Peripheral chemoreceptor hypersensitivity: an ominous sign in patients with chronic heart failure

Carotid Body-Mediated Chemoreflex Drive in The Setting of low and High Output Heart Failure

Enhanced carotid body (CB) chemoreflex function is strongly related to cardiorespiratory disorders and disease progression in heart failure (HF). The mechanisms underlying CB sensitization during HF are not fully understood, however previous work indicates blood flow per se can affect CB function. Then, we hypothesized that the CB-mediated chemoreflex drive will be enhanced only in low output HF but not in high output HF. Myocardial infarcted rats and aorto-caval fistulated rats were used as a low output HF model (MI-CHF) and as a high output HF model (AV-CHF), respectively. Blood flow supply to the CB region was decreased only in MI-CHF rats compared to Sham and AV-CHF rats. MI-CHF rats exhibited a significantly enhanced hypoxic ventilatory response compared to AV-CHF rats. However, apnea/hypopnea incidence was similarly increased in both MI-CHF and AV-CHF rats compared to control. Kruppel-like factor 2 expression, a flow sensitive transcription factor, was reduced in the CBs of MI-CHF rats but not in AV-CHF rats. Our results indicate that in the setting of HF, potentiation of the CB chemoreflex is strongly associated with a reduction in cardiac output and may not be related to other pathophysiological consequences of HF.

Sympathetic Activation in Chronic Heart Failure: Potential Benefits of Interventional Therapies

Heart failure (HF) is a major and growing public health problem. This condition is associated with poor prognosis, a high rate of mortality, frequent hospitalization and increasing costs to health care systems. Pharmacological approaches aimed at reducing morbidity and mortality in HF have primarily focused on inhibition of the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system (SNS), both of which have been associated with disease development, progression and adverse cardiovascular (CV) outcomes. The increasing number of hospitalizations for HF decompensation suggests the failure of available treatment options, indicating the necessity for alternative therapeutic approaches. Alongside pharmacological and cardiac resynchronization therapies in selected patients with arrhythmia, recent advancements in the management of HF have been directed at inhibiting relevant neurogenic pathways underlying disease development and progression. Initial evidence regarding the safety and effectiveness of interventional procedures suggests that HF patients may benefit from novel adjunctive therapies. Here we review the critical role of sympathetic activation in HF and the rationale for therapeutic interventions including device-based and interventional approaches aimed at restoring autonomic neural balance in this condition.

Carotid Body Denervation Markedly Improves Survival in Rats With Hypertensive Heart Failure

BACKGROUND

Hypertension is a major cause of heart failure. Excessive sympathoexcitation in patients with heart failure leads to poor prognosis. Since carotid body denervation (CBD) has been shown to reduce sympathetic nerve activity in animal models of hypertension and heart failure, we examined if bilateral CBD attenuates the progression of hypertensive heart failure and improves survival.

METHODS

We randomly allocated Dahl salt-sensitive rats fed a high-salt diet from 6 weeks of age into CBD (n = 31) and sham-operation (SHAM; n = 50) groups, and conducted CBD or SHAM at 7 weeks of age. We examined the time course of 24-hour urinary norepinephrine (uNE) excretion, blood pressure (BP) and the percent fractional shortening assessed by echocardiography, and estimated the pressure-natriuresis relationship at 14 weeks of age. Finally, we assessed hemodynamics, histological findings, and survival at 16 weeks of age.

RESULTS

Compared to SHAM, CBD significantly reduced 24-hour uNE at 12, 14, and 16 weeks of age, shifted the pressure-natriuresis relationship leftward without changing its slope, and attenuated the increase in BP. CBD preserved percent fractional shortening (34.2 ± 1.2 vs. 29.1 ± 1.3%, P < 0.01) and lowered left ventricular end-diastolic pressure (5.0 ± 0.9 vs. 9.0 ± 1.4 mm Hg, P < 0.05). Furthermore, CBD significantly attenuated myocardial hypertrophy (P < 0.01) and fibrosis (P < 0.01). Consequently, CBD markedly improved survival (relative risk reduction: 64.8%).

CONCLUSIONS

CBD attenuated the progression of hypertension and worsening of heart failure possibly through sympathoinhibition, and markedly improved survival in a rat model of hypertensive heart failure.

Effect of dead space on breathing stability at exercise in hypoxia

Recent studies have shown that normal subjects exhibit periodic breathing when submitted to concomitant environmental (hypoxia) and physiological (exercise) stresses. A mathematical model including mass balance equations confirmed the short period of ventilatory oscillations and pointed out an important role of dead space in the genesis of these phenomena. Ten healthy subjects performed mild exercise on a cycloergometer in different conditions: rest/exercise, normoxia/hypoxia and no added dead space/added dead space (aDS). Ventilatory oscillations (V˙E peak power) were augmented by exercise, hypoxia and aDS (P<0.001, P<0.001 and P<0.01, respectively) whereas V˙E period was only shortened by exercise (P<0.001), with an 11-s period. aDS also increased V˙E (P<0.001), tidal volume (VT, P<0.001), and slightly augmented PETCO₂ (P<0.05) and the respiratory frequency (P<0.05). These results confirmed our previous model, showing an exacerbation of breathing instability by increasing dead space. This underlines opposite effects observed in heart failure patients and normal subjects, in which added dead space drastically reduced periodic breathing and sleep apneas. It also points out that alveolar ventilation remains very close to metabolic needs and is not affected by an added dead space. Clinical Trial reg. n°: NCT02201875.

The human ventilatory response to stress: rate or depth?

Many stressors cause an increase in ventilation in humans. This is predominantly reported as an increase in minute ventilation (V̇E). But, the same V̇E can be achieved by a wide variety of changes in the depth (tidal volume, V_T ) and number of breaths (respiratory frequency, ƒ_R ). This review investigates the impact of stressors including: cold, heat, hypoxia, pain and panic on the contributions of ƒ_R and V_T to V̇E to see if they differ with different stressors. Where possible we also consider the potential mechanisms that underpin the responses identified, and propose mechanisms by which differences in ƒ_R and V_T are mediated. Our aim being to consider if there is an overall differential control of ƒ_R and V_T that applies in a wide range of conditions. We consider moderating factors, including exercise, sex, intensity and duration of stimuli. For the stressors reviewed, as the stress becomes extreme V̇E generally becomes increased more by ƒ_R than V_T . We also present some tentative evidence that the pattern of ƒ_R and V_T could provide some useful diagnostic information for a variety of clinical conditions. In The Physiological Society's year of 'Making Sense of Stress', this review has wide-ranging implications that are not limited to one discipline, but are integrative and relevant for physiology, psychophysiology, neuroscience and pathophysiology.

Resonance as the Mechanism of Daytime Periodic Breathing in Patients with Heart Failure

RATIONALE

In patients with chronic heart failure, daytime oscillatory breathing at rest is associated with a high risk of mortality. Experimental evidence, including exaggerated ventilatory responses to CO₂ and prolonged circulation time, implicates the ventilatory control system and suggests feedback instability (loop gain > 1) is responsible. However, daytime oscillatory patterns often appear remarkably irregular versus classic instability (Cheyne-Stokes respiration), suggesting our mechanistic understanding is limited.

