Emilio Macchi

Potential fields on the ventricular surface of the exposed dog heart during normal excitation.

We studied the normal spread of excitation on the anterior and posterior ventricular surface of open-chest dogs by recording unipolar electrograms from an array of 1124 electrodes spaced 2 mm apart. The array had the shape of the ventricular surface of the heart. The electrograms were processed by a computer and displayed as epicardial equipotential maps at 1-msec intervals. Isochrone maps also were drawn. Several new features of epicardial potential fields were identified: (1) a high number of breakthrough points; (2) the topography, apparent widths, velocities of the wavefronts and the related potential drop; (3) the topography of positive potential peaks in relation to the wavefronts. Fifteen to 24 breakthrough points were located on the anterior, and 10 to 13 on the posterior ventricular surface. Some were in previously described locations and many others in new locations. Specifically, 3 to 5 breakthrough points appeared close to the atrioventricular groove on the anterior right ventricle and 2 to 4 on the posterior heart aspect; these basal breakthrough points appeared when a large portion of ventricular surface was still unexcited. Due to the presence of numerous breakthrough points on the anterior and posterior aspect of the heart which had not previously been described, the spread of excitation on the ventricular surface was “mosaic-like,” with activation wavefronts spreading in all directions, rather than radially from the two breakthrough points, as traditionally described. The positive potential peaks which lay ahead of the expanding wavefronts moved along preferential directions which were probably related to the myocardial fiber direction.

Potential fields generated by oblique dipole layers modeling excitation wavefronts in the anisotropic myocardium. Comparison with potential fields elicited by paced dog hearts in a volume conductor.

The potential distribution in a homogeneous, cylindrical volume conductor surrounding an isolated paced dog heart was first measured and then calculated by using a mathematical model that simulates an anisotropic excitation wavefront spreading through the heart muscle. The study was performed with a view to establish to what extent the anisotropy of cardiac generators affects the potential field in the extra-cardiac conducting media at a great distance from the heart. The model considers an oblique dipole layer on the wavefront which, assuming axial symmetry of the electrical properties of the fibers, can be viewed as the superposition of an axial and a transverse dipole layer. These layers are, respectively, parallel and perpendicular to the local fiber direction. A notable feature of the model is that, in the case of axial symmetry, the potential field due to such an oblique distribution is also equivalent to the sum of the potentials generated, respectively, by a normal and an axial dipole layer. In this form, the model generalizes the classical, uniform double layer model, upon which the solid angle theory is based, by adding to it an axial component. The features of the measured potential fields, which could not be interpreted on the basis of the solid angle theory, were satisfactorily reproduced by the model, at least on a qualitative basis. The results clearly showed the dominant role played by the axial component of the potential field even at a considerable distance from the heart.

Nε-lysine acetylation determines dissociation from GAP junctions and lateralization of connexin 43 in normal and dystrophic heart

Wanting to explore the epigenetic basis of Duchenne cardiomyopathy, we found that global histone acetylase activity was abnormally elevated and the acetylase P300/CBP-associated factor (PCAF) coimmunoprecipitated with connexin 43 (Cx43), which was Nε-lysine acetylated and lateralized in mdx heart. This observation was paralleled by Cx43 dissociation from N-cadherin and zonula occludens 1, whereas pp60-c-Src association was unaltered. In vivo treatment of mdx with the pan-histone acetylase inhibitor anacardic acid significantly reduced Cx43 Nε-lysine acetylation and restored its association to GAP junctions (GJs) at intercalated discs. Noteworthy, in normal as well as mdx mice, the class IIa histone deacetylases 4 and 5 constitutively colocalized with Cx43 either at GJs or in the lateralized compartments. The class I histone deacetylase 3 was also part of the complex. Treatment of normal controls with the histone deacetylase pan-inhibitor suberoylanilide hydroxamic acid (MC1568) or the class IIa-selective inhibitor 3-{4-[3-(3-fluorophenyl)-3-oxo-1-propen-1-yl]-1-methyl-1H-pyrrol-2-yl}-N-hydroxy-2-propenamide (MC1568) determined Cx43 hyperacetylation, dissociation from GJs, and distribution along the long axis of ventricular cardiomyocytes. Consistently, the histone acetylase activator pentadecylidenemalonate 1b (SPV106) hyperacetylated cardiac proteins, including Cx43, which assumed a lateralized position that partly reproduced the dystrophic phenotype. In the presence of suberoylanilide hydroxamic acid, cell to cell permeability was significantly diminished, which is in agreement with a Cx43 close conformation in the consequence of hyperacetylation. Additional experiments, performed with Cx43 acetylation mutants, revealed, for the acetylated form of the molecule, a significant reduction in plasma membrane localization and a tendency to nuclear accumulation. These results suggest that Cx43 Nε-lysine acetylation may have physiopathological consequences for cell to cell coupling and cardiac function.

Epicardial and intramural excitation during ventricular pacing: effect of myocardial structure.

