Potential clinical relevance of the 'little brain' on the mammalian heart

Publication guidelines for human heart rate and heart rate variability studies in psychophysiology-Part 1: Physiological underpinnings and foundations of measurement

This Committee Report provides methodological, interpretive, and reporting guidance for researchers who use measures of heart rate (HR) and heart rate variability (HRV) in psychophysiological research. We provide brief summaries of best practices in measuring HR and HRV via electrocardiographic and photoplethysmographic signals in laboratory, field (ambulatory), and brain-imaging contexts to address research questions incorporating measures of HR and HRV. The Report emphasizes evidence for the strengths and weaknesses of different recording and derivation methods for measures of HR and HRV. Along with this guidance, the Report reviews what is known about the origin of the heartbeat and its neural control, including factors that produce and influence HRV metrics. The Report concludes with checklists to guide authors in study design and analysis considerations, as well as guidance on the reporting of key methodological details and characteristics of the samples under study. It is expected that rigorous and transparent recording and reporting of HR and HRV measures will strengthen inferences across the many applications of these metrics in psychophysiology. The prior Committee Reports on HR and HRV are several decades old. Since their appearance, technologies for human cardiac and vascular monitoring in laboratory and daily life (i.e., ambulatory) contexts have greatly expanded. This Committee Report was prepared for the Society for Psychophysiological Research to provide updated methodological and interpretive guidance, as well as to summarize best practices for reporting HR and HRV studies in humans.

The autonomic nervous system and the origins of neurocardiology

Following new concepts by Bichat in the early 19th century, of organic and animal life centered around the ganglionic nervous system, over 100 years of anatomic studies and physiologic experimentation eventually resulted in Gaskell's 1916 book entitled "The Involuntary Nervous System" and Langley's 1921 book entitled "The Autonomic Nervous System." Neurology and cardiology emerged as specialties of medicine in the early 20th century. Although neurology made several prominent discoveries in neurophysiology during the first half of the 20th century, cardiology developed coronary care units and cardiac catheterization in the 1960s. Programmed electrical stimulation of the heart and noninvasive ambulatory monitoring provided new methodologies to study clinical cardiac arrhythmias. Experimentally, direct cardiac nerve stimulation of sympathetic nerve endings, as well as parasympathetic control of the atrioventricular node, provided the background to new detailed autonomic studies of the heart. Neurocardiology, perhaps initially more directed towards our understanding of sudden cardiac death, ultimately embraced an even significantly more complex scheme of local circuit neurons and near-endless loops of interconnecting neurons in the heart. Intrathoracic extracardiac and intracardiac ganglia have been recharacterized, both anatomically and physiologically, laying the groundwork for potential new therapies of cardiac neuromodulation.

Molecular and cellular neurocardiology in heart disease

This paper updates and builds on a previous White Paper in this journal that some of us contributed to concerning the molecular and cellular basis of cardiac neurobiology of heart disease. Here we focus on recent findings that underpin cardiac autonomic development, novel intracellular pathways and neuroplasticity. Throughout we highlight unanswered questions and areas of controversy. Whilst some neurochemical pathways are already demonstrating prognostic viability in patients with heart failure, we also discuss the opportunity to better understand sympathetic impairment by using patient specific stem cells that provides pathophysiological contextualization to study 'disease in a dish'. Novel imaging techniques and spatial transcriptomics are also facilitating a road map for target discovery of molecular pathways that may form a therapeutic opportunity to treat cardiac dysautonomia.

Electrophysiology and 3D-imaging reveal properties of human intracardiac neurons and increased excitability with atrial fibrillation

Altered autonomic input to the heart plays a major role in atrial fibrillation (AF). Autonomic neurons termed ganglionated plexi (GP) are clustered on the heart surface to provide the last point of neural control of cardiac function. To date the properties of GP neurons in humans are unknown. Here we have addressed this knowledge gap in human GP neuron structure and physiology in patients with and without AF. Human right atrial GP neurons embedded in epicardial adipose tissue were excised during open heart surgery performed on both non-AF and AF patients and then characterised physiologically by whole cell patch clamp techniques. Structural analysis was also performed after fixation at both the single cell and at the entire GP levels via three-dimensional confocal imaging. Human GP neurons were found to exhibit unique properties and structural complexity with branched neurite outgrowth. Significant differences in excitability were revealed between AF and non-AF GP neurons as measured by lower current to induce action potential firing, a reduced occurrence of low action potential firing rates, decreased accommodation and increased synaptic density. Visualisation of entire GPs showed almost all neurons are cholinergic with a small proportion of noradrenergic and dual phenotype neurons. Phenotypic distribution differences occurred with AF including decreased cholinergic and dual phenotype neurons, and increased noradrenergic neurons. These data show both functional and structural differences occur between GP neurons from patients with and without AF, highlighting that cellular plasticity occurs in neural input to the heart that could alter autonomic influence on atrial function. KEY POINTS: The autonomic nervous system plays a critical role in regulating heart rhythm and the initiation of AF; however, the structural and functional properties of human autonomic neurons in the autonomic ganglionated plexi (GP) remain unknown. Here we perform the first whole cell patch clamp electrophysiological and large tissue confocal imaging analysis of these neurons from patients with and without AF. Our data show human GP neurons are functionally and structurally complex. Measurements of action potential kinetics show higher excitability in GP neurons from AF patients as measured by lower current to induce action potential firing, reduced low firing action potential rates, and decreased action potential accommodation. Confocal imaging shows increased synaptic density and noradrenergic phenotypes in patients with AF. Both functional and structural differences occur in GP neurons from patients with AF that could alter autonomic influence on atrial rhythm.

From Psychostasis to the Discovery of Cardiac Nerves: The Origins of the Modern Cardiac Neuromodulation Concept

This review explores the historical development of cardiology knowledge, from ancient Egyptian psychostasis to the modern comprehension of cardiac neuromodulation. In ancient Egyptian religion, psychostasis was the ceremony in which the deceased was judged before gaining access to the afterlife. This ritual was also known as the "weighing of the heart" or "weighing of the soul". The Egyptians believed that the heart, not the brain, was the seat of human wisdom, emotions, and memory. They were the first to recognize the cardiocentric nature of the body, identifying the heart as the center of the circulatory system. Aristotle (fourth century BC) considered the importance of the heart in human physiology in his philosophical analyses. For Galen (third century AD), the heart muscle was the site of the vital spirit, which regulated body temperature. Cardiology knowledge advanced significantly in the 15th century, coinciding with Leonardo da Vinci and Vesalius's pioneering anatomical and physiological studies. It was William Harvey, in the 17th century, who introduced the concept of cardiac circulation. Servet's research and Marcello Malpighi's discovery of arterioles and capillaries provided a more detailed understanding of circulation. Richard Lower emerged as the foremost pioneer of experimental cardiology in the late 17th century. He demonstrated the heart's neural control by tying off the vagus nerve. In 1753, Albrecht von Haller, a professor at Göttingen, was the first to discover the heart's automaticity and the excitation of muscle fibers. Towards the end of the 18th century, Antonio Scarpa challenged the theories of Albrecht von Haller and Johann Bernhard Jacob Behrends, who maintained that the myocardium possessed its own "irritability", on which the heartbeat depended, and was independent of neuronal sensitivity. Instead, Scarpa argued that the heart required innervation to maintain life, refuting Galenic notions. In contemporary times, the study of cardiac innervation has regained prominence, particularly in understanding the post-acute sequelae of SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2) infection (PASC), which frequently involves cardiorespiratory symptoms and dysregulation of the intrinsic cardiac innervation. Recently, it has been recognized that post-acute sequelae of acute respiratory infections (ARIs) due to other pathogens can also be a cause of long-term vegetative and somatic symptoms. Understanding cardiac innervation and modulation can help to recognize and treat long COVID and long non-COVID-19 (coronavirus disease 2019) ARIs. This analysis explores the historical foundations of cardiac neuromodulation and its contemporary relevance. By focusing on this concept, we aim to bridge the gap between historical understanding and modern applications. This will illuminate the complex interplay between cardiac function, neural modulation, cardiovascular health, and disease management in the context of long-term cardiorespiratory symptoms and dysregulation of intrinsic cardiac innervations.

