Long-term Head-down Tilt Bed Rest Research Articles

Arterial Stiffness Alterations in Simulated Microgravity and Reactive Sledge as a Countermeasure.

Experiments during spaceflight and simulated microgravity as head-down tilt bedrest, demonstrated the role of arterial stiffness among others, in microgravity induced cardiovascular pathologies and emphasized the need for a robust countermeasure. The purpose of the present study was to evaluate the use of a new countermeasure, consisting of a high intensity Reactive Sledge (RSL) jumps training protocol, to counteract changes in arterial stiffness during long term head down tilt bedrest (LTBR). The participants enrolled in the study were 23 male, healthy volunteers, aged between 20 and 45years, subjected to LTBR for 60days and randomly assigned either to a control (11) or to a training sledge (12) group using RSL 3-4 times per week, as a countermeasure. Recorded values were systolic and diastolic blood pressure, heart rate and the user's arterial stiffness index. Compared to baseline measurements, there was a deterioration in the values of arterial stiffness, systolic and diastolic blood pressure and heart rate, in both groups until day 35 of LTBR, interpreted as adaptation to the microgravity environment. From this day until the end of the experiment, arterial stiffness of the control group was constantly fluctuating, while constantly improving for the training group. During the recovery period, arterial stiffness values returned to the pre-experimental levels in both groups. Overall, arterial stiffness increased the longer the time spent in LTBR and the countermeasure was partially effective in preventing the observed phenomenon. German Clinical Trials Register (DRKS), DRKS00012946, September 18, 2017, retrospectively registered.

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.

Intracranial pressure and visual acuity: The final frontier?

Intracranial pressure and visual acuity: The final frontier?

Incretion of Insulin by Pancreas and Its Inactivation in Women during Long-Term Head-down Tilt Bed Rest

An increase in insular activity in response to hyperglycemia when performing a glucose tolerance test (GTT) during normal physical activity in eight women volunteers was found to be associated with increased blood concentrations of insulin provided by an increase in secretion rate and a decrease in inactivation rate. After 81 days of stay in long-term –6° head-down tilt bed rest (bed rest), flattened glycemic curves (with low hyperglycemic lifting) observed during the GTT, what was indicated high insular activity. An increase in the insular activity was evidencing also by a smaller peak of the blood concentration of glucose from 5 min after intravenous nfusion of 20 mL of 40% glucose solution at 120 minute of the GTT. In bed rest on empty stomach no significant changes in the concentrations of insulin and C-peptide were observed. The dynamics of hormone during the GTT was showed that the appearance of flattened glycemic curves when performing GTT during bed rest was associated with more pronounced, than under the usual conditions, an increase in insulin concentration in response to hyperglycemia. An increase in the blood insulin concentration when performing GTT in bed rest was mainly provided by increased hormone secretion rate in the pancreas, but not by a decrease in its inactivation rate. If you use regular exercises for prevention of hypokinesia under conditions of bed rest, an increase in insulin concentration during GTT was mainly provided by a decrease in its inactivation rate and not by an increase in secretion rate.

Structural Brain Changes following Long-Term 6° Head-Down Tilt Bed Rest as an Analog for Spaceflight.

Following long-term spaceflight, a subset of the National Aeronautics and Space Administration astronauts present with visual impairment and increased intracranial pressure, known as visual impairment and intracranial pressure syndrome. We investigated structural brain changes following long-term head-down tilt bed rest as a spaceflight analog. Volumetric analysis was performed on structural pre- and post-bed rest brain MR images. Comparing post-bed rest to pre-bed rest images, we found the following: 1) no significant group differences in GM, WM, CSF, or ventricular volumes; 2) shift of the center of mass of the brain upward and posterior rotation of the brain relative to the skull; 3) a significant correlation between posterior brain rotation and changes in ventricular volume; and 4) significant increases in brain tissue density in regions at the vertex, including the frontoparietal lobes, with contraction of adjacent extra-axial CSF spaces, and significant decreases in tissue density in areas along the base of the brain, including the orbitofrontal cortex. We observed widespread morphologic changes with brain tissue redistribution in response to gravity changes; possible associated functional changes are unknown. The observation that ventricular change is correlated to posterior brain rotation suggests an alteration in CSF homeostasis. Ultimately, to elucidate any structural changes that may play a role in visual impairment and intracranial pressure syndrome, volumetric analysis of pre- and postflight structural scans of astronauts is needed.

