Attenuated respiratory modulation of chemoreflex-mediated sympathoexcitation in patients with chronic heart failure

Clinical Implications of Changes in Respiratory Instability Following Transcatheter Aortic Valve Replacement

BACKGROUND

Respiratory instability, which can be quantified using respiratory stability time (RST), is associated with the severity and prognostic impact of the disease in patients with chronic heart failure. However, its clinical implications in patients with severe aortic stenosis receiving transcatheter aortic valve replacement (TAVR) remain unknown.

METHODS

Patients who received TAVR and had paired measurements of RST at a baseline and one week following TAVR were prospectively included. Changes in RST following TAVR and its impact on post-TAVR heart failure readmissions were investigated.

RESULTS

Seventy-one patients (median age, 86 years old; 35% men) were included. The baseline RST was correlated with the severity of heart failure including elevated levels of plasma B-type natriuretic peptide (p < 0.05 for all). RST improved significantly following TAVR from 34 (26, 37) s to 36 (33, 38) s (p < 0.001). Post-TAVR lower RST (<33 s, n = 18) was associated with a higher 2-year cumulative incidence of heart failure readmission (21% vs. 8%, p = 0.039) with a hazard ratio of 5.47 (95% confidence interval 0.90-33.2).

CONCLUSION

Overall, respiratory instability improved following TAVR. Persistent respiratory instability following TAVR was associated with heart failure recurrence.

Sympathetic activation: a potential link between comorbidities and COVID-19

In coronavirus disease 2019 (COVID-19), higher morbidity and mortality are associated with age, male gender, and comorbidities, such as chronic lung diseases, cardiovascular pathologies, hypertension, kidney diseases, diabetes mellitus, and obesity. All of the above conditions are characterized by increased sympathetic discharge, which may exert significant detrimental effects on COVID-19 patients, through actions on the lungs, heart, blood vessels, kidneys, metabolism, and/or immune system. Furthermore, COVID-19 may also increase sympathetic discharge, through changes in blood gases (chronic intermittent hypoxia, hyperpnea), angiotensin-converting enzyme (ACE)1/ACE2 imbalance, immune/inflammatory factors, or emotional distress. Nevertheless, the potential role of the sympathetic nervous system has not yet been considered in the pathophysiology of COVID-19. In our opinion, sympathetic overactivation could represent a so-far undervalued mechanism for a vicious circle between COVID-19 and comorbidities.

Independent prognostic importance of respiratory instability and sympathetic nerve activity in patients with chronic heart failure

BACKGROUND

Respiratory instability in chronic heart failure (CHF) is characterized by irregularly rapid respiration or non-periodic breathing rather than by Cheyne-Stokes respiration. We developed a new quantitative measure of respiratory instability (RSI) and examined its independent prognostic impact upon CHF.

METHODS

In 87 patients with stable CHF, respiratory flow and muscle sympathetic nerve activity (MSNA) were simultaneously recorded. RSI was calculated from the frequency distribution of respiratory spectral components and very low frequency components.

RESULTS

During a mean follow-up of 85±38 months, 24 patients died. Sixteen patients who died of cardiac causes had a lower RSI (16±6 vs. 30±21, p<0.01), a lower specific activity scale (4.3±1.4 Mets vs. 5.7±1.4 Mets, p<0.005), a higher MSNA burst area (16±5% vs. 11±4%, p<0.001), and a higher brain natriuretic peptide (BNP) level (514±559pg/ml vs. 234±311pg/ml, p<0.05) than 71 patients who did not die of cardiac causes. Multivariate analysis revealed that RSI (p=0.015), followed by MSNA burst area (p=0.033), was an independent predictor of subsequent all-cause deaths and that RSI (p=0.026), MSNA burst area (p=0.001), and BNP (p=0.048) were independent predictors of cardiac deaths. Patients at very high risk of fatal outcome could be identified by an RSI<20.

CONCLUSIONS

The daytime respiratory instability quantified by a new measure of RSI has prognostic importance independent of sympathetic nerve activation in patients with clinically stable CHF. An RSI of <20 identifies patients at very high risk for subsequent all-cause and cardiovascular death.

