Comparison of Autonomic Control of Blood Pressure During Standing and Artificial Gravity Induced via Short-Arm Human Centrifuge

The combined effects of artificial gravity, temperature, and hypoxia on haemodynamic responses and limb blood flow

Under simultaneous environmental and gravitational stressors, integrated vascular responses maintain homeostatic balance via coordinated baro- and thermo-regulatory action. The effect of temperature and hypoxia at an elevated gravitational vector on the interaction of these systems was examined. Ten male participants experienced either cool (18.4 °C) or warm (29.1 °C) ambient temperatures in normoxia (partial pressure of oxygen, PIO2 = 133 mmHg) or hypoxia (PIO2 = 92 mmHg). Cardiovascular (heart rate, HR; arterial pressure, MAP; cardiac output, CO; stroke volume, SV; skin blood flow, SkBF) and thermoregulatory (skin temperature; core temperature) responses were monitored during standing (NG), and supine centrifugation at ground reaction forces (GRF) measured with a force platform at 1GRF and 2GRF. At 2GRF, warm and hypoxic conditions reduced the test duration by 16%. No differences were observed between NG and 1GRF in any variable; however, 2GRF significantly raised HR by 29.3% and MAP by 12.6%, and lowered SV by 22.2%. Warm condition significantly increased HR, and significantly decreased MAP and SV compared to the cool condition, by 17.8%, 6.1%, and 5.8%, respectively. Hypoxia had no effect on any variable. Arm SkBF significantly decreased by 33.3% with increasing artificial gravity, whereas leg SkBF increased by 38.7%. Higher ambient temperatures had no effect on leg SkBF, but significantly increased arm SkBF by 38.7%. Human tolerance to passive centrifugation is significantly lower at 2GRF, and further affected by the ambient conditions. Haemodynamic and leg SkBF responses in higher temperature and Gz conditions were frequently unable to prevent pre-syncopal symptoms. Finally, arm SkBF was modulated by both baroreflex and thermoregulation, and the baroreflex alone in leg SkBF.

Comparison of joint kinematics between upright front squat exercise and horizontal squat exercise performed on a short arm human centrifugation

This study compared the joint kinematics between the front squat (FS) conducted in the upright (natural gravity) position and in the supine position on a short arm human centrifuge (SAHC). Male participants (N = 12) with no prior experience exercising on a centrifuge completed a FS in the upright position before (PRE) and after (POST) a FS exercise conducted on the SAHC while exposed to artificial gravity (AG). Participants completed, in randomized order, three sets of six repetitions with a load equal to body weight or 1.25 × body weight for upright squats, and 1 g and 1.25 g at the center of gravity (COG) for AG. During the terrestrial squats, the load was applied with a barbell. Knee (left/right) and hip (left/right) flexion angles were recorded with a set of inertial measurement units. AG decreased the maximum flexion angle (MAX) of knees and hips as well as the range of motion (ROM), both at 1 and 1.25 g. Minor adaptation was observed between the first and the last repetition performed in AG. AG affects the ability to FS in naïve participants by reducing MAX, MIN and ROM of the knees and hip.

