Model-Based Design and Experimental Validation of Control Modules for Neuromodulation Devices

International audienceGoal - The goal of this paper is to propose a model-based control design framework, adapted to the development of control modules for medical devices. A particular example is presented in which instantaneous heart rate is regulated in real-time, by modulating, in an adaptive manner, the current delivered to the vagus nerve by a neuromodulator. Methods - The proposed framework couples a control module, based on a classical PI controller, a mathematical model of the medical device, and a physiological model representing the cardiovascular responses to vagus nerve stimulation (VNS). In order to analyze and evaluate the behavior of the device, different control parameters are tested on a "virtual population," generated with the model, according to the Latin Hypercube sampling method. In particular, sensitivity analyses are applied for the identification of a domain of interest in the space of the control parameters. The obtained control parameter domain has been validated in an experimental evaluation on six sheep. Results - A range of control parameters leading to accurate results was successfully estimated by the proposed model-based design method. Experimental evaluation of the control parameters inside such a domain led to the best compromise between accuracy and time response of the VNS control. Conclusion - The feasibility and usefulness of the proposed model-based design method were shown, leading to a functional, real-time closed-loop control of the VNS for the regulation of heart rate

Cardiac autonomic modulation in drug-resistant epilepsy patients after vagus nerve stimulation therapy

The positive effect of vagus nerve stimulation (VNS) in patients with drug-resistant epilepsy is considered to be mediated by the afferent pathways of the vagus nerve, but the efferent pathways may influence the cardiac autonomic activity.Aim of the study. To assess the effects of VNS on cardiac autonomic modulation in epilepsy patients, over three months of neurostimulation.Clinical rationale for the study. Linear and non-linear heart rate variability (HRV) analysis can provide information on the sympathovagal balance and reveal particularities of the central control of the autonomic cardiovascular function.Materials and Methods. Using Biopac Acquisition System, we analysed HRV parameters in resting condition and during sympathetic and parasympathetic activation tests in five patients with drug-resistant epilepsy, who underwent VNS procedure.Results. During the sympathetic and vagal activation tests, all five patients presented normal responses of cardiac autonomic activity, reflected in RMSSD, HFnu and LF/HF dynamics in both HRV evaluations. No bradycardia, cardiac arrhythmia or orthostatic hypotension was registered during the two evaluations.Conclusions. Our results indicate that VNS appears not to alter the cardiac autonomic function after three months of neurostimulation. HRV analysis is a useful tool for evaluating cardiac autonomic modulation in epilepsy patients during VNS therapy.Clinical Implications. Patients with decreased HRV should be periodically monitored. Cardiac changes in patients with epilepsy are important because of the additional risk of arrhythmias mediated through the autonomic dysfunction

Vagus nerve stimulation: State of the art of stimulation and recording strategies to address autonomic function neuromodulation

International audienceObjective. Neural signals along the vagus nerve (VN) drive many somatic and autonomic functions. The clinical interest of VN stimulation (VNS) is thus potentially huge and has already been demonstrated in epilepsy. However, side effects are often elicited, in addition to the targeted neuromodulation. Approach. This review examines the state of the art of VNS applied to two emerging modulations of autonomic function: heart failure and obesity, especially morbid obesity. Main results. We report that VNS may benefit from improved stimulation delivery using very advanced technologies. However, most of the results from fundamental animal studies still need to be demonstrated in humans

Neural regulation of cardiovascular response to exercise: role of central command and peripheral afferents

During dynamic exercise, mechanisms controlling the cardiovascular apparatus operate to provide adequate oxygen to fulfill metabolic demand of exercising muscles and to guarantee metabolic end-products washout. Moreover, arterial blood pressure is regulated to maintain adequate perfusion of the vital organs without excessive pressure variations. The autonomic nervous system adjustments are characterized by a parasympathetic withdrawal and a sympathetic activation. In this review, we briefly summarize neural reflexes operating during dynamic exercise. The main focus of the present review will be on the central command, the arterial baroreflex and chemoreflex, and the exercise pressure reflex. The regulation and integration of these reflexes operating during dynamic exercise and their possible role in the pathophysiology of some cardiovascular diseases are also discusse

Stabilization of the Cardiac Nervous System During Cardiac Stress Induces Cardioprotection

