Increased mechanically-induced ectopy in the hypertrophied heart

Assessing Post-TAVR Cardiac Conduction Abnormalities Risk Using a Digital Twin of a Beating Heart

Transcatheter aortic valve replacement (TAVR) has rapidly displaced surgical aortic valve replacement (SAVR). However, certain post-TAVR complications persist, with cardiac conduction abnormalities (CCA) being one of the major ones. The elevated pressure exerted by the TAVR stent onto the conduction fibers situated between the aortic annulus and the His bundle, in proximity to the atrioventricular (AV) node, may disrupt the cardiac conduction leading to the emergence of CCA. In his study, an in-silico framework was developed to assess the CCA risk, incorporating the effect of a dynamic beating heart and pre-procedural parameters such as implantation depth and preexisting cardiac asynchrony in the new onset of post-TAVR CCA. A self-expandable TAVR device deployment was simulated inside an electro-mechanically coupled beating heart model in five patient scenarios, including three implantation depths, and two preexisting cardiac asynchronies: (i) a right bundle branch block (RBBB) and (ii) a left bundle branch block (LBBB). Subsequently, several biomechanical parameters were analyzed to assess the post-TAVR CCA risk. The results manifested a lower cumulative contact pressure on the conduction fibers following TAVR for aortic deployment (0.018 MPa) compared to baseline (0.29 MPa) and ventricular deployment (0.52 MPa). Notably, the preexisting RBBB demonstrated a higher cumulative contact pressure (0.34 MPa) compared to the baseline and preexisting LBBB (0.25 MPa). Deeper implantation and preexisting RBBB cause higher stresses and contact pressure on the conduction fibers leading to an increased risk of post-TAVR CCA. Conversely, implantation above the MS landmark and preexisting LBBB reduces the risk.

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

Response of non-failing hypertrophic rat hearts to prostaglandin F2α

Background

Prostaglandin F2α (PGF2α) has a positively inotropic effect on right ventricular (RV) trabeculae from healthy adult rat hearts, and may therefore be therapeutically useful as a non-catecholaminergic inotrope. These provide additional contractile support for the heart without the added energetic demand of increased heart rate, and are also suitable for patients with reduced β adrenergic receptor (β-AR) responsiveness, or impaired mitochondrial energy supply. However, the response of hypertrophied rat hearts to PGF2α has not previously been examined. Our aim was therefore to determine the effect of PGF2α on isolated perfused rat hearts with RV hypertrophy following induction of pulmonary artery hypertension.

Methods

Male Wistar rats (300 g) were injected with either 60 mg kg⁻¹ of monocrotaline (MCT, n = 10) or sterile saline as control (CON, n = 11). Four weeks post injection; hearts were isolated and Langendorff-perfused in sinus rhythm. Measurement of left ventricular (LV) pressure and the electrocardiogram were made and the response to 0.3 μM PGF2α was determined.

Results

PGF2α increased LV developed pressure in CON and in 60% MCT hearts, with no change in heart rate. However, 40% of MCT hearts developed arrhythmias during the peak inotropic response. For comparison, the response to 0.03 μM isoproterenol (ISO) was also investigated. Peak LV pressure developed sooner in response to ISO compared to PGF2α in both rat groups, although the inotropic response to ISO was reduced in MCT hearts. Analysis of fixed ventricular tissue confirmed that only RV myocytes were hypertrophied in MCT hearts. Our study showed that PGF2α was positively inotropic for healthy hearts, but found it generated arrhythmias in 40% of MCT hearts at the dose investigated. However, a more physiological dose of PGF2α may be a useful alternative without the added energetic cost of catecholaminergic inotropes.

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PGF2α elicits a positive inotropic response in isolated, perfused healthy and hypertrophic rat hearts, with no chronotropic effects, unlike β-AR stimulation.

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The dose of 0.3 μM PGF2α investigated also triggered sustained, slow onset, arrhythmic activity in 40% of hypertrophic MCT hearts.

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The peak inotropic response to PGF2α is slower to establish in comparison to the characteristic response to β-AR stimulation, which suggests PGF2α acts via a separate signalling pathway within cardiomyocytes.