Peripheral skin cooling during hyper-gravity: hemodynamic reactions

Introduction: Orthostatic dysregulation occurs during exposure to an increased gravitational vector and is especially common upon re-entering standard Earth gravity (1 g) after an extended period in microgravity (0 g). External peripheral skin cooling (PSC) has recently been described as a potent countermeasure against orthostatic dysregulation during heat stress and in lower body negative pressure (LBNP) studies. We therefore hypothesized that PSC may also be an effective countermeasure during hyper-gravity exposure (+Gz). Methods: To investigate this, we designed a randomized short-arm human centrifuge (SAHC) experiment ("Coolspin") to investigate whether PSC could act as a stabilizing factor in cardiovascular function during +Gz. Artificial gravity between +1 g and +4 g was generated by a SAHC. 18 healthy male volunteers completed two runs in the SAHC. PSC was applied during one of the two runs and the other run was conducted without cooling. Each run consisted of a 10-min baseline trial followed by a +Gz step protocol marked by increasing g-forces, with each step being 3 min long. The following parameters were measured: blood pressure (BP), heart rate (HR), stroke volume (SV), total peripheral resistance (TPR), cardiac output (CO). Furthermore, a cumulative stress index for each subject was calculated. Results: +Gz led to significant changes in primary as well as in secondary outcome parameters such as HR, SV, TPR, CO, and BP. However, none of the primary outcome parameters (HR, cumulative stress-index, BP) nor secondary outcome parameters (SV, TPR, CO) showed any significant differences-whether the subject was cooled or not cooled. Systolic BP did, however, tend to be higher amongst the PSC group. Conclusion: In conclusion, PSC during +Gz did not confer any significant impact on hemodynamic activity or orthostatic stability during +Gz. This may be due to lower PSC responsiveness of the test subjects, or an insufficient level of body surface area used for cooling. Further investigations are warranted in order to comprehensively pinpoint the exact degree of PSC needed to serve as a useful countermeasure system during +Gz.

Canadian aging and inactivity study: Spaceflight-inspired exercises during head-down tilt bedrest blunted reductions in muscle-pump but not cardiac baroreflex in older persons

As part of the first Canadian aging and inactivity study (CAIS) we assessed the efficacy of space-based exercise countermeasures for maintenance of cardiac and muscle-pump baroreflex in older persons during bedrest. An initiative of the Canadian Space Agency, Canadian Institutes of Health Research and the Canadian Frailty Network, CAIS involved 14 days of 6-degree head-down tilt bedrest (HDBR) with (Exercise) or without (Control) combined upper and lower body strength, aerobic, and high-intensity interval training exercise countermeasures. Twenty healthy men and women aged 55 to 65, randomly divided into control and exercise groups (male control (MC, n = 5), male exercise (ME, n = 5), female control (FC, n = 6), female exercise (FE, n = 4)) (age: 58.7 ± 0.5 years, height: 1.67 ± 0.02 m, body mass: 70.2 ± 3.2 kg; mean ± SEM), completed the study. Cardiac and muscle-pump baroreflex activity were assessed with supine-to-stand tests. Wavelet transform coherence was used to characterise cardiac and muscle-pump baroreflex fraction time active (FTA) and gain values, and convergent cross-mapping was used to investigate causal directionality between blood pressure (BP) and heart rate, as well as BP and lower leg muscle electromyography (EMG). Seven of the twenty participants were unable to stand for 6 minutes after HDBR, with six of those being female. Our findings showed that 2 weeks of bedrest impaired skeletal muscle’s ability to return blood to the venous circulation differently across various sexes and intervention groups. Comparing values after bed rest with before bed rest values, there was a significant increase in heart rates (∆ of +25%; +17% in MC to +33% in FC; p < 0.0001), beat-to-beat EMG decreased (∆ of −43%; −25% in ME to −58% in MC; p < 0.02), while BP change was dependent on sex and intervention groups. Unlike their male counterparts, in terms of muscle-pump baroreflex, female participants had considerably decreased FTA after HDBR (p < 0.01). All groups except female control demonstrated parallel decreases in cardiac active gain and causality, while the FC demonstrated an increase in cardiac causality despite a similar decline in cardiac active gain. Results showed that the proposed exercises may alleviate muscle-pump baroreflex declines but could not influence the cardiac baroreflex decline from 14 days of inactivity in older adults.

Mechanical deconditioning of the heart due to long-term bed rest as observed on seismocardiogram morphology

