Simvastatin treatment attenuates increased respiratory variability and apnea/hypopnea index in rats with chronic heart failure

Commonalities and differences in carotid body dysfunction in hypertension and heart failure

Carotid body pathophysiology is associated with many cardiovascular-respiratory-metabolic diseases. This pathophysiology reflects both hyper-sensitivity and hyper-tonicity. From both animal models and human patients, evidence indicates that amelioration of this pathophysiological signalling improves disease states such as a lowering of blood pressure in hypertension, a reduction of breathing disturbances with improved cardiac function in heart failure (HF) and a re-balancing of autonomic activity with lowered sympathetic discharge. Given this, we have reviewed the mechanisms of carotid body hyper-sensitivity and hyper-tonicity across disease models asking whether there is uniqueness related to specific disease states. Our analysis indicates some commonalities and some potential differences, although not all mechanisms have been fully explored across all disease models. One potential commonality is that of hypoperfusion of the carotid body across hypertension and HF, where the excessive sympathetic drive may reduce blood flow in both models and, in addition, lowered cardiac output in HF may potentiate the hypoperfusion state of the carotid body. Other mechanisms are explored that focus on neurotransmitter and signalling pathways intrinsic to the carotid body (e.g. ATP, carbon monoxide) as well as extrinsic molecules carried in the blood (e.g. leptin); there are also transcription factors found in the carotid body endothelium that modulate its activity (Krüppel-like factor 2). The evidence to date fully supports that a better understanding of the mechanisms of carotid body pathophysiology is a fruitful strategy for informing potential new treatment strategies for many cardiovascular, respiratory and metabolic diseases, and this is highly relevant clinically.

Chronic intermittent hypoxia promotes glomerular hyperfiltration and potentiates hypoxia-evoked decreases in renal perfusion and PO2

Introduction: Sleep apnea (SA) is highly prevalent in patients with chronic kidney disease and may contribute to the development and/or progression of this condition. Previous studies suggest that dysregulation of renal hemodynamics and oxygen flux may play a key role in this process. The present study sought to determine how chronic intermittent hypoxia (CIH) associated with SA affects regulation of renal artery blood flow (RBF), renal microcirculatory perfusion (RP), glomerular filtration rate (GFR), and cortical and medullary tissue PO2 as well as expression of genes that could contribute to renal injury. We hypothesized that normoxic RBF and tissue PO2 would be reduced after CIH, but that GFR would be increased relative to baseline, and that RBF, RP, and tissue PO2 would be decreased to a greater extent in CIH vs. sham during exposure to intermittent asphyxia (IA, FiO2 0.10/FiCO2 0.03). Additionally, we hypothesized that gene programs promoting oxidative stress and fibrosis would be activated by CIH in renal tissue. Methods: All physiological variables were measured at baseline (FiO2 0.21) and during exposure to 10 episodes of IA (excluding GFR). Results: GFR was higher in CIH-conditioned vs. sham (p < 0.05), whereas normoxic RBF and renal tissue PO2 were significantly lower in CIH vs. sham (p < 0.05). Reductions in RBF, RP, and renal tissue PO2 during IA occurred in both groups but to a greater extent in CIH (p < 0.05). Pro-oxidative and pro-fibrotic gene programs were activated in renal tissue from CIH but not sham. Conclusion: CIH adversely affects renal hemodynamic regulation and oxygen flux during both normoxia and IA and results in changes in renal tissue gene expression.

Autonomic and respiratory consequences of altered chemoreflex function: clinical and therapeutic implications in cardiovascular diseases

The importance of chemoreflex function for cardiovascular health is increasingly recognized in clinical practice. The physiological function of the chemoreflex is to constantly adjust ventilation and circulatory control to match respiratory gases to metabolism. This is achieved in a highly integrated fashion with the baroreflex and the ergoreflex. The functionality of chemoreceptors is altered in cardiovascular diseases, causing unstable ventilation and apnoeas and promoting sympathovagal imbalance, and it is associated with arrhythmias and fatal cardiorespiratory events. In the last few years, opportunities to desensitize hyperactive chemoreceptors have emerged as potential options for treatment of hypertension and heart failure. This review summarizes up to date evidence of chemoreflex physiology/pathophysiology, highlighting the clinical significance of chemoreflex dysfunction, and lists the latest proof of concept studies based on modulation of the chemoreflex as a novel target in cardiovascular diseases.

A Methodological Perspective on the Function and Assessment of Peripheral Chemoreceptors in Heart Failure: A Review of Data from Clinical Trials

Augmented peripheral chemoreceptor sensitivity (PChS) is a common feature of many sympathetically mediated diseases, among others, and it is an important mechanism of the pathophysiology of heart failure (HF). It is related not only to the greater severity of symptoms, especially to dyspnea and lower exercise tolerance but also to a greater prevalence of complications and poor prognosis. The causes, mechanisms, and impact of the enhanced activity of peripheral chemoreceptors (PChR) in the HF population are subject to intense research. Several methodologies have been established and utilized to assess the PChR function. Each of them presents certain advantages and limitations. Furthermore, numerous factors could influence and modulate the response from PChR in studied subjects. Nevertheless, even with the impressive number of studies conducted in this field, there are still some gaps in knowledge that require further research. We performed a review of all clinical trials in HF human patients, in which the function of PChR was evaluated. This review provides an extensive synthesis of studies evaluating PChR function in the HF human population, including methods used, factors potentially influencing the results, and predictors of increased PChS.

