Vagally-mediated heart block after myocardial infarction associated with plasticity of epicardial neurons controlling the atrioventricular node.

Imbalances in the opposing actions of sympathetic and parasympathetic nerves controlling the heart enhance risk for arrhythmia and sudden cardiac death after myocardial infarction (MI). Plasticity in peripheral neuron function may underlie the observed changes in cardiomotor nerve activity. We studied vagal control of the heart in pigs after chronic infarction of the left ventricle. Stimulation of the cervical vagus nerve produced greater bradycardic responses 8-weeks after MI. Recordings of epicardial electrocardiograms demonstrate increased severity and duration of atrioventricular (AV) block in MI-pigs during 20 Hz vagal stimulation. Intracellular voltage recordings from isolated neurons of the inferior vena cava-inferior left atrium (IVC-ILA) ganglionated plexus, a cluster of epicardial neurons receiving innervation from the vagus known to regulate the AV node, were used to assess plasticity of membrane and synaptic physiology of intrinsic cardiac neurons (ICNs) after MI. Changes to both passive and active membrane properties were observed, including more negative resting membrane potentials and greater input resistances in MI-pig ICNs, concomitant with a depression of neuronal excitability. Immunoreactivity to pituitary adenylate cyclase-activating polypeptide (PACAP), a cardiotropic peptide known to modulate cardiac neuron excitability, was localized to perineuronal varicosities surrounding pig IVC-ILA neurons. Exogenous application of PACAP increased excitability of control but not MI-ICNs. Stimulation (20 Hz) of interganglionic nerves in the ex vivo whole-mount preparations elicited slow excitatory postsynaptic potentials (sEPSPs) which persisted in hexamethonium (500 µM), but were blocked by atropine (1 µM), indicating muscarinic receptor-mediated inhibition of M-current. Extracellular application of 1 mM BaCl2 to inhibit M-current increased neuronal excitability. The muscarine-sensitive sEPSPs were observed more frequently and were of larger amplitude in IVC-ILA neurons from MI animals. In conclusion, we suggest the increased probability of muscarinic sEPSPs play a role in the potentiation of the vagus nerve mediated-slowing of AV nodal conduction following chronic MI. We identify both a novel role of a muscarinic sensitive current in the regulation of synaptic strength at ICNs projecting to the AV node, and demonstrate changes to both intrinsic plasticity and synaptic plasticity of IVC-ILA neurons which may contribute to greater risk for heart block and sudden cardiac death after MI.

Scalable and reversible axonal neuromodulation of the sympathetic chain for cardiac control.

Maladaptation of the sympathetic nervous system contributes to the progression of cardiovascular disease and risk for sudden cardiac death, the leading cause of mortality worldwide. Axonal modulation therapy (AMT) directed at the paravertebral chain blocks sympathetic efferent outflow to the heart and maybe a promising strategy to mitigate excess disease-associated sympathoexcitation. The present work evaluates AMT, directed at the sympathetic chain, in blocking sympathoexcitation using a porcine model. In anesthetized porcine (n = 14), we applied AMT to the right T1-T2 paravertebral chain and performed electrical stimulation of the distal portion of the right sympathetic chain (RSS). RSS-evoked changes in heart rate, contractility, ventricular activation recovery interval (ARI), and norepinephrine release were examined with and without kilohertz frequency alternating current block (KHFAC). To evaluate efficacy of AMT in the setting of sympathectomy, evaluations were performed in the intact state and repeated after left and bilateral sympathectomy. We found strong correlations between AMT intensity and block of sympathetic stimulation-evoked changes in cardiac electrical and mechanical indices (r = 0.83-0.96, effect size d = 1.9-5.7), as well as evidence of sustainability and memory. AMT significantly reduced RSS-evoked left ventricular interstitial norepinephrine release, as well as coronary sinus norepinephrine levels. Moreover, AMT remained efficacious following removal of the left sympathetic chain, with similar mitigation of evoked cardiac changes and reduction of catecholamine release. With growth of neuromodulation, an on-demand or reactionary system for reversible AMT may have therapeutic potential for cardiovascular disease-associated sympathoexcitation.NEW & NOTEWORTHY Autonomic imbalance and excess sympathetic activity have been implicated in the pathogenesis of cardiovascular disease and are targets for existing medical therapy. Neuromodulation may allow for control of sympathetic projections to the heart in an on-demand and reversible manner. This study provides proof-of-concept evidence that axonal modulation therapy (AMT) blocks sympathoexcitation by defining scalability, sustainability, and memory properties of AMT. Moreover, AMT directly reduces release of myocardial norepinephrine, a mediator of arrhythmias and heart failure.

Autonomic Nervous System Dysfunction: JACC Focus Seminar.

