Impact of heart failure and exercise capacity on sympathetic response to handgrip exercise

Dissecting the exercise pressor reflex in heart failure: A multi-step failure

The contribution of neural feedback originating from exercising limb muscles to the cardiovascular response to exercise was first recognized nearly 100 years ago. Today, it is well established that this influence is initiated by the activation of group III and IV sensory neurons with terminal endings located within contracting skeletal muscle. During exercise, these sensory neurons project feedback related to intramuscular mechanical and metabolic perturbations to medullary neural circuits which reflexively evoke decreases in parasympathetic and increases in sympathetic nervous system activity with the purpose of optimizing central and peripheral hemodynamics. Considerable evidence from animal and human studies suggests that the function of this regulatory control system, known as the exercise pressor reflex (EPR), is abnormal in heart failure and exaggerates sympatho-excitation which impairs the hemodynamic response to exercise and contributes to the functional limitations characterizing these patients. This review briefly introduces the key determinants of EPR control in health and covers the impact of heart failure on the integrity of each of its components and overall function. These include the sensitivity of group III/IV muscle afferents, afferent signal transmission in the spinal cord, and the central integration and processing of sensory feedback within the brainstem. Importantly, although most data relevant for this review come from studies in HFrEF, the limited HFpEF-specific insights are included when available. While arguably not part of the EPR, we also discuss the impact of heart failure on the exercise-induced increase of intramuscular stimuli of group III/IV muscle afferents and end-organ responsiveness to sympathetic/neurochemical stimulation.

Mechanisms mediating muscle metaboreflex control of cardiac output during exercise: Impaired regulation in heart failure

The ability to increase cardiac output during dynamic exercise is paramount for the ability to maintain workload performance. Reflex control of the cardiovascular system during exercise is complex and multifaceted involving multiple feedforward and feedback systems. One major reflex thought to mediate the autonomic adjustments to exercise is termed the muscle metaboreflex and is activated via afferent neurons within active skeletal muscle which respond to the accumulation of interstitial metabolites during exercise when blood flow and O2 delivery are insufficient to meet metabolic demands. This is one of the most powerful cardiovascular reflexes capable of eliciting profound increases in sympathetic nerve activity, arterial blood pressure, central blood volume mobilization, heart rate and cardiac output. This review summarizes the mechanisms meditating muscle metaboreflex-induced increases in cardiac output. Although much has been learned from studies using anaesthetized and/or decerebrate animals, we focus on studies in conscious animals and humans performing volitional exercise. We discuss the separate and interrelated roles of heart rate, ventricular contractility, ventricular preload and ventricular-vascular coupling as well as the interaction with other cardiovascular reflexes which modify muscle metaboreflex control of cardiac output. We discuss how these mechanisms may be altered in subjects with heart failure with reduced ejection fraction and offer suggestions for future studies.

Mechanisms of sympathetic excitation during cycling exercise in heart failure with reduced ejection fraction

Leg muscle sympathetic nerve activity (MSNA) diminishes in healthy (HC) individuals during mild dynamic exercise but not in age-matched patients with heart failure due to reduced ejection fraction (HFrEF). To elucidate the neural mechanisms responsible for such sympathetic excitation, we studied 20 stable HFrEF patients (6F; mean age 62 ± 8 SD years) and 15 age-matched HC (6F; mean age 59 ± 7). We quantified peak oxygen uptake (V ̇ O 2 peak ${{\dot{V}}_{{{{\mathrm{O}}}_{2{\mathrm{peak}}}}}}$ ) and separately, fibular MSNA (microneurography) at rest and during one-leg cycling (2 min each, mild (unloaded) and moderate intensity (loaded = 30-40%V ̇ O 2 peak ${{\dot{V}}_{{{{\mathrm{O}}}_{2{\mathrm{peak}}}}}}$ )) throughout three interventions: (1) post-exercise circulatory occlusion (PECO), which isolates the leg muscle metaboreflex (MMR); (2) supine posture, which stimulates cardiopulmonary baroreceptors (CPB); and (3) 32% inspired oxygen, to supress the peripheral chemoreflex (PC). One-leg cycling increased MSNA and activated the leg MMR in patients with HFrEF but not HC. MSNA at rest and during mild exercise was lower when supine than seated in both cohorts. Breathing 32% oxygen lowered the MSNA of HC but not HFrEF. In both groups, hyperoxia decreased burst frequency during low-intensity cycling. Hyperoxia abolished the 'paradoxical' sympatho-excitation of HFrEF. Thirteen participants with HFrEF were reassessed after 4 months of conventional cardiopulmonary rehabilitation. Exercise training improvedV ̇ O 2 peak ${{\dot{V}}_{{{{\mathrm{O}}}_{2{\mathrm{peak}}}}}}$ by 17% and attenuated the leg MMR response without altering CPB activation or PC suppression. We conclude that in HFrEF, all three autonomic reflexes are engaged to a varying degree by one-leg cycling. Patient training attenuates the leg MMR without affecting CPB or PC modulation of MSNA during exercise. KEY POINTS: In HFrEF patients, an exaggerated leg MMR is the dominant sympatho-excitatory reflex during one-leg cycling at moderate work rates; with their MSNA response relating inversely toV ̇ O 2 peak ${{\dot{V}}_{{{{\mathrm{O}}}_{2{\mathrm{peak}}}}}}$ . Activation of the cardiopulmonary baroreflex and peripheral chemoreflex by exercise also contribute, suggesting that exercising supine or while breathing 32% O2 may complement conventional training protocols. An exercise-based cardiac rehabilitation programme lowers sympathetic discharge at rest and during mild intensity cycling by abolishing specifically the leg MMR response.

Cerebrovascular responses to muscle metaboreflex activation in patients living with heart failure with reduced ejection fraction

Impaired cerebrovascular control in patients with heart failure with reduced ejection fraction (HFrEF) has been attributed to cardiac impairment and exaggerated sympathetic-mediated cerebral vasoconstriction. The goal of this study was to examine the effect of muscle metaboreflex activation (MMA) on cerebrovascular hemodynamics in patients with HFrEF under conditions of preserved cardiac output. It was hypothesized that reductions in the index of cerebral blood flow and cerebrovascular conductance (CVCi) during MMA would be exaggerated in HFrEF and independent of reduced cardiac output. Middle cerebral blood velocity (MCAVmean; transcranial Doppler), blood pressure, cardiac output (Finometer), and end-tidal CO2 were examined at rest, during isometric handgrip, and during muscle MMA (postexercise circulatory occlusion) in 18 patients with HFrEF and 21 healthy, sex-, and age-matched controls. To minimize differences in β-adrenergic control, patients with HFrEF withdrew from β-blockade medications before the study. Cardiac index and blood pressure were not significantly different between groups under any condition. The MCAVmean was lower at rest and during exercise in HFrEF. The CVCi (MCAVmean/mean arterial pressure) and MCAVmean decreased during MMA in the control group. In contrast, the CVCi remained unchanged and MCAVmean increased during MMA in the HFrEF group. Despite similar systemic hemodynamics, patients with HFrEF display lower MCAVmean at rest and an increase in MCAVmean during MMA. These novel findings implicate aspects other than reduced cardiac output or exaggerated sympathetic constriction as underlying causes of altered cerebrovascular regulation in HFrEF.NEW & NOTEWORTHY Compared with controls, patients with heart failure with reduced ejection fraction (HFrEF) displayed reduced indices of cerebral perfusion at rest and increases in perfusion in response to postexercise circulatory occlusion (PECO, method to isolate muscle metaboreflex activation). This occurred despite similar cardiac output and blood pressure values between groups. Thus, lower resting indices of cerebral perfusion and increased perfusion during sympathoexcitation in HFrEF may occur independently from differences in systemic hemodynamics.

Brain structural and functional changes in patients with chronic heart failure

Heart failure (HF) frequently suffers from brain abnormalities and cognitive impairments. This study aims to investigate brain structure and function alteration in patients with chronic HF. This retrospective study included 49 chronic HF and 49 health controls (HCs). Voxel-based morphometry was conducted on structural MRI to quantify gray matter volume (GMV), and functional connectivity (FC) was assessed with seed-based analysis using resting-state fMRI. White matter microstructure integrity was also evaluated through tract-based spatial statistics employing DTI. Correlations between multimodal MRI features and cognitive performance were further investigated in patients with chronic HF. Patients with chronic HF exhibited significantly reduced regional GMV, white matter microstructure injury (Family wise error correction, p＜0.05), and decreased FC in multiple brain regions involved in cognition, sensorimotor, visual function (Gaussian random field correction, voxel level p＜0.0001 and cluster-level p＜0.01). There was no observed increases in GMV or FC compared with HCs. Decreased GMV showed positive correlations with cognitive performance (r = 0.025-0.577, p = 0.025-0.001), while decreased fractional anisotropy was negatively correlated with anxiety scores (r = -0.339, p = 0.040) in patients with chronic HF. This study revealed that patients with chronic HF exhibited brain structure injury affecting gray matter and white matter, as well as FC abnormalities of brain regions responsible for cognition, sensorimotor and visual function. These findings suggest GMV could serve as a neuroimaging biomarker for cognitive impairments and a potential target for neuroprotective therapies in patients with chronic HF.