Potential consequences of cardioneuroablation for vasovagal syncope: A call for appropriately designed, sham-controlled clinical trials

Cardioneuroablation (CNA) is being increasingly used to treat patients with vasovagal syncope (VVS). Bradycardia, in the cardioinhibitory subtype of VVS, results from transient parasympathetic overactivity leading to sinus bradycardia and/or atrioventricular block. By mitigating parasympathetic overactivity, CNA has been shown to improve VVS symptoms in clinical studies with relatively small sample sizes and short follow-up periods (<5 years) at selected centers. However, CNA may potentially tip the autonomic balance to a state of sympathovagal imbalance with attenuation of cardiac parasympathetic activity. A higher heart rate is associated with adverse cardiovascular events and increased mortality in healthy populations without cardiovascular diseases. Chronic sympathovagal imbalance may also affect the pathophysiology of spectra of cardiovascular disorders including atrial and ventricular arrhythmias. This review addresses potential long-term pathophysiological consequences of CNA for VVS.

The vagus nerve in cardiovascular physiology and pathophysiology: From evolutionary insights to clinical medicine

The parasympathetic nervous system via the vagus nerve exerts profound influence over the heart. Together with the sympathetic nervous system, the parasympathetic nervous system is responsible for fine-tuned regulation of all aspects of cardiovascular function, including heart rate, rhythm, contractility, and blood pressure. In this review, we highlight vagal efferent and afferent innervation of the heart, with a focus on insights from comparative biology and advances in understanding the molecular and genetic diversity of vagal neurons, as well as interoception, parasympathetic dysfunction in heart disease, and the therapeutic potential of targeting the parasympathetic nervous system in cardiovascular disease.

The Coding Logic of Interoception

Interoception, the ability to precisely and timely sense internal body signals, is critical for life. The interoceptive system monitors a large variety of mechanical, chemical, hormonal, and pathological cues using specialized organ cells, organ innervating neurons, and brain sensory neurons. It is important for maintaining body homeostasis, providing motivational drives, and regulating autonomic, cognitive, and behavioral functions. However, compared to external sensory systems, our knowledge about how diverse body signals are coded at a system level is quite limited. In this review, we focus on the unique features of interoceptive signals and the organization of the interoceptive system, with the goal of better understanding the coding logic of interoception.

The Intrinsic Cardiac Nervous System: From Pathophysiology to Therapeutic Implications

The cardiac autonomic nervous system (CANS) plays a pivotal role in cardiac homeostasis as well as in cardiac pathology. The first level of cardiac autonomic control, the intrinsic cardiac nervous system (ICNS), is located within the epicardial fat pads and is physically organized in ganglionated plexi (GPs). The ICNS system does not only contain parasympathetic cardiac efferent neurons, as long believed, but also afferent neurons and local circuit neurons. Thanks to its high degree of connectivity, combined with neuronal plasticity and memory capacity, the ICNS allows for a beat-to-beat control of all cardiac functions and responses as well as integration with extracardiac and higher centers for longer-term cardiovascular reflexes. The present review provides a detailed overview of the current knowledge of the bidirectional connection between the ICNS and the most studied cardiac pathologies/conditions (myocardial infarction, heart failure, arrhythmias and heart transplant) and the potential therapeutic implications. Indeed, GP modulation with efferent activity inhibition, differently achieved, has been studied for atrial fibrillation and functional bradyarrhythmias, while GP modulation with efferent activity stimulation has been evaluated for myocardial infarction, heart failure and ventricular arrhythmias. Electrical therapy has the unique potential to allow for both kinds of ICNS modulation while preserving the anatomical integrity of the system.

Non-invasive Neuromodulation of Arrhythmias

Dysfunction of the cardiac autonomic nervous system (CANS) is associated with various cardiac arrhythmias. Subsequently, invasive techniques have successfully targeted the CANS for the treatment of certain arrhythmias, such as sympathetic denervation for ventricular tachycardia storm. Non-invasive strategies capable of modulating the CANS for arrhythmia treatment have begun to gain interest due to their low-risk profile and applicability as an adjuvant therapy. This review provides an evidence-based overview of the currently studied technologies capable of non-invasively modulating CANS for the suppression of atrial fibrillation and ventricular arrhythmias.

Respiratory sinus arrhythmia (RSA), vagal tone and biobehavioral integration: Beyond parasympathetic function

Linchpin to the entire area of psychophysiological research and discussion of the vagus is the respiratory and cardiovascular phenomenon known as respiratory sinus arrhythmia (RSA; often synonymous with high-frequency heart-rate variability when it is specifically linked to respiratory frequency), i.e. rhythmic fluctuations in heart rate synchronized to inspiration and expiration. This article aims 1) to clarify concepts, terms and measures commonly employed during the last half century in the scientific literature, which relate vagal function to psychological processes and general aspects of health; and 2) to expand upon an earlier theoretical model, emphasizing the importance of RSA well beyond the current focus upon parasympathetic mechanisms. A close examination of RSA and its relations to the vagus may 1) dispel certain commonly held beliefs about associations between psychological functioning, RSA and the parasympathetic nervous system (for which the vagus nerve plays a major role), and 2) offer fresh perspectives about the likely functions and adaptive significance of RSA, as well as RSA's relationship to vagal control. RSA is neither an invariably reliable index of cardiac vagal tone nor of central vagal outflow to the heart. The model here presented posits that RSA represents an evolutionarily entrenched, cardiovascular and respiratory phenomenon that significantly contributes to meeting continuously changing metabolic, energy and behavioral demands.

Differential developmental blueprints of organ-intrinsic nervous systems

The organ-intrinsic nervous system is a major interface between visceral organs and the brain, mediating important sensory and regulatory functions in the body-brain axis and serving as critical local processors for organ homeostasis. Molecularly, anatomically, and functionally, organ-intrinsic neurons are highly specialized for their host organs. However, the underlying mechanism that drives this specialization is largely unknown. Here, we describe the differential strategies utilized to achieve organ-specific organization between the enteric nervous system (ENS) 1 and the intrinsic cardiac nervous system (ICNS) 2 , a neuronal network essential for heart performance but poorly characterized. Integrating high-resolution whole-embryo imaging, single-cell genomics, spatial transcriptomics, proteomics, and bioinformatics, we uncover that unlike the ENS which is highly mobile and colonizes the entire gastrointestinal (GI) tract, the ICNS uses a rich set of extracellular matrix (ECM) genes that match with surrounding heart cells and an intermediate dedicated neuronal progenitor state to stabilize itself for a 'beads-on-the-necklace' organization on heart atria. While ICNS- and ENS-precursors are genetically similar, their differentiation paths are influenced by their host-organs, leading to distinct mature neuron types. Co-culturing ENS-precursors with heart cells shifts their identity towards the ICNS and induces the expression of heart-matching ECM genes. Our cross-organ study thus reveals fundamental principles for the maturation and specialization of organ-intrinsic neurons.

A novel conceptual model of heart rate autonomic modulation based on a small-world modular structure of the sinoatrial node

The present view on heartbeat initiation is that a primary pacemaker cell or a group of cells in the sinoatrial node (SAN) center paces the rest of the SAN and the atria. However, recent high-resolution imaging studies show a more complex paradigm of SAN function that emerges from heterogeneous signaling, mimicking brain cytoarchitecture and function. Here, we developed and tested a new conceptual numerical model of SAN organized similarly to brain networks featuring a modular structure with small-world topology. In our model, a lower rate module leads action potential (AP) firing in the basal state and during parasympathetic stimulation, whereas a higher rate module leads during β-adrenergic stimulation. Such a system reproduces the respective shift of the leading pacemaker site observed experimentally and a wide range of rate modulation and robust function while conserving energy. Since experimental studies found functional modules at different scales, from a few cells up to the highest scale of the superior and inferior SAN, the SAN appears to feature hierarchical modularity, i.e., within each module, there is a set of sub-modules, like in the brain, exhibiting greater robustness, adaptivity, and evolvability of network function. In this perspective, our model offers a new mainframe for interpreting new data on heterogeneous signaling in the SAN at different scales, providing new insights into cardiac pacemaker function and SAN-related cardiac arrhythmias in aging and disease.

