Functional selectivity of cardiac preganglionic sympathetic neurones in the rabbit heart

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.

The Effects of Vagus Nerve Stimulation on Ventricular Electrophysiology and Nitric Oxide Release in the Rabbit Heart

Background: Abnormal autonomic activity including impaired parasympathetic control is a known hallmark of heart failure (HF). Vagus nerve stimulation (VNS) has been shown to reduce the susceptibility of the heart to ventricular fibrillation, however the precise underlying mechanisms are not well understood and the detailed stimulation parameters needed to improve patient outcomes clinically are currently inconclusive.

Objective: To investigate NO release and cardiac electrophysiological effects of electrical stimulation of the vagus nerve at varying parameters using the isolated innervated rabbit heart preparation.

Methods: The right cervical vagus nerve was electrically stimulated in the innervated isolated rabbit heart preparation (n = 30). Heart rate (HR), effective refractory period (ERP), ventricular fibrillation threshold (VFT) and electrical restitution were measured as well as NO release from the left ventricle.

Results: High voltage with low frequency VNS resulted in the most significant reduction in HR (by −20.6 ± 3.3%, −25.7 ± 3.0% and −30.5 ± 3.0% at 0.1, 1 and 2 ms pulse widths, with minimal increase in NO release. Low voltage and high frequency VNS significantly altered NO release in the left ventricle, whilst significantly flattening the slope of restitution and significantly increasing VFT. HR changes however using low voltage, high frequency VNS were minimal at 20Hz (to 138.5 ± 7.7 bpm (−7.3 ± 2.0%) at 1 ms pulse width and 141.1 ± 6.6 bpm (−4.4 ± 1.1%) at 2 ms pulse width).

Conclusion: The protective effects of the VNS are independent of HR reductions demonstrating the likelihood of such effects being as a result of the modulation of more than one molecular pathway. Altering the parameters of VNS impacts neural fibre recruitment in the ventricle; influencing changes in ventricular electrophysiology, the protective effect of VNS against VF and the release of NO from the left ventricle.

Limited Left Thoracoscopic Sympathectomy Effectively Silences Refractory Electrical Storm

BACKGROUND

Electrical Storm is a life-threatening condition that affects up to 20% of patients with ICDs. In this small retrospective study, we report our results with left video-assisted thoracoscopic sympathectomy/ganglionectomy (VATSG) to treat refractory electrical storm in low ejection fraction patients who were not candidates for catheter ablations (CA).

METHODS

We identified 12 patients who presented with electrical storm and underwent a total of 14 video assisted thoracoscopic sympathectomy/ganglionectomy, including three patients on venoarterial extracorporeal membrane oxygenation (VA ECMO). We reviewed demographic data, survival to discharge, number of cardioversions (before and after VATSG), need for readmissions, and need for right-sided procedures.

RESULTS

In the 30 days prior to a left VATSG the mean number of shocks was 22.67 for all patients. For the patients who survived to discharge the mean was 3.55 since surgery with a median of zero shocks after a median follow up of 358 days. Six patients have not experienced any further cardioversions since their last VATSG and five have never been readmitted for VT. Two patients had staged bilateral procedures due to recurrences and of those, one never required any further cardioversions.

CONCLUSIONS

Limited left VATSG is an appropriate and effective initial treatment for ES patients who are not candidates for CA, including patients on VA ECMO for hemodynamic support.

Cardiac innervation in acute myocardial ischaemia/reperfusion injury and cardioprotection

Acute myocardial infarction (AMI) and the heart failure (HF) that often complicates this condition, are among the leading causes of death and disability worldwide. To reduce myocardial infarct (MI) size and prevent heart failure, novel therapies are required to protect the heart against the detrimental effects of acute ischaemia/reperfusion injury (IRI). In this regard, targeting cardiac innervation may provide a novel therapeutic strategy for cardioprotection. A number of cardiac neural pathways mediate the beneficial effects of cardioprotective strategies such as ischaemic preconditioning and remote ischaemic conditioning, and nerve stimulation may therefore provide a novel therapeutic strategy for cardioprotection. In this article, we provide an overview of cardiac innervation and its impact on acute myocardial IRI, the role of extrinsic and intrinsic cardiac neural pathways in cardioprotection, and highlight peripheral and central nerve stimulation as a cardioprotective strategy with therapeutic potential for reducing MI size and preventing HF following AMI. This article is part of a Cardiovascular Research Spotlight Issue entitled 'Cardioprotection Beyond the Cardiomyocyte', and emerged as part of the discussions of the European Union (EU)-CARDIOPROTECTION Cooperation in Science and Technology (COST) Action, CA16225.

Potential role of sympathetic activity on the pathogenesis of massive pulmonary embolism with circulatory shock in rabbits

BACKGROUND

We recently showed that intravenous sodium nitroprusside treatment (SNP) could relieve the pulmonary vasospasm of pulmonary embolism (PE) and non-pulmonary embolism (non-PE) regions in a rabbit massive pulmonary embolism (MPE) model associated with shock. The present study explored the potential role of cardiopulmonary sympathetic activity on the pathogenesis and the impact of vasodilators on cardiopulmonary sympathetic activity in this model.

METHODS

Rabbits were randomly divided into sham operation group (S group, n = 8), model group (M, equal volume of saline intravenously, n = 11), SNP group (3.5 μg/kg/min intravenously, n = 10) and diltiazem group (DLZ, 6.0 μg/kg/min intravenously, n = 10).

RESULTS

MPE resulted in reduced mean arterial pressure and increased mean pulmonary arterial pressure as well as reduced PaO2 in the M, SNP and DLZ groups. Tyrosine hydroxylase (TH), neuropeptide Y (NPY) and endothelin-1 (ET-1) expression levels were significantly increased, while nitric oxide (NO) levels were reduced in both PE and non-PE regions in the M group. Both SNP and DLZ decreased mean pulmonary arterial pressure, reversed shock status, downregulated the expression of TH, NPY and ET-1, and increased NO levels in PE and non-PE regions.

CONCLUSION

Present results indicate that upregulation of the sympathetic medium transmitters TH and NPY in whole lung tissues serves one of the pathological features of MPE. The vasodilators SNP and DLZ could relieve pulmonary vasospasm in both embolization and non-embolization regions and reverse circulatory shock, thereby indirectly downregulating the sympathetic activation of the whole lung tissues and breaking a vicious cycle related to sympathetic activation in this model.

Data highlighting the effects of spinal segmental stimulation of preganglionic sympathetic neurons on the electrophysiology of the rabbit heart

This article presents data highlighting the functional selectivity of cardiac preganglionic sympathetic neurons in the rabbit heart. Specifically, the data draw attention to the role of each spinal segmental outflow on cardiac electrophysiology and the influence of each segment on cardiac excitability through investigating markers of arrhythmia such as electrical restitution. This data holds importance for exploring whether the preganglionic sympathetic neurons have functionally distinct pathways to the heart and whether some spinal segmental outflows have a greater potential for arrhythmia generation than others. Discussion of the data can be found in Chauhan et al. (2018) [1].