This, in turn, can have a positive impact on patients suffering from cardiac arrhythmias, ultimately leading to better clinical outcomes. Purpose We propose a novel reconstruction method to estimate omnipolar EGM that overcomes the angle dependence weakness of the current method to enhance the reconstruction of electrophysiological signals. Methodology Prior experiments were conducted on isolated rabbit hearts using a 128-electrode array with electrodes spaced 1mm apart over the ventricular epicardium. Four-electrode cliques were designed from these experiments, with different inter-electrode distances as depicted in Figure 1A, to reconstruct the current and novel omnipolar configurations. Metrics were designed to assess the quality of the electrogram signals, including the bipolar loop area and the ratio of the bipolar components of the omnipole (see fig. 2). The bipoles were calculated from unipolar electrograms, and the signal was corrected with a 45º offset in the cross. The propagation angle was then detected, and the local activation time (LAT) was estimated. Results In the bipole plots, it was observed that the morphology of the bipolar loop becomes distorted with an increase in the inter-electrode distance, becoming thinner for distances less than 2mm. There is a direct relation between the areas in the bipolar loop and the inter-electrode distance (see Figure 2A). Furthermore, the values of the area in the cross are always comparable to the best-performing triangular clique. For the ratio of bipoles, the relation is inversely related to distance (see Figure 2B). Similarly, the performance of the cross is comparable to the best-performing triangular omnipole. Conclusion This study highlights the importance of accurate reconstruction of omnipolar EGMs for the diagnosis and treatment of cardiac arrhythmias. To achieve precise reconstruction, inter-electrode distances smaller than 2mm are recommended. The proposed reconstruction method based on the cross configuration is more accurate and robust to wavefront propagation, provided that the recommended distance criterion is met. This study opens up new possibilities for the design of new devices and practices in the clinic.Figure 1Figure 2

Assessment of the Interelectrode Distance Effect over the Omnipole with High Multielectrode Arrays.

The development of high-density multielectrode catheters has significantly advanced cardiac electrophysiology mapping. High-density grid catheters have enabled the creation of a novel technique for reconstructing electrogram (EGM) signals known as "omnipole," which is believed to be more reliable than other methods, especially in terms of orientation independence. This study aims to evaluate how distance affects the omnipolar reconstruction of EGMs by comparing different configurations. Using an animal set up of perfused isolated rabbit hearts, recordings were taken using an ad hoc high-density epicardial multielectrode catheter. Inter-electrode distances ranging from 1 to 4 mm were analysed for their effect on the quality of resulting EGMs. Two biomarkers were computed to evaluate the robustness of the reconstructions: the areas contained within the bipolar loops and the amplitudes of the omnipoles. We hypothesised that both bipolar and omnipolar electrograms would be more robust at shorter inter-electrode distances. The results showed that an increase in distance triggers an increase in loop areas and amplitudes, which supports the hypothesis. This finding provides a more reliable estimate of wavefront propagation for the cross-omnipolar reconstruction method. These results emphasise the importance of distance in cardiac electrophysiology mapping and provide valuable insights into the use of high-density multielectrode catheters for EGM reconstruction.Clinical Relevance- The results of this study have direct clinical relevance in the application of the described techniques to recording systems in the cardiac electrophysiology laboratory, enabling clinicians to obtain more precise characterisation of signals in the myocardium.

Study of the omnipolar EGM reconstruction for robustness against wavefront propagation in epicardial signals