Published studies show that ventricular pacing in canine hearts produces three distinct patterns of epicardial excitation: elliptical isochrones near an epicardial pacing site, with asymmetric bulges; areas with high propagation velocity, up to 2 or 3 m/s and numerous breakthrough sites; and lower velocity areas (<1 m/s), where excitation moves across the epicardial projection of the septum. With increasing pacing depth, the magnitude of epicardial potential maxima becomes asymmetric. The electrophysiological mechanisms that generate the distinct patterns have not been fully elucidated. In this study, we investigated those mechanisms experimentally. Under pentobarbital anesthesia, epicardial and intramural excitation isochrone and potential maps have been recorded from 22 exposed or isolated dog hearts, by means of epicardial electrode arrays and transmural plunge electrodes. In five experiments, a ventricular cavity was perfused with diluted Lugol solution. The epicardial bulges result from electrotonic attraction from the helically shaped subepicardial portions of the wave front. The high-velocity patterns and the associated multiple breakthroughs are due to involvement of the Purkinje network. The low velocity at the septum crossing is due to the missing Purkinje involvement in that area. The asymmetric magnitude of the epicardial potential maxima and the shift of the breakthrough sites provoked by deep stimulation are a consequence of the epi-endocardial obliqueness of the intramural fibers. These results improve our understanding of intramural and epicardial propagation during premature ventricular contractions and paced beats. This can be useful for interpreting epicardial maps recorded at surgery or inversely computed from body surface ECGs.

The histone deacetylase inhibitor suberoylanilide hydroxamic acid reduces cardiac arrhythmias in dystrophic mice

AIMS The effect of histone deacetylase inhibitors on dystrophic heart function is not established. To investigate this aspect, dystrophic mdx mice and wild-type (WT) animals were treated 90 days either with suberoylanilide hydroxamic acid (SAHA, 5 mg/kg/day) or with an equivalent amount of vehicle. METHODS AND RESULTS The following parameters were evaluated: (i) number of ventricular arrhythmias in resting and stress conditions (restraint test) or after aconitine administration; (ii) cardiac excitability, conduction velocity, and refractoriness; (iii) expression and distribution of connexins (Cxs) and Na(v)1.5 sodium channel. Ventricular arrhythmias were negligible in all resting animals. During restraint, however, an increase in the number of arrhythmias was detected in vehicle-treated mdx mice (mdx-V) when compared with SAHA-treated mdx (mdx-SAHA) mice or normal control (WT-V). Interestingly, aconitine, a sodium channel pharmacologic opener, induced ventricular arrhythmias in 83% of WT-V mice, 11% of mdx-V, and in 57% of mdx-SAHA. Epicardial multiple lead recording revealed a prolongation of the QRS complex in mdx-V mice in comparison to WT-V and WT-SAHA mice, paralleled by a significant reduction in impulse propagation velocity. These alterations were efficiently counteracted by SAHA. Molecular analyses revealed that in mdx mice, SAHA determined Cx remodelling of Cx40, Cx37 and Cx32, whereas expression levels of Cx43 and Cx45 were unaltered. Remarkably, Cx43 lateralization observed in mdx control animals was reversed by SAHA treatment which also re-induced Na(v)1.5 expression. CONCLUSION SAHA attenuates arrhythmias in mdx mice by a mechanism in which Cx remodelling and sodium channel re-expression could play an important role.

Ventricular activation is impaired in aged rat hearts.

Ventricular arrhythmias are frequently observed in the elderly population secondary to alterations of electrophysiological properties that occur with the normal aging process of the heart. However, the underlying mechanisms remain poorly understood. The aim of the present study was to determine specific age-related changes in electrophysiological properties and myocardial structure in the ventricles that can be related to a structural-functional arrhythmogenic substrate. Multiple unipolar electrograms were recorded in vivo on the anterior ventricular surface of four control and seven aged rats during normal sinus rhythm and ventricular pacing. Electrical data were related to morphometric and immunohistochemical parameters of the underlying ventricular myocardium. In aged hearts total ventricular activation time was significantly delayed (QRS duration: +69%), while ventricular conduction velocity did not change significantly compared with control hearts. Moreover, ventricular activation patterns displayed variable numbers of epicardial breakthrough points whose appearance could change with time. Morphological analysis in aged rats revealed that heart weight and myocyte transverse diameter increased significantly, scattered microfoci of interstitial fibrosis were mostly present in the ventricular subendocardium, and gap junction connexin expression decreased significantly in ventricular myocardium compared with control rats. Our results show that in aged hearts delayed total ventricular activation time and abnormal activation patterns are not due to delayed myocardial conduction and suggest the occurrence of impaired impulse propagation through the conduction system leading to uncoordinated myocardial excitation. Impaired interaction between the conduction system and ventricular myocardium might create a potential reentry substrate, contributing to a higher incidence of ventricular arrhythmias in the elderly population.

Epicardial excitation during ventricular pacing. Relative independence of breakthrough sites from excitation sequence in canine right ventricle.

