Aldosterone impairs baroreflex sensitivity in healthy adults

Animal studies suggest that acute and chronic aldosterone administration impairs baroreceptor/baroreflex responses. We tested the hypothesis that aldosterone impairs baroreflex control of cardiac period [cardiovagal baroreflex sensitivity (BRS)] and muscle sympathetic nerve activity (MSNA, sympathetic BRS) in humans. Twenty-six young (25 +/- 1 yr old, mean +/- SE) adults were examined in this study. BRS was determined by using the modified Oxford technique (bolus infusion of nitroprusside, followed 60 s later by bolus infusion of phenylephrine) in triplicate before (Pre) and 30-min after (Post) beginning aldosterone (experimental, 12 pmol.kg(-1).min(-1); n = 10 subjects) or saline infusion (control; n = 10). BRS was quantified from the R-R interval-systolic blood pressure (BP) (cardiovagal BRS) and MSNA-diastolic BP (sympathetic BRS) relations. Aldosterone infusion increased serum aldosterone levels approximately fourfold (P < 0.05) and decreased (P < 0.05) cardiovagal (19.0 +/- 2.3 vs. 15.6 +/- 1.7 ms/mmHg Pre and Post, respectively) and sympathetic BRS [-4.4 +/- 0.4 vs. -3.0 +/- 0.4 arbitrary units (AU).beat(-1).mmHg(-1)]. In contrast, neither cardiovagal (19.3 +/- 3.3 vs. 20.2 +/- 3.3 ms/mmHg) nor sympathetic BRS (-3.8 +/- 0.5 vs. -3.6 +/- 0.5 AU.beat(-1).mmHg(-1)) were altered (Pre vs. Post) in the control group. BP, heart rate, and MSNA at rest were similar in experimental and control subjects before and after the intervention. Additionally, neural and cardiovascular responses to a cold pressor test and isometric handgrip to fatigue were unaffected by aldosterone infusion (n = 6 subjects). These data provide direct experimental support for the concept that aldosterone impairs baroreflex function (cardiovagal and sympathetic BRS) in humans. Therefore, aldosterone may be an important determinant/modulator of baroreflex function in humans.     

Effect of losartan, an angiotensin II type 1 receptor antagonist on cardiac autonomic functions of rats during acute and chronic inhibition of nitric oxide synthesis

We studied the effect of losartan on baroreflex sensitivity (BRS) and heart rate variability (HRV) of adult Wistar rats during acute and chronic inhibition of nitric oxide synthesis by N(G)-nitro-L-arginine methyl ester (L-NAME). Chronic L-NAME administration (50 mg/kg per day for 7 days, orally through gavage) increased mean arterial pressure (MAP), heart rate but significantly decreased BRS. In addition, a significant fall of standard deviation of normal RR intervals, total spectral power, high frequency spectral power and a rise of low frequency to high frequency (LF: HF) ratio was seen. Acute L-NAME administration (30 mg/kg, i.v. bolus dose) also raised MAP and impaired HRV but it was associated with augmented BRS for bradycardia reflex. Losartan treatment (10 mg/kg, i.v.) in both acute and chronic L-NAME treated rats, decreased MAP but the difference was not significant. On the other hand, losartan administration normalized depressed BRS for bradycardia reflex and significantly reduced LF to HF ratio in chronic L-NAME treated rats. But this improvement was not observed in acute L-NAME group. These results indicate importance of mechanisms other than renin-angiotensin system in the pressor response of both acute as well as chronic L-NAME. However, autonomic dysregulation especially following chronic L-NAME appears to be partly angiotensin dependent.     

Age dependency of cardiovascular autonomic responses to head-up tilt in healthy subjects.