Abstract Funding Acknowledgements Type of funding sources: Public Institution(s). Main funding source(s): National Research Program, Ministerio de Ciencia e Innovación, Spanish Government, Instituto de Salud Carlos III. Background Characterisation of electrophysiological substrate is commonly performed by examining cardiac tissue locally by using high-density (HD) arrays of equispaced electrodes – e.g. Advisor™ HD Grid Mapping Catheter, Sensor Enabled™ (SE). These allow creating activation maps of local tissue and substrate exploration. Recently, a novel operation mode arouse: the omnipole, which overcomes many limitations of bipolar and unipolar electrograms, as they are claimed to be orientation independent. Nevertheless, the current technique to reconstruct omnipolar EGMs has pitfalls as the estimation of some incidence angles of the propagation wavefront. We propose a novel reconstruction method to estimate omnipolar EGMs which overcomes the angle dependence weakness of the current one, providing a more robust EGM representation. We hypothesise that this technique will improve reconstruction of electrophysiological signals. Methods Prior electrophysiological experiments on isolated rabbit hearts were used [1], including recordings with a 128-electrode array 1mm apart over ventricular epicardium at 37ºC, with bipolar electrode stimulation. Four-electrodes cliques were designed from these experiments, with different inter-electrode distances as depicted in fig. 1A, reconstructing the current, and novel omnipolar configuration. Metrics to assess the quality of the EGM signals were designed, including the bipolar loop area, and the ratio of the bipolar components of the omnipole (ox and oy) – see fig. 1B. The bipoles were calculated from unipolar EGMs, the signal was corrected with the 45º offset in the cross, the propagation angle was then detected, and the local activation time (LAT) was estimated. Results In the bipole plots, it was observed that the morphology of the bipolar loop gets distorted with an increase in the interelectrode distance – thinner for distances &amp;lt;2mm. There is a direct relation between the areas in the bipolar loop and the inter-electrode distance (see fig. 2A). Furthermore, the values of the area in the cross are always comparable to the best performing triangular clique. For the ratio of bipoles, the relation is inversely to distance (see fig. 2B), similarly, the performance of the cross is comparable to the best performing triangular omnipole. Conclusion For reconstructing omnipolar EGMs, inter-electrode distances smaller than 2mm should be employed. Furthermore, if that condition is satisfied, the novel omnipolar reconstruction method proposed based on the cross configuration is more accurate and robust to wavefront propagation, given that the omnipolar EGM performs similarly or better than the best triangular configuration – and which triangle is fixed for the whole procedure, being the current reconstruction method less robust. Thus, a new standpoint to omnipolar reconstruction in high density electrode catheters is open, towards the design of new devices, and the practice in the clinic.

Attenuation of stretch-induced arrhythmias following chemical ablation of Purkinje fibres, in isolated rabbit hearts.

Purkinje fibres (PFs) play an important role in some ventricular arrhythmias and acute ventricular stretch can evoke mechanically-induced arrhythmias. We tested whether Purkinje fibres, play a role in these arrhythmias. Pseudo-ECGs were recorded in isolated, Langendorff-perfused, rabbit hearts in which the left ventricular endocardial surface was also irrigated with Tyrode, via an indwelling catheter placed in the left ventricular lumen. The number and period of ectopic activations was measured during left ventricular lumen inflation via an indwelling fluid-filled balloon (500μL added over 2s and maintained for 15s in total). Mechanically-induced arrhythmias occurred in 70% of balloon inflations: they were maximal in the first 5s and ceased within 15s. Brief, (10s) irrigation of the left ventricular lumen with Lugol solution (IK/I2), via the indwelling catheter, reduced inflation-induced ectopics by 98% (p < 0.05). Ablation of endocardial PFs by Lugol was confirmed by Triphenyltetrazolium Chloride staining. Optical mapping revealed the left ventricular epicardial activation patterns of ectopics could have PF-mediated and focal sources. In silico modelling predicted ectopic sources originating in the endocardial region propagate to and through the Purkinje fibres network. Acute distention-induced ectopics are multi-focal, their attenuation by Lugol, their activation patterns and in silico modelling indicate a participation of Purkinje fibres in these arrhythmias.

A novel ex-vivo isolated rabbit heart preparation to explore the cardiac effects of cervical and cardiac vagus nerve stimulation

The cardiac responses to vagus nerve stimulation (VNS) are still not fully understood, partly due to uncontrollable confounders in the in-vivo experimental condition. Therefore, an ex-vivo Langendorff-perfused rabbit heart with intact vagal innervation is proposed to study VNS in absence of cofounding anesthetic or autonomic influences. The feasibility to evoke chronotropic responses through electrical stimulation ex-vivo was studied in innervated isolated rabbit hearts (n = 6). The general nerve excitability was assessed through the ability to evoke a heart rate (HR) reduction of at least 5 bpm (physiological threshold). The excitability was quantified as the charge needed for a 10-bpm HR reduction. The results were compared to a series of in-vivo experiments rabbits (n = 5). In the ex-vivo isolated heart, the baseline HR was about 20 bpm lower than in-vivo (158 ± 11 bpm vs 181 ± 19 bpm). Overall, the nerve remained excitable for about 5 h ex-vivo. The charges required to reduce HR by 5 bpm were 9 ± 6 µC and 549 ± 370 µC, ex-vivo and in-vivo, respectively. The charges needed for a 10-bpm HR reduction, normalized to the physiological threshold were 1.78 ± 0.8 and 1.22 ± 0.1, in-vivo and ex-vivo, respectively. Overall, the viability of this ex-vivo model to study the acute cardiac effects of VNS was demonstrated.

