Head-down Tilt Bed Rest Research Articles

Altered cerebral perfusion in response to chronic mild hypercapnia and head-down tilt Bed rest as an analog for Spaceflight.

Following prolonged stays on the International Space Station (ISS), some astronauts exhibit visual acuity changes, ophthalmological findings, and mildly elevated intracranial pressures as part of a novel process called spaceflight-associated neuro-ocular syndrome (SANS). To determine the pathophysiology of SANS, NASA conducted a multi-investigator study in which 11 healthy participants underwent head-down tilt bed rest, mimicking microgravity-induced cephalad fluid shifts, combined with elevated ambient CO2 levels similar to those on the ISS (HDT+CO2). As part of that study, we examined the effects of HDT+CO2 on cerebral perfusion. Using arterial spin labeling, we compared cerebral perfusion before, during, and after HDT+CO2 in participants who developed SANS (n = 5) with those who did not (n = 6). All participants demonstrated a decrease in perfusion during HDT+CO2 (mean decrease of 25.1% at HDT7 and 16.2% at HDT29); however, the timing and degree of change varied between the groups. At day 7 of HDT+CO2, the SANS group experienced a greater reduction in perfusion than the non-SANS group (p =.05, 95% CI:-0.19 to 16.11, d=.94, large effect). Conversely, by day 29 of HDT+CO2, the SANS group had significantly higher perfusion (approaching their baseline) than the non-SANS group (p = .04, 95% CI:0.33 to 13.07, d=1.01, large effect). Compared with baseline and recovery, HDT+CO2 resulted in reduced cerebral perfusion which varied based on SANS status. Further studies are needed to unravel the relative role of HDT vs hypercapnia, to determine if these perfusion changes are clinically relevant, and whether perfusion changes contribute to the development of SANS during spaceflight.

Evaluation of Cardiac Circadian Rhythm Deconditioning Induced by 5-to-60 Days of Head-Down Bed Rest.

Head-down tilt (HDT) bed rest elicits changes in cardiac circadian rhythms, generating possible adverse health outcomes such as increased arrhythmic risk. Our aim was to study the impact of HDT duration on the circadian rhythms of heart beat (RR) and ventricular repolarization (QTend) duration intervals from 24-h Holter ECG recordings acquired in 63 subjects during six different HDT bed rest campaigns of different duration (two 5-day, two 21-day, and two 60-day). Circadian rhythms of RR and QTend intervals series were evaluated by Cosinor analysis, resulting in a value of midline (MESOR), oscillation amplitude (OA) and acrophase (φ). In addition, the QTc (with Bazett correction) was computed, and day-time, night-time, maximum and minimum RR, QTend and QTc intervals were calculated. Statistical analysis was conducted, comparing: (1) the effects at 5 (HDT5), 21 (HDT21) and 58 (HDT58) days of HDT with baseline (PRE); (2) trends in recovery period at post-HDT epochs (R) in 5-day, 21-day, and 60-day HDT separately vs. PRE; (3) differences at R + 0 due to bed rest duration; (4) changes between the last HDT acquisition and the respective R + 0 in 5-day, 21-day, and 60-day HDT. During HDT, major changes were observed at HDT5, with increased RR and QTend intervals’ MESOR, mostly related to day-time lengthening and increased minima, while the QTc shortened. Afterward, a progressive trend toward baseline values was observed with HDT progression. Additionally, the φ anticipated, and the OA was reduced during HDT, decreasing system’s ability to react to incoming stimuli. Consequently, the restoration of the orthostatic position elicited the shortening of RR and QTend intervals together with QTc prolongation, notwithstanding the period spent in HDT. However, the magnitude of post-HDT changes, as well as the difference between the last HDT day and R + 0, showed a trend to increase with increasing HDT duration, and 5/7 days were not sufficient for recovering after 60-day HDT. Additionally, the φ postponed and the OA significantly increased at R + 0 compared to PRE after 5-day and 60-day HDT, possibly increasing the arrhythmic risk. These results provide evidence that continuous monitoring of astronauts’ circadian rhythms, and further investigations on possible measures for counteracting the observed modifications, will be key for future missions including long periods of weightlessness and gravity transitions, for preserving astronauts’ health and mission success.

