End–diastolic myofiber stress and ejection strain increase with ventricular volume overload

OUP accepted manuscript

An abundance of literature describes physiological and pathological determinants of cardiac performance, building on the principles of excitation–contraction coupling. However, the mutual influencing of excitation–contraction and mechano-electrical feedback in the beating heart, here designated ‘electromechanical reciprocity’, remains poorly recognized clinically, despite the awareness that external and cardiac-internal mechanical stimuli can trigger electrical responses and arrhythmia. This review focuses on electromechanical reciprocity in the long-QT syndrome (LQTS), historically considered a purely electrical disease, but now appreciated as paradigmatic for the understanding of mechano-electrical contributions to arrhythmogenesis in this and other cardiac conditions. Electromechanical dispersion in LQTS is characterized by heterogeneously prolonged ventricular repolarization, besides altered contraction duration and relaxation. Mechanical alterations may deviate from what would be expected from global and regional repolarization abnormalities. Pathological repolarization prolongation outlasts mechanical systole in patients with LQTS, yielding a negative electromechanical window (EMW), which is most pronounced in symptomatic patients. The electromechanical window is a superior and independent arrhythmia-risk predictor compared with the heart rate-corrected QT. A negative EMW implies that the ventricle is deformed—by volume loading during the rapid filling phase—when repolarization is still ongoing. This creates a ‘sensitized’ electromechanical substrate, in which inadvertent electrical or mechanical stimuli such as local after-depolarizations, after-contractions, or dyssynchrony can trigger abnormal impulses. Increased sympathetic-nerve activity and pause-dependent potentiation further exaggerate electromechanical heterogeneities, promoting arrhythmogenesis. Unraveling electromechanical reciprocity advances the understanding of arrhythmia formation in various conditions. Real-time image integration of cardiac electrophysiology and mechanics offers new opportunities to address challenges in arrhythmia management.

An Augmented Negative Force-Frequency Relationship and Slowed Mechanical Restitution Are Associated With Increased Susceptibility to Drug-Induced Torsade de Pointes Arrhythmias in the Chronic Atrioventricular Block Dog

Introduction: In the chronic AV-block (CAVB) dog model, structural, contractile, and electrical remodeling occur, which predispose the heart to dofetilide-induced Torsade de Pointes (TdP) arrhythmias. Previous studies found a relation between electrical remodeling and inducibility of TdP, while structural remodeling is not a prerequisite for arrhythmogenesis. In this study, we prospectively assessed the relation between in vivo markers of contractile remodeling and TdP inducibility.

Methods: In 18 anesthetized dogs, the maximal first derivative of left ventricular pressure (LV dP/dt_max) was assessed at acute AV-block (AAVB) and after 2 weeks of chronic AV-block (CAVB2). Using pacing protocols, three markers of contractile remodeling, i.e., force-frequency relationship (FFR), mechanical restitution (MR), and post-extrasystolic potentiation (PESP) were determined. Infusion of dofetilide (0.025 mg/kg in 5 min) was used to test for TdP inducibility.

Results: After infusion of dofetilide, 1/18 dogs and 12/18 were susceptible to TdP-arrhythmias at AAVB and CAVB2, respectively (p = 0.001). The inducible dogs at CAVB2 showed augmented contractility at a CL of 1200 ms (2354 ± 168 mmHg/s in inducible dogs versus 1091 ± 59 mmHg/s in non-inducible dogs, p < 0.001) with a negative FFR, while the non-inducible dogs retained their positive FFR. The time constant (TC) of the MR curve was significantly higher in the inducible dogs (158 ± 7 ms versus 97 ± 8 ms, p < 0.0001). Furthermore, a linear correlation was found between a weighted score of the number and severity of arrhythmias and contractile parameters, i.e., contractility at CL of 1200 ms (r = 0.73, p = 0.002), the slope of the FFR (r = -0.58, p = 0.01) and the TC of MR (r = 0.66, p = 0.003). Thus, more severe arrhythmias were seen in dogs with the most pronounced contractile remodeling.

