Spontaneous Baroreflex Slope Research Articles

The Association Between Cardiovagal Baroreceptor Sensitivity and Hippocampal Tissue Integrity in Young and Middle-aged Adults

Prior research has investigated the association of cardiovagal baroreceptor sensitivity (BRS) with white matter neuronal integrity and cerebral perfusion using magnetic resonance imaging (MRI) techniques such as diffusion tensor imaging (DTI) and arterial spin labeling (ASL); however, less is known about the association with specific regions of gray matter (i.e, hippocampus) involved in memory formation and recall. MR elastography (MRE) has emerged as a constructive tool for assessing the viscoelastic mechanical properties of the brain which are believed to reflect the microstructural integrity of neuronal tissue. PURPOSE: To investigate the association between cardiovagal BRS and the viscoelastic properties of the brain, with a sub goal of examining how advanced age affects this association. We hypothesized that there would be a positive relation between cardiovagal BRS and hippocampal (HC) viscoelastic properties that strengthens with age, indicating a greater influence of blood pressure control on HC microstructural integrity. Methods: Ten young (Yng, 25 ± 2 years) and ten middle-aged adults (MA, 55 ± 3 years) laid in supine position for 10 minutes while arterial blood pressure (ABP) and heart rate (HR) were measured. R-R intervals and systolic blood pressures were plotted within a linear regression to calculate the spontaneous baroreflex slope. Subjects went in an MRI scanner to measure hippocampal viscoelastic properties using MRE. Results: As expected, we observed a lower cBRS in the middle-aged group compared with young (MA: 12.51±4.41 vs. Yng: 25.21±8.77 ms/mmHg, p≤0.05). There were no significant differences in HC stiffness or damping ratio when comparing between age groups (MA: 3.06±0.32 kPa vs. Yng: 3.02±0.09 kPa, p=0.69; MA: 0.2±0.02 vs. Yng: 0.2±0.03, p=0.56). However, a multiple linear regression with age included as a categorical covariate revealed a trend towards a stronger association between HC stiffness and cBRS in the middle-aged compared to the young group (p=0.07). CONCLUSION: In contrast to our hypothesis, preserved BRS was associated with lower HC stiffness in the middle-aged group; however, the physiological importance of this finding needs to be more completely explored. Our findings indicate that the association between short-term blood pressure regulation via cardiovagal BRS may be more closely linked to HC tissue integrity with advancing age. These mechanisms should be explored in a larger cohort including older individuals. Supported by UD Research Foundation 2020 Pilot Grant and P20GM113125. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

CCISS, Vascular and BP Reg: Canadian space life science research on ISS

A comprehensive goal of the Canadian Space Agency studies (CCISS, Vascular and BP Reg) has been to investigate the efficacy of current exercise countermeasures to maintain cardiovascular and cerebrovascular health on return to Earth after up to 6-months in space. Results from the CCISS experiments revealed no significant change of in-flight heart rate during daily activities or sleep, and small, but variable between astronauts, post-flight elevation. The between astronaut differences were exaggerated during measurement of spontaneous baroreflex slope, which was reduced post-flight (P<0.05) during paced breathing with 3 astronauts having significant correlations between reduced baroreflex and reduced RR-interval (consistent with reduced fitness). Cerebrovascular autoregulation and CO2 response were mildly impaired after flight. Some loss of in-flight fitness of astronauts in Vascular was reflected by the increase in HR at a work rate of 161±46W of 12.3±10.5bpm, 10.4±5.9bpm and 13.4±5.7bpm for early-flight, late-flight and R+1, respectively. On return to gravity, changes in resting heart rate for supine (5.9±3.5bpm), sit (8.1±3.3bpm) and stand (10.3±10.0bpm) were small but variable between individuals (from −5bpm to +20bpm in post-flight standing) and not related to the change in exercise heart rate. In Vascular astronauts, pulse wave transit time measured to the finger tended to be reduced post-flight and carotid artery distensibility was significantly reduced (P=0.03, and n=6). The heart rate and baroreflex data suggest that some astronauts return with cardiovascular deconditioning in spite of the exercise regimes. However, greater arterial stiffness is common among all astronauts studied to date. The new CSA project, BP Reg, will monitor inflight blood pressure in an attempt to identify astronauts in greater need for countermeasures. Future research should focus on whether Vascular changes in astronauts might make them an appropriate model to study the mechanisms of arterial aging on Earth.