Circulating MiR-21 expression is upregulated after 30days of head-down tilt bed rest.

Circulating miRNAs (c-miRNAs) have potential as biomarkers of cellular activity, and they may play a role in cell-to-cell communication. The purpose of this study was to examine c-miRNA and bone marker responses to a 30-day six-degree head-down bed rest protocol at an ambient 0.5% CO2. Eleven participants (6 males/5 females, 25-50years) had fasting blood draws taken 3days before and immediately after completing the 30-day bed rest protocol at the Institute for Aerospace Medicine in Germany. Serum relative expression of miRNAs associated with bone function (miR-21-5p, -100-5p, -125b-5p, -126-3p) were analyzed using qPCR, and serum bone markers were quantitated using ELISA. Serum bone markers, sclerostin, and calcium significantly increased (p ≤ 0.036), and total hip aBMD significantly decreased (p = 0.003) post bed rest. Serum miR-21-5p relative expression was significantly upregulated (p = 0.018) post bed rest. Fold changes in miR-126-3p (r = 0.82, p = 0.002) and miR-21-5p (r = 0.62, p = 0.042) were positively correlated with absolute change in serum calcium. There were no sex differences in miRNA responses; women had greater percent increases in TRAP5b (37.3% vs. 16.9% p = 0.021) and greater percent decreases in total hip aBMD (- 2.15% vs. - 0.69%, p = 0.034) than men. c-miR-21-5p has potential as a biomarker of bone resorption and bone loss in an unloading condition. The upregulation of miR-21-5p may reflect an increase in osteoclast activity after bed rest, which is corroborated by the increase in TRAP5b.

Regular exercise counteracts circadian shifts in core body temperature during long-duration bed rest

With NASA’s plans for the human exploration of Mars, astronauts will be exposed to mission durations much longer than current spaceflight missions on the International Space Station. These mission durations will increase the risk for circadian misalignment. Exercise has gained increasing interest as a non-pharmacological aid to entrain the circadian system. To assess the potential of exercise as a countermeasure to mitigate the risk for circadian disorders during spaceflight, we investigated the effects of long-term head-down tilt bed rest (HDBR) with and without exercise on the circadian rhythm of core body temperature. Core body temperature was recorded for 24 h using a rectal probe in sixteen healthy men (age: 30.5 ± 7.5 years (mean ± SD)) after 7 days and 49 days of HDBR. Five participants underwent HDBR only (CTR), five participants underwent HDBR and performed resistive exercises (RE), and six participants underwent HDBR and performed resistive exercises superimposed with vibrations (RVE). The exercise was scheduled three times per week. CTR showed a phase delay of 0.69 h. In contrast, both exercise groups were characterized by a phase advance (0.45 h for RE and 0.45 h for RVE; p = 0.026 for interaction between time and group). These findings suggest that resistive exercise (with or without vibration) may also serve as a countermeasure during spaceflight to mitigate circadian misalignments. The results could also be important for increasing awareness about the role of circadian disorders in long-term bedridden patients.

The Effect of Afferentation of Various Sensory Systems on the Otolith-Ocular Reflex in a Real and Simulated Weightlessness