OBJECTIVES

We propose that daytime ventilatory oscillations generally result from a chemoreflex resonance, in which spontaneous biological variations in ventilatory drive repeatedly induce temporary and irregular ringing effects. Importantly, the ease with which spontaneous biological variations induce irregular oscillations (resonance "strength") rises profoundly as loop gain rises toward 1. We tested this hypothesis through a comparison of mathematical predictions against actual measurements in patients with heart failure and healthy control subjects.

METHODS

In 25 patients with chronic heart failure and 25 control subjects, we examined spontaneous oscillations in ventilation and separately quantified loop gain using dynamic inspired CO₂ stimulation.

MEASUREMENTS AND MAIN RESULTS

Resonance was detected in 24 of 25 patients with heart failure and 18 of 25 control subjects. With increased loop gain-consequent to increased chemosensitivity and delay-the strength of spontaneous oscillations increased precipitously as predicted (r = 0.88), yielding larger (r = 0.78) and more regular (interpeak interval SD, r = -0.68) oscillations (P < 0.001 for all, both groups combined).

CONCLUSIONS

Our study elucidates the mechanism underlying daytime ventilatory oscillations in heart failure and provides a means to measure and interpret these oscillations to reveal the underlying chemoreflex hypersensitivity and reduced stability that foretells mortality in this population.

Central Sleep Apnea in Patients with Congestive Heart Failure

Central sleep apnea and Cheyne-Stokes respiration are commonly observed breathing patterns during sleep in patients with congestive heart failure. Common risk factors are male gender, older age, presence of atrial fibrillation, and daytime hypocapnia. Proposed mechanisms include augmented peripheral and central chemoreceptor sensitivity, which increase ventilator instability during both wakefulness and sleep; diminished cerebrovascular reactivity and increased circulation time, which impair the normal buffering of Paco₂ and hydrogen ions and delay the detection of changes in Paco₂ during sleep; and rostral fluid shifts that predispose to hypocapnia.

Exercise training improves characteristics of exercise oscillatory ventilation in chronic heart failure

Background Exercise oscillatory ventilation in chronic heart failure has been suggested as a factor related to adverse cardiac events, aggravated prognosis and higher mortality. Exercise training is well known to affect exercise capacity and mechanisms of pathophysiology beneficially in chronic heart failure. Little is known, however, about the exercise training effects on characteristics of exercise oscillatory ventilation in chronic heart failure patients. Design and methods Twenty (out of 38) stable chronic heart failure patients exhibited exercise oscillatory ventilation (age 54 ± 11 years, peak oxygen uptake 15.0 ± 5.0 ml/kg per minute). Patients attended 36 sessions of high intensity interval exercise. All patients underwent cardiopulmonary exercise testing before and after the programme. Assessment of exercise oscillatory ventilation was based on the amplitude of cyclic fluctuations in breathing during rest and exercise. All values are mean ± SD. Results Exercise training reduced ( P < 0.05) the percentage of exercise oscillatory ventilation duration (79.0 ± 13.0 to 50.0 ± 25.0%), while average amplitude (5.2 ± 2.0 to 4.9 ± 1.6 L/minute) and length (44.0 ± 10.9 to 41.0 ± 6.7 seconds) did not change ( P > 0.05). Exercise oscillatory ventilation patients also increased exercise capacity ( P < 0.05). Conclusions A rehabilitation programme based on high intensity interval training improved exercise oscillatory ventilation observed in chronic heart failure patients, as well as cardiopulmonary efficiency and functional capacity.

The Influence of Age on Interaction between Breath-Holding Test and Single-Breath Carbon Dioxide Test

Introduction. The aim of the study was to compare the breath-holding test and single-breath carbon dioxide test in evaluation of the peripheral chemoreflex sensitivity to carbon dioxide in healthy subjects of different age. Methods. The study involved 47 healthy volunteers between ages of 25 and 85 years. All participants were divided into 4 groups according to age: 25 to 44 years (n = 14), 45 to 60 years (n = 13), 60 to 75 years (n = 12), and older than 75 years (n = 8). Breath-holding test was performed in the morning before breakfast. The single-breath carbon dioxide (SB-CO2) test was performed the following day. Results. No correlation was found between age and duration of breath-holding (r = 0.13) and between age and peripheral chemoreflex sensitivity to CO2 (r = 0.07). In all age groups there were no significant differences in the mean values from the breath-holding test and peripheral chemoreflex sensitivity tests. In all groups there was a strong significant inverse correlation between breath-holding test and SB-CO2 test. Conclusion. A breath-holding test reflects the sensitivity of the peripheral chemoreflex to carbon dioxide in healthy elderly humans. Increasing age alone does not alter the peripheral ventilatory response to hypercapnia.

Insulin resistance: a new consequence of altered carotid body chemoreflex?

Metabolic diseases affect millions of individuals across the world and represent a group of chronic diseases of very high prevalence and relatively low therapeutic success, making them suitable candidates for pathophysiological studies. The sympathetic nervous system (SNS) contributes to the regulation of energy balance and energy expenditure both in physiological and pathological states. For instance, drugs that stimulate sympathetic activity decrease food intake, increase resting metabolic rate and increase the thermogenic response to food, while pharmacological blockade of the SNS has opposite effects. Likewise, dysmetabolic features such as insulin resistance, dyslipidaemia and obesity are characterized by a basal overactivation of the SNS. Recently, a new line of research linking the SNS to metabolic diseases has emerged with the report that the carotid bodies (CBs) are involved in the development of insulin resistance. The CBs are arterial chemoreceptors that classically sense changes in arterial blood O2 , CO2 and pH levels and whose activity is known to be increased in rodent models of insulin resistance. We have shown that selective bilateral resection of the nerve of the CB, the carotid sinus nerve (CSN), totally prevents diet-induced insulin resistance, hyperglycaemia, dyslipidaemia, hypertension and sympathoadrenal overactivity. These results imply that the beneficial effects of CSN resection on insulin action and glucoregulation are modulated by target-related efferent sympathetic nerves through a reflex that is initiated in the CBs. It also highlights modulation of CB activity as a putative future therapeutic intervention for metabolic diseases.

Endurance training attenuates the increase in peripheral chemoreflex sensitivity with intermittent hypoxia

Patients with heart failure and sleep apnea have greater chemoreflex sensitivity, presumably due to intermittent hypoxia (IH), and this is predictive of mortality. We hypothesized that endurance training would attenuate the effect of IH on peripheral chemoreflex sensitivity in healthy humans. Fifteen young healthy subjects (9 female, 26 ± 1 yr) participated. Between visits, 11 subjects underwent 8 wk of endurance training that included running four times/wk at 80% predicted maximum heart rate and interval training, and four control subjects did not change activity. Chemoreflex sensitivity (the slope of ventilation responses to serial oxygen desaturations), blood pressure, heart rate, and muscle sympathetic nerve activity (MSNA) were assessed before and after 30 min of IH. Endurance training decreased resting systolic blood pressure (119 ± 3 to 113 ± 3 mmHg; P = 0.027) and heart rate (67 ± 3 to 61 ± 2 beats/min; P = 0.004) but did not alter respiratory parameters at rest (P > 0.2). Endurance training attenuated the IH-induced increase in chemoreflex sensitivity (pretraining: Δ 0.045 ± 0.026 vs. posttraining: Δ -0.028 ± 0.040 l·min^-1·% O₂ desaturation^-1; P = 0.045). Furthermore, IH increased mean blood pressure and MSNA burst rate before training (P < 0.05), but IH did not alter these measures after training (P > 0.2). All measurements were similar in the control subjects at both visits (P > 0.05). Endurance training attenuates chemoreflex sensitization to IH, which may partially explain the beneficial effects of exercise training in patients with cardiovascular disease.