In elderly subjects, heart rate responses to postural change are attenuated, whereas their vascular responses are augmented. Altered strategy in maintaining blood pressure homeostasis during upright position may result from various cardiovascular changes, including age-related cardiovascular autonomic dysfunction. This exploratory study was conducted to evaluate impact of age on cardiovascular autonomic responses to head-up tilt (HUT) in healthy subjects covering a wide age range. The study population consisted of 63 healthy, normal-weight, nonsmoking subjects aged 23-77 yr. Five-minute electrocardiogram and finger blood pressure recordings were performed in the supine position and in the upright position 5 min after 70 degrees HUT. Stroke volume was assessed from noninvasive blood pressure signals by the arterial pulse contour method. Heart rate variability (HRV) and systolic blood pressure variability (SBPV) were analyzed by using spectral analysis, and baroreflex sensitivity (BRS) was assessed by using sequence and cross-spectral methods. Cardiovascular autonomic activation during HUT consisted of decreases in HRV and BRS and an increase in SBPV. These changes became attenuated with aging. Age correlated significantly with amplitude of HUT-stimulated response of the high-frequency component (r = -0.61, P < 0.001) and the ratio of low-frequency to high-frequency power of HRV (r = -0.31, P < 0.05) and indexes of BRS (local BRS: r = -0.62, P < 0.001; cross-spectral baroreflex sensitivity in the low-frequency range: r = -0.38, P < 0.01). Blood pressure in the upright position was maintained well irrespective of age. However, the HUT-induced increase in heart rate was more pronounced in the younger subjects, whereas the increase in peripheral resistance was predominantly observed in the older subjects. Thus it is likely that whereas the dynamic capacity of cardiac autonomic regulation decreases, vascular responses related to vasoactive mechanisms and vascular sympathetic regulation become augmented with increasing age.     

Controlled 5-mo aerobic training improves heart rate but not heart rate variability or baroreflex sensitivity.

Endurance-trained athletes have increased heart rate variability (HRV), but it is not known whether exercise training improves the HRV and baroreflex sensitivity (BRS) in sedentary persons. We compared the effects of low- and high-intensity endurance training on resting heart rate, HRV, and BRS. The maximal oxygen uptake and endurance time increased significantly in the high-intensity group compared with the control group. Heart rate did not change significantly in the low-intensity group but decreased significantly in the high-intensity group (-6 beats/min, 95% confidence interval; -10 to -1 beats/min, exercise vs. control). No significant changes occurred in either the time or frequency domain measures of HRV or BRS in either of the exercise groups. Exercise training was not able to modify the cardiac vagal outflow in sedentary, middle-aged persons.     

AT1 receptors in the nucleus tractus solitarii mediate the interaction between the baroreflex and the cardiac sympathetic afferent reflex in anesthetized rats

The cardiac "sympathetic afferent" reflex (CSAR) has been reported to increase sympathetic outflow and depress baroreflex function via a central angiotensin II (ANG II) mechanism. In the present study, we examined the role of ANG II type 1 (AT(1)) receptors in the nucleus tractus solitarii (NTS) in mediating the interaction between the CSAR and the baroreflex in anesthetized rats. We examined the effects of bilateral microinjection of AT(1) receptor antagonist losartan (100 pmol) into the NTS on baroreflex control of renal sympathetic nerve activity (RSNA) before and after CSAR activation by epicardial application of capsaicin (0.4 microg). Using single-unit extracellular recording, we further examined the effects of CSAR activation on the barosensitivity of barosensitive NTS neurons and the effects of intravenous losartan (2 mg/kg) on CSAR-induced changes in activity of NTS barosensitive neurons. Bilateral NTS microinjection of losartan significantly attenuated the increases in arterial pressure, heart rate, and RSNA evoked by capsaicin but also markedly (P < 0.01) reversed the CSAR-induced blunted baroreflex control of RSNA (Gain(max) from 1.65 +/- 0.10 to 2.22 +/- 0.11%/mmHg). In 17 of 24 (70.8%) NTS barosensitive neurons, CSAR activation significantly (P < 0.01) inhibited the baseline neuronal activity and attenuated the neuronal barosensitivity. In 11 NTS barosensitive neurons, intravenous losartan effectively (P < 0.01) normalized the decreased neuronal barosensitivity induced by CSAR activation. In conclusion, blockade of NTS AT(1) receptors improved the blunted baroreflex during CSAR activation, suggesting that the NTS plays an important role in processing the interaction between the baroreflex and the CSAR via an AT(1) receptor-dependent mechanism.     