Weightlessness can affect the vestibular function, primarily the otolith function, both directly due to the loss of gravitational influence and by eliminating the support and minimizing proprioceptive afferentation, influencing indirectly through the central integrative multisensory structures of the central nervous system (CNS), where the convergence of afferent signals of various sensory modality (primarily visual, vestibular, support and motor) occurs. All this prompted the study of the influence of various afferentations on the otolith-ocular reflex under the conditions of real and simulated weightlessness. Using the video oculography method, 58 Russian cosmonauts, members of long-term missions to the ISS (125–219 days, ~175 days on average), were examined by a static torsional otolith-cervical-ocular reflex (OCOR) with a head tilt to the shoulder at an angle of 30 deg, the otolith-ocular reflex (OOR) when the body is tilted at an angle of 30 deg, and the otolith-ocular reflex when exposed to centrifugal acceleration using a centrifuge (OORCF). Nine out of 58 cosmonauts were examined in microgravity (175 days) as part of the Virtual space experiment (Stage 1). The study of simulation of the spaceflight (SF) factors involved 30 subjects during five- or seven-day horizontal “dry” immersion (DI) and 12 subjects in 60-day head-down tilt (–8 deg) bed rest (BR). The procedure and the corresponding hardware and software used both in studies with the participation of cosmonauts and in model experiments were completely identical. In weightlessness, throughout the whole SF, all the cosmonauts surveyed had a significant OCOR decrease with the presence (from 11 to 55%, depending on the flight day) of the atypical (inversion or absence) otolith-ocular reflex. After SF, on days 1–4 of examination, 80% of the cosmonauts exhibited a significant OCOR/OOR decrease; 34% of the cosmonauts had atypical (inversion or absence of the otolith reflex) vestibular reactions, while no differences were found between OCOR and OOR. Statistically significant differences were found between the values ​of the OCOR/OOR and OORCF reflexes before and on days 4–5, 9–12 after SF. On days 2–3 after SF, no difference between OORCF and OCOR/OOR was observed. The return of the otolith reflex to the baseline took place only on day 9 after SF. The OCOR/OOR study on days 1–3 after DI showed that the otolith-ocular reflex in 47% of those examined was significantly reduced; in 13% of the subjects atypical reactions (absence or inversion of the reflex) were recorded. On days 4–6 after DI, OCOR/OOR returned to the baseline in all subjects. Analysis of OCOR/OOR after the BR experiment showed that only 14% of the surveyed had a significant decrease in OCOR/OOR on day 1 after the experiment. The atypical form of OCOR/OOR (inversion or complete absence of the otolith reflex) after BR was not revealed in contrast to the testers after DI. The results obtained demonstrated the influence of nonvestibular afferentation on the intensity of the otolith-ocular reflex and its dependence on other sensory inputs.

Sixty days of head-down tilt bed rest with or without artificial gravity do not affect the neuromuscular secretome

Artificial gravity is a potential countermeasure to attenuate effects of weightlessness during long-term spaceflight, including losses of muscle mass and function, possibly to some extent attributable to disturbed neuromuscular interaction. The 60-day AGBRESA bed-rest study was conducted with 24 participants (16 men, 8 women; 33 ± 9 years; 175 ± 9 cm; 74 ± 10 kg; 8 control group, 8 continuous (cAG) and 8 intermittent (iAG) centrifugation) to assess the impact of bed rest with or without daily 30-min continuous/intermittent centrifugation with 1G at the centre of mass. Fasting blood samples were collected before and on day 6, 20, 40 and 57 during 6° head-down tilt bed rest. Concentrations of circulating markers of muscle wasting (GDF-8/myostatin; slow skeletal muscle troponin T; prostaglandin E2), neurotrophic factors (BDNF; GDNF) and C-terminal Agrin Fragment (CAF) were determined by ELISAs. Creatine kinase activity was assessed by colorimetric enzyme assay. Repeated-measures ANOVAs were conducted with TIME as within-subject, and INTERVENTION and SEX as between-subject factors. The analyses revealed no significant effect of bed rest or sex on any of the parameters. Continuous or intermittent artificial gravity is a safe intervention that does not have a negative impact of the neuromuscular secretome.

Strategies for studying bone loss in microgravity

Astronauts are prone to a condition known as disuse osteoporosis as the microgravity environment negates the need for skeletal weight bearing. Recently, the development of new strategies to study bone loss in microgravity has been advancing at a rapid pace. As a result, several emerging technologies have paved the way for new research into the cellular and physiological mechanisms involved in disuse osteoporosis. In this review, we discuss the most impactful and current methodologies and technologies for both in vivo and in vitro studies of bone loss in space and with simulators on Earth from the past decade. We cover research performed on the International Space Station, uncrewed satellites, head-down tilt bed rest, rodent hindlimb unloading, and 2D/3D clinorotation for cell culture which are all established methods to mechanically unload the skeleton and/or bone cells. We also summarize the experimental findings documenting the changes that occur following exposure to unloading on a macroscopic scale, such as morphometric changes to the bone structure, and on the microscopic scale, such as effects on bone-forming osteoblasts, bone-resorbing osteoclasts, and mechanical stress-sensing osteocytes.