Impact of sleep disordered breathing on carotid body size

We tested the hypotheses that: (1) carotid body size can be measured by computed tomographic angiography (CTA) with high inter-observer agreement, and (2) patients with sleep apnea exhibit larger carotid bodies than those without sleep apnea. A chart review was conducted from patients who underwent neck CTA and polysomnography at the Mayo Clinic between January 2000 and February 2015. Widest axial measurements of the carotid bodies, performed independently by two radiologists, were possible in 81% of patients. Intra-class correlation coefficients ranged from 0.93 to 0.95 (Right carotid body: 0.93; Left: 0.94; Average: 0.95). Widest axial measurements of the carotid bodies were greater in patients with sleep apnea (n=32) compared to controls (n=46, P-value range 0.02-0.04). After adjusting for age, no differences in carotid body size were observed between the patient groups (P-value range 0.45-0.59). We conclude carotid body size can be detected by CTA with high inter-observer agreement; however, carotid body size is not increased in patients with sleep apnea.

Ventilatory oscillations at exercise in hypoxia: A mathematical model

We evaluated the mechanisms responsible for the instability of ventilation control system under simultaneous metabolic (exercise) and environmental (hypoxia) stresses, promoting the genesis of periodic breathing. A model following the main concepts of ventilatory control has been tested, including cardiovascular and respiratory parameters, characteristics of peripheral and central chemoreceptors, at mild exercise in hypoxia (FIO₂=0.145). Interaction between O₂ and CO₂ sensing was introduced following three different modalities. A sensitivity and multivariate regression analyses closely matched with physiological data for magnitude and period of oscillations. Low FIO₂ and long circulatory delay from lungs to peripheral chemoreceptors (DeltaTp) lengthen the period of oscillations, while high peripheral and central chemoresponses to O₂ and CO₂, low FIO₂ and high DeltaTp increased their magnitude. Peripheral and central O₂/CO₂ interactions highlight the role of CO₂ on peripheral gain to O₂ and the contribution of peripheral afferences on central gain to CO₂. Our model supports the key role of peripheral chemoreceptors in the genesis of ventilatory oscillations. Differences in the dynamics of central and peripheral components might be determinant for the system stability.

Unilateral Carotid Body Resection in Resistant Hypertension: A Safety and Feasibility Trial

Animal and human data indicate pathological afferent signaling emanating from the carotid body that drives sympathetically mediated elevations in blood pressure in conditions of hypertension. This first-in-man, proof-of-principle study tested the safety and feasibility of unilateral carotid body resection in 15 patients with drug-resistant hypertension. The procedure proved to be safe and feasible. Overall, no change in blood pressure was found. However, 8 patients showed significant reductions in ambulatory blood pressure coinciding with decreases in sympathetic activity. The carotid body may be a novel target for treating an identifiable subpopulation of humans with hypertension.

Reductions in carotid chemoreceptor activity with low-dose dopamine improves baroreflex control of heart rate during hypoxia in humans

The purpose of the present investigation was to examine the contribution of the carotid body chemoreceptors to changes in baroreflex control of heart rate with exposure to hypoxia. We hypothesized spontaneous cardiac baroreflex sensitivity (scBRS) would be reduced with hypoxia and this effect would be blunted when carotid chemoreceptor activity was reduced with low-dose dopamine. Fifteen healthy adults (11 M/4 F) completed two visits randomized to intravenous dopamine or placebo (saline). On each visit, subjects were exposed to 5-min normoxia (~99% SpO2), followed by 5-min hypoxia (~84% SpO2). Blood pressure (intra-arterial catheter) and heart rate (ECG) were measured continuously and scBRS was assessed by spectrum and sequence methodologies. scBRS was reduced with hypoxia (P < 0.01). Using the spectrum analysis approach, the fall in scBRS with hypoxia was attenuated with infusion of low-dose dopamine (P < 0.01). The decrease in baroreflex sensitivity to rising pressures (scBRS "up-up") was also attenuated with low-dose dopamine (P < 0.05). However, dopamine did not attenuate the decrease in baroreflex sensitivity to falling pressures (scBRS "down-down"; P > 0.05). Present findings are consistent with a reduction in scBRS with systemic hypoxia. Furthermore, we show this effect is partially mediated by the carotid body chemoreceptors, given the fall in scBRS is attenuated when activity of the chemoreceptors is reduced with low-dose dopamine. However, the improvement in scBRS with dopamine appears to be specific to rising blood pressures. These results may have important implications for impairments in baroreflex function common in disease states of acute and/or chronic hypoxemia, as well as the experimental use of dopamine to assess such changes.

Chemosensitivity, Cardiovascular Risk, and the Ventilatory Response to Exercise in COPD

COPD is associated with elevated cardiovascular risk and a potentiated ventilatory response to exercise. Enhanced carotid chemoreceptor (CC) activity/sensitivity is present in other clinical conditions, has been shown to contribute to sympathetic vasoconstrictor outflow, and is predictive of mortality. CC activity/sensitivity, and the resulting functional significance, has not been well examined in COPD. We hypothesized that CC activity/sensitivity would be elevated in COPD, and related to increased pulse wave velocity (a marker of CV risk) and the ventilatory response to exercise.

METHODS

30 COPD patients and 10 healthy age-matched controls were examined. Participants performed baseline cardiopulmonary exercise and pulmonary function testing. CC activity was later evaluated by the drop in ventilation with breathing 100% O2, and CC sensitivity was then assessed by the ventilatory response to hypoxia (ΔVE/ΔSpO2). Peripheral arterial stiffness was subsequently evaluated by measurement of pulse wave velocity (PWV) using applanation tonometry while the subjects were breathing room air, and then following chemoreceptor inhibition by breathing 100% O2 for 2 minutes.

RESULTS

CC activity, CC sensitivity, PWV and the ventilatory response to exercise were all increased in COPD relative to controls. CC sensitivity was related to PWV; however, neither CC activity nor CC sensitivity was related to the ventilatory response to exercise in COPD. CC inhibition by breathing 100% O2 normalized PWV in COPD, while no effect was observed in controls.

CONCLUSION

CC activity and sensitivity are elevated in COPD, and appear related to cardiovascular risk; however, CC activity/sensitivity does not contribute to the potentiated ventilatory response to exercise.