Digestive system functioning during simulation of microravity effects on humans by means of immersion

The functioning of the digestive system was investigated in ten volunteers after a seven-day dry immersion (DI). The experimental conditions were found to raise the secretory activities of the stomach, pancreas, and liver and to increase the spectral indices of the gastrointestinal tract’s (GIT) electrical activity against the background of an increased insular secretion and decreased gastrin secretion. The elevated GIT electrical activities and changes in their relationships were determined by an increased gastric secretion and elevated intestine tone in fasting subjects and displayed a close similarity to changes induced by caffeine stimulation and those arising during long-term head-down tilt bed rest (HDTBR) or spaceflight.

Stress and recovery assessment during simulated microgravity: Effects of exercise during a long-term head-down tilt bed rest in women

The aim of this study was to determine the effects of a 60-day head-down tilt long-term bed rest (HDT) on stress and recovery in sixteen healthy female volunteers. Participants were randomly assigned to either an exercise group (Exe) that followed a training program combining resistive and aerobic exercises, or to a no-exercise control group (Ctl). Psychological states were assessed using the Rest-Q, a validated questionnaire based on stress–recovery responses. A longitudinal analysis revealed significant changes in the general and specific-stress scales for all participants throughout the experiment with a critical stage from supine to standing posture leading to a significant decrease in physical recovery. During HDT, Exe reported higher scores on stress subscales, as well as lower recovery scores compared to the Ctl. During the post-HDT ambulatory recovery period, the exercisers still reported higher scores than the non-exercisers on the Lack of energy stress-related scale, along with lower scores in general well-being and personal accomplishment. The present findings show that simulated weightlessness such as HDT may induce psychological stress and lead to subsequent alterations in perceived recovery. Exercise did not reduce HDT impaired effects on stress and recovery states. Several suggestions, including psychological preparation, are proposed to support the beneficial effects of exercise on psychological factors.

Personality, social support and affective states during simulated microgravity in healthy women

This study investigated the time-course of stress and recovery states and their relations to social support and personality traits in healthy women during a long-term head-down tilt bed rest. Personality, social support and affective states were assessed in 16 women exposed to simulated microgravity for a 60-day duration involving three stages: a 20-day baseline control period (BDC), a 60-day head-down tilt bed rest (HDT) and a 20-day post-HDT ambulatory recovery period (R+). Participants were divided into two groups: an exercise (Exe, n = 8) and a control group (Ctl, n = 8). All the participants experienced significantly more stress during the HDT period. But exercise did not improve the impaired effects of simulated microgravity. The Exe group perceived more stress and less recovery than the Ctl group during the HDT period. Among the five major personality factors, only Neuroticism was related to both social and affective variables. Neuroticism was positively associated with stress and negatively associated with recovery and social support (S-SSQ). Practical implications in psychological countermeasures for better dealing with the key human factor in spaceflights are discussed.

Long-term bed rest-induced reductions in stroke volume during rest and exercise: cardiac dysfunction vs. volume depletion

Long-term head-down-tilt bed rest (HDT) causes cardiovascular deconditioning, attributed to reflex dysfunctions, plasma volume reduction, or cardiac impairments. Our objective with the present study was to evaluate the functional importance and relative contribution of these during rest and exercise in supine and upright postures. We studied six subjects before (baseline), during [days 60 (D60) and 113 (D113)], and after [recovery days 0 (R0), 3 (R3), and 15 (R15)] 120 days of -6 degrees HDT. We determined cardiac output, stroke volume (SV), mean arterial pressure, and heart rate during rest and exercise in supine and upright postures. Cardiac output and SV decreased significantly in all four conditions, but the time courses differed for rest and exercise. Upright resting SV was decreased by 24 +/- 9% at D60 compared with baseline but had recovered already at R3. Supine exercise SV decreased more slowly (by 5 +/- 8% at D60 and by 18 +/- 4% at D113) and recovered more slowly after HDT termination. Steady-state mean arterial pressure showed no changes. Heart rate had increased by 18 +/- 4% at D60 and had recovered partially at R3. Our data indicate that long-term HDT causes both a rapid, preload-dependent reduction in SV, most evident during rest in the upright position, and a more slowly developing cardiac dysfunction, most evident during supine exercise. However, the ability to maintain blood pressure and to perform sustained low levels of dynamic exercise is not influenced by HDT.