Levosimendan attenuates electrical alternans and prevents ventricular arrhythmia during therapeutic hypothermia in isolated rabbit hearts

Therapeutic hypothermia (TH) increases the susceptibility to ventricular arrhythmias (VAs) by prolonging action potential duration (APD) and facilitating arrhythmogenic spatially discordant alternans (SDA). Levosimendan, a calcium sensitizer, has been reported to shorten APD by enhancing the adenosine triphosphate (ATP)-sensitive K current. The purpose of this study was to test the hypothesis that, during TH, levosimendan shortens the already prolonged APD, attenuates SDA, and prevents VA. Langendorff-perfused isolated rabbit hearts were subjected to TH (30°C) for 15 minutes, followed by treatment with either levosimendan 0.5 μM (n = 9) or vehicle (n = 8) for an additional 30 minutes under TH. Using an optical mapping system, epicardial APD was evaluated by S1 pacing. SDA threshold was defined as the longest pacing cycle length (PCL) that induces the phenomenon of SDA. Ventricular fibrillation (VF) inducibility was evaluated by burst pacing for 30 seconds at the shortest PCL that achieved 1:1 ventricular capture. During TH, levosimendan shortened ventricular APD (PCL 400 ms; from 259 ± 8 ms to 241 ± 18 ms; P = .036) and decreased SDA threshold (from 327 ± 88 ms to 311 ± 68 ms; P = .011). VF inducibility was lowered from 39% ± 30% to 14% ± 12% with levosimendan (P = .018), whereas APD at PCL 400 ms (P = .161), SDA threshold (P = 1), and VF inducibility (P = .173) were not changed by vehicle. During TH, levosimendan could protect hearts against VA by shortening APD and decreasing SDA threshold. Enhancing ATP-sensitive K current with levosimendan might be a novel approach to preventing VA during TH.

Cardiovascular Effects of Nigella Sativa on Isolated Rabbit Heart and its Impact on Cardiac Remodeling

Background: Black Cumin/Nigella sativa (NS) which belongs to the botanical family of Ranunculaceae commonly grows in Eastern Europe, the Middle East, and Western Asia. Its prolonged use can produce physiological changes with or without affecting the architecture of different organs like the heart (cardiac remodeling). The data for the cardiovascular benefits of black cumin are not well-established scientifically. Objectives: To determine the direct cardiovascular effects of Nigella Sativa extract on heart rate, cardiac contractility (apical force), ECG, and coronary flow in the normal heart with and without cardiac remodeling. Methods: This experimental study was conducted on forty-two (42) rabbits. These rabbits were divided into seven groups, each comprising six animals (Group I-VI without cardiac remodeling and Group VII with cardiac remodeling). NS was given to these groups in different doses i.e., Group I (NS=10ug), Group II (NS=30ug), Group III (NS=100ug), Group IV (NS=300ug), Group V (NS=3000ug), Group VI (NS=10000ug) and VII (NS=300ug). Radnoti's working heart system was used to determine the effects of NS on heart rate, cardiac contractility (apical Force), ECG, and coronary flow in a normal heart with and without cardiac remodeling. The results were analyzed using SPSS version 28. Results: Results of this study revealed negative chronotropic and positive inotropic effects without ECG changes in the normal heart and with ECG changes in the remodeled heart. Conclusions: Prolonged use of Nigella sativa can lead to disturbed ECG by affecting the conducting tissue.