Peripheral chemoreflex modulation of renal hemodynamics and renal tissue PO2 in chronic heart failure with reduced ejection fraction

Aberrant carotid body chemoreceptor (CBC) function contributes to increased sympathetic nerve activity (SNA) and reduced renal blood flow (RBF) in chronic heart failure (CHF). Intermittent asphyxia (IA) mimicking sleep apnea is associated with additional increases in SNA and may worsen reductions in RBF and renal PO2 (RPO2) in CHF. The combined effects of decreased RBF and RPO2 may contribute to biochemical changes precipitating renal injury. This study sought to determine the role of CBC activity on glomerular filtration rate (GFR), RBF and RPO2 in CHF, and to assess the additive effects of IA. Furthermore, we sought to identify changes in gene expression that might contribute to renal injury. We hypothesized that GFR, RBF, and RPO2 would be reduced in CHF, that decreases in RBF and RPO2 would be worsened by IA, and that these changes would be ameliorated by CBC ablation (CBD). Finally, we hypothesized that CHF would be associated with pro-oxidative pro-fibrotic changes in renal gene expression that would be ameliorated by CBD. CHF was induced in adult male Sprague Dawley rats using coronary artery ligation (CAL). Carotid body denervation was performed by cryogenic ablation. GFR was assessed in conscious animals at the beginning and end of the experimental period. At 8-weeks post-CAL, cardiac function was assessed via echocardiography, and GFR, baseline and IA RBF and RPO2 were measured. Renal gene expression was measured using qRT-PCR. GFR was lower in CHF compared to sham (p < 0.05) but CBD had no salutary effect. RBF and RPO2 were decreased in CHF compared to sham (p < 0.05), and this effect was attenuated by CBD (p < 0.05). RBF and RPO2 were reduced to a greater extent in CHF vs. sham during exposure to IA (p < 0.05), and this effect was attenuated by CBD for RBF (p < 0.05). Downregulation of antioxidant defense and fibrosis-suppressing genes was observed in CHF vs. sham however CBD had no salutary effect. These results suggest that aberrant CBC function in CHF has a clear modulatory effect on RBF during normoxia and during IA simulating central sleep apnea.

Contribution of Peripheral Chemoreceptors to Exercise Intolerance in Heart Failure

Peripheral chemoreceptors (PChRs), because of their strategic localization at the bifurcation of the common carotid artery and along the aortic arch, play an important protective role against hypoxia. Stimulation of PChRs evokes hyperventilation and hypertension to maintain adequate oxygenation of critical organs. A relationship between increased sensitivity of PChRs (hyperreflexia) and exercise intolerance (ExIn) in patients with heart failure (HF) has been previously reported. Moreover, some studies employing an acute blockade of PChRs (e.g., using oxygen or opioids) demonstrated improvement in exercise capacity, suggesting that hypertonicity is also involved in the development of ExIn in HF. Nonetheless, the precise mechanisms linking dysfunctional PChRs to ExIn remain unclear. From the clinical perspective, there are two main factors limiting exercise capacity in HF patients: subjective perception of dyspnoea and muscle fatigue. Both have many determinants that might be influenced by abnormal signalling from PChRs, including: exertional hyperventilation, oscillatory ventilation, ergoreceptor oversensitivity, and augmented sympathetic tone. The latter results in reduced muscle perfusion and altered muscle structure. In this review, we intend to present the milieu of abnormalities tied to malfunctioning PChRs and discuss their role in the complex relationships leading, ultimately, to ExIn.

Exercise training reduces brainstem oxidative stress and restores normal breathing function in heart failure

Enhanced central chemoreflex drive and irregular breathing are both hallmarks in heart failure (HF) and closely related to disease progression. Central chemoreceptor neurons located within the retrotrapezoid nucleus (RTN) are known to play a role in breathing alterations in HF. It has been shown that exercise (EX) effectively reduced reactive oxygen species (ROS) in HF rats. However, the link between EX and ROS, particularly at the RTN, with breathing alterations in HF has not been previously addressed. Accordingly, we aimed to determine: i) ROS levels in the RTN in HF and its association with chemoreflex drive, ii) whether EX improves chemoreflex/breathing function by reducing ROS levels, and iii) determine molecular alterations associated with ROS generation within the RTN of HF rats and study EX effects on these pathways. Adult male Sprague-Dawley rats were allocated into 3 experimental groups: Sham (n = 5), volume overloaded HF (n = 6) and HF (n = 8) rats that underwent EX training for 6 weeks (60 min/day, 25 m/min, 10% inclination). At 8 weeks post-HF induction, breathing patterns and chemoreflex function were analyzed by unrestrained plethysmography. ROS levels and anti/pro-oxidant enzymes gene expression were analyzed in the RTN. Our results showed that HF rats have high ROS levels in the RTN which were closely linked to the enhanced central chemoreflex and breathing disorders. Also, HF rats displayed decreased expression of antioxidant genes in the RTN compared with control rats. EX training increases antioxidant defense in the RTN, reduces ROS formation and restores normal central chemoreflex drive and breathing regularity in HF rats. This study provides evidence for a role of ROS in central chemoreception in the setting of HF and support the use of EX to reduce ROS in the brainstem of HF animals and reveal its potential as an effective mean to normalize chemoreflex and breathing function in HF.

