Cardiac mechano-electric feedback: past, present, and prospect

Commotio Cordis: A Comprehensive Review

Commotio cordis is a rare, however, potentially fatal, cardiovascular phenomenon arising from direct chest wall trauma, causing sudden cardiac arrest and potentially death. It is primarily seen in young athletes who participate in contact and organized sports. Though debated, the cause of commotio cordis is believed to involve specific timing of chest impact during ventricular electrical activity leading to severe arrhythmic events. Due to sudden onset, the first step in management is immediate and effective basic life support with automated external defibrillation, followed by advanced cardiac life support. Future considerations should include secondary prevention measures such as protective padding in contact sports. It is paramount that clinicians are vigilant in recognizing potential cases of commotio cordis in the field and provide immediate care. This review consolidates the current understanding of commotio cordis, emphasizing the importance of awareness and early intervention. Future research is warranted, including retrospective and observational studies to identify high-risk patterns or trends associated with the condition.

Correlation properties and respiratory frequency of ECG-derived heart rate variability during multiple race-pace running intervals in female and male long-distance runners

Aim was to evaluate alterations of the non-linear short-term scaling exponent alpha1 of detrended fluctuation analysis (DFAa1) of heart rate (HR) variability (HRV) as a sensitive marker for assessing global physiological demands during multiple running intervals. As a secondary analysis, agreement of ECG-derived respiratory frequency (EDR) compared to respiratory frequency (RF) derived from the metabolic cart was evaluated with the same chest belt device. Fifteen trained female and male long-distance runners completed four running bouts over 5 min on a treadmill at marathon pace. During the last 3 min of each bout gas exchange data and a single-channel ECG for the determination of HR, DFAa1 of HRV, EDR and RF were analyzed. Additionally, blood lactate concentration (BLC) was determined and rating of perceived exertion (RPE) was requested. DFAa1, oxygen consumption, BLC, and RPE showed stable behaviors comparing the running intervals. Only HR (p < 0.001, d = 0.17) and RF (p = 0.012, d = 0.20) indicated slight increases with small effect sizes. In addition, results point towards inter-individual differences in all internal load metrics. The comparison of EDR with RF during running revealed high correlations (r = 0.80, p < 0.001, ICC3,1 = 0.87) and low mean differences (1.8 ± 4.4 breaths/min), but rather large limits of agreement with 10.4 to -6.8 breaths/min. Results show the necessity of EDR methodology improvement before being used in a wide range of individuals and sports applications. Relationship of DFAa1 to other internal load metrics, including RF, in quasi-steady-state conditions bears the potential for further evaluation of exercise prescription and may enlighten decoupling mechanisms during prolonged exercise bouts.

Vascularisation of pluripotent stem cell-derived myocardium: biomechanical insights for physiological relevance in cardiac tissue engineering

The myocardium is a diverse environment, requiring coordination between a variety of specialised cell types. Biochemical crosstalk between cardiomyocytes (CM) and microvascular endothelial cells (MVEC) is essential to maintain contractility and healthy tissue homeostasis. Yet, as myocytes beat, heterocellular communication occurs also through constantly fluctuating biomechanical stimuli, namely (1) compressive and tensile forces generated directly by the beating myocardium, and (2) pulsatile shear stress caused by intra-microvascular flow. Despite endothelial cells (EC) being highly mechanosensitive, the role of biomechanical stimuli from beating CM as a regulatory mode of myocardial-microvascular crosstalk is relatively unexplored. Given that cardiac biomechanics are dramatically altered during disease, and disruption of myocardial-microvascular communication is a known driver of pathological remodelling, understanding the biomechanical context necessary for healthy myocardial-microvascular interaction is of high importance. The current gap in understanding can largely be attributed to technical limitations associated with reproducing dynamic physiological biomechanics in multicellular in vitro platforms, coupled with limited in vitro viability of primary cardiac tissue. However, differentiation of CM from human pluripotent stem cells (hPSC) has provided an unlimited source of human myocytes suitable for designing in vitro models. This technology is now converging with the diverse field of tissue engineering, which utilises in vitro techniques designed to enhance physiological relevance, such as biomimetic extracellular matrix (ECM) as 3D scaffolds, microfluidic perfusion of vascularised networks, and complex multicellular architectures generated via 3D bioprinting. These strategies are now allowing researchers to design in vitro platforms which emulate the cell composition, architectures, and biomechanics specific to the myocardial-microvascular microenvironment. Inclusion of physiological multicellularity and biomechanics may also induce a more mature phenotype in stem cell-derived CM, further enhancing their value. This review aims to highlight the importance of biomechanical stimuli as determinants of CM-EC crosstalk in cardiac health and disease, and to explore emerging tissue engineering and hPSC technologies which can recapitulate physiological dynamics to enhance the value of in vitro cardiac experimentation.

Direct in vivo assessment of global and regional mechanoelectric feedback in the intact human heart

Background

Inhomogeneity of ventricular contraction is associated with sudden cardiac death, but the underlying mechanisms are unclear. Alterations in cardiac contraction impact electrophysiological parameters through mechanoelectric feedback. This has been shown to promote arrhythmias in experimental studies, but its effect in the in vivo human heart is unclear.

Objective

The purpose of this study was to quantify the impact of regional myocardial deformation provoked by a sudden increase in ventricular loading (aortic occlusion) on human cardiac electrophysiology.

Methods

In 10 patients undergoing open heart cardiac surgery, left ventricular (LV) afterload was modified by transient aortic occlusion. Simultaneous assessment of whole-heart electrophysiology and LV deformation was performed using an epicardial sock (240 electrodes) and speckle-tracking transesophageal echocardiography. Parameters were matched to 6 American Heart Association LV model segments. The association between changes in regional myocardial segment length and activation-recovery interval (ARI; a conventional surrogate for action potential duration) was studied using mixed-effect models.