Investigation of the cardiac depressant effect of Caralluma tuberculate N.E.Br on isolated rabbit heart

Purpose: To investigate the histopathological and cardiac depressant effect of the aqueous methanol extract of Caralluma tuberculata N.E. Br (AMECT) (family: Asclepiadaceae)’ and to determine if there is a scientific basis for its cardiovascular diseases-related folkloric use. &#x0D; Methods: The effect of AMECT in different concentrations ranging from 0.00001 to 1.0 mg/mL were evaluated in isolated perfused rabbit heart to assess their effect on the force of contraction and heart rate using Langendorff’s apparatus. Atropine and adrenaline were used to identify the underlying mechanism of response produced by AMECT. The extract was studied for its possible mechanism in the absence and presence of atropine and adrenaline. In addition, sub-chronic toxicity and histopathological study of heart tissues in rats were assessed by administering 500 mg/kg of extract. &#x0D; Results: At all concentrations, AMECT produced significant (p &lt; 0.001) negative ionotropic and negative chronotropic effects. The most significant effect was observed at 0.001 mg/mL and higher concentrations hence 0.001 mg/mL was selected for further studies. Pre-incubation with atropine did not significantly inhibit the effects of AMECT. However, AMECT significantly (p &lt; 0.01) blocked the cardiac stimulant effect of adrenaline. In the histopathological studies, AMECT did not produce any significant cellular changes or signs of toxicity in the sub-chronic toxicity study. &#x0D; Conclusion: The cardiac-depressant responses of AMECT may involve the β-adrenergic receptors in the myocardium of isolated rabbit heart thus confirming the rationale for its use in ethnomedicine for cardiac diseases.

Safe electrophysiologic profile of the neprilysin-inhibitor sacubitril in combination with different antiarrhythmic drugs

Abstract Background Observational clinical data suggest a direct effect of sacubitril on cardiac electrophysiology. However, data concerning the impact of combination of sacubitril with antiarrhythmic drug therapy is sparse. Purpose We aimed at characterizing the electrophysiologic effects of a combination therapy of sacubitril with the class III-drug sotalol and the class IB-agent mexiletine in a sensitive, experimental whole-heart model. Methods and results 25 isolated rabbit hearts were retrogradely perfused. Hearts were treated with mexiletine (25 μM, 13 hearts) or sotalol (100 μM, 12 hearts) after generating baseline data. Eight endo- and epicardial monophasic action potentials and ECG recordings demonstrated an abbreviation of action potential duration (APD90, −21 ms, p&amp;lt;0.01) and no significant changes of QT interval (+10ms, p=ns) after administration of mexiletine. Spatial dispersion of repolarization (SDR) remained stable after mexiletine treatment (+6 ms, p=ns) whereas effective refractory periods (ERP) were significantly prolonged (+80 ms, p&amp;lt;0.01). Sotalol prolonged cardiac repolarization (APD90: +1 ms, p=ns; QT: +24 ms, p&amp;lt;0.01) and amplified spatial dispersion (+19 ms, p&amp;lt;0.01) without changing ERP (+8 ms, p=ns). Additional treatment with sacubitril (5 μM) led to a significant reduction of APD90 (−12 ms, p&amp;lt;0.01), QT interval (−20 ms, p&amp;lt;0.01) and ERP (−23 ms, p&amp;lt;0.01) in the presence of a stable SDR (−4 ms, p=ns) in the mexiletine group. In the sotalol group, additional administration of sacubitril abbreviated APD90 (−9 ms, p&amp;lt;0.05) and QT interval (−13 ms, p&amp;lt;0.01) and reduced ERP (−18 ms, p&amp;lt;0.01) without affecting SDR (−3 ms, p=ns). Ventricular vulnerability was assessed by a predefined pacing protocol employing premature extra stimuli and burst stimulation. After lowering the potassium concentration, 30 episodes of torsade de pointes tachycardia were observed after sotalol treatment (vs. 0 episodes under baseline, p&amp;lt;0.05). Additional sacubitril treatment significantly suppressed torsade de pointes tachycardia (8 episodes, p&amp;lt;0.05) in the sotalol-group. No proarrhythmic effect was observed after mexiletine treatment (8 episodes vs. 3 episodes under baseline conditions, p=ns). Additional sacubitril treatment did not increase ventricular vulnerability (32 episods, p=ns). Conclusion Sacubitril in combination with antiarrhythmic drugs demonstrates a safe electrophysiologic profile. In class IB- and class III-pretreated hearts, sacubitril abbreviates cardiac repolarization duration and effective refractory periods without changing spatial dispersion of repolarization. Thereby, sacubitril reduces the occurrence of torsade de pointes-tachycardia in antiarrhythmic pretreated hearts. Funding Acknowledgement Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): Deutsche Stiftung für Herzforschung (to G.F.)

