Cardiac magnetic resonance features of left dominant arrhythmogenic cardiomyopathy: differential diagnosis with myocarditis

Objectives To identify potential imaging features at cardiac magnetic resonance (CMR) specific for left-dominant arrhythmogenic cardiomyopathy (LDAC) diagnosis. Materials and methods Between January 2011 and May 2016, we considered 36 consecutive stable patients with a recent diagnosis of significant VA and ECG morphology consistent with a LV origin, detection of potential LV arrhythmic substrate at CMR, undergoing a clinically-indicated LV endomyocardial biopsy. Exclusion criteria were history of known cardiac disease, contraindications to CMR and impaired CMR image quality. After application of these criteria, in 9 patients endomyocardial biopsy showed tissue abnormalities consistent with the diagnosis of LDAC. From the same CMR-endomyocardial biopsy registry, a second group of 9 consecutive patients with a histological diagnosis of previous myocarditis were identified. Results Mid-wall LGE in the interventricular septum was detected in 5 myocarditis, without findings in LDAC group (p=0.03), whereas subepicardial LGE at the level of posterolateral wall of LV was detected in 8 cases of LDAC vs. 2 cases of myocarditis (p=0.02). Fat infiltration, and particularly subepicardial posterolateral fat infiltration, was found in all LDAC patients vs. one myocarditis only (p<0.01). No differences in other CMR findings or in any clinical or echocardiographic parameters were found between patients with a biopsy consistent with LDAC vs. patients in whom biopsy suggested myocarditis. Conclusions In patients with significant VA and ECG morphology consistent with a LV origin, identification of morpho-functional involvement of the subepicardial layer of LV posterolateral wall at CMR (LGE, fat infiltration, wall dyskinesis) is consistent with a diagnosis of LDAC. Funding Acknowledgement Type of funding source: None

Interaction of phase singularities on the spiral wave tail: reconsideration of capturing the excitable gap

The action mechanism of stimulation toward spiral waves (SWs) owing to the complex excitation patterns that occur just after point stimulation has not yet been experimentally clarified. This study sought to test our hypothesis that the effect of capturing excitable gap of SWs by stimulation can also be explained as the interaction of original phase singularity (PS) and PSs induced by the stimulation on the wave tail (WT) of the original SW. Phase variance analysis was used to quantitatively analyze the postshock PS trajectories. In a two-dimensional subepicardial layer of Langendorff-perfused rabbit hearts, optical mapping was used to record the excitation pattern during stimulation. After a SW was induced by S1–S2 shock, single biphasic point stimulation S3 was applied. In 70 of the S1-S2-S3 stimulation episodes applied on 6 hearts, the original PS was clearly observed just before the S3 point stimulation in 37 episodes. Pairwise PSs were newly induced by the S3 in 20 episodes. The original PS collided with the newly induced PSs in 16 episodes; otherwise, they did not interact with the original PS. SW shift occurred most efficiently when the S3 shock was applied at the relative refractory period, and PS shifted in the direction of the WT. In conclusion, quantitative tracking of PS clarified that stimulation in desirable conditions induces pairwise PSs on WT and that the collision of PSs causes SW shift along the WT. The results of this study indicate the importance of the interaction of shock-induced excitation with the WT for effective stimulation. NEW & NOTEWORTHY The quantitative analysis of spiral wave dynamics during stimulation clarified the action mechanism of capturing the excitable gap, i.e., the induction of pairwise phase singularities on the wave tail and spiral wave shift along the wave tail as a result of these interactions. The importance of the wave tail for effective stimulation was revealed.