Inhibition of Brainstem Endoplasmic Reticulum Stress Rescues Cardiorespiratory Dysfunction in High Output Heart Failure

Recent evidence shows that chronic activation of catecholaminergic neurons of the rostral ventrolateral medulla is crucial in promoting autonomic imbalance and cardiorespiratory dysfunction in high output heart failure (HF). Brainstem endoplasmic reticulum stress (ERS) is known to promote cardiovascular dysfunction; however, no studies have addressed the potential role of brainstem ERS in cardiorespiratory dysfunction in high output HF. In this study, we assessed the presence of brainstem ERS and its potential role in cardiorespiratory dysfunction in an experimental model of HF induced by volume overload. High output HF was surgically induced via creation of an arterio-venous fistula in adult male Sprague-Dawley rats. Tauroursodeoxycholic acid (TUDCA), an inhibitor of ERS, or vehicle was administered intracerebroventricularly for 4 weeks post-HF induction. Compared with vehicle treatment, TUDCA improved cardiac autonomic balance (LF_HRV/HF_HRV ratio, 3.02±0.29 versus 1.14±0.24), reduced cardiac arrhythmia incidence (141.5±26.7 versus 35.67±12.5 events/h), and reduced abnormal respiratory patterns (Apneas: 11.83±2.26 versus 4.33±1.80 events/h). TUDCA administration (HF+Veh versus HF+TUDCA, P<0.05) attenuated cardiac hypertrophy (HW/BW 4.4±0.3 versus 4.0±0.1 mg/g) and diastolic dysfunction. Analysis of rostral ventrolateral medulla gene expression confirmed the presence of ERS, inflammation, and activation of renin-angiotensin system pathways in high output HF and showed that TUDCA treatment completely abolished ERS and ERS-related signaling. Taken together, these results support the notion that ERS plays a role in cardiorespiratory dysfunction in high output HF and more importantly that reducing brain ERS with TUDCA treatment has a potent salutary effect on cardiac function in this model.

The need for specificity in quantifying neurocirculatory vs. respiratory effects of eucapnic hypoxia and transient hyperoxia

KEY POINTS

The carotid chemoreceptor mediates the ventilatory and muscle sympathetic nerve activity (MSNA) responses to hypoxia and contributes to tonic sympathetic and respiratory drives. It is often presumed that both excitatory and inhibitory tests of chemoreflex function show congruence in the end-organ responses. Ventilatory and neurocirculatory (MSNA, blood pressure and heart rate) responses to chemoreflex inhibition elicited by transient hyperoxia and to chemoreflex excitation produced by steady-state eucapnic hypoxia were measured in a cohort of 82 middle-aged individuals. Ventilatory and MSNA responsiveness to hyperoxia and hypoxia were not significantly correlated within individuals. It was concluded that ventilatory responses to hypoxia and hyperoxia do not predict MSNA responses and it is recommended that tests using the specific outcome of interest, i.e. MSNA or ventilation, are required. Transient hyperoxia is recommended as a sensitive and reliable means of quantifying tonic chemoreceptor-driven levels of sympathetic nervous system activity and respiratory drive.

ABSTRACT

Hypersensitivity of the carotid chemoreceptor leading to sympathetic nervous system activation and ventilatory instability has been implicated in the pathogenesis and consequences of several common clinical conditions. A variety of treatment approaches aimed at lessening chemoreceptor-driven sympathetic overactivity are now under investigation; thus, the ability to quantify this outcome variable with specificity and precision is crucial. Accordingly, we measured ventilatory and neurocirculatory responses to chemoreflex inhibition elicited by transient hyperoxia and chemoreflex excitation produced by exposure to graded, steady-state eucapnic hypoxia in middle-aged men and women (n = 82) with continuous positive airway pressure-treated obstructive sleep apnoea. Progressive, eucapnic hypoxia produced robust and highly variable increases in ventilation (+83 ± 59%) and muscle sympathetic nerve activity (MSNA) burst frequency (+55 ± 31%), whereas transient hyperoxia caused marked reductions in these variables (-35 ± 14% and -42 ± 16%, respectively). Coefficients of variation for ventilatory and MSNA burst frequency responses, indicating test-retest reproducibility, were respectively 9% and 24% for hyperoxia and 35% and 28% for hypoxia. Based on statistical measures of rank correlation or even comparisons across quartiles of corresponding ventilatory and MSNA responses, we found that the magnitudes of ventilatory inhibition with hyperoxia or excitation with eucapnic hypoxia were not correlated with corresponding MSNA responses within individuals. We conclude that, in conscious, behaving humans, ventilatory sensitivities to progressive, steady-state, eucapnic hypoxia and transient hyperoxia do not predict MSNA responsiveness. Our findings also support the use of transient hyperoxia as a reliable, sensitive, measure of the carotid chemoreceptor contribution to tonic sympathetic nervous system activity and respiratory drive.

Peripheral-to-central immune communication at the area postrema glial-barrier following bleomycin-induced sterile lung injury in adult rats

The pathways for peripheral-to-central immune communication (P → C I-comm) following sterile lung injury (SLI) are unknown. SLI evokes systemic and central inflammation, which alters central respiratory control and viscerosensory transmission in the nucleus tractus solitarii (nTS). These functional changes coincide with increased interleukin-1 beta (IL-1β) in the area postrema, a sensory circumventricular organ that connects P → C I-comm to brainstem circuits that control homeostasis. We hypothesize that IL-1β and its downstream transcriptional target, cyclooxygenase-2 (COX-2), mediate P → C I-comm in the nTS. In a rodent model of SLI induced by intratracheal bleomycin (Bleo), the sigh frequency and duration of post-sigh apnea increased in Bleo- compared to saline- treated rats one week after injury. This SLI-dependent change in respiratory control occurred concurrently with augmented IL-1β and COX-2 immunoreactivity (IR) in the funiculus separans (FS), a barrier between the AP and the brainstem. At this barrier, increases in IL-1β and COX-2 IR were confined to processes that stained for glial fibrillary acidic protein (GFAP) and that projected basolaterally to the nTS. Further, FS radial-glia did not express TNF-α or IL-6 following SLI. To test our hypothesis, we blocked central COX-1/2 activity by intracerebroventricular (ICV) infusion of Indomethacin (Ind). Continuous ICV Ind treatment prevented Bleo-dependent increases in GFAP + and IL-1β + IR, and restored characteristics of sighs that reset the rhythm. These data indicate that changes in sighs following SLI depend partially on activation of a central COX-dependent P → C I-comm via radial-glia of the FS.