Conclusion: Contractile remodeling is concomitantly observed with susceptibility to dofetilide-induced TdP-arrhythmias. The inducible dogs show augmented contractile remodeling compared to non-inducible dogs, as seen by a negative FFR, higher maximal response of MR and PESP and slowed MR kinetics. These altered contractility parameters could reflect disrupted Ca²⁺ handling and Ca²⁺-overload, which predispose the heart to delayed- and early afterdepolarizations that could trigger TdP-arrhythmias.

Beat-to-Beat Variability in Preload Unmasks Latent Risk of Torsade de Pointes in Anesthetized Chronic Atrioventricular Block Dogs

BACKGROUND

Beat-to-beat variability in ventricular repolarization (BVR) associates with increased arrhythmic risk. Proarrhythmic remodeling in the dog with chronic AV-block (CAVB) compromises repolarization reserve and associates with increased BVR, which further increases upon dofetilide infusion and correlates with Torsade de Pointes (TdP) arrhythmias. It was hypothesized that these pro-arrhythmia-associated increases in BVR are induced by beat-to-beat variability in preload.

METHODS AND RESULTS

Left ventricular monophasic action potential duration (LVMAPD) was recorded in acute (AAVB) and CAVB dogs, before and after dofetilide infusion. BVR was quantified as short-term variability of LVMAPD. The PQ-interval was controlled by pacing: either a constant or an alternating preload pattern was established, verified by PV-loop. The effect of the stretch-activated channel blocker, streptomycin, on BVR was evaluated in a second CAVB group. At alternating preload only, BVR was increased after proarrhythmic remodeling (0.45±0.14 ms AAVB vs. 2.2±1.1 ms CAVB, P<0.01). At CAVB, but not at AAVB, dofetilide induced significant proarrhythmia. Preload variability augmented the dofetilide-induced BVR increase at CAVB (+1.5±0.8 ms vs. +0.9±0.9 ms, P=0.058). In the second group, the increase in baseline BVR by alternating preload (0.3±0.03 ms to 1.0±0.8 ms, P<0.01) was abolished by streptomycin (0.5±0.2 ms, P<0.05).

CONCLUSIONS

In CAVB dogs, the inverse relation between BVR and repolarization reserve originates from an augmented sensitivity of ventricular repolarization to beat-to-beat preload changes. Stretch-activated channels appear to be involved in the mechanism of BVR. (Circ J 2016; 80: 1336-1345).

The cardiac muscle duplex as a method to study myocardial heterogeneity

This paper reviews the development and application of paired muscle preparations, called duplex, for the investigation of mechanisms and consequences of intra-myocardial electro-mechanical heterogeneity. We illustrate the utility of the underlying combined experimental and computational approach for conceptual development and integration of basic science insight with clinically relevant settings, using previously published and new data. Directions for further study are identified.

Control of whole heart geometry by intramyocardial mechano-feedback: a model study

Geometry of the heart adapts to mechanical load, imposed by pressures and volumes of the cavities. We regarded preservation of cardiac geometry as a homeostatic control system. The control loop was simulated by a chain of models, starting with geometry of the cardiac walls, sequentially simulating circulation hemodynamics, myofiber stress and strain in the walls, transfer of mechano-sensed signals to structural changes of the myocardium, and finalized by calculation of resulting changes in cardiac wall geometry. Instead of modeling detailed mechano-transductive pathways and their interconnections, we used principles of control theory to find optimal transfer functions, representing the overall biological responses to mechanical signals. As biological responses we regarded tissue mass, extent of contractile myocyte structure and extent of the extra-cellular matrix. Mechano-structural stimulus-response characteristics were considered to be the same for atrial and ventricular tissue. Simulation of adaptation to self-generated hemodynamic load rendered physiologic geometry of all cardiac cavities automatically. Adaptation of geometry to chronic hypertension and volume load appeared also physiologic. Different combinations of mechano-sensors satisfied the condition that control of geometry is stable. Thus, we expect that for various species, evolution may have selected different solutions for mechano-adaptation.