Sex differences in blood pressure control during 6° head-down tilt bed rest

Spaceflight-induced orthostatic intolerance has been studied for decades. Although ∼22% of the astronaut corps are women, most mechanistic studies use mostly male subjects, despite known sex differences in autonomic control and postflight orthostatic intolerance. We studied adrenergic, baroreflex, and autonomic indexes during continuous infusions of vasoactive drugs in men and women during a 60-day head-down bed rest. Volunteers were tested before bed rest (20 men and 10 women) and around day 30 (20 men and 10 women) and day 60 (16 men and 8 women) of bed rest. Three increasing doses of phenylephrine (PE) and sodium nitroprusside were infused for 10 min after an infusion of normal saline. A 20-min rest period separated the phenylephrine and sodium nitroprusside infusions. Autonomic activity was approximated by spectral indexes of heart rate and blood pressure variability, and baroreflex sensitivity was measured by the spontaneous baroreflex slope. Parasympathetic modulation and baroreflex sensitivity decreased with bed rest, with women experiencing a larger decrease in baroreflex sensitivity by day 30 than men. The sympathetic activation of men and parasympathetic responsiveness of women in blood pressure control during physiological stress were preserved throughout bed rest. During PE infusions, women experienced saturation of the R-R interval at high frequency, whereas men did not, revealing a sex difference in the parabolic relationship between high-frequency R-R interval, a measurement of respiratory sinus arrhythmia, and R-R interval. These sex differences in blood pressure control during simulated microgravity reveal the need to study sex differences in long-duration spaceflight to ensure the health and safety of the entire astronaut corps.

Gender Differences in Baroreflex Sensitivity after Bed Rest

Two potential contributing factors to post-spaceflight orthostatic intolerance are decreases in baroreflex sensitivity (BRS) and sympathetic nervous system response. PURPOSE: The purpose of this study was to examine the shape of the BRS curve and sympathetic response to a wide range of blood pressures (BP) before and during 6° head-down bed rest (BR). METHODS: Normal volunteers were tested 1 day before BR (20M, 10W) and near BR days 30 (20M, 10W), 60 (16M, 8W), and 90 (10M, 5W). BP was pharmacologically manipulated by 10-min infusions of phenylephrine (PE) and sodium nitroprusside (SNP) at 3 increasing concentrations, with a 20-min rest between PE and SNP. Electrocardiogram and continuous finger blood pressure were recorded. A blood sample was drawn at the end of each infusion to measure plasma norepinephrine levels. The spontaneous baroreflex slope (SBS), a measure of BRS, was calculated as the slope of a sequence of 3 or more beats in which the systolic BP (SBP) and following R-R interval (RR) both increased or decreased. The data included saturated responses at the upper but not the lower end of the BP range. Mean response curves were constructed using second-order mixed model analysis. Results are based on term significance in the models. RESULTSRR: RR was lower during BR than before BR (p<0.001). Before BR, men were modeled by a linear RR response to SBP (p=0.000) whereas women had a quadratic response that saturated at high SBP (p=0.019). By day 30, both genders were modeled by a linear response; compared to men, women had an attenuated (lower slope) RR response to changes in SBP (p=0.031). SBS: SBS vs SBP analysis showed a lower SBS during BR (p<0.001) than before BR. Before BR, women had a higher SBS than men (p=0.006). Women exhibited saturating SBS at higher SBP (p=0.016) on day 30, whereas men were modeled by a linear SBS response to SBP (p=0.035). NE: The NE response of women to diastolic BP was different from that of men before BR (p=0.035) and on day 30 (p=0.005). CONCLUSION: NE, RR, and SBS responses to BP are affected by gender and BR. Not only are there main effects of gender and BR, but gender and BR also influence the slope and saturation of the BRS curves. Attenuated and saturating RR and SBS responses, as well as differences in baseline values, may contribute to the higher rates of orthostatic intolerance in women and after bed rest.