Autonomic control of the respiratory-hemodynamic system in 8-to 11-year-old children with different baroreflex sensitivities

The efficiency of baroreflex control depends on the baroreflex sensitivity (BRS), which is defined as the ratio of the change in the heart rate (HR) to the change in the blood pressure (BP). The BRS value may be used for assessing the autonomic control of the cardiovascular system and the degree of autonomic dysfunction. Until recently, the baroreflex had not been assessed in a large population of healthy subjects. In this study, the BRS was estimated by the ratio of the low-frequency component of the HR spectrum and the low-frequency component of the rhythm of the systolic BP. For assessing the arterial baroreflex in children, the BRSs for spontaneous and induced baroreflexes were compared. Sex-and age-related differences in BRS were found in 8-to-11-year-old children, and correlations between BRS and some spectral components of HR variability (HRV) and BP rhythm variability were determined. Cluster analysis of the BRS calculated for the spontaneous baroreflex at rest was used to distinguish three clusters of subjects (with high, medium, and low BRSs). These clusters differed in the variability of the basic parameter and size and showed sex-related differences.     

Impaired cardiac and sympathetic autonomic control in rats differing in acetylcholine receptor sensitivity

Acetylcholine receptors (AChR) are important in premotor and efferent control of autonomic function; however, the extent to which cardiovascular function is affected by genetic variations in AChR sensitivity is unknown. We assessed heart rate variability (HRV) and baroreflex sensitivity (BRS) in rats bred for resistance (FRL) or sensitivity (FSL) to cholinergic agents compared with Sprague-Dawley rats (SD), confirmed by using hypothermic responses evoked by the muscarinic agonist oxotremorine (0.2 mg/kg i.p.) (n > or = 9 rats/group). Arterial pressure, ECG, and splanchnic sympathetic (SNA) and phrenic (PNA) nerve activities were acquired under anesthesia (urethane 1.3 g/kg i.p.). HRV was assessed in time and frequency domains from short-term R-R interval data, and spontaneous heart rate BRS was obtained by using a sequence method at rest and after administration of atropine methylnitrate (mATR, 2 mg/kg i.v.). Heart rate and SNA baroreflex gains were assessed by using conventional pharmacological methods. FRL and FSL were normotensive but displayed elevated heart rates, reduced HRV and HF power, and spontaneous BRS compared with SD. mATR had no effect on these parameters in FRL or FSL, indicating reduced cardiovagal tone. FSL exhibited reduced PNA frequency, longer baroreflex latency, and reduced baroreflex gain of heart rate and SNA compared with FRL and SD, indicating in FSL dual impairment of cardiac and circulatory baroreflexes. These findings show that AChR resistance results in reduced cardiac muscarinic receptor function leading to cardiovagal insufficiency. In contrast, AChR sensitivity results in autonomic and respiratory abnormalities arising from alterations in central muscarinic and or other neurotransmitter receptors.     