Brain connectivity and behavioral changes in a spaceflight analog environment with elevated CO2

Astronauts are exposed to microgravity and elevated CO2 levels onboard the International Space Station. Little is known about how microgravity and elevated CO2 combine to affect the brain and sensorimotor performance during and after spaceflight. Here we examined changes in resting-state functional connectivity (FC) and sensorimotor behavior associated with a spaceflight analog environment. Participants underwent 30 days of strict 6o head-down tilt bed rest with elevated ambient CO2 (HDBR+CO2). Resting-state functional magnetic resonance imaging and sensorimotor assessments were collected 13 and 7 days prior to bed rest, on days 7 and 29 of bed rest, and 0, 5, 12, and 13 days following bed rest. We assessed the time course of FC changes from before, during, to after HDBR+CO2. We then compared the observed connectivity changes with those of a HDBR control group that underwent HDBR in standard ambient air. Moreover, we assessed associations between post-HDBR+CO2 FC changes and alterations in sensorimotor performance. HDBR+CO2 was associated with significant changes in functional connectivity between vestibular, visual, somatosensory and motor brain areas. Several of these sensory and motor regions showed post-HDBR+CO2 FC changes that were significantly associated with alterations in sensorimotor performance. We propose that these FC changes reflect multisensory reweighting associated with adaptation to the HDBR+CO2 microgravity analog environment. This knowledge will further improve HDBR as a model of microgravity exposure and contribute to our knowledge of brain and performance changes during and after spaceflight.

Bone Marrow Reconversion With Reambulation: A Prospective Clinical Trial.

In a prospective clinical trial, 20 healthy men participated in a 60-day, 6-degree head-down tilt bed rest study. Serial 3-T magnetic resonance (MR) imaging measures of the lumbar spine were performed at baseline, after 57 days of bed rest, and at 30, 360, and 720 days of reambulation (100 MR imaging scans). Proton density with and without fat saturation, 2-point Dixon, and single-voxel MR spectroscopy techniques were used to assess bone marrow composition (300 measures). Erythropoiesis was measured using hematocrit, reticulocyte, and ferritin. Also, participants randomly received either a nutritional intervention composed of polyphenols, omega-3, vitamin E, and selenium or a normal diet. Thirty days of reambulation after 60 days of bed rest caused a marked decrease of the mean lumbar vertebral fat fraction (VFF) (-9.2 ± 1.6 percentage points, -8.0 ± 1.3 percentage points, and -12.7 ± 1.2 percentage points compared with baseline using proton density, Dixon, MR spectroscopy, respectively; all 3, P < 0.05). Reambulation also decreased the fat saturation index (-5.3 ± 1.1 percentage points compared with baseline; P < 0.05). These coincided with lower hematocrit and ferritin and with increased reticulocytes at reambulation day 13 compared with baseline (all 3, P < 0.05). After 57 days of bed rest, the VFF was unchanged from baseline (all 3 MR techniques, P > 0.05); reambulation for 2 years returned the lumbar VFF to baseline values. This longitudinal trial established that 30 days of reambulation after 60 days of bed rest constituted a powerful stimulus for bone marrow reconversion. In this model, the enhanced erythropoiesis coupled with preferential consumption of fatty acids from regulated marrow adipose tissue to supply energy for erythropoiesis and bone anabolism may explain the lumbar vertebrae reconversion. These results will help interpreting bone marrow signal in ambulatory patients after long periods of bed rest.

Daily generation of a footward fluid shift attenuates ocular changes associated with head-down tilt bed rest.

