The influence of the parasympathetic nervous system on atrioventricular conduction

Neuroimaging Studies of the Neural Correlates of Heart Rate Variability: A Systematic Review

Direct and indirect links between brain regions and cardiac function have been reported. We performed a systematic literature review to summarize current knowledge regarding the associations of heart rate variability (HRV) and brain region morphology, activity and connectivity involved in autonomic control at rest in healthy subjects. Both positive and negative correlations of cortical thickness and gray matter volumes of brain structures with HRV were observed. The strongest were found for a cluster located within the cingulate cortex. A decline in HRV, as well as cortical thickness with increasing age, especially in the orbitofrontal cortex were noted. When associations of region-specific brain activity with HRV were examined, HRV correlated most strongly with activity in the insula, cingulate cortex, frontal and prefrontal cortices, hippocampus, thalamus, striatum and amygdala. Furthermore, significant correlations, largely positive, between HRV and brain region connectivity (in the amygdala, cingulate cortex and prefrontal cortex) were observed. Notably, right-sided neural structures may be preferentially involved in heart rate and HRV control. However, the evidence for left hemispheric control of cardiac vagal function has also been reported. Our findings provide support for the premise that the brain and the heart are interconnected by both structural and functional networks and indicate complex multi-level interactions. Further studies of brain-heart associations promise to yield insights into their relationship to health and disease.

An atrioventricular node model incorporating autonomic tone

The response to atrial fibrillation (AF) treatment is differing widely among patients, and a better understanding of the factors that contribute to these differences is needed. One important factor may be differences in the autonomic nervous system (ANS) activity. The atrioventricular (AV) node plays an important role during AF in modulating heart rate. To study the effect of the ANS-induced activity on the AV nodal function in AF, mathematical modelling is a valuable tool. In this study, we present an extended AV node model that incorporates changes in autonomic tone. The extension was guided by a distribution-based sensitivity analysis and incorporates the ANS-induced changes in the refractoriness and conduction delay. Simulated RR series from the extended model driven by atrial impulse series obtained from clinical tilt test data were qualitatively evaluated against clinical RR series in terms of heart rate, RR series variability and RR series irregularity. The changes to the RR series characteristics during head-down tilt were replicated by a 10% decrease in conduction delay, while the changes during head-up tilt were replicated by a 5% decrease in the refractory period and a 10% decrease in the conduction delay. We demonstrate that the model extension is needed to replicate ANS-induced changes during tilt, indicating that the changes in RR series characteristics could not be explained by changes in atrial activity alone.

Hypothalamic miR-204 Induces Alteration of Heart Electrophysiology and Neurogenic Hypertension by Regulating the Sympathetic Nerve Activity: Potential Role of Microbiota

There is abundant evidence demonstrating the association between gut dysbiosis and neurogenic diseases such as hypertension. A common characteristic of resistant hypertension is the chronic elevation in sympathetic nervous system (SNS) activity accompanied by increased release of norepinephrine (NE), indicating a neurogenic component that contributes to the development of hypertension. Factors that modulate the sympathetic tone to the cardiovascular system in hypertensive patients are still poorly understood. Research has identified an interaction between the brain and the gut, and this interaction plays a possible role in the mechanism of heart damage-induced hypertension. Data, however, remain scarce, and further study is required to define the role of microbiota in sympathetic neural function and its relationship with heart damage and blood pressure (BP) control. Experimental evidence has pointed toward a bidirectional relationship between alterations in the types of bacteria present in the gut and neurogenic diseases, such as hypertension. Our published data showed that miR-204, a microRNA that plays an important role in the CNS function, is affected by gut dysbiosis. Therefore, miR-204 could be a key element that regulates normal sinus rhythm and neuronal hypertension. In this review, we will shed light on the potential mechanism by which microbiota affects hypothalamic miR-204, which in turn, could hinder the sympathetic nerve drive to the cardiovascular system leading to arrhythmia and hypertension.

Safety and efficacy of neurostimulation with a miniaturised vagus nerve stimulation device in patients with multidrug-refractory rheumatoid arthritis: a two-stage multicentre, randomised pilot study

Background The inflammatory reflex plays a role in regulating innate and adaptive immunity by modulating cellular and molecular inflammatory pathways. The vagus nerve is a major constituent of the inflammatory reflex and studies have shown that the reflex can be activated by electrical stimulation of the vagus nerve. In this first in-human pilot study, we assessed the safety and efficacy of a novel miniaturised vagus nerve stimulation (VNS) device for the treatment of multidrug-refractory rheumatoid arthritis.