Dynamic time-varying analysis of heart rate and blood pressure variability in cats exposed to short-term chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) contributes to the development of hypertension in patients with obstructive sleep apnea and animal models. However, the early cardiovascular changes that precede CIH-induced hypertension are not completely understood. Nevertheless, it has been proposed that one of the possible contributing mechanisms to CIH-induced hypertension is a potentiation of carotid body (CB) hypoxic chemoreflexes. Therefore, we studied the dynamic responses of heart rate, blood pressure, and their variabilities during acute exposure to different levels of hypoxia after CIH short-term preconditioning (4 days) in cats. In addition, we measured baroreflex sensitivity (BRS) on the control of heart rate by noninvasive techniques. To assess the relationships among these indexes and CB chemoreflexes, we also recorded CB chemosensory discharges. Our data show that short-term CIH reduced BRS, potentiated the increase in heart rate induced by acute hypoxia, and was associated with a dynamic shift of heart rate variability (HRV) spectral indexes toward the low-frequency band. In addition, we found a striking linear correlation (r = 0.97) between the low-to-high frequency ratio of HRV and baseline. CB chemosensory discharges in the CIH-treated cats. Thus, our results suggest that cyclic hypoxic stimulation of the CB by short-term CIH induces subtle but clear selective alterations of HRV and BRS in normotensive cats.     

Improvement of Left Ventricular Function under Cardiac Resynchronization Therapy Goes along with a Reduced Incidence of Ventricular Arrhythmia

Objectives

The beneficial effects of cardiac resynchronization therapy (CRT) are thought to result from favorable left ventricular (LV) reverse remodeling, however CRT is only successful in about 70% of patients. Whether response to CRT is associated with a decrease in ventricular arrhythmias (VA) is still discussed controversially. Therefore, we investigated the incidence of VA in CRT responders in comparison with non-responders.

Methods

In this nonrandomized, two-center, observational study patients with moderate-to-severe heart failure, LV ejection fraction (LVEF) ≤35%, and QRS duration >120 ms undergoing CRT were included. After 6 months patients were classified as CRT responders or non-responders. Incidence of VA was compared between both groups by Kaplan-Meier analysis and Cox regression analysis. ROC analysis was performed to determine the aptitude of LVEF cut-off values to predict VA.

Results

In total 126 consecutive patients (64±11years; 67%male) were included, 74 were classified as responders and 52 as non-responders. While the mean LVEF at baseline was comparable in both groups (25±7% vs. 24±8%; P = 0.4583) only the responder group showed an improvement of LVEF (36±6% vs. 24±7; p<0.0001) under CRT. In total in 56 patients VA were observed during a mean follow-up of 28±14 months, with CRT responders experiencing fewer VA than non-responders (35% vs. 58%, p<0.0061). Secondary preventive CRT implantation was associated with a higher likelihood of VA. As determined by ROC analysis an increase of LVEF by >7% was found to be a predictor of a significantly lower incidence of VA (AUC = 0.606).

Conclusions

Improvement of left ventricular function under cardiac resynchronization therapy goes along with a reduced incidence of ventricular arrhythmia.     

Postnatal intermittent hypoxia alters baroreflex function in adult rats

Chronic perinatal intermittent hypoxia (IH) could have long-term cardiovascular effects by altering baroreflex function. To examine this hypothesis, we exposed rats (n = 6/group) for postnatal days 1-30 or prenatal embryonic days 5-21 to IH (8% ambient O2 for 90 s after 90 s of 21% of O2, 12 h/day) or to normoxia (control). Baroreflex sensitivity (BRS) and cardiac chronotropic responses were examined in anesthetized animals 3.5-5 mo later by infusing phenylephrine or sodium nitroprusside (6-12 microg/min iv, 1-2 min) during normoxia and after 18 min of acute IH (IHA). In controls after IHA, baroreflex gain was 42% (P < 0.05) less than during normoxia. BRS in the postnatal IH group during normoxia was approximately 50% less than in control rats and similar to controls after IHA. The heart rate response to phenylephrine in the IH group was also less than in controls (P < 0.05) and was not changed by IHA. BRS and heart rate responses in the prenatal IH group were similar to the normoxic control group. Vagal efferent projections to atrial ganglia neurons in rats after postnatal IH (n = 4) were examined by injecting tracer into the left nucleus ambiguous. After 35 days of postnatal IH, basket ending density was reduced by 17% (P < 0.001) and vagal axon varicose contacts by 56% (P < 0.001) compared with controls. We conclude that reduction of vagal efferent projections in cardiac ganglia could be a cause of long-term modifications in baroreflex function.     