Central Chemoreceptor Sensitivity Is Not Enhanced in Contemporary Patients With Chronic Systolic Heart Failure Receiving Optimal Treatment

BACKGROUND

Clinical and prognostic consequences of enhanced central chemosensitivity in the contemporary optimally treated patients with chronic heart failure (CHF) are unknown.

METHODS AND RESULTS

We studied central chemosensitivity (defined as hypercapnic ventilatory response [HCVR; L/min/mmHg]) in 161 CHF patients (mean left ventricular ejection fraction [LVEF] 31 ± 6%, all receiving a combination of angiotensin-converting enzyme inhibitor/angiotensin receptor blocker and beta-blocker) and 55 sex- and age-matched healthy controls. HCVR did not differ between CHF patients and controls (median 0.63 vs 0.57 L/min^-1/mmHg^-1, P = .76). When the CHF patients were divided into tertiles according to their HCVR values, there were no significant differences in clinical characteristics (except for ischemic etiology, which was more frequent in those with the highest HCVR), results of the cardiopulmonary exercise testing, and indices of heart rate variability. During the follow-up (median 28 months, range 1-48 months, ≥15 months in all survivors), 21 patients died. HCVR was not related to survival in the Cox proportional hazards analysis.

CONCLUSIONS

Central chemosensitivity is not enhanced in contemporary, optimally treated CHF patients and its assessment does not provide significant clinical or prognostic information.

Restrictive Lung Function Is Related to Sympathetic Hyperactivity in Patients With Heart Failure

BACKGROUND

Sympathoexcitation and impaired lung function are common in patients with severe heart failure (HF). However, the association between impaired lung function and sympathoexcitation remains unknown.

METHODS AND RESULTS

Muscle sympathetic nerve activity (MSNA) and clinical variables were determined in 83 HF patients with left ventricular ejection fraction (LVEF) <0.45. Restrictive and obstructive changes on spirometry were defined as reduced forced vital capacity (FVC) of <80% of predicted and a ratio of forced expiratory volume in the first second to FVC of <70%, respectively. Restrictive and obstructive changes were identified in 17 and 21 patients, respectively. MSNA was higher in patients with restrictive changes than in those without restrictive changes (84 vs 66 bursts per 100 beats; P < .01), but was similar in those with and without obstructive changes. Univariate analyses showed that FVC, estimated glomerular filtration rate (eGFR), specific activity scale, B-type natriuretic peptide level, LVEF, age, and use of aldosterone receptor blockers were significant predictors of MSNA burst incidence. Multivariate analysis revealed that FVC, LVEF, and eGFR were independent factors for increased burst incidence. Changes in FVC during follow-up negatively correlated with changes in burst rate (n = 11; P < .01).

CONCLUSION

Restrictive lung function was associated with increased sympathetic nerve activity independently from HF severity.

Slow and deep respiration suppresses steady-state sympathetic nerve activity in patients with chronic heart failure: from modeling to clinical application

Influences of slow and deep respiration on steady-state sympathetic nerve activity remain controversial in humans and could vary depending on disease conditions and basal sympathetic nerve activity. To elucidate the respiratory modulation of steady-state sympathetic nerve activity, we modeled the dynamic nature of the relationship between lung inflation and muscle sympathetic nerve activity (MSNA) in 11 heart failure patients with exaggerated sympathetic outflow at rest. An autoregressive exogenous input model was utilized to simulate entire responses of MSNA to variable respiratory patterns. In another 18 patients, we determined the influence of increasing tidal volume and slowing respiratory frequency on MSNA; 10 patients underwent a 15-min device-guided slow respiration and the remaining 8 had no respiratory modification. The model predicted that a 1-liter, step increase of lung volume decreased MSNA dynamically; its nadir (−33 ± 22%) occurred at 2.4 s; and steady-state decrease (−15 ± 5%), at 6 s. Actually, in patients with the device-guided slow and deep respiration, respiratory frequency effectively fell from 16.4 ± 3.9 to 6.7 ± 2.8/min ( P < 0.0001) with a concomitant increase in tidal volume from 499 ± 206 to 1,177 ± 497 ml ( P < 0.001). Consequently, steady-state MSNA was decreased by 31% ( P < 0.005). In patients without respiratory modulation, there were no significant changes in respiratory frequency, tidal volume, and steady-state MSNA. Thus slow and deep respiration suppresses steady-state sympathetic nerve activity in patients with high levels of resting sympathetic tone as in heart failure.