Cardiovascular autonomic neuropathy and other complications in type 1 diabetes

This study deals with cardiovascular autonomic neuropathy (CAN) in type 1 diabetic patients and its association with other complications. We searched for CAN in 684 patients (age, 47 +/- 12 years; diabetes duration, 22 +/- 11 years) by cardiovascular responses to deep breathing and standing. Patients considered as positive had laboratory evaluation: "Ewing" tests (deep breathing, Valsalva, stand test, hand grip); heart rate variability (HRV) [low frequency (LF) and high frequency (HF) power] and spontaneous baroreflex slope (SBS). Logistic regression was used to identify the combination of patient characteristics, including other complications, most associated with CAN severity according to Ewing Score (ES 0-5). 66.2% presented no significant abnormality (ES 0-0.5), 21.5 % had mild abnormalities (ES 1-2), and 12.3% had confirmed autonomic failure (ES > 2). Decrease in LF, HF and SBS was highly correlated to CAN severity. In the stepwise regression, age, retinopathy, nephropathy, bladder dysfunction, erectile dysfunction, peripheral neuropathy and hypertension remained correlated with CAN, whereas digestive neuropathy, BMI and HbA1c were excluded. Despite a small number of events, we found a significant association between coronary disorders and CAN severity. Simple bedside tests can detect CAN. HRV and SBS provide additional elements on CAN severity. Diabetes duration did not discriminate sufficiently patients with CAN. The association with retinopathy is in favor of the role of poor glycemic control in CAN development. This study shows the interest of CAN detection and the need to look for extracardiac autonomic neuropathy and silent myocardial ischemia in patients with confirmed CAN.

Baroreflex Sensitivity Decreases During 90-day Bed Rest

Baroreflex sensitivity (BRS) decreases during spaceflight and simulated spaceflight (head down bed rest [BR]). However, previous studies have only examined BRS in response to a limited blood pressure (BP) range or to a single sudden change in BP. PURPOSE: The purpose of this study was to examine BRS during 90 days of 6deg head-down tilt BR over a broad range of BP perturbations. METHODS: Nineteen normal volunteers (12M, 7F) were tested one day before BR, and then near BR days 30, 60 and 90. BP was pharmacologically altered by continuous infusions of phenylephrine (PE) and sodium nitroprusside (SNP). Electrocardiogram and continuous BP were collected during 10 min of normal saline (NS), followed by increasing concentrations of PE (10 min each of 0.4, 0.8 and 1.6 micro-g/kg/min). After a 20 min break, NS was infused again for 10 min, followed by increasing concentrations of SNP (10 min each of 0.4, 0.8, 1.2 micro-g/kg/min). Baroreceptor sensitivity was measured as the slope of a sequence of 3 or more beats in which the systolic BP and following R-R interval (RR) both increased or decreased. Spectral heart rate variability (HRV) and mean RR were analyzed using data from only the NS infusions. Two-way repeated-measures analysis of variance was performed to examine the effects of BR and gender. RESULTS: RR decreased (p<0.001) from pre- BR across BR days. High frequency in normalized units, a measure of parasympathetic activity, decreased with BR (p=0.027) and was lower (p=0.046) in men (0.39+/-0.02, mean+/-SEM) than women (0.48+/-0.02). The spontaneous baroreflex slope, our measure of BRS, increased with PE and decreased with SNP across BR (p<0.001). The percentage decrease in BRS from pre- to post-BR appeared to be larger in women (43.6+/-7.0%) than in men (31.3+/-3.9%, p=0.06). CONCLUSION: Parasympathetic activity and baroreflex sensitivity decrease during 90 days of BR, and BRS tends to diminish more in women than in men.