During head-down tilt bed rest (HDT) the cardiovascular system is subject to headward fluid shifts. The fluid shift phenomenon is analogous to weightlessness experienced during spaceflight microgravity. The purpose of this study was to investigate the effect of prolonged 60-day bed rest on the mechanical performance of the heart using the morphology of seismocardiography (SCG). Three-lead electrocardiogram (ECG), SCG and blood pressure recordings were collected simultaneously from 20 males in a 60-day HDT study (MEDES, Toulouse, France). The study was divided into two campaigns of ten participants. The first commenced in January, and the second in September. Signals were recorded in the supine position during the baseline data collection (BDC) before bed rest, during 6° HDT bed rest and during recovery (R), post-bed rest. Using SCG and blood pressure at the finger, the following were determined: Pulse Transit Time (PTT); and left-ventricular ejection time (LVET). SCG morphology was analyzed using functional data analysis (FDA). The coefficients of the model were estimated over 20 cycles of SCG recordings of BDC12 and HDT52. SCG fiducial morphology AO (aortic valve opening) and AC (aortic valve closing) amplitudes showed significant decrease between BDC12 and HDT52 (p < 0.03). PTT and LVET were also found to decrease through HDT bed rest (p < 0.01). Furthermore, PTT and LVET magnitude of response to bed rest was found to be different between campaigns (p < 0.001) possibly due to seasonal effects on of the cardiovascular system. Correlations between FDA and cardiac timing intervals PTT and LVET using SCG suggests decreases in mechanical strength of the heart and increased arterial stiffness due to fluid shifts associated with the prolonged bed rest.

Indirect measurement of absolute cardiac output during exercise in simulated altered gravity is highly dependent on the method

PURPOSE

Altered gravity environments introduce cardiovascular changes that may require continuous hemodynamic monitoring in both spaceflight and terrestrial analogs. Conditions in such environments are often prohibitive to direct/invasive methods and therefore, indirect measurement techniques must be used. This study compares two common cardiac measurement techniques used in the human spaceflight domain, pulse contour analysis (PCA-Nexfin) and inert gas rebreathing (IGR-Innocor), in subjects completing ergometer exercise under altered gravity conditions simulated using a tilt paradigm.

METHODS

Seven subjects were tilted to three different angles representing Martian, Lunar, and microgravity conditions in the rostrocaudal direction. They completed a 36-min submaximal cardiovascular exercise protocol in each condition. Hemodynamics were continuously monitored using Nexfin and Innocor.

RESULTS

Linear mixed-effects models revealed a significant bias of [Formula: see text] ml ([Formula: see text]) in stroke volume and [Formula: see text] l/min ([Formula: see text]) in cardiac output, with Nexfin measuring greater than Innocor in both variables. These values are in agreement with a Bland-Altman analysis. The correlation of stroke volume and cardiac output measurements between Nexfin and Innocor were [Formula: see text] ([Formula: see text]) and [Formula: see text] ([Formula: see text]) respectively.

CONCLUSION

There is a poor agreement in absolute stroke volume and cardiac output values between measurement via PCA (Nexfin) and IGR (Innocor) in subjects who are exercising in simulated altered gravity environments. These results suggest that the chosen measurement method and device greatly impacts absolute measurements of cardiac output. However, there is a good level of agreement between the two devices when measuring relative changes. Either of these devices seem adequate to capture cardiac changes, but should not be solely relied upon for accurate measurement of absolute cardiac output.

Gravity Threshold and Dose Response Relationships: Health Benefits Using a Short Arm Human Centrifuge

Purpose

Increasing the level of gravity passively on a centrifuge, should be equal to or even more beneficial not only to astronauts living in a microgravity environment but also to patients confined to bed. Gravity therapy (GT) may have beneficial effects on numerous conditions, such as immobility due to neuromuscular disorders, balance disorders, stroke, sports injuries. However, the appropriate configuration for administering the Gz load remains to be determined.

Methods

To address these issues, we studied graded G-loads from 0.5 to 2.0g in 24 young healthy, male and female participants, trained on a short arm human centrifuge (SAHC) combined with mild activity exercise within 40–59% MHR, provided by an onboard bicycle ergometer. Hemodynamic parameters, as cardiac output (CO), stroke volume (SV), mean arterial pressure (MAP), systolic blood pressure (SBP), diastolic blood pressure (DBP), and heart rate (HR) were analyzed, as well as blood gas analysis. A one-way repeated measures ANOVA and pairwise comparisons were conducted with a level of significance p < 0.05.