Ascorbic acid increases cardiovagal baroreflex sensitivity in healthy older men

Cardiovagal baroreflex sensitivity (BRS) declines with advancing age in healthy men. We tested the hypothesis that oxidative stress contributes mechanistically to this age-associated reduction. Eight young (23 +/- 1 yrs, means +/- SE) and seven older (63 +/- 3) healthy men were studied. Cardiovagal BRS was assessed using the modified Oxford technique (bolus infusion of 50-100 microg sodium nitroprusside, followed 60 s later by a 100- to 150-microg bolus of phenylephrine hydrochloride) in triplicate at baseline and during acute intravenous ascorbic acid infusion. At baseline, cardiovagal BRS (slope of the linear portion of the R-R interval-systolic blood pressure relation during pharmacological changes in arterial blood pressure) was 56% lower (P < 0.01) in older (8.3 +/- 1.6 ms/mmHg) compared with young (19.0 +/- 3.1 ms/mmHg) men. Ascorbic acid infusion increased plasma concentrations similarly in young (62 +/- 9 vs. 1,249 +/- 72 micromol/l for baseline and during ascorbic acid; P < 0.05) and older men (62 +/- 4 vs. 1,022 +/- 55 micromol/l; P < 0.05) without affecting baseline blood pressure, heart rate, carotid artery compliance, or the magnitude of change in systolic blood pressure in response to bolus sodium nitroprusside and phenylephrine hydrochloride infusion. Ascorbic acid (vitamin C) infusion increased cardiovagal BRS in older (Delta58 +/- 16%; P < 0.01), but not younger (Delta - 4 +/- 4%) men. These data provide experimental support for the concept that oxidative stress contributes mechanistically to age-associated reductions in cardiovagal BRS in healthy men.     

Autonomic control of the heart is altered in Sprague-Dawley rats with spontaneous hydronephrosis

The renal medulla plays an important role in cardiovascular regulation, through interactions with the autonomic nervous system. Hydronephrosis is characterized by substantial loss of renal medullary tissue. However, whether alterations in autonomic control of the heart are observed in this condition is unknown. Thus we assessed resting hemodynamics and baroreflex sensitivity (BRS) for control of heart rate in urethane/chloralose-anesthetized Sprague-Dawley rats with normal or hydronephrotic kidneys. While resting arterial pressure was similar, heart rate was higher in rats with hydronephrosis (290 ± 12 normal vs. 344 ± 11 mild/moderate vs. 355 ± 13 beats/min severe; P < 0.05). The evoked BRS to increases, but not decreases, in pressure was lower in hydronephrotic rats (1.06 ± 0.06 normal vs. 0.72 ± 0.10 mild/moderate vs. 0.63 ± 0.07 ms/mmHg severe; P < 0.05). Spectral analysis methods confirmed reduced parasympathetic function in hydronephrosis, with no differences in measures of indirect sympathetic activity among conditions. As a secondary aim, we investigated whether autonomic dysfunction in hydronephrosis is associated with activation of the renin-angiotensin system (RAS). There were no differences in circulating angiotensin peptides among conditions, suggesting that the impaired autonomic function in hydronephrosis is independent of peripheral RAS activation. A possible site for angiotensin II-mediated BRS impairment is the solitary tract nucleus (NTS). In normal and mild/moderate hydronephrotic rats, NTS administration of the angiotensin II type 1 receptor antagonist candesartan significantly improved the BRS, suggesting that angiotensin II provides tonic suppression to the baroreflex. In contrast, angiotensin II blockade produced no significant effect in severe hydronephrosis, indicating that at least within the NTS baroreflex suppression in these animals is independent of angiotensin II.     

Attenuated autonomic function in multiple organ dysfunction syndrome across three age groups.

