Novel insights into the role of the sympathetic nervous system in cardiac arrhythmogenesis

Inhibition of a TREK-like K+ channel current by noradrenaline requires both β1- and β2-adrenoceptors in rat atrial myocytes

AIMS

Noradrenaline plays an important role in the modulation of atrial electrophysiology. However, the identity of the modulated channels, their mechanisms of modulation, and their role in the action potential remain unclear. This study aimed to investigate the noradrenergic modulation of an atrial steady-state outward current (IKss).

METHODS AND RESULTS

Rat atrial myocyte whole-cell currents were recorded at 36°C. Noradrenaline potently inhibited IKss (IC50 = 0.90 nM, 42.1 ± 4.3% at 1 µM, n = 7) and potentiated the L-type Ca(2+) current (ICaL, EC50 = 136 nM, 205 ± 40% at 1 µM, n = 6). Noradrenaline-sensitive IKss was weakly voltage-dependent, time-independent, and potentiated by the arachidonic acid analogue, 5,8,11,14-eicosatetraynoic acid (EYTA; 10 µM), or by osmotically induced membrane stretch. Noise analysis revealed a unitary conductance of 8.4 ± 0.42 pS (n = 8). The biophysical/pharmacological properties of IKss indicate a TREK-like K(+) channel. The effect of noradrenaline on IKss was abolished by combined β1-/β2-adrenoceptor antagonism (1 µM propranolol or 10 µM β1-selective atenolol and 100 nM β2-selective ICI-118,551 in combination), but not by β1- or β2-antagonist alone. The action of noradrenaline could be mimicked by β2-agonists (zinterol and fenoterol) in the presence of β1-antagonist. The action of noradrenaline on IKss, but not on ICaL, was abolished by pertussis toxin (PTX) treatment. The action of noradrenaline on ICaL was mediated by β1-adrenoceptors via a PTX-insensitive pathway. Noradrenaline prolonged APD30 by 52 ± 19% (n = 5; P < 0.05), and this effect was abolished by combined β1-/β2-antagonism, but not by atenolol alone.

CONCLUSION

Noradrenaline inhibits a rat atrial TREK-like K(+) channel current via a PTX-sensitive mechanism involving co-operativity of β1-/β2-adrenoceptors that contributes to atrial APD prolongation.

Sudden cardiac death and diabetes mellitus

Sudden cardiac death (SCD) affects a significant percentage of diabetic patients. SCD in these patients can be due to several factors, such as diastolic dysfunction, heart failure, altered platelet function, inflammation, sympathetic nervous stimulation and other factors. In the present review, we discuss the association between diabetes mellitus and SCD.

Long-term mortality risk in patients with an implantable cardioverter-defibrillator: Influence of heart rate and QRS duration

BACKGROUND

A paucity of studies has investigated the role of autonomic cardiac regulation as well as cardiac conduction in relation to prognosis in implantable cardioverter-defibrillator (ICD) patients. Therefore, we examined the association of heart rate and QRS duration with long-term mortality risk in first-time ICD patients, adjusting also for measures of emotional distress.

METHODS

Resting heart rate and QRS duration were assessed prior to ICD implantation in 448 patients. Primary study endpoint was all-cause mortality (up to 6.0 year follow-up, median follow-up of 5.6 years (IQR: 1.9)). The impact of heart rate and QRS duration on time to all-cause mortality was separately assessed with Cox proportional hazard regression analysis, adjusting for clinical factors and symptoms of depression and anxiety.

RESULTS

Mean (SD) heart rate was 68.0 ± 13.3 bpm and mean QRS duration was 130.9 ± 36.9 ms. Heart rate of ≥80 bpm was associated with increased risk of mortality (HR=1.86; 95% CI=1.15-3.00; p=.011) in unadjusted analysis. In adjusted analyses, this relationship remained significant both with depression (HR=1.86, 95% CI=1.12-3.09; p=.017) and anxiety (HR=1.82, 95% CI=1.10-3.03; p=.021) and clinical measures as covariates. QRS duration of ≥120 ms was associated with impaired prognosis in unadjusted analysis (HR=2.00, 95% CI=1.27-3.14; p=.003), but was reduced to non-significance in adjusted analysis when medical comorbidities were included (HR=1.15, 95% CI=0.70-1.89; p=.60).

CONCLUSIONS

This study shows that increased heart rate is associated with impaired prognosis. Since heart rate is a relatively easy measurable parameter of autonomic functioning, heart rate should be included as a measure for risk stratification in daily clinical practice.