Results

Significant changes in heart rate variability (HRV) and its spectral components (Class, Fmax, and VHF) were found in all g loads when compared to standing (p < 0.001), except in 1.7 and 2.0g. There were significant changes in CO, cardiac index (CI), and cardiac power (CP) (p < 0.001), and in MAP (p = 0.003) at different artificial gravity (AG) levels. Dose-response curves were determined based on statistically significant changes in cardiovascular parameters, as well as in identifying the optimal G level for training, as well as the optimal G level for training. There were statistically significant gender differences in Cardiac Output/CO (p = 0.002) and Cardiac Power/CP (p = 0.016) during the AG training as compared to standing. More specifically, these cardiovascular parameters were significantly higher for male than female participants. Also, there was a statistically significant (p = 0.022) gender by experimental condition interaction, since the high-frequency parameter of the heart rate variability was attenuated during AG training as compared to standing but only for the female participants (p = 0.004).

Conclusion

The comprehensive cardiovascular evaluation of the response to a range of graded AG loads, as compared to standing, in male and female subjects provides the dose-response framework that enables us to explore and validate the usefulness of the centrifuge as a medical device. It further allows its use in precisely selecting personalized gravity therapy (GT) as needed for treatment or rehabilitation of individuals confined to bed.

Quantitative Evaluation of a Telerobotic System for Vascular Ultrasound Measurement on a Short Arm Human Centrifuge

Artificial Gravity generated by Short Arm Human Centrifuges is a promising multi-system countermeasure for physiological deconditioning during long duration space flights. To allow a continuous assessment of cardiovascular hemodynamics during centrifugation, a telerobotic robotic system holding an ultrasound probe has been installed on a Short Arm Human Centrifuge. A feasibility study was conducted to define the use capabilities and limitations of such a novel method. The objective of this study is to estimate the reproducibility and precision of remotely controlled vascular ultrasound assessment under centrifugation by assessing peripheral vascular diameter and wall distension. Four repeated centrifugation runs of 5 min, with 2.4 g at feet level, were performed including a 15 min rest between each run for a group of eight healthy male volunteers. Vascular diameter and distention were assessed for the common carotid artery (CCA) and the femoral artery (FA) by ultrasound imaging using a 10 MHz linear array probe (Mylab1, Esaote). Ultrasound measurements were consecutively performed: a) by an expert user in hand-held mode in standing as well as supine position, b) using the telerobotic arm without centrifugation as baseline and c) using the telerobotic arm during centrifugation. Vascular responses were compared between baseline and under centrifugation. Inter-, intra-registration and group variability have been assessed for hand-held and remotely controlled examination. The results show that intra-registration variability, σ _h , was always smaller than inter-registration variability, σ _m, that is in turned smaller than the inter-subject variability σ _g (σ _h < σ _m < σ _g). Centrifugation caused no significant changes in CCA diameter but a lower carotid distension compared to manual and robotic ultrasound in supine position (p < 0.05). Femoral diameter was significantly decreased in hypergravity compared to robotic sonography without centrifugation. A good reproducibility and precision of the remotely controlled vascular ultrasound assessment under centrifugation could be demonstrated. In conclusion, arterial wall dynamics can be precisely assessed for the CCA and femoral artery during centrifugation using a telerobotic ultrasound measurement system. Potential improvements to further enhance reproducibility and safety of the system are discussed.

Tolerability of daily intermittent or continuous short-arm centrifugation during 60-day 6o head down bed rest (AGBRESA study)