Selective subepicardial localization of monocyte subsets in response to progressive coronary artery constriction

Following myocardial infarction and atherosclerotic lesion development, monocytes contribute to myocardial protection and repair, while also partaking in myocardial ischemic injury. The balance of proinflammatory and reparative monocyte subsets is crucial in governing these therapeutic and pathological outcomes. Myocardial ischemic damage displays heterogeneity across the myocardium, whereby the subendocardium shows greatest vulnerability to ischemic damage. In this study we examined the transmural distribution of monocyte subsets in response to gradual coronary artery occlusion. CD14+monocytes were isolated from peripheral blood of New Zealand White rabbits and divided into two subgroups based on the expression of CD62L. We employed a rabbit model of progressive coronary artery obstruction to induce chronic myocardial ischemia and reinfused fluorescently labeled autologous monocytes. The distribution of fluorescently labeled autologous monocytes was examined with a high-resolution three-dimensional imaging cryomicrotome. The subepicardial layer contained the largest infiltration of both monocyte subgroups, with a significantly greater proportion of CD14+CD62L+monocytes at the time when the ischemic area was at a maximum. By targeting CD13+angiogenic vessels, we confirmed the presence of angiogenesis in epicardial and midmyocardial regions. These myocardial regions demonstrated the highest level of infiltration of both monocyte subsets. Furthermore, CD14+CD62L+monocytes showed significantly greater migration towards monocyte chemoattractant protein-1, greater adhesive capacity, and higher expression of C-C chemokine receptor type-2 relative to CD14+CD62L−monocytes. In conclusion, we note selective subepicardial distribution of monocyte subpopulations, with changes in proportion depending on the time after onset of coronary narrowing. Selective homing is supported by divergent migratory properties of each respective monocyte subgroup.

Abstract 12721: Compensatory Mechanism in HCM Patients With Preserved LV Ejection Fraction Determined by 2D Speckle Tracking Echocardiographic LV Myocardial Strain Gradient Using a Novel Multi-Layer Technique

Introduction: A novel multi-layer technique on speckle tracking transthoracic echocardiography (TTE) can achieve quantitative strain measurements of all, subendocardial, and subepicardial layer of left ventricle (LV), respectively. Aim: To evaluate compensatory mechanism in hypertrophic cardiomyopathy (HCM) subjects with preserved LV ejection fraction (EF), we measured myocardial strain gradient by 2D LV global longitudinal (GLS) and circumferential strain (GCS) using a multi-layer technique on speckle tracking TTE. Methods: 60 subjects (40 HCM (30 male; age 62 ± 15 years, LV EF >50%, and 20 age matched controls (10 male; age 59 ± 10 years)) underwent TTE (Vivid E9, GE). Apical 4-, 2-, 3-chamber views for GLS and parasternal short-axis views at the level of mitral valve, papillary muscle, and apex for GCS were acquired. We defined myocardial strain gradient as the ratio of subendocardial strain to subepicardial strain. Results: GLS from each view were significantly smaller in HCM than controls in all, subendocardial, and subepicardial layers. GCS at all levels were significantly smaller in HCM than controls in all and subepicardial layer; however there were no significant differences in subendocardial GCS between both groups at mitral valve and papillary muscle levels. GLS gradient was significant greater in HCM than controls in GLS from 4 chamber views. GCS gradients were significantly greater in HCM than in controls at all levels. In HCM, subendocardial GCS at mitral valve and papillary muscle levels significantly positive correlated with LV EF (R=0.446, 0.34, respectively) and subendocardial GLS from 4-, and 2- chamber views, and subendocardial GCS at mitral valve and papillary muscles levels significantly negatively and GCS gradients at mitral valve and papillary muscle levels significantly positively correlated with interventricular septum thickness in end diastole (R=-0.44, -0.347, -0.335, -0.359, 0.363, and 0.373, respectively). Conclusions: In HCM subjects with preserved LV EF, GLS was significantly smaller than in controls, but GLS and GCS gradients tended to be greater than controls. We speculate that in HCM, subendocardial GCS was maintained in compensation for subendocardial GLS decrement; thus GCS gradient may increase.

