Effects of central sympathetic activation on repolarization-dispersion during short-term myocardial ischemia in anesthetized rats

Air Pollution and Arrhythmias

Air pollution is commonly defined as the contamination of the air we breathe by any chemical, physical, or biological agent that is potentially threatening to human and ecosystem health. The common pollutants known to be disease-causing are particulate matter, ground-level ozone, sulphur dioxide, nitrogen dioxide, and carbon monoxide. Although the association between increasing concentrations of these pollutants and cardiovascular disease is now accepted, the association of air pollution and arrhythmias is less well established. In this review we provide an in-depth discussion of the association of acute and chronic air pollution exposure and arrhythmia incidence, morbidity, and mortality, and the purported pathophysiological mechanisms. Increases in concentrations of air pollutants have multiple proarrhythmic mechanisms including systemic inflammation (via increases in reactive oxygen species, tumour necrosis factor, and direct effects from translocated particulate matter), structural remodelling (via an increased risk of atherosclerosis and myocardial infarction or by affecting the cell-to-cell coupling and gap junction function), and mitochondrial and autonomic dysfunction. Furthermore, we describe the associations of air pollution and arrhythmias. There is a strong correlation of acute and chronic air pollutant exposure and the incidence of atrial fibrillation. Acute increases in air pollution increase the risk of emergency room visits and hospital admissions for atrial fibrillation and the risk of stroke and mortality in patients with atrial fibrillation. Similarly, there is a strong correlation of increases of air pollutants and the risk of ventricular arrhythmias, out-of-hospital cardiac arrest, and sudden cardiac death.

Air Pollution and Cardiac Arrhythmias: From Epidemiological and Clinical Evidences to Cellular Electrophysiological Mechanisms

Cardiovascular disease is the leading cause of death worldwide and kills over 17 million people per year. In the recent decade, growing epidemiological evidence links air pollution and cardiac arrhythmias, suggesting a detrimental influence of air pollution on cardiac electrophysiological functionality. However, the proarrhythmic mechanisms underlying the air pollution-induced cardiac arrhythmias are not fully understood. The purpose of this work is to provide recent advances in air pollution-induced arrhythmias with a comprehensive review of the literature on the common air pollutants and arrhythmias. Six common air pollutants of widespread concern are discussed, namely particulate matter, carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen dioxide, and ozone. The epidemiological and clinical reports in recent years are reviewed by pollutant type, and the recently identified mechanisms including both the general pathways and the direct influences of air pollutants on the cellular electrophysiology are summarized. Particularly, this review focuses on the impaired ion channel functionality underlying the air pollution-induced arrhythmias. Alterations of ionic currents directly by the air pollutants, as well as the alterations mediated by intracellular signaling or other more general pathways are reviewed in this work. Finally, areas for future research are suggested to address several remaining scientific questions.

Endothelin receptors in the brain modulate autonomic responses and arrhythmogenesis during acute myocardial infarction in rats

AIMS

Endothelin has been implicated in various processes in the brain, including the modulation of sympathetic responses. The present study examined the pathophysiologic role of brain endothelin-receptors in the setting of acute myocardial infarction, characterized by high incidence of ventricular tachyarrhythmias.

MAIN METHODS

We investigated the effects of intracerebroventricular administration of antagonists of endothelin-receptors ET_A, ET_B, or both, during a 24 h-observation period post-coronary ligation in (n = 70) rats. Continuous recording was performed via implanted telemetry transmitters, followed by arrhythmia-analysis and calculation of autonomic indices derived from heart rate variability. The regional myocardial electrophysiologic properties were assessed by monophasic action potentials and multi-electrode recordings.

KEY FINDINGS

Sympathetic-activity was decreased and vagal-activity was enhanced after intracerebroventricular ET_A-receptor blockade, thus attenuating regional myocardial repolarization inhomogeneity. As a result, the incidence of ventricular tachyarrhythmias was markedly lower in this group. Such effects were also observed after intracerebroventricular blockade of ET_B-, or both, ET_A- and ET_B-receptors, although to a lesser extent.

SIGNIFICANCE

ET_A-receptors in the brain modulate sympathetic and vagal responses and alter arrhythmogenesis during evolving myocardial necrosis in rats. These findings provide insights into arrhythmogenic mechanisms during acute myocardial infarction and call for further investigation on the role of endothelin in the central autonomic network.