Results

Increased ventricular loading reduced longitudinal shortening (P = .01) and shortened ARI (P = .02), but changes were heterogeneous between cardiac segments. Increased regional longitudinal shortening was associated with ARI shortening (effect size 0.20 [0.01–0.38] ms/%; P = .04) and increased local ARI dispersion (effect size –0.13 [–0.23 to –0.03] ms/%; P = .04). At the whole organ level, increased mechanical dispersion translated into increased dispersion of repolarization (correlation coefficient r = 0.81; P = .01).

Conclusion

Mechanoelectric feedback can establish a potentially proarrhythmic substrate in the human heart and should be considered to advance our understanding and prevention of cardiac arrhythmias.

A Simulation Study of the Role of Mechanical Stretch in Arrhythmogenesis during Cardiac Alternans

The deformation of the heart tissue due to the contraction can modulate the excitation, a phenomenon referred to as mechanoelectrical feedback (MEF), via stretch-activated channels. The effects of MEF on the electrophysiology at high pacing rates are shown to be proarrhythmic in general. However, more studies need to be done to elucidate the underlying mechanism. In this work, we investigate the effects of MEF on cardiac alternans, which is an alternation in the width of the action potential that typically occurs when the heart is paced at high rates, using a biophysically detailed electromechanical model of cardiac tissue. We observe that the transition from spatially concordant alternans to spatially discordant alternans, which is more arrhythmogenic than concordant alternans, may occur in the presence of MEF and when its strength is sufficiently large. We show that this transition is due to the increase of the dispersion of conduction velocity. In addition, our results also show that the MEF effects, depending on the stretch-activated channels’ conductances and reversal potentials, can result in blocking action potential propagation.

Delayed global feedback in the genesis and stability of spatiotemporal excitation patterns in paced biological excitable media

Biological excitable media, such as cardiac or neural cells and tissue, exhibit memory in which a change in the present excitation may affect the behaviors in the next excitation. For example, a change in calcium (Ca2+) concentration in a cell in the present excitation may affect the Ca2+ dynamics in the next excitation via bi-directional coupling between voltage and Ca2+, forming a delayed feedback loop. Since the Ca2+ dynamics inside the excitable cells are spatiotemporal while the membrane voltage is a global signal, the feedback loop is then a delayed global feedback (DGF) loop. In this study, we investigate the roles of DGF in the genesis and stability of spatiotemporal excitation patterns in periodically-paced excitable media using mathematical models with different levels of complexity: a model composed of coupled FitzHugh-Nagumo units, a 3-dimensional physiologically-detailed ventricular myocyte model, and a coupled map lattice model. We investigate the dynamics of excitation patterns that are temporal period-2 (P2) and spatially concordant or discordant, such as subcellular concordant or discordant Ca2+alternans in cardiac myocytes or spatially concordant or discordant Ca2+ and repolarization alternans in cardiac tissue. Our modeling approach allows both computer simulations and rigorous analytical treatments, which lead to the following results and conclusions. When DGF is absent, concordant and discordant P2 patterns occur depending on initial conditions with the discordant P2 patterns being spatially random. When the DGF is negative, only concordant P2 patterns exist. When the DGF is positive, both concordant and discordant P2 patterns can occur. The discordant P2 patterns are still spatially random, but they satisfy that the global signal exhibits a temporal period-1 behavior. The theoretical analyses of the coupled map lattice model reveal the underlying instabilities and bifurcations for the genesis, selection, and stability of spatiotemporal excitation patterns.

Upregulation of miR-133b and miR-328 in Patients With Atrial Dilatation: Implications for Stretch-Induced Atrial Fibrillation

Atrial stretch and dilatation are common features of many clinical conditions predisposing to atrial fibrillation (AF). MicroRNAs (miRs) are emerging as potential molecular determinants of AF, but their relationship with atrial dilatation (AD) is poorly understood. The present study was designed to assess the specific miR expression profiles associated with AD in human atrial tissue. The expressions of a preselected panel of miRs, previously described as playing a role in cardiac disease, were quantified by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) in atrial tissue samples from 30 cardiac surgery patients, who were characterized by different grades of AD and arrhythmic profiles. Our results showed that AD per se was associated with significant up-regulation of miR-328-3p and miR-133b (p < 0.05) with respect to controls, with a fold-change of 1.53 and 1.74, respectively. In a multivariate model including AD and AF as independent variables, miR-328-3p expression was mainly associated with AD grade (p < 0.05), while miR-133b was related to both AD (p < 0.005) and AF (p < 0.05), the two factors exerting opposite modulation effects. The presence of AF was associated with significant (p < 0.05) up-regulation of the expression level of miR-1-3p, miR-21-5p, miR-29a-3p, miR-208b-3p, and miR-590-5p. These results showed the existence of specific alterations of miR expression associated with AD, which may pave the way to future experimental studies to test the involvement of post-transcriptional mechanisms in the stretch-induced formation of a pro-arrhythmic substrate.

Effects of mechano-electrical feedback on the onset of alternans: A computational study

Cardiac alternans is a heart rhythm instability that is associated with cardiac arrhythmias and may lead to sudden cardiac death. The onset of this instability, which is linked to period-doubling bifurcation and may be a route to chaos, is of particular interest. Mechano-electric feedback depicts the effects of tissue deformation on cardiac excitation. The main effect of mechano-electric feedback is delivered via the so-called stretch-activated ion channels and is caused by stretch-activated currents. Mechano-electric feedback, which is believed to have proarrhythmic and antiarrhythmic effects on cardiac electrophysiology, affects the action potential duration in a manner dependent on cycle length, but the mechanisms by which this occurs remain to be elucidated. In this study, a biophysically detailed electromechanical model of cardiac tissue is employed to show how a stretch-activated current can affect the action potential duration at cellular and tissue levels, illustrating its effects on the onset of alternans. Also, using a two-dimensional iterated map that incorporates stretch-activated current effects, we apply linear stability analysis to study the stability of the bifurcation. We show that alternans bifurcation can be prevented depending on the strength of the stretch-activated current.