Update on Chemoreception: Influence on Cardiorespiratory Regulation and Pathophysiology

We examine recent findings that have revealed interdependence of function within the chemoreceptor pathway regulating breathing and sympathetic vasomotor activity and the hypersensitization of these reflexes in chronic disease states. Recommendations are made as to how these states of hyperreflexia in chemoreceptors and muscle afferents might be modified in treating sleep apnea, drug-resistant hypertension, chronic heart failure-induced sympathoexcitation, and the exertional dyspnea of chronic obstructive pulmonary disease.

Rostral ventrolateral medullary catecholaminergic neurones mediate irregular breathing pattern in volume overload heart failure rats

KEY POINTS

A strong association between disordered breathing patterns, elevated sympathetic activity, and enhanced central chemoreflex drive has been shown in experimental and human heart failure (HF). The aim of this study was to determine the contribution of catecholaminergic rostral ventrolateral medulla catecholaminergic neurones (RVLM-C1) to both haemodynamic and respiratory alterations in HF. Apnoea/hypopnoea incidence (AHI), breathing variability, respiratory-cardiovascular coupling, cardiac autonomic control and cardiac function were analysed in HF rats with or without selective ablation of RVLM-C1 neurones. Partial lesion (∼65%) of RVLM-C1 neurones reduces AHI, respiratory variability, and respiratory-cardiovascular coupling in HF rats. In addition, the deleterious effects of central chemoreflex activation on cardiac autonomic balance and cardiac function in HF rats was abolished by ablation of RVLM-C1 neurones. Our findings suggest that RVLM-C1 neurones play a pivotal role in breathing irregularities in volume overload HF, and mediate the sympathetic responses induced by acute central chemoreflex activation.

ABSTRACT

Rostral ventrolateral medulla catecholaminergic neurones (RVLM-C1) modulate sympathetic outflow and breathing under normal conditions. Heart failure (HF) is characterized by chronic RVLM-C1 activation, increased sympathetic activity and irregular breathing patterns. Despite studies showing a relationship between RVLM-C1 and sympathetic activity in HF, no studies have addressed a potential contribution of RVLM-C1 neurones to irregular breathing in this context. Thus, the aim of this study was to determine the contribution of RVLM-C1 neurones to irregular breathing patterns in HF. Sprague-Dawley rats underwent surgery to induce volume overload HF. Anti-dopamine β-hydroxylase-saporin toxin (DβH-SAP) was used to selectively lesion RVLM-C1 neurones. At 8 weeks post-HF induction, breathing pattern, blood pressures (BP), respiratory-cardiovascular coupling (RCC), central chemoreflex function, cardiac autonomic control and cardiac function were studied. Reduction (∼65%) of RVLM-C1 neurones resulted in attenuation of irregular breathing, decreased apnoea-hypopnoea incidence (11.1 ± 2.9 vs. 6.5 ± 2.5 events h-1 ; HF+Veh vs. HF+DβH-SAP; P < 0.05) and improved cardiac autonomic control in HF rats. Pathological RCC was observed in HF rats (peak coherence >0.5 between breathing and cardiovascular signals) and was attenuated by DβH-SAP treatment (coherence: 0.74 ± 0.12 vs. 0.54 ± 0.10, HF+Veh vs. HF+DβH-SAP rats; P < 0.05). Central chemoreflex activation had deleterious effects on cardiac function and cardiac autonomic control in HF rats that were abolished by lesion of RVLM-C1 neurones. Our findings reveal that RVLM-C1 neurones play a major role in irregular breathing patterns observed in volume overload HF and highlight their contribution to cardiac dysautonomia and deterioration of cardiac function during chemoreflex activation.

The effect of pulmonary rehabilitation on carotid chemoreceptor activity and sensitivity in chronic obstructive pulmonary disease

Recent work demonstrates that carotid chemoreceptor (CC) activity/sensitivity is elevated in patients with chronic obstructive pulmonary disease (COPD) compared with healthy controls, and this elevated chemoreception appears to contribute to increased cardiovascular risk. Exercise training has been shown to normalize CC activity/sensitivity in other populations, and therefore, the purpose of this study was to determine whether pulmonary rehabilitation (PR) can reduce CC activity/sensitivity in COPD. Forty-five COPD patients [mean FEV₁ (forced expiratory volume in 1 s) = 56.6% predicted] completed PR, while 15 COPD patients (mean FEV₁ = 74.6% predicted) served as non-PR controls. CC activity was determined by the reduction in ventilation while breathing transient hyperoxia ([Formula: see text] = 1.0); CC sensitivity was evaluated by the increase in ventilation relative to the drop in arterial saturation while breathing hypoxia. Dyspnea, six-minute walk and autonomic function data were also obtained. PR improved 6-minute walk distance (P < 0.001) and dyspnea (P = 0.04); however, there was no effect on CC activity (P = 0.60), sensitivity (P = 0.69), or autonomic function (P > 0.05 for all). Subgroup analyses indicated that PR reduced CC activity in those with elevated baseline CC activity, independent of changes in autonomic function. No change in dyspnea (P = 0.24), CC activity (P = 0.19), sensitivity (P = 0.80), or autonomic function (P > 0.05 for all) was observed in the control group. Despite improvements in exercise tolerance and dyspnea, PR appears to be generally ineffective at reducing CC sensitivity in stable COPD patients; while PR reduced CC activity in those with elevated basal CC activity, the physiological significance of this is unclear. Further investigations aimed at improving CC function in COPD are needed.NEW & NOTEWORTHY While work in other chronic diseases has shown that exercise training may help normalize carotid chemoreceptor (CC) activity/sensitivity, the current study found that exercise training through pulmonary rehabilitation did not consistently reduce CC activity/sensitivity in patients with chronic obstructive pulmonary disease (COPD). These results suggest that other interventions are needed to normalize CC activity/sensitivity in COPD.