Astronauts have presented with a constellation of visual changes referred to as spaceflight-associated neuro-ocular syndrome (SANS). However, neither have early markers of microgravity-induced optic remodeling been fully identified nor have countermeasures been developed. To identify early markers of SANS, we studied 10 subjects with optical coherence tomography and ultrasonography when upright and supine and again after 24 h of 6° head-down tilt (HDT) bed rest. Upon acute transition from the upright to the supine position, choroid area (2.24 ± 0.53 to 2.28 ± 0.52 mm2, P = 0.001) and volume (9.51 ± 2.08 to 9.73 ± 2.08 mm3, P = 0.002) increased. After 24 h of HDT bed rest, subfoveal choroidal thickness (372 ± 93 to 381 ± 95 µm, P = 0.02), choroid area (2.25 ± 0.52 to 2.33 ± 0.54 mm2, P = 0.08), and volume (9.64 ± 2.03 to 9.82 ± 2.08 mm3, P = 0.08) increased relative to the supine position. Subsequently, seven subjects spent 3 days in -6°HDT bed rest to assess whether low-level lower body negative pressure (LBNP) could prevent the observed choroidal engorgement during bed rest. Maintaining the -6° HDT position for 3 days caused choroid area (Δ0.11 mm2, P = 0.05) and volume (Δ0.45 mm3, P = 0.003) to increase. When participants also spent 8 h daily under -20 mmHg LBNP, choroid volume still increased, but substantially (40%) less than in the control trial (Δ0.27 mm3, P = 0.05). Moreover, the increase in choroid area was diminished (Δ0.03 mm2, P = 0.13), indicating that low-level LBNP attenuates the choroid expansion associated with 3 days of -6° HDT bed rest. These data suggest that low-level LBNP may be an effective countermeasure for SANS.NEW & NOTEWORTHY Choroid measurements appear to be sensitive to changes in gravitational gradients, as well as periods of head-down tilt (HDT) bed rest, suggesting that they are potential indicators of early ocular remodeling and could serve to evaluate the efficacy of countermeasures for SANS. Eight hours of lower body negative pressure (LBNP) daily attenuates the choroid expansion associated with 3 days of strict -6° HDT bed rest, indicating that LBNP may be an effective countermeasure for SANS.

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

PurposeLong 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.MethodsTwenty 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.ResultsOn 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).ConclusionDaily 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.

Head-down tilt bed rest with or without artificial gravity is not associated with motor unit remodeling

PurposeThe objective of this study was to assess whether artificial gravity attenuates any long-duration head-down 60 bed rest (HDBR)-induced alterations in motor unit (MU) properties.MethodsTwenty-four healthy participants (16 men; 8 women; 26–54 years) underwent 60-day HDBR with (n = 16) or without (n = 8) 30 min artificial gravity daily induced by whole-body centrifugation. Compound muscle action potential (CMAP), MU number (MUNIX) and MU size (MUSIX) were estimated using the method of Motor Unit Number Index in the Abductor digiti minimi and tibialis anterior muscles 5 days before (BDC-5), and during day 4 (HDT4) and 59 (HDT59) of HDBR.ResultsThe CMAP, MUNIX, and MUSIX at baseline did not change significantly in either muscle, irrespective of the intervention (p > 0.05). Across groups, there were no significant differences in any variable during HDBR, compared to BDC-5.ConclusionSixty days of HDBR with or without artificial gravity does not induce alterations in motor unit number and size in the ADM or TA muscles in healthy individuals.

Intracranial pressure and visual acuity: The final frontier?

Intracranial pressure and visual acuity: The final frontier?

Neural Working Memory Changes During a Spaceflight Analog With Elevated Carbon Dioxide: A Pilot Study.