METHODS

Participants with moderately to severely active rheumatoid arthritis and prior insufficient response to two or more biological disease-modifying anti-rheumatic drugs or Janus kinase inhibitors with at least two different modes of action were enrolled in a two-stage study done at five clinical research sites in the USA. Stage 1 was open label; participants were implanted with a miniaturised VNS device, which was activated for 1 min once a day. In stage 2, participants were randomly assigned (1:1:1) to receive active stimulation (1 min once a day or 1 min four times a day) or sham stimulation (device implanted but not activated), with the sites and participants masked to treatment assignment. The primary outcome was incidence of treatment-emergent adverse events. Clinical efficacy was assessed as a key secondary outcome. The study was registered with ClinicalTrials.gov, NCT03437473.

FINDINGS

14 patients were enrolled between March 13 and Aug 8, 2018. Three patients received stimulation in stage 1 and, following safety review board approval, the remaining 11 patients were implanted during stage 2 and randomly assigned to receive 1 min of stimulation once daily (n=3), 1 min of stimulation four times daily (n=4), or no stimulation (n=4) for 12 weeks. There were no device-related or treatment-related serious adverse events. Surgery-related adverse events were Horner's syndrome and vocal cord paralysis (in one patient each), which resolved without clinically significant sequelae. No deaths were recorded.

INTERPRETATION

VNS with a miniaturised neurostimulator was safe and well tolerated and reduced signs and symptoms of rheumatoid arthritis in patients with multidrug-refractory disease. These results support further evaluation in a larger randomised sham-controlled study.

FUNDING

SetPoint Medical.

Differential Effects of Vagal Activation on the Sinus and Atrioventricular Nodes: Report of 2 Cases

Vagal activation usually affects both sinus and atrioventricular (AV) nodes, manifesting as sinus slowing accompanied by varying degrees of AV block. AV block accompanying sinus acceleration as during treadmill testing is usually considered pathologic. We report 2 cases of vagally mediated reflex AV block accompanied by sinus tachycardia and acceleration. (Level of Difficulty: Intermediate.)

Epigastric Pain, Vomiting, and Cardiac Arrhythmias

A woman with epigastric pain and nausea had atrioventricular block confined to episodes of vomiting.

Resolution of AV block after ablation for atrial fibrillation

We report a case with complete atrioventricular block (AVB) present both during sinus rhythm and with atrial fibrillation (AF). He had declined pacemaker placement and opted for ablation for the symptomatic AF. He underwent staged hybrid approach with video-assisted thoracoscopic surgical maze (VATS) followed by endocardial ablation. VATS included ganglionic plexi cauterization. One month after the endocardial procedure, he had complete resolution of AF and AVB and remains so at 18 months follow up. We discuss possible abolition of vagal input for this improvement in AV conduction.

Neurocardiology

Neurocardiology refers to the interplay between the nervous system and the cardiovascular system. Stress-related cardiomyopathy exemplifies the brain-heart connection and occurs in several conditions with acute brain injury that share oversympathetic activation. The brain's influences on the heart can include elevated cardiac markers, arrhythmias, repolarization abnormalities on electrocardiogram, myocardial necrosis, and autonomic dysfunction. The neurogenic stunned myocardium in aneurysmal subarachnoid hemorrhage represents one end of the spectrum, and is associated with an explosive rise in intracranial pressure that results in excess catecholamine state and possibly CBN. A brain-heart link is more known to cardiologists than neurologists. This chapter provides some insight into the pathophysiology of these pathologic neurocardiac states and their most appropriate management relevant to neurologists.

Common and rare variants in SCN10A modulate the risk of atrial fibrillation

BACKGROUND

Genome-wide association studies have shown that the common single nucleotide polymorphism rs6800541 located in SCN10A, encoding the voltage-gated Nav1.8 sodium channel, is associated with PR-interval prolongation and atrial fibrillation (AF). Single nucleotide polymorphism rs6800541 is in high linkage disequilibrium with the nonsynonymous variant in SCN10A, rs6795970 (V1073A, r(2)=0.933). We therefore sought to determine whether common and rare SCN10A variants are associated with early onset AF.