Sympathetic Response to 1-Leg Cycling Exercise Predicts Exercise Capacity in Patients With Heart Failure With Preserved Ejection Fraction

BACKGROUND

In heart failure, sympathetic excess and exercise intolerance impair quality of life. In heart failure with reduced ejection fraction, exercise stimulates a reflex increase in muscle sympathetic nerve activity (MSNA) that relates inversely to peak oxygen uptake (V̇O2peak). Whether similar sympathoexcitatory responses are present in heart failure with preserved EF (HFpEF) and relate to V̇O2peak are unknown.

METHODS

In 13 patients with HFpEF (70±6 years), 17 comorbidity-matched controls (CMC; 67±8 years), and 18 healthy controls (65±8 years), we measured heart rate, blood pressure, and MSNA (microneurography) during (1) 7-minute baseline; (2) 2-minute isometric handgrip (40% maximal voluntary contraction) or rhythmic handgrip (50% and 30% maximal voluntary contraction) exercise, followed by 2-minute postexercise circulatory occlusion; and (3) 4-minute 1-leg cycling (2 minutes each at mild and moderate intensity). V̇O2peak was obtained by open-circuit spirometry.

RESULTS

Resting MSNA was higher and V̇O2peak was lower in HFpEF versus CMCs and healthy controls (all P<0.05). During handgrip, MSNA increased in all groups (all P<0.05); in HFpEF, MSNA was greater than CMCs and healthy controls during HG and postexercise circulatory occlusion at 40% isometric handgrip (all P<0.05) and HG only at 50% and 30% rhythmic handgrip (all P<0.05). During cycling, MSNA (bursts·min-1) decreased during mild (-4±4; P=0.01) and moderate (-8±6; P<0.001) cycling in healthy controls, was unchanged during mild (+1±7; P=0.42) and moderate (+2±8; P=0.28) cycling in CMCs, yet increased in HFpEF during mild (+8±8; P<0.001) and moderate (+9±10; P<0.001) cycling. In HFpEF, the change in MSNA during moderate cycling related inversely to relative (r=-0.72; R 2=0.51; P<0.01) and percent-predicted (r=-0.63; R 2=0.39; P=0.03) V̇O2peak. No statistically significant relationships were detected in controls (P>0.05).

CONCLUSIONS

In contrast to CMCs, patients with HFpEF exhibit augmented MSNA at rest and during exercise. The magnitude of such paradoxical sympathoexcitation during dynamic cycling relates inversely to V̇O2peak, consistent with a neurogenic, vasoconstrictor limit on exercise capacity in HFpEF.

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.

Consequences of group III/IV afferent feedback and respiratory muscle work on exercise tolerance in heart failure with reduced ejection fraction

Exercise intolerance and exertional dyspnoea are the cardinal symptoms of heart failure with reduced ejection fraction (HFrEF). In HFrEF, abnormal autonomic and cardiopulmonary responses arising from locomotor muscle group III/IV afferent feedback is one of the primary mechanisms contributing to exercise intolerance. HFrEF patients also have pulmonary system and respiratory muscle abnormalities that impair exercise tolerance. Thus, the primary impetus for this review was to describe the mechanistic consequences of locomotor muscle group III/IV afferent feedback and respiratory muscle work in HFrEF. To address this, we first discuss the abnormal autonomic and cardiopulmonary responses mediated by locomotor muscle afferent feedback in HFrEF. Next, we outline how respiratory muscle work impairs exercise tolerance in HFrEF through its effects on locomotor muscle O2 delivery. We then discuss the direct and indirect evidence supporting an interaction between locomotor muscle group III/IV afferent feedback and respiratory muscle work during exercise in HFrEF. Last, we outline future research directions related to locomotor and respiratory muscle abnormalities to progress the field forward in understanding the pathophysiology of exercise intolerance in HFrEF. NEW FINDINGS: What is the topic of this review? This review is focused on understanding the role that locomotor muscle group III/IV afferent feedback and respiratory muscle work play in the pathophysiology of exercise intolerance in patients with heart failure. What advances does it highlight? This review proposes that the concomitant effects of locomotor muscle afferent feedback and respiratory muscle work worsen exercise tolerance and exacerbate exertional dyspnoea in patients with heart failure.

Neural control of the circulation during exercise in heart failure with reduced and preserved ejection fraction

Patients with heart failure with reduced (HFrEF) and preserved ejection fraction (HFpEF) exhibit severe exercise intolerance that may be due, in part, to inappropriate cardiovascular and hemodynamic adjustments to exercise. Several neural mechanisms and locally released vasoactive substances work in concert through complex interactions to ensure proper adjustments to meet the metabolic demands of the contracting skeletal muscle. Specifically, accumulating evidence suggests that disease-related alterations in neural mechanisms (e.g., central command, exercise pressor reflex, arterial baroreflex, and cardiopulmonary baroreflex) contribute to heightened sympathetic activation and impaired ability to attenuate sympathetic vasoconstrictor responsiveness that may contribute to reduced skeletal muscle blood flow and severe exercise intolerance in patients with HFrEF. In contrast, little is known regarding these important aspects of physiology in patients with HFpEF, though emerging data reveal heightened sympathetic activation and attenuated skeletal muscle blood flow during exercise in this patient population that may be attributable to dysregulated neural control of the circulation. The overall goal of this review is to provide a brief overview of the current understanding of disease-related alterations in the integrative neural cardiovascular responses to exercise in both HFrEF and HFpEF phenotypes, with a focus on sympathetic nervous system regulation during exercise.

Normal and excessive muscle sympathetic nerve activity in heart failure: implications for future trials of therapeutic autonomic modulation

AIMS

Patients with sympathetic excess are those most likely to benefit from novel interventions targeting the autonomic nervous system. To inform such personalized therapy, we identified determinants of augmented muscle sympathetic nerve activity (MSNA) in heart failure, versus healthy controls.

METHODS AND RESULTS

We compared data acquired in 177 conventionally-treated, stable non-diabetic patients in sinus rhythm, aged 18-79 years (149 males; 28 females; left ventricular ejection fraction [LVEF] 25 ± 11% [mean ± standard deviation]; range 5-60%), and, concurrently, under similar conditions, in 658 healthy, normotensive volunteers (398 males; aged 18-81 years). In heart failure, MSNA ranged between 7 and 90 bursts·min^-1 , proportionate to heart rate (p < 0.0001) and body mass index (BMI) (p = 0.03), but was unrelated to age, blood pressure, or drug therapy. Mean MSNA, adjusted for age, sex, BMI, and heart rate, was greater in heart failure (+14.2 bursts·min^-1 ; 95% confidence interval [CI] 12.1-16.3; p < 0.0001), but lower in women (-5.0 bursts·min^-1 ; 95% CI 3.4-6.6; p < 0.0001). With spline modeling, LVEF accounted for 9.8% of MSNA variance; MSNA related inversely to LVEF below an inflection point of ∼21% (p < 0.006), but not above. Burst incidence was greater in ischaemic than dilated cardiomyopathy (p = 0.01), and patients with sleep apnoea (p = 0.03). Burst frequency correlated inversely with stroke volume (p < 0.001), cardiac output (p < 0.001), and peak oxygen consumption (p = 0.002), and directly with norepinephrine (p < 0.0001) and peripheral resistance (p < 0.001).

CONCLUSION

Burst frequency and incidence exceeded normative values in only ∼53% and ∼33% of patients. Such diversity encourages selective deployment of sympatho-modulatory therapies. Clinical characteristics can highlight individuals who may benefit from future personalized interventions targeting pathological sympathetic activation.

Sympathetic and hemodynamic responses to exercise in heart failure with preserved ejection fraction

Excessive sympathetic activity during exercise causes heightened peripheral vasoconstriction, which can reduce oxygen delivery to active muscles, resulting in exercise intolerance. Although both patients suffering from heart failure with preserved and reduced ejection fraction (HFpEF and HFrEF, respectively) exhibit reduced exercise capacity, accumulating evidence suggests that the underlying pathophysiology may be different between these two conditions. Unlike HFrEF, which is characterized by cardiac dysfunction with lower peak oxygen uptake, exercise intolerance in HFpEF appears to be predominantly attributed to peripheral limitations involving inadequate vasoconstriction rather than cardiac limitations. However, the relationship between systemic hemodynamics and the sympathetic neural response during exercise in HFpEF is less clear. This mini review summarizes the current knowledge on the sympathetic (i.e., muscle sympathetic nerve activity, plasma norepinephrine concentration) and hemodynamic (i.e., blood pressure, limb blood flow) responses to dynamic and static exercise in HFpEF compared to HFrEF, as well as non-HF controls. We also discuss the potential of a relationship between sympathetic over-activation and vasoconstriction leading to exercise intolerance in HFpEF. The limited body of literature indicates that higher peripheral vascular resistance, perhaps secondary to excessive sympathetically mediated vasoconstrictor discharge compared to non-HF and HFrEF, drives exercise in HFpEF. Excessive vasoconstriction also may primarily account for over elevations in blood pressure and concomitant limitations in skeletal muscle blood flow during dynamic exercise, resulting in exercise intolerance. Conversely, during static exercise, HFpEF exhibit relatively normal sympathetic neural reactivity compared to non-HF, suggesting that other mechanisms beyond sympathetic vasoconstriction dictate exercise intolerance in HFpEF.