Artificial gravity through short-arm centrifugation has potential as a multi-system countermeasure for deconditioning and cranial fluid shifts that may underlie ocular issues in microgravity. However, the optimal short-arm centrifugation protocol that is effective whilst remaining tolerable has yet to be determined. Given that exposure to centrifugation is associated with presyncope and syncope and in addition motion sickness an intermittent protocol has been suggested to be more tolerable. Therefore, we assessed cardiovascular loading and subjective tolerability of daily short arm centrifugation with either an intermittent or a continuous protocol during long-term head-down bed rest as model for microgravity exposure in a mixed sex cohort. During the Artificial Gravity Bed Rest with European Space Agency (AGBRESA) 60 day 6° head down tilt bed rest study we compared the tolerability of daily +1 Gz exposure at the center of mass centrifugation, either performed continuously for 30 minutes, or intermittedly (6 x 5 minutes). Heart rate and blood pressure were assessed daily during centrifugation along with post motion sickness scoring and rate of perceived exertion. During bed rest, 16 subjects (6 women, 10 men), underwent 960 centrifuge runs in total. Ten centrifuge runs had to be terminated prematurely, 8 continuous runs and 2 intermittent runs, mostly due to pre-syncopal symptoms and not motion sickness. All subjects were, however, able to resume centrifuge training on subsequent days. We conclude that both continuous and intermittent short-arm centrifugation protocols providing artificial gravity equivalent to +1 Gz at the center of mass is tolerable in terms of cardiovascular loading and motion sickness during long-term head down tilt bed rest. However, intermittent centrifugation appears marginally better tolerated, albeit differences appear minor.

Cardiorespiratory and Neuromuscular Demand of Daily Centrifugation: Results From the 60-Day AGBRESA Bed Rest Study

Purpose

Long stays in space require countermeasures for the degrading effects of weightlessness on the human body, and artificial gravity (AG) has been proposed as an integrated countermeasure. The aim of this study was to assess the cardiorespiratory and neuromuscular demand of AG elicited via daily centrifugation during 60 days of bed rest.

Methods

Twenty four participants (33 ± 9 y, 175 ± 9 cm, 74 ± 10 kg, 8 female) were subjected to 60 days of strict six-degree head-down tilt (HDT) bed rest and were randomly allocated to one of three experimental groups: 30 min of daily centrifugation with an acceleration of 1 g at the center of mass and 2 g at the feet applied continuously (cAG) or intermittently in 6 epochs of 5 min each, separated by 3 min breaks (iAG), or non-centrifuged control (CTRL). Cardiorespiratory demand during centrifugation was assessed at the beginning (HDT3) and end (HDT60) of the bed rest phase via spirometry and heart rate monitoring, leg muscle activation was monitored via electromyography.

Results

On average, analyses of variance revealed that heart rate during centrifugation increased by 40% (iAG) and 60% (cAG) compared to resting values (p < 0.001), while oxygen uptake did not change significantly (p = 0.96). There was a preference for calf over knee extensor muscle activation (active time soleus 57 ± 27%, gastrocnemius medialis 45 ± 27% and vastus lateralis 27 ± 27%, p < 0.001), with large inter-individual differences in leg muscle active time. AG could not prevent the increase in resting heart rate after bed rest. For most of the recorded parameters, there were little differences between cAG and iAG, with the increase in heart rate during centrifugation being a notable exception (greater increase for cAG, p = 0.01).

Conclusion

Daily 30 min bouts of artificial gravity elicited by centrifugation put a substantial demand on the heart as a pump without increasing oxygen consumption. If centrifugation is to be used as a countermeasure for the deteriorating effects of microgravity on physical performance, we recommend combining it with strenuous exercise.

Cardio-postural interactions and muscle-pump baroreflex are severely impacted by 60-day bedrest immobilization

To understand fundamental mechanisms associated with post-flight orthostatic intolerance we investigated the interaction between the cardiovascular and postural functions before and after 60 days of head down bedrest (HDBR). Twenty healthy young males (35.0 ± 1.7 years) were subjected to 60-day HDBR at 6˚ to simulate spaceflight-induced fluid shifts. A supine-to-stand (STS) test was conducted to evaluate cardio-postural control before and after (R) HDBR while an assessment of cardiovascular function was performed during HDBR. Beat-to-beat heart period, systolic blood pressure, and electromyography impulses were derived for wavelet transform coherence and causality analyses of the cardio-postural control and used to assess changes in the muscle-pump baroreflex. During quiet stand of the STS test, compared to baseline, heart rate was 50% higher on the day of exit from bedrest (R0) and 20% higher eight days later (R8). There was a 50% increase in deoxygenated hemoglobin on R0 and R8. Leg muscle activity reduced, and postural sway increased after HDBR. Causality of the muscle-pump baroreflex was reduced on R0 (0.73 ± 0.2) compared to baseline (0.87 ± 0.2) with complete recovery by R8. The muscle-pump baroreflex also had decreased gain and fraction time active following HDBR. Overall, our data show a significantly impaired muscle-pump baroreflex following bedrest.