Maturation of Cardiomyocytes Derived from Human Pluripotent Stem Cells: Current Strategies and Limitations

The capacity of differentiation of human pluripotent stem cells (hPSCs), which include both embryonic stem cells and induced pluripotent stem cells, into cardiomyocytes (CMs) in vitro provides an unlimited resource for human CMs for a wide range of applications such as cell based cardiac repair, cardiac drug toxicology screening, and human cardiac disease modeling. However, their applicability is significantly limited by immature phenotypes. It has been well known that currently available CMs derived from hPSCs (hPSC-CMs) represent immature embryonic or fetal stage CMs and are functionally and structurally different from mature human CMs. To overcome this critical issue, several new approaches aiming to generate more mature hPSC-CMs have been developed. This review describes recent approaches to generate more mature hPSC-CMs including their scientific principles, advantages, and limitations.

Interregional electro-mechanical heterogeneity in the rabbit myocardium

BACKGROUND

Increased electrical heterogeneity has been causatively linked to arrhythmic disorders, yet the knowledge about physiological heterogeneity remains incomplete. This study investigates regional electro-mechanical heterogeneities in rabbits, one of the key animal models for arrhythmic disorders.

METHODS AND FINDINGS

7 wild-type rabbits were examined by phase-contrast magnetic resonance imaging in vivo to assess cardiac wall movement velocities. Using a novel data-processing algorithm regional contraction-like profiles were calculated. Contraction started earlier and was longer in left ventricular (LV) apex than base. Patch clamp recordings showed longer action potentials (AP) in LV apex compared to the base of LV, septum, and right ventricle. Western blots of cardiac ion channels and calcium handling proteins showed lower expression of Cav1.2, KvLQT1, Kv1.4, NCX and Phospholamban in LV apex vs. base. A single-cell in silico model integrating the quantitative regional differences in ion channels reproduced a longer contraction and longer AP in apex vs. base.

CONCLUSIONS

Apico-basal electro-mechanical heterogeneity is physiologically present in the healthy rabbit heart. An apico-basal electro-mechanical gradient exists with longer APD and contraction duration in the apex and associated regionally heterogeneous expression of five key proteins. This pattern of apical mechanical dominance probably serves to increase pumping efficiency.

Optical Mapping of Membrane Potential and Epicardial Deformation in Beating Hearts

Cardiac optical mapping uses potentiometric fluorescent dyes to image membrane potential (Vm). An important limitation of conventional optical mapping is that contraction is usually arrested pharmacologically to prevent motion artifacts from obscuring Vm signals. However, these agents may alter electrophysiology, and by abolishing contraction, also prevent optical mapping from being used to study coupling between electrical and mechanical function. Here, we present a method to simultaneously map Vm and epicardial contraction in the beating heart. Isolated perfused swine hearts were stained with di-4-ANEPPS and fiducial markers were glued to the epicardium for motion tracking. The heart was imaged at 750 Hz with a video camera. Fluorescence was excited with cyan or blue LEDs on alternating camera frames, thus providing a 375-Hz effective sampling rate. Marker tracking enabled the pixel(s) imaging any epicardial site within the marked region to be identified in each camera frame. Cyan- and blue-elicited fluorescence have different sensitivities to Vm, but other signal features, primarily motion artifacts, are common. Thus, taking the ratio of fluorescence emitted by a motion-tracked epicardial site in adjacent frames removes artifacts, leaving Vm (excitation ratiometry). Reconstructed Vm signals were validated by comparison to monophasic action potentials and to conventional optical mapping signals. Binocular imaging with additional video cameras enabled marker motion to be tracked in three dimensions. From these data, epicardial deformation during the cardiac cycle was quantified by computing finite strain fields. We show that the method can simultaneously map Vm and strain in a left-sided working heart preparation and can image changes in both electrical and mechanical function 5 min after the induction of regional ischemia. By allowing high-resolution optical mapping in the absence of electromechanical uncoupling agents, the method relieves a long-standing limitation of optical mapping and has potential to enhance new studies in coupled cardiac electromechanics.

Mechano-electric coupling, heterogeneity in repolarization and the electrocardiographic T-wave

Stretch influences repolarization by mechano-electric coupling (MEC) and contributes to arrhythmogenesis. Although there is an abundance of research on electrophysiological effects of MEC, it is still unclear how MEC translates to the ECG. We aim to provide an overview of the MEC research focused on the ECG and the underlying changes in electrophysiology. In addition, we present new data on the effect of left ventricular pressure on the electrocardiographic T-wave. We show that an increase in left ventricular pressure leads to prolonged QT-intervals with increased amplitudes of the STT-segment. This corresponds to a prolongation in repolarization and an increased interventricular dispersion of repolarization. MEC is dependent on timing, intensity and modality of stretch and these three factors should be taken into account to analyse the effects of MEC on the heart and on the ECG. In addition, the deformation of the heart itself should be considered, since it influences the amplitude of the STT-segment. Because the electrocardiographic T-wave represents heterogeneity in repolarization, left ventricular pressure increases may have significant influence on the inducibility of (re-entrant) arrhythmias.