Impaired sympathetic neural recruitment during exercise pressor reflex activation in women with post-traumatic stress disorder

Muscle sympathetic nerve activity (MSNA) increases during isometric exercise via increased firing of low-threshold action potentials (AP), recruitment of larger, higher-threshold APs, and synaptic delay modifications. Recent work found that women with post-traumatic stress disorder (PTSD) demonstrate exaggerated early-onset MSNA responses to exercise; however, it is unclear how PTSD affects AP recruitment patterns during fatiguing exercise. We hypothesized that women with PTSD (n = 11, 43 [11] [SD] years) would exhibit exaggerated sympathetic neural recruitment compared to women without PTSD (controls; n = 13, 40 [8] years). MSNA and AP discharge patterns (via microneurography and a continuous wavelet transform) were measured during 1 min of baseline, isometric handgrip exercise (IHG) to fatigue, 2 min of post-exercise circulatory occlusion (PECO), and 3 min of recovery. Women with PTSD were unable to increase AP content per burst compared to controls throughout IHG and PECO (main effect of group: P = 0.026). Furthermore, relative to controls, women with PTSD recruited fewer AP clusters per burst during the first (controls: ∆1.3 [1.2] vs. PTSD: ∆-0.2 [0.8]; P = 0.016) and second minute (controls: ∆1.2 [1.1] vs. PTSD: ∆-0.1 [0.8]; P = 0.022) of PECO, and fewer subpopulations of larger, previously silent axons during the first (controls: ∆5 [4] vs. PTSD: ∆1 [2]; P = 0.020) and second minute (controls: ∆4 [2] vs. PTSD: ∆1 [2]; P = 0.021) of PECO. Conversely, PTSD did not modify the AP cluster size-latency relationship during baseline, the end of IHG, or PECO (all P = 0.658-0.745). Collectively, these data indicate that women with PTSD demonstrate inherent impairments in the fundamental neural coding patterns elicited by the sympathetic nervous system during IHG and exercise pressor reflex activation.

Heart failure-specific inverse relationship between the muscle sympathetic response to dynamic leg exercise and V̇O2peak

During 1-leg cycling, contralateral muscle sympathetic nerve activity (MSNA) falls in healthy adults but increases in most with reduced ejection fraction heart failure (HFrEF). We hypothesized that their peak oxygen uptake (V̇O2peak) relates inversely to their MSNA response to exercise. Twenty-nine patients (6 women; 63 ± 9 years; left ventricular ejection fraction: 30 ± 7%; V̇O2peak: 78 ± 23 percent age-predicted (%V̇O2peak); mean ± SD) and 21 healthy adults (9 women; 58 ± 7 years; 115 ± 29%V̇O2peak) performed 2 min of mild- (“loadless”) and moderate-intensity (“loaded”) 1-leg cycling. Heart rate, blood pressure (BP), contralateral leg MSNA and perceived exertion rate (RPE) were recorded. Resting MSNA burst frequency (BF) was higher (p < 0.01) in HFrEF (51 ± 11 vs 44 ± 7 bursts·min−1). Exercise heart rate, BP and RPE responses at either intensity were similar between groups. In minute 2 of “loadless” and “loaded” cycling, group mean BF fell from baseline values in controls (−5 ± 6 and −7 ± 7 bursts·min−1, respectively) but rose in HFrEF (+5 ± 7 and +5 ± 10 bursts·min−1). However, in 10 of the latter cohort, BF fell, similarly to controls. An inverse relationship between ΔBF from baseline to “loaded” cycling and %V̇O2peak was present in patients (r = −0.43, p < 0.05) but absent in controls (r = 0.07, p = 0.77). In HFrEF, ∼18% of variance in %V̇O2peak can be attributed to the change in BF elicited by exercise.

Novelty: Unlike healthy individuals, in the majority of heart failure patients with reduced ejection fraction (HFrEF), 1-leg cycling increases muscle sympathetic nerve activity (MSNA). In HFrEF, ∼18% of age-predicted peak oxygen uptake (V̇O2peak) can be attributed to changes in MSNA elicited by low-intensity exercise. This relationship is absent in healthy adults.

The 2021 Carl Ludwig Lecture. Unsympathetic autonomic regulation in heart failure: patient-inspired insights

Defined as a structural or functional cardiac abnormality accompanied by symptoms, signs, or biomarkers of altered ventricular pressures or volumes, heart failure also is a state of autonomic disequilibrium. A large body of evidence affirms that autonomic disturbances are intrinsic to heart failure; basal or stimulated sympathetic nerve firing or neural norepinephrine (NE) release more often than not exceed homeostatic need, such that an initially adaptive adrenergic or vagal reflex response becomes maladaptive. The magnitude of such maladaptation predicts prognosis. This Ludwig lecture develops two theses: the elucidation and judiciously targeted amelioration of maladaptive autonomic disturbances offers opportunities to complement contemporary guideline-based heart failure therapy, and serendipitous single-participant insights, acquired in the course of experimental protocols with entirely different intent, can generate novel insight, inform mechanisms, and launch entirely new research directions. I précis six elements of our current synthesis of the causes and consequences of maladaptive sympathetic disequilibrium in heart failure, shaped by patient-inspired epiphanies: arterial baroreceptor reflex modulation, excitation stimulated by increased cardiac filling pressure, paradoxical muscle sympathetic activation as a peripheral neurogenic constraint on exercise capacity, renal sympathetic restraint of natriuresis, coexisting sleep apnea, and augmented chemoreceptor reflex sensitivity and then conclude by envisaging translational therapeutic opportunities.

Muscle metaboreflex adaptations to exercise training in health and disease

Abnormalities in the muscle metaboreflex concur to exercise intolerance and greater cardiovascular risk. Exercise training benefits neurocardiovascular function at rest and during exercise, but its role in favoring muscle metaboreflex in health and disease remains controversial. While some authors demonstrated that exercise training enhanced the sensitization of muscle metabolically afferents and improved neurocardiovascular responses to muscle metaboreflex activation, others reported unaltered responses. This narrative review aimed to: (a) highlight the current evidence on the effects of exercise training upon cardiovascular and autonomic responses to muscle metaboreflex activation; (b) analyze the role of training components and indicate potential mechanisms of metaboreflex adaptations; and (c) address key methodological features for future research. Though limited, accumulated evidence suggests that muscle metaboreflex adaptations depend on the individual clinical status, exercise modality, and training duration. In healthy populations, most trials negated the hypothesis of metaboreflex improvement due to chronic exercise, irrespective of the training duration. Favorable changes in patients with impaired metaboreflex, particularly chronic heart failure, mostly resulted from long-term interventions (> 16 weeks) including aerobic exercise of moderate to high intensity, performed in isolation or within multimodal training. Potential mechanisms of metaboreflex improvements include enhanced sensitivity of channels and receptors, greater antioxidant capacity, lower metabolite accumulation, increased functional sympatholysis, and muscle perfusion. Future research should investigate: (1) the dose-response relationship of training components within different exercise modalities to elicit improvements in individuals showing intact or impaired muscle metaboreflex; and (2) potential and specific underlying mechanisms of metaboreflex improvements in individuals with different medical conditions.

Sympathetic neural responses in heart failure during exercise and after exercise training

The sympathetic nervous system coordinates the cardiovascular response to exercise. This regulation is impaired in both experimental and human heart failure with reduced ejection fraction (HFrEF), resulting in a state of sympathoexcitation which limits exercise capacity and contributes to adverse outcome. Exercise training can moderate sympathetic excess at rest. Recording sympathetic nerve firing during exercise is more challenging. Hence, data acquired during exercise are scant and results vary according to exercise modality. In this review we will: (1) describe sympathetic activity during various exercise modes in both experimental and human HFrEF and consider factors which influence these responses; and (2) summarise the effect of exercise training on sympathetic outflow both at rest and during exercise in both animal models and human HFrEF. We will particularly highlight studies in humans which report direct measurements of efferent sympathetic nerve traffic using intraneural recordings. Future research is required to clarify the neural afferent mechanisms which contribute to efferent sympathetic activation during exercise in HFrEF, how this may be altered by exercise training, and the impact of such attenuation on cardiac and renal function.

Central Command and the Regulation of Exercise Heart Rate Response in Heart Failure With Preserved Ejection Fraction

BACKGROUND

Chronotropic incompetence is common in heart failure with preserved ejection fraction (HFpEF) and is linked to impaired aerobic capacity. Whether upstream autonomic signaling pathways responsible for raising exercise heart rate are impaired in HFpEF is unknown. We investigated the integrity of central command and muscle metaboreceptor function, 2 predominant mechanisms responsible for exertional increases in heart rate, in patients with HFpEF and senior controls.