In a previous investigation, epicardial recordings with 1,124 closely spaced electrodes revealed 20-35 breakthrough (BKT) sites and an equal number of separate wave fronts on the ventricular surface of exposed dog hearts during normal sinus rhythm. In the present study we tried 1) to determine whether ventricular pacing also produced multiple BKTs and wave fronts and 2) to determine whether the number and location of BKTs were, to some degree, independent of pacing site. The study mainly focused on right ventricular BKTs observed during right ventricular pacing. To test hypotheses 1 and 2 we identified many breakthrough sites during sinus rhythm in seven exposed dog hearts and then paced the heart from several BKT and non-BKT sites on the right ventricle. Epicardial potential maps and excitation time maps were obtained by using 1,124 epicardial electrodes covering the anterior right ventricle and part of the anterior left ventricle. A primary wave front spread radially for several centimeters from the pacing site, and no BKTs appeared in the areas covered by the primary wave front. In the remaining areas (secondary areas), multiple BKTs appeared; their number was close to that observed during sinus rhythm in the same areas (113 versus 115, respectively, in 12 paced beats). The majority of paced BKTs (83 out of 115, or 72%) occurred exactly at the same locations where they appeared during sinus rhythm. However, 30 right ventricular BKTs observed during sinus rhythm disappeared in the secondary areas and were replaced by approximately the same number of new BKTs. Many areas without BKTs in normal beats remained so in paced beats.(ABSTRACT TRUNCATED AT 250 WORDS)

Body surface maps in left bundle branch block uncomplicated or complicated by myocardial infarction, left ventricular hypertrophy or myocardial ischemia☆

We provided a topographic and quantitative description of body surface maps (BSM) during the entire QRST interval in seven uncomplicated LBBBs and 31 LBBBs complicated by: myocardial infarction (MI, seven cases), left ventricular hypertrophy (LVH, eight cases), myocardial ischemia (IS, seven cases), MI + LVH (six cases) and LVH + IS (three cases). In all patients we observed abnormal map configurations attributable to the LBBB. We were unable to identify consistent effects of the complicating heart condition(s) on the general pattern of chest potentials. Conversely, the surface voltages were generally decreased by MI and IS and increased by LVH. By considering the 38 patients as a preliminary learning set we applied a stepwise discriminant analysis to 77 voltage-related variables derived from BSM to produce a model for discriminating between LBBBs with and without MI. We properly allocated more than 90% of the patients. We also attempted to classify the patients into four groups: pure LBBB, LBBB + MI, LBBB + LVH and LBBB + IS, with a percentage of correct classification of about 80%. The two classifying procedures were applied to ten new LBBB patients with results similar to those obtained in the 38 of the study group.

Differential Structural Remodeling of the Left‐Atrial Posterior Wall in Patients Affected by Mitral Regurgitation with or Without Persistent Atrial Fibrillation: A Morphological and Molecular Study

Structural Remodeling in Atrial Fibrillation. Introduction: Atrial fibrillation (AF) in mitral regurgitation (MR) is a complex disease where multiple factors may induce left‐atrial structural remodeling (SR). We explored the differential SR of the left‐atrial posterior wall (LAPW) of patients affected by MR with or without persistent AF, and the expression of key proteins involved in its pathogenesis.

Structural and electrical myocardial remodeling in a rodent model of depression

Objective Despite a well-documented association between stress and depression with cardiac morbidity and mortality, there is no satisfactory explanation for the mechanisms linking affective and cardiac disorders. This study investigated cardiac electrophysiological properties in an animal model of depression. Methods Depression-relevant physiological and behavioral parameters were measured in adult male wild-type rats during and after a period of intermittent social defeat stress (n = 12) or empty cage exposure (control, n = 11). Nine days after the last defeat/empty cage exposure, high-definition epicardial mapping was performed under anesthesia. Results Stressed animals versus controls displayed a larger reduction in the circadian amplitude of heart rate (−32% [3%] versus −13 [2%]; p = .001) and body temperature (−33% [4%] versus −5% [2%]; p = .001) rhythms, had smaller body weight gain (+11% [1%] versus +17% [1%]; p < .001), and showed a larger reduction in sucrose solution intake (−19% [6%] versus −7% [4%]; p = .006). Epicardial mapping analysis revealed a decrease in the transversal conduction velocity of the wavefront (0.23 [0.0] versus 0.27 [0.1] m/s; p = .02) and a shortening of the effective refractory period (86.8 [2.1] versus 95.9 [3.0] milliseconds; p = .01) in stressed animals. Upon killing, moderate left ventricular fibrosis was observed in the stressed group. Conclusions Intermittent social stress procedure is associated with depression-like symptoms and altered myocardial electrical stability in a potentially proarrhythmic manner. In particular, reduced myocardial refractoriness and impaired conduction, which are considered major determinants of arrhythmogenesis, represent possible mechanisms underlying cardiac vulnerability.