The cardiac nervous system consists of nested reflex feedback loops that interact to regulate regional heart function. Cardiac disease affects multiple components of the cardiac nervous system and the myocytes themselves. This study aims to determine: 1) how select components of the cardiac nervous system respond to acute cardiac stress, including myocardial ischemia (MI) and induced neural imbalance leading to cardiac electrical instability, and 2) how neuromodulation can affect neural-myocyte interactions to induce cardioprotection. Thoracic spinal cord stimulation (SCS) is recognized for its anti-anginal effects and ability to reduce apoptosis in response to acute MI, primarily via modulation of adrenergic efferent systems. The data presented here suggest that cervical SCS exerts similar cardioprotective effects in response to MI, but in contradistinction to thoracic SCS, uses both adrenergic and cholinergic efferent mechanisms to stabilize cardiomyocytes and the arrhythmogenic potential. SCS potentially can use efferent and/or anti-dromically activated cardiac afferents to mediate its cardioprotection. Thoracic SCS mitigates the MI-induced activation of both nodose and dorsal root ganglia cardiac-related afferents, doing so without antidromic activation of the primary cardiac afferents. Instead, thoracic SCS acts through altering the cardiac milieu thereby secondarily affecting the primary afferent sensory transduction. In response to cardiac stressors, reflex activation of efferent activity modifies mechanical and electrical functions of the heart. Excessive activation of neuronal input to the cardiac nervous system can induce arrhythmias. Stimulation of intrathoracic mediastinal nerves directly activates subpopulations of intrinsic cardiac neurons, thereby inducing atrial arrhythmias. Neuromodulation, either thoracic SCS or hexamethonium, suppressed mediastinal nerve stimulation (MSNS)-induced activation of intrinsic cardiac neurons and correspondingly reduced the arrhythmogenic potential. SCS exerted its stabilizing effects on neural processing and subsequent effects on atrial electrical function by selectively targeting local circuit neurons within the intrinsic cardiac nervous system. Together these data indicate that neuromodulation therapy, using SCS, can mitigate the imbalances in cardiac reflex control arising from acute cardiac stress and thereby has the potential to slow the progression of chronic heart disease

Alterations in intrinsic heart rate in endotoxemia

After much early debate, it is now well recognized that the origin of the heart beat is myogenic, or intrinsic, to the heart. It has been felt that the intrinsic heart rate (IHR) is constant and that the observed heart rate is controlled by tonal alterations in the activity of the autonomic nervous system in response to various stimuli. Some investigators have recognized, however, that the intrinsic heart rate (IHR) can be altered in certain conditions. Sepsis, or the host response to bacterial infection, is recognized as a stress to the host that results in decreased myocardial contractility and tachycardia. Some previous work has suggested that this tachycardia may exist outside the tonal regulation of the autonomic nervous system, raising the possibility of an elevation of the IHR. Utilizing an E. coli lipopolysaccharide (LPS) endotoxin model of sepsis in the rat, this work demonstrates an elevation of the IHR in sepsis. The experimental evidence further suggests that the beta-adrenoceptor participates in the induction of the elevation in IHR early in the septic process. By sixteen hours after LPS administration the beta-adrenoceptor is, however, no longer required for the ongoing elevation of IHR. It is also demonstrated that the elevation in IHR is more prolonged than the contractile dysfunction induced by LPS

Magnesium sulphate reversal of established bupivacaine electrophysiological cardiotoxicity

The results of this study show that in intact rats magnesium produces a more rapid resolution of bupivacaine induced electrophysiological changes than placebo. The improvements are in rhythm and electrical conduction, although this is often at the expense of potentiating the bradycardic effects of bupivacaine toxicity. Whilst the bradycardia remains a problem it is potentially more amenable to therapy than the changes in rhythm and conduction which magnesium sulphate reversed. The opportunity therefore exists to explore the possibility of combining magnesium with a positive chronotrophic agent such as dobutamine

The brain beating and heart breathing: a unifying theory of the neuro- cardiac- respiratory control in infant and adult sudden unexpected deaths