The Impact of the Nervous System on Arteries and the Heart: The Neuroimmune Cardiovascular Circuit Hypothesis

Three systemic biological systems, i.e., the nervous, the immune, and the cardiovascular systems, form a mutually responsive and forward-acting tissue network to regulate acute and chronic cardiovascular function in health and disease. Two sub-circuits within the cardiovascular system have been described, the artery brain circuit (ABC) and the heart brain circuit (HBC), forming a large cardiovascular brain circuit (CBC). Likewise, the nervous system consists of the peripheral nervous system and the central nervous system with their functional distinct sensory and effector arms. Moreover, the immune system with its constituents, i.e., the innate and the adaptive immune systems, interact with the CBC and the nervous system at multiple levels. As understanding the structure and inner workings of the CBC gains momentum, it becomes evident that further research into the CBC may lead to unprecedented classes of therapies to treat cardiovascular diseases as multiple new biologically active molecules are being discovered that likely affect cardiovascular disease progression. Here, we weigh the merits of integrating these recent observations in cardiovascular neurobiology into previous views of cardiovascular disease pathogeneses. These considerations lead us to propose the Neuroimmune Cardiovascular Circuit Hypothesis.

Mapping of Neuro-Cardiac Electrophysiology: Interlinking Epilepsy and Arrhythmia

The interplay between neurology and cardiology has gained significant attention in recent years, particularly regarding the shared pathophysiological mechanisms and clinical comorbidities observed in epilepsy and arrhythmias. Neuro-cardiac electrophysiology mapping involves the comprehensive assessment of both neural and cardiac electrical activity, aiming to unravel the intricate connections and potential cross-talk between the brain and the heart. The emergence of artificial intelligence (AI) has revolutionized the field by enabling the analysis of large-scale data sets, complex signal processing, and predictive modeling. AI algorithms have been applied to neuroimaging, electroencephalography (EEG), electrocardiography (ECG), and other diagnostic modalities to identify subtle patterns, classify disease subtypes, predict outcomes, and guide personalized treatment strategies. In this review, we highlight the potential clinical implications of neuro-cardiac mapping and AI in the management of epilepsy and arrhythmias. We address the challenges and limitations associated with these approaches, including data quality, interpretability, and ethical considerations. Further research and collaboration between neurologists, cardiologists, and AI experts are needed to fully unlock the potential of this interdisciplinary field.

The distribution of sinoatrial nodal cells and their innervation in the pig

The sinoatrial node (SAN) has been the object of interest of various studies. In experimental neurocardiology, the real challenge is the choice of the most appropriate animal model. Pig is routinely used animal due to its size and physiological features. Despite this, the anatomy and innervation of the pig SAN are not completely examined. This study analyses the distribution of SAN cells and their innervation in whole-mount preparations and the cross-sections of the pig right atrium. Our findings revealed the differences in the distribution of the SAN cells and their innervation pattern between pigs and other animals. The pig SAN myocytes were distributed around the root of the anterior vena cava. A meshwork of nerve fibers (NFs) in this area was four-fold denser compared to other right atrial areas and contained the adrenergic (positive for TH), cholinergic (positive for ChAT), nitrergic (positive for nNOS), and potentially sensory (positive for SP) NFs. The SAN area contained 98 ± 10 ganglia that involved 21 ± 2 neuronal somata per ganglion. The determined chemical phenotypes of ganglionic cells demonstrate their diversity in the pig SAN area as there were identified neuronal somata positive for ChAT, nNOS, TH, and simultaneously for ChAT/nNOS and ChAT/TH. Small intensively fluorescent cells were also abundant. The broad distribution of SAN cells, the chemical diversity, and the high density of neural components in the SAN area are comparable to the human one and, therefore, the pig may be considered as the appropriate animal model for experimental cardiology.

Speech-induced atrial tachycardia: A narrative review of putative mechanisms implicating the autonomic nervous system

Despite being uncommon, speech-induced atrial tachycardias carry significant morbidity and affect predominantly healthy individuals. Little is known about their mechanism, treatment, and prognosis. In this review, we seek to identify the underlying connections and pathophysiology between speech and arrhythmias while providing an informed approach to evaluation and management.

Cardiac innervations in diabetes mellitus-Anatomical evidence of neuropathy

The extensive innervations of the heart include a complex network of sympathetic, parasympathetic, and sensory nerves connected in loops that serve to regulate cardiac output. Metabolic dysfunction in diabetes affects many different organ systems, including the cardiovascular system; it causes cardiac arrhythmias, silent myocardial ischemia, and sudden cardiac death, among others. These conditions are associated with damage to the nerves that innervate the heart, cardiac autonomic neuropathy (CAN), which is caused by various pathophysiological mechanisms. In this review, the main facts about the anatomy of cardiac innervations and the current knowledge of CAN, its pathophysiological mechanisms, and its diagnostic approach are discussed. In addition, anatomical evidence for CAN from human and animal studies has been summarized.

Ganglionated Plexus Ablation Procedures to Treat Vasovagal Syncope

Vasovagal syncope (VVS) refers to a heterogeneous group of conditions whereby the cardiovascular reflexes normally controlling the circulation are interrupted irregularly in response to a trigger, resulting in vasodilation, bradycardia, or both. VVS affects one-third of the population at least once in their lifetime or by the age of 60, reduces the quality of life, and may cause disability affecting certain routines. It poses a considerable economic burden on society, and, despite its prevalence, there is currently no proven pharmacological treatment for preventing VVS. The novel procedure of ganglionated plexus (GP) ablation has emerged rapidly in the past two decades, and has been proven successful in treating syncope. Several parameters influence the success rate of GP ablation, including specific ablation sites, localization and surgical techniques, method of access, and the integration of other interventions. This review aims to provide an overview of the existing literature on the physiological aspects and clinical effectiveness of GP ablation in the treatment of VVS. Specifically, we explore the association between GPs and VVS and examine the impact of GP ablation procedures as reported in human clinical trials. Our objective is to shed light on the therapeutic significance of GP ablation in eliminating VVS and restoring normal sinus rhythm, particularly among young adults affected by this condition.

Case report: Anxiety and depression as initial symptoms in a patient with acute hypoxia and patent foramen ovale

The prevalence of patent foramen ovale (PFO) is 15-35% among adults. The role of right-to-left shunting through the PFO, anxiety, depression, and hypoxemia in the systemic circulation remains poorly understood. Herein, we present the case of a 52-year-old woman with no heart or lung disease, who was admitted due to anxiety for 5 months and had symptom exacerbation with dizziness for 4 days and presented with cyanosis. She was noted to have acute hypoxemia, with an oxygen saturation of 94.48% on room air, and arterial blood gas showed an oxygen tension of 65.64 mmHg. Agitated saline contrast echocardiography showed right-to-left shunting due to PFO. Arteriovenous fistula, pneumonia, pulmonary embolism, pulmonary hypertension, congestion peripheral cyanosis, ischemic peripheral cyanosis, and methemoglobin were excluded. Additionally, the patient improved by taking Paroxetine, Oxazepam, and Olanzapine. Her oxygen tension returned to 90.42 mmHg, and her symptoms resolved. In the case of severe anxiety and depression, right-to-left shunting through the PFO may cause acute systemic hypoxemia via a flow-driven mechanism, occasionally manifesting as cyanosis. When anxiety improved, hypoxia also improved. Thus, the treatment of anxiety and depression seems effective in improving hypoxemia. Notably, this is a rare report, and we hope to draw the attention of psychosomatic specialists, psychiatrists, and clinicians to seek the relationship between anxiety appearing as acute stress and PFO. This may be a new therapeutic method for treating severe anxiety disorder.