Naturally Engineered Maturation of Cardiomyocytes

Ischemic heart disease remains one of the most prominent causes of mortalities worldwide with heart transplantation being the gold-standard treatment option. However, due to the major limitations associated with heart transplants, such as an inadequate supply and heart rejection, there remains a significant clinical need for a viable cardiac regenerative therapy to restore native myocardial function. Over the course of the previous several decades, researchers have made prominent advances in the field of cardiac regeneration with the creation of in vitro human pluripotent stem cell-derived cardiomyocyte tissue engineered constructs. However, these engineered constructs exhibit a functionally immature, disorganized, fetal-like phenotype that is not equivalent physiologically to native adult cardiac tissue. Due to this major limitation, many recent studies have investigated approaches to improve pluripotent stem cell-derived cardiomyocyte maturation to close this large functionality gap between engineered and native cardiac tissue. This review integrates the natural developmental mechanisms of cardiomyocyte structural and functional maturation. The variety of ways researchers have attempted to improve cardiomyocyte maturation in vitro by mimicking natural development, known as natural engineering, is readily discussed. The main focus of this review involves the synergistic role of electrical and mechanical stimulation, extracellular matrix interactions, and non-cardiomyocyte interactions in facilitating cardiomyocyte maturation. Overall, even with these current natural engineering approaches, pluripotent stem cell-derived cardiomyocytes within three-dimensional engineered heart tissue still remain mostly within the early to late fetal stages of cardiomyocyte maturity. Therefore, although the end goal is to achieve adult phenotypic maturity, more emphasis must be placed on elucidating how the in vivo fetal microenvironment drives cardiomyocyte maturation. This information can then be utilized to develop natural engineering approaches that can emulate this fetal microenvironment and thus make prominent progress in pluripotent stem cell-derived maturity toward a more clinically relevant model for cardiac regeneration.

Study of the union method of microelectrode array and AFM for the recording of electromechanical activities in living cardiomyocytes

Electrophysiology and mechanics are two essential components in the functions of cardiomyocytes and skeletal muscle cells. The simultaneous recording of electrophysiological and mechanical activities is important for the understanding of mechanisms underlying cell functions. For example, on the one hand, mechanisms under cardiovascular drug effects will be investigated in a comprehensive way by the simultaneous recording of electrophysiological and mechanical activities. On the other hand, computational models of electromechanics provide a powerful tool for the research of cardiomyocytes. The electrical and mechanical activities are important in cardiomyocyte models. The simultaneous recording of electrophysiological and mechanical activities can provide much experimental data for the models. Therefore, an efficient method for the simultaneous recording of the electrical and mechanical data from cardiomyocytes is required for the improvement of cardiac modeling. However, as far as we know, most of the previous methods were not easy to be implemented in the electromechanical recording. For this reason, in this study, a union method of microelectrode array and atomic force microscope was proposed. With this method, the extracellular field potential and beating force of cardiomyocytes were recorded simultaneously with a low root-mean-square noise level of 11.67 μV and 60 pN. Drug tests were conducted to verify the feasibility of the experimental platform. The experimental results suggested the method would be useful for the cardiovascular drug screening and refinement of the computational cardiomyocyte models. It may be valuable for exploring the functional mechanisms of cardiomyocytes and skeletal muscle cells under physiological or pathological conditions.

L-Type Calcium Channels Do Not Play a Critical Role in Chest Blow Induced Ventricular Fibrillation: Commotio Cordis

Background. In a commotio cordis swine model, ventricular fibrillation (VF) can be induced by a ball blow to the chest believed secondary to activation of mechanosensitive ion channels. The purpose of the current study is to evaluate whether stretch induced activation of the L-type calcium channel may cause intracellular calcium overload and underlie the VF in commotio cordis. Method and Results. Anesthetized juvenile swine received 6 chest wall strikes with a 17.9 m/s lacrosse ball timed to the vulnerable period for VF induction. Animals were randomized to IV verapamil (n = 6) or placebo (n = 6). There was no difference in the observed frequency of VF between verapamil (19/26: 73%) and placebo (20/36: 56%) treated animals (p = 0.16). There was also no significant difference in the combined endpoint of VF or nonsustained VF (21/26: 81% in verapamil versus 24/36: 67% in controls, p = 0.22). Conclusions. In this experimental model of commotio cordis, verapamil did not prevent VF induction. Thus, in commotio cordis it is unlikely that stretch activation of the L-type calcium channel with resultant intracellular calcium overload plays a prominent role.

Axial stretch-dependent cation entry in dystrophic cardiomyopathy: Involvement of several TRPs channels

In Duchenne muscular dystrophy (DMD), deficiency of the cytoskeletal protein dystrophin leads to well-described defects in skeletal muscle but also to dilated cardiomyopathy (DCM). In cardiac cells, the subsarcolemmal localization of dystrophin is thought to protect the membrane from mechanical stress. The dystrophin deficiency leads to membrane instability and a high stress-induced Ca(2+) influx due to dysregulation of sarcolemmal channels such as stretch-activated channels (SACs). In this work divalent cation entry has been explored in isolated ventricular Wild Type (WT) and mdx cardiomyocytes in two different conditions: at rest and during the application of an axial stretch. At rest, our results suggest that activation of TRPV2 channels participates to a constitutive basal cation entry in mdx cardiomyocytes.Using microcarbon fibres technique, an axial stretchwas applied to mimic effects of physiological conditions of ventricular filling and study on cation influx bythe Mn(2+)-quenching techniquedemonstrated a high stretch-dependentcationic influx in dystrophic cells, partially due to SACs. Involvement of TRPs channels in this excessive Ca(2+) influx has been investigated using specific modulators and demonstratedboth sarcolemmal localization and an abnormal activity of TRPV2 channels. In conclusion, TRPV2 channels are demonstrated here to play a key role in cation influx and dysregulation in dystrophin deficient cardiomyocytes, enhanced in stretching conditions.