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Hypertrophic MCT hearts had a reduced inotropic response to β-AR stimulation, which illustrates the importance of developing non-catecholaminergic inotropes which will eliminate the increased energetic cost and improve myocardial performance.

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).

Mechanically stimulated contraction of engineered cardiac constructs using a microcantilever

The beating heart undergoes cyclic mechanical and electrical activity during systole and diastole. The interaction between mechanical stimulation and propagation of the depolarization wavefront is important for understanding not just normal sinus rhythm, but also mechanically induced cardiac arrhythmia. This study presents a new platform to study mechanoelectrical coupling in a 3-D in vitro model of the myocardium. Cardiomyocytes and cardiac fibroblasts are seeded within extracellular matrix proteins and form constructs constrained by microfabricated tissue gauges that provide in situ measurement of contractile function. The microcantilever of an atomic force microscope is indented into the construct at varying magnitudes and frequencies to cause a coordinated contraction. The results indicate that changes in indentation depth and frequency do not significantly affect the magnitude of contraction, but increasing indentation frequency significantly increases the contractile velocity. Overall, this study demonstrates the validity of this platform as a means to study mechanoelectrical coupling in a 3-D setting, and to investigate the mechanism underlying mechanically stimulated contraction.

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.

Streptomycin inhibits electrophysiological changes induced by stretching of chronically infarcted rat hearts

OBJECTIVE

To investigate stretch-induced electrophysiological changes in chronically infarcted hearts and the effect of streptomycin (SM) on these changes in vivo.

METHODS

Sixty Wistar rats were divided randomly into four groups: a control group (n=15), an SM group (n=15), a myocardial infarction (MI) group (n=15), and an MI+SM group (n=15). Chronic MI was obtained by ligating the left anterior descending branch (LAD) of rat hearts for eight weeks. The in vivo blockade of stretch-activated ion channels (SACs) was achieved by intramuscular injection of SM (180 mg/(kg∙d)) for seven days after operation. The hearts were stretched for 5 s by occlusion of the aortic arch. Suction electrodes were placed on the anterior wall of left ventricle to record the monophasic action potential (MAP). The effect of stretching was examined by assessing the 90% monophasic action potential duration (MAPD90), premature ventricular beats (PVBs), and ventricular tachycardia (VT).

RESULTS

The MAPD90 decreased during stretching in both the control (from (50.27±5.61) ms to (46.27±4.51) ms, P<0.05) and MI groups (from (65.47±6.38) ms to (57.47±5.76 ms), P<0.01). SM inhibited the decrease in MAPD90 during inflation ((46.27±4.51) ms vs. (49.53±3.52) ms, P<0.05 in normal hearts; (57.47±5.76) ms vs. (61.87±5.33) ms, P<0.05 in MI hearts). The occurrence of PVBs and VT in the MI group increased compared with that in the control group (PVB: 7.93±1.66 vs. 1.80±0.86, P<0.01; VT: 7 vs. 1, P<0.05). SM decreased the occurrence of PVBs in both normal and MI hearts (0.93±0.59 vs. 1.80±0.86 in normal hearts, P<0.05; 5.40±1.18 vs. 7.93±1.66 in MI hearts, P<0.01).

CONCLUSIONS

Stretch-induced MAPD90 changes and arrhythmias were observed in chronically infarcted myocardium. The use of SM in vivo decreased the incidence of PVBs but not of VT. This suggests that SACs may be involved in mechanoelectric feedback (MEF), but that there might be other mechanisms involved in causing VT in chronic MI.

Can preload-reducing therapy prevent disease progression in arrhythmogenic right ventricular cardiomyopathy? Experimental evidence and concept for a clinical trial

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited cardiomyopathy and a leading cause of sudden cardiac death in a young population. ARVC is especially common in young athletes. Mutations in different desmosomal genes have been identified causing dysfunctional cell-cell contacts. Reduced myocardial expression of plakoglobin in cell-cell contact complexes appears to associate with disease manifestation in patients harbouring mutations within other cell-cell contact genes. Experimental data suggest that preload reduction may be a simple and effective intervention to prevent disease progression and ventricular arrhythmias in ARVC. This review discusses the potential effects of this innovative approach and describes the design of the first controlled trial of preload-reducing therapy in patients with ARVC.