Hemodynamic, autonomic and baroreflex changes after one night sleep deprivation in healthy volunteers

Background Sleep disorders are associated to a number of cardiovascular disturbances that might increase cardiovascular risk. Sleep deprivation, in particular, might, by inducing autonomic dysregulation, raise arterial pressure and hypertensive risk. Available evidence however is contradictory. Methods We tested the main hypothesis that one night sleep deprivation in 24 volunteers might alter hemodynamics (heart rate and Arterial Pressure — AP), autonomic regulation (mono and bivariate spectral analysis of RR and non invasive AP variability) and baroreflex control (spectral index alpha and spontaneous baroreflex slope), performance indices (reaction time) and subjective stress (questionnaires and salivary cortisol). Volunteers were studied in normal living conditions and while kept in isolation and confinement, to test the presence of possible bias related to environmental stress. Results Results indicate that there were no differences between normal living conditions and isolation and confinement (Intraclass Correlation Coefficient > 0.75 for most variables). Conversely, after one night sleep deprivation subjects felt tired ( p < 0.05), and performance deteriorated ( p < 0.05), while cortisol profile was substantially maintained, hemodynamic parameters did not change and HRV and index alpha increased slightly. Conclusions Findings support the contention that one night sleep deprivation, in absence of significant additional stress or disturbances, does not lead to increased arterial pressure values or to changes in autonomic or baroreflex profiles that could conceivably favor hypertension development, but induces the expected increase in tiredness and reduction in performance.

Analysis of Heart Period and Arterial Pressure Variability in Childhood Hypertension

In adults, initial stages of hypertension are associated with elevated sympathetic drive and significant alterations in indirect autonomic markers. There is growing evidence that children in the highest-pressure percentiles will be more likely to develop hypertension in adulthood, although mechanisms are not understood. We assessed whether computer analysis of RR interval and arterial blood pressure variability could detect early autonomic alterations in childhood hypertension, as assessed by noninvasive time and frequency domain measures of baroreflex regulation. We studied 75 children, subdivided in 3 subgroups of similar age (9.7+/-0.2 years): control subjects, prehypertensive children (ie, children with arterial pressure values >90th and <95th percentile for age, gender, and height), and children in the hypertensive range (ie, >95th percentile; systolic arterial pressure: 97+/-3/57+/-2, 121+/-5/70+/-1, and 128+/-2/80+/-2 mm Hg, respectively). We observed that hypertensive children demonstrate a significant impairment of the baroreflex as compared with control subjects (index alpha: 20+/-2 and 40+/-4 ms/mm Hg; spontaneous baroreflex slope: 20+/-2 and 37+/-5; ms/mm Hg; P<0.05 in both cases) and reduced RR variance. A similar baroreflex impairment is also observed in children whose arterial pressure falls short of this limit, ie, in the prehypertensive range. In conclusion, hypertensive children display a marked baroreflex impairment. A similar baroreflex impairment is also observed in the prehypertensive state. Baroreflex assessment could furnish additional information in the clinical assessment of pediatric hypertension.

Renin Enhancer Is Critical for Control of Renin Gene Expression and Cardiovascular Function

Renin Enhancer Is Critical for Control of Renin Gene Expression and Cardiovascular Function

Spontaneous baroreflex cardiac sensitivity in end-stage liver disease

End-stage liver disease is associated with an imbalance in the autonomic nervous system. The purpose of this study was to estimate the effect of liver transplantation on this imbalance. The study involved 10 patients undergoing liver transplantation and 9 patients without liver impairment undergoing liver surgery. The spontaneous baroreflex sensitivity was measured before and 1 month after surgery for the liver surgery group; before and 1, 3, 6, 12 and 18 months after orthotopic liver transplantation. The spontaneous baroreflex slope of patients with end-stage liver disease was decreased before liver transplantation compared to the liver surgery group (3.9 +/- 2.5 ms mmHg(-1) vs. 9.9 +/- 5.0 ms mmHg(-1), P = 0.002). The mean slope was significantly increased at 12 and 18 months compared to the pre-transplantation value (3.9 +/- 2.5 ms mmHg(-1) vs. 8.1 +/- 6.6 ms mmHg(-1) and 7.4 +/- 4.8 ms mmHg(-1), respectively; P = 0.042). Nevertheless, further analysis of individual data showed that only four patients exhibited a marked increase in their baroreflex slope 12 months after the liver transplantation whereas it remained decreased in the six others. These results confirm that the baroreflex sensitivity is depressed in end-stage liver disease in line with an autonomic nervous system imbalance. The liver transplantation reverses this disturbance only in some patients.