Clinical magnetocardiography: the unshielded bet-past, present, and future

Magnetocardiography (MCG), which is nowadays 60 years old, has not yet been fully accepted as a clinical tool. Nevertheless, a large body of research and several clinical trials have demonstrated its reliability in providing additional diagnostic electrophysiological information if compared with conventional non-invasive electrocardiographic methods. Since the beginning, one major objective difficulty has been the need to clean the weak cardiac magnetic signals from the much higher environmental noise, especially that of urban and hospital environments. The obvious solution to record the magnetocardiogram in highly performant magnetically shielded rooms has provided the ideal setup for decades of research demonstrating the diagnostic potential of this technology. However, only a few clinical institutions have had the resources to install and run routinely such highly expensive and technically demanding systems. Therefore, increasing attempts have been made to develop cheaper alternatives to improve the magnetic signal-to-noise ratio allowing MCG in unshielded hospital environments. In this article, the most relevant milestones in the MCG's journey are reviewed, addressing the possible reasons beyond the currently long-lasting difficulty to reach a clinical breakthrough and leveraging the authors' personal experience since the early 1980s attempting to finally bring MCG to the patient's bedside for many years thus far. Their nearly four decades of foundational experimental and clinical research between shielded and unshielded solutions are summarized and referenced, following the original vision that MCG had to be intended as an unrivaled method for contactless assessment of the cardiac electrophysiology and as an advanced method for non-invasive electroanatomical imaging, through multimodal integration with other non-fluoroscopic imaging techniques. Whereas all the above accounts for the past, with the available innovative sensors and more affordable active shielding technologies, the present demonstrates that several novel systems have been developed and tested in multicenter clinical trials adopting both shielded and unshielded MCG built-in hospital environments. The future of MCG will mostly be dependent on the results from the ongoing progress in novel sensor technology, which is relatively soon foreseen to provide multiple alternatives for the construction of more compact, affordable, portable, and even wearable devices for unshielded MCG inside hospital environments and perhaps also for ambulatory patients.

Autonomic control of ventricular function in health and disease: current state of the art

PURPOSE

Cardiac autonomic dysfunction is one of the main pillars of cardiovascular pathophysiology. The purpose of this review is to provide an overview of the current state of the art on the pathological remodeling that occurs within the autonomic nervous system with cardiac injury and available neuromodulatory therapies for autonomic dysfunction in heart failure.

METHODS

Data from peer-reviewed publications on autonomic function in health and after cardiac injury are reviewed. The role of and evidence behind various neuromodulatory therapies both in preclinical investigation and in-use in clinical practice are summarized.

RESULTS

A harmonic interplay between the heart and the autonomic nervous system exists at multiple levels of the neuraxis. This interplay becomes disrupted in the setting of cardiovascular disease, resulting in pathological changes at multiple levels, from subcellular cardiac signaling of neurotransmitters to extra-cardiac, extra-thoracic remodeling. The subsequent detrimental cycle of sympathovagal imbalance, characterized by sympathoexcitation and parasympathetic withdrawal, predisposes to ventricular arrhythmias, progression of heart failure, and cardiac mortality. Knowledge on the etiology and pathophysiology of this condition has increased exponentially over the past few decades, resulting in a number of different neuromodulatory approaches. However, significant knowledge gaps in both sympathetic and parasympathetic interactions and causal factors that mediate progressive sympathoexcitation and parasympathetic dysfunction remain.

CONCLUSIONS

Although our understanding of autonomic imbalance in cardiovascular diseases has significantly increased, specific, pivotal mediators of this imbalance and the recognition and implementation of available autonomic parameters and neuromodulatory therapies are still lagging.

Spinal neuromodulation mitigates myocardial ischemia-induced sympathoexcitation by suppressing the intermediolateral nucleus hyperactivity and spinal neural synchrony

Introduction

Myocardial ischemia disrupts the cardio-spinal neural network that controls the cardiac sympathetic preganglionic neurons, leading to sympathoexcitation and ventricular tachyarrhythmias (VTs). Spinal cord stimulation (SCS) is capable of suppressing the sympathoexcitation caused by myocardial ischemia. However, how SCS modulates the spinal neural network is not fully known.

Methods

In this pre-clinical study, we investigated the impact of SCS on the spinal neural network in mitigating myocardial ischemia-induced sympathoexcitation and arrhythmogenicity. Ten Yorkshire pigs with left circumflex coronary artery (LCX) occlusion-induced chronic myocardial infarction (MI) were anesthetized and underwent laminectomy and a sternotomy at 4-5 weeks post-MI. The activation recovery interval (ARI) and dispersion of repolarization (DOR) were analyzed to evaluate the extent of sympathoexcitation and arrhythmogenicity during the left anterior descending coronary artery (LAD) ischemia. Extracellular in vivo and in situ spinal dorsal horn (DH) and intermediolateral column (IML) neural recordings were performed using a multichannel microelectrode array inserted at the T2-T3 segment of the spinal cord. SCS was performed for 30 min at 1 kHz, 0.03 ms, 90% motor threshold. LAD ischemia was induced pre- and 1 min post-SCS to investigate how SCS modulates spinal neural network processing of myocardial ischemia. DH and IML neural interactions, including neuronal synchrony as well as cardiac sympathoexcitation and arrhythmogenicity markers were evaluated during myocardial ischemia pre- vs. post-SCS.

Results

ARI shortening in the ischemic region and global DOR augmentation due to LAD ischemia was mitigated by SCS. Neural firing response of ischemia-sensitive neurons during LAD ischemia and reperfusion was blunted by SCS. Further, SCS showed a similar effect in suppressing the firing response of IML and DH neurons during LAD ischemia. SCS exhibited a similar suppressive impact on the mechanical, nociceptive and multimodal ischemia sensitive neurons. The LAD ischemia and reperfusion-induced augmentation in neuronal synchrony between DH-DH and DH-IML pairs of neurons were mitigated by the SCS.

Discussion

These results suggest that SCS is decreasing the sympathoexcitation and arrhythmogenicity by suppressing the interactions between the spinal DH and IML neurons and activity of IML preganglionic sympathetic neurons.

Cardiovascular Brain Circuits

The cardiovascular system is hardwired to the brain via multilayered afferent and efferent polysynaptic axonal connections. Two major anatomically and functionally distinct though closely interacting subcircuits within the cardiovascular system have recently been defined: The artery-brain circuit and the heart-brain circuit. However, how the nervous system impacts cardiovascular disease progression remains poorly understood. Here, we review recent findings on the anatomy, structures, and inner workings of the lesser-known artery-brain circuit and the better-established heart-brain circuit. We explore the evidence that signals from arteries or the heart form a systemic and finely tuned cardiovascular brain circuit: afferent inputs originating in the arterial tree or the heart are conveyed to distinct sensory neurons in the brain. There, primary integration centers act as hubs that receive and integrate artery-brain circuit-derived and heart-brain circuit-derived signals and process them together with axonal connections and humoral cues from distant brain regions. To conclude the cardiovascular brain circuit, integration centers transmit the constantly modified signals to efferent neurons which transfer them back to the cardiovascular system. Importantly, primary integration centers are wired to and receive information from secondary brain centers that control a wide variety of brain traits encoded in engrams including immune memory, stress-regulating hormone release, pain, reward, emotions, and even motivated types of behavior. Finally, we explore the important possibility that brain effector neurons in the cardiovascular brain circuit network connect efferent signals to other peripheral organs including the immune system, the gut, the liver, and adipose tissue. The enormous recent progress vis-à-vis the cardiovascular brain circuit allows us to propose a novel neurobiology-centered cardiovascular disease hypothesis that we term the neuroimmune cardiovascular circuit hypothesis.

The Autonomic Coumel Triangle: A New Way to Define the Fascinating Relationship between Atrial Fibrillation and the Autonomic Nervous System

Arrhythmogenic substrate, modulating factors, and triggering factors (the so-called Coumel's triangle concept) play a primary role in atrial fibrillation (AF) pathophysiology. Several years have elapsed since Coumel and co-workers advanced the concept of the relevance of autonomic nervous system (ANS) influences on atrial cells' electrophysiological characteristics. The ANS is not only associated with cardiac rhythm regulation but also exerts an important role in the triggering and maintenance of atrial fibrillation. This review aims to describe in detail the autonomic mechanisms involved in the pathophysiology of atrial fibrillation (AF), starting from the hypothesis of an "Autonomic Coumel Triangle" that stems from the condition of the fundamental role played by the ANS in all phases of the pathophysiology of AF. In this article, we provide updated information on the biomolecular mechanisms of the ANS role in Coumel's triangle, with the molecular pathways of cardiac autonomic neurotransmission, both adrenergic and cholinergic, and the interplay between the ANS and cardiomyocytes' action potential. The heterogeneity of the clinical spectrum of the ANS and AF, with the ANS playing a relevant role in situations that may promote the initiation and maintenance of AF, is highlighted. We also report on drug, biological, and gene therapy as well as interventional therapy. On the basis of the evidence reviewed, we propose that one should speak of an "Autonomic Coumel's Triangle" instead of simply "Coumel's Triangle".