Feasibility of simultaneous multichannel magnetocardiographic mapping, multiple monophasic action potential recording and laser photocoagulation of isolated and perfused rabbit ventricular myocardium

Abstract Background Multichannel magnetocardiographic mapping (MCG) has been validated as an accurate non-invasive method for 3D localization of arrhythmogenic substrates suitable for ablation treatment and of intracardiac electrophysiology catheters (1,2). High resolution (HR) MCG-guided multiple simultaneous monophasic action potential (multi-MAP) recording and pacing with a single patented amagnetic catheter (AC) detects local arrhythmogenic mechanisms (3). Laser photocoagulation (LP) providing more controlled and homogeneous myocardial lesions (4), may be an alternative to radiofrequency (RF) ablation, without unwanted electromagnetic noise effect during real-time MCG. Purpose This study aimed to experimentally test the feasibility of simultaneous multi-MAP recording and LP guided by real-time MCG. Methods An unshielded 36 channels SQUID system (sensitivity of 30 fT/Hz1/2, at 1 Hz) (CMI, USA) was used to study Langendorff-perfused isolated rabbit heart with MCG (Figure 1A). Four epicardial MAPs were simultaneously recorded (inter MAP distance: 1 mm) with a single multi-MAP AC (bandpass: DC-500 Hz; sampling frequency: 2 kHz) (GE-Prucka Cardiolab). LP was performed with a NdYAG laser (Dornier Medilas 5100) and a catheter system for safe myocardium LP (5). The study was carried out according to NIH guidelines for animal care and use. Results MCG imaging of rabbit ventricular magnetic field distribution and inverse source localization with the magnetic dipole model are shown in Figure 1B. The difference in morphology of local HR ventricular multi-MAP signals was compatible with myocardial anisotropy (Figure 1C). With appropriate cooling of the optic fiber-tissue interface, no epicardial damage was induced by LP, whereas deeper well defined intramural myocardial lesions were appreciable, of different size as a function of the energy delivered (Figure 1D). Multi-MAP recording was not significantly disturbed by LP. Keeping the Medilas unit at least 5 meters away from the MCG sensors, no electromagnetic artifacts were observed during MCG. Conclusion MCG, single catheter multi-MAP recording and well-defined LP lesions of ventricular myocardium were simultaneously feasible on Langendorff-perfused isolated rabbit heart, without major electromagnetic interferences, in an unshielded catheterization laboratory. With appropriate technological developments, safe MCG-guided single-catheter laser ablation of focal arrhythmogenic substrates is foreseen as a possible minimally invasive alternative to current interventional approaches. Funding Acknowledgement Type of funding sources: None.

Electrophysiological Safety Profile of Antiestrogenic Therapies in the Isolated Rabbit Heart

Introduction: Hormone-mediated therapies are on the rise and are key therapies in the treatment of some of the most common cancer entities. Proarrhythmic effects have been described in patients treated with SERMs while little to none is known about the electrophysiological effects of the potentially less arrhythmogenic selective estrogen receptor degraders. Methods: Twenty hearts of female New Zealand White rabbits were excised and retrogradely perfused employing a Langendorff setup. An electrophysiology study was performed to obtain CL-dependent action potential duration at 90% of repolarization (APD90), QT interval, effective refractory period (ERP), and post-repolarization refractoriness (PRR = ERP-APD90). After generating baseline data, the hearts were assigned to two groups and perfused with (n = 10) increasing doses of fulvestrant (1 µM and 5 µM; n = 10) or tamoxifen (2.5 µM and 7.5 µM; n = 10), and the protocol was repeated again. Results: Fulvestrant led to a decrease in APD90 and QT interval and an increased PRR. The inducibility of ventricular tachycardia (VT) episodes was unaltered. Tamoxifen showed similar effects, resulting in a shortened APD90 and QT interval and no increased VT incidence in the setting of a prolonged PRR. Conclusion: The present study shows no acute proarrhythmic effect of tamoxifen and fulvestrant in an established whole heart model when employing clinically relevant concentrations.

The Effects of Vagus Nerve Stimulation on Ventricular Electrophysiology and Nitric Oxide Release in the Rabbit Heart.