Background: Sudden Infant Death Syndrome (SIDS) is characterized by the death of an infant that cannot be explained, despite a systematic case examination, including death scene investigation, autopsy and review of the clinical history. Nowadays, Sudden Unexpected Infant Death (SUID) is a wide-ranging concept used to describe any sudden and unexpected death, whether explained or unexplained, including SIDS, which occurs during the first year of life. Several differing and sometimes contradictory hypotheses of the underlying mechanisms of SIDS have been proposed. The most reliable seems to be the “triple risk hypothesis”. Based on this theory, unexpected infant deaths might arise as a consequence of the combination of three factors coming together: a vulnerable infant, a vulnerable phase of development and a final insult occurring in this window of vulnerability. Recently, a unified neuropathological theory contributes to describing SIDS. According to this, serotonergic neurons play a crucial homeostatic function in the cardiorespiratory brainstem centres. A high incidence of morphological abnormalities and biochemical defects of serotoninergic neurotransmission have been reported in the brainstem of SIDS victims. This brain region includes the main nuclei and structures that coordinate the vital activities, such as cardiovascular function and breathing, perinatal and after birth. Nevertheless, evidence suggests likely genomic complexity and a degree of overlap among SIDS, Sudden Intrauterine Death (SIUD), Sudden Cardiac Death (SCD) and Sudden Unexpected Death in Epilepsy (SUDEP). In SUDEP, which has clinical parallels with SIDS, alterations to medullary serotoninergic neural populations and autonomic dysregulation have been shown too. Molecular profiling of SUDEP cases and the investigation of genetic models have directed to the identification of putative SUDEP genes of which most are ion channel active along the neurocardiac, neuroautonomic, and neurorespiratory pathway. Concurrently, anomalous time- activation, transcription or regional expression of candidate neuro-cardiac-respiratory genes implicated for SUDEP, could be similarly involved in other unexpected sudden deaths. A small but significant proportion of infants who die suddenly and unexpectedly have been shown on postmortem genetic testing to have Developmental Serotonopathies, Cardiac Channelopathies and Autonomic Nervous System Dysregulation, with considerable implications for surviving and future family members. This has led to the demonstration that neuro-cardiac genes are expressed in both tissues (brain and heart) and recently in the respiratory system. Aim: Despite their decreasing incidence, SIDS and SUDEP are still important causes of death. There are many nuclei in the cardio and respiratory centres of the brain involved in unexpected and sudden deaths. Cardiac, sympathetic, and respiratory motor activities can be viewed as a unified rhythm controlled by brainstem neural circuits for effective and efficient gas exchange. We aim to describe abnormalities in these nuclei, in part because robust molecular or functional examination of these nuclei has not been carefully performed. We intend to perform detailed functional mapping of these brainstem nuclei. Specifically, the cardiorespiratory and cardioventilatory coupling can be understood as a unified vital rhythm controlled by brainstem neural circuits. By cardiorespiratory coupling, we mean the Respiratory Sinus Arrhythmia (RSA) that is characterized by a heart rate (HR) increasing during inspiration and an HR decrease during expiration. Conversely, Cardioventilatory coupling (CVC) is considered the influence of heartbeats and arterial pulse pressure on respiration with the tendency for the next inspiration to start at a preferred latency after the last heartbeat in expiration. We hypothesized that these two reflex systems are not separate, but constitute an integrated network. We defined this last concept as "unifying theory". By studying all the maps of the cardiorespiratory nuclei of the Literature, we integrated this concept into a reworking map of brainstem nuclei that could also explain the gasping and blocking cardiorespiratory of sudden deaths. The theory of a unique, unifying cardiorespiratory network, it has been recently demonstrated in some cases of arrhythmia, in some cases of SUDEP with striking systolic hypotensive changes and in some cases of SIDS too. Material and Methods: We investigated articles, reviews indexed in PubMed describing putative neuro-cardiac-respiratory genes and cardiorespiratory, and cardioventilatory coupling theories. Specifically, we evaluated cardiorespiratory brainstem nuclei and whole brains of fetal, infant and adult autopsies respectively to detect congenital errors in the cerebral development or malformations, but also to identify the “normal” or “dysplastic” brainstem centres. Results: Based on the Literature, we identified a brain-heart gene mapping and a scheme of cardiorespiratory brainstem nuclei network. Contemporary, we collected a large pool of fetal brain malformations and cardiorespiratory nuclei dysgenesis both in infants both in adult sudden deaths. We found dysgenesia, agenesia and hypoplasia of brainstem nuclei associated with SIDS cases, compared with post-mortem infant control cases. However, the arcuate nucleus showed insignificant inter-variations regarding adults autoptic cases. Discussion: Many intrinsic and extrinsic factors increase fetal, perinatal, infant, and adult sudden death susceptibility. The final common pathway for SIDS and SUDEP involves a failure to arouse and autoresuscitate in response to environmental challenge. The different risk factors, among these a prone position, can directly alter the function of cardiorespiratory nuclei and impair the ability of this network to coordinate cardiorespiratory–cardioventilatory coupling. Conclusions: Neuropathological analysis of the infant brainstem and neuro-cardiac-respiratory gene mapping represents a good tool to infer on the final events of SIDS and SUDEP, although nothing it is clear regarding the role of adult cardiorespiratory centres. An integrated study of postmortem neuropathology and molecular autopsies could help to understand the network of this beating-breathing-thinking unit