Exercise training attenuates the pressor response evoked by peripheral chemoreflex in rats with heart failure

The effects of exercise training (ExT) on the pressor response elicited by potassium cyanide (KCN) in the rat model of ischemia-induced heart failure (HF) are unknown. We evaluated the effects of ExT on chemoreflex sensitivity and its interaction with baroreflex in rats with HF. Wistar rats were divided into four groups: trained HF (Tr-HF), sedentary HF (Sed-HF), trained sham (Tr-Sham), and sedentary sham (Sed-Sham). Trained animals underwent to a treadmill running protocol for 8 weeks (60 m/day, 5 days/week, 16 m/min). After ExT, arterial pressure (AP), baroreflex sensitivity (BRS), peripheral chemoreflex (KCN: 100 μg/kg body mass), and cardiac function were evaluated. The results demonstrate that ExT induces an improvement in BRS and attenuates the pressor response to KCN relative to the Sed-HF group (P < 0.05). The improvement in BRS was associated with a reduction in the pressor response following ExT in HF rats (P < 0.05). Moreover, ExT induced a reduction in left ventricular end-diastolic pressure and pulmonary congestion compared with the Sed-HF group (P < 0.05). The pressor response to KCN in the hypotensive state is decreased in sedentary HF rats. These results suggest that ExT improves cardiac function and BRS and attenuates the pressor response evoked by KCN in HF rats.

Cardiopulmonary Exercise Testing in Heart Failure

Exercise intolerance, indicated by dyspnea and fatigue during exertion, is a cardinal manifestation of heart failure (HF). Cardiopulmonary exercise testing (CPET) precisely defines maximum exercise capacity through measurement of peak oxygen uptake (VO2). Peak VO2 values have a critical role in informing patient selection for advanced HF interventions such as heart transplantation and ventricular assist devices. Oxygen uptake and ventilatory patterns obtained during the submaximal portion of CPET are also valuable to recognize because of their ease of ascertainment during low-level exercise, relevance to ability to perform activities of daily living, independence from volitional effort, and strong relationship to prognosis in HF. The ability of peak VO2 and other CPET variables to be measured reproducibly and to accurately reflect HF severity is increasingly recognized and endorsed by scientific statements. Integration of CPET with invasive hemodynamic monitoring and cardiac imaging during exercise provides comprehensive characterization of multisystem reserve capacity that can inform prognosis and the need for cardiac interventions. Here, we review both practical aspects of conducting CPETs in patients with HF for clinical and research purposes as well as interpretation of gas exchange patterns across the spectrum of preclinical HF to advanced HF.

Prediction of the Chemoreflex Gain by Common Clinical Variables in Heart Failure

BACKGROUND

Peripheral and central chemoreflex sensitivity, assessed by the hypoxic or hypercapnic ventilatory response (HVR and HCVR, respectively), is enhanced in heart failure (HF) patients, is involved in the pathophysiology of the disease, and is under investigation as a potential therapeutic target. Chemoreflex sensitivity assessment is however demanding and, therefore, not easily applicable in the clinical setting. We aimed at evaluating whether common clinical variables, broadly obtained by routine clinical and instrumental evaluation, could predict increased HVR and HCVR.

METHODS AND RESULTS

191 patients with systolic HF (left ventricular ejection fraction--LVEF--<50%) underwent chemoreflex assessment by rebreathing technique to assess HVR and HCVR. All patients underwent clinical and neurohormonal evaluation, comprising: echocardiogram, cardiopulmonary exercise test (CPET), daytime cardiorespiratory monitoring for breathing pattern evaluation. Regarding HVR, multivariate penalized logistic regression, Bayesian Model Averaging (BMA) logistic regression and random forest analysis identified, as predictors, the presence of periodic breathing and increased slope of the relation between ventilation and carbon dioxide production (VE/VCO2) during exercise. Again, the above-mentioned statistical tools identified as HCVR predictors plasma levels of N-terminal fragment of proBNP and VE/VCO2 slope.

CONCLUSIONS

In HF patients, the simple assessment of breathing pattern, alongside with ventilatory efficiency during exercise and natriuretic peptides levels identifies a subset of patients presenting with increased chemoreflex sensitivity to either hypoxia or hypercapnia.

Carotid body modulation in systolic heart failure from the clinical perspective

Augmented sensitivity of peripheral chemoreceptors (PChS) is a common finding in systolic heart failure (HF). It is related to lower left ventricle systolic function, higher plasma concentrations of natriuretic peptides, worse exercise tolerance and greater prevalence of atrial fibrillation compared to patients with normal PChS. The magnitude of ventilatory response to the activation of peripheral chemoreceptors is proportional to the level of heart rate (tachycardia) and blood pressure (hypertension) responses. All these responses can be measured non-invasively in a safe and reproducible fashion using different methods employing either hypoxia or hypercapnia. Current interventions aimed at modulation of peripheral chemoreceptors in HF are focused on carotid bodies (CBs). There is a clear link between afferent signalling from CBs and sympathetic overactivity, which remains the priority target of modern HF treatment. However, CB modulation therapies may face several potential obstacles: (1) As evidenced by HF trials, an excessive inhibition of sympathetic system may be harmful. (2) Proximity of critical anatomical structures (important vessels and nerves) makes surgical and transcutaneous interventions on CB technically demanding. (3) Co-existence of atherosclerosis in the area of carotid artery bifurcation increases the risk of central embolic events related to CB modulation. (4) The relative contribution of CBs vs. aortic bodies to sympathetic activation in HF patients is unclear. (5) Choosing optimal candidates for CB modulation from the population of HF patients may be problematic. (6) There is a risk of nocturnal hypoxia following CB ablation - mostly after bilateral procedures and in patients with concomitant obstructive sleep apnoea.

Exercise oscillatory ventilation: Mechanisms and prognostic significance

Alteration in breathing patterns characterized by cyclic variation of ventilation during rest and during exercise has been recognized in patients with advanced heart failure (HF) for nearly two centuries. Periodic breathing (PB) during exercise is known as exercise oscillatory ventilation (EOV) and is characterized by the periods of hyperpnea and hypopnea without interposed apnea. EOV is a non-invasive parameter detected during submaximal cardiopulmonary exercise testing. Presence of EOV during exercise in HF patients indicates significant impairment in resting and exercise hemodynamic parameters. EOV is also an independent risk factor for poor prognosis in HF patients both with reduced and preserved ejection fraction irrespective of other gas exchange variables. Circulatory delay, increased chemosensitivity, pulmonary congestion and increased ergoreflex signaling have been proposed as the mechanisms underlying the generation of EOV in HF patients. There is no proven treatment of EOV but its reversal has been noted with phosphodiesterase inhibitors, exercise training and acetazolamide in relatively small studies. In this review, we discuss the mechanistic basis of PB during exercise and the clinical implications of recognizing PB patterns in patients with HF.

Carotid body chemoreceptors, sympathetic neural activation, and cardiometabolic disease

The carotid body (CB) is the main peripheral chemoreceptor that senses the arterial PO₂, PCO₂ and pH. In response to hypoxemia, hypercapnia and acidosis, carotid chemosensory discharge elicits reflex respiratory, autonomic and cardiovascular adjustments. The classical construct considers the CB as the main peripheral oxygen sensor, triggering reflex physiological responses to acute hypoxemia and facilitating the ventilatory acclimation to chronic hypoxemia at high altitude. However, a growing body of experimental evidence supports the novel concept that an abnormally enhanced CB chemosensory input to the brainstem contributes to overactivation of the sympathetic nervous system, and consequent pathology. Indeed, the CB has been implicated in several diseases associated with increases in central sympathetic outflow. These include hypertension, heart failure, sleep apnea, chronic obstructive pulmonary disease and metabolic syndrome. Indeed, ablation of the CB has been proposed for the treatment of severe and resistant hypertension in humans. In this review, we will analyze and discuss new evidence supporting an important role for the CB chemoreceptor in the progression of autonomic and cardiorespiratory alterations induced by heart failure, obstructive sleep apnea, chronic obstructive pulmonary disease and metabolic syndrome.