Background: Abnormal autonomic activity including impaired parasympathetic control is a known hallmark of heart failure (HF). Vagus nerve stimulation (VNS) has been shown to reduce the susceptibility of the heart to ventricular fibrillation, however the precise underlying mechanisms are not well understood and the detailed stimulation parameters needed to improve patient outcomes clinically are currently inconclusive. Objective: To investigate NO release and cardiac electrophysiological effects of electrical stimulation of the vagus nerve at varying parameters using the isolated innervated rabbit heart preparation. Methods: The right cervical vagus nerve was electrically stimulated in the innervated isolated rabbit heart preparation (n = 30). Heart rate (HR), effective refractory period (ERP), ventricular fibrillation threshold (VFT) and electrical restitution were measured as well as NO release from the left ventricle. Results: High voltage with low frequency VNS resulted in the most significant reduction in HR (by −20.6 ± 3.3%, −25.7 ± 3.0% and −30.5 ± 3.0% at 0.1, 1 and 2 ms pulse widths, with minimal increase in NO release. Low voltage and high frequency VNS significantly altered NO release in the left ventricle, whilst significantly flattening the slope of restitution and significantly increasing VFT. HR changes however using low voltage, high frequency VNS were minimal at 20Hz (to 138.5 ± 7.7 bpm (−7.3 ± 2.0%) at 1 ms pulse width and 141.1 ± 6.6 bpm (−4.4 ± 1.1%) at 2 ms pulse width). Conclusion: The protective effects of the VNS are independent of HR reductions demonstrating the likelihood of such effects being as a result of the modulation of more than one molecular pathway. Altering the parameters of VNS impacts neural fibre recruitment in the ventricle; influencing changes in ventricular electrophysiology, the protective effect of VNS against VF and the release of NO from the left ventricle.

Intra-cardiac transfer of fatty acids from capillary to cardiomyocyte.

Blood-borne fatty acids (Fa) are important substrates for energy conversion in the mammalian heart. After release from plasma albumin, Fa traverse the endothelium and the interstitial compartment to cross the sarcolemma prior to oxidation in the cardiomyocytal mitochondria. The aims of the present study were to elucidate the site with lowest Fa permeability (i.e., highest Fa resistance) in the overall Fa trajectory from capillary to cardiomyocyte and the relative contribution of unbound Fa (detach pathway, characterized by the dissociation time constant τAlbFa) and albumin-bound Fa (contact pathway, characterized by the membrane reaction rate parameter dAlb) in delivering Fa to the cellular membranes. In this study, an extensive set of 34 multiple indicator dilution experiments with radiolabeled albumin and palmitate on isolated rabbit hearts was analysed by means of a previously developed mathematical model of Fa transfer dynamics. In these experiments, the ratio of the concentration of palmitate to albumin was set at 0.91. The analysis shows that total cardiac Fa permeability, Ptot, is indeed related to the albumin concentration in the extracellular compartment as predicted by the mathematical model. The analysis also reveals that the lowest permeability may reside in the boundary zones containing albumin in the microvascular and interstitial compartment. However, the permeability of the endothelial cytoplasm, Pec, may influence overall Fa permeability, Ptot, as well. The model analysis predicts that the most likely value of τAlbFa ranges from about 200 to 400 ms. In case τAlbFa is fast, i.e., about 200 ms, the extracellular contact pathway appears to be of minor importance in delivering Fa to the cell membrane. If Fa dissociation from albumin is slower, e.g. τAlbFa equals 400 ms, the contribution of the contact pathway may vary from minimal (dAlb≤5 nm) to substantial (dAlb about 100 nm). In the latter case, the permeability of the endothelial cytoplasm varies from infinite (no hindrance) to low (substantial hindrance) to keep the overall Fa flux at a fixed level. Definitive estimation of the impact of endothelial permeability on Ptot and the precise contribution of the contact pathway to overall transfer of Fa in boundary zones containing albumin requires adequate physicochemical experimentation to delineate the true value of, among others, τAlbFa, under physiologically relevant circumstances. Our analysis also implies that concentration differences of unbound Fa are the driving force of intra-cardiac Fa transfer; an active, energy requiring transport mechanism is not necessarily involved. Membrane-associated proteins may facilitate Fa transfer in the boundary zones containing albumin by modulating the membrane reaction rate parameter, dAlb, and, hence, the contribution of the contact pathway to intra-cardiac Fa transfer.