Multiple organ dysfunction syndrome (MODS) is the failure of several organs after a trigger event. The mortality is high, at up to 70%. We hypothesize that autonomic dysfunction may substantially contribute to the development of MODS and speculate that there is an age dependence of autonomic dysfunction in MODS. A total of 90 consecutively admitted MODS patients were assigned to this study. Three variables of autonomic function were analyzed: heart rate variability (HRV), baroreflex sensitivity (BRS) and chemoreflex sensitivity (CRS). The patient cohort was divided into three age groups. The main finding was that BRS, CRS and almost all indices of HRV were attenuated in comparison to normal range data and there was no age dependence for HRV indices or CRS, but there was for BRS. In conclusion, autonomic function in MODS is attenuated. The influence of MODS on autonomic function overwhelms the age dependence of autonomic function observed in healthy subjects.     

Baroreflex sensitivity, blood pressure buffering, and resonance: what are the links? Computer simulation of healthy subjects and heart failure patients.

The arterial baroreflex buffers slow (<0.05 Hz) blood pressure (BP) fluctuations, mainly by controlling peripheral resistance. Baroreflex sensitivity (BRS), an important characteristic of baroreflex control, is often noninvasively assessed by relating heart rate (HR) fluctuations to BP fluctuations; more specifically, spectral BRS assessment techniques focus on the BP-to-HR transfer function around 0.1 Hz. Skepticism about the relevance of BRS to characterize baroreflex-mediated BP buffering is based on two considerations: 1) baroreflex-modulated peripheral vasomotor function is not necessarily related to baroreflex-HR transfer; and 2) although BP fluctuations around 0.1 Hz (Mayer waves) might be related to baroreflex BP buffering, they are merely a not-intended side effect of a closed-loop control system. To further investigate the relationship between BRS and baroreflex-mediated BP buffering, we set up a computer model of baroreflex BP control to simulate normal subjects and heart failure patients. Output variables for various randomly chosen combinations of feedback gains in the baroreflex arms were BP resonance, BP-buffering capacity, and BRS. Our results show that BP buffering and BP resonance are related expressions of baroreflex BP control and depend strongly on the sympathetic gain to the peripheral resistance. BRS is almost uniquely determined by the vagal baroreflex gain to the sinus node. In conclusion, BP buffering and BRS are unrelated unless coupled gains in all baroreflex limbs are assumed. Hence, the clinical benefit of a high BRS is most likely to be attributed to vagal effects on the heart instead of to effective BP buffering.     

Reduced baroreflex sensitivity in alcoholic cirrhosis: relations to hemodynamics and humoral systems

In cirrhosis, arterial vasodilatation leads to central hypovolemia and activation of the sympathetic nervous and renin-angiotensin-aldosterone systems. As the liver disease and circulatory dysfunction may affect baroreflex sensitivity (BRS), we assessed BRS in a large group of patients with cirrhosis and in controls who were all supine and some after 60 degrees passive head-up and 30 degrees head-down tilting in relation to central hemodynamics and activity of the sympathetic nervous and renin-angiotensin-aldosterone systems. One-hundred and five patients (Child classes A/B/C: 21/55/29) and 25 (n=11 + 14) controls underwent a full hemodynamic investigation. BRS was assessed by cross-spectral analysis of variabilities between blood pressure and heart rate time series. The median BRS was significantly lower in the supine cirrhotic patients, 3.7 (range 0.3-30.7) ms/mmHg than in matched controls (n=11): 14.3 (6.1-23.6) ms/mmHg, P<0.001. A stepwise multiple-regression analysis revealed that serum sodium (P=0.044), heart rate (P=0.027), and central circulation time (P=0.034) independently correlated with BRS. Head-down tilting had no effects on BRS, but, after head-up tilting, BRS was similar in the patients (n=23) and controls (n=14). In conclusion, BRS is reduced in cirrhosis in the supine position and relates to various aspects of cardiovascular dysfunction, but no further reduction was observed in parallel with the amelioration of the hyperdynamic circulation after head-up tilting. The results indicate that liver dysfunction and compensatory mechanisms to vasodilatation may be involved in the low BRS, which may contribute to poor cardiovascular adaptation in cirrhosis.     