Spaceflight missions to the International Space Station (ISS) expose astronauts to microgravity, radiation, isolation, and elevated carbon dioxide (CO2), among other factors. Head down tilt bed rest (HDBR) is an Earth-based analog for spaceflight used to study body unloading, fluid shifts, and other factors unrelated to gravitational changes. While in space, astronauts need to use mental rotation strategies to facilitate their adaptation to the ISS environment. Therefore, spatial working memory is essential for crewmember performance. Although the effects of HDBR on spatial working memory have recently been studied, the results are still inconclusive. Here, we expand upon past work and examine the effects of HDBR with elevated CO2 (HDBR + CO2) on brain activation patterns during spatial working memory performance. In addition, we compare brain activation between 30 days of HDBR + CO2 and 70 days of HDBR to test the isolated effect of CO2. Eleven subjects (6 males, 5 females; mean age = 34 ± 8 years) underwent six functional magnetic resonance imaging (fMRI) sessions pre-, during, and post-HDBR + CO2. During the HDBR + CO2 intervention, we observed decreasing activation in the right middle frontal gyrus and left regions of the cerebellum, followed by post-intervention recovery. We detected several correlations between brain and behavioral slopes of change with the HDBR + CO2 intervention. For example, greater increases in activation in frontal, temporal and parietal regions were associated with larger spatial working memory improvements. Comparing the HDBR + CO2 group to data from our previous 70-day HDBR study, we found greater decreases in activation in the right hippocampus and left inferior temporal gyrus for the HDBR + CO2 group over the course of the intervention. Together, these findings increase our understanding of the neural mechanisms of HDBR, elevated levels of CO2 and spaceflight-related changes in spatial working memory performance.

Impacts of Microgravity Analogs to Spaceflight on Cerebral Autoregulation.

It is well known that exposure to microgravity in astronauts leads to a plethora physiological responses such as headward fluid shift, body unloading, and cardiovascular deconditioning. When astronauts return to Earth, some encounter problems related to orthostatic intolerance. An impaired cerebral autoregulation (CA), which could be compromised by the effects of microgravity, has been proposed as one of the mechanisms responsible for orthostatic intolerance. CA is a homeostatic mechanism that maintains cerebral blood flow for any variations in cerebral perfusion pressure by adapting the vascular tone and cerebral vessel diameter. The ground-based models of microgravity are useful tools for determining the gravitational impact of spaceflight on human body. The head-down tilt bed rest (HDTBR), where the subject remains in supine position at −6 degrees for periods ranging from few days to several weeks is the most commonly used ground-based model of microgravity for cardiovascular deconditioning. head-down bed rest (HDBR) is able to replicate cephalic fluid shift, immobilization, confinement, and inactivity. Dry immersion (DI) model is another approach where the subject remains immersed in thermoneutral water covered with an elastic waterproof fabric separating the subject from the water. Regarding DI, this analog imitates absence of any supporting structure for the body, centralization of body fluids, immobilization and hypokinesia observed during spaceflight. However, little is known about the impact of microgravity on CA. Here, we review the fundamental principles and the different mechanisms involved in CA. We also consider the different approaches in order to assess CA. Finally, we focus on the effects of short- and long-term spaceflight on CA and compare these findings with two specific analogs to microgravity: HDBR and DI.

Measuring Arterial Stiffness in a Head-Down Tilt Bed Rest Study: A Multisensor Approach.

Arterial stiffness is an important indicator for vascular aging and an independent predictor for cardiovascular diseases. During space flights or simulated space flights by prolonged head-down tilt bed rest, major cardiovascular alterations occur. However, the changes in arterial stiffness are not fully understood yet. Thus, we aimed to develop a setup for the measurement of arterial stiffness during prolonged head-down tilt bed rest, which incorporates several combinations of biosignals and measurement locations for the determination of pulse transit times. By performing measurements using this setup on female and male subjects, we intend to deepen the understanding of changes in arterial stiffness during prolonged head-down tilt bed rest. This work describes and visualizes the complete setup as well as our measurement protocols and algorithms used. The result section shows the successful recording of baseline signals before the bed rest study and visualizes the synchronized recordings of pulse waves measured on different sides of the body. Thus, it is feasible to use the presented setup in bed rest studies.

Ground-Based Analogs for Human Spaceflight.

This mini-review provides an updated summary of various analogs for adaptations of humans to the microgravity of space. Microgravity analogs discussed in this paper include dry immersion, wet immersion, unilateral lower-extremity limb suspension, head down tilt (HDT), and supine bed rest. All Earth-based analogs are imperfect simulations of microgravity with their own advantages and disadvantages. This paper compares these five frequently used microgravity analogs to offer insights into their usefulness for various physiological systems. New developments for each human microgravity analog are explored and advantages of one analog are evaluated against other analogs. Furthermore, the newly observed risk of Spaceflight Associated Neuro-Ocular Syndrome (SANS) is included in this mini review with a discussion of the advantages and disadvantages of each method of simulation for the relatively new risk of SANS. Overall, the best and most integrated analog for Earth-based studies of the microgravity of space flight appears to be head-down tilt bed rest.