Relationship between Ischemic Stroke and Pulse Rate Variability as a Surrogate of Heart Rate Variability

Autonomic reflex ascertains cardiovascular homeostasis during standing. Impaired autonomic reflex could lead to dizziness and falls while standing; this is prevalent in stroke survivors. Pulse rate variability (PRV) has been utilized in the literature in lieu of heart rate variability (HRV) for ambulatory and portable monitoring of autonomic reflex predominantly in young, healthy individuals. Here, we compared the PRV with gold standard HRV for monitoring autonomic reflex in ischemic stroke survivors. Continuous blood pressure and electrocardiography were acquired from ischemic stroke survivors (64 ± 1 years) and age-matched controls (65 ± 2 years) during a 10-minute sit-to-stand test. Beat-by-beat heart period (represented by RR and peak-to-peak (PP) intervals), systolic blood pressure (SBP), diastolic blood pressure (DBP), and pulse arrival time (PAT), an indicator of arterial stiffness, were derived. Time and frequency domain HRV (from RR intervals) and PRV (from PP intervals) metrics were extracted. PAT was lower (248 ± 7 ms vs. 270 ± 8 ms, p < 0.05) suggesting higher arterial stiffness in stroke survivors compared to controls during standing. Further, compared to controls, the agreement between HRV and PRV was impaired in stroke survivors while standing. The study outcomes suggest that caution should be exercised when considering PRV as a surrogate of HRV for monitoring autonomic cardiovascular control while standing in stroke survivors.

The effects of varying gravito-inertial stressors on grip strength and hemodynamic responses in men and women

Purpose

The body behaves as a global system with many interconnected subsystems. While the effects of a gravitational change on body responses have been extensively studied in isolation, we are not aware of any study that has examined these two types of body responses concurrently. Here, we examined how the cognitive and cardiovascular systems respond during application of varying gravito-inertial stressors in men and women.

Methods

Ten men and nine women underwent three 5-min centrifugation sessions (2.4 g at the feet, 1.5 g at the heart) in which participants rhythmically moved a hand-held object for 20 s. Grip force and hemodynamic responses were continuously measured during centrifugation and rest periods.

Result

Men optimized the modulation between grip force and the destabilizing load force, but not women. Exposure to artificial gravity induced higher heart rate and mean arterial pressure in both sexes compared to baseline. However, during artificial gravity exposure, only women decreased heart rate across sessions. Interestingly, we found that finishers of the protocol (mostly men) and Non-finishers (mostly women) exhibited divergent patterns of hemodynamic responses.

Conclusion

We speculate that the lack of grip force adaptation reported in women could be linked to the challenged hemodynamic responses during artificial gravity. By deriving a simple model to predict failure to complete the protocol, we found that mean arterial pressure—and not sex of the participant—was the most relevant factor. As artificial gravity is being proposed as a countermeasure in long-term manned missions, the observed effects in grip force adaptation and hemodynamic responses during varying gravito-inertial stressors application are particularly important.

Galanin and Adrenomedullin Plasma Responses During Artificial Gravity on a Human Short-Arm Centrifuge