Autonomic cardiac control in animal models of cardiovascular diseases. I. Methods of variability analysis.

Analysis of heart rate variability (HRV) and blood pressure variability (BPV) and baroreceptor sensitivity (BRS) has become a proven tool in clinical cardiovascular diagnostics and risk stratification. In the present work, traditional and new methodological approaches for analysis of HRV, BPV, and BRS data are summarized. HRV, BPV, and BRS parameters were obtained from animal studies designed to study pathogenetic mechanisms of distinct cardiovascular diseases. Different non-linear approaches for HRV and BPV analysis are presented here, in particular measures of complexity based on symbolic dynamics. The dual sequence method (DSM) was employed for BRS analysis. In comparison to the classical measure of BRS using the average slope [ms/mm Hg], DSM offers additional information about the time-variant coupling between BPV and HRV. Since cardiovascular regulation shares common features among different species, data on HRV and BPV, as well as BRS, in animal models might be useful for understanding the pathogenetic mechanisms of cardiovascular diseases in humans and in the development of new diagnostic approaches.     

Increased sympathetic and decreased parasympathetic cardiovascular modulation in normal humans with acute sleep deprivation.

Cardiovascular autonomic modulation during 36 h of total sleep deprivation (SD) was assessed in 18 normal subjects (16 men, 2 women, 26.0 +/- 4.6 yr old). ECG and continuous blood pressure (BP) from radial artery tonometry were obtained at 2100 on the first study night (baseline) and every subsequent 12 h of SD. Each measurement period included resting supine, seated, and seated performing computerized tasks and measured vigilance and executive function. Subjects were not supine in the periods between measurements. Spectral analysis of heart rate variability (HRV) and BP variability (BPV) was computed for cardiac parasympathetic modulation [high-frequency power (HF)], sympathetic modulation [low-frequency power (LF)], sympathovagal balance (LF/HF power of R-R variability), and BPV sympathetic modulation (at LF). All spectral data were expressed in normalized units [(total power of the components/total power-very LF) x 100]. Spontaneous baroreflex sensitivity (BRS), based on systolic BP and pulse interval powers, was also measured. Supine and sitting, BPV LF was significantly increased from baseline at 12, 24, and 36 h of SD. Sitting, HRV LF was increased at 12 and 24 h of SD, HRV HF was decreased at 12 h SD, and HRV LF/HF power of R-R variability was increased at 12 h of SD. BRS was decreased at 24 h of SD supine and seated. During the simple reaction time task (vigilance testing), the significantly increased sympathetic and decreased parasympathetic cardiac modulation and BRS extended through 36 h of SD. In summary, acute SD was associated with increased sympathetic and decreased parasympathetic cardiovascular modulation and decreased BRS, most consistently in the seated position and during simple reaction-time testing.     

Age dependency and correlation of heart rate variability,blood pressure variability and baroreflex sensitivity

Simultaneous analysis of heart rate variability (HRV), blood pressure variability (BPV) and baroreflex sensitivity (BRS) with different types of measures may provide non-duplicative information about autonomic cardiovascular regulation. Therefore, a multiple signal analysis of cardiovascular time series will enhance the physiological understanding of neuro cardiovascular regulation with deconditioning in bedrest or related gravitational physiological studies. It has been shown that age is an important determinant of HRV and BRS in healthy subjects. Whereas in the case of BPV, the effect of aging seems to depend upon the activity status of the subjects. In view of the facts that most of the previous works were dealing with only the variability of one kind of cardiovascular parameters in one study with conventional time-domain and/or frequency-domain analysis, we therefore designed the present work to compare the HRV, BPV and BRS between young and middle-aged male healthy subjects in one study with the same subjects using various techniques, including the approximate entropy (ApEn) measurement, a statistic quantifying HRV "complexity" derived from non-linear dynamics.     