METHODS

Fourteen healthy senior controls (7 men, 7 women) and 20 carefully screened patients with HFpEF (8 men, 12 women) underwent cardiopulmonary exercise testing (peak Vo₂) and static handgrip exercise at 40% of maximal voluntary contraction to fatigue with postexercise circulatory arrest for 2 minutes to assess central command and metaboreceptor function, respectively.

RESULTS

Peak Vo₂ (13.1±3.4 versus 22.7±4.0 mL/kg/min; P<0.001) and heart rate (122±20 versus 155±14 bpm; P<0.001) were lower in patients with HFpEF than senior controls. There were no significant differences in peak heart rate response during static handgrip between groups (patients with HFpEF versus controls: 90±13 versus 93±10 bpm; P=0.49). Metaboreceptor function, defined as mean arterial blood pressure at the end of postexercise circulatory arrest, was not significantly different between groups.

CONCLUSIONS

Central command (vagally mediated) and metaboreceptor function (sympathetically mediated) in patients with HFpEF were not different from those in healthy senior controls despite significantly lower peak whole-body exercise heart rates. These results demonstrate key reflex autonomic pathways regulating exercise heart rate responsiveness are intact in HFpEF.

Recent advances in exercise pressor reflex function in health and disease

Autonomic alterations at the onset of exercise are critical to redistribute cardiac output towards the contracting muscles while preventing a fall in arterial pressure due to excessive vasodilation within the contracting muscles. Neural mechanisms responsible for these adjustments include central command, the exercise pressor reflex, and arterial and cardiopulmonary baroreflexes. The exercise pressor reflex evokes reflex increases in sympathetic activity to the heart and systemic vessels and decreases in parasympathetic activity to the heart, which increases blood pressure (BP), heart rate, and total peripheral resistance through vasoconstriction of systemic vessels. In this review, we discuss recent advancements in our understanding of exercise pressor reflex function in health and disease. Specifically, we discuss emerging evidence suggesting that sympathetic vasoconstrictor drive to the contracting and non-contracting skeletal muscle is differentially controlled by central command and the metaboreflex in healthy conditions. Further, we discuss evidence from animal and human studies showing that cardiovascular diseases, including hypertension, diabetes, and heart failure, lead to an altered exercise pressor reflex function. We also provide an update on the mechanisms thought to underlie this altered exercise pressor reflex function in each of these diseases. Although these mechanisms are complex, multifactorial, and dependent on the etiology of the disease, there is a clear consensus that several mechanisms are involved. Ultimately, approaches targeting these mechanisms are clinically significant as they provide alternative therapeutic strategies to prevent adverse cardiovascular events while also reducing symptoms of exercise intolerance.

Grip Exercise of Non-Paretic Hand Can Improve Venous Return in the Paretic Arm in Stroke Patients: An Experimental Study in the Supine and Sitting Positions

Objective: This study aims to determine the effect of grip exercise by the non-paretic hand on venous return in the paretic arm in stroke in sitting and supine positions.

Methods: The study population included 21 stroke patients (mean age, 59.5 years). The diameter (mm) and time-averaged mean velocity (TAMV) (cm/s) of the axillary vein on the paretic side were measured by ultrasound during three distinct conditions: resting, rhythmic non-resistive grip exercise, and resistive exercise (30% of maximum grip strength) in supine and sitting positions. The venous flow volume (ml/min) was calculated using the obtained data.

Results: In the supine and sitting positions, the venous flow volume during rhythmic non-resistive and resistive exercises was increased in comparison to resting, which resulted in more increased venous flow volume by rhythmic resistive grip exercise than by non-resistive grip exercise (both, p=0.01).

Conclusion: Grip exercise by the non-paretic hand was found to be effective for increasing the venous flow volume in the paretic hand, and resistive grip exercise caused the greatest increase. Our results suggest that rhythmic handgrip exercise may be clinically useful for reducing the incidence of hand edema in stroke patients.

Cardiovascular autonomic reflex function after bilateral cardiac sympathetic denervation for ventricular arrhythmias

BACKGROUND

Bilateral cardiac sympathetic denervation (BCSD) is an effective therapy for ventricular arrhythmias (VAs) in cardiomyopathies (CMPs). After BCSD, residual autonomic nervous system (ANS) function is unknown.

OBJECTIVE

The purpose of this study was to assess ANS responses in patients with CMP before and after BCSD as compared with demographically matched healthy controls.

METHODS

Patients with CMP undergoing BCSD and matched healthy controls were recruited. Noninvasive measures-finger cuff beat-to-beat blood pressure (BP), electrocardiography, palmar electrodermal activity (EDA), and finger pulse volume (FPV)-were obtained at rest and during autonomic stressors-posture change, handgrip, and mental stress. Maximal as well as specific responses to stressors were compared.

RESULTS

Eighteen patients with CMP (mean age 54 ± 14 years; 16 men, 89%; left ventricular ejection fraction 36% ± 14%) with refractory VAs and 8 matched healthy controls were studied; 9 patients with CMP underwent testing before and after (median 28 days) BCSD, with comparable ongoing medication. Before BCSD, patients with CMP (n = 13) had lower resting systolic BP and FPV than did healthy controls (P < .01). Maximal FPV and systolic BP reflex responses, expressed as percent change were similar, while diastolic BP, mean BP, and EDA responses were blunted. After BCSD, resting measurements were unchanged relative to presurgical baseline (n = 9). EDA responses to stressors were abolished, confirming BCSD, while maximal FPV and BP responses were preserved. Diastolic BP, mean BP, and FPV responses to orthostatic challenge pointed toward a better tolerance of active standing after BCSD as compared with before. Responses to other stressors remained unchanged.

CONCLUSION

Patients with CMP and refractory VAs on optimal medical therapy have detectable but blunted adrenergic responses, which are not disrupted by BCSD.

Brain structural changes associated with aberrant functional responses to the Valsalva maneuver in heart failure

Heart failure (HF) patients show inability to regulate autonomic functions in response to autonomic challenges. The autonomic deficits may stem from brain tissue injury in central autonomic regulatory areas, resulting from ischemic and hypoxic processes accompanying the condition. However, the direct evaluation of correlations between brain structural injury and functional timing and magnitude of neural signal patterns within affected areas, which may lead to impaired autonomic outflow, is unclear. In this study, we evaluate neural responses to the Valsalva maneuver with blood oxygen level-dependent functional magnetic resonance imaging in 29 HF patients and 35 control subjects and brain structural changes using diffusion tensor imaging-based mean diffusivity in a subsample of 19 HF and 24 control subjects. HF showed decreased neural activation in multiple autonomic and motor control areas, including cerebellum cortices, vermis, left insular, left putamen, and bilateral postcentral gyrus. Structural brain changes emerged in similar autonomic, as well as cognitive and mood regulation areas. Functional MRI responses in cerebellum and insula in HF subjects are delayed or decreased in magnitude to the challenge. The impaired functional responses of insular and cerebellar sites are correlated with the severity of tissue changes. These results indicate that the functions of insular and cerebellar regions, sites that are involved in autonomic regulation, are compromised, and that autonomic deficits in these areas have brain structural basis for impaired functions. Our study enhanced our understanding of brain structural and functional alterations underlying impaired autonomic regulations in HF subjects.

Clinical safety of blood flow-restricted training? A comprehensive review of altered muscle metaboreflex in cardiovascular disease during ischemic exercise

Blood flow restriction training (BFRT) is an increasingly widespread method of exercise that involves imposed restriction of blood flow to the exercising muscle. Blood flow restriction is achieved by inflating a pneumatic pressure cuff (or a tourniquet) positioned proximal to the exercising muscle before, and during, the bout of exercise (i.e., ischemic exercise). Low-intensity BFRT with resistance training promotes comparable increases in muscle mass and strength observed during high-intensity exercise without blood flow restriction. BFRT has expanded into the clinical research setting as a potential therapeutic approach to treat functionally impaired individuals, such as the elderly, and patients with orthopedic and cardiovascular disease/conditions. However, questions regarding the safety of BFRT must be fully examined and addressed before the implementation of this exercise methodology in the clinical setting. In this respect, there is a general concern that BFRT may generate abnormal reflex-mediated cardiovascular responses. Indeed, the muscle metaboreflex is an ischemia-induced, sympathoexcitatory pressor reflex originating in skeletal muscle, and the present review synthesizes evidence that BFRT may elicit abnormal cardiovascular responses resulting from increased metaboreflex activation. Importantly, abnormal cardiovascular responses are more clearly evidenced in populations with increased cardiovascular risk (e.g., elderly and individuals with cardiovascular disease). The evidence provided in the present review draws into question the cardiovascular safety of BFRT, which clearly needs to be further investigated in future studies. This information will be paramount for the consideration of BFRT exercise implementation in clinical populations.

