The autonomic nervous system (ANS) has a profound influence on cardiac function. Alterations in ANS balance has tremendous impact on the initiation and progression of cardiac pathology. Current standards of care (pharmacology, surgery, devices) have proven efficacy, but well-known limitations as well. Bioelectronic therapy has the potential to provide a reversible, on demand, scalable approach to cardiac control. The hypothesis of this thesis is that imbalances in information processing between different levels of the cardiac neuraxis is a primary determinant in mediating electrical instability of the heart and progression into heart failure. As a corollary, mitigating imposed neural imbalances within the hierarchy for cardiac control is cardioprotective.

To investigate bioelectronic modulation of the parasympathetic control of cardiac pathology, vagal nerve stimulation (VNS) was employed in a guinea pig pressure overload (PO) model of heart failure with preserved ejection fraction (HFpEF). Guinea pigs were subjected to aortic constriction, in the presence or absence of VNS. The effects on disease were assessed in the whole animal (echocardiography) and at the neural (intracellular current clamp recordings of intrinsic cardiac (IC) neurons) and myocyte (protein expression) levels.

To investigate bioelectronic modulation of sympathetic drive to the heart, proof of concept studies involving kilohertz frequency alternating current (KHFAC) and charge-balanced direct current carousel (CBDCC) were employed in an anesthetized porcine model. CBDCC was also used to determine impact on arrhythmia potential in a porcine myocardial infarction (MI) model.

With VNS therapy, PO-induced hemodynamic responses (increased cardiac output, systolic/diastolic left ventricular volumes) were prevented. Enhancements in synaptic efficacy muscarinic sensitivity were also mitigated. Myocyte hypertrophy was prevented using right-sided VNS. PO-induced hypertrophic myocardium also abnormal energetics, which was modulated with VNS application.

For sympathetic control, KHFAC and CBDCC demonstrated reversible, current-dependent impact on cardiac indices (activation recovery intervals (ARI), heart rate, and contractility). In the MI model, extrasystolic (S1-S2) stimulations induced ventricular arrhythmias in all animals (n=7), whereas right-sided CBDCC application reduced inducibility by 83%. The ventricular effective refractory period (VERP) was prolonged with CBDCC (323�26 ms) compared to baseline (271�32 ms).

In conclusion, the data within this thesis provides validation for a mechanism-based approach to cardiac therapy that combines the potential of bioelectronic therapy with a detailed knowledge of cardiac neuraxis structure/function, addressing a significant unmet need in managing both arrhythmia burden and the progression of heart failure.

The autonomic nervous system (ANS) regulates normal cardiovascular function and plays a critical role in the pathophysiology of cardiovascular disease. Neuroablative techniques to modulate control of cardiac function are currently being studied in patients, albeit with variable and sometimes deleterious results. Further understanding of the interplay between the ANS and cardiovascular system holds promise for the development and refinement of neuroscience-based cardiovascular therapeutics. To this end, techniques to image myocardial innervation will help provide a basis for understanding the fundamental underpinnings of cardiac neural control. The evolution of gross and microscopic anatomical studies for functional mapping of cardiac neuroanatomy are reviewed here. Recognizing the major gaps in our understanding of cardiac neural control, the neural regulation of impulse initiation in the sinoatrial node as an initial discovery step is evaluated. This represents an in-depth, multi-scale structural and functional characterization of the innervation of the sinoatrial node (SAN) by the right atrial ganglionated plexus (RAGP) in porcine and human hearts. Combining intersectional strategies including tissue clearing, immunohistochemical and ultrastructural techniques, a comprehensive neuroanatomic atlas of the RAGP-SAN complex is delineated. The RAGP shows significant phenotypic diversity of neurons while maintaining predominant cholinergic innervation. Cellular and tissue-level electrophysiological mapping and ablation studies demonstrate interconnected ganglia with synaptic convergence within the RAGP to modulate SAN automaticity, atrioventricular conduction and left ventricular contractility. Using this approach, intrinsic cardiac neurons’ influence on the pacemaking site in the heart is comprehensively demonstrated. This provides an experimental demonstration of a discrete neuronal population controlling a specific geographic region of the heart (SAN) that can serve as a framework for further exploration of other parts of the intrinsic cardiac nervous system in mammalian hearts and for developing targeted therapies.

Autonomic dysregulation in cardiovascular disease plays a major role in the pathogenesis of arrhythmias. Cardiac neural control relies on complex feedback loops consisting of efferent and afferent limbs, which carry sympathetic and parasympathetic signals from the brain to the heart and sensory signals from the heart to the brain. Cardiac disease leads to neural remodeling and sympathovagal imbalances with arrhythmogenic effects. Preclinical studies of modulation at central and peripheral levels of the cardiac autonomic nervous system have yielded promising results, leading to early-stage clinical studies of these techniques in atrial fibrillation and refractory ventricular arrhythmias, particularly in patients with inherited primary arrhythmia syndromes and structural heart disease. However, significant knowledge gaps in basic cardiac neurophysiology limit the success of these neuromodulatory therapies. At the level of the myocardium, remodeling of injured sympathetic nerves on the heart after myocardial infarction (MI) contributes to adverse outcomes such as sudden arrhythmic death, yet the underlying structural mechanisms are poorly understood. We sought to examine microstructural changes on the post-MI heart and to directly link these changes with electrical dysfunction. We developed a high-resolution pipeline for anatomically precise alignment of electrical maps with structural myofiber and nerve-fiber maps created by customized computer vision algorithms. Using this integrative approach in a mouse model, we identified distinct structure-function correlates to objectively delineate the infarct border zone, a known source of post-MI arrhythmias. During tyramine-induced sympathetic nerve activation, we demonstrated regional patterns of altered electrical conduction aligned directly with altered neuroeffector junction distribution, pointing to potential neural substrates for cardiac arrhythmia. The pathophysiological alterations in sympathetic function of border zone nerve sprouts led us to initiate further mechanistic studies of how altered neural patterning fundamentally affects cardiac electrophysiology. Overall, this dissertation establishes a synergistic framework for examining structure-function relationships after MI with microscopic precision, which has potential to advance understanding of arrhythmogenic mechanisms.