Episodic stimulation of central chemoreceptor neurons elicits disordered breathing and autonomic dysfunction in volume overload heart failure

Enhanced central chemoreflex (CC) gain is observed in volume overload heart failure (HF) and is correlated with autonomic dysfunction and breathing disorders. The aim of this study was to determine the role of the CC in the development of respiratory and autonomic dysfunction in HF. Volume overload was surgically created to induce HF in male Sprague-Dawley rats. Radiotelemetry transmitters were implanted for continuous monitoring of blood pressure and heart rate. After recovering from surgery, conscious unrestrained rats were exposed to episodic hypercapnic stimulation [EHS; 10 cycles/5 min, inspiratory fraction of carbon dioxide (FICO2) 7%] in a whole body plethysmograph for recording of cardiorespiratory function. To determine the contribution of CC to cardiorespiratory variables, selective ablation of chemoreceptor neurons within the retrotrapezoid nucleus (RTN) was performed via injection of saporin toxin conjugated to substance P (SSP-SAP). Vehicle-treated rats (HF+Veh and Sham+Veh) were used as controls for SSP-SAP experiments. Sixty minutes post-EHS, minute ventilation was depressed in sham animals relative to HF animals (ΔV̇e: -5.55 ± 2.10 vs. 1.24 ± 1.35 mL/min 100 g, P < 0.05; Sham+Veh vs. HF+Veh). Furthermore, EHS resulted in autonomic imbalance, cardiorespiratory entrainment, and ventilatory disturbances in HF+Veh but not Sham+Veh rats, and these effects were significantly attenuated by SSP-SAP treatment. Also, the apnea-hypopnea index (AHI) was significantly lower in HF+SSP-SAP rats compared with HF+Veh rats (AHI: 5.5 ± 0.8 vs. 14.4 ± 1.3 events/h, HF+SSP-SAP vs. HF+Veh, respectively, P < 0.05). Finally, EHS-induced respiratory-cardiovascular coupling in HF rats depends on RTN chemoreceptor neurons because it was reduced by SSP-SAP treatment. Overall, EHS triggers ventilatory plasticity and elicits cardiorespiratory abnormalities in HF that are largely dependent on RTN chemoreceptor neurons.

Cognitive impairment in heart failure is associated with altered Wnt signaling in the hippocampus

Age represents the highest risk factor for death due to cardiovascular disease. Heart failure (HF) is the most common cardiovascular disease in elder population and it is associated with cognitive impairment (CI), diminishing learning and memory process affecting life quality and mortality in these patients. In HF, CI has been associated with inadequate O₂ supply to the brain; however, an important subset of HF patients displays CI with almost no alteration in cerebral blood flow. Importantly, nothing is known about the pathophysiological mechanisms underpinning CI in HF with no change in brain tissue perfusion. Here, we aimed to study memory performance and learning function in a rodent model of HF that shows no change in blood flow going to the brain. We found that HF rats presented learning impairments and memory loss. In addition, HF rats displayed a decreased level of Wnt/β-catenin signaling downstream elements in the hippocampus, one pathway implicated largely in aging diseases. Taken together, our results suggest that in HF rats CI is associated with dysfunction of the Wnt/β-catenin signaling pathway. The mechanisms involved in the alterations of Wnt/β-catenin signaling in HF and its contribution to the development/maintenance of CI deserves future investigations.

Central sleep apnea: misunderstood and mistreated!

Central sleep apnea is prevalent in patients with heart failure, healthy individuals at high altitudes, and chronic opiate users and in the initiation of "mixed" (that is, central plus obstructive apneas). This brief review focuses on (a) the causes of repetitive, cyclical central apneas as mediated primarily through enhanced sensitivities in the respiratory control system and (b) treatment of central sleep apnea through modification of key components of neurochemical control as opposed to the current universal use of positive airway pressure.

Ablation of brainstem C1 neurons improves cardiac function in volume overload heart failure

Activation of the sympathetic nervous system is a hallmark of heart failure (HF) and is positively correlated with disease progression. Catecholaminergic (C1) neurons located in the rostral ventrolateral medulla (RVLM) are known to modulate sympathetic outflow and are hyperactivated in volume overload HF. However, there is no conclusive evidence showing a contribution of RVLM-C1 neurons to the development of cardiac dysfunction in the setting of HF. Therefore, the aim of this study was to determine the role of RVLM-C1 neurons in cardiac autonomic control and deterioration of cardiac function in HF rats. A surgical arteriovenous shunt was created in adult male Sprague-Dawley rats to induce HF. RVLM-C1 neurons were selectively ablated using cell-specific immunotoxin (dopamine-β hydroxylase saporin [DβH-SAP]) and measures of cardiac autonomic tone, function, and arrhythmia incidence were evaluated. Cardiac autonomic imbalance, arrhythmogenesis and cardiac dysfunction were present in HF rats and improved after DβH-SAP toxin treatment. Most importantly, the progressive decline in fractional shortening observed in HF rats was reduced by DβH-SAP toxin. Our results unveil a pivotal role played by RVLM-C1 neurons in cardiac autonomic imbalance, arrhythmogenesis and cardiac dysfunction in volume overload-induced HF.