Abstract 3491: Endo-Epicardial Dissociation of Electrical Activity Increases During the Development of the Substrate for Atrial Fibrillation in the Goat

Background: The high incidence of transmural conduction of fibrillation waves (breakthroughs) in a complex substrate for atrial fibrillation (AF) implies the presence of electrical dissociation between the subepicardial layer (Epi) and the endocardial bundle network (Endo). The presence of this Endo/Epi dissociation (EED) in remodeled atria and its role in the progressive stabilization of AF over time has not been studied yet. Methods: We developed a mapping tool for synchronous Endo/Epi mapping (spatial resolution 1.6mm) with 90 exactly opposing electrode pairs (open chest experiment). We included 3 groups of goats: C = Control (acute AF induced by 50Hz burst pacing, n=7), 3wk = 3weeks AF (n=7) and 6mo = 6months AF (n=7). Dissociated activity was postulated when either activation times differed by more than 12ms vertically or 8ms horizontally (indicating a local conduction velocity < 20cm/s) or local direction of propagation between Endo and Epi differed by more than 90 degrees. To monitor AF stability, repetitive in-vivo cardioversion experiments with class 1C drugs were performed in 6 of the 6mo goats at 2,6,10 and 14wk AF. Results: Applying the time criterion, EED increased from 15±4% (C) to 22±11% (3wk) and 35±13% (6mo, p=0.002 vs. C). Also the differences in the direction of propagation significantly contributed to EED. Using the combined criterion, EED increased from 38±5% (C) to 46±10% (3wk) and 53±11% (6mo, p=0.007 vs. C). Dissociation within the epicardial and the endocardial layer (time criterion) increased to a comparable extent (19±8% vs. 27±14% vs. 37± 7%, p<0.001 C vs. 6mo). Mean Endo/Epi activation time differences were close to 0ms in all three groups (−1.0±15ms vs. −0.8±16ms vs. −0.3±20ms), ruling out preferential conduction from Endo to Epi or vice versa. Success rate of cardioversion experiments decreased from 83% (2w) to 33% (6wk) to 16% (10wk) to 0% (14wk) indicating increasing stability of AF over time. Conclusion: During AF, pronounced EED occurs. EED (like dissociation within Endo and Epi) increases over time, contributing to the progressive stabilization of AF. Enhanced EED might explain the high incidence of transmural conduction (breakthroughs) in a complex substrate for AF.

Heterogeneous fatty acid uptake early after reperfusion in rat hearts

We determined whether spatial distributions of substrate uptake are heterogeneous within the area at risk during reperfusion. Quantitative autoradiography with imaging plates and two long-lived radioisotopes was applied to 15 open-chest, anesthetized rats subjected to 30 min of coronary artery ligation and 30 min of reperfusion. Regions showing increased β-methyl-[1-14C]heptadecanoic acid ([14C]BMHDA) uptake (166 ± 17% of that in the nonischemic area) appeared at the lateral borders and subepicardial layer within the area at risk, and 2-deoxy-d-[1-3H]glucose ([3H]DG) uptake was 103 ± 24% in these regions. Regions with decreased [14C]BMHDA uptake (28 ± 11%) occupied the midmyocardial layer except at the lateral borders within the area at risk, and [3H]DG uptake was 62 ± 18% in these regions. The percentage interregional coefficientsof variation (index of heterogeneity) in [14C]BMHDA uptake, [3H]DG uptake, and blood flow were higher in the area at risk than in the nonischemic area (76 ± 23 vs. 21 ± 7%, 39 ± 10 vs. 21 ± 7%, and 49 ± 19 vs. 14 ± 4%, respectively). Heterogeneous distributions of substrate uptake may explain the conflicting results concerning substrate metabolism during reperfusion.