Increased body fat is associated with potentiation of blood pressure response to hypoxia in healthy men: relations with insulin and leptin

Background

Increased peripheral chemosensitivity (PChS) has been proposed as mechanism underlying obesity-related sympathoactivation, with insulin and/or leptin as possible mediators. However, human data on PChS in obesity are scarce. Therefore, we explored this issue in a sample of 41 healthy men aged 30–59 years, divided according to body fat percentage (fat %) into two groups: <25 and ≥25 %.

Methods

PChS was assessed using transient hypoxia method [respiratory (PChS-MV), heart rate (PChS-HR), and blood pressure (PChS-SBP) responses were calculated]. Baroreflex sensitivity (BRS-Seq) was assessed using sequence method. Fasting plasma insulin and leptin levels were measured. Homeostatic model assessment (HOMA) was used to assess insulin sensitivity/resistance.

Results

Individuals with ≥25 % body fat demonstrated increased PChS-SBP (p < 0.01), but unchanged PChS-MV and PChS-HR (both p > 0.4). PChS-SBP was related positively with anthropometric characteristics (e.g. waist circumference, fat %), plasma insulin and HOMA (all p < 0.05), and negatively with BRS-Seq (p = 0.001), but not with plasma leptin (p = 0.27).

Conclusions

In healthy men, overweight/obesity is accompanied by augmented blood pressure response from peripheral chemoreceptors, while respiratory and heart rate responses remain unaltered. Hyperinsulinaemia and insulin resistance (but not hyperleptinaemia) are associated with augmented pressure response from chemoreceptors.

Electronic supplementary material

The online version of this article (doi:10.1007/s10286-015-0338-2) contains supplementary material, which is available to authorized users.

Peripheral reflex feedbacks in chronic heart failure: Is it time for a direct treatment?

Despite repeated attempts to develop a unifying hypothesis that explains the clinical syndrome of heart failure (HF), no single conceptual paradigm for HF has withstood the test of time. The last model that has been developed, the neurohormonal model, has the great virtue of highlighting the role of the heart as an endocrine organ, as well as to shed some light on the key role on HF progression of neurohormones and peripheral organs and tissues beyond the heart itself. However, while survival in clinical trials based on neurohormonal antagonist drugs has improved, HF currently remains a lethal condition. At the borders of the neurohormonal model of HF, a partially unexplored path trough the maze of HF pathophysiology is represented by the feedback systems. There are several evidences, from both animal studies and humans reports, that the deregulation of baro-, ergo- and chemo-reflexes in HF patients elicits autonomic imbalance associated with parasympathetic withdrawal and increased adrenergic drive to the heart, thus fundamentally contributing to the evolution of the disease. Hence, on top of guideline-recommended medical therapy, mainly based on neurohormonal antagonisms, all visceral feedbacks have been recently considered in HF patients as additional potential therapeutic targets.

Relevance of the Carotid Body Chemoreflex in the Progression of Heart Failure

Chronic heart failure (CHF) is a global health problem affecting millions of people. Autonomic dysfunction and disordered breathing patterns are commonly observed in patients with CHF, and both are strongly related to poor prognosis and high mortality risk. Tonic activation of carotid body (CB) chemoreceptors contributes to sympathoexcitation and disordered breathing patterns in experimental models of CHF. Recent studies show that ablation of the CB chemoreceptors improves autonomic function and breathing control in CHF and improves survival. These exciting findings indicate that alterations in CB function are critical to the progression of CHF. Therefore, better understanding of the physiology of the CB chemoreflex in CHF could lead to improvements in current treatments and clinical management of patients with CHF characterized by high chemosensitivity. Accordingly, the main focus of this brief review is to summarize current knowledge of CB chemoreflex function in different experimental models of CHF and to comment on their potential translation to treatment of human CHF.

The role of the renal afferent and efferent nerve fibers in heart failure

Renal nerves contain afferent, sensory and efferent, sympathetic nerve fibers. In heart failure (HF) there is an increase in renal sympathetic nerve activity (RSNA), which can lead to renal vasoconstriction, increased renin release and sodium retention. These changes are thought to contribute to renal dysfunction, which is predictive of poor outcome in patients with HF. In contrast, the role of the renal afferent nerves remains largely unexplored in HF. This is somewhat surprising as there are multiple triggers in HF that have the potential to increase afferent nerve activity, including increased venous pressure and reduced kidney perfusion. Some of the few studies investigating renal afferents in HF have suggested that at least the sympatho-inhibitory reno-renal reflex is blunted. In experimentally induced HF, renal denervation, both surgical and catheter-based, has been associated with some improvements in renal and cardiac function. It remains unknown whether the effects are due to removal of the efferent renal nerve fibers or afferent renal nerve fibers, or a combination of both. Here, we review the effects of HF on renal efferent and afferent nerve function and critically assess the latest evidence supporting renal denervation as a potential treatment in HF.

The sympathetic/parasympathetic imbalance in heart failure with reduced ejection fraction

Cardiovascular autonomic imbalance, a cardinal phenotype of human heart failure, has adverse implications for symptoms during wakefulness and sleep; for cardiac, renal, and immune function; for exercise capacity; and for lifespan and mode of death. The objectives of this Clinical Review are to summarize current knowledge concerning mechanisms for disturbed parasympathetic and sympathetic circulatory control in heart failure with reduced ejection fraction and its clinical and prognostic implications; to demonstrate the patient-specific nature of abnormalities underlying this common phenotype; and to illustrate how such variation provides opportunities to improve or restore normal sympathetic/parasympathetic balance through personalized drug or device therapy.

Effects of carotid body tumor resection on the blood pressure of essential hypertensive patients

Removal of carotid body (CB) improves animal models of hypertension (HTN) and heart failure, via withdrawal of chemoreflex-induced sympathetic activation. Effect of CB tumor (CBT) resection on blood pressure (BP) in subjects with HTN is unknown. A retrospective analysis of 20 subjects with HTN (BP≥140/90 mmHg or anti-hypertensives use) out of 134 with CBT resection. Short-term (30 days from surgery) and long-term (slope of regressions on time over the entire follow-up) changes in BP and heart rate were adjusted for covariates (interval between readings, total follow-up, number of readings and changes in therapy). Age and duration of HTN were 56±4 and 9±5 years. Adjusted short-term decreases in systolic (SBP: -9.9±3.1, p<0.001) and pulse pressures (PP: -7.9±2.7, p<0.002) were significant and correlated with their respective long-term changes (SBP: r=0.47, p=0.047; PP: r=0.54, p=0.019). There was a strong relationship between adjusted short-term changes in SBP and PP (r=0.64, p<0.004). Six (50% of responders or 33% of the total) had short-term falls of SBP ≥10 mmHg and of PP ≥ 5 mmHg. First study to show that unilateral CBT resection is associated with sustained reduction of BP in hypertensive patients. Targeted CB chemoreflex removal could play a role in the therapy of human HTN.