The autonomic nervous system regulates cardiac excitability and genesis of ventricular tachyarrhythmias (VT/VF). Myocardial infarction leads to sympathetic activation and parasympathetic dysfunction, which act in concert to increase risk of sudden death due to VT/VF. The mechanism behind parasympathetic dysfunction and the electrophysiological effects of vagal nerve stimulation are poorly characterized.

The vago-symapthetic trunks carries both efferent cardiomotor as well as cardiac afferent fibers that transduce mechano-chemical changes. To better delineate and compare their functional effects, in normal porcine hearts in vivo, right and left vagal nerve stimulation (VNS) was performed and hemodynamic parameters and activation recovery interval mapping (ARI) performed. ARI is a validated surrogate of local action potential duration. Subsequently, role of afferent fiber activation during VNS on efferent control of cardiac function was assessed by transection of right and/or left, and ispilateral, contralateral, and/or bilateral trunks. Finally, mechanism behind neural parasympathetic remodeling due to myocardial infarction (MI) and the anti-arrhythmic effects of VNS were assessed in a chronic porcine infarct model.

In normal porcine hearts, right and left VNS significantly decreased heart rate and dP/dt max and prolonged regional ARI. No anterior-posterior-lateral ventricular regional differences in prolongation of ARI were found. However, endocaridal ARI increased more than epicardial ARI, and apical ARI more than basal ARI, during right and left VNS. With regards to afferent fiber activation during VNS, an augmentation in hemodynamic and electrophysiological effects of right VNS after ipsilateral vagal nerve transection was observed. Similar changes were suggested by left VNS after left vagal nerve transection, however, contralateral vagal nerve transection did not modify VNS response. Finally, measurement of regional acetylcholine, the primary neurotransmitter of cardiac parasympathetic neurons, showed that this neurotransmitter is primarily preserved in infarcted hearts. However, direct neural recordings from intrinsic cardiac ganglia showed significant alterations in the basal activity of parasympathetic efferent neurons that receive input from the left vagal trunk, suggesting a decrease in central parasympathetic drive. Augmentation of parasympathetic drive with VNS reduced ventricular tachy-arrhythmia inducibility by decreasing dispersion of repolarization in border zone regions of infarcted hearts.

In conclusion, both right and left vagi provided significant innervation to the ventricular myocardium, modulating both cardiac electrical and mechanical function. VNS activated both afferent and efferent fibers in the ispilateral vagal trunk, and afferent fiber activation reduced efferent physiological effects. Myocardial infarction led to significant neural remodeling and alteration in the behavior of parasympathetic neurons in the cardiac ganglia, changes that were suggestive of decreased central parasympathetic drive. VNS restored this drive and electrically stabilized infarct border zones, reducing ventricular arrhythmias.

Cardiovascular diseases including hypertension, arrhythmias, and heart failure are the leading causes of morbidity and mortality in the world. The autonomic nervous system (ANS) regulates the electrical and mechanical function of the heart, and its dysfunction plays an important role in the pathophysiology of cardiovascular diseases, representing an emerging target for therapeutic intervention. However, fundamental gaps in our knowledge regarding autonomic control of the heart in health and the adverse remodeling that occurs in disease have impeded the development of neuromodulation therapies. The cardiac ANS consists of neurons located from the level of the brain to that of the heart itself. At the organ level, the intrinsic cardiac nervous system (ICNS), a distributed network of ganglia and interconnecting nerves, serves as the final common pathway for the integration of neural inputs to the heart. Despite the importance of the ICNS, the impact of acute and chronic stress on the function of this neural network is not well-characterized. Therefore, we investigated the acute effects of premature ventricular contractions (PVCs) and the chronic remodeling induced by myocardial infarction (MI) on the ICNS by obtaining in vivo neuronal recordings from beating hearts. Both PVCs and MI affect a critical population of neurons within the ICNS—local circuit neurons. Given that they receive a convergence of afferent and efferent inputs and have intra- and interganglionic projections, local circuit neurons integrate information and coordinate regional cardiac function. Further, MI results in the formation of a neural sensory border zone, characterized by diminished afferent signals from the infarct and preserved signals from border and remote regions of the heart. These neural effects may underlie, at least in part, the mechanical dyssynchrony, electrical instability, and reflex activation of the ANS noted with PVCs and after MI. In addition, we demonstrated that the evoked cardiac response to vagus nerve stimulation, a neuromodulation therapy being evaluated for cardiac conditions, represents the dynamic interplay between afferent and efferent vagal fibers and reflex responses of the ANS. In conclusion, a better understanding of autonomic regulation of the heart will lead to the development of rationale neuroscience-based therapies for cardiovascular diseases.