Cerebrovascular reactivity: a new frontier for measuring cognitive health in models of accelerated ageing?

Cerebrovascular reactivity: a new frontier for measuring cognitive health in models of accelerated ageing?

Analysis of changes in cardiac circadian rhythms of RR and QT induced by a 60-day head-down bed rest with and without nutritional countermeasure.

Prolonged weightlessness exposure generates cardiovascular deconditioning, with potential implications on ECG circadian rhythms. Head-down (-6°) tilt (HDT) bed rest is a ground-based analogue model for simulating the effects of reduced motor activity and fluids redistribution occurring during spaceflight. Our aim was to evaluate the impact of 60-day HDT on the circadianity of RR and ventricular repolarization (QTend) intervals extracted from 24-h Holter ECG recordings, scheduled 9days before HDT (BDC-9), the 5th (HDT5), 21st (HDT21) and 58th (HDT58) day of HDT, the 1st (R + 0) and 8th (R + 7) day after HDT. Also, the effectiveness of a nutritional countermeasure (CM) in mitigating the HDT-related changes was tested. RR and QTend circadian rhythms were evaluated by Cosinor analysis, resulting in maximum and minimum values, MESOR (a rhythm-adjusted mean), oscillation amplitude (OA, half variation within a night-day cycle), and acrophase (φ, the time at which the fitting sinusoid's amplitude is maximal) values. RR and QTend MESOR increased at HDT5, and the OA was reduced along the HDT period, mainly due to the increase of the minima. At R + 0, QTend OA increased, particularly in the control group. The φ slightly anticipated during HDT and was delayed at R + 0. 60-Day HDT affects the characteristics of cardiac circadian rhythm by altering the physiological daily cycle of RR and QTend intervals. Scheduled day-night cycle and feeding time were maintained during the experiment, thus inferring the role of changes in the gravitational stimulus to determine these variations. The applied nutritional countermeasure did not show effectiveness in preventing such changes.

Effects of short-term −12° head-down tilt on cognitive performance

BackgroundMicrogravity-induced fluid shifts and the resulting changes in cerebral hemodynamics may be linked to cognitive performance alterations in spaceflight. Head-down tilt (HDT) bed rest serves as a well-validated spaceflight analog for microgravity-induced cephalic fluid shifts. This study examined the effects of short-term (t < 4 h) changes in body position (supine or −12° HDT) relative to a 45° upright position on cognitive performance. MethodsIn this within-subject cross-over study, ten male subjects without significant medical comorbidities (mean ± standard deviation [SD] age: 34 years ±3, weight: 81 kg ± 11, height: 1.77 m ± 0.06, body mass index: 26 kg/m2 ± 3) performed the Cognition test battery on an iPad at baseline (45° upright), after 1, 2, 3, and 4 h in the −12° HDT or supine position, and after 30-minute recovery in 45° upright position. The order of conditions (−12° HDT or supine) was randomized, with >1 week washout between test days. ResultsMixed model analyses of z-transformed Cognition test scores indicate a small (0.21–0.25 SD) decrease in accuracy that did not survive adjustment for multiple testing and no change in speed (0.00–0.12 SD) across cognitive domains in the supine and −12° HDT position relative to upright. A true decrease in speed may have been masked by practice effects, as speed was found to increase by 0.06 SD per test bout in condition (95% confidence interval 0.03; 0.10). Performance differences between −12° HDT and supine positions across cognitive domains were less than small (<0.2 SD) according to conventional standards. DiscussionThis study on short-term positional changes on cognitive performance suggests that performing Cognition in the supine or −12° HDT position on a tablet is associated with small decreases in accuracy and, if practice effects are taken into account, potentially also speed, but with no relevant differences between positions. These findings help interpret bed rest studies that use the new −12° HDT paradigm.