Autonomic nervous system control of the heart is a dynamic process in both health and disease. A multilevel neural network is responsible for control of chronotropy, lusitropy, dromotropy, and inotropy. Intrinsic autonomic dysfunction arises from diseases that directly affect the autonomic nerves, such as diabetes mellitus and the syndromes of primary autonomic failure. Extrinsic autonomic dysfunction reflects the changes in autonomic function that are secondarily induced by cardiac or other disease. An array of tests interrogate various aspects of cardiac autonomic control in either resting conditions or with physiological perturbations from resting conditions. The prognostic significance of these assessments have been well established. Clinical usefulness has not been established, and the precise mechanistic link to mortality is less well established. Further efforts are required to develop optimal approaches to delineate cardiac autonomic dysfunction and its adverse effects to develop tools that can be used to guide clinical decision-making.

Bioelectronic block of paravertebral sympathetic nerves mitigates post-myocardial infarction ventricular arrhythmias.

BACKGROUND: Autonomic dysfunction contributes to induction of ventricular tachyarrhythmia (VT). OBJECTIVE: To determine the efficacy of charge-balanced direct current (CBDC), applied to the T1-T2 segment of the paravertebral sympathetic chain, on VT inducibility post-myocardial infarction (MI). METHODS: In a porcine model, CBDC was applied in acute animals (n = 7) to optimize stimulation parameters for sympathetic blockade and in chronic MI animals (n = 7) to evaluate the potential for VTs. Chronic MI was induced by microsphere embolization of the left anterior descending coronary artery. At termination, in anesthetized animals and following thoracotomy, an epicardial sock array was placed over both ventricles and a quadripolar carousel electrode positioned underlying the right T1-T2 paravertebral chain. In acute animals, the efficacy of CBDC carousel (CBDCC) block was assessed by evaluating cardiac function during T2 paravertebral ganglion stimulation with and without CBDCC. In chronic MI animals, VT inducibility was assessed by extrasystolic (S1-S2) stimulations at baseline and under >66% CBDCC blockade of T2-evoked sympathoexcitation. RESULTS: CBDCC demonstrated a current-dependent and reversible block without impacting basal cardiac function. VT was induced at baseline in all chronic MI animals. One animal died after baseline induction. Of the 6 remaining animals, only 1 was reinducible with simultaneous CBDCC application (P < .002 from baseline). The ventricular effective refractory period (VERP) was prolonged with CBDCC (323 ± 26 ms) compared to baseline (271 ± 32 ms) (P < .05). CONCLUSIONS: Axonal block of the T1-T2 paravertebral chain with CBDCC reduced VT in a chronic MI model. CBDCC prolonged VERP, without altering baseline cardiac function, resulting in improved electrical stability.

Effect of Thoracic Epidural Anesthesia on Ventricular Excitability in a Porcine Model.

BACKGROUND: Imbalances in the autonomic nervous system, namely, excessive sympathoexcitation, contribute to ventricular tachyarrhythmias. While thoracic epidural anesthesia clinically suppresses ventricular tachyarrhythmias, its effects on global and regional ventricular electrophysiology and electrical wave stability have not been fully characterized. The authors hypothesized that thoracic epidural anesthesia attenuates myocardial excitability and the proarrhythmic effects of sympathetic hyperactivity. METHODS: Yorkshire pigs (n = 15) had an epidural catheter inserted (T1 to T4) and a 56-electrode sock placed on the heart. Myocardial excitability was measured by activation recovery interval, dispersion of repolarization, and action potential duration restitution at baseline and during programed ventricular extrastimulation or left stellate ganglion stimulation, before and 30 min after thoracic epidural anesthesia (0.25% bupivacaine). RESULTS: After thoracic epidural anesthesia infusion, there was no change in baseline activation recovery interval or dispersion of repolarization. During programmed ventricular extrastimulation, thoracic epidural anesthesia decreased the maximum slope of ventricular electrical restitution (0.70 ± 0.24 vs. 0.89 ± 0.24; P = 0.021) reflecting improved electrical wave stability. Thoracic epidural anesthesia also reduced myocardial excitability during left stellate ganglion stimulation-induced sympathoexcitation through attenuated shortening of activation recovery interval (-7 ± 4% vs. -4 ± 3%; P = 0.001), suppression of the increase in dispersion of repolarization (313 ± 293% vs. 185 ± 234%; P = 0.029), and reduction in sympathovagal imbalance as measured by heart rate variability. CONCLUSIONS: Our study describes the electrophysiologic mechanisms underlying antiarrhythmic effects of thoracic epidural anesthesia during sympathetic hyperactivity. Thoracic epidural anesthesia attenuates ventricular myocardial excitability and induces electrical wave stability through its effects on activation recovery interval, dispersion of repolarization, and the action potential duration restitution slope.

Bioelectronic neuromodulation of the paravertebral cardiac efferent sympathetic outflow and its effect on ventricular electrical indices.

