Emerging excitatory role of cardiovascular sympathetic afferents in pathophysiological conditions

Comparison of muscle sympathetic activity in ischemic and nonischemic heart failure

BACKGROUND

The magnitude of sympathetic activation in chronic heart failure is assumed to be independent of its cause. However, because a higher sympathetic component of heart rate variability (HRV) in patients with ischemic cardiomyopathy (ICM) has been reported, we hypothesized that patients with ICM would have a higher resting muscle sympathetic nerve activity (MSNA) than patients with nonischemic dilated cardiomyopathy (DCM).

METHODS AND RESULTS

Resting MSNA was assessed by microneurography and HRV concurrently by coarse-graining spectral analysis in 30 treated normotensive patients with chronic heart failure (12 with ICM and 18 with DCM), matched for age and left ventricular ejection fraction, and 23 healthy normal control subjects, matched for age and blood pressure. Peak oxygen uptake was determined during graded cycling (17 W/min) to maximum effort. MSNA was significantly different between groups (P < .001; ICM 60 +/- 3; DCM 47 +/- 3; control subjects 35 +/- 3 bursts/min). Compared with control subjects, the total spectral power and the high-frequency component of HRV were lower in both ICM and DCM groups, but fractal and low-frequency power were lower only in the ICM group (P < .05). Peak oxygen uptake (milliliters per kilogram of body weight per minute) was significantly less in the ICM group than in the DCM group (P = .04) and lower in both groups than in the control subjects (P < .001).

CONCLUSIONS

These observations suggest an additional ischemic stimulus to sympathetic activation in heart failure, which may impair exercise capacity reflexively.

Sympathetic neural responses to coronary occlusion during balloon angioplasty

OBJECTIVE

The sympathetic nervous system is a key modulator of prognosis and outcome in cardiac ischaemia and infarction. The effects of acute cardiac ischaemia on sympathetic neural traffic in humans are unknown. We tested the hypothesis that angioplasty, and associated transient myocardial ischaemia, elicits changes in neural circulatory control, including direct intraneural measures of sympathetic traffic.

METHODS

We measured muscle sympathetic nerve activity (MSNA), blood pressure and heart rate in 12 patients (11 men, one woman) undergoing clinically indicated elective coronary angioplasty of the left anterior descending (n = 7) or circumflex (n = 5) coronary artery. Baseline data were obtained for 2 min before occlusion. Each balloon inflation was designed to last up to 120 s.

RESULTS

Coronary occlusion had no significant effect on blood pressure or heart rate. In contrast, occlusion resulted in a significant increase in MSNA, in 10 of the 12 patients, and in all seven of those patients undergoing angioplasty of the left anterior descending artery. The group mean increase in MSNA during occlusion was 36 +/- 11% (P = 0.008 versus preocclusion). MSNA increased within 60 s of occlusion (129 +/- 12% of baseline; n = 12; P = 0.04), and increased further during the next 60 s to 141 +/- 12% of baseline levels (n = 10; P < 0.001). Increases in MSNA were similar in patients with and without significant chest pain (39 +/- 9% versus 34 +/- 13%, respectively; P = 0.84).

CONCLUSION

Acute coronary occlusion during angioplasty increases central sympathetic outflow, but there is no systematic change in heart rate. The increase in sympathetic nerve traffic cannot be explained by blood pressure changes or occlusion-related chest pain.

NAD(P)H oxidase in paraventricular nucleus contributes to the effect of angiotensin II on cardiac sympathetic afferent reflex

We previously reported that reactive oxygen species (ROS) in paraventricular nucleus (PVN) modulated cardiac sympathetic afferent reflex (CSAR) and mediated the effect of angiotensin II (Ang II) in the PVN on the CSAR. In the present study, we investigated whether the NAD(P)H oxidase in the PVN was a key source of ROS which modulated the CSAR and contributed to the effect of Ang II on the CSAR. In anesthetized rats with sinoaortic denervation and vagotomy, renal sympathetic nerve activity (RSNA) and arterial pressure were recorded. The CSAR was evaluated by the RSNA response to epicardial application of bradykinin (BK). The NAD(P)H oxidase activity in the PVN was measured with lucigenin-enhanced chemiluminescent method. Microinjection of the NAD(P)H oxidase inhibitor, either apocynin (1.0 nmol) or phenylarsine oxide (PAO, 1.0 nmol), into the PVN significantly inhibited the CSAR. Microinjection of Ang II (0.3 nmol) into the PVN significantly augmented the CSAR. The effects of Ang II were not only abolished by pretreatment with either apocynin or PAO in the PVN but also partially inhibited by xanthine oxidase inhibitor allopurinol. Either epicardial application of BK or microinjection of Ang II into the PVN significantly increased NAD(P)H oxidase activity in the PVN. The effect of Ang II on NAD(P)H oxidase activity was abolished by pretreatment with AT(1) receptor antagonist losartan in the PVN. These findings suggested that NAD(P)H oxidase in the PVN was a major source of the ROS in modulating the CSAR, and the NAD(P)H oxidase contributes to the effect of Ang II on the CSAR.