Differential effects of lipophilic and hydrophilic statins on muscle sympathetic nerve activity in heart failure with preserved left ventricular ejection fraction

Augmented sympathetic nerve activity is associated with heart failure with preserved left ventricular ejection fraction (HFpEF). Lipophilic statins reduce sympathetic nerve activity in patients with heart failure with reduced left ventricular ejection fraction. However, little is known about whether all types of statins, regardless of solubility, reduce sympathetic nerve activity in HFpEF. We evaluated the effect of atorvastatin, a lipophilic statin, and rosuvastatin, a hydrophilic statin, on muscle sympathetic nerve activity (MSNA) in HFpEF patients. This study was conducted as a prospective, randomized, open-label, crossover trial. Ten HFpEF patients with untreated hyperlipidemia participated in this study. Subjects were assigned to either the atorvastatin (lipophilic) or the rosuvastatin (hydrophilic) group with each drug administered for 8 weeks. Atorvastatin and rosuvastatin treatment resulted in a similar reduction in low-density lipoprotein cholesterol (LDL-C) levels. There was no difference in the effect of either treatment on blood pressure, heart rate, or left ventricular function. Atorvastatin significantly decreased MSNA frequency compared with baseline (31.5 ± 6.3 vs. 47.5 ± 10.7 bursts/min, p < 0.01), but rosuvastatin had no effect on MSNA (40.9 ± 7.3 bursts/min). MSNA was significantly lower in the atorvastatin group than rosuvastatin group (p < 0.05). However, the reduction in MSNA seen in either group did not correlate with the reduction in LDL-C. No significant differences were observed in either the baroreflex control of heart rate or MSNA between the two groups. These results suggest that lipophilic statins have a favorable effect on sympathetic nerve activity beyond lowering LDL-C in HFpEF, but hydrophilic statins do not.

Revisiting the physiological effects of exercise training on autonomic regulation and chemoreflex control in heart failure: does ejection fraction matter?

Heart failure (HF) is a global public health problem that, independent of its etiology [reduced (HFrEF) or preserved ejection fraction (HFpEF)], is characterized by functional impairments of cardiac function, chemoreflex hypersensitivity, baroreflex sensitivity (BRS) impairment, and abnormal autonomic regulation, all of which contribute to increased morbidity and mortality. Exercise training (ExT) has been identified as a nonpharmacological therapy capable of restoring normal autonomic function and improving survival in patients with HFrEF. Improvements in autonomic function after ExT are correlated with restoration of normal peripheral chemoreflex sensitivity and BRS in HFrEF. To date, few studies have addressed the effects of ExT on chemoreflex control, BRS, and cardiac autonomic control in HFpEF; however, there are some studies that have suggested that ExT has a beneficial effect on cardiac autonomic control. The beneficial effects of ExT on cardiac function and autonomic control in HF may have important implications for functional capacity in addition to their obvious importance to survival. Recent studies have suggested that the peripheral chemoreflex may also play an important role in attenuating exercise intolerance in HFrEF patients. The role of the central/peripheral chemoreflex, if any, in mediating exercise intolerance in HFpEF has not been investigated. The present review focuses on recent studies that address primary pathophysiological mechanisms of HF (HFrEF and HFpEF) and the potential avenues by which ExT exerts its beneficial effects.

KLF2 mediates enhanced chemoreflex sensitivity, disordered breathing and autonomic dysregulation in heart failure

KEY POINTS

Enhanced carotid body chemoreflex activity contributes to development of disordered breathing patterns, autonomic dysregulation and increases in incidence of arrhythmia in animal models of reduced ejection fraction heart failure. Chronic reductions in carotid artery blood flow are associated with increased carotid body chemoreceptor activity. Krüppel-like Factor 2 (KLF2) is a shear stress-sensitive transcription factor that regulates the expression of enzymes which have previously been shown to play a role in increased chemoreflex sensitivity. We investigated the impact of restoring carotid body KLF2 expression on chemoreflex control of ventilation, sympathetic nerve activity, cardiac sympatho-vagal balance and arrhythmia incidence in an animal model of heart failure. The results indicate that restoring carotid body KLF2 in chronic heart failure reduces sympathetic nerve activity and arrhythmia incidence, and improves cardiac sympatho-vagal balance and breathing stability. Therapeutic approaches that increase KLF2 in the carotid bodies may be efficacious in the treatment of respiratory and autonomic dysfunction in heart failure.