METHODS AND RESULTS

SCN10A was sequenced in 225 AF patients in whom there was no evidence of other cardiovascular disease or dysfunction (lone AF). In an association study of the rs6795970 single nucleotide polymorphism variant, we included 515 AF patients and 2 control cohorts of 730 individuals free of AF and 6161 randomly sampled individuals. Functional characterization of SCN10A variants was performed by whole-cell patch-clamping. In the lone AF cohort, 9 rare missense variants and 1 splice site donor variant were detected. Interestingly, AF patients were found to have higher G allele frequency of rs6795970, which encodes the alanine variant at position 1073 (described from here on as A1073, odds ratio =1.35 [1.16-1.54]; P=2.3×10(-5)). Both of the common variants, A1073 and P1092, induced a gain-of-channel function, whereas the rare missense variants, V94G and R1588Q, resulted in a loss-of-channel function.

CONCLUSIONS

The common variant A1073 is associated with increased susceptibility to AF. Both rare and common variants have effect on the function of the channel, indicating that these variants influence susceptibility to AF. Hence, our study suggests that SCN10A variations are involved in the genesis of AF.

Muscarinic receptor agonists and antagonists: effects on cardiovascular function

Muscarinic receptor activation plays an essential role in parasympathetic regulation of cardiovascular function. The primary effect of parasympathetic stimulation is to decrease cardiac output by inhibiting heart rate. However, pharmacologically, muscarinic agonists are actually capable of producing both inhibitory and stimulatory effects on the heart as well as vasculature. This reflects the fact that muscarinic receptors are expressed throughout the cardiovascular system, even though they are not always involved in mediating parasympathetic responses. In the heart, in addition to regulating heart rate by altering the electrical activity of the sinoatrial node, activation of M₂ receptors can affect conduction of electrical impulses through the atrioventricular node. These same receptors can also regulate the electrical and mechanical activity of the atria and ventricles. In the vasculature, activation of M₃ and M₅ receptors in epithelial cells can cause vasorelaxation, while activation of M₁ or M₃ receptors in vascular smooth muscle cells can cause vasoconstriction in the absence of endothelium. This review focuses on our current understanding of the signaling mechanisms involved in mediating these responses.

The PR interval-fetal heart rate relationship during repetitive umbilical cord occlusions in immature fetal sheep

OBJECTIVE

To evaluate the relationship of the PR interval and fetal heart rate during repetitive umbilical cord occlusions in immature sheep fetuses.

STUDY DESIGN

In seven chronically cannulated immature sheep fetuses [gestational age 90.6 days (mean)], we analyzed continuous fetal electrocardiogram recordings during repetitive cord occlusions for 2 out of every 5 min until fetal mean arterial pressure dropped to 50% of baseline value. PR interval-fetal heart rate correlation coefficients (Pearson) was measured on consecutive blocks of 2.5 min. R-values of the baseline and the repetitive occlusion period were compared by Fisher's exact test.

RESULTS

Repetitive cord occlusions resulted in acidosis and hypotension. Two fetuses died at the end of the repetitive occlusion period. Four out of seven fetuses showed a significant change from a negative relationship between the PR interval and fetal heart rate during baseline to a predominantly positive relationship during the repetitive occlusion period.

CONCLUSION

In immature fetal sheep, a change from a negative relationship between the PR interval and fetal heart rate to a predominantly positive relationship between the PR interval and fetal heart rate was observed in four out of seven fetuses following the initiation of repetitive umbilical cord occlusions.

Dynamic counterbalance between direct and indirect vagal controls of atrioventricular conduction in cats

The vagal system regulates the atrioventricular conduction time (TAV) via two opposing mechanisms: a direct effect on the atrioventricular node and an indirect effect through changes in heart period (TAA). To evaluate how dynamic vagal activation affects TAV, we stimulated the vagal nerve with frequency-modulated Gaussian white noise and estimated the transfer function from vagal stimulation to the TAV response under conditions of no pacing and constant pacing in anesthetized cats. The effect of changes in TAA on TAV was estimated by a random-pacing protocol. The transfer function from vagal stimulation to TAV has low-pass filter characteristics. Constant pacing increased the maximum step response in TAV (2.4 +/- 1.2 vs. 6.3 +/- 2.2 ms/Hz, P < 0.01). The time constant did not differ between the vagal effect on TAV and that on TAA (2.9 +/- 1.2 vs. 2.3 +/- 0.5 s). Because changes in TAA reciprocally affected TAV without significant delay, the direct and indirect effects were dynamically counterbalanced and exerted stable TAV transient response during vagal stimulation under normal sinus rhythm.