Unpacking the multimodal, multi-scale data of the fast and slow lanes of the cardiac vagus through computational modelling

NEW FINDINGS

What is the topic of this review? The vagus nerve is a crucial regulator of cardiovascular homeostasis, and its activity is linked to heart health. Vagal activity originates from two brainstem nuclei: the nucleus ambiguus (fast lane) and the dorsal motor nucleus of the vagus (slow lane), nicknamed for the time scales that they require to transmit signals. What advances does it highlight? Computational models are powerful tools for organizing multi-scale, multimodal data on the fast and slow lanes in a physiologically meaningful way. A strategy is laid out for how these models can guide experiments aimed at harnessing the cardiovascular health benefits of differential activation of the fast and slow lanes.

ABSTRACT

The vagus nerve is a key mediator of brain-heart signaling, and its activity is necessary for cardiovascular health. Vagal outflow stems from the nucleus ambiguus, responsible primarily for fast, beat-to-beat regulation of heart rate and rhythm, and the dorsal motor nucleus of the vagus, responsible primarily for slow regulation of ventricular contractility. Due to the high-dimensional and multimodal nature of the anatomical, molecular and physiological data on neural regulation of cardiac function, data-derived mechanistic insights have proven elusive. Elucidating insights has been complicated further by the broad distribution of the data across heart, brain and peripheral nervous system circuits. Here we lay out an integrative framework based on computational modelling for combining these disparate and multi-scale data on the two vagal control lanes of the cardiovascular system. Newly available molecular-scale data, particularly single-cell transcriptomic analyses, have augmented our understanding of the heterogeneous neuronal states underlying vagally mediated fast and slow regulation of cardiac physiology. Cellular-scale computational models built from these data sets represent building blocks that can be combined using anatomical and neural circuit connectivity, neuronal electrophysiology, and organ/organismal-scale physiology data to create multi-system, multi-scale models that enable in silico exploration of the fast versus slow lane vagal stimulation. The insights from the computational modelling and analyses will guide new experimental questions on the mechanisms regulating the fast and slow lanes of the cardiac vagus toward exploiting targeted vagal neuromodulatory activity to promote cardiovascular health.

Closed-loop modeling of central and intrinsic cardiac nervous system circuits underlying cardiovascular control

The baroreflex is a multi-input, multi-output control physiological system that regulates blood pressure by modulating nerve activity between the brainstem and the heart. Existing computational models of the baroreflex do not explictly incorporate the intrinsic cardiac nervous system (ICN), which mediates central control of the heart function. We developed a computational model of closed-loop cardiovascular control by integrating a network representation of the ICN within central control reflex circuits. We examined central and local contributions to the control of heart rate, ventricular functions, and respiratory sinus arrhythmia (RSA). Our simulations match the experimentally observed relationship between RSA and lung tidal volume. Our simulations predicted the relative contributions of the sensory and the motor neuron pathways to the experimentally observed changes in the heart rate. Our closed-loop cardiovascular control model is primed for evaluating bioelectronic interventions to treat heart failure and renormalize cardiovascular physiology.

Catecholaminergic axon innervation and morphology in flat-mounts of atria and ventricles of mice

Sympathetic efferent axons regulate cardiac functions. However, the topographical distribution and morphology of cardiac sympathetic efferent axons remain insufficiently characterized due to the technical challenges involved in immunohistochemical labeling of the thick walls of the whole heart. In this study, flat-mounts of the left and right atria and ventricles of FVB mice were immunolabeled for tyrosine hydroxylase (TH), a marker of sympathetic nerves. Atrial and ventricular flat-mounts were scanned using a confocal microscope to construct montages. We found (1) In the atria: A few large TH-immunoreactive (IR) axon bundles entered both atria, branched into small bundles and then single axons that eventually formed very dense terminal networks in the epicardium, myocardium and inlet regions of great vessels to the atria. Varicose TH-IR axons formed close contact with cardiomyocytes, vessels, and adipocytes. Multiple intrinsic cardiac ganglia (ICG) were identified in the epicardium of both atria, and a subpopulation of the neurons in the ICG were TH-IR. Most TH-IR axons in bundles traveled through ICG before forming dense varicose terminal networks in cardiomyocytes. We did not observe varicose TH-IR terminals encircling ICG neurons. (2) In the left and right ventricles and interventricular septum: TH-IR axons formed dense terminal networks in the epicardium, myocardium, and vasculature. Collectively, TH labeling is achievable in flat-mounts of thick cardiac walls, enabling detailed mapping of catecholaminergic axons and terminal structures in the whole heart at single-cell/axon/varicosity scale. This approach provides a foundation for future quantification of the topographical organization of the cardiac sympathetic innervation in different pathological conditions.

A novel ex-vivo isolated rabbit heart preparation to explore the cardiac effects of cervical and cardiac vagus nerve stimulation

The cardiac responses to vagus nerve stimulation (VNS) are still not fully understood, partly due to uncontrollable confounders in the in-vivo experimental condition. Therefore, an ex-vivo Langendorff-perfused rabbit heart with intact vagal innervation is proposed to study VNS in absence of cofounding anesthetic or autonomic influences. The feasibility to evoke chronotropic responses through electrical stimulation ex-vivo was studied in innervated isolated rabbit hearts (n = 6). The general nerve excitability was assessed through the ability to evoke a heart rate (HR) reduction of at least 5 bpm (physiological threshold). The excitability was quantified as the charge needed for a 10-bpm HR reduction. The results were compared to a series of in-vivo experiments rabbits (n = 5). In the ex-vivo isolated heart, the baseline HR was about 20 bpm lower than in-vivo (158 ± 11 bpm vs 181 ± 19 bpm). Overall, the nerve remained excitable for about 5 h ex-vivo. The charges required to reduce HR by 5 bpm were 9 ± 6 µC and 549 ± 370 µC, ex-vivo and in-vivo, respectively. The charges needed for a 10-bpm HR reduction, normalized to the physiological threshold were 1.78 ± 0.8 and 1.22 ± 0.1, in-vivo and ex-vivo, respectively. Overall, the viability of this ex-vivo model to study the acute cardiac effects of VNS was demonstrated.

Hemodynamic responses to low-level transcutaneous auricular nerve stimulation in young volunteers

Objectives

The aim of this study was to characterize cardiovascular autonomic responses during two constant current intensities below sensory threshold of transcutaneous auricular nerve stimulation (taNS). On this basis, a protocol for taNS with autonomic modulatory potential could be proposed.

Subjects and methods

We included 26 men and 24 women, mean age 26. Data were collected during three randomly allocated 20-minute right tragus stimulation sessions - a) no-stimulation (sham), b) 90 µA (arbitrary), and c) 130 µA (near the lowest sensory threshold in majority). Stimulation was 20 Hz, rectangular pulse width of 2 ms, duty cycle 2-second on/off. To assess autonomic responses, we continuously recorded ECG, non-invasive arterial blood pressure (BP) and thoracic impedance cardiography data. Ten-minute data were compared. Fast Fourier transform of RR intervals was performed on 10-minute recordings as well. Low frequency and high frequency power spectra were calculated. Friedman test or one-way ANOVA for repeated measurements and Mann-Whitney or Wilcoxon's signed-rank test, or t-test were carried out. P < 0.05 was considered significant.

Results

At 130 µA stimulation, cardiac output significantly decreased (p < 0.05), driven by significant heart rate drop in women, and stroke volume and contractility drop in men, pointing to a gender-related autonomic responses. We observed no significant changes in BP, or variability parameters. Significantly higher body size and BP were found in men, as expected.

Conclusions

It seems that tested taNS protocol has a potential for cardiac autonomic modulation in majority of young healthy men as well as women. Further studies are however needed to prove the therapeutic potential of this stimulation protocol in different patient groups.