Reactive oxygen species in paraventricular nucleus modulates cardiac sympathetic afferent reflex in rats

Our previous studies showed that angiotensin II (Ang II) in the paraventricular nucleus (PVN) potentiated the cardiac sympathetic afferent reflex (CSAR) in rats. This study investigated whether the reactive oxygen species (ROS) in the PVN modulated the CSAR and contributed to the effect of Ang II on the CSAR in rats. Under alpha-chloralose and urethane anesthesia, renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP) and heart rate were recorded in sinoaortic-denervated and cervical-vagotomized rats. The CSAR was evaluated by the RSNA response to epicardial application of bradykinin (0.04 and 0.4 microCompared with microinjection of saline into the PVN, superoxide anion scavenger, either tempol (20 nmol) or tiron (10 nmol), significantly decreased the CSAR (P < 0.05). Conversely, superoxide dismutase (SOD) inhibitor diethyldithio-carbamic acid (DETC, 10 nmol) potentiated the CSAR (P < 0.05). Microinjection of Ang II (0.3 nmol) into the PVN resulted in an enhanced CSAR (P < 0.05). The effect of Ang II on the CSAR was completely inhibited by pretreatment with either tempol or tiron (P < 0.05) but was not affected by DETC. On the other hand, either tempol or tiron decreased the RSNA (P < 0.05), but DETC increased the RSNA (P < 0.05). Ang II increased the RSNA (P < 0.05) and MAP (P < 0.05). The effect of Ang II on the RSNA and MAP was abolished by pretreatment with either tempol or tiron but was not affected by DETC. These results indicated that the ROS in the PVN modulated the CSAR and contributed to the effect of Ang II in the PVN on the CSAR.

α7-Nicotinic acetylcholine receptor subunit is not required for parasympathetic control of the heart in the mouse

Nicotinic acetylcholine receptors (nAChR) are assembled from a pool of nine α-subunits and three β-subunits into functional pentamers in peripheral autonomic neurons. The contribution of different subunits to native, physiologically important nAChR for synaptic transmission in autonomic ganglia is unclear. Here, we examined the importance of the α7-subunit for parasympathetic innervation of the heart. Normal (C57BL/6J), α7-deficient (Chrna7), and wild-type littermate mice were implanted with telemetry devices, and, under conscious, unsedated conditions, ECG recordings were obtained at baseline and after atropine, propranolol, and hexamethonium bromide administration. Spectral analysis of heart rate variability [power spectral analysis (PSA)] was performed for the evaluation of resting autonomic tone to the heart. At the completion of conscious studies, animals were anesthetized and underwent electrical stimulation of the vagus nerve (VS) while R-R intervals were recorded. Heart rate at baseline and after atropine, propranolol, or hexamethonium was similar in all three groups of animals. PSA curves were similar between normal, wild-type, and Chrna7 mice. VS showed no difference between control and Chrna7 mice throughout the range of stimulation (5–20 Hz). Mice deficient in the α7-nAChR subunit do not display differences in resting autonomic tone to the heart at baseline or under conditions of single and combined autonomic blockade. VS showed no difference in heart rate responses between normal and α7-deficient mice. These data support previous findings in vitro and highlight the important differences in function between nicotinic receptor subtypes because α3-deficient mice display major autonomic dysfunction. We conclude that the α7-subunit does not contribute critically to resting parasympathetic control of the heart.

Mechanisms of neurohormonal activation in chronic congestive heart failure: pathophysiology and therapeutic implications

Patients with chronic congestive heart failure have a sequential and incessant activation of those neurohormonal systems, which control body fluids, cardiac output and systemic blood pressure. Neurohormonal activation is initially selective and regional. Generalized activation is a late event in the natural history of congestive heart failure. Although the ultimate stimulus responsible for the activation of these neurohormonal systems is unknown, a decreased cardiac output and diminished effective blood volume have been proposed as the responsible mechanisms. However, extensive clinical and experimental research suggest that cardiac remodeling and loading of low-pressure cardiac receptors with sympathetic afferents could be the triggering events followed by unloading of high-pressure carotid receptors by decreased cardiac output and diminished effective blood volume.