Augmented fear bradycardia in rats with heart failure

In congestive heart failure (CHF), while resting parasympathetic activity becomes reduced, parasympathetically-mediated responses to stressors have not been described. This study aimed to (1) elucidate the effect of CHF on fear bradycardia, a parasympathetically-mediated response, and (2) examine if brain oxidative stress of CHF mediates fear bradycardia. White noise sound (WNS) exposure to conscious rats induced freezing behavior and elicited bradycardia. WNS exposure-elicited bradycardia was greater in rats with CHF than in controls. Superoxide dismutase mimetics administered in the lateral/ventrolateral midbrain periaqueductal gray (l/vlPAG), a region that contributes to the generation of fear bradycardia, had no effect on the bradycardia response in control and CHF rats. Dihydroethidium staining in situ showed that superoxide generation in the l/vlPAG of CHF rats was increased as compared to controls. These results demonstrate that CHF leads to the augmentation of fear bradycardia. Moreover, oxidative stress in the l/vlPAG of CHF unlikely mediates the augmented fear bradycardia.

Training heart failure patients with reduced ejection fraction attenuates muscle sympathetic nerve activation during mild dynamic exercise

Muscle sympathetic nerve activity (MSNA) decreases during low-intensity dynamic one-leg exercise in healthy subjects but increases in patients with heart failure with reduced ejection fraction (HFrEF). We hypothesized that increased peak oxygen uptake (V̇o_2peak) after aerobic training would be accompanied by less sympathoexcitation during both mild and moderate one-leg dynamic cycling, an attenuated muscle metaboreflex, and greater skin vasodilation. We studied 27 stable, treated HFrEF patients (6 women; mean age: 65 ± 2 SE yr; mean left ventricular ejection fraction: 30 ± 1%) and 18 healthy age-matched volunteers (6 women; mean age: 57 ± 2 yr). We assessed V̇o_2peak (open-circuit spirometry) and the skin microcirculatory response to reactive hyperemia (laser flowmetry). Fibular MSNA (microneurography) was recorded before and during one-leg cycling (2 min unloaded and 2 min at 50% of V̇o_2peak) and, to assess the muscle metaboreflex, during posthandgrip ischemia (PHGI). HFrEF patients were evaluated before and after 6 mo of exercise-based cardiac rehabilitation. Pretraining V̇o_2peak and skin vasodilatation were lower (P < 0.001) and resting MSNA higher (P = 0.01) in HFrEF than control subjects. Training improved V̇o_2peak (+3.0 ± 1.0 mL·kg^-1·min^-1; P < 0.001) and cutaneous vasodilation and diminished resting MSNA (-6.0 ± 2.0, P = 0.01) plus exercise MSNA during unloaded (-4.0 ± 2.5, P = 0.04) but not loaded cycling (-1.0 ± 4.0 bursts/min, P = 0.34) and MSNA during PHGI (P < 0.05). In HFrEF patients, exercise training lowers MSNA at rest, desensitizes the sympathoexcitatory metaboreflex, and diminishes MSNA elicited by mild but not moderate cycling. Training-induced downregulation of resting MSNA and attenuated reflex sympathetic excitation may improve exercise capacity and survival.

Aberrant Brain Functional Connectivity Dynamic Responses to the Valsalva Maneuver in Heart Failure

BACKGROUND

Patients with heart failure (HF) show abnormal autonomic activities, which may stem from altered functional connectivity (FC) between different brain sites.

METHODS AND RESULTS

We evaluate insular and cerebellar FC with other brain areas, before, during, and after the Valsalva challenge, with functional magnetic resonance imaging in 35 HF and 35 control subjects. Significant insular FC emerged with striatum, thalamus, and anterior cingulate. While left and right cerebellar cortices showed significant FC with each other constituting the cerebellum network, the insula and cerebellum networks showed significant negative FC with each other at baseline, challenge, and recovery phases. The challenge induced increased FC within the insula and the cerebellum networks in both HF and controls. However, patients with HF showed more increased insular network FC, but less enhanced cerebellar FC. During the recovery phase, the negative FC between the insular network and cerebellum enhanced significantly in controls, but not in HF. Lower left ventricle ejection fraction was correlated with lower insula network FC, and impaired negative FC between cerebellum and the insula network in HF.

CONCLUSIONS

Increased insular FC in patients with HF might contribute to exaggerated sympathetic tone. While impaired cerebellar FC and diminished negative interactions between cerebellum and insular systems may indicate impaired parasympathetic functions in HF.

Skeletal Muscle Fatigability in Heart Failure

Evidence suggests that heart failure (HF) patients experience skeletal muscle fatigability in the lower extremity during single-limb tasks. The contribution of skeletal muscle fatigability to symptoms of exercise intolerance (perceived fatigue and dyspnea) is relatively unclear. Symptomatic or ‘perceived’ fatigue is defined by the sensations of exhaustion or tiredness that patients experience either at rest or while performing a motor task. Although factors that contribute to symptoms of fatigue in patients with HF are multifactorial; the skeletal muscle likely plays a major role. Skeletal muscle fatigability, as opposed to symptomatic fatigue, is an objective measure of a reduction in muscle force or power or reduced ability of the muscles to perform over time. Indeed, evidence suggests that patients with HF experience greater skeletal muscle fatigability which may contribute to a diminution in motor performance and the overall symptomatology that is hallmark of exercise intolerance in HF. This review will discuss (1) skeletal muscle fatigability in patients with HF, (2) the mechanisms contributing to locomotor skeletal muscle fatigability in HF and (3) the relationship of fatigability to symptoms of perceived fatigue and exercise intolerance in HF patients. Evidence suggests that cardiac dysfunction alone does not contribute to exercise intolerance. Therefore, mechanisms of skeletal muscle fatigability and their contribution to symptoms of fatigue and exercise intolerance, is an increasingly important consideration as we develop rehabilitative strategies for improving motor performance and functional capacity in patients with HF.

α-Adrenergic receptor regulation of skeletal muscle blood flow during exercise in heart failure patients with reduced ejection fraction

Patients suffering from heart failure with reduced ejection fraction (HFrEF) experience impaired limb blood flow during exercise, which may be due to a disease-related increase in α-adrenergic receptor vasoconstriction. Thus, in eight patients with HFrEF (63 ± 4 yr) and eight well-matched controls (63 ± 2 yr), we examined changes in leg blood flow (Doppler ultrasound) during intra-arterial infusion of phenylephrine (PE; an α₁-adrenergic receptor agonist) and phentolamine (Phen; a nonspecific α-adrenergic receptor antagonist) at rest and during dynamic single-leg knee-extensor exercise (0, 5, and 10 W). At rest, the PE-induced reduction in blood flow was significantly attenuated in patients with HFrEF (-15 ± 7%) compared with controls (-36 ± 5%). During exercise, the controls exhibited a blunted reduction in blood flow induced by PE (-12 ± 4, -10 ± 4, and -9 ± 2% at 0, 5, and 10 W, respectively) compared with rest, while the PE-induced change in blood flow was unchanged compared with rest in the HFrEF group (-8 ± 5, -10 ± 3, and -14 ± 3%, respectively). Phen administration increased leg blood flow to a greater extent in the HFrEF group at rest (+178 ± 34% vs. +114 ± 28%, HFrEF vs. control) and during exercise (36 ± 6, 37 ± 7, and 39 ± 6% vs. 13 ± 3, 14 ± 1, and 8 ± 3% at 0, 5, and 10 W, respectively, in HFrEF vs. control). Together, these findings imply that a HFrEF-related increase in α-adrenergic vasoconstriction restrains exercising skeletal muscle blood flow, potentially contributing to diminished exercise capacity in this population.

Functional neuroimaging of the central autonomic network: recent developments and clinical implications

Purpose

The central autonomic network (CAN) is an intricate system of brainstem, subcortical, and cortical structures that play key roles in the function of the autonomic nervous system. Prior to the advent of functional neuroimaging, in vivo studies of the human CAN were limited. The purpose of this review is to highlight the contribution of functional neuroimaging, specifically functional magnetic resonance imaging (fMRI), to the study of the CAN, and to discuss recent advances in this area. Additionally, we aim to emphasize exciting areas for future research.

Methods

We reviewed the existing literature in functional neuroimaging of the CAN. Here, we focus on fMRI research conducted in healthy human subjects, as well as research that has been done in disease states, to understand CAN function. To minimize confounding, papers examining CAN function in the context of cognition, emotion, pain, and affective disorders were excluded.

Results

fMRI has led to significant advances in the understanding of human CAN function. The CAN is composed of widespread brainstem and forebrain structures that are intricately connected and play key roles in reflexive and modulatory control of autonomic function.

Conclusions

fMRI technology has contributed extensively to current knowledge of CAN function. It holds promise to serve as a biomarker in disease states. With ongoing advancements in fMRI technology, there is great opportunity and need for future research involving the CAN.