Autonomic nervous system and cardiac neuro-signaling pathway modulation in cardiovascular disorders and Alzheimer's disease

The heart is a functional syncytium controlled by a delicate and sophisticated balance ensured by the tight coordination of its several cell subpopulations. Accordingly, cardiomyocytes together with the surrounding microenvironment participate in the heart tissue homeostasis. In the right atrium, the sinoatrial nodal cells regulate the cardiac impulse propagation through cardiomyocytes, thus ensuring the maintenance of the electric network in the heart tissue. Notably, the central nervous system (CNS) modulates the cardiac rhythm through the two limbs of the autonomic nervous system (ANS): the parasympathetic and sympathetic compartments. The autonomic nervous system exerts non-voluntary effects on different peripheral organs. The main neuromodulator of the Sympathetic Nervous System (SNS) is norepinephrine, while the principal neurotransmitter of the Parasympathetic Nervous System (PNS) is acetylcholine. Through these two main neurohormones, the ANS can gradually regulate cardiac, vascular, visceral, and glandular functions by turning on one of its two branches (adrenergic and/or cholinergic), which exert opposite effects on targeted organs. Besides these neuromodulators, the cardiac nervous system is ruled by specific neuropeptides (neurotrophic factors) that help to preserve innervation homeostasis through the myocardial layers (from epicardium to endocardium). Interestingly, the dysregulation of this neuro-signaling pathway may expose the cardiac tissue to severe disorders of different etiology and nature. Specifically, a maladaptive remodeling of the cardiac nervous system may culminate in a progressive loss of neurotrophins, thus leading to severe myocardial denervation, as observed in different cardiometabolic and neurodegenerative diseases (myocardial infarction, heart failure, Alzheimer's disease). This review analyzes the current knowledge on the pathophysiological processes involved in cardiac nervous system impairment from the perspectives of both cardiac disorders and a widely diffused and devastating neurodegenerative disorder, Alzheimer's disease, proposing a relationship between neurodegeneration, loss of neurotrophic factors, and cardiac nervous system impairment. This overview is conducive to a more comprehensive understanding of the process of cardiac neuro-signaling dysfunction, while bringing to light potential therapeutic scenarios to correct or delay the adverse cardiovascular remodeling, thus improving the cardiac prognosis and quality of life in patients with heart or neurodegenerative disorders.

Ganglionic Plexus Ablation: A Step-by-step Guide for Electrophysiologists and Review of Modalities for Neuromodulation for the Management of Atrial Fibrillation

As the most common sustained arrhythmia, AF is a complex clinical entity which remains a difficult condition to durably treat in the majority of patients. Over the past few decades, the management of AF has focused mainly on pulmonary vein triggers for its initiation and perpetuation. It is well known that the autonomic nervous system (ANS) has a significant role in the milieu predisposing to the triggers, perpetuators and substrate for AF. Neuromodulation of ANS - ganglionated plexus ablation, vein of Marshall ethanol infusion, transcutaneous tragal stimulation, renal nerve denervation, stellate ganglion block and baroreceptor stimulation - constitute an emerging therapeutic approach for AF. The purpose of this review is to summarise and critically appraise the currently available evidence for neuromodulation modalities in AF.

Effects of Spinal Cord Injury Site on Cardiac Autonomic Regulation: Insight from Analysis of Cardiovascular Beat by Beat Variability during Sleep and Orthostatic Challenge

The goal of this study on Spinal Cord Injury (SCI) patients with cervical or thoracic lesion was to assess whether disturbances of ANS control, according to location, might differently affect vagal and sympatho-vagal markers during sleep and orthostatic challenge. We analyzed with linear and nonlinear techniques beat-by-beat RR and arterial pressure (and respiration) variability signals, extracted from a polysomnographic study and a rest-tilt test. We considered spontaneous or induced sympathetic excitation, as obtained shifting from non-REM to REM sleep or from rest to passive tilt. We obtained evidence of ANS cardiac (dys)regulation, of greater importance for gradually proximal location (i.e., cervical) SCI, compatible with a progressive loss of modulatory role of sympathetic afferents to the spinal cord. Furthermore, in accordance with the dual, vagal and sympathetic bidirectional innervation, the results suggest that vagally mediated negative feedback baroreflexes were substantially maintained in all cases. Conversely, the LF and HF balance (expressed specifically by normalized units) appeared to be negatively affected by SCI, particularly in the case of cervical lesion (group p = 0.006, interaction p = 0.011). Multivariate analysis of cardiovascular variability may be a convenient technique to assess autonomic responsiveness and alteration of functionality in patients with SCI addressing selectively vagal or sympathetic alterations and injury location. This contention requires confirmatory studies with a larger population.

Emergence of heartbeat frailty in advanced age I: perspectives from life-long EKG recordings in adult mice

The combined influences of sinoatrial nodal (SAN) pacemaker cell automaticity and its response to autonomic input determine the heart's beating interval variability and mean beating rate. To determine the intrinsic SAN and autonomic signatures buried within EKG RR interval time series change in advanced age, we measured RR interval variability before and during double autonomic blockade at 3-month intervals from 6 months of age until the end of life in long-lived (those that achieved the total cohort median life span of 24 months and beyond) C57/BL6 mice. Prior to 21 months of age, time-dependent changes in intrinsic RR interval variability and mean RR interval were relatively minor. Between 21 and 30 months of age, however, marked changes emerged in intrinsic SAN RR interval variability signatures, pointing to a reduction in the kinetics of pacemaker clock mechanisms, leading to reduced synchronization of molecular functions within and among SAN cells. This loss of high-frequency signal processing within intrinsic SAN signatures resulted in a marked increase in the mean intrinsic RR interval. The impact of autonomic signatures on RR interval variability were net sympathetic and partially compensated for the reduced kinetics of the intrinsic SAN RR interval variability signatures, and partially, but not completely, shifted the EKG RR time series intervals to a more youthful pattern. Cross-sectional analyses of other subsets of C57/BL6 ages indicated that at or beyond the median life span of our longitudinal cohort, noncardiac, constitutional, whole-body frailty was increased, energetic efficiency was reduced, and the respiratory exchange ratio increased. We interpret the progressive reduction in kinetics in intrinsic SAN RR interval variability signatures in this context of whole-body frailty beyond 21 months of age to be a manifestation of "heartbeat frailty."

Human in vitro neurocardiac coculture (ivNCC) assay development for evaluating cardiac contractility modulation

Two of the most prominent organ systems, the nervous and the cardiovascular systems, are intricately connected to maintain homeostasis in mammals. Recent years have shown tremendous efforts toward therapeutic modulation of cardiac contractility and electrophysiology by electrical stimulation. Neuronal innervation and cardiac ganglia regulation are often overlooked when developing in vitro models for cardiac devices, but it is likely that peripheral nervous system plays a role in the clinical effects. We developed an in vitro neurocardiac coculture (ivNCC) model system to study cardiac and neuronal interplay using human induced pluripotent stem cell (hiPSC) technology. We demonstrated significant expression and colocalization of cardiac markers including troponin, α-actinin, and neuronal marker peripherin in neurocardiac coculture. To assess functional coupling between the cardiomyocytes and neurons, we evaluated nicotine-induced β-adrenergic norepinephrine effect and found beat rate was significantly increased in ivNCC as compared to monoculture alone. The developed platform was used as a nonclinical model for the assessment of cardiac medical devices that deliver nonexcitatory electrical pulses to the heart during the absolute refractory period of the cardiac cycle, that is, cardiac contractility modulation (CCM) therapy. Robust coculture response was observed at 14 V/cm (5 V, 64 mA), monophasic, 2 ms pulse duration for pacing and 20 V/cm (7 V, 90 mA) phase amplitude, biphasic, 5.14 ms pulse duration for CCM. We observed that the CCM effect and kinetics were more pronounced in coculture as compared to cardiac monoculture, supporting a hypothesis that some part of CCM mechanism of action can be attributed to peripheral nervous system stimulation. This study provides novel characterization of CCM effects on hiPSC-derived neurocardiac cocultures. This innervated human heart model can be further extended to investigate arrhythmic mechanisms, neurocardiac safety, and toxicity post-chronic exposure to materials, drugs, and medical devices. We present data on acute CCM electrical stimulation effects on a functional and optimized coculture using commercially available hiPSC-derived cardiomyocytes and neurons. Moreover, this study provides an in vitro human heart model to evaluate neuronal innervation and cardiac ganglia regulation of contractility by applying CCM pulse parameters that closely resemble clinical setting. This ivNCC platform provides a potential tool for investigating aspects of cardiac and neurological device safety and performance.