Autonomic Recovery Is Delayed in Chinese Compared with Caucasian following Treadmill Exercise

Caucasian populations have a higher prevalence of cardiovascular disease (CVD) when compared with their Chinese counterparts and CVD is associated with autonomic function. It is unknown whether autonomic function during exercise recovery differs between Caucasians and Chinese. The present study investigated autonomic recovery following an acute bout of treadmill exercise in healthy Caucasians and Chinese. Sixty-two participants (30 Caucasian and 32 Chinese, 50% male) performed an acute bout of treadmill exercise at 70% of heart rate reserve. Heart rate variability (HRV) and baroreflex sensitivity (BRS) were obtained during 5-min epochs at pre-exercise, 30-min, and 60-min post-exercise. HRV was assessed using frequency [natural logarithm of high (LnHF) and low frequency (LnLF) powers, normalized high (nHF) and low frequency (nLF) powers, and LF/HF ratio] and time domains [Root mean square of successive differences (RMSSD), natural logarithm of RMSSD (LnRMSSD) and R–R interval (RRI)]. Spontaneous BRS included both up-up and down-down sequences. At pre-exercise, no group differences were observed for any HR, HRV and BRS parameters. During exercise recovery, significant race-by-time interactions were observed for LnHF, nHF, nLF, LF/HF, LnRMSSD, RRI, HR, and BRS (up-up). The declines in LnHF, nHF, RMSSD, RRI and BRS (up-up) and the increases in LF/HF, nLF and HR were blunted in Chinese when compared to Caucasians from pre-exercise to 30-min to 60-min post-exercise. Chinese exhibited delayed autonomic recovery following an acute bout of treadmill exercise. This delayed autonomic recovery may result from greater sympathetic dominance and extended vagal withdrawal in Chinese.Trial Registration: Chinese Clinical Trial Register ChiCTR-IPR-15006684     

Baroreflex sensitivity in obesity: relationship with cardiac autonomic nervous system activity

Objective: The aim of this study was to test the hypothesis that baroreflex sensitivity (BRS), assessed by indirect measurement of aortic pressure, is blunted in obesity. Additionally, the potential effect of cardiac autonomic nervous system (ANS) activity, aortic compliance, and metabolic parameters on BRS of obese subjects was investigated. Research Methods and Procedures: A group of 30 women with BMI >30 kg/m² and a group of 30 controls with BMI <25 kg/m² were examined. BRS was estimated by the sequence technique, cardiac ANS activity by short‐term spectral analysis of heart rate variability (HRV), and aortic compliance by the method of applanation tonometry. Results: BRS was lower in obese women (9.18 ± 3.77 vs. 19.63 ± 9.16 ms/mm Hg, p < 0.001). The median values (interquartile range) of the power of both the high‐frequency and low‐frequency components of the HRV were higher in the lean than in the obese participants [1079.2 (202.7 to 1716.9) vs. 224.1 (72.7 to 539.6) msec², p = 0.001 and 411.8 (199.3 to 798.0) vs. 235.8 (99.4 to 424.5) msec², p = 0.01 respectively]. Low‐to‐high‐frequency ratio values were higher in the obese subjects [0.82 (0.47 to 2.1) vs. 0.57 (0.28 to 0.89), p = 0.02]. Aortic augmentation values were not significantly different between lean and obese subjects. Multivariate analysis demonstrated a significant and independent association between BRS and age (p = 0.003), BMI (p < 0.001), and high‐frequency power of HRV (p < 0.001). These variables explained 72% of the variation of BRS values. Discussion: BRS is severely reduced in obese subjects. BMI, age, and the parasympathetic nervous system activity are the main determinants of BRS. Baroreflex behavior is of clinical relevance because an attenuated BRS represents a negative prognostic factor in cardiovascular diseases, which are common in obesity.