ABSTRACT

Oscillatory breathing and increased sympathetic nerve activity (SNA) are associated with increased arrhythmia incidence and contribute to mortality in chronic heart failure (CHF). Increased carotid body chemoreflex (CBC) sensitivity plays a role in this process and can be precipitated by chronic blood flow reduction. We hypothesized that downregulation of a shear stress-sensitive transcription factor, Krüppel-like Factor 2 (KLF2), mediates increased CBC sensitivity in CHF and contributes to associated autonomic, respiratory and cardiac sequelae. Ventilation (Ve), renal SNA (RSNA) and ECG were measured at rest and during CBC activation in sham and CHF rabbits. Oscillatory breathing was quantified as the apnoea-hypopnoea index (AHI) and respiratory rate variability index (RRVI). AHI (control 6 ± 1/h, CHF 25 ± 1/h), RRVI (control 9 ± 3/h, CHF 29 ± 3/h), RSNA (control 22 ± 2% max, CHF 43 ± 5% max) and arrhythmia incidence (control 50 ± 10/h, CHF 300 ± 100/h) were increased in CHF at rest ( FIO2 21%), as were CBC responses (Ve, RSNA) to 10% FIO2 (all P < 0.05 vs. control). In vivo adenoviral transfection of KLF2 to the carotid bodies in CHF rabbits restored KLF2 expression, and reduced AHI (7 ± 2/h), RSNA (18 ± 2% max) and arrhythmia incidence (46 ± 13/h) as well as CBC responses to hypoxia (all P < 0.05 vs. CHF empty virus). Conversely, lentiviral KLF2 siRNA in the carotid body decreased KLF2 expression, increased chemoreflex sensitivity, and increased AHI (6 ± 2/h vs. 14 ± 3/h), RRVI (5 ± 3/h vs. 20 ± 3/h) and RSNA (24 ± 4% max vs. 34 ± 5% max) relative to scrambled-siRNA rabbits. In conclusion, down-regulation of KLF2 in the carotid body increases CBC sensitivity, oscillatory breathing, RSNA and arrhythmia incidence during CHF.

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.

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.

Central TrkB blockade attenuates ICV angiotensin II-hypertension and sympathetic nerve activity in male Sprague-Dawley rats

Increased sympathetic nerve activity and the activation of the central renin-angiotensin system are commonly associated with cardiovascular disease states such as hypertension and heart failure, yet the precise mechanisms contributing to the long-term maintenance of this sympatho-excitation are incompletely understood. Due to the established physiological role of neurotrophins contributing toward neuroplasticity and neuronal excitability along with recent evidence linking the renin-angiotensin system and brain-derived neurotrophic factor (BDNF) along with its receptor (TrkB), it is likely the two systems interact to promote sympatho-excitation during cardiovascular disease. However, this interaction has not yet been fully demonstrated, in vivo. Thus, we hypothesized that central angiotensin II (Ang II) treatment will evoke a sympatho-excitatory state mediated through the actions of BDNF/TrkB. We infused Ang II (20ng/min) into the right lateral ventricle of male Sprague-Dawley rats for twelve days with or without the TrkB receptor antagonist, ANA-12 (50ng/h). We found that ICV infusion of Ang II increased mean arterial pressure (+40.4mmHg), increased renal sympathetic nerve activity (+19.4% max activity), and induced baroreflex dysfunction relative to vehicle. Co-infusion of ANA-12 attenuated the increase in blood pressure (-20.6mmHg) and prevented the increase in renal sympathetic nerve activity (-22.2% max) and baroreflex dysfunction relative to Ang II alone. Ang II increased thirst and decreased food consumption, and Ang II+ANA-12 augmented the thirst response while attenuating the decrease in food consumption. We conclude that TrkB signaling is a mediator of the long-term blood pressure and sympathetic nerve activity responses to central Ang II activity. These findings demonstrate the involvement of neurotrophins such as BDNF in promoting Ang II-induced autonomic dysfunction and further implicate TrkB signaling in modulating presympathetic autonomic neurons during cardiovascular disease.

Cardiac diastolic and autonomic dysfunction are aggravated by central chemoreflex activation in heart failure with preserved ejection fraction rats

KEY POINTS

Heart failure with preserved ejection fraction (HFpEF) is associated with disordered breathing patterns, and sympatho-vagal imbalance. Although it is well accepted that altered peripheral chemoreflex control plays a role in the progression of heart failure with reduced ejection fraction (HFrEF), the pathophysiological mechanisms underlying deterioration of cardiac function in HFpEF are poorly understood. We found that central chemoreflex is enhanced in HFpEF and neuronal activation is increased in pre-sympathetic regions of the brainstem. Our data showed that activation of the central chemoreflex pathway in HFpEF exacerbates diastolic dysfunction, worsens sympatho-vagal imbalance and markedly increases the incidence of cardiac arrhythmias in rats with HFpEF.

ABSTRACT

Heart failure (HF) patients with preserved ejection fraction (HFpEF) display irregular breathing, sympatho-vagal imbalance, arrhythmias and diastolic dysfunction. It has been shown that tonic activation of the central and peripheral chemoreflex pathway plays a pivotal role in the pathophysiology of HF with reduced ejection fraction. In contrast, no studies to date have addressed chemoreflex function or its effect on cardiac function in HFpEF. Therefore, we tested whether peripheral and central chemoreflexes are hyperactive in HFpEF and if chemoreflex activation exacerbates cardiac dysfunction and autonomic imbalance. Sprague-Dawley rats (n = 32) were subjected to sham or volume overload to induce HFpEF. Resting breathing variability, chemoreflex gain, cardiac function and sympatho-vagal balance, and arrhythmia incidence were studied. HFpEF rats displayed [mean ± SD; chronic heart failure (CHF) vs. Sham, respectively] a marked increase in the incidence of apnoeas/hypopnoeas (20.2 ± 4.0 vs. 9.7 ± 2.6 events h^-1 ), autonomic imbalance [0.6 ± 0.2 vs. 0.2 ± 0.1 low/high frequency heart rate variability (LF/HF_HRV )] and cardiac arrhythmias (196.0 ± 239.9 vs. 19.8 ± 21.7 events h^-1 ). Furthermore, HFpEF rats showed increase central chemoreflex sensitivity but not peripheral chemosensitivity. Accordingly, hypercapnic stimulation in HFpEF rats exacerbated increases in sympathetic outflow to the heart (229.6 ± 43.2% vs. 296.0 ± 43.9% LF/HF_HRV , normoxia vs. hypercapnia, respectively), incidence of cardiac arrhythmias (196.0 ± 239.9 vs. 576.7 ± 472.9 events h^-1 ) and diastolic dysfunction (0.008 ± 0.004 vs. 0.027 ± 0.027 mmHg μl^-1 ). Importantly, the cardiovascular consequences of central chemoreflex activation were related to sympathoexcitation since these effects were abolished by propranolol. The present results show that the central chemoreflex is enhanced in HFpEF and that acute activation of central chemoreceptors leads to increases of cardiac sympathetic outflow, cardiac arrhythmogenesis and impairment in cardiac function in rats with HFpEF.