Ultrasound-induced modulation of cardiac rhythm in neonatal rat ventricular cardiomyocytes

Isolated neonatal rat ventricular cardiomyocytes were used to study the influence of ultrasound on the chronotropic response in a tissue culture model. The beat frequency of the cells, varying from 40 to 90 beats/min, was measured based upon the translocation of the nuclear membrane captured by a high-speed camera. Ultrasound pulses (frequency = 2.5 MHz) were delivered at 300-ms intervals [3.33 Hz pulse repetition frequency (PRF)], in turn corresponding to 200 pulses/min. The intensity of acoustic energy and pulse duration were made variable, 0.02-0.87 W/cm(2) and 1-5 ms, respectively. In 57 of 99 trials, there was a noted average increase in beat frequency of 25% with 8-s exposures to ultrasonic pulses. Applied ultrasound energy with a spatial peak time average acoustic intensity (Ispta) of 0.02 W/cm(2) and pulse duration of 1 ms effectively increased the contraction rate of cardiomyocytes (P < 0.05). Of the acoustic power tested, the lowest level of acoustic intensity and shortest pulse duration proved most effective at increasing the electrophysiological responsiveness and beat frequency of cardiomyocytes. Determining the optimal conditions for delivery of ultrasound will be essential to developing new models for understanding mechanoelectrical coupling (MEC) and understanding novel nonelectrical pacing modalities for clinical applications.

Cardiac contraction induces discordant alternans and localized block

In this paper we use a simplified model of cardiac excitation-contraction coupling to study the effect of tissue deformation on the dynamics of alternans, i.e., alternations in the duration of the cardiac action potential, that occur at fast pacing rates and are known to be proarrhythmic. We show that small stretch-activated currents can produce large effects and cause a transition from in-phase to off-phase alternations (i.e., from concordant to discordant alternans) and to conduction blocks. We demonstrate numerically and analytically that this effect is the result of a generic change in the slope of the conduction velocity restitution curve due to electromechanical coupling. Thus, excitation-contraction coupling can potentially play a relevant role in the transition to reentry and fibrillation.

Pathophysiology, prevention, and treatment of commotio cordis

Commotio cordis is increasing described and it is now clear that this phenomenon is an important cause of sudden cardiac death on the playing field. Victims are predominantly young, male, and struck in the left chest with a ball. An animal model has been developed and utilized to explore the important variables and mechanism of commotio cordis. Impact during a narrow window of repolarization causes ventricular fibrillation. Other important variables include location, velocity, shape, and hardness of the impact object. Biological characteristics such as gender, pliability of the chest wall, and genetic susceptibility also play a role in commotio cordis. The mechanism of ventricular fibrillation appears to be an increase in heterogeneity of repolarization caused by induced abnormalities of ion channels activated by abrupt increases in left ventricular pressure. In the setting of altered repolarization a trigger of ventricular depolarization (premature ventricular depolarization caused directly by the chest blow) initiates a spiral wave that quickly breaks down into ventricular fibrillation. Prevention of commotio cordis is possible. Improved recognition and resuscitation have led to an improvement in outcome.

Systems approach to the study of stretch and arrhythmias in right ventricular failure induced in rats by monocrotaline

We demonstrate the synergistic benefits of using multiple technologies to investigate complex multi-scale biological responses. The combination of reductionist and integrative methodologies can reveal novel insights into mechanisms of action by tracking changes of in vivo phenomena to alterations in protein activity (or vice versa). We have applied this approach to electrical and mechanical remodelling in right ventricular failure caused by monocrotaline-induced pulmonary artery hypertension in rats. We show arrhythmogenic T-wave alternans in the ECG of conscious heart failure animals. Optical mapping of isolated hearts revealed discordant action potential duration (APD) alternans. Potential causes of the arrhythmic substrate; structural remodelling and/or steep APD restitution and dispersion were observed, with specific remodelling of the Right Ventricular Outflow Tract. At the myocyte level, [Ca(2+)]i transient alternans were observed together with decreased activity, gene and protein expression of the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA). Computer simulations of the electrical and structural remodelling suggest both contribute to a less stable substrate. Echocardiography was used to estimate increased wall stress in failure, in vivo. Stretch of intact and skinned single myocytes revealed no effect on the Frank-Starling mechanism in failing myocytes. In isolated hearts acute stretch-induced arrhythmias occurred in all preparations. Significant shortening of the early APD was seen in control but not failing hearts. These observations may be linked to changes in the gene expression of candidate mechanosensitive ion channels (MSCs) TREK-1 and TRPC1/6. Computer simulations incorporating MSCs and changes in ion channels with failure, based on altered gene expression, largely reproduced experimental observations.

Molecular candidates for cardiac stretch-activated ion channels

The heart is a mechanically-active organ that dynamically senses its own mechanical environment. This environment is constantly changing, on a beat-by-beat basis, with additional modulation by respiratory activity and changes in posture or physical activity, and further overlaid with more slowly occurring physiological (e.g. pregnancy, endurance training) or pathological challenges (e.g. pressure or volume overload). Far from being a simple pump, the heart detects changes in mechanical demand and adjusts its performance accordingly, both via heart rate and stroke volume alteration. Many of the underlying regulatory processes are encoded intracardially, and are thus maintained even in heart transplant recipients. Over the last three decades, molecular substrates of cardiac mechanosensitivity have gained increasing recognition in the scientific and clinical communities. Nonetheless, the processes underlying this phenomenon are still poorly understood. Stretch-activated ion channels (SAC) have been identified as one contributor to mechanosensitive autoregulation of the heartbeat. They also appear to play important roles in the development of cardiac pathologies – most notably stretch-induced arrhythmias. As recently discovered, some established cardiac drugs act, in part at least, via mechanotransduction pathways suggesting SAC as potential therapeutic targets. Clearly, identification of the molecular substrate of cardiac SAC is of clinical importance and a number of candidate proteins have been identified. At the same time, experimental studies have revealed variable–and at times contrasting–results regarding their function. Further complication arises from the fact that many ion channels that are not classically defined as SAC, including voltage and ligand-gated ion channels, can respond to mechanical stimulation. Here, we summarise what is known about the molecular substrate of the main candidates for cardiac SAC, before identifying potential further developments in this area of translational research.