The heart is known to adapt size of the cavities and thickness of the walls to the pumping requirements set by blood pressure and blood flow. We think that mechanical load of the cardiac tissue provides feedback signals for adaptation of mass and thickness of the cardiac walls. Many cellular mechanisms are known where mechanical load initiates a cascade of chemical reactions, eventually affecting structure and mass of the tissue. Because these mechanisms interact intricately, understanding of the system of adaptation as a whole is tremendously complicated. We present a novel approach by considering adaptation as a control system. Using the principle that control should converge to a stable end state, general rules are found that should be satisfied on transfer of mechanical load to structural adaptation in the cells of the tissue. We think that deeper understanding of the mechanism of adaptation requires that knowledge on mechano-transductive pathways is placed in the context of regarding adaptation as a system. Knowledge on adaptation of cardiac geometry to mechanical load is crucial in predicting long term effects of pathologic disorders or therapeutic interventions that chronically affect blood pressure or blood flow.

Three-wall segment (TriSeg) model describing mechanics and hemodynamics of ventricular interaction

A mathematical model (TriSeg model) of ventricular mechanics incorporating mechanical interaction of the left and right ventricular free walls and the interventricular septum is presented. Global left and right ventricular pump mechanics were related to representative myofiber mechanics in the three ventricular walls, satisfying the principle of conservation of energy. The walls were mechanically coupled satisfying tensile force equilibrium in the junction. Wall sizes and masses were rendered by adaptation to normalize mechanical myofiber load to physiological standard levels. The TriSeg model was implemented in the previously published lumped closed-loop CircAdapt model of heart and circulation. Simulation results of cardiac mechanics and hemodynamics during normal ventricular loading, acute pulmonary hypertension, and chronic pulmonary hypertension (including load adaptation) agreed with clinical data as obtained in healthy volunteers and pulmonary hypertension patients. In chronic pulmonary hypertension, the model predicted right ventricular free wall hypertrophy, increased systolic pulmonary flow acceleration, and increased right ventricular isovolumic contraction and relaxation times. Furthermore, septal curvature decreased linearly with its transmural pressure difference. In conclusion, the TriSeg model enables realistic simulation of ventricular mechanics including interaction between left and right ventricular pump mechanics, dynamics of septal geometry, and myofiber mechanics in the three ventricular walls.

Computational modeling of volumetric soft tissue growth: application to the cardiac left ventricle

As an initial step to investigate stimulus-response relations in growth and remodeling (G&R) of cardiac tissue, this study aims to develop a method to simulate 3D-inhomogeneous volumetric growth. Growth is regarded as a deformation that is decomposed into a plastic component which describes unconstrained growth and an elastic component to satisfy continuity of the tissue after growth. In current growth models, a single reference configuration is used that remains fixed throughout the entire growth process. However, considering continuous turnover to occur together with growth, such a fixed reference is unlikely to exist in reality. Therefore, we investigated the effect of tissue turnover on growth by incrementally updating the reference configuration. With both a fixed reference and an updated reference, strain-induced cardiac growth in magnitude of 30% could be simulated. However, with an updated reference, the amplitude of the stimulus for growth decreased over time, whereas with a fixed reference this amplitude increased. We conclude that, when modeling volumetric growth, the choice of the reference configuration is of great importance for the computed growth.

Computational analysis of the myocardial structure: adaptation of cardiac myofiber orientations through deformation

Deformation and structure of the cardiac wall can be assessed non-invasively by imaging techniques such as magnetic resonance imaging. Understanding the (patho-)physiology that underlies the observed deformation and structure is critical for clinical diagnosis. However, much about the genesis of deformation and structure is unknown. In the present computational model study, we hypothesize that myofibers locally adapt their orientation to achieve minimal fiber-cross fiber shear strain during the cardiac cycle. This hypothesis was tested in a 3D finite element model of left ventricular (LV) mechanics by computation of tissue deformations and subsequent adaptation of initial myofiber orientations towards those in the deformed tissue. As a consequence of adaptation, local tissue peak stress, strain during ejection and stroke work density were all found to increase by at least 10%, as well as to become 50% more homogeneous throughout the wall. Global LV work (peak systolic pressure, stroke volume and stroke work) increased significantly as well (>9%). The model-predicted myofiber orientations were found to be similar to those in experiments. To the best of our knowledge the presented model is the first that is able to simultaneously predict a realistic myocardial structure as well as to account for the experimentally observed homogeneity in local mechanics.