VE/VCO2 slope and its prognostic value in patients with chronic heart failure

The minute ventilation/carbon dioxide production (VE/VCO₂) slope has been widely demonstrated to have strong prognostic value in patients with chronic heart failure (CHF), and the risk of mortality is believed to increase when the VE/VCO₂ slope is >32.8; however, there is little evidence concerning the prognostic value of the VE/VCO₂ slope in Chinese patients. In the present study, the prognostic value of the VE/VCO₂ slope was investigated in patients with CHF. A total of 258 subjects underwent symptom-limited cardiopulmonary exercise testing (CPET) and were divided into CHF (113 males and 16 females; LVEF <0.49) and control (106 males and 23 females) groups. The cardiac-related events over a median 33.7-month follow-up period subsequent to the CPET were evaluated using receiver operating characteristic curve analysis. The VE/VCO₂ slope was significantly different between the CHF and control groups (P<0.001). The area under the curve (AUC) for the VE/VCO₂ slope in predicting cardiac-related mortalities in the patients with CHF was 0.670 (P<0.05), and the sensitivity and specificity of the VE/VCO₂ slope were 0.667 and 0.620, respectively. The optimal threshold of the VE/VCO₂ slope for predicting cardiac-related mortalities in patients with CHF was ≥39.3. The AUC for the VE/VCO₂ slope in predicting cardiac-related hospitalizations in patients with CHF was 0.682 (P<0.05), and the sensitivity and specificity of the VE/VCO₂ slope were 0.631 and 0.778, respectively. The optimal threshold of the VE/VCO₂ slope for predicting cardiac-related hospitalizations in patients with CHF was ≥32.9. In conclusion, ventilatory efficiency decreases in patients with CHF. The VE/VCO₂ slope is a strong predictor of cardiac-related mortalities in the patients with CHF analyzed.

Peripheral chemoreceptor control of cardiovascular function at rest and during exercise in heart failure patients

Peripheral chemoreceptor activity/sensitivity is enhanced in chronic heart failure (HF), and sensitivity is linked to greater mortality. This study aimed to determine the role of the peripheral chemoreceptor in cardiovascular control at rest and during exercise in HF patients and controls. Clinically stable HF patients (n = 11; ejection fraction: 39 ± 5%) and risk-matched controls (n = 10; ejection fraction: 65 ± 2%) performed randomized trials with or without dopamine infusion (2 μg·min(-1)·kg(-1)) at rest and during 40% maximal voluntary contraction handgrip (HG) exercise, and a resting trial of 2 min of inspired 100% oxygen. Both dopamine and hyperoxia were used to inhibit the peripheral chemoreceptor. At rest in HF patients, dopamine decreased ventilation (P = 0.02), decreased total peripheral resistance index (P = 0.003), and increased cardiac and stroke indexes (P ≤ 0.01), yet there was no effect of dopamine on these variables in controls (P ≥ 0.7). Hyperoxia lowered ventilation in HF (P = 0.01), but not in controls (P = 0.9), indicating suppression of the peripheral chemoreceptors in HF. However, no decrease of total peripheral resistance index was observed in HF. As expected, HG increased heart rate, ventilation, and brachial conductance of the nonexercising arm in controls and HF patients. During dopamine infusion, there were no changes in mean arterial pressure, heart rate, or ventilation responses to HG in either group (P ≥ 0.26); however, brachial conductance increased with dopamine in the control group (P = 0.004), but decreased in HF (P = 0.02). Our findings indicate that the peripheral chemoreceptor contributes to cardiovascular control at rest in HF patients and during exercise in risk-matched controls.

Mechanisms of carotid body chemoreflex dysfunction during heart failure

NEW FINDINGS

What is the topic of this review? Carotid body chemoreceptor activity is tonically elevated in heart failure and contributes to morbidity due to the reflex activation of sympathetic nerve activity and destabilization of breathing. The potential causes for the enhanced chemoreceptor activation in heart failure are discussed. What advances does it highlight? The role of a chronic reduction in blood flow to the carotid body due to cardiac failure and its impact on signalling pathways in the carotid body is discussed. Recent advances have attracted interest in the potential for carotid body (CB) ablation or desensitization as an effective strategy for clinical treatment and management of cardiorespiratory diseases, including hypertension, heart failure, diabetes mellitus, metabolic syndrome and renal failure. These disease states have in common sympathetic overactivity, which plays an important role in the development and progression of the disease and is often associated with breathing dysregulation, which in turn is likely to mediate or aggravate the autonomic imbalance. Evidence from both chronic heart failure (CHF) patients and animal models indicates that the CB chemoreflex is enhanced in CHF and contributes to the tonic elevation in sympathetic activity and the development of periodic breathing associated with the disease. Although this maladaptive change is likely to derive from altered function at all levels of the reflex arc, a tonic increase in afferent activity from CB glomus cells is likely to be a main driving force. This report focuses on our understanding of mechanisms that alter CB function in CHF and their potential translational impact on treatment of CHF.

Does impaired O2 delivery during exercise accentuate central and peripheral fatigue in patients with coexistent COPD-CHF?

Impairment in oxygen (O2) delivery to the central nervous system ("brain") and skeletal locomotor muscle during exercise has been associated with central and peripheral neuromuscular fatigue in healthy humans. From a clinical perspective, impaired tissue O2 transport is a key pathophysiological mechanism shared by cardiopulmonary diseases, such as chronic obstructive pulmonary disease (COPD) and chronic heart failure (CHF). In addition to arterial hypoxemic conditions in COPD, there is growing evidence that cerebral and muscle blood flow and oxygenation can be reduced during exercise in both isolated COPD and CHF. Compromised cardiac output due to impaired cardiopulmonary function/interactions and blood flow redistribution to the overloaded respiratory muscles (i.e., ↑work of breathing) may underpin these abnormalities. Unfortunately, COPD and CHF coexist in almost a third of elderly patients making these mechanisms potentially more relevant to exercise intolerance. In this context, it remains unknown whether decreased O2 delivery accentuates neuromuscular manifestations of central and peripheral fatigue in coexistent COPD-CHF. If this holds true, it is conceivable that delivering a low-density gas mixture (heliox) through non-invasive positive pressure ventilation could ameliorate cardiopulmonary function/interactions and reduce the work of breathing during exercise in these patients. The major consequence would be increased O2 delivery to the brain and active muscles with potential benefits to exercise capacity (i.e., ↓central and peripheral neuromuscular fatigue, respectively). We therefore hypothesize that patients with coexistent COPD-CHF stop exercising prematurely due to impaired central motor drive and muscle contractility as the cardiorespiratory system fails to deliver sufficient O2 to simultaneously attend the metabolic demands of the brain and the active limb muscles.