Increased cardiac sympathetic nerve activity following acute myocardial infarction in a sheep model

The time course of cardiac sympathetic nerve activity (CSNA) following acute myocardial infarction (MI) is unknown. We therefore undertook serial direct recordings of CSNA, arterial blood pressure (MAP) and heart rate (HR) in 11 conscious sheep before and after MI, and compared them with 10 controls. Conscious CSNA recordings were taken daily from electrodes glued into the thoracic cardiac nerves. Infarction was induced under pethidine and diazepam analgesia by applying tension to a coronary suture. MI size was assessed by left ventricular planimetry (%) at postmortem, peak troponin T and brain natriuretic peptide levels (BNP). Baroreflex slopes were assessed daily using phenylephrine-nitroprusside ramps. The mean infarcted area was 14.4 +/- 2.9%, troponin T 1.88 +/- 0.39 microg l(-1) and BNP 8.4 +/- 1.3 pmol l(-1). There were no differences in haemodynamic parameters or CSNA between groups at baseline. MAP and HR remained constant following MI. CSNA burst frequency increased from baseline levels of 55.8 +/- 7.1 bursts min(-1) to levels of 77.5 +/- 8.7 bursts min(-1) at 2 h post-MI, and remained elevated for 2 days (P < 0.001). CSNA burst area also increased and was sustained for 7 days following MI (P= 0.016). Baroreflex slopes for pulse interval and CSNA did not change. CSNA increases within 1 h of the onset of MI and is sustained for at least 7 days. The duration of this response may be longer because the recording fields decrease with time. This result is consistent with a sustained cardiac excitatory sympathetic reflex.

Heart rate variability: from bench to bedside

Power spectrum analysis of cardiovascular signal variability, and in particular of the RR period (heart rate variability, HRV), is a widely used methodology for investigating autonomic neural regulation in health and disease that can quantify the sympathovagal balance modulating the sinus node pacemaker. In some cases, it can also quantify the neural regulation of other organs or apparatuses. However, use of the correct methodology is crucial to extract the information embedded in the frequency domain. In numerous abnormal conditions, such as essential arterial hypertension, acute myocardial infarction and heart failure, the sympathovagal balance may be altered in basal conditions. However, a reduced responsiveness to an excitatory stimulus is the most common feature that characterizes numerous pathophysiological states. The attenuation of an oscillatory pattern can also reflect an altered target function, thus providing important prognostic markers. The general features of this approach correspond well to the needs of an internist attempting to envisage the involvement of the whole organism in a disease process.

Paradoxical respiratory sinus arrhythmia in the anesthetized rat

This study examines respiratory sinus arrhythmia (RSA) in the isoflurane-anesthetized rat. In fifteen female Sprague-Dawley rats, we recorded continuous ECG and respiratory airflow before and after bilateral vagotomy. RSA was assessed using power spectral analysis and by plotting the normalised changes in heart period as a function of the time during the respiratory cycle. Contrary to descriptions of RSA in conscious rats, we observed in all rats in the current study a 'reversed' pattern of RSA in which heart rate decelerated during inspiration. Elimination of vagal efferent fibres to the heart by vagotomy did not abolish the presence of reversed RSA suggesting that the pattern of heart period variation is not neural, and may be largely mechanical. Vagotomy altered breathing by increasing respiratory period, tidal volume, and the time to peak inspiratory flow. These changes did not alter the magnitude of RSA but reduced the latency period between inspiratory onset and the onset of respiratory related prolongation of heart period. Periods of positive pressure ventilation were associated with reversal of the inspiratory cardiac-deceleration pattern of RSA to resemble the more widely described pattern of inspiratory cardiac-acceleration. We conclude that RSA is not a suitable measure of vagal tone during anesthesia in the rat and reiterate the caution that needs to be taken when working with anesthetized experimental models of cardiac control.

Different spectral components of 24 h heart rate variability are related to different modes of death in chronic heart failure

AIMS

To assess whether analysis of heart rate variability (HRV) from 24 h Holter recordings provides information about the mode of death (pump failure vs. sudden death) in chronic heart failure (CHF).