BACKGROUND: Neuromodulation of the paravertebral ganglia by using symmetric voltage controlled kilohertz frequency alternating current (KHFAC) has the potential to be a reversible alternative to surgical intervention in patients with refractory ventricular arrhythmias. KHFAC creates scalable focal inhibition of action potential conduction. OBJECTIVE: The purpose of this article was to evaluate the efficacy of KHFAC when applied to the T1-T2 paravertebral chain to mitigate sympathetic outflow to the heart. METHODS: In anesthetized, vagotomized, porcine subjects, the heart was exposed via a midline sternotomy along with paravertebral chain ganglia. The T3 paravertebral ganglion was electrically stimulated, and activation recovery intervals (ARIs) were obtained from a 56-electrode sock placed over both ventricles. A bipolar Ag electrode was wrapped around the paravertebral chain between T1 and T2 and connected to a symmetric voltage controlled KHFAC generator. A comparison of cardiac indices during T3 stimulation conditions, with and without KHFAC, provided a measure of block efficacy. RESULTS: Right-sided T3 stimulation (at 4 Hz) was titrated to produce reproducible ARI changes from baseline (52 ± 30 ms). KHFAC resulted in a 67% mitigation of T3 electrical stimulation effects on ARI (18.5 ± 22 ms; P < .005). T3 stimulation repeated after KHFAC produced equivalent ARI changes as control. KHFAC evoked a transient functional sympathoexcitation at onset that was inversely related to frequency and directly related to intensity. The optimum block threshold was 15 kHz and 15 V. CONCLUSION: KHFAC applied to nexus (convergence) points of the cardiac nervous system produces a graded and reversible block of underlying axons. As such, KHFAC has the therapeutic potential for on-demand and reversible mitigation of sympathoexcitation.

Targeted stellate decentralization: Implications for sympathetic control of ventricular electrophysiology.

BACKGROUND: Selective bilateral cervicothoracic sympathectomy has proven to be effective for managing ventricular arrhythmias in the setting of structural heart disease. In the procedure currently used, the caudal portions of both stellate ganglia along with thoracic chain ganglia down to T4 ganglia are removed. OBJECTIVE: The purpose of this study was to define the relative contributions of the T1-T2 and T3-T4 paravertebral ganglia in modulating ventricular electrical function. METHODS: In anesthetized vagotomized porcine subjects (n = 8), the heart was exposed via sternotomy along with right and left paravertebral sympathetic ganglia to the T4 level. A 56-electrode epicardial sock was placed over both ventricles to assess epicardial activation-recovery intervals (ARIs) in response to individually stimulating right and left stellate vs T3 paravertebral ganglia. Responses to T3 stimuli were repeated after surgical removal of the caudal portions of stellate ganglia and T2 bilaterally. RESULTS: In intact preparations, stellate ganglion vs T3 stimuli (4 Hz, 4-ms duration) were titrated to produce equivalent decreases in global ventricular ARIs (right side: 85 ± 6 ms vs 55 ± 10 ms; left side: 24 ± 3 ms vs 17 ± 7 ms). Threshold of stimulus intensity applied to T3 ganglia to achieve threshold was 3 times that of T1 threshold. ARIs in unstimulated states were unaffected by bilateral stellate-T2 ganglion removal. After acute decentralization, T3 stimulation failed to change ARIs. CONCLUSION: Preganglionic sympathetic efferents arising from the T1-T4 spinal cord that project to the heart transit through stellate ganglia via the paravertebral chain. Thus, T1-T2 surgical excision is sufficient to functionally interrupt central control of peripheral sympathetic efferent activity.

Autonomic Regulation Therapy in Heart Failure.

Autonomic regulation therapy (ART) is a rapidly emerging therapy in the management of congestive heart failure secondary to systolic dysfunction. Modulation of the cardiac neuronal hierarchy can be achieved with bioelectronics modulation of the spinal cord, cervical vagus, baroreceptor, or renal nerve ablation. This review will discuss relevant preclinical and clinical research in ART for systolic heart failure. Understanding mechanistically what is being stimulated within the autonomic nervous system by such device-based therapy and how the system reacts to such stimuli is essential for optimizing stimulation parameters and for the future development of effective ART.

Post mortem study of the depth and circumferential location of sympathetic nerves in human renal arteries--implications for renal denervation catheter design.