Excitation-contraction coupling between human atrial myocytes with fibroblasts and stretch activated channel current: a simulation study

Myocytes have been regarded as the main objectives in most cardiac modeling studies and attracted a lot of attention. Connective tissue cells, such as fibroblasts (Fbs), also play crucial role in cardiac function. This study proposed an integrated myocyte-I_sac-Fb electromechanical model to investigate the effect of Fbs and stretch activated ion channel current (I_sac) on cardiac electrical excitation conduction and mechanical contraction. At the cellular level, an active Fb model was coupled with a human atrial myocyte electrophysiological model (including I_sac) and a mechanical model. At the tissue level, electrical excitation conduction was coupled with an elastic mechanical model, in which finite difference method (FDM) was used to solve the electrical excitation equations, while finite element method (FEM) was used for the mechanics equations. The simulation results showed that Fbs and I_sac coupling caused diverse effects on action potential morphology during repolarization, depolarized the resting membrane potential of the human atrial myocyte, slowed down wave propagation, and decreased strains in fibrotic tissue. This preliminary simulation study indicates that Fbs and I_sac have important implications for modulating cardiac electromechanical behavior and should be considered in future cardiac modeling studies.

Effects of mechano-electric feedback on scroll wave stability in human ventricular fibrillation

Recruitment of stretch-activated channels, one of the mechanisms of mechano-electric feedback, has been shown to influence the stability of scroll waves, the waves that underlie reentrant arrhythmias. However, a comprehensive study to examine the effects of recruitment of stretch-activated channels with different reversal potentials and conductances on scroll wave stability has not been undertaken; the mechanisms by which stretch-activated channel opening alters scroll wave stability are also not well understood. The goals of this study were to test the hypothesis that recruitment of stretch-activated channels affects scroll wave stability differently depending on stretch-activated channel reversal potential and channel conductance, and to uncover the relevant mechanisms underlying the observed behaviors. We developed a strongly-coupled model of human ventricular electromechanics that incorporated human ventricular geometry and fiber and sheet orientation reconstructed from MR and diffusion tensor MR images. Since a wide variety of reversal potentials and channel conductances have been reported for stretch-activated channels, two reversal potentials, −60 mV and −10 mV, and a range of channel conductances (0 to 0.07 mS/µF) were implemented. Opening of stretch-activated channels with a reversal potential of −60 mV diminished scroll wave breakup for all values of conductances by flattening heterogeneously the action potential duration restitution curve. Opening of stretch-activated channels with a reversal potential of −10 mV inhibited partially scroll wave breakup at low conductance values (from 0.02 to 0.04 mS/µF) by flattening heterogeneously the conduction velocity restitution relation. For large conductance values (>0.05 mS/µF), recruitment of stretch-activated channels with a reversal potential of −10 mV did not reduce the likelihood of scroll wave breakup because Na channel inactivation in regions of large stretch led to conduction block, which counteracted the increased scroll wave stability due to an overall flatter conduction velocity restitution.

A fully coupled model for electromechanics of the heart

We present a fully coupled electromechanical model of the heart. The model integrates cardiac electrophysiology and cardiac mechanics through excitation-induced contraction and deformation-induced current. Numerical schemes based on finite element were implemented in a supercomputer. Numerical examples were presented using a thin cardiac tissue and a dog ventricle with realistic geometry. Performance of the parallel simulation scheme was studied. The model provides a useful tool to understand cardiovascular dynamics.

The ischemic heart: what causes ectopic beating?

The mechanisms by which spontaneous electrical activity originates in the ischemic heart and leads to arrhythmia remain unknown, however mechanical stretch of the diseased region has been hypothesized to play a role. The goal of this study is to investigate the conditions that favor the initiation of stretch-induced premature beats in the ischemic heart. We employ a mathematical model of the ischemic cell subjected to stretch. The study found that upon stretch, spontaneous beats occur in the ischemic cell, which are due to the stretch-induced re-activation of the L-type calcium current.

Mechano-electrical feedback explains T-wave morphology and optimizes cardiac pump function: insight from a multi-scale model

In the ECG, T- and R-wave are concordant during normal sinus rhythm (SR), but discordant after a period of ventricular pacing (VP). Experiments showed that the latter phenomenon, called T-wave memory, is mediated by a mechanical stimulus. By means of a mathematical model, we investigated the hypothesis that slow acting mechano-electrical feedback (MEF) explains T-wave memory. In our model, electromechanical behavior of the left ventricle (LV) was simulated using a series of mechanically and electrically coupled segments. Each segment comprised ionic membrane currents, calcium handling, and excitation-contraction coupling. MEF was incorporated by locally adjusting conductivity of L-type calcium current (g(CaL)) to local external work. In our set-up, g(CaL) could vary up to 25%, 50%, 100% or unlimited amount around its default value. Four consecutive simulations were performed: normal SR (with MEF), acute VP, sustained VP (with MEF), and acutely restored SR. MEF led to T-wave concordance in normal SR and to discordant T-waves acutely after restoring SR. Simulated ECGs with a maximum of 25-50% adaptation closely resembled those during T-wave memory experiments in vivo and also provided the best compromise between optimal systolic and diastolic function. In conclusion, these simulation results indicate that slow acting MEF in the LV can explain a) the relatively small differences in systolic shortening and mechanical work during SR, b) the small dispersion in repolarization time, c) the concordant T-wave during SR, and d) T-wave memory. The physiological distribution in electrophysiological properties, reflected by the concordant T-wave, may serve to optimize cardiac pump function.