METHODS AND RESULTS

We analysed 24 h HRV in 330 consecutive CHF patients in sinus rhythm. Indices derived from time domain, spectral domain, and fractal analyses of 24 h automatic HRV were evaluated. Data from clinical assessment, echocardiography, right heart catheterization, exercise test, blood biochemical examination, and arrhythmia pattern were analysed. Patients were followed up for 3 years. Two simple multivariable models, both including 24 h spectral indices, were able to identify patients at higher risk of progressive pump failure and sudden death, respectively. Depressed power of night-time HRV (< or = 509 ms(2)) below 0.04 Hz [very low frequency (VLF)], high pulmonary wedge pressure (PWP > or = 18 mm Hg) and low left ventricular ejection fraction (LVEF < or = 24%) were independently related to death for progressive pump failure, while the reduction of power between 0.04 and 0.15 Hz at night (LF < or = 20 ms(2)) and increased left ventricular end-systolic diameter (LVESD > or = 61 mm) were linked to sudden mortality.

CONCLUSION

Automatic spectral analysis of 24 h HRV provides independent risk indices related to mode of death in sinus rhythm CHF patients.

The origin of sympathetic outflow in heart failure: the roles of angiotensin II and nitric oxide

The regulation of sympathetic nerve activity in chronic heart failure (CHF) has been an area of renewed investigation. Understanding the central mechanisms that are responsible for sympatho-excitation in this disease state may help in reducing the deleterious effects of chronic sympatho-excitation. This review will summarize our understanding of abnormal reflex control of the circulation in CHF. The roles of the arterial baroreflex, the chemoreflex, the cardiac sympathetic afferent reflex and the cardiopulmonary reflex are discussed. New experimental techniques that allow genetic manipulation of substances such as nitric oxide synthase in discrete areas of the brain aid in clarifying the role of NO in the modulation of sympathetic tone in the CHF state. Lastly, clinical implications of this work are discussed.

Heart Rate Variability and Diastolic Heart Failure

Diastolic heart failure accounts for up to 40% of patients with congestive heart failure (CHF), and is associated with a better prognosis as compared to patients with systolic dysfunction. Nevertheless, patients with diastolic dysfunction have a significantly higher mortality as compared to the normal population. Reduced heart rate variability (HRV), a marker of autonomic dysfunction, is associated with increased mortality in patients with systolic heart failure. We therefore sought to determine to what extent HRV is altered in a population of patients with diastolic heart failure. Twenty-four hour ambulatory (Holter) recordings were performed in 19 consecutive patients with diastolic heart failure, in 9 patients with systolic heart failure, as well as in 9 healthy volunteers (normal controls). Time and frequency domain HRV variables were obtained for all three groups of patients. Both Time and Frequency domain variables were found to be reduced in both heart failure groups compared to normal controls. When compared with each other, patients with diastolic function had relatively higher values of HRV variables, compared to those with systolic dysfunction (SDNN, Total power, ULF power, all P <or= 0.05). Patients with diastolic dysfunction have reduced HRV, suggesting a disturbed sympathetic-parasympathetic balance. Nevertheless, values for HRV are not as profoundly reduced as in patients with systolic dysfunction. The relative preservation of sympathetic-parasympathetic balance may explain the better prognosis in this patient population.

Role of short-term cardiovascular regulation in heart period variability: a modeling study

A mathematical model of short-term cardiovascular regulation is used to investigate how heart period variability reflects the action of the autonomic regulatory mechanisms (vagal and sympathetic). The model includes the pulsating heart, the systemic (splanchnic and extrasplanchnic) and pulmonary circulation, the mechanical effect of respiration on venous return, two groups of receptors (arterial baroreceptors and lung stretch receptors), the sympathetic and vagal efferent branches, and a very low-frequency (LF) vasomotor noise. All model parameters were given on the basis of physiological data from the literature. We used data from humans whenever possible, whereas parameters for the regulation loops are derived from dog experiments. The model, with basal parameter values, produces a heart period power spectrum with two distinct peaks [a high frequency (HF) peak at the respiratory rate and a LF peak at approximately 0.1 Hz]. Sensitivity analysis on the mechanism gains suggests that the HF peak is mainly affected by the vagal mechanism, whereas the LF peak is increased by a high sympathetic gain and reduced by a high vagal gain. Moreover, the LF peak depends significantly on the reactivity of resistance vessels and is affected by noise, amplified by the sympathetic control loop at its resonance frequency. The model may represent a new tool to study alterations in the heart period spectrum on the basis of quantitative physiological hypotheses.