OBJECTIVES: The aims of this study were to examine human renal arteries and to accurately characterize their sympathetic innervation and location using CD-56 immunohistochemistry stains to highlight Neural Cell Adhesion Molecules (N-CAM). BACKGROUND: Porcine models have often formed the basis for design of denervation technology, with only a limited number of human studies available to detail the complex microarray of renal sympathetic nerves. METHODS: Post-mortem renal arteries (N = 14) were harvested and prepared into three sections (proximal, mid, and distal), and then stained using Hematoxylin and Eosin, followed by immunohistochemistry to characterize the expression of CD-56 renal neural tissue. Digital micro calipers were then used to measure the nerve distances and locations within the vessels. RESULTS: (i) Approximately 77% of nerves are located between 0.5 and 2.5 mm from the tunica intima layer, with 22.5% occurring in the 2.5-5.0 mm range, (ii) nerve bundles occur in 3-dimensional arborized arrays, (iii) the nerve bundles are evenly distributed throughout the proximal and distal vessel in this human study. Thickness of vessel wall correlated with proximity of the nerve bundles (r = 0.74, P < 0.01), and nerve bundle thickness (r = 0.62, P = 0.04). The larger the internal and external diameters and areas of the vessel were, the further the distance to the nearest nerve bundles were (r = 0.752, P =<0.01). CONCLUSIONS: In human renal arteries with larger diameters and thicker vessel parenchyma, the innervation is found further from the lumen, and the nerves increase in thickness. This has implications for catheter and system design, as well as depth and duration of energy required for effective ablations. Effective percutaneous transluminal denervation procedures in this population would need to be circumferential rather than interrupted, and to mediate tissue damage to depths beyond 2.5 mm from the tunica intima.

Deficiencies in patients' comprehension of implantable cardioverter defibrillator therapy.

BACKGROUND: Patients receive education before implantable cardioverter defibrillator (ICD) implantation. Patients' understanding of ICD therapy requires investigation. METHODS: A retrospective cohort study was carried out at two implant centers where patients are educated during a consenting process pre-ICD implantation. Questionnaires examining understanding of ICD therapy were completed during telephone interviews of patients with ICDs. RESULTS: Of 75 patients interviewed, 62 (83%) were male. The median age at time of ICD implantation was 64 years (standard deviation [SD] = 9.4; range: 29-82 years). The median interval from implantation to interview was 3 years (SD = 1.9; range: 0.1-9.0 years). Despite 83% (62 of 75) claiming to understand the reason for ICD implantation, no patient suggested arrhythmia termination when describing the indication. Of shock recipients, 60% (12 of 20) felt poorly prepared for shock therapy. Of patients who experienced a device-related complication, 83% (10 of 12) reported feeling inadequately forewarned of complications. Excluding patients with cardiac resynchronization therapy defibrillators (n = 6), 65% (45 of 69), 52% (36 of 69), 50% (35 of 69), and 61% (42 of 69) believe their ICD reduces risk of heart attack and improves breathing, exercise capacity, and heart function, respectively. Ninety-three percent (70 of 75) are satisfied with their decision to accept ICD therapy. Only 12% (9 of 75) believe they will want to inactivate therapies in setting of terminal illness. CONCLUSIONS: Despite preimplantation education, patient comprehension of the risks and benefits of ICD therapy is poor. Patients' expectations of ICD therapy may be inappropriate. Education strategies before and after implantation require improvement.

Cost implications of defibrillator lead failures.

AIMS: The prevalence of lead failures is increasing with a growing population of implantable cardioverter defibrillator (ICD) recipients. The cost of managing defibrillator lead failures requires investigation. METHODS AND RESULTS: A retrospective cohort study of patients requiring lead replacement for defibrillator lead failure was performed. Details pertaining to admissions were recorded. The cost per lead replacement was determined. Twenty-three patients {mean age [standard deviation (SD); range] = 56 (17; 18-83) years; 87% male} underwent lead replacement at a mean (SD; range) interval from implant of 3.0 (1.8; 0.9-9.0) years. The median (SD; range) length of hospital stay was 4.5 (8.6; 1-43) days. Procedure-related complications were recorded for three (13%) patients. Thirty days and 1-year mortality were 0 and 4% (1 of 23). The median (SD; range) cost per lead replacement was €7660 (€10 964; €1472-39 663). Bed day costs accounted for 54% of overall costs. Extraction of the failed lead by manual traction at time of lead replacement did not significantly increase costs. The median (SD; range) cost of lead replacement was higher in patients receiving a new ICD generator (n= 6), compared with patients retaining existing generators (n= 17): €23 394 (€5026; €17 266-31 245) vs. €4470 (€9080; €1472-39 663); P= 0.005. The median (SD; range) cost of lead replacement among patients who remained in hospital pending lead replacement (n= 16) was higher than for patients who underwent replacement on an emergent outpatient basis (n= 7): €8508 (€11 920; €1472-39 663) vs. €4372 (€7256; €1555-20 478); however, this observation was not statistically significant, P= 0.21. CONCLUSIONS: Defibrillator lead failures incur significant cost and are likely to undermine overall cost effectiveness of ICDs. Cost-effectiveness analyses of device therapy should include costs related to such complications.