The Association of Abnormal Ventricular Wall Motion and Increased Dispersion of Repolarization in Humans is Independent of the Presence of Myocardial Infarction

Abnormal ventricular wall motion is a strong clinical predictor of sudden, arrhythmic, cardiac death. Dispersion in repolarization is a prerequisite for the initiation of re-entrant arrhythmia. We hypothesize that regionally decreased wall motion is associated with heterogeneity of repolarization. We measured local activation times, activation-recovery intervals (ARIs, surrogate for action potential duration), and repolarization times using a multielectrode grid at nine segments on the left ventricular epicardium in 23 patients undergoing coronary artery surgery. Regional wall motion was simultaneously assessed using intraoperative transesophageal echocardiography. Three groups were discriminated: (1) Patients with normal wall motion (n = 11), (2) Patients with one or more hypokinetic segments (n = 6), (3) Patients with one or more akinetic or dyskinetic segments (n = 6). The average ARI was similar in all groups (251 ± 3.7 ms, ±SEM). Dispersion of ARIs between the nine segments was significantly increased in the hypokinetic (84 ± 7.4 ms, p < 0.005) and akinetic/dyskinetic group (94 ± 3.5 ms, p < 0.0005) compared with the normal group (49 ± 5.1 ms), independent from the presence of myocardial infarction. Repolarization heterogeneity occurred primarily in the normally contracting regions of the hearts with abnormal wall motion. An almost maximal increased dispersion of repolarization was observed when there was only a single hypokinetic segment. We conclude that inhomogeneous wall motion abnormality of even moderate severity is associated with increased repolarization inhomogeneity, independent from the presence of infarction.

Cardiac alternans annihilation by distributed mechano-electric feedback (MEF)

The presence of the electrical alternans induces, through the mechanism of the excitation-contraction coupling, an alternation in the heart muscle contractile activity. In this work, we demonstrate the cardiac alternans annihilation by applied mechanical perturbation. In particular, we address annihilation of alternans in realistic heart size tissue by considering ionic currents suggested by Luo-Rudy-1 (LR1) model, in which the control algorithm involves a combined electrical boundary pacing control and a spatially distributed calcium based control which perturbs the calcium in the cells. Complimentary to this, we also address a novel mechanism of alternans annihilation which uses a Nash Panfilov model coupled with the stress equilibrium equations. The coupled model includes an additional variable to represent the active stress which defines the mechanical properties of the tissue.

Bacterial mechanosensitive channels as a paradigm for mechanosensory transduction

Research on bacterial mechanosensitive (MS) channels has since their discovery been at the forefront of the MS channel field due to extensive studies of the structure and function of MscL and MscS, two of the several different types of MS channels found in bacteria. Just a few years after these two MS channels were cloned their 3D structure was solved by X-ray crystallography. Today, the repertoire of multidisciplinary approaches used in experimental and theoretical studies following the cloning and crystallographic determination of the MscL and MscS structure has expanded by including electronparamagnetic resonance (EPR) and Förster resonance energy transfer (FRET) spectroscopy aided by computational modelling employing molecular dynamics as well as Brownian dynamics simulations, which significantly advanced the understanding of structural determinants of the gating and conduction properties of these two MS channels. These extensive multidisciplinary studies of MscL and MscS have greatly contributed to elucidation of the basic physical principles of MS channel gating by mechanical force. This review summarizes briefly the major experimental and conceptual advancements, which helped in establishing MscL and MscS as a major paradigm of mechanosensory transduction in living cells.

TREK-1 K(+) channels in the cardiovascular system: their significance and potential as a therapeutic target

Potassium (K(+) ) channels are important in cardiovascular disease both as drug targets and as a cause of underlying pathology. Voltage-dependent K(+) (K(V) ) channels are inhibited by the class III antiarrhythmic agents. Certain vasodilators work by opening K(+) channels in vascular smooth muscle cells (VSMCs), and K(+) channel activation may also be a route to improving endothelial function. The two-pore domain K(+) (K(2P) ) channels form a group of 15 known channels with an expanding list of functions in the cardiovascular system. One of these K(2P) channels, TREK-1, is the focus of this review. TREK-1 channel activity is tightly regulated by intracellular and extracellular pH, membrane stretch, polyunsaturated fatty acids (PUFAs), temperature, and receptor-coupled second messenger systems. TREK-1 channels are also activated by volatile anesthetics and some neuroprotectant agents, and they are inhibited by selective serotonin reuptake inhibitors (SSRIs) as well as amide local anesthetics. Some of the clinical cardiovascular effects and side effects of these drugs may be through their actions on TREK-1 channels. It has recently been suggested that TREK-1 channels have a role in mechano-electrical coupling in the heart. They also seem important in the vascular responses to PUFAs, and this may underlie some of the beneficial cardiovascular effects of the essential dietary fatty acids. Development of selective TREK-1 openers and inhibitors may provide promising routes for intervention in cardiovascular diseases.

Canonical TRP channels and mechanotransduction: from physiology to disease states

Mechano-gated ion channels play a key physiological role in cardiac, arterial, and skeletal myocytes. For instance, opening of the non-selective stretch-activated cation channels in smooth muscle cells is involved in the pressure-dependent myogenic constriction of resistance arteries. These channels are also implicated in major pathologies, including cardiac hypertrophy or Duchenne muscular dystrophy. Seminal work in prokaryotes and invertebrates highlighted the role of transient receptor potential (TRP) channels in mechanosensory transduction. In mammals, recent findings have shown that the canonical TRPC1 and TRPC6 channels are key players in muscle mechanotransduction. In the present review, we will focus on the functional properties of TRPC1 and TRPC6 channels, on their mechano-gating, regulation by interacting cytoskeletal and scaffolding proteins, physiological role and implication in associated diseases.