Assessing autonomic disturbances of hypertension in the general practitioner's office: a transtelephonic approach to spectral analysis of heart rate variability

OBJECTIVES

Spectral analysis of heart rate variability (HRV) provides potentially useful information on autonomic disturbances of human hypertension. However, its practical use outside the clinical laboratory is largely unexplored. The aim of this study was to test the feasibility of assessing HRV from the general practitioner's (GP's) office, using a novel transtelephonic approach.

DESIGN

Parallel study on two similar groups: 150 subjects (100 hypertensives and 50 normotensives) studied in our hospital clinic and compared with 150 subjects studied in an out-of-hospital practice (100 hypertensive subjects from GPs' surgeries and 50 normotensive subjects participating in a multinational company's health maintenance programme).

METHODS

Spectral analysis of RR interval variability was used to assess autonomic regulation of the sino-atrial (SA) node, both at rest and during standing. For the out-of-hospital practice, strings of continuous electrocardiograms (ECGs), recorded with a microminiature instrument, were fed, off line, through standard telephone lines to our laboratory for subsequent spectral analysis.

RESULTS

RR spectral profiles obtained from healthy controls or hypertensive patients examined in their GPs' surgeries were similar to those obtained in a hospital clinic, both at rest and when standing up.

CONCLUSIONS

The present study shows the feasibility of using a transtelephonic approach to perform spectral analysis of heart rate variability from physicians' surgeries. This method could favour a more widespread use of autonomic assessment in the clinical management of hypertension.

A thermodynamic model of the sympathetic and parasympathetic nervous systems

In light of the nonequilibrium thermodynamics by I. Prigogine, the autonomic nervous system as a whole may be viewed as a dissipative structure progressively assembled in the course of evolution, plastically and rhythmically interfaced between forebrain, internal and external environments, to regulate energy, matter and information exchanges. In the present paper, this hypothesis is further pursued to verify whether the two main divisions of the autonomic nervous system, the sympathetic and parasympathetic systems, may support different types of exchange with the external environment. Previous data from hypothalamic stimulation experiments, studies of locus coeruleus function and available data on behavioral functional organization indicate that (1) tight engagement with the external environment, (2) high level of energy mobilization and utilization and (3) information mainly related to exteroceptive sensory stimulation characterize a behavioral prevalence of sympathoadrenal activation. On the other hand, (1) disengagement from the external environment, (2) low levels of internal energy and (3) dominance of proprioceptive information characterize a behavioral prevalence of vagal tone. Behavioral matter exchanges such as feeding, drinking, micturition and defecation are equally absent at the extreme of sympathoadrenal and vagally driven behaviors. The autonomic nervous system as a whole is genetically determined, but the sympathoadrenal system has been mainly designed to organize the visceral apparatus for an action to be performed by the biological system in the external environment and to deal with the novelty of task and of the environment, while the functional role of the parasympathetic is to prepare the visceral apparatus for an action to be performed by the biological system on itself, for recovery and self-protection (homeostasis), and is reinforced by repetition of phylo- and ontogenetically determined patterns. The available clinical data further support this interpretation indicating that an increased sympathetic and a decreased vagal tone may represent a consistent risk factor for cardiovascular diseases.

Sympathetic nervous system activation, antivenin administration and cardiovascular manifestations of scorpion envenomation

We performed two-dimensional echocardiograms and determined plasma norepinephrine levels on admission and at 24h after hospitalization, in 16 children with scorpion envenomation. All patients came from areas where scorpions have been identified as Tityus zulianus and received antivenin at the site of the accident or upon admission. Based on the presence or absence of cardiovascular manifestations, patients were divided into two groups.

GROUP A

10 patients had cardiovascular manifestations of pulmonary edema. Four patients had mild pulmonary edema (Left ventricular ejection fraction: 0.43+/-0.19) and six had moderate to severe pulmonary edema (Ejection fraction: 0.31+/-0.09. p=NS, M+/-SD). Plasma norepinephrine was elevated on admission (1279+/-824) and decreased at 24h in seven of eight patients (474+/-140 pg/ml, p<0.03).

GROUP B

Six patients had no cardiovascular manifestations. These patients had normal chest X-rays and normal echocardiograms. Plasma norepinephrine was not elevated (188+/-180 pg/ml). Time interval from the accident to antivenin administration was significantly longer in Group A compared to Group B (4.5+/-3.3 vs 1.2+/-0.4h, p<0.03) and correlated directly with the absolute change in plasma norepinephrine (r=0.76, p<001). Consequently, we strongly recommend very early administration of antivenin in the medical management of scorpion envenomation by T. zulianus.