Cardiovascular profile of the canine torsades de pointes arrhythmia model assessed by echocardiographic and haemodynamic methods

Chronic atrioventricular block dogs have been established as an in vivo model of drug-induced torsades de pointes arrhythmias. We compared the cardiovascular profile of the canine model with that of sham-operated animals using echocardiographic and haemodynamic methods. In the echocardiographic study, the larger diameters of the left atria, inferior vena cava and left ventricle in end-diastole in addition to greater fractional shortening, end-diastolic volume, stroke volume and ejection fraction were more often detected in the chronic atrioventricular block dogs than in the sham-operated animals. During haemodynamic examination, lower cardiac output and higher pulmonary capillary wedge pressure were detected in chronic atrioventricular block dogs more than in sham-operated animals; however, these changes were within the physiological limits, and the results suggest that the chronic atrioventricular block dogs have a pathophysiological profile of chronic compensated heart failure.

High-Septal Pacing Reduces Ventricular Electrical Remodeling and Proarrhythmia in Chronic Atrioventricular Block Dogs

OBJECTIVES

This study was designed to analyze the relevance of ventricular activation patterns for ventricular electrical remodeling after atrioventricular (AV) block in dogs.

BACKGROUND

Bradycardia is thought to be the main contributor to ventricular electrical remodeling after complete AV block. However, an altered ventricular activation pattern or AV dyssynchrony may also contribute.

METHODS

For 4 weeks, AV block dogs were either paced from the high-ventricular septum near the His bundle at lowest captured rate (n = 9, high-septal pacing [HSP]) or kept at idioventricular rate without controlled activation (n = 14, chronic AV block [CAVB]). Multiple electrocardiographic and electrophysiological parameters were measured under anesthesia at 0 and 4 weeks. Proarrhythmia was tested at 4 weeks by I(Kr) block (25 mug/kg dofetilide intravenous).

RESULTS

At 0 weeks, the 2 groups were comparable, whereas after 4 weeks of similar bradycardia, QT duration at unpaced conditions had increased from 300 +/- 5 to 395 +/- 18 ms in CAVB (+32 +/- 6%) and from 307 +/- 8 ms to 357 +/- 11 ms in HSP (+17 +/- 4%; p < 0.05). Frequency dependency of repolarization was less steep in HSP compared to CAVB dogs after 4 weeks remodeling. Beat-to-beat variability of repolarization, a proarrhythmic parameter, increased only in CAVB from 0 to 4 weeks. Torsades de pointes arrhythmias were induced at 4 weeks in 44% HSP versus 78% CAVB dogs (p = 0.17). Cumulative duration of arrhythmias per inducible dog was 87 +/- 36 s in CAVB and 30 +/- 21 s in HSP (p < 0.05).

CONCLUSIONS

High-septal pacing reduces the magnitude of ventricular electrical remodeling and proarrhythmia in AV block dogs, suggesting a larger role for altered ventricular activation pattern in the generation of ventricular electrical remodeling than previously assumed.

Serial left-ventricular biopsy sampling using a minimally invasive trans-thoracic approach in adult dogs

Myocardial biopsies are an increasingly important tool to unravel the molecular mechanisms of cardiac disease. We evaluate a novel minimally invasive trans-thoracic approach for left-ventricular (LV) intra-mural biopsies, which enables repetitive individual sampling in adult dogs. Forty three generally anaesthesised dogs were studied during sinus rhythm (SR, control) and multiple times after the induction of volume overload hypertrophy (complete atrioventricular block [AVB]). Through a small cutaneous incision, an automatic biopsy needle was advanced into the apicolateral LV, guided by fluoroscopy. Electrocardiography (ECG), LV pressure and echocardiography served to monitor the procedure. One hundred eighty-eight intra-mural LV biopsies were obtained in 82 serial experiments (usually SR, 1, 2 and 6 weeks AVB) with a maximum of 8 repeated biopsies. All biopsies ( approximately 10 mm(3)) were suitable for simultaneous application of different cell-biological (light and electron microscopy, immunohistochemistry) and molecular techniques (PCR, Western blotting). In chronic experiments, repeated biopsy sampling did not influence haemodynamics, mechanics, electrocardiographic parameters or myocardial remodelling during SR or AVB. The rate of significant complications was as low as 4% of experiments. Minimally invasive sampling of LV needle biopsies enables serial assessment of myocardial remodelling using different molecular techniques in individual animals. The technique is safe and has no long-term effects on cardiac function or structure.