Altered arterial baroreflex-muscle metaboreflex interaction in heart failure

Two powerful reflexes controlling cardiovascular function during exercise are the muscle metaboreflex and arterial baroreflex. In heart failure (HF), the strength and mechanisms of these reflexes are altered. Muscle metaboreflex activation (MMA) in normal subjects increases mean arterial pressure (MAP) primarily via increases in cardiac output (CO), whereas in HF the mechanism shifts to peripheral vasoconstriction. Baroreceptor unloading increases MAP via peripheral vasoconstriction, and this pressor response is blunted in HF. Baroreceptor unloading during MMA in normal animals elicits an enormous pressor response via combined increases in CO and peripheral vasoconstriction. The mode of interaction between these reflexes is intimately dependent on the parameter (e.g., MAP and CO) being investigated. The interaction between the two reflexes when activated simultaneously during dynamic exercise in HF is unknown. We activated the muscle metaboreflex in chronically instrumented dogs during mild exercise (via graded reductions in hindlimb blood flow) followed by baroreceptor unloading [via bilateral carotid occlusion (BCO)] before and after induction of HF. We hypothesized that BCO during MMA in HF would cause a smaller increase in MAP and a larger vasoconstriction of ischemic hindlimb vasculature, which would attenuate the restoration of blood flow to ischemic muscle observed in normal dogs. We observed that BCO during MMA in HF increases MAP by substantial vasoconstriction of all vascular beds, including ischemic active muscle, and that all cardiovascular responses, except ventricular function, exhibit occlusive interaction. We conclude that vasoconstriction of ischemic active skeletal muscle in response to baroreceptor unloading during MMA attenuates restoration of hindlimb blood flow. NEW & NOTEWORTHY We found that baroreceptor unloading during the muscle metaboreflex in heart failure results in occlusive interaction (except for ventricular function) with significant vasoconstriction of all vascular beds. In addition, restoration of blood flow to ischemic active muscle, via preferentially larger vasoconstriction of nonischemic beds, is significantly attenuated in heart failure.

Skeletal Muscle Myopathy in Heart Failure: the Role of Ejection Fraction

PURPOSE OF REVIEW

This review summarizes: (1) the structural and functional features coupled with pathophysiological factors responsible of skeletal muscle myopathy (SMM) in both heart failure with reduced (HFrEF) and preserved (HFpEF) ejection fraction and (2) the role of exercise as treatment of SMM in these HF-related phenotypes.

RECENT FINDINGS

The recent literature showed two main phenotypes of heart failure (HF): (1) HFrEF primarily due to a systolic dysfunction of the left ventricle and (2) HFpEF, mainly related to a diastolic dysfunction. Exercise intolerance is one of most disabling symptoms of HF and it is shown that persists after the normalization of the central hemodynamic impairments by therapy and/or cardiac surgery including heart transplant. A specific skeletal muscle myopathy (SMM) has been defined as one of the main causes of exercise intolerance in HF. The SMM has been well described in the last 20 years in the HFrEF; on the contrary, few studies are available in HFpEF. Recent evidences have revealed that exercise training counteracts HF-related SMM and in turn ameliorates exercise intolerance.

TRPV1 and BDKRB2 receptor polymorphisms can influence the exercise pressor reflex

KEY POINTS

The mechanisms responsible for the high inter-individual variability in blood pressure responses to exercise remain unclear. Common genetic variants of genes related to the vascular transduction of sympathetic outflow have been investigated, but variants influencing skeletal muscle afferent feedback during exercise have not been explored. Single nucleotide polymorphisms in TRPV1 rs222747 and BDKRB2 rs1799722 receptors present in skeletal muscle were associated with differences in the magnitude of the blood pressure response to static handgrip exercise but not mental stress. The combined effects of TRPV1 rs222747 and BDKRB2 rs1799722 on blood pressure and heart rate responses during exercise were additive, and primarily found in men. Genetic differences in skeletal muscle metaboreceptors may be a risk factor for exaggerated blood pressure responses to exercise.

ABSTRACT

Exercise blood pressure (BP) responses demonstrate high inter-individual variability, which could relate to differences in metabolically sensitive afferent feedback from the exercising muscle. We hypothesized that single-nucleotide polymorphisms (SNPs) in genes encoding metaboreceptors present in group III/IV skeletal muscle afferents can influence the exercise pressor response. Two hundred men and women underwent measurements of continuous BP and heart rate at baseline and during 2 min of static handgrip exercise (30% maximal volitional contraction), post-exercise circulatory occlusion and mental stress (serial subtraction; internal control). Participants were genotyped for SNPs in TRPV1 (rs222747; G/C), ASIC3 (rs2288645; G/A), BDKRB2 (rs1799722; C/T), PTGER2 (rs17197; A/G) and P2RX4 (rs25644; A/G). Exercise systolic BP (19 ± 10 vs. 22 ± 10 mmHg, P = 0.03) was lower in GG versus GC/CC minor allele carriers for TRPV1 rs222747, while exercise diastolic BP (14 ± 7 vs. 17 ± 7 mmHg, P = 0.007) and heart rate (12 ± 8 vs. 15 ± 9 beats min^-1 , P = 0.03) were lower in CC versus CT/TT minor allele carriers for BDKRB2 rs1799722. Individuals carrying both minor alleles for TRPV1 rs222747 and BDKRB2 rs1799722 had greater systolic (22 ± 11 vs. 17 ± 10 mmHg, P = 0.04) and diastolic (18 ± 7 vs. 14 ± 7 mmHg, P = 0.01) BP responses than those with no minor alleles; these differences were larger in men. No differences in BP or heart rate responses were detected during static handgrip with ASIC3 rs2288645, PTGER2 rs17197 or P2RX4 rs25644. None of the selected SNPs were associated with differences during mental stress. These findings demonstrate that variants in TRPV1 and BDKRB2 receptors can contribute to BP differences during static exercise in an additive manner.

Angiotensin II, Oxidative Stress, and Sympathetic Nervous System Hyperactivity in Heart Failure

In congestive heart failure (CHF), sympathetic nervous system is hyperactive. This article reviews current understandings about central and peripheral neural mechanisms underlying sympathetic hyperactivation in this pathological condition. During the development of CHF, renin-angiotensin system (RAS) activities and angiotensin II-mediated oxidative stress become enhanced. Here, on the basis of findings obtained from animal studies, it is examined how RAS overactivation and oxidative stress in central and peripheral nervous systems of CHF mediate sympathetic hyperactivation. Mechanisms by which exercise training in CHF ameliorates RAS overactivation, oxidative stress and sympathetic hyperactivation are also investigated.

Metaboreceptor activation in heart failure with reduced ejection fraction: Linking cardiac and peripheral vascular haemodynamics

NEW FINDINGS

What is the central question of this research? Do patients with heart failure with reduced ejection fraction (HFrEF) exhibit a greater dependence on cardiac or peripheral vascular haemodynamics across multiple levels of muscle metaboreflex activation provoked by postexercise circulatory occlusion? What is the main finding and its importance? The metaboreflex-induced pressor response in HFrEF patients is governed almost entirely by the peripheral circulation, which places a substantial haemodynamic load on the failing heart. This maladaptive response exacerbates the disease-related impairment of systolic function that is a hallmark feature of HFrEF and may therefore contribute to exercise intolerance in this patient group.

ABSTRACT

We sought to evaluate the muscle metaboreflex in heart failure with reduced ejection fraction (HFrEF) patients, with an emphasis on the interaction between cardiac and peripheral vascular haemodynamics across multiple levels of metaboreceptor activation. In 23 HFrEF patients (63 ± 2 years of age) and 15 healthy control subjects (64 ± 3 years of age), we examined changes in mean arterial pressure, cardiac output, systemic vascular conductance, effective arterial elastance, stroke work and forearm deoxyhaemoglobin concentration during metaboreceptor activation elicited by postexercise circulatory occlusion (PECO) after three levels of static-intermittent handgrip exercise (15, 30 and 45% maximal voluntary contraction). Across workloads, the metaboreflex-induced increase in deoxyhaemoglobin and mean arterial pressure were similar between groups. However, in control subjects, the pressor response was driven by changes (Δ) in cardiac output  (Δ495 ± 155, Δ564 ± 156 and Δ666 ± 217 ml min-1 ), whereas this change was accomplished by intensity-dependent reductions in systemic vascular conductance in patients with HFrEF (Δ-4.9 ± 1.5, Δ-9.1 ± 1.9 and Δ-12.7 ± 1.8 ml min mmHg-1 ). This differential response contributed to the exaggerated increases in effective arterial elastance in HFrEF patients compared with control subjects, coupled with a blunted response in stroke work in the HFrEF patients. Together, these findings indicate a preserved role of the metaboreflex-induced pressor response in HFrEF but suggest that this response is governed by changes in the peripheral circulation. The net effect of this response appears to be maladaptive, as it places a substantial haemodynamic load on the left ventricle that may exacerbate left ventricular systolic dysfunction and contribute to exercise intolerance in this patient population.