Role of the autonomic nervous system in atrial fibrillation: pathophysiology and therapy

Autonomic nervous system activation can induce significant and heterogeneous changes of atrial electrophysiology and induce atrial tachyarrhythmias, including atrial tachycardia and atrial fibrillation (AF). The importance of the autonomic nervous system in atrial arrhythmogenesis is also supported by circadian variation in the incidence of symptomatic AF in humans. Methods that reduce autonomic innervation or outflow have been shown to reduce the incidence of spontaneous or induced atrial arrhythmias, suggesting that neuromodulation may be helpful in controlling AF. In this review, we focus on the relationship between the autonomic nervous system and the pathophysiology of AF and the potential benefit and limitations of neuromodulation in the management of this arrhythmia. We conclude that autonomic nerve activity plays an important role in the initiation and maintenance of AF, and modulating autonomic nerve function may contribute to AF control. Potential therapeutic applications include ganglionated plexus ablation, renal sympathetic denervation, cervical vagal nerve stimulation, baroreflex stimulation, cutaneous stimulation, novel drug approaches, and biological therapies. Although the role of the autonomic nervous system has long been recognized, new science and new technologies promise exciting prospects for the future.

Vagal modulation of resting heart rate in rats: the role of stress, psychosocial factors, and physical exercise

In humans, there are large individual differences in the levels of vagal modulation of resting heart rate (HR). High levels are a recognized index of cardiac health, whereas low levels are considered an important risk factor for cardiovascular morbidity and mortality. Several factors are thought to contribute significantly to this inter-individual variability. While regular physical exercise seems to induce an increase in resting vagal tone, chronic life stress, and psychosocial factors such as negative moods and personality traits appear associated with vagal withdrawal. Preclinical research has been attempting to clarify such relationships and to provide insights into the neurobiological mechanisms underlying vagal tone impairment/enhancement. This paper focuses on rat studies that have explored the effects of stress, psychosocial factors and physical exercise on vagal modulation of resting HR. Results are discussed with regard to: (i) individual differences in resting vagal tone, cardiac stress reactivity and arrhythmia vulnerability; (ii) elucidation of the neurobiological determinants of resting vagal tone.

Role of nuclear cardiology for guiding device therapy in patients with heart failure

Heart failure is a dynamic condition with high morbidity and mortality and its prognosis should be reassessed frequently, particularly in patients for whom critical treatment decisions may depend on the results of prognostication. In patients with heart failure, nuclear cardiology techniques are useful to establish the etiology and the severity of the disease, while fewer studies have explored the potential capability of nuclear cardiology to guide cardiac resynchronization therapy (CRT) and to select patients for implantable cardioverter defibrillators (ICD). Left ventricular synchrony may be assessed by radionuclide angiography or gated single-photon emission computed tomography myocardial perfusion scintigraphy. These modalities have shown promise as predictors of CRT outcome using phase analysis. Combined assessment of myocardial viability and left ventricular dyssynchrony is feasible using positron emission tomography and could improve conventional response prediction criteria for CRT. Preliminary data also exists on integrated positron emission tomography/computed tomography approach for assessing myocardial viability, identifying the location of biventricular pacemaker leads, and obtaining left ventricular functional data, including contractile phase analysis. Finally, cardiac imaging with autonomic radiotracers may be useful in predicting CRT response and for identifying patients at risk for sudden cardiac death, therefore potentially offering a way to select patients for both CRT and ICD therapy. Prospective trials where imaging is combined with image-test driven therapy are needed to better define the role of nuclear cardiology for guiding device therapy in patients with heart failure.

Periodic limb movements during sleep and cardiac arrhythmia in older men (MrOS sleep)

STUDY OBJECTIVES

To determine if periodic limb movements during sleep (PLMS) are associated with nocturnal cardiac arrhythmia.

METHODS

2,793 community-dwelling older men underwent polysomnography with measurement of limb movements and EKG. Logistic regression assessed association of periodic limb movement index and periodic limb movement arousal index with arrhythmia including atrial fibrillation and non-sustained ventricular tachycardia detected by polysomnography. Models were adjusted for age, race, cardiovascular risk factors, and clinic site. Secondary analyses were subset to men without calcium channel/β-adrenergic medication usage, and stratified by congestive heart failure or myocardial infarction history.