Biogenic amine localization in cardiac ganglion intrinsic neurons: electron microscopic histochemistry of SIF cells

The parasympathetic cardiac ganglion in the mudpuppy, N. maculosus, contains postganglionic nerve cells and intrinsic neurons, many of which are small intensely fluorescent (SIF) cells. Several bioactive substances have been localized in the intrinsic nerve cells which may have integrative effects at synapses within the ganglion. Ganglionic intrinsic neurons can be identified electron microscopically by the presence of numerous cytoplasmic granular vesicles 80-120 nm in diameter. Throughout the ganglion there are bundles of unmyelinated fibers some of which are filled with granular and agranular vesicles and axosomatic terminals with similar vesicles synapsing on principal parasympathetic nerve cells. To understand the aminergic contribution to ganglionic synaptic circuitry the chromaffin reaction was used. The intrinsic neurons (i.e., SIF cells) were readily identified by their characteristic intracellular granule population. All intrinsic nerve cells identified showed granules which were positively labelled by the chromaffin reaction. Granular vesicles in synaptic profiles on principal cells (P cells) were also labelled indicating a direct aminergic synaptic innervation to these cells. The cell bodies of intrinsic neurons, ensheathed with supportive glial-like cellular processes, rarely received synapses. Elemental microanalysis was used to verify the chromium content of the electron dense product within the granular vesicles. These studies demonstrated direct aminergic synaptic input to at least a subpopulation of principal parasympathetic cells in the cardiac ganglion of mudpuppy.

A case of swallow syncope induced by vagotonic visceral reflex resulting in atrioventricular node suppression

A 48-year-old man repeatedly experienced syncope associated with paroxysmal atrioventricular block (PAVB) while swallowing. PAVB ("Mobitz type II" AH block) occurred only when "balloon-like" deformity of the lower esophagus developed. Balloon inflation in the lower esophagus induced PAVB. However, inflation in the upper or mid portion, or other vagal maneuvers, caused only sinus slowing. PAVB was not induced after intravenous atropine. The PAVB in this patient was probably caused by a vagotonic reflex triggered by tensoreceptors in the lower esophagus, resulting in selective suppression of the atrioventricular node.

Vagotonia in infants, children, adolescents and young adults

Twenty-four patients with various manifestations of increased vagal tone were seen between 1975 and 1981. Three distinct groups could be identified by clinical characteristics and to some extent by age. The first group showed evidence of prolonged atrioventricular conduction on their electrocardiograms but were asymptomatic. The others presented with syncope, the etiology of which was determined to be vasovagal. Some patients were older and more athletic. They suffered from syncope associated with exercise or heavy exertion. The youngest patients, however, experienced frequent prolonged episodes of syncope often mistaken for seizures. Treatment for the older patients with syncope consisted mainly of behavior modification, while 5 out of 6 younger patients required the implantation of a ventricular demand pacemaker to prevent repeated and problematic recurrences of syncope.

Unidirectional retrograde atrioventricular nodal block in man: determinants of reversibility by vagal antagonism

The mechanism of unidirectional retrograde atrioventricular (AV) nodal block remains largely unknown. In this study, factors determining the reversal of the unidirectional block by atropine were evaluated in 12 patients who had no demonstrable ventriculoatrial (VA) conduction during ventricular pacing. Six patients demonstrated 1:1 VA conduction after atropine (group I), while the remaining six patients continued to show VA block (group II). During the control study there was no significant difference in the sinus cycle length and AH interval between the two groups. The percent decrease in sinus cycle length after atropine was also similar in groups I and II (i.e., 23 +/- 12 and 26 +/- 6, respectively). The effect on antegrade AV nodal conduction (i.e., the percent decrease in AH interval), however, was significantly greater in group I (24 +/- 9) as compared to group II (9 +/- 5) (p less than 0.004). The onset of VA conduction appeared to correlate with the improvement of antegrade conduction. The ratio of these two effects of atropine (i.e., percent decrease in AH interval to percent decrease in sinus cycle length) was higher when VA conduction was first demonstrated in group I (2.3 +/- 1.1) than at the maximal effect of atropine (1.2 +/- 0.3), reflecting a relatively greater decrease in sinus cycle length. Three of six group I patients redeveloped VA block at maximal effect of atropine. The results suggest a functional and dynamic nature of the unidirectional AV nodal block, possibly caused by vagal influence exaggerating the well-known directional asymmetry of AV nodal conduction in man.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of brief bursts of carotid sinus stimuli on heart rate and atrioventricular conduction