Development Of A Cold Pressor Test Protocol To Evaluate Cardiac Autonomic Function

PURPOSE This study tested the reliability and reproducibility of a cold pressor test protocol for the evaluation of cardiac autonomic function. Secondary goals were to investigate the effects of gender and posture on cardiac autonomic function. METHODS Twenty-four normotensive men (n = 12) and women (n = 12) completed 3 trials of the study protocol that included heart rate, blood pressure and ventilatory data collection during 3 postures (left lateral lying, standing and seated for 15 minutes each) and during the cold pressor test (hand immersed in 5°Celsius water for 6 minutes). Spectral analysis of heart rate variability and spontaneous baroreflex sensitivity were used to identify variables indicative of of cardiac autonomic function. RESULTS Betwen trials 1 and 2, there were no significant differences in variables indicative of cardiac autonomic function during the cold pressor test and postural changes. Significant correlations were observed in all heart rate variability and spontaneous baroreflex sensitivity variables, except for low frequency power in the lying posture and during the cold pressor test. Bland-Altman plots constructed for heart rate variability and spontaneous baroreflex variables in trials 1 versus 2 revealed no significant systematic biases. Heart rate and blood pressure were significantly higher and total power, high frequency power, and low frequency power were significantly lower during the cold pressor test compared to the seated posture. The sympathetic nervous system (SNS) indicator increased significantly from a baseline value of 0.45±0.06 to 0.82±0.2 during the cold pressor test. Significant changes were also observed in the parasympathetic nervous system (PNS) indicator, which decreased from a baseline value of 0.87±0.10 to 0.45±0.30 in response to the cold pressor test. In comparison to the left lateral lying posture, both genders showed a significant increase in the SNS indicator and decreases in the R-R interval and spontaneous baroreflex slope when standing. The only significant gender difference was in the PNS indicator during the left lateral lying posture, with women having a higher value than men (0.50±0.05 vs. 0.29±0.04). CONCLUSIONS Findings support the hypothesis that the cold pressor test is a reliable and reproducible method for testing cardiac autonomic function. Cardiac autonomic function exhibits gender and postural differences in humans. Supported by The Physicians' Services Incorporated Foundation (Canada), the Garfield Kelly Cardiovascular Research and Development Fund, and Advisory Research Committee (Queen's University)

Spontaneous baroreflex sensitivity after coronary artery bypass graft surgery as a function of gender and age.

The effects of coronary artery bypass graft (CABG) surgery on spontaneous baroreflex (SBR) sensitivity and heart rate variability were examined in 11 women and 23 men preoperatively and 5 days postoperatively. Electrocardiograph R-R interval and beat-by-beat arterial blood pressure data were collected continuously for 20 min in the supine and standing postures. Coarse graining spectral analysis was performed on the heart rate variability data. Spontaneous baroreflex sensitivity declined after surgery with a differential influence of gender. Men showed a decrease in SBR slope following surgery, with a greater decrease in the standing posture; the parasympathetic (PNS) indicator was lower postoperatively and in the standing posture; the reduction in low-frequency (LF) power was greater for the younger men. In women, the PNS indicator was lower in the standing posture. Both men and women showed a decrease in high-frequency power following CABG surgery, which decreased the sensitivity of the short-term cardiac control mechanisms that modulate heart rate, with the greater effects occurring in men. The reduction in SBR sensitivity indicates that the ability of the cardiovascular system to respond rapidly to changing stimuli was compromised. The decline in the PNS indicator implies that patients were vulnerable to the risks of myocardial ischemia, sympathetically mediated cardiac dysrhythmias, and sudden cardiac death.

Heart rate variability after prolonged spaceflights

Astronauts returning from spaceflight often experience post-flight orthostatic intolerance. This study was designed to determine whether cosmonauts with post-flight syncope could be distinguished from those with no post-flight syncope. The autonomic function was determined in a group of ten subjects, with no previous history of syncope, during a stand test before and after a long-term spaceflight (90 to 198 days). Heart rate (HR) and systolic blood pressure (SBP) were measured beat-by-beat, pre- and post-flight and the spontaneous baroreflex sensitivity and HR variability were studied. Individuals were categorized according to their ability to remain standing for 5 min the day after landing. Three of the ten cosmonauts failed to finish the standing test performed the day after landing (nonfinishers). The spontaneous baroreflex slope was reduced in both groups after the spaceflight. The non-finisher group had a lower SBP (P < 0.05) at rest in pre-flight tests than the group that completed the test (finisher group). The non-finisher group also had higher indicators of parasympathetic activity when supine, both pre- and post-flight, but this difference disappeared with standing. At the end of the stand test, SBP and HR were lower in non-finisher cosmonauts than the finishers, while HR did not increase compared to early measurements in the stand test of the finisher group. These results suggest an impairment in autonomic control of HR, which might contribute to the fainting response.