RESULTS

In the overall cohort, periodic limb movement index, and periodic limb movement arousal index were not associated with ventricular or atrial arrhythmia after considering potential confounders. In men not taking calcium channel/β-blocking medication, increased adjusted odds of non-sustained ventricular tachycardia were observed for periodic limb movement index (OR = 1.30 per SD increase; 95% CI 1.00, 1.68) and periodic limb movement arousal index (OR = 1.29 per SD increase; 95% CI 1.03, 1.62). In men with CHF or MI, there was a suggested association of atrial fibrillation with periodic limb movement index (OR = 1.29, 95% CI 0.96, 1.73 per SD increase; p = 0.09) or periodic limb movement arousal index (OR = 1.21, 95% CI 0.94, 1.57 per SD increase; p = 0.14), although results were not statistically significant.

CONCLUSIONS

There is not an association between PLMS and cardiac arrhythmia in all older men but in subsets of men, particularly those with structural heart disease and not on calcium channel or β-adrenergic medication, cardiac arrhythmia does associate with PLMS.

The role and management of sympathetic overactivity in cardiovascular and renal complications of diabetes

Feedback activation of neurohormonal pathways in the setting of kidney or heart failure contributes to the development and progression of dysfunction in the other. Diabetes and its management independently activate these same pathogenic pathways, feeding into this vicious cycle and contributing to a poor prognosis. One of the most important of these neurohormonal pathways is the sympathetic nervous system (SNS). The activity of the SNS in increased in patients with chronic kidney disease, even in the absence of renal impairment or heart failure. There is a strong relationship between SNS overactivity and prognosis, and evidence that blockade of SNS reduces morbidity and mortality in patients with diabetes. However, modulation of SNS is underutilised as a strategy to protect both the diabetic kidney and the heart. This is partly because of the historically poor tolerability, adverse haemodynamic and metabolic effects, lack of selectivity of β-blockers and the lack of specificity of other interventions that might modify SNS activation. The advent of "vasodilating β-blockers" with better tolerability as well as more favourable effects on renal function and metabolic profiles opens the door for their more widespread utility in patients with diabetes. Radiofrequency renal sympathectomy and baroreflex activation technologies also offer exciting new ways to tackle the challenge of sympathetic overactivity.

Autonomic cardiac innervation: development and adult plasticity

Autonomic cardiac neurons have a common origin in the neural crest but undergo distinct developmental differentiation as they mature toward their adult phenotype. Progenitor cells respond to repulsive cues during migration, followed by differentiation cues from paracrine sources that promote neurochemistry and differentiation. When autonomic axons start to innervate cardiac tissue, neurotrophic factors from vascular tissue are essential for maintenance of neurons before they reach their targets, upon which target-derived trophic factors take over final maturation, synaptic strength and postnatal survival. Although target-derived neurotrophins have a central role to play in development, alternative sources of neurotrophins may also modulate innervation. Both developing and adult sympathetic neurons express proNGF, and adult parasympathetic cardiac ganglion neurons also synthesize and release NGF. The physiological function of these “non-classical” cardiac sources of neurotrophins remains to be determined, especially in relation to autocrine/paracrine sustenance during development.

 

Cardiac autonomic nerves are closely spatially associated in cardiac plexuses, ganglia and pacemaker regions and so are sensitive to release of neurotransmitter, neuropeptides and trophic factors from adjacent nerves. As such, in many cardiac pathologies, it is an imbalance within the two arms of the autonomic system that is critical for disease progression. Although this crosstalk between sympathetic and parasympathetic nerves has been well established for adult nerves, it is unclear whether a degree of paracrine regulation occurs across the autonomic limbs during development. Aberrant nerve remodeling is a common occurrence in many adult cardiovascular pathologies, and the mechanisms regulating outgrowth or denervation are disparate. However, autonomic neurons display considerable plasticity in this regard with neurotrophins and inflammatory cytokines having a central regulatory function, including in possible neurotransmitter changes. Certainly, neurotrophins and cytokines regulate transcriptional factors in adult autonomic neurons that have vital differentiation roles in development. Particularly for parasympathetic cardiac ganglion neurons, additional examinations of developmental regulatory mechanisms will potentially aid in understanding attenuated parasympathetic function in a number of conditions, including heart failure.