Sympathetic activation in heart failure

Sympathetic activation has been long appreciated exclusively as a fundamental compensatory mechanism of the failing heart and, thus, welcome and to be supported. In the initial clinical phases of heart failure (HF), the sympathetic nervous system overdrive plays a compensatory function aimed at maintaining an adequate cardiac output despite the inotropic dysfunction affecting the myocardium. However, when the sympathetic reflex response is exaggerated it triggers a sequence of unfavourable remodelling processes causing a further contractile deterioration that unleashes major adverse cardiovascular consequences, favouring the HF progression and the occurrence of fatal events. Eventually, the sympathetic nervous system in HF was demonstrated to be a ‘lethality factor’ and thus became a prominent therapeutic target. The existence of an effective highly specialized intracardiac neuronal network immediately rules out the old concept that sympathetic activation in HF is merely the consequence of a drop in cardiac output. When a cardiac damage occurs, such as myocardial ischaemia or a primary myocardial disorder, the adaptive capability of the system may be overcame, leading to excessive sympatho-excitation coupled with attenuation till to abolishment of central parasympathetic drive. Myocardial infarction causes, within a very short time, both a functional and anatomical remodelling with a diffuse up-regulation of nerve growth factor (NGF). The subsequent nerve sprouting signal, facilitated by a rise in the levels of NGF in the left stellate ganglion and in the serum, triggers an increase in cardiac nerve density in both peri-infarct and non-infarcted areas. Finally, NFG production decreases over time, supposedly as an adaptative response to the prolonged exposure to sympathetic overactivity, leading in the end to a reduction in sympathetic nerve density. Accordingly, NGF levels were markedly reduced in patients with severe congestive heart failure. The kidney is the other key player of the sympathetic response to HF as it indeed reacts to under-perfusion and to loop diuretics to preserve filtration at the cost of many pathological consequences on its physiology. This vicious loop ultimately participates to the chronic and disruptive sympathetic overdrive.

In conclusion, sympathetic activation is the natural physiological consequence to life stressors but also to any condition that may harm our body. It is the first system of reaction to any potential life-threatening event. However, in any aspect of life over reaction is never effective but, in many instances, is, actually, life threatening. One for all is the case of ischaemia-related ventricular fibrillation which is, strongly facilitated by sympathetic hyperactivity. The take home message? When, in a condition of harm, everybody is yelling failure is just around the corner.

Anatomical characterization of vagal nodose afferent innervation and ending morphologies at the murine heart using a transgenic approach

Heart is an extensively innervated organ and its function is strictly coordinated by autonomic neural circuits. After pathological events such as myocardial infarction (MI), cardiac nerves undergo a structural and functional remodeling contributing to cardiac dysfunction. Although the efferent component of the cardiac nerves has been well described, sensory innervation of the heart has not been defined in detail. Considering its importance, comprehensive description of vagal afferent innervation on the whole heart would enable a better description of autonomic imbalances manifesting as sympathoexcitation and vagal withdrawal in post-ischemic states. To address this issue, we globally mapped the vagal nodose afferent fibers innervating the whole murine heart with unprecedented resolution. By using the Phox2b-Cre::tdTomato transgenic mouse line, we described the detailed distribution and distinct vagal sensory ending morphologies at both the dorsal and ventral sides of the mouse heart. By neural tracing analysis, we quantitated the distribution and prevalence of vagal afferent nerve fibers with varying diameters across dorsal and ventral surfaces of the heart. Moreover, we demonstrated that vagal afferents formed flower spray and end-net-like endings within the atria and ventricles. As distinct from the atria, vagal afferents formed intramuscular array-like endings within the ventricles. Furthermore, we showed that vagal afferents undergo structural remodeling by forming axonal sprouts around the infarct area in post-MI hearts. These findings improve our understanding of the potential effect of vagal afferent remodeling on autonomic imbalance and generation of cardiac arrhythmias and could prospectively contribute to the development of more effective neuromodulatory therapies.

Heart Rate Variability Reveals Altered Autonomic Regulation in Response to Myocardial Infarction in Experimental Animals

The analysis of beating rate provides information on the modulatory action of the autonomic nervous system on the heart, which mediates adjustments of cardiac function to meet hemodynamic requirements. In patients with myocardial infarction, alterations of heart rate variability (HRV) have been correlated to the occurrence of arrhythmic events and all-cause mortality. In the current study, we tested whether experimental rodent models of myocardial infarction recapitulate dynamics of heart rate variability observed in humans, and constitute valid platforms for understanding mechanisms linking autonomic function to the development and manifestation of cardiovascular conditions. For this purpose, HRV was evaluated in two engineered mouse lines using electrocardiograms collected in the conscious, restrained state, using a tunnel device. Measurements were obtained in naïve mice and animals at 3–∼28 days following myocardial infarction, induced by permanent coronary artery ligation. Two mouse lines with inbred and hybrid genetic background and, respectively, homozygous (Homo) and heterozygous (Het) for the MerCreMer transgene, were employed. In the naïve state, Het female and male mice presented prolonged RR interval duration (∼9%) and a ∼4-fold increased short- and long-term RR interval variability, with respect to sex-matched Homo mice. These differences were abrogated by pharmacological interventions inhibiting the sympathetic and parasympathetic axes. At 3–∼14 days after myocardial infarction, RR interval duration increased in Homo mice, but was not affected in Het animals. In contrast, Homo mice had minor modifications in HRV parameters, whereas substantial (> 50%) reduction of short- and long-term RR interval variation occurred in Het mice. Interestingly, ex vivo studies in isolated organs documented that intrinsic RR interval duration increased in infarcted vs. non-infarcted Homo and Het hearts, whereas RR interval variation was not affected. In conclusion, our study documents that, as observed in humans, myocardial infarction in rodents is associated with alterations in heart rhythm dynamics consistent with sympathoexcitation and parasympathetic withdrawal. Moreover, we report that mouse strain is an important variable when evaluating autonomic function via the analysis of HRV.

Drivers of Sinoatrial Node Automaticity in Zebrafish: Comparison With Mechanisms of Mammalian Pacemaker Function

Cardiac excitation originates in the sinoatrial node (SAN), due to the automaticity of this distinct region of the heart. SAN automaticity is the result of a gradual depolarisation of the membrane potential in diastole, driven by a coupled system of transarcolemmal ion currents and intracellular Ca²⁺ cycling. The frequency of SAN excitation determines heart rate and is under the control of extra- and intracardiac (extrinsic and intrinsic) factors, including neural inputs and responses to tissue stretch. While the structure, function, and control of the SAN have been extensively studied in mammals, and some critical aspects have been shown to be similar in zebrafish, the specific drivers of zebrafish SAN automaticity and the response of its excitation to vagal nerve stimulation and mechanical preload remain incompletely understood. As the zebrafish represents an important alternative experimental model for the study of cardiac (patho-) physiology, we sought to determine its drivers of SAN automaticity and the response to nerve stimulation and baseline stretch. Using a pharmacological approach mirroring classic mammalian experiments, along with electrical stimulation of intact cardiac vagal nerves and the application of mechanical preload to the SAN, we demonstrate that the principal components of the coupled membrane- Ca²⁺ pacemaker system that drives automaticity in mammals are also active in the zebrafish, and that the effects of extra- and intracardiac control of heart rate seen in mammals are also present. Overall, these results, combined with previously published work, support the utility of the zebrafish as a novel experimental model for studies of SAN (patho-) physiological function.

From Mice to Mainframes: Experimental Models for Investigation of the Intracardiac Nervous System

The intracardiac nervous system (IcNS), sometimes referred to as the "little brain" of the heart, is involved in modulating many aspects of cardiac physiology. In recent years our fundamental understanding of autonomic control of the heart has drastically improved, and the IcNS is increasingly being viewed as a therapeutic target in cardiovascular disease. However, investigations of the physiology and specific roles of intracardiac neurons within the neural circuitry mediating cardiac control has been hampered by an incomplete knowledge of the anatomical organisation of the IcNS. A more thorough understanding of the IcNS is hoped to promote the development of new, highly targeted therapies to modulate IcNS activity in cardiovascular disease. In this paper, we first provide an overview of IcNS anatomy and function derived from experiments in mammals. We then provide descriptions of alternate experimental models for investigation of the IcNS, focusing on a non-mammalian model (zebrafish), neuron-cardiomyocyte co-cultures, and computational models to demonstrate how the similarity of the relevant processes in each model can help to further our understanding of the IcNS in health and disease.

The "heart brain" and neuromodulation for vasovagal syncope

It is well known that the autonomic nervous system (ANS) is a major contributor in etiopathogenesis of vasovagal syncope (VVS). Catheter based neuromodulation (CNA) of the intrinsic cardiac ANS has evolved rapidly from being an experimental unproven procedure to its current status as an increasingly performed ablation procedure in many major hospitals worldwide. The present review aims to bring the anatomical elements of intrinsic cardiac ANS and clinical application of intrinsic cardiac neuromodulation together, by reviewing anatomical terminologies and clinical data, in order to provide a practical assistance to the electrophysiology community.