Effects of human pregnancy on cardiac autonomic function above and below the ventilatory threshold.

This study examined the effects of human pregnancy on heart rate variability (HRV), spontaneous baroreflex (SBR) sensitivity, and plasma catecholamines at rest and during exercise. Subjects were 14 healthy, physically active pregnant women (PG; mean gestational age = 33.9 +/- 1.0 wk). Results were compared with an age-matched nonpregnant control group (NPG; n = 14) with similar characteristics. The electrocardiographic R-wave-R-wave interval and systolic blood pressure (via finger plethysmograph) were measured on a beat-to-beat basis at rest and during upright cycling at 60 and 110% of the ventilatory threshold (T(vent)). Parasympathetic nervous system (PNS) modulation (as reflected by HRV high-frequency/total power and SBR slope) was significantly reduced at rest in the PG vs. the NPG. During exercise, PNS modulation decreased significantly in both groups, but the magnitude of PNS withdrawal from rest to 110% T(vent) was smaller in the PG vs. NPG. Sympathetic nervous system (SNS) modulation (reflected by the low-frequency power-to-high-frequency power ratio) increased above resting values at 60 and 110% T(vent) in the NPG. SNS modulation at 110% T(vent) was significantly lower in the PG compared with the NPG. Plasma norepinephrine and epinephrine levels were also lower at 110% T(vent) in the PG. It was concluded that healthy pregnant women exhibit lower PNS modulation at rest and blunted SNS modulation during exercise above T(vent) in late gestation.

EFFECTS OF HUMAN PREGNANCY ON CARDIAC AUTONOMIC ACTIVITY ABOVE AND BELOW THE VENTILATORY ANAEROBIC THRESHOLD 23

This study was conducted to test the hypothesis that cardiac autonomic balance is altered by human pregnancy. Subjects were 5 physically active pregnant volunteers studied between 27-37 weeks gestation. A reference group of 5 nonpregnant subjects matched according to age, height, pre-pregnant body mass and parity was also studied. Subjects exercised on a constant work rate cycle ergometer for 4 minutes at 20 watts followed by a ramp increase in work rate of 20 watts/min until fatigue. Ventilatory threshold (Tvent) was determined using the V-slope method (J. Appl. Physiol. 60:2020, 1986). Subjects then performed 2 additional submaximal exercise tests on a separate day. The testing protocol involved 4 minutes of pedalling at a work rate of 20 watts followed by a ramp increase to a level which corresponds to 60 or 110% of the work rate at Tvent. R-R interval data were collected (≥ 512 cycles) and stored using a computerized system. Fast Fourier Transform Analysis was performed to plot the R-R interval spectrum. Low frequency(0-0.15 Hz) and high frequency (0.15-0.5 Hz) power were calculated from data obtained at rest and at each level of exercise. The ratio of high frequency power:total power was used as an index of cardiac parasympathetic activity and the ratio of low to high frequency power was used as an index of sympathetic activity (J. Appl. Physiol. 71: 1136, 1991). Blood pressure in the finger (Ohmeda 2300 Finapres) was measured at rest and during exercise on a beat-to-beat basis for evaluation of spontaneous baroreflex function(Hypertension 12:214, 1988). In the resting state, cardiac parasympathetic modulation (as reflected by HRV high frequency power/total power and spontaneous baroreflex slope) were reduced and cardiac sympathetic modulation (as reflected by low frequency power/high frequency power) was increased in late gestation (p<0.05). At work rates above Tvent, cardiac sympathetic modulation was reduced in late gestation vs. the nonpregnant state. These findings support our original experimental hypothesis.