Galanin and adrenomedullin plasma responses to head-up tilt and lower body negative pressure have been studied previously. However, to what extent short-arm human centrifugation (SAHC) affects these responses is not known. In this study, we assessed how the application of variable gradients of accelerations (ΔGz ) via shifting of the rotation axis during centrifugation affects selected hormonal responses. Specifically, we tested the hypothesis, that cardiovascular modulating hormones such as galanin and adrenomedullin will be higher in non-finishers (participants in whom at least one of the pre-defined criteria for presyncope was fulfilled) when compared to finishers (participants who completed the entire protocol in both sessions) during SAHC exposure. Twenty healthy subjects (10 women and 10 men) were exposed to two g-levels [1 Gz and 2.4 Gz at the feet (Gz_Feet)] in two positions (axis of rotation placed above the head and axis of rotation placed at the heart level). Elevated baseline levels of galanin appeared to predict orthostatic tolerance (p = 0.054) and seemed to support good orthostatic tolerance during 1 Gz_Feet SAHC (p = 0.034). In finishers, 2.4 Gz_Feet SAHC was associated with increased galanin levels after centrifugation (p = 0.007). For adrenomedullin, the hypothesized increases were observed after centrifugation at 1 Gz_Feet (p = 0.031), but not at 2.4 Gz_Feet, suggesting that other central mechanisms than local distribution of adrenomedullin predominate when coping with central hypovolemia induced by SAHC (p > 0.14). In conclusion, baseline galanin levels could potentially be used to predict development of presyncope in subjects. Furthermore, galanin levels increase during elevated levels of central hypovolemia and galanin responses appear to be important for coping with such challenges. Adrenomedullin release depends on degree of central hypovolemia induced fluid shifts and a subject's ability to cope with such challenges. Our results suggest that the gradient of acceleration (ΔGz ) is an innovative approach to quantify the grade of central hypovolemia and to assess neurohormonal responses in those that can tolerate (finishers) or not tolerate (non-finishers) artificial gravity (AG). As AG is being considered as a preventing tool for spaceflight induced deconditioning in future missions, understanding effects of AG on hormonal responses in subjects who develop presyncope is important.

Effect of Aging on Muscle-Pump Baroreflex of Individual Leg Muscles During Standing

Activation of leg muscles is an important component in the regulation of blood pressure during standing, failure of which could result in syncope and falls. Our previous work demonstrated baroreflex mediated activation of leg muscles (muscle-pump baroreflex) as an important factor in the regulation of blood pressure during standing; however, the effect of aging on the muscle-pump baroreflex of individual leg muscles during standing remains to be understood. Here, the interaction between systolic blood pressure (SBP) and the activation of lateral gastrocnemius (LG), medial gastrocnemius (MG), tibialis anterior (TA), and soleus (SOL) muscles during standing was quantified. Beat-to-beat heart period (RR interval), SBP, electromyography impulse (EMG_imp) were derived from continuously acquired electrocardiography, finger blood pressure, and calf-electromyography, respectively. The cardiac baroreflex (SBP→RR) causality (0.88 ± 0.08 vs. 0.94 ± 0.03, p = 0.01), percent time with significant coherence (%SC: 50.95 ± 23.31 vs. 76.75 ± 16.91, p = 0.001), and gain (4.39 ± 4.38 vs. 13.05 ± 8.11, p < 0.001) was lower in older (69 ± 4 years) compared to young (26 ± 2 years) persons. Muscle-pump baroreflex (SBP→EMG_imp) causality of LG (0.81 ± 0.08 vs. 0.88 ± 0.05, p = 0.01) and SOL (0.79 ± 0.11 vs. 0.88 ± 0.04, p = 0.01) muscles was lower in older compared to young persons. %SC was lower for all muscles in the older group (LG, p < 0.001; MG, p = 0.01; TA, p = 0.01; and SOL, p < 0.001) compared to young. The study outcomes highlighted impairment in muscle-pump baroreflex with age in addition to cardiac baroreflex. The findings of the study can assist in the development of an effective system for monitoring orthostatic tolerance via cardiac and muscle-pump baroreflexes to mitigate syncope and falls.