Coronary hyperperfusion augments myocardial oxygen consumption

This study was performed to test the hypothesis that increases in myocardial oxygen consumption (MVo2) and myocardial contractile function during exercise are flow limited. Studies were performed in 15 chronically instrumented normal dogs. MVo2 and regional percent systolic wall thickening were measured during control conditions and during maximal vasodilation produced by infusion of adenosine (20-75 micrograms.kg-1.min-1) or adenosine combined with nitroglycerin (0.4 micrograms.kg-1.min-1; TNG) into the left anterior descending coronary artery during a three-stage graded treadmill exercise protocol. Adenosine and adenosine plus TNG significantly increased coronary blood flow by 298 +/- 26 and 306 +/- 24%, respectively, at rest and by 134 +/- 7 and 145 +/- 9%, respectively, during the heaviest level of exercise (each P < 0.01). Adenosine and adenosine plus TNG increased MVo2 at rest, but this was associated with a parallel increase in heart rate, so that MVo2 per beat was not significantly changed. Systolic wall thickening was also not changed by hyperperfusion during resting conditions. However, MVo2 per beat was increased by 12 +/- 4% with adenosine and by 13 +/- 5% with adenosine plus TNG during moderate exercise and by 23 +/- 5% with adenosine and by 27 +/- 4% with adenosine plus TNG during the heaviest level of exercise (each P < 0.05). Systolic thickening of the full left ventricular wall did not change during hyperperfusion, but thickening in the subepicardial layer was increased by 14 +/- 3% with adenosine and 18 +/- 3% with adenosine plus TNG during the heaviest level of exercise (each P < 0.05). There was no difference in wall thickening between adenosine and adenosine plus TNG. These findings imply that the increases in MVo2 which occur during exercise are limited by coronary blood flow.

Transmural bioenergetic responses of normal myocardium to high workstates

The response of myocardial high-energy and inorganic phosphates (HEP and Pi, respectively) and associated changes in myocardial blood flow, lactate uptake, and O2 consumption (MVo2) rates were examined in an open-chest canine model during progressively increasing workloads achieved by catecholamine infusion. HEP and Pi levels (measured with transmurally localized 31P-nuclear magnetic resonance spectroscopy) were unaffected by moderate increases in the level of energy expenditure but were significantly altered by high workloads, especially in the subepicardium. The MVo2 and HEP data from three different protocols that utilized pharmacological augmentation of blood flow demonstrated that the maximal rate of myocardial energy production during inotropic stimulation was dictated by perfusion limitation. This limitation was more severe in the subepicardial layer at the high workloads despite equivalent or even higher increases in blood flow to this layer, reflecting a preferential enhancement of demand in the outer layer by catecholamines. In contrast, under basal conditions, existence of a marginal perfusion limitation was evident in the inner but not in the outer layer.

Defects in heart and lung development in compound heterozygotes for two different targeted mutations at the N-myc locus

Two types of mutant allele, one leaky and one null, have been generated by gene targeting at the N-myc locus in embryonic stem cells and the phenotypes of mice homozygous for these mutations have been described. These mutations have shown that N-myc has a number of functions during development, including a role in branching morphogenesis in the lung, which manifests itself at birth in mice homozygous for the leaky allele, and roles in the development of the mesonephric tubules, the neuroepithelium, the sensory ganglia, the gut and the heart, which become evident at midgestation in embryos homozygous for the null allele. In an attempt to define roles for N-myc at other stages of development, we have combined the two types of N-myc mutant allele in a compound heterozygote that as a result contains approximately 15% of normal levels of N-Myc protein. Compound heterozygotes died during gestation at a time intermediate to the times of death of embryos homozygous for either mutation individually, and their death appeared to result from cardiac failure stemming from hypoplasia of the compact subepicardial layer of the myocardium. Investigation of the expression pattern of N-myc and various markers of differentiation in wild-type and compound heterozygote mutant hearts has suggested that N-myc may function in maintaining the proliferation and/or preventing the differentiation of compact layer myocytes. This study illustrates the importance of generating different mutations at a given locus to elucidate fully the function of a particular gene during development.