Effect of chronic ethanol exposure on rat ventilatory responses to hypoxia and hypercapnia

OBJECTIVE

The effect of chronic ethanol exposure on chemoreflexes has not been extensively studied in experimental animals. Therefore, this study tested the hypothesis that known ethanol-induced autonomic, neuroendocrine and cardiovascular changes coincide with increased chemoreflex sensitivity, as indicated by increased ventilatory responses to hypoxia and hypercapnia.

METHODS

Male Wistar rats were subjected to increasing ethanol concentrations in their drinking water (first week: 5% v/v, second week: 10% v/v, third and fourth weeks: 20% v/v). At the end of each week of ethanol exposure, ventilatory parameters were measured under basal conditions and in response to hypoxia (evaluation of peripheral chemoreflex sensitivity) and hypercapnia (evaluation of central chemoreflex sensitivity).

RESULTS

Decreased respiratory frequency was observed in rats exposed to ethanol from the first until the fourth week, whereas minute ventilation remained unchanged. Moreover, we observed an increased tidal volume in the second through the fourth week of exposure. The minute ventilation responses to hypoxia were attenuated in the first through the third week but remained unchanged during the last week. The respiratory frequency responses to hypoxia in ethanol-exposed rats were attenuated in the second through the third week but remained unchanged in the first and fourth weeks. There was no significant change in tidal volume responses to hypoxia. With regard to hypercapnic responses, no significant changes in ventilatory parameters were observed.

CONCLUSIONS

Our data are consistent with the notion that chronic ethanol exposure does not increase peripheral or central chemoreflex sensitivity.

Parasympathetic activation by pyridostigmine on chemoreflex sensitivity in heart-failure rats

We evaluated the effects of parasympathetic activation by pyridostigmine (PYR) on chemoreflex sensitivity in a rat model of heart failure (HF rats). HF rats demonstrated higher pulmonary ventilation (PV), which was not affected by PYR. When HF and control rats treated or untreated with PYR were exposed to 15% O2, all groups exhibited prompt increases in respiratory frequency (RF), tidal volume (TV) and PV. When HF rats were exposed to 10% O2 they showed greater PV response which was prevented by PYR. The hypercapnia triggered by either 5% CO2 or 10% CO2 promoted greater RF and PV responses in HF rats. PYR blunted the RF response in HF rats but did not affect the PV response. In conclusion, PYR prevented increased peripheral chemoreflex sensitivity, partially blunted central chemoreflex sensitivity and did not affect basal PV in HF rats.

Sympathetic nervous system activation in human heart failure: clinical implications of an updated model

Disturbances in cardiovascular neural regulation, influencing both disease course and survival, progress as heart failure worsens. Heart failure due to left ventricular systolic dysfunction has long been considered a state of generalized sympathetic activation, itself a reflex response to alterations in cardiac and peripheral hemodynamics that is initially appropriate, but ultimately pathological. Because arterial baroreceptor reflex vagal control of heart rate is impaired early in heart failure, a parallel reduction in its reflex buffering of sympathetic outflow has been assumed. However, it is now recognized that: 1) the time course and magnitude of sympathetic activation are target organ-specific, not generalized, and independent of ventricular systolic function; and 2) human heart failure is characterized by rapidly responsive arterial baroreflex regulation of muscle sympathetic nerve activity (MSNA), attenuated cardiopulmonary reflex modulation of MSNA, a cardiac sympathoexcitatory reflex related to increased cardiopulmonary filling pressure, and by individual variation in nonbaroreflex-mediated sympathoexcitatory mechanisms, including coexisting sleep apnea, myocardial ischemia, obesity, and reflexes from exercising muscle. Thus, sympathetic activation in the setting of impaired systolic function reflects the net balance and interaction between appropriate reflex compensatory responses to impaired systolic function and excitatory stimuli that elicit adrenergic responses in excess of homeostatic requirements. Recent observations have been incorporated into an updated model of cardiovascular neural regulation in chronic heart failure due to ventricular systolic dysfunction, with implications for the clinical evaluation of patients, application of current treatment, and development of new therapies.