Central Angiotensin-II Increases Blood Pressure and Sympathetic Outflow via Rho Kinase Activation in Conscious Rabbits

Elevated sympathetic tone and activation of the renin-angiotensin system are pathophysiologic hallmarks of hypertension, and the interactions between these systems are particularly deleterious. The importance of Rho kinase as a mediator of the effects of angiotensin-II (AngII) in the periphery is clear, but the role of Rho kinase in sympathoexcitation caused by central AngII is not well established. We hypothesized that AngII mediates its effects in the brain by the activation of the RhoA/Rho kinase pathway. Chronically instrumented, conscious rabbits received the following intracerebroventricular infusion treatments for 2 weeks via osmotic minipump: AngII, Rho kinase inhibitor Fasudil, AngII plus Fasudil, or a vehicle control. AngII increased mean arterial pressure over the course of the infusion, and this effect was prevented by the coadministration of Fasudil. AngII increased cardiac and vascular sympathetic outflow as quantified by the heart rate response to metoprolol and the depressor effect of hexamethonium; coadministration of Fasudil abolished both of these effects. AngII increased baseline renal sympathetic nerve activity in conscious animals and impaired baroreflex control of sympathetic nerve activity; again Fasudil coinfusion prevented these effects. Each of these end points showed a statistically significant interaction between AngII and Fasudil. Quantitative immunofluorescence of brain slices confirmed that Rho kinase activity was increased by AngII and decreased by Fasudil. Taken together, these data indicate that hypertension, elevated sympathetic outflow, and baroreflex dysfunction caused by central AngII are mediated by Rho kinase activation and suggest that Rho kinase inhibition may be an important therapeutic target in sympathoexcitatory cardiovascular diseases.

Carotid Body Ablation Abrogates Hypertension and Autonomic Alterations Induced by Intermittent Hypoxia in Rats

Chronic intermittent hypoxia (CIH), the main feature of obstructive sleep apnea, enhances carotid body (CB) chemosensory responses to hypoxia and produces autonomic dysfunction, cardiac arrhythmias, and hypertension. We tested whether autonomic alterations, arrhythmogenesis, and the progression of hypertension induced by CIH depend on the enhanced CB chemosensory drive, by ablation of the CB chemoreceptors. Male Sprague-Dawley rats were exposed to control (Sham) conditions for 7 days and then to CIH (5% O2, 12/h 8 h/d) for a total of 28 days. At 21 days of CIH exposure, rats underwent bilateral CB ablation and then exposed to CIH for 7 additional days. Arterial blood pressure and ventilatory chemoreflex response to hypoxia were measured in conscious rats. In addition, cardiac autonomic imbalance, cardiac baroreflex gain, and arrhythmia score were assessed during the length of the experiments. In separate experimental series, we measured extracellular matrix remodeling content in cardiac atrial tissue and systemic oxidative stress. CIH induced hypertension, enhanced ventilatory response to hypoxia, induced autonomic imbalance toward sympathetic preponderance, reduced baroreflex gain, and increased arrhythmias and atrial fibrosis. CB ablation normalized blood pressure, reduced ventilatory response to hypoxia, and restored cardiac autonomic and baroreflex function. In addition, CB ablation reduced the number of arrhythmias, but not extracellular matrix remodeling or systemic oxidative stress, suggesting that reductions in arrhythmia incidence during CIH were related to normalization of cardiac autonomic balance. Present results show that autonomic alterations induced by CIH are critically dependent on the CB and support a main role for the CB in the CIH-induced hypertension.

Contribution of peripheral and central chemoreceptors to sympatho-excitation in heart failure

Chronic heart failure (CHF) is a major public health problem. Tonic hyper-activation of sympathetic neural outflow is commonly observed in patients with CHF. Importantly, sympatho-excitation in CHF exacerbates its progression and is strongly related to poor prognosis and high mortality risk. Increases in both peripheral and central chemoreflex drive are considered markers of the severity of CHF. The principal peripheral chemoreceptors are the carotid bodies (CBs) and alteration in their function has been described in CHF. Mainly, during CHF the CB chemosensitivity is enhanced leading to increases in ventilation and sympathetic outflow. In addition to peripheral control of breathing, central chemoreceptors (CCs) are considered a dominant mechanism in ventilatory regulation. Potentiation of the ventilatory and sympathetic drive in response to CC activation has been shown in patients with CHF as well as in animal models. Therefore, improving understanding of the contribution of the peripheral and central chemoreflexes to augmented sympathetic discharge in CHF could help in developing new therapeutic approaches intended to attenuate the progression of CHF. Accordingly, the main focus of this review is to discuss recent evidence that peripheral and central chemoreflex function are altered in CHF and that they contribute to autonomic imbalance and progression of CHF.

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.

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.

Cheyne-Stokes respiration during sleep: mechanisms and potential interventions

Cheyne-Stokes respiration is characterized by a typical waxing and waning pattern in breathing amplitude, interspersed with central apnoeas or hypopnoeas. This article reviews current knowledge regarding Cheyne-Stokes respiration with a particular emphasis on the mechanisms and latest methods of intervention.

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.

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.

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.