Generation of histo-anatomically representative models of the individual heart: tools and application

This paper presents methods to build histo-anatomically detailed individualized cardiac models. The models are based on high-resolution three-dimensional anatomical and/or diffusion tensor magnetic resonance images, combined with serial histological sectioning data, and are used to investigate individualized cardiac function. The current state of the art is reviewed, and its limitations are discussed. We assess the challenges associated with the generation of histo-anatomically representative individualized in silico models of the heart. The entire processing pipeline including image acquisition, image processing, mesh generation, model set-up and execution of computer simulations, and the underlying methods are described. The multifaceted challenges associated with these goals are highlighted, suitable solutions are proposed, and an important application of developed high-resolution structure-function models in elucidating the effect of individual structural heterogeneity upon wavefront dynamics is demonstrated.

Sensing pressure in the cardiovascular system: Gq-coupled mechanoreceptors and TRP channels

Despite the central physiological importance of cardiovascular mechanotransduction, the molecular identities of the sensors and the signaling pathways have long remained elusive. Indeed, how pressure is transduced into cellular excitation has only recently started to emerge. In both arterial and cardiac myocytes, the diacylglycerol-sensitive canonical transient receptor potential (TRPC) subunits are proposed to underlie the stretch-activated depolarizing cation channels. An indirect mechanism of activation through a ligand-independent conformational switch of Gq-coupled receptors by mechanical stress is invoked. Such a mechanism involving the angiotensin type 1 receptor and TRPC6 is proposed to trigger the arterial myogenic response to intraluminal pressure. TRPC6 is also involved in load-induced cardiac hypertrophy. In this review, we will focus on the molecular basis of pressure sensing in the cardiovascular system and associated disease states.

Cardiorespiratory interactions in patients with atrial flutter

Respiratory sinus arrhythmia (RSA) is generally known as the autonomically mediated modulation of the sinus node pacemaker frequency in synchrony with respiration. Cardiorespiratory interactions have been largely investigated during sinus rhythm, whereas little is known about interactions during reentrant arrhythmias. In this study, cardiorespiratory interactions at the atrial and ventricular level were investigated during atrial flutter (AFL), a supraventricular arrhythmia based on a reentry, by using cross-spectral analysis and computer modeling. The coherence and phase between respiration and atrial (γ[Formula: see text], φAA) and ventricular (γ[Formula: see text], φRR) interval series were estimated in 20 patients with typical AFL (68.0 ± 8.8 yr) and some degree of atrioventricular (AV) conduction block. In all patients, atrial intervals displayed oscillations strongly coupled and in phase with respiration (γ[Formula: see text]= 0.97 ± 0.05, φAA = 0.71 ± 0.31 rad), corresponding to a paradoxical lengthening of intervals during inspiration. The modulation pattern was frequency independent, with in-phase oscillations and short time delays (0.40 ± 0.15 s) for respiratory frequencies in the range 0.1–0.4 Hz. Ventricular patterns were affected by AV conduction type. In patients with fixed AV conduction, ventricular intervals displayed oscillations strongly coupled (γ[Formula: see text]= 0.97 ± 0.03) and in phase with respiration (φRR = 1.08 ± 0.80 rad). Differently, in patients with variable AV conduction, respiratory oscillations were secondary to Wencheback rhythmicity, resulting in a decreased level of coupling (γ[Formula: see text]= 0.50 ± 0.21). Simulations with a simplified model of AV conduction showed ventricular patterns to originate from the combination of a respiratory modulated atrial input with the functional properties of the AV node. The paradoxical frequency-independent modulation pattern of atrial interval, the short time delays, and the complexity of ventricular rhythm characterize respiratory arrhythmia during AFL and distinguish it from normal RSA. These peculiar features can be explained by assuming a direct mechanical action of respiration on AFL reentrant circuit.

Repolarization changes induced by mental stress in normal subjects and patients with coronary artery disease: effect of nitroglycerine

OBJECTIVES

Mental stress can significantly affect ventricular repolarization, which could potentially trigger arrhythmias. We compared the effect of mental stress on repolarization indexed by the amplitude and area of the T wave in patients with coronary artery disease (CAD) and healthy subjects.

METHODS

Fourteen healthy controls (11 M, mean age 42 years) and 14 patients with stable CAD (12 M, mean age 64) underwent a mental stress protocol consisting of mental arithmetic followed by a speech (5 minutes each), which was performed on two occasions following either nitroglycerine (NTG) or placebo. Multiple 12-lead electrocardiograms were acquired and repolarization was analyzed using automatically measured T wave amplitude (T(amp)) and area (T(area)).

RESULTS

When preceded by placebo the overall effect of mental stress, whether induced by arithmetic or speech, was significantly different in CAD patients compared with controls, with a decrease in T(amp) and T(area) in controls and an increase in patients; e.g., change in T(amp) during arithmetic -20 +/- 3 microV in controls versus 4 +/- 2 microV in patients, p < .001, and during speech -9 +/- 3 microV in controls versus 7 +/- 1 microV in patients, p < .001. Following NTG, the effect of stress on repolarization was similar in the 2 groups, with a reversed effect, i.e., decrease instead of increase in T(amp) and T(area) in CAD patients.

CONCLUSIONS

The effect of mental stress on ventricular repolarization is significantly different in CAD patients compared with healthy controls. These differences are considerably reduced by NTG.