High-intensity muscle metaboreflex activation attenuates cardiopulmonary baroreflex-mediated inhibition of muscle sympathetic nerve activity

Previous studies have shown that muscle sympathetic nerve activity (MSNA) is reduced during low- and mild-intensity dynamic leg exercise. It has been suggested that such inhibition is mediated by loading of the cardiopulmonary baroreceptors and that this effect is overridden by muscle metaboreflex activation with higher-intensity exercise. However, limited data are available regarding the interaction between the cardiopulmonary baroreflex and the muscle metaboreflex. Therefore, we tested the hypothesis that cardiopulmonary baroreflex-mediated inhibition of MSNA is attenuated during high-intensity muscle metaboreflex activation. In nine young men, MSNA (right peroneal nerve), mean arterial pressure (MAP), and thoracic impedance were recorded. Graded isolation of muscle metaboreflex activation was achieved via postexercise ischemia (PEI) following low (PEI-L)-, moderate (PEI-M)-, and high (PEI-H)-intensity isometric handgrip performed at 20, 30, and 40% maximum voluntary contraction, respectively. Lower-body positive pressure (LBPP, +10 Torr) was applied at rest and during PEI, to load the cardiopulmonary baroreceptors. Handgrip exercise elicited intensity-dependent increases in MSNA and MAP that were maintained during PEI, indicating a graded muscle metaboreflex activation. LBPP at rest significantly decreased MSNA burst frequency (BF: -36.7 ± 4.7%, mean ± SE, P < 0.05), whereas MAP was unchanged. When LBPP was applied during PEI, MSNA BF decreased significantly at PEI-L (-40.0 ± 9.2%, P < 0.05) and PEI-M (-27.0 ± 6.3%, P < 0.05), but not at PEI-H (+1.9 ± 7.1%, P > 0.05). These results suggest that low- and moderate-intensity muscle metaboreflex activation does not modulate the inhibition of MSNA by cardiopulmonary baroreceptor loading, whereas high-intensity metaboreflex activation can override cardiopulmonary baroreflex-mediated inhibition of sympathetic vasomotor outflow. NEW & NOTEWORTHY The interaction between the sympathoinhibitory influence of cardiopulmonary baroreflex and sympathoexcitatory effect of skeletal muscle metaboreflex is not completely understood. In the current study, light- to moderate-intensity muscle metaboreflex activation did not modulate the suppression of muscle sympathetic nerve activity by cardiopulmonary baroreceptor loading, whereas high-intensity muscle metaboreflex activation attenuated the cardiopulmonary baroreflex-mediated inhibition of muscle sympathetic nerve activity. These results provide important information concerning the neural reflex mechanisms regulating sympathetic vasomotor outflow during exercise.

Peripheral vascular function, oxygen delivery and utilization: the impact of oxidative stress in aging and heart failure with reduced ejection fraction

The aging process appears to be a precursor to many age-related diseases, perhaps the most impactful of which is cardiovascular disease (CVD). Heart disease, a manifestation of CVD, is the leading cause of death in the USA, and heart failure (HF), a syndrome that develops as a consequence of heart disease, now affects almost six million American. Importantly, as this is an age-related disease, this number is likely to grow along with the ever-increasing elderly population. Hallmarks of the aging process and HF patients with a reduced ejection fraction (HFrEF) include exercise intolerance, premature fatigue, and limited oxygen delivery and utilization, perhaps as a consequence of diminished peripheral vascular function. Free radicals and oxidative stress have been implicated in this peripheral vascular dysfunction, as a redox imbalance may directly impact the function of the vascular endothelium. This review aims to bring together studies that have examined the impact of oxidative stress on peripheral vascular function and oxygen delivery and utilization with both healthy aging and HFrEF.

Interindividual variability in muscle sympathetic responses to static handgrip in young men: evidence for sympathetic responder types?

Negative and positive muscle sympathetic nerve activity (MSNA) responders have been observed during mental stress. We hypothesized that similar MSNA response patterns could be identified during the first minute of static handgrip and contribute to the interindividual variability throughout exercise. Supine measurements of multiunit MSNA (microneurography) and continuous blood pressure (Finometer) were recorded in 29 young healthy men during the first (HG1) and second (HG2) minute of static handgrip (30% maximal voluntary contraction) and subsequent postexercise circulatory occlusion (PECO). Responders were identified on the basis of differences from the typical error of baseline total MSNA: 7 negative, 12 positive, and 10 nonresponse patterns. Positive responders demonstrated larger total MSNA responses during HG1 ( P < 0.01) and HG2 ( P < 0.0001); however, the increases in blood pressure throughout handgrip exercise were similar between all groups, as were the changes in heart rate, stroke volume, cardiac output, total vascular conductance, and respiration (all P > 0.05). Comparing negative and positive responders, total MSNA responses were similar during PECO ( P = 0.17) but opposite from HG2 to PECO (∆40 ± 46 vs. ∆-21 ± 62%, P = 0.04). Negative responders also had a shorter time-to-peak diastolic blood pressure during HG1 (20 ± 20 vs. 44 ± 14 s, P < 0.001). Total MSNA responses during HG1 were associated with responses to PECO ( r = 0.39, P < 0.05), the change from HG2 to PECO ( r = -0.49, P < 0.01), and diastolic blood pressure time to peak ( r = 0.50, P < 0.01). Overall, MSNA response patterns during the first minute of static handgrip contribute to interindividual variability and appear to be influenced by differences in central command, muscle metaboreflex activation, and rate of loading of the arterial baroreflex.

Muscle metaboreflex-induced vasoconstriction in the ischemic active muscle is exaggerated in heart failure

When oxygen delivery to active muscle is insufficient to meet the metabolic demand during exercise, metabolites accumulate and stimulate skeletal muscle afferents, inducing a reflex increase in blood pressure, termed the muscle metaboreflex. In healthy individuals, muscle metaboreflex activation (MMA) during submaximal exercise increases arterial pressure primarily via an increase in cardiac output (CO), as little peripheral vasoconstriction occurs. This increase in CO partially restores blood flow to ischemic muscle. However, we recently demonstrated that MMA induces sympathetic vasoconstriction in ischemic active muscle, limiting the ability of the metaboreflex to restore blood flow. In heart failure (HF), increases in CO are limited, and metaboreflex-induced pressor responses occur predominantly via peripheral vasoconstriction. In the present study, we tested the hypothesis that vasoconstriction of ischemic active muscle is exaggerated in HF. Changes in hindlimb vascular resistance [femoral arterial pressure ÷ hindlimb blood flow (HLBF)] were observed during MMA (via graded reductions in HLBF) during mild exercise with and without α₁-adrenergic blockade (prazosin, 50 µg/kg) before and after induction of HF. In normal animals, initial HLBF reductions caused metabolic vasodilation, while reductions below the metaboreflex threshold elicited reflex vasoconstriction, in ischemic active skeletal muscle, which was abolished after α₁-adrenergic blockade. Metaboreflex-induced vasoconstriction of ischemic active muscle was exaggerated after induction of HF. This heightened vasoconstriction impairs the ability of the metaboreflex to restore blood flow to ischemic muscle in HF and may contribute to the exercise intolerance observed in these patients. We conclude that sympathetically mediated vasoconstriction of ischemic active muscle during MMA is exaggerated in HF. NEW & NOTEWORTHY We found that muscle metaboreflex-induced vasoconstriction of the ischemic active skeletal muscle from which the reflex originates is exaggerated in heart failure. This results in heightened metaboreflex activation, which further amplifies the reflex-induced vasoconstriction of the ischemic active skeletal muscle and contributes to exercise intolerance in patients.

Neural Contributions to Muscle Fatigue: From the Brain to the Muscle and Back Again

: During exercise, there is a progressive reduction in the ability to produce muscle force. Processes within the nervous system as well as within the muscles contribute to this fatigue. In addition to impaired function of the motor system, sensations associated with fatigue and impairment of homeostasis can contribute to the impairment of performance during exercise. This review discusses some of the neural changes that accompany exercise and the development of fatigue. The role of brain monoaminergic neurotransmitter systems in whole-body endurance performance is discussed, particularly with regard to exercise in hot environments. Next, fatigue-related alterations in the neuromuscular pathway are discussed in terms of changes in motor unit firing, motoneuron excitability, and motor cortical excitability. These changes have mostly been investigated during single-limb isometric contractions. Finally, the small-diameter muscle afferents that increase firing with exercise and fatigue are discussed. These afferents have roles in cardiovascular and respiratory responses to exercise, and in the impairment of exercise performance through interaction with the motor pathway, as well as in providing sensations of muscle discomfort. Thus, changes at all levels of the nervous system, including the brain, spinal cord, motor output, sensory input, and autonomic function, occur during exercise and fatigue. The mix of influences and the importance of their contribution vary with the type of exercise being performed.

Central vs. peripheral determinants of sympathetic neural recruitment: insights from static handgrip exercise and postexercise circulatory occlusion

Sympathetic outflow is modified during acute homeostatic stress through increased firing of low-threshold axons, recruitment of latent axons, and synaptic delay modifications. However, the role of central mechanisms versus peripheral reflex control over sympathetic recruitment remains unknown. Here, we examined sympathetic discharge patterns during fatiguing static handgrip (SHG) exercise and postexercise circulatory occlusion (PECO) to study the central vs. peripheral reflex elements of sympathetic neural coding. Muscle sympathetic nerve activity (MSNA; microneurography) was measured in six males (25 ± 3 yr) at baseline (3 min) and during 5 min of SHG exercise completed at 20% maximal voluntary contraction. Isolation of the peripheral metaboreflex component was achieved by PECO for 3 min. Action potential (AP) patterns were studied using wavelet-based methodology. Compared with baseline, total MSNA increased by minute 3 of SHG, remaining elevated throughout the duration of exercise and PECO (all P < 0.05). The AP content per burst increased above baseline by minute 4 of SHG (Δ4 ± 2), remaining elevated at minute 5 (Δ6 ± 4) and PECO (Δ4 ± 4; all P < 0.05). Similarly, total AP clusters increased by minute 4 of SHG (Δ5 ± 5) and remained elevated at minute 5 (Δ6 ± 3) and PECO (Δ7 ± 5; all P < 0.01), indicating recruitment of latent subpopulations. Finally, the AP cluster size-latency profile was shifted downward during minutes 4 (-32 ± 22 ms) and 5 (-49 ± 17 ms; both P < 0.05) of SHG but was not different than baseline during PECO (P > 0.05). Our findings suggest that central perceptual factors play a specific role in the synaptic delay aspect of sympathetic discharge timing, whereas peripheral reflex mechanisms affect recruitment of latent axons.