[Molecular and genetic background of sudden cardiac death]

Despite recent findings on the functional, structural and genetic background of sudden cardiac death, the incidence is still relatively high in the entire population. A thorough knowledge on susceptibility, as well as pathophysiology behind the development of malignant arrhythmias will help us to identify individuals at risk and prevent sudden cardiac death. This article presents a review of the current literature on the role of altered intracellular Ca2+ handling, acute myocardial ischaemia, cardiac autonomic innervation, renin-angiotensin-aldosterone system, monogenic and complex heritability in the pathogenesis of sudden cardiac death.

Exercise pressor response and arterial baroreflex unloading during exercise in chronic kidney disease

Patients with chronic kidney disease (CKD) have poor exercise capacity, which contributes to cardiovascular risk. We sought to determine whether patients with stage 2 or stage 3 CKD have an augmented blood pressure (BP) response during exercise, and if so, whether overactivation of the sympathetic nervous system (SNS) during exercise might play a role. In 13 patients with CKD and hypertension and 13 controls with hypertension, we measured hemodynamics and muscle sympathetic nerve activity (MSNA) during the following maneuvers: low-level rhythmic handgrip (RHG 20%), which primarily stimulates mechanoreceptors, and moderate static handgrip exercise (SHG 30%) followed by posthandgrip circulatory arrest (PHGCA), which isolates metaboreceptors. During baseline studies, patients with CKD had significantly greater increases in mean arterial pressure (MAP) during SHG 30% (P = 0.045), RHG 20% (P = 0.031), and PHGCA (P = 0.043); however, the MSNA response was not augmented in patients with CKD compared with controls. We hypothesized that an augmented SNS response during exercise might be revealed in CKD if arterial baroreflex constraint was equalized using nitroprusside (NTP). These exercise maneuvers were repeated in patients with CKD during NTP infusion to equalize the BP response between groups, thereby relieving baroreflex-mediated suppression of SNS activity. With NTP infusion, patients with CKD had significantly increased MSNA responses during SHG 30% (P = 0.0044), and RHG 20% (P = 0.0064), but not during PHGCA (P > 0.05), suggesting increased reflex activation of the SNS during exercise, which may be mediated by mechanoreceptors but not metaboreceptors. Patients with CKD have an exaggerated BP response during rhythmic and static exercise with underlying SNS overactivation that is revealed during arterial baroreflex unloading during exercise.

The placental origins of sudden cardiac death

OBJECTIVE

Most sudden cardiac deaths are due to cardiac arrhythmias, and abnormalities in the autonomic nervous system could underlie them. There is growing evidence that coronary heart disease is associated with alterations of fetal development as a result of variations in the processes of placentation that control fetal nutrition. We hypothesized that placental size would be associated with sudden cardiac death.

METHODS

We examined sudden cardiac death within the Helsinki Birth Cohort of 13 345 men and women.

RESULTS

One hundred eighty-seven (2.7%) men and 47 (0.7%) women had sudden unexplained cardiac death outside hospital. Sudden death was associated with a thin placenta, the hazard ratio being 1.47 [95% confidence interval (CI) 1.11-1.93, P = 0.006] for each g/cm(2) decrease in thickness. Sudden death was independently associated with poor educational attainment (P < 0.0001). Both of these associations were independent of socio-economic status in later life.

CONCLUSION

Sudden death may be initiated by impaired development of the autonomic nervous system in utero as a result of shallow invasion of the spiral arteries in the maternal endometrium and consequent fetal malnutrition.

Cardiovascular risk in chronic kidney disease: role of the sympathetic nervous system

Patients with chronic kidney disease are at significantly increased risk for cardiovascular disease and sudden cardiac death. One mechanism underlying increased cardiovascular risk in patients with renal failure includes overactivation of the sympathetic nervous system (SNS). Multiple human and animal studies have shown that central sympathetic outflow is chronically elevated in patients with both end-stage renal disease (ESRD) and chronic kidney disease (CKD). SNS overactivation, in turn, increases the risk of cardiovascular disease and sudden death by increasing arterial blood pressure, arrythmogenicity, left ventricular hypertrophy, and coronary vasoconstriction and contributes to the progression renal disease. This paper will examine the evidence for SNS overactivation in renal failure from both human and experimental studies and discuss mechanisms of SNS overactivity in CKD and therapeutic implications.