Selective carotid body ablation in experimental heart failure: a new therapeutic tool to improve cardiorespiratory control

NEW FINDINGS

What is the topic of this review? This review summarizes the physiological role played by the carotid body in the autonomic dysregulation and breathing disturbances during the progression of chronic heart failure and the therapeutic potential of carotid body ablation to control cardiorespiratory imbalance and improve survival in heart failure. What advances does it highlight? Carotid body ablation markedly improves breathing stability and normalizes autonomic function in chronic heart failure. More importantly, if carotid body ablation is performed early during the progression of the disease it significantly improves animal survival. Chronic heart failure (CHF) is a leading medical problem worldwide. Common hallmarks of CHF include autonomic imbalance and breathing disorders, both of which are closely related to the progression of the disease and strongly predict mortality in CHF patients. The role played by the carotid body (CB) chemoreceptors in the progression of CHF has received attention because enhanced carotid chemoreflex drive is thought to contribute to autonomic dysfunction, abnormal breathing patterns and increased mortality in CHF. Therefore, therapeutic tools intended to normalize CB-mediated chemoreflex drive could have the potential to improve quality of life and decrease mortality of CHF patients. In experimental CHF, an enhancement of the CB chemoreflex drive, elevated sympathetic outflow, increased resting breathing variability, increased incidence of apnoea and desensitization of the baroreflex have been shown. Notably, selective elimination of the CB reduced central presympathetic neuronal activation, normalized sympathetic outflow and baroreflex sensitivity and stabilized breathing function in CHF. More remarkably, CB ablation has been shown to be a valuable therapeutic tool that significantly reduced aberrant cardiac remodelling, improved left ventriclular ejection fraction and reduced cardiac arrhythmogenesis. Most importantly, animals with CHF that underwent CB ablation showed a marked improvement in survival rate. Interestingly, a case report from a heart failure patient in whom unilateral CB ablation was performed showed promising results, with significant improvement in autonomic balance and breathing variability. Together, the CHF data from experimental animals as well as humans unveil a major role for the CB chemoreceptors in the progression of heart failure and support the notion that CB ablation could represent a novel therapeutic strategy to reduce cardiorespiratory dysfunction and improve survival during heart failure.

Role of the Carotid Body Chemoreflex in the Pathophysiology of Heart Failure: A Perspective from Animal Studies

The treatment and management of chronic heart failure (CHF) remains an important focus for new and more effective clinical strategies. This important goal, however, is dependent upon advancing our understanding of the underlying pathophysiology. In CHF, sympathetic overactivity plays an important role in the development and progression of the cardiac and renal dysfunction and is often associated with breathing dysregulation, which in turn likely mediates or aggravates the autonomic imbalance. In this review we will summarize evidence that in CHF, the elevation in sympathetic activity and breathing instability that ultimately lead to cardiac and renal failure are driven, at least in part, by maladaptive activation of the carotid body (CB) chemoreflex. This maladaptive change derives from a tonic increase in CB afferent activity. We will focus our discussion on an understanding of mechanisms that alter CB afferent activity in CHF and its consequence on reflex control of autonomic, respiratory, renal, and cardiac function in animal models of CHF. We will also discuss the potential translational impact of targeting the CB in the treatment of CHF in humans, with relevance to other cardio-respiratory diseases.

Age-related reflex responses from peripheral and central chemoreceptors in healthy men

Objective

The study aimed: (i) to characterize reflex responses from peripheral and central chemoreceptors in different age groups of healthy men (<50 years old vs ≥50 years old) and, (ii) to assess, within these groups, whether there is any relationship between ventilatory and hemodynamic responses from chemoreceptors and indices of autonomic nervous system (ANS).

Methods

Peripheral chemoreflex sensitivity was assessed by the transient hypoxia method and respiratory, heart rate (HR) and blood pressure responses were calculated. Central chemoreflex sensitivity was assessed by the rebreathing method and respiratory response was calculated. ANS was assessed using heart rate variability indices and baroreflex sensitivity (BRS).

Results

Sixty-seven healthy men were divided into 2 groups: <50 years (n = 38, mean age: 32 ± 10 years) and ≥50 years (n = 29, mean age: 61 ± 8 years). There were no differences in respiratory response from central and peripheral chemoreceptors between the older and younger groups of healthy males. We found a significantly different pattern of hemodynamic responses from peripheral chemoreceptors between the older and the younger groups. The former expressed attenuated HR acceleration and exaggerated blood pressure increase in response to transient hypoxia. Blunted HR response was related to reduced BRS and sympathovagal imbalance characterized by reduced vagal tone. Blood pressure responses seemed to be independent of sympathovagal balance and BRS.

Interpretation

Ageing impacts hemodynamic rather than respiratory response from chemoreceptors. Impaired arterial baroreflex and sympathovagal imbalance related to ageing may contribute to decreased heart rate response, but not to increased blood pressure response from peripheral chemoreceptors.

Central role of carotid body chemoreceptors in disordered breathing and cardiorenal dysfunction in chronic heart failure

Oscillatory breathing (OB) patterns are observed in pre-term infants, patients with cardio-renal impairment, and in otherwise healthy humans exposed to high altitude. Enhanced carotid body (CB) chemoreflex sensitivity is common to all of these populations and is thought to contribute to these abnormal patterns by destabilizing the respiratory control system. OB patterns in chronic heart failure (CHF) patients are associated with greater levels of tonic and chemoreflex-evoked sympathetic nerve activity (SNA), which is associated with greater morbidity and poor prognosis. Enhanced chemoreflex drive may contribute to tonic elevations in SNA by strengthening the relationship between respiratory and sympathetic neural outflow. Elimination of CB afferents in experimental models of CHF has been shown to reduce OB, respiratory-sympathetic coupling, and renal SNA, and to improve autonomic balance in the heart. The CB chemoreceptors may play an important role in progression of CHF by contributing to respiratory instability and OB, which in turn further exacerbates tonic and chemoreflex-evoked increases in SNA to the heart and kidney.

Dead space: the physiology of wasted ventilation

An elevated physiological dead space, calculated from measurements of arterial CO2 and mixed expired CO2, has proven to be a useful clinical marker of prognosis both for patients with acute respiratory distress syndrome and for patients with severe heart failure. Although a frequently cited explanation for an elevated dead space measurement has been the development of alveolar regions receiving no perfusion, evidence for this mechanism is lacking in both of these disease settings. For the range of physiological abnormalities associated with an increased physiological dead space measurement, increased alveolar ventilation/perfusion ratio (V'A/Q') heterogeneity has been the most important pathophysiological mechanism. Depending on the disease condition, additional mechanisms that can contribute to an elevated physiological dead space measurement include shunt, a substantial increase in overall V'A/Q' ratio, diffusion impairment, and ventilation delivered to unperfused alveolar spaces.

Carotid body: a new target for rescuing neural control of cardiorespiratory balance in disease

Significant insight into the mechanisms involved in chronic heart failure (CHF) have been provided by Schultz and his associates at the University of Nebraska Medical Center with the use of pacing-induced heart failure rabbits. Critical among the CHF mechanisms was the role of the carotid body (CB). The stimulated CB produces a wide array of systemic reflex responses; certainly those in the cardiopulmonary (CP) system are the most important in CHF. This generates a question as to whether the CB could serve as a target for some kind of treatment to reestablish control of cardiorespiratory balance in CHF. Any treatment would have to be based on a solid understanding of the mechanisms of chemosensing by the CB as well as the transducing of that sensing into neural activity sent to the medullary centers and regions of autonomic outflow to the periphery. Two avenues of treatment could be to (1) silence or attenuate the CB's neural output pharmacologically and (2) excise the CBS. There is a long history of CB removal mostly as a remedy for chronic obstructive lung disease. Results have been inconclusive as to the effectiveness of this procedure. But if carefully planned, the procedure might be a helpful treatment.