Reduced spontaneous baroreflex response slope during lower body negative pressure after 28 days of head-down bed rest

Effects of 28 days of continuous 6 degrees head-down tilt bed rest on spontaneous vagally mediated baroreflex response slope were evaluated from beat-by-beat relationships between R-R interval and systolic arterial blood pressure. Twelve healthy men (age 27-42 yr) were assigned to either countermeasure (CM) or no-countermeasure (no-CM) groups. CM consisted of strenuous short-term exercise once per day 6 days/wk from days 7 to 28 and lower body negative pressure (LBNP) for 15 min on days 16, 18, 20, and 22-28. Spontaneous baroreflex slope was evaluated by application of linear regression to sequences of at least three beats in which systolic blood pressure and R-R interval changed in the same direction. Measurements were made pre-, mid- (day 15), and post-bed rest at rest and during progressive LBNP tests (3 min at each of -20, -30, -40, and -50 mmHg). R-R interval decreased progressively and significantly (P < 0.0001) over duration of bed rest. Spontaneous baroreflex slope at rest in pre-bed rest was 18.5 +/- 2.1 ms/mm Hg for CM and 14.9 +/- 1.6 ms/mmHg for no-CM. There was a significant reduction in baroreflex slope as a function of bed rest, and it was further reduced during LBNP (P < 0.0001). Between CM and no-CM groups differences existed, but these were present pre-bed rest and appeared unaffected by countermeasures.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered autonomic regulation of cardiac function during head‐up tilt after 28‐day head‐down bed‐rest with counter‐measures

The effects of 28 days continuous 6 degrees head-down tilt bed-rest on heart rate variability and the slope of the spontaneous arterial baroreflex were evaluated during supine rest and the first 10 min of 60 degrees head-up tilt. Twelve healthy men were assigned to either a no counter-measure (No-CM), or a counter-measure (CM) group so that there was no difference in maximal oxygen uptake. Counter-measures consisted of short-term, high resistance exercise for 6 days per week from days 7-28, and lower body negative pressure (-28 mmHg) for 15 min on days 16, 18, 20 and 22-28. In spite of balanced between-group fitness, mean RR-interval was different between the No-CM and the CM group prior to bed-rest, but neither this nor any other variables showed significant counter-measure by bed-rest interaction effects. Therefore, all data presented are from the main effects of bed-rest or tilt from the analysis of variance. RR-interval was reduced significantly by bed-rest and by tilt (P < 0.0001). Indicators from spectral analysis of heart-rate variability suggested reduced parasympathetic nervous system activity with bed-rest (P < 0.01) and head-up tilt (P < 0.05), and increased sympathetic nervous system activity after bed-rest (P < 0.01). An indicator of complexity of cardiovascular control mechanisms, taken from the slope (beta) of log spectral power vs. log frequency relationship, suggested reduced complexity with bed-rest (P < 0.05) and head-up tilt (P < 0.01). The spontaneous baroreflex slope was reduced significantly by bed-rest (P < 0.03) and by head-up tilt (P < 0.04). Taken together, these data support the concept of altered autonomic nervous system function in the aetiology of cardiovascular deconditioning with bed-rest or space travel; and it would appear that no benefit is derived from these specific counter-measures.

Evaluation of spontaneous baroreflex response after 28 days head down tilt bedrest

The spontaneous baroreflex response was evaluated during supine rest and head up tilt (60°) before and immediately after a 28 day 6° HDT bedrest in 6 healthy adult men (age 30–42 years). Sequences of 3 or more beats where RR-interval and systolic blood pressure changed in the same direction were used to evaluate baroreflex response slope (BRS). Prior to bedrest, the mean BRS and RR-interval were 18.0 ± 3.9 ms/mm Hg and 926 ± 61 ms at rest and 10.5 ± 2.5 ms/mm Hg and 772 ± 63 ms during the first 10 min of 60° tilt. Following bedrest, these values changed to 15.6 ± 2.7 ms/mm Hg and 780 ± 53 ms at rest, and to 6.5 ± 1.2 ms/mm Hg and 636 ± 44 ms during tilt. Thus, (1) the spontaneous baroreflex can be evaluated in human subjects during experiments of orthostatic stress; (2) the baroreflex slope was reduced on going from supine to the head up tilt position; and (3) 28 days of bedrest reduced the spontaneous baroreflex slope.