Neurohumoral Cardiac Regulation: Optogenetics Gets Into the Groove

The cardiac autonomic nervous system (ANS) is the main modulator of heart function, adapting contraction force, and rate to the continuous variations of intrinsic and extrinsic environmental conditions. While the parasympathetic branch dominates during rest-and-digest sympathetic neuron (SN) activation ensures the rapid, efficient, and repeatable increase of heart performance, e.g., during the "fight-or-flight response." Although the key role of the nervous system in cardiac homeostasis was evident to the eyes of physiologists and cardiologists, the degree of cardiac innervation, and the complexity of its circuits has remained underestimated for too long. In addition, the mechanisms allowing elevated efficiency and precision of neurogenic control of heart function have somehow lingered in the dark. This can be ascribed to the absence of methods adequate to study complex cardiac electric circuits in the unceasingly moving heart. An increasing number of studies adds to the scenario the evidence of an intracardiac neuron system, which, together with the autonomic components, define a little brain inside the heart, in fervent dialogue with the central nervous system (CNS). The advent of optogenetics, allowing control the activity of excitable cells with cell specificity, spatial selectivity, and temporal resolution, has allowed to shed light on basic neuro-cardiology. This review describes how optogenetics, which has extensively been used to interrogate the circuits of the CNS, has been applied to untangle the knots of heart innervation, unveiling the cellular mechanisms of neurogenic control of heart function, in physiology and pathology, as well as those participating to brain-heart communication, back and forth. We discuss existing literature, providing a comprehensive view of the advancement in the understanding of the mechanisms of neurogenic heart control. In addition, we weigh the limits and potential of optogenetics in basic and applied research in neuro-cardiology.

β-Adrenergic Stimulation Synchronizes a Broad Spectrum of Action Potential Firing Rates of Cardiac Pacemaker Cells toward a Higher Population Average

The heartbeat is initiated by pacemaker cells residing in the sinoatrial node (SAN). SAN cells generate spontaneous action potentials (APs), i.e., normal automaticity. The sympathetic nervous system increases the heart rate commensurate with the cardiac output demand via stimulation of SAN β-adrenergic receptors (βAR). While SAN cells reportedly represent a highly heterogeneous cell population, the current dogma is that, in response to βAR stimulation, all cells increase their spontaneous AP firing rate in a similar fashion. The aim of the present study was to investigate the cell-to-cell variability in the responses of a large population of SAN cells. We measured the βAR responses among 166 single SAN cells isolated from 33 guinea pig hearts. In contrast to the current dogma, the SAN cell responses to βAR stimulation substantially varied. In each cell, changes in the AP cycle length were highly correlated (R2 = 0.97) with the AP cycle length before βAR stimulation. While, as expected, on average, the cells increased their pacemaker rate, greater responses were observed in cells with slower basal rates, and vice versa: cells with higher basal rates showed smaller responses, no responses, or even decreased their rate. Thus, βAR stimulation synchronized the operation of the SAN cell population toward a higher average rate, rather than uniformly shifting the rate in each cell, creating a new paradigm of βAR-driven fight-or-flight responses among individual pacemaker cells.

Synchronized Cardiac Impulses Emerge From Heterogeneous Local Calcium Signals Within and Among Cells of Pacemaker Tissue

OBJECTIVES

This study sought to identify subcellular Ca²⁺ signals within and among cells comprising the sinoatrial node (SAN) tissue.

BACKGROUND

The current paradigm of SAN impulse generation: 1) is that full-scale action potentials (APs) of a common frequency are initiated at 1 site and are conducted within the SAN along smooth isochrones; and 2) does not feature fine details of Ca²⁺ signaling present in isolated SAN cells, in which small subcellular, subthreshold local Ca²⁺ releases (LCRs) self-organize to generate cell-wide APs.

METHODS

Immunolabeling was combined with a novel technique to detect the occurrence of LCRs and AP-induced Ca²⁺ transients (APCTs) in individual pixels (chronopix) across the entire mouse SAN images.

RESULTS

At high magnification, Ca²⁺ signals appeared markedly heterogeneous in space, amplitude, frequency, and phase among cells comprising an HCN4⁺/CX43^- cell meshwork. The signaling exhibited several distinguishable patterns of LCR/APCT interactions within and among cells. Rhythmic APCTs that were apparently conducted within the meshwork were transferred to a truly conducting HCN4^-/CX43⁺ network of striated cells via narrow functional interfaces where different cell types intertwine, that is, the SAN anatomic/functional unit. At low magnification, the earliest APCT of each cycle occurred within a small area of the HCN4 meshwork, and subsequent APCT appearance throughout SAN pixels was discontinuous and asynchronous.

CONCLUSIONS

The study has discovered a novel, microscopic Ca²⁺ signaling paradigm of SAN operation that has escaped detection using low-resolution, macroscopic tissue isochrones employed in prior studies: synchronized APs emerge from heterogeneous subcellular subthreshold Ca²⁺ signals, resembling multiscale complex processes of impulse generation within clusters of neurons in neuronal networks.

The cardiac autonomic nervous system: an introduction

In recent decades, numerous anatomical and physiological studies of the cardiac autonomic nervous system (ANS) have investigated the complex relationships between the brain and the heart. Autonomic activation not only alters heart rate, conduction, and hemodynamics, but also cellular and subcellular properties of individual myocytes. Moreover, the cardiac ANS plays an essential role in cardiac arrhythmogenesis. There is mounting evidence that neural modulation either by ablation or stimulation can effectively control a wide spectrum of cardiac arrhythmias. This article discusses anatomic aspects of the cardiac ANS, focusing on how autonomic activities influence cardiac electrophysiology. Specific autonomic triggers of various cardiac arrhythmias, in particular atrial fibrillation (AF) and ventricular arrhythmias, are also briefly discussed. Studies with heart-rate variability analysis indicate that, rather than being triggered by either vagal or sympathetic activity, the onset of AF can be associated with simultaneous discharge of both limbs, leading to an imbalance between these two arms of the cardiac ANS. At the same time, sudden cardiac death resulting from ventricular arrhythmias continues to be a significant health and societal burden. These nerve activities of the cardiac ANS can be targeted for the treatment for cardiac arrhythmias, in particular AF and ventricular tachyarrhythmias.

Pulmonary vein isolation in a real-world population does not influence QTc interval

AIMS

We aimed to examine whether routine pulmonary vein isolation (PVI) induces significant ventricular repolarization changes as suggested earlier.

METHODS AND RESULTS

Five-minute electrocardiograms were recorded at hospital's admission (T-1d), 1 day after the PVI-procedure (T+1d) and at 3 months post-procedure (T+3m) from a registry of consecutive atrial fibrillation (AF) patients scheduled for routine PVI with different PVI modalities (radiofrequency, cryo-ablation, and hybrid). Only patients who were in sinus rhythm at all three recordings (n = 117) were included. QT-intervals and QT-dispersion were evaluated with custom-made software and QTc was calculated using Bazett's, Fridericia's, Framingham's, and Hodges' formulas. Both QT- and RR-intervals were significantly shorter at T+1d (399 ± 37 and 870 ± 141 ms) and T+3m (407 ± 36 and 950 ± 140 ms) compared with baseline (417 ± 36 and 1025 ± 164 ms). There was no statistically significant within-subject difference in QTc Fridericia (T-1d 416 ± 28 ms, T+1d 419 ± 33 ms, and T+3m 414 ± 25 ms) and QT-dispersion (T-1d 18 ± 12 ms, T+1d 21 ± 19 ms, and T+3m 17 ± 12 ms) between the recordings. A multiple linear regression model with age, sex, AF type, ablation technique, first/re-do ablation, and AF recurrence to predict the change in QTc at T+3m with respect to QTc at T-1d did not reach significance which indicates that the change in QTc does not differ between all subgroups (age, sex, AF type, ablation technique, first/re-do ablation, and AF recurrence).

CONCLUSION

Based on our data a routine PVI does not result in a prolongation of QTc in a real-world population. These findings, therefore, suggest that there is no need to intensify post-PVI QT-interval monitoring.