Gravitational stress during parabolic flights reduces the number of circulating innate and adaptive leukocyte subsets in human blood

Gravitational stress occurs during space flights or certain physical activities including extreme sports, where the change in experienced gravitational acceleration can reach large magnitudes. These changes include reduction and increase in the physical forces experienced by the body and may potentially induce pathogenic alterations of physiological processes. The immune system is known to regulate most functions in the human organism and previous studies suggest an impairment of the immune function under gravitational stress. However, systematic studies aiming to investigate the effect of gravitational stress on cellular immune response in humans are lacking. Since parabolic flights are considered as feasible model to investigate a short-term impact of gravitational changes, we evaluated the influence of gravitational stress on the immune system by analyzing leukocyte numbers before and after parabolic flight maneuvers in human blood. To correct for circadian effects, samples were taken at the corresponding time points on ground the day before the flight. The parabolic flight maneuvers led to changes in numbers of different leukocyte subsets. Naïve and memory T and B cell subsets decreased under gravitational stress and lower numbers of basophils and eosinophils were observed. Only circulating neutrophils increased during the parabolic flight. The observed changes could not be attributed to stress-induced cortisol effects, since cortisol levels were not affected. Our data demonstrate that the gravitational stress by parabolic flights can affect all parts of the human immune system. Consequently, it is possible that gravitational stress can have clinically relevant impacts on the control of immune responses.

Short-Term Cardiovascular Response to Short-Radius Centrifugation With and Without Ergometer Exercise

Artificial gravity (AG) has often been proposed as an integrated multi-system countermeasure to physiological deconditioning associated with extended exposure to reduced gravity levels, particularly if combined with exercise. Twelve subjects underwent short-radius centrifugation along with bicycle ergometry to quantify the short-term cardiovascular response to AG and exercise across three AG levels (0 G or no rotation, 1 G, and 1.4 G; referenced to the subject's feet and measured in the centripetal direction) and three exercise intensities (25, 50, and 100 W). Continuous cardiovascular measurements were collected during the centrifugation sessions using a non-invasive monitoring system. The cardiovascular responses were more prominent at higher levels of AG and exercise intensity. In particular, cardiac output, stroke volume, pulse pressure, and heart rate significantly increased with both AG level (in most of exercise group combinations, showing averaged increments across exercise conditions of 1.4 L/min/g, 7.6 mL/g, 5.22 mmHg/g, and 2.0 bpm/g, respectively), and workload intensity (averaged increments across AG conditions of 0.09 L/min/W, 0.17 mL/W, 0.22 mmHg/W, and 0.74 bpm/W respectively). These results suggest that the addition of AG to exercise can provide a greater cardiovascular benefit than exercise alone. Hierarchical regression models were fitted to the experimental data to determine dose-response curves of all cardiovascular variables as a function of AG-level and exercise intensity during short-radius centrifugation. These results can inform future studies, decisions, and trade-offs toward potential implementation of AG as a space countermeasure.

Short-Term Cardiovascular Response to Short-Radius Centrifugation With and Without Ergometer Exercise

Artificial gravity (AG) has often been proposed as an integrated multi-system countermeasure to physiological deconditioning associated with extended exposure to reduced gravity levels, particularly if combined with exercise. Twelve subjects underwent short-radius centrifugation along with bicycle ergometry to quantify the short-term cardiovascular response to AG and exercise across three AG levels (0 G or no rotation, 1 G, and 1.4 G; referenced to the subject's feet and measured in the centripetal direction) and three exercise intensities (25, 50, and 100 W). Continuous cardiovascular measurements were collected during the centrifugation sessions using a non-invasive monitoring system. The cardiovascular responses were more prominent at higher levels of AG and exercise intensity. In particular, cardiac output, stroke volume, pulse pressure, and heart rate significantly increased with both AG level (in most of exercise group combinations, showing averaged increments across exercise conditions of 1.4 L/min/g, 7.6 mL/g, 5.22 mmHg/g, and 2.0 bpm/g, respectively), and workload intensity (averaged increments across AG conditions of 0.09 L/min/W, 0.17 mL/W, 0.22 mmHg/W, and 0.74 bpm/W respectively). These results suggest that the addition of AG to exercise can provide a greater cardiovascular benefit than exercise alone. Hierarchical regression models were fitted to the experimental data to determine dose-response curves of all cardiovascular variables as a function of AG-level and exercise intensity during short-radius centrifugation. These results can inform future studies, decisions, and trade-offs toward potential implementation of AG as a space countermeasure.