Medium-term Electrophysiologic Effects of a Cellularized Scaffold Implanted in Rats After Myocardial Infarction

Background Cardiac repair strategies are being evaluated for myocardial infarctions, but the safety issues regarding their arrhythmogenic potential remain unresolved. By utilizing the in-vivo rat model, we have examined the medium-term electrophysiologic effects of a biomaterial scaffold that has been cellularized with spheroids of human adipose tissue, derived from mesenchymal stem cells and umbilical vein endothelial cells. Methods Mesenchymal stem cells, which exhibit adequate differentiation capacity, were co-cultured with umbilical vein endothelial cells and were seeded on an alginate based scaffold. After in-vitro characterization, the cellularized scaffold was implanted in (n=15) adult Wistar rats 15 min post ligation of the left coronary artery, with an equal number of animals serving as controls. Two weeks thereafter, monophasic action potentials were recorded and activation-mapping was performed with a multi-electrode array. An arrhythmia score for inducible ventricular tachyarrhythmias was calculated after programmed electrical stimulation. Results The arrhythmia score was comparable between the treated animals and controls. No differences were detected in the local conduction at the infarct border and in the voltage rise in monophasic action potential recordings. Treatment did not affect the duration of local repolarization, but tended to enhance its dispersion. Conclusions The fabricated bi-culture cellularized scaffold displayed favorable properties after in-vitro characterization. Medium-term electrophysiologic assessment after implantation in the infarcted rat myocardium revealed low arrhythmogenic potential, but the long-term effects on repolarization dispersion will require further investigation.

Spatial indices of repolarization correlate with non-ST elevation myocardial ischemia in patients with chest pain

Mild-to-moderate ischemia does not result in ST segment elevation on the electrocardiogram (ECG), but rather non-specific changes in the T wave, which are frequently labeled as non-diagnostic for ischemia. Robust methods to quantify such T wave heterogeneity can have immediate clinical applications. We sought to evaluate the effects of spontaneous ischemia on the evolution of spatial T wave changes, based on the eigenvalues of the spatial correlation matrix of the ECG, in patients undergoing nuclear cardiac imaging for evaluating intermittent chest pain. We computed T wave complexity (TWC), the ratio of the second to the first eigenvalue of repolarization, from 5-min baseline and 5-min peak-stress Holter ECG recordings. Our sample included 30 males and 20 females aged 63 ± 11 years. Compared to baseline, significant changes in TWC were only seen in patients with ischemia (n = 10) during stress testing, but not among others. The absolute changes in TWC were significantly larger in the ischemia group compared to others, with a pattern that seemed to depend on the severity or anatomic distribution of ischemia. Our results demonstrate that ischemia-induced changes in T wave morphology can be meaningfully quantified from the surface 12-lead ECG, suggesting an important opportunity for improving diagnostics in patients with chest pain.

Prolonged intra-myocardial growth hormone administration ameliorates post-infarction electrophysiologic remodeling in rats

Experimental studies indicate improved ventricular function after treatment with growth hormone (GH) post-myocardial infarction, but its effect on arrhythmogenesis is unknown. Here, we assessed the medium-term electrophysiologic remodeling after intra-myocardial GH administration in (n = 33) rats. GH was released from an alginate scaffold, injected around the ischemic myocardium after coronary ligation. Two weeks thereafter, ventricular tachyarrhythmias were induced by programmed electrical stimulation. Monophasic action potentials were recorded from the infarct border, coupled with evaluation of electrical conduction and repolarization from a multi-electrode array. The arrhythmia score was lower in GH-treated rats than in alginate-treated rats or controls. The shape and the duration of the action potential at the infarct border were preserved, and repolarization-dispersion was attenuated after GH; moreover, voltage rise was higher and activation delay was shorter. GH normalized also right ventricular parameters. Intra-myocardial GH preserved electrical conduction and repolarization-dispersion at the infarct border and decreased the incidence of induced tachyarrhythmias in rats post-ligation. The long-term antiarrhythmic potential of GH merits further study.

Local conduction during acute myocardial infarction in rats: Interplay between central sympathetic activation and endothelin

We investigated the effects of autonomic dysfunction and endothelin on local conduction and arrhythmogenesis during myocardial infarction. We recorded ventricular tachyarrhythmias, monophasic action potentials, and activation sequences in wild-type and ET_B-deficient rats displaying high endothelin levels. Central sympathetic inputs were examined after clonidine administration. Clonidine mitigated early and delayed arrhythmogenesis in ET_B-deficient and wild-type rats, respectively. The right ventricular activation delay increased in clonidine-treated ET_B-deficient rats and slightly decreased in wild-type rats. The left ventricular voltage rise decreased in all groups, whereas the activation delay increased mainly in clonidine-treated ET_B-deficient rats. Central sympathetic activation and endothelin modulate ischemia-induced arrhythmogenesis. Ischemia alters excitability, whereas endothelin impairs local conduction, an action partly counterbalanced by central sympathetic activity.