Muscle sympathetic nerve activity and ventilation during exercise in subjects with and without chronic heart failure

BACKGROUND

Changes within skeletal muscle, including augmentation of its capacity to elicit reflex increases in both efferent muscle sympathetic nerve activity (MSNA) and ventilation during work, contribute significantly to exercise intolerance in heart failure (HF). Previously, we demonstrated that peak oxygen uptake (pVO(2)) in HF relates inversely to MSNA at rest and during exercise.

OBJECTIVE

To test the hypothesis that there is an independent positive relationship between resting MSNA and the ratio of ventilation to carbon dioxide output during exercise (VE/VCO(2)) that is augmented in HF.

METHODS

MSNA at rest and VE/VCO(2)) during stationary cycling were measured in 30 patients (27 men) with HF (mean +/- SD ejection fraction 20+/-6%) and in 31 age-matched controls (29 men).

RESULTS

MSNA was higher in HF patients than in controls (51.5+/-14.3 bursts/min versus 33.0+/-11.1 bursts/min; P<0.0001). The VE/VCO(2) slope was also higher in HF patients than in controls (33.7+/-5.7 versus 26.0+/-3.5; P<0.0001), whereas pVO(2) was lower in HF patients than in controls (18.6+/-6.6 versus 31.4+/-8.4 mL/kg/min; P<0.0001). There were significant relationships between MSNA and VE/VCO(2) in both HF (r=0.50; P=0.005) and control subjects (r=0.36; P=0.046). The slope of this regression equation was steeper in HF (0.20 versus 0.11 x MSNA; P=0.001). An analysis of covariance for main effects, including age and pVO(2), identified a significant independent relationship between MSNA burst frequency and VE/VCO(2) (P=0.013) that differed between HF and controls (P<0.01).

CONCLUSIONS

The magnitude of resting sympathetic activity correlates positively with the VE/VCO(2) slope. Augmentation of this relationship in HF patients is consistent with the concept that enhanced mechanoreceptor reflex activity exaggerates their ventilatory response to exercise.

Blunted muscle vasodilatation during chemoreceptor stimulation in patients with heart failure

Chemoreflex control of sympathetic nerve activity is exaggerated in heart failure (HF) patients. However, the vascular implications of the augmented sympathetic activity during chemoreceptor activation in patients with HF are unknown. We tested the hypothesis that the muscle blood flow responses during peripheral and central chemoreflex stimulation would be blunted in patients with HF. Sixteen patients with HF (49 +/- 3 years old, Functional Class II-III, New York Heart Association) and 11 age-paired normal controls were studied. The peripheral chemoreflex control was evaluated by inhalation of 10% O(2) and 90% N(2) for 3 min. The central chemoreflex control was evaluated by inhalation of 7% CO(2) and 93% O(2) for 3 min. Muscle sympathetic nerve activity (MSNA) was directly evaluated by microneurography. Forearm blood flow was evaluated by venous occlusion plethysmography. Baseline MSNA were significantly greater in HF patients (33 +/- 3 vs. 20 +/- 2 bursts/min, P = 0.001). Forearm vascular conductance (FVC) was not different between the groups. During hypoxia, the increase in MSNA was significantly greater in HF patients than in normal controls (9.0 +/- 1.6 vs. 0.8 +/- 2.0 bursts/min, P = 0.001). The increase in FVC was significantly lower in HF patients (0.00 +/- 0.10 vs. 0.76 +/- 0.25 units, P = 0.001). During hypercapnia, MSNA responses were significantly greater in HF patients than in normal controls (13.9 +/- 3.2 vs. 2.1 +/- 1.9 bursts/min, P = 0.001). FVC responses were significantly lower in HF patients (-0.29 +/- 0.10 vs. 0.37 +/- 0.18 units, P = 0.001). In conclusion, muscle vasodilatation during peripheral and central chemoreceptor stimulation is blunted in HF patients. This vascular response seems to be explained, at least in part, by the exaggerated MSNA responses during hypoxia and hypercapnia.