Cardioprotection afforded by exercise training prior to myocardial infarction is associated with autonomic function improvement

BACKGROUND

It has been suggested that exercise training (ET) protects against the pathological remodeling and ventricular dysfunction induced by myocardial infarction (MI). However, it remains unclear whether the positive adjustments on baroreflex and cardiac autonomic modulations promoted by ET may afford a cardioprotective mechanism. The aim of this study was to evaluate the effects of aerobic ET, prior to MI, on cardiac remodeling and function, as well as on baroreflex sensitivity and autonomic modulation in rats.

METHODS

Male Wistar rats were divided into 4 groups: sedentary rats submitted to Sham surgery (C); trained rats submitted to Sham surgery (TC); sedentary rats submitted to MI (I), trained rats submitted to MI (TI). Sham and MI were performed after ET period. After surgeries, echocardiographic, hemodynamic and autonomic (baroreflex sensitivity, cardiovascular autonomic modulation) evaluations were conducted.

RESULTS

Prior ET prevented an additional decline in exercise capacity in TI group in comparison with I. MI area was not modified by previous ET. ET was able to increase the survival and prevent additional left ventricle dysfunction in TI rats. Although changes in hemodynamic evaluations were not observed, ET prevented the decrease of baroreflex sensitivity, and autonomic dysfunction in TI animals when compared with I animals. Importantly, cardiac improvement was associated with the prevention of cardiac autonomic impairment in studied groups.

CONCLUSIONS

Prior ET was effective in changing aerobic capacity, left ventricular morphology and function in rats undergoing MI. Furthermore, these cardioprotective effects were associated with attenuated cardiac autonomic dysfunction observed in trained rats. Although these cause-effect relationships can only be inferred, rather than confirmed, our study suggests that positive adaptations of autonomic function by ET can play a vital role in preventing changes associated with cardiovascular disease, particularly in relation to MI.

Role of exercise training on autonomic changes and inflammatory profile induced by myocardial infarction

The cardiovascular autonomic imbalance in patients after myocardial infarction (MI) provides a significant increase in mortality rate, and seems to precede metabolic, hormonal, and immunological changes. Moreover, the reduction in the parasympathetic function has been associated with inflammatory response in different pathological conditions. Over the years, most of the studies have indicated the exercise training (ET) as an important nonpharmacological tool in the management of autonomic dysfunction and reduction in inflammatory profile after a myocardial infarction. In this work, we reviewed the effects of ET on autonomic imbalance after MI, and its consequences, particularly, in the post-MI inflammatory profile. Clinical and experimental evidence regarding relationship between alterations in autonomic regulation and local or systemic inflammation response after MI were also discussed.

Tonic arterial chemoreceptor activity contributes to cardiac sympathetic activation in mild ovine heart failure

Heart failure (HF) is associated with a large increase in cardiac sympathetic nerve activity (CSNA), which has detrimental effects on the heart and promotes arrhythmias and sudden death. There is increasing evidence that arterial chemoreceptor activation plays an important role in stimulating renal sympathetic nerve activity (RSNA) and muscle sympathetic nerve activity in HF. Given that sympathetic nerve activity to individual organs is differentially controlled, we investigated whether tonic arterial chemoreceptor activation contributes to the increased CSNA in HF. We recorded CSNA and RSNA in conscious normal sheep and in sheep with mild HF induced by rapid ventricular pacing (ejection fraction <40%). Tonic arterial chemoreceptor function was evaluated by supplementing room air with 100% intranasal oxygen (2-3 l min(-1)) for 20 min, thereby deactivating chemoreceptors. The effects of hyperoxia on resting levels and baroreflex control of heart rate, CSNA and RSNA were determined. In HF, chemoreceptor deactivation induced by hyperoxia significantly reduced CSNA [90 ± 2 versus 75 ± 5 bursts (100 heart beats)(-1), P < 0.05, n = 10; room air versus hyperoxia] and heart rate (96 ± 4 versus 85 ± 4 beats min(-1), P < 0.001, n = 12). There was no change in RSNA burst incidence [93 ± 4 versus 92 ± 4 bursts (100 heart beats)(-1), n = 7], although due to the bradycardia the RSNA burst frequency was decreased (90 ± 8 versus 77 ± 7 bursts min(-1), P < 0.001). In normal sheep, chemoreceptor deactivation reduced heart rate without a significant effect on CSNA or RSNA. In summary, deactivation of peripheral chemoreceptors during HF reduced the elevated levels of CSNA, indicating that tonic arterial chemoreceptor activation plays a critical role in stimulating the elevated CSNA in HF.

Role of the carotid body in the pathophysiology of heart failure

Important recent advances implicate a role of the carotid body (CB) chemoreflex in sympathetic and breathing dysregulation in several cardio-respiratory diseases, drawing renewed interest in its potential implications for clinical treatment. Evidence from both chronic heart failure (CHF) patients and animal models indicates that the CB chemoreflex is enhanced in CHF, and contributes to the tonic elevation in sympathetic nerve activity (SNA) and periodic breathing associated with the disease. Although this maladaptive change likely derives from altered function at all levels of the reflex arc, a change in afferent function of the CB is likely to be a main driving force. This review will focus on recent advances in our understanding of the pathophysiological mechanisms that alter CB function in CHF and their potential translational impact on treatment of chronic heart failure (CHF).

Ventilatory responses to chemoreflex stimulation are not enhanced by angiotensin II in healthy humans

The chemoreflexes exert significant control over respiration and sympathetic outflow. Abnormalities in chemoreflex function may contribute to various disease processes. Based on prior animal studies, we developed the hypothesis that acutely elevating circulating angiotensin II levels into the pathophysiological range increases chemoreflex responsiveness in healthy humans. Eighteen adults were studied before (Pre) and during (Post) low (protocol 1; 2ng/kg/min; n=9) or high (protocol 2; 5ng/kg/min; n=9) dose angiotensin II infusion (study day 1). Chemoreflex responses were quantified by the pure nitrogen breathing method [slope of the minute ventilation vs. arterial oxygen saturation plot generated during a series (n=10) of 100% inspired nitrogen exposures (1-8 breaths)] and by measuring responses to hypercapnia (7% inspired carbon dioxide). Responses to a non-chemoreflex stimulus were also determined (cold pressor test). Measurements were repeated on a subsequent day (study day 2) before and during infusion of a control vasoconstrictor (phenylephrine) infused at a dose (0.6-1.2μg/kg/min) sufficient to increase blood pressure to the same degree as that achieved during angiotensin II infusion. We found that despite increasing plasma angiotensin II levels to pathophysiological levels responses to pure nitrogen breathing, hypercapnia, and the cold pressor test were unchanged by low (2ng/kg/min) and high dose (5ng/kg/min) angiotensin II infusion (protocols 1 and 2). Similarly, responses measured during phenylephrine infusion (Post) were unchanged (from Pre). These findings indicate that acutely increasing plasma angiotensin II levels to levels observed in disease states, such as human heart failure, do not increase chemoreflex responsiveness in healthy humans.