Prolonged atrial fibrillation following generalized tonic-clonic seizures

We describe two male patients with focal epilepsy in whom transitory episodes of atrial fibrillation (AF) lasting for up to 25h were detected in the context of generalized tonic-clonic seizures (GTCSs). In five of seven previously published cases of transitory AF associated with epileptic seizures, AF was also associated with GTCS, suggesting a pathophysiological link via GTCS-related increase in sympathetic tone and release of catecholamines. Importantly, AF increases the risk of thromboembolic cerebral ischemia, prompting the question of whether antithrombotic preventive treatment should be initiated in people with pharmacoresistant epilepsy and prolonged peri-ictal AF. Furthermore, AF can considerably impair cardiac output and may, via this mechanism, contribute to the risk of sudden unexpected death in epilepsy following GTCS.

Association of heart rate variability with arrhythmic events in patients with arrhythmogenic right ventricular cardiomyopathy/dysplasia

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) is associated with an increased risk of sudden cardiac death (SCD). Risk stratification of ARVC/D patients, however, remains an unresolved issue. In this study we investigated whether heart rate variability (HRV) can be helpful in identifying ARVC/D patients with increased risk of arrhythmic events.

METHODS AND RESULTS

We studied 30 consecutive patients (17 males; 45.4 ± 18 years) with ARVC/D, diagnosed according to guideline criteria; 15 patients (50%) had received an implantable cardioverter defibrillator (ICD) for primary SCD prevention. HRV was assessed on 24-h ECG Holter monitoring. The primary endpoint was the occurrence of major arrhythmic events (SCD, sustained ventricular tachycardia (VT), ICD therapy for sustained VT or ventricular fibrillation (VF)). During the follow-up period (19 ± 7 months), no deaths occurred, but 5 patients (17%) experienced arrhythmic events (4 VTs and 1 VF, all in the ICD group). All HRV parameters were significantly lower in patients with, compared with those without, arrhythmic events. Low-frequency amplitude was the most significant HRV variable associated with arrhythmic events in univariate Cox regression analysis (P=0.017), and was the only significant predictor of arrhythmic events in multivariable regression analysis (hazard ratio 0.88, P=0.047), together with unexplained syncope (hazard ratio 16.1, P=0.039).

CONCLUSIONS

Our data show that among ARVC/D patients HRV analysis might be helpful in identifying those with increased risk of major arrhythmic events.

Cerebrospinal fluid catecholamine levels as predictors of outcome in subarachnoid hemorrhage

OBJECTIVE

Subarachnoid hemorrhage (SAH) is associated with marked sympathetic activation at the time of ictus. The purpose of this study is to determine whether early central catecholamine levels measured from cerebrospinal fluid (CSF) relate to outcome in patients with SAH.

METHODS

Observational study of consecutive SAH grade 3-5 patients who underwent ventriculostomy placement, but did not undergo open craniotomy for aneurysm obliteration. CSF samples were obtained during the first 48 h following symptom onset and assayed for catecholamine levels. Statistical analyses were performed to determine whether the levels predicted mortality by day 15 or mortality/disability by day 30.

RESULTS

For the 102 patients included, mean age was 58, and 73% were female - 21% experienced day-15 mortality, and 32% experienced mortality/disability by day 30. Early mortality was related to Hunt-Hess (H/H) grade (p < 0.001), neurogenic cardiomyopathy (NC) (p = 0.003), cerebral infarction (p = 0.001), elevated intracranial pressure (ICP) (p = 0.029), epinephrine (EPI) level (p = 0.002) and norepinephrine/3,4-dihydroxyphenylglycol (NE/DHPG) ratio (p = 0.003). Mortality/disability was related to H/H grade (p < 0.001), NC (p = 0.018), infarction (p < 0.001), elevated ICP (p = 0.002), EPI (p = 0.004) and NE/DHPG (p = 0.014). Logistic regression identified age [OR 1.09 (95% CI 1.01-1.17)], H/H grade [9.52 (1.19-77)], infarction [10.87 (1.22-100)], ICP elevation [32.26 (2-500)], EPI [1.06 (1.01-1.10)], and (inversely) DHPG [0.99 (0.99-1.00)] as independent predictors of early mortality. For mortality/disability, H/H grade [OR 21.74 (95% CI 5.62-83)], ICP elevation [18.52 (1.93-166)], and EPI [1.05 (1.02-1.09)] emerged as independent predictors. Proportional-hazards analysis revealed age [HR 1.041 (95% CI 1.003-1.08)], H/H grade [6.9 (1.54-31.25)], NC [4.31 (1.5-12.35)], and EPI [1.032 (1.009-1.054)] independently predicted early mortality.

CONCLUSIONS

CSF catecholamine levels are elevated in SAH patients who experience early mortality or disability. EPI may potentially serve as useful index of outcome in this population of patients with SAH.