Microneurography as a tool in clinical neurophysiology to investigate peripheral neural traffic in humans

Microneurography is a method using metal microelectrodes to investigate directly identified neural traffic in myelinated as well as unmyelinated efferent and afferent nerves leading to and coming from muscle and skin in human peripheral nerves in situ. The present paper reviews how this technique has been used in clinical neurophysiology to elucidate the neural mechanisms of autonomic regulation, motor control and sensory functions in humans under physiological and pathological conditions. Microneurography is particularly important to investigate efferent and afferent neural traffic in unmyelinated C fibers. The recording of efferent discharges in postganglionic sympathetic C efferent fibers innervating muscle and skin (muscle sympathetic nerve activity; MSNA and skin sympathetic nerve activity; SSNA) provides direct information about neural control of autonomic effector organs including blood vessels and sweat glands. Sympathetic microneurography has become a potent tool to reveal neural functions and dysfunctions concerning blood pressure control and thermoregulation. This recording has been used not only in wake conditions but also in sleep to investigate changes in sympathetic neural traffic during sleep and sleep-related events such as sleep apnea. The same recording was also successfully carried out by astronauts during spaceflight. Recordings of afferent discharges from muscle mechanoreceptors have been used to understand the mechanisms of motor control. Muscle spindle afferent information is particularly important for the control of fine precise movements. It may also play important roles to predict behavior outcomes during learning of a motor task. Recordings of discharges in myelinated afferent fibers from skin mechanoreceptors have provided not only objective information about mechanoreceptive cutaneous sensation but also the roles of these signals in fine motor control. Unmyelinated mechanoreceptive afferent discharges from hairy skin seem to be important to convey cutaneous sensation to the central structures related to emotion. Recordings of afferent discharges in thin myelinated and unmyelinated fibers from nociceptors in muscle and skin have been used to provide information concerning pain. Recordings of afferent discharges of different types of cutaneous C-nociceptors identified by marking method have become an important tool to reveal the neural mechanisms of cutaneous sensations such as an itch. No direct microneurographic evidence has been so far proved regarding the effects of sympathoexcitation on sensitization of muscle and skin sensory receptors at least in healthy humans.

Central Sympathetic Inhibition Augments Sleep-Related Ultradian Rhythm of Parasympathetic Tone in Patients With Chronic Heart Failure

BACKGROUND

Abnormal sleep dynamics in patients with heart failure is one of the mechanisms for the relative predominance of central sympathetic outflow over parasympathetic tone. This study was designed to examine whether central sympathoinhibition could improve the sympathovagal imbalance related to rapid-eye-movement (REM)/non-REM ultradian sleep rhythm in these patients.

METHODS AND RESULTS

Beat-by-beat RR intervals of overnight electrocardiogram were serially subject to power spectral analysis in 14 patients with chronic heart failure and 13 age-matched subjects with normal cardiac function. To assess autonomic sleep dynamics, the ultradian rhythm was extracted from all-night consecutive high-frequency (HF) components of heart rate variability (HRV) before and after administration of an (alpha2)-adrenergic agonist, guanfacine. Night-time HRV in heart failure was characterized by an attenuated ultradian rhythm of HF-components with a concomitant reduction in averaged HF power. Guanfacine reduced blood pressure, heart rate, and plasma norepinephrine concentrations by 7%, 8%, and 34% (p < 0.01), respectively. After guanfacine, HF power rose by 154% (p < 0.01) with a prominent augmentation of the all-night ultradian rhythm (+361%, p < 0.01).

CONCLUSIONS

Central sympathoinhibition augments a sleep-related ultradian rhythm of parasympathetic tone, suggesting a potential benefit to autonomic balancing and sleep quality in patients with chronic heart failure.