Ventilatory responses to muscle metaboreflex activation in chronic obstructive pulmonary disease

KEY POINTS

Recent evidence indicates a role for group III/IV muscle afferents in reflex control of the human ventilatory response to exercise. Dyspnoea in chronic obstructive pulmonary disease (COPD) may be linked to this reflex response. This study shows that activation of the muscle metaboreflex causes a ventilatory response in COPD patients but not in healthy controls. This indicates abnormal involvement of muscle afferents in the control of ventilation in COPD which may be a contributing factor to exercise dyspnoea.

ABSTRACT

Blockade of thin fibre muscle afferent feedback during dynamic exercise reduces exercise hyperpnoea in health and chronic obstructive pulmonary disease (COPD). Therefore, we hypothesised that activation of the muscle metaboreflex at rest would cause hyperpnoea. We evaluated the effect of muscle metaboreflex activation on ventilation, in resting COPD patients and healthy participants. Following a bout of rhythmic hand grip exercise, post exercise circulatory occlusion (PECO) was applied to the resting forearm to sustain activation of the muscle metaboreflex, in 18 COPD patients (FEV₁ /FVC ratio < 70%), 9 also classified as chronically hypercapnic, and 9 age- and gender-matched controls. The cardiovascular response to exercise and the sustained blood pressure elevation during PECO was similar in patients and controls. During exercise ventilation increased by 6.64 ± 0.84 in controls and significantly (P < 0.05) more, 8.38 ± 0.81 l min^-1 , in patients. During PECO it fell to baseline levels in controls but remained significantly (P < 0.05) elevated by 2.78 ± 0.51 l min^-1 in patients until release of circulatory occlusion, with no significant difference in responses between patient groups. Muscle metaboreflex activation causes increased ventilation in COPD patients but not in healthy participants. Chronic hypercapnia in COPD patients does not exaggerate this response. The muscle metaboreflex appears to be abnormally involved in the control of ventilation in COPD and may be a contributing factor to exercise dyspnoea.

Blood flow restriction training and the exercise pressor reflex: a call for concern

Blood flow restriction (BFR) training (also known as Kaatsu training) is an increasingly common practice employed during resistance exercise by athletes attempting to enhance skeletal muscle mass and strength. During BFR training, blood flow to the exercising muscle is mechanically restricted by placing flexible pressurizing cuffs around the active limb proximal to the working muscle. This maneuver results in the accumulation of metabolites (e.g., protons and lactic acid) in the muscle interstitium that increase muscle force and promote muscle growth. Therefore, the premise of BFR training is to simulate and receive the benefits of high-intensity resistance exercise while merely performing low-intensity resistance exercise. This technique has also been purported to provide health benefits to the elderly, individuals recovering from joint injuries, and patients undergoing cardiac rehabilitation. Since the seminal work of Alam and Smirk in the 1930s, it has been well established that reductions in blood flow to exercising muscle engage the exercise pressor reflex (EPR), a reflex that significantly contributes to the autonomic cardiovascular response to exercise. However, the EPR and its likely contribution to the BFR-mediated cardiovascular response to exercise is glaringly missing from the scientific literature. Inasmuch as the EPR has been shown to generate exaggerated increases in sympathetic nerve activity in disease states such as hypertension (HTN), heart failure (HF), and peripheral artery disease (PAD), concerns are raised that BFR training can be used safely for the rehabilitation of patients with cardiovascular disease, as has been suggested. Abnormal BFR-induced and EPR-mediated cardiovascular complications generated during exercise could precipitate adverse cardiovascular or cerebrovascular events (e.g., cardiac arrhythmia, myocardial infarction, stroke and sudden cardiac death). Moreover, although altered EPR function in HTN, HF, and PAD underlies our concern for the widespread implementation of BFR, use of this training mechanism may also have negative consequences in the absence of disease. That is, even normal, healthy individuals performing resistance training exercise with BFR are potentially at increased risk for deleterious cardiovascular events. This review provides a brief yet detailed overview of the mechanisms underlying the autonomic cardiovascular response to exercise with BFR. A more complete understanding of the consequences of BFR training is needed before this technique is passively explored by the layman athlete or prescribed by a health care professional.

Heart Rate Recovery as a Preoperative Test of Perioperative Complication Risk

The autonomic nervous system plays important physiologic roles in a variety of organ systems. Autonomic dysfunction has been shown to be predictive of increased mortality in patients with cardiovascular disease. Its importance in patients with chronic respiratory disorders has been described in recent years. Here, we summarize the prognostic value of autonomic dysfunction, as reflected by impaired heart rate recovery (HRR), in patients with chronic respiratory disorders, including chronic obstructive pulmonary disease, interstitial lung disease, and lung cancer. We suggest that HRR may be clinically useful in the preoperative physiologic evaluation, specifically in lung cancer patients being considered for surgery.

Heart failure induces changes in acid-sensing ion channels in sensory neurons innervating skeletal muscle

Heart failure is associated with diminished exercise capacity, which is driven, in part, by alterations in exercise-induced autonomic reflexes triggered by skeletal muscle sensory neurons (afferents). These overactive reflexes may also contribute to the chronic state of sympathetic excitation, which is a major contributor to the morbidity and mortality of heart failure. Acid-sensing ion channels (ASICs) are highly expressed in muscle afferents where they sense metabolic changes associated with ischaemia and exercise, and contribute to the metabolic component of these reflexes. Therefore, we tested if ASICs within muscle afferents are altered in heart failure. We used whole-cell patch clamp to study the electrophysiological properties of acid-evoked currents in isolated, labelled muscle afferent neurons from control and heart failure (induced by myocardial infarction) mice. We found that the percentage of muscle afferents that displayed ASIC-like currents, the current amplitudes, and the pH dose-response relationships were not altered in mice with heart failure. On the other hand, the biophysical properties of ASIC-like currents were significantly different in a subpopulation of cells (40%) from heart failure mice. This population displayed diminished pH sensitivity, altered desensitization kinetics, and very fast recovery from desensitization. These unique properties define these channels within this subpopulation of muscle afferents as being heteromeric channels composed of ASIC2a and -3 subunits. Heart failure induced a shift in the subunit composition of ASICs within muscle afferents, which significantly altered their pH sensing characteristics. These results might, in part, contribute to the changes in exercise-mediated reflexes that are associated with heart failure.

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.

Cardiovascular responses to forearm muscle metaboreflex activation during hypercapnia in humans

We characterized the cardiovascular responses to forearm muscle metaboreflex activation during hypercapnia. Ten healthy males participated under three experimental conditions: 1) hypercapnia (HCA, PetCO2 : +10 mmHg, by inhalation of a CO2-enriched gas mixture); 2) muscle metaboreflex activation (MMA, by 5 min of local circulatory occlusion after 1 min of 50% maximum voluntary contraction isometric handgrip under normocapnia); and 3) HCA+MMA. We measured mean arterial pressure (MAP), heart rate (HR), and cardiac output (CO); calculated stroke volume (SV), and total peripheral resistance (TPR); and evaluated myocardial oxygen consumption (MV̇o2) and cardiac work (CW) noninvasively. MAP increased in the three experimental conditions but HCA+MMA led to the highest MAP, CO, and HR. Moreover, HCA+MMA increased SV and was associated with the highest MV̇o2 and CW. HCA and MMA exhibited inhibitory interactions with MAP, HR, TPR, MV̇o2, and CW, increases of which were smaller during HCA+MMA than the sum of the increases during HCA and MMA alone (MAP: +28 ± 2 vs. +34 ± 2 mmHg, P < 0.001; HR: +15 ± 2 vs. +22 ± 3 bpm, P < 0.01; TPR: +1.1 ± 1.4 vs. +3.0 ± 1.5 mmHg·l·min(-1), P < 0.05; MV̇o2: +50.25 ± 4.74 vs. +59.48 ± 5.37 mmHg·min(-1)·10(-2), P < 0.01; CW: +59.10 ± 7.52 vs. +63.67 ± 7.71 ml mmHg·min(-1)·10(-4), P < 0.05). Oppositely, HCA and MMA interactions were linearly additive for CO (+2.3 ± 0.4 l/min) and SV (+13 ± 4 ml). We showed that muscle metaboreflex and hypercapnia interact in healthy humans, reducing vasoconstriction but enhancing SV.