Impact of acute and enduring volume overload on mechanotransduction and cytoskeletal integrity of canine left ventricular myocardium

It is poorly understood how mechanical stimuli influence in vivo myocardial remodeling during chronic hemodynamic overload. Combined quantitation of ventricular mechanics and expression of key proteins involved in mechanotransduction can improve fundamental understanding. Adult anesthetized dogs ( n = 20) were studied at sinus rhythm (SR) and 0, 3, 10, and 35 days of complete atrioventricular block (AVB). Serial left ventricular (LV) myofiber mechanics were measured. Repeated LV biopsies were analyzed for mRNA and/or protein expression of β1D-integrin, melusin, Akt, GSK3β, muscle LIM protein (MLP), four-and-a-half LIM protein 2 (fhl2), desmin, and calpain. Upon AVB, increased ejection strain (0.29 ± 0.01 vs. 0.13 ± 0.02, SR) and end-diastolic stress (4.8 ± 1.1 vs. 2.7 ± 0.4 kPa) dominated mechanical changes. Brain natriuretic peptide plasma levels were correspondingly high (33 ± 4 vs. 19 ± 1 pg/ml, SR). β1D-Integrin protein expression increased chronically after AVB. Melusin was temporarily overexpressed (+33 ± 9%, 3 days AVB vs. SR), followed by elevated ratios of phosphorylated (P)-Akt to Akt and P-GSK3β to GSK3β (+26 ± 6% and +30 ± 8% at 10 days AVB vs. SR). These changes corresponded to peak hypertrophic growth at 3 to 10 days. MLP increased gradually to maxima at chronic AVB (+36 ± 7%). In contrast, fhl2 (−22 ± 3%, 3 days) and desmin (−30 ± 9%, 10 days AVB) transiently declined but recovered at chronic AVB. Calpain protein expression remained unaltered. In conclusion, volume overload after AVB causes a transient compromise of cytoskeletal integrity based, at least partly, on transcriptional downregulation. Subsequent cytoskeletal reorganization coincides with the upregulation of melusin, P-Akt, P-GSK3β, and MLP, indicating a strong drive to compensated hypertrophy.

Regional cardiac dysfunction is associated with specific alterations in inflammatory cytokines and matrix metalloproteinases after acute myocardial infarction in sheep

Cardiac remodeling following myocardial infarction (MI) is a maladaptive process, fundamental to the progression of ischemic heart failure. The extent of remodeling is influenced by mechanical stress, inflammatory response and activation of matrix metalloproteinases (MMPs). This study examined regional association between these parameters in response to acute MI. Coronary ligation was performed in ten sheep. Sonomicrometer transducers measured segmental length in the infarcted, border and non-infarcted region. Regional tissue samples obtained 3 h post MI from six sheep were analysed using RT-PCR, gelatin zymography and Western blot. Six sham-operated sheep served as controls.Region-specific dilation and reduced contraction was associated with corresponding alterations in matrix molecules.IL-6 and MMP-9 mRNA were increased in the infarcted and border regions compared to controls.MMP-2 and TIMP-1 mRNA increased in non-infarcted myocardium and both correlated positively with segmental shortening. IL-6 mRNA levels, in contrast, were negatively associated with segmental shortening. In summary, inflammatory cytokines and MMPs are altered early after MI in a region-specific manner, and these changes correspond to acute regional myocardial dysfunction. Therapies for LV remodeling from the time of reperfusion may benefit from further understanding this portfolio of acute alterations.