The effect of a single brief stimulus burst applied simultaneously to both carotid sinus nerves on atrioventricular conduction (PR) was examined in paced and unpaced preparations of anesthetized open-chest dogs. The relative timing of the stimulus burst was varied to encompass the complete cardiac cycle. Carotid sinus/vagal effect curves were constructed to identify the time course of the response. In paced preparations the maximum increase in PR was 20.3 +/- 2.7 msec and this occurred 458.0 +/- 22.8 msec after the stimulus. There was a latency of 246.0 +/- 12.4 msec after the electrical stimulus before the PR began to increase. In unpaced heart preparations the effect of single carotid sinus nerve stimuli on heart period (PP) and PR was also determined. PP was maximally lengthened by 251.4 +/- 49.4 msec at 644.2 +/- 50.9 msec after the stimulus. There was a latency of 251.4 +/- 10.5 msec before the first noticeable change in PP occurred. The PR response was biphasic. The maximum lengthening of the PR interval was 16.2 +/- 4.2 msec. This response occurred at 361 +/- 22.0 msec after the electrical stimulus. The PR decreased to a minimum value of 13.6 +/- 2.2 msec below control values at 767.1 +/- 44.0 msec after the stimulus. The overall effect of carotid sinus activity on atrioventricular conduction depends not only on the direct nodal effect of acetylcholine but also on the indirect heart rate changes. We conclude that brief bursts of carotid sinus nerve stimulation produce cardiac electrophysiological effects qualitatively similar to the effects of direct vagal stimulation, differing only in having a lower amplitude, a longer latency and a somewhat wider time dispersion.

Multiple atrioventricular nodal pathways--a new electrophysiological phenomenon in children

Multiple atrioventricular (A-V) nodal pathways are described in five dysrhythmia-free children with congenital heart defects. The five were some (2.9%) of 175 children who underwent diagnostic cardiac catheterization and electrophysiological evaluation over a three year period. Supraventricular tachycardia was not induced during the electrophysiological evaluation. Medical follow-up is recommended since multiple A-V nodal pathways have been demonstrated in adults with A-V nodal reentrant tachycardia.

Exercise-induced distal atrioventricular block

Three patients with 1:1 atrioventricular (AV) conduction at rest developed fixed 2:1 or 3:1 AV block during treadmill exercise testing. Electrophysiologic study documented block distal to the AV node in all three patients, and suggested that the exercise-induced block occurred because of increased atrial rate and abnormal refractoriness of the His-Purkinje conduction system. The findings in these three patients suggest that high grade AV block appearing during exercise reflects conduction disease of the His-Purkinje system rather than of the AV node, even in the absence of bundle branch block. Patients with this diagnosis should be considered for permanent cardiac pacing.

Normal electrophysiological findings in a patient with symptomatic intermittent advanced atrioventricular block

This report describes an otherwise healthy young woman who presented with syncope during episodes of advanced atrioventricular (AV) block. The His bundle recordings during normal sinus rhythm and atrial and ventricular pacing were normal. Carotid sinus massage produced no abnormality. Subsequently, the patient received a permanent pacemaker and has been free of symptoms. Intermittent advanced AV block has been observed on follow-up electrocardiograms. This unique case demonstrates a potential limitation of routine electrophysiologic investigation.

The mechanical atrioventricular time of isolated hearts as a correlate of the atrioventricular conduction time

The time difference between onset of contraction in right atrium and left ventricle of isolated guinea pig hearts (mechanical AV time) was measured continuously by an impulse-triggered interval counter and taken as an equivalent of the electrophysiological AV conduction time derived from the epicardial ECG. Excellent correlation between these two parameters was established over a broad range of experimentally modified conduction times. The method was used to analyze the influence of electrical pacing, epinephrine, Ca2+ ions, and adenosine both on the AV conduction and on functional parameters of the hearts and revealed advantages in comparison to separate tests of single parameters in different experimental arrangements.

Disorders of cardiac conduction accompany the dive reflex in man

The faces of 19 healthy subjects were immersed in water at the temperatures varying between 10 degrees and 30 degrees C and at different degrees of lung inflation. Several abnormalities of cardiac conduction were noted. They occurred with the greatest frequency in the coldest water and when there was relatively little air in the lungs at the moment of immersion. There was fragmentary evidence, confirmatory of earlier studies, that heart-rate slowing was accentuated by fear and that very little slowing occurred when the subject was distracted or preoccupied. Various conduction abnormalities were recorded. The most striking finding was the wide difference from person to person in the occurrence of conduction disturbances under more or less comparable circumstances. Moreover, the patterns of conduction alterations, differing from person to person, were nevertheless relatively consistent from dive to dive for the same individual. To ascertain whether or not such idiosyncratic responses may have prognostic significance calls for a long-term, prospective study.