Heart rate variability under acute simulated microgravity during daytime waking state and nocturnal sleep: comparison of horizontal and 6 degrees head-down bed rest

This study examined the acute effect of cephalad fluid shift under simulated microgravity on heart rate variability (HRV) during both daytime waking state and nocturnal sleep. Seven healthy male volunteers (21-31 years) underwent a series of experiments involving 6 degrees head-down bed rest (HD) for 3 days. A control experiment on the same subjects was conducted under horizontal bed rest (HZ) in the same series. HRV from electrocardiogram signals was periodically calculated by the MemCalc method during daytime on the first and second days of both conditions. Nocturnal sleep on the first night of bed rest was monitored by polysomnography. HRV during stage 2 sleep and REM sleep were assessed in the former and latter halves of the sleep period time. Nocturnal sleep architecture under both conditions was normal, but a slight decrease in stage 4 sleep and an increase in the number of arousals occurred under HD. On both the first and second days, HRV during the daytime did not differ between HZ and HD. In contrast, high frequency components in HRV during sleep stage 2 were significantly higher in the latter half of sleep under HD than under HZ, although there were no differences in the ratio of low frequency to high frequency components during both stage 2 and the REM stage between the conditions. These results suggest that the acute effect of the cephalad fluid shift on cardiac autonomic nervous activity might be affected by the sleep/wake state modulating the dominance between sympathetic and parasympathetic nervous activity.     

Neuropsychological and emotional changes during simulated microgravity: effects of triiodothyronine alendronate, and testosterone.

INTRODUCTION: We present the results of a two-experiment study designed to evaluate the neurocognitive and psychological effects of six-degree head-down bedrest and pharmacologic interventions (3,5,3'-triiodothyronine; T3) implemented to enhance the muscle and bone atrophy associated with simulated microgravity. Subsequently, the effects of countermeasures (alendronate and testosterone) administered to retard or reverse these T3 plus bedrest enhanced atrophic changes, were evaluated. Each participant was tested weekly for 5 weeks during Bedrest or non-bedrest (Up) conditions with the Neurobehavioral Evaluation System 2 (NES2), the Symptom Check List 90 Revised (SCL-90-R), and the Coping Responses Inventory (CRI). Resultant data were subjected to repeated measures, between groups analysis of variance testing for all 82 neurocognitive and psychological test measures. RESULTS: In Experiment 1, participants in the Placebo-Bedrest condition performed better on several neurocognitive measures compared to participants in the T3-Up condition. However, participants in the Placebo-Bedrest condition also reported more confusion. In Experiment 2 (countermeasure trials), superior coordination was observed for participants in the Testosterone-T3 condition over those in the Alendronate-T3 condition, but just the opposite for reaction time. Also, testosterone and to a lesser degree, alendronate, were associated with less self-reported emotional distress than T3 plus bedrest alone. CONCLUSION: Triiodothyronine, alendronate, and testosterone each influence participant response to simulated microgravity. Between group differences for significant findings were substantial and averaged 1.62 standard deviations. Although the observed neurocognitive effects likely pose no immediate danger for research participants, the significantly greater level of self-reported psychological symptoms by T3-Placebo and Placebo-Bedrest treated participants is of clinical importance.     

Centrifugation as a countermeasure during actual and simulated microgravity: a review

This paper summarizes what has been learned from studies of the effects of artificial gravity generated by centrifugation in actual and simulated weightless conditions. The experience of artificial gravity during actual space flight in animals and humans are discussed. Studies using intermittent centrifugation during bed rest and water immersion, as a way to maintain orthostatic tolerance and exercise capacity, are reviewed; their results indicate that intermittent centrifugation is a potential countermeasure for maintaining the integrity of these physiological functions in extended space missions. These results can help set guidelines for future experiments aimed at validating the regimes of centrifugation as a countermeasure for space missions. Current and future research projects using artificial gravity conditions in humans are discussed.     

Cardiovascular control and stabilization via inclination and mobilization during bed rest

Cardiovascular deconditioning has long been recognized as a characteristic of the physiological adaptation to long-term bed rest in patients. The process is thought to contribute to orthostatic intolerance and enhance secondary complications in a significant way. Mobilization is a cost-effective and simple method to maintain the cardiovascular parameters (i.e., blood pressure, heart rate) stable, counter orthostatic intolerance and reduce the risk of secondary problems in patients during long-term immobilization. The aim of this project is to control the cardiovascular parameters such as heart rate and blood pressure of bed rest patients via automated leg mobilization and body tilting. In a first step, a nonlinear model predictive control strategy was designed and evaluated on five healthy subjects and 11 bed rest patients. In a next step, a clinically feasible study was conducted on two patients. The mean values differed on average less than 1 bpm from the predetermined heart rate and less than 2.5 mmHg from the desired blood pressure values. These results of the feasibility study are promising, although heterogeneous disease etiologies and individual medication strongly influence the mechanically induced reactions. The long-term goal is an automation of the control of physiological signals and the mobilization of bed rest patients in an early phase of the rehabilitation process. Therefore, this new approach could help to strengthen the cardiovascular system and prevent secondary health problems arising from long-term bed rest.     

Screening of lower back pain, low back pain clinic. The clinical experience

Acute low back pain is a common complaint with high prevalence in society. Orthopaedic and specialist spinal services may be overwhelmed by large numbers of patients with low back pain who do not require investigation or surgical intervention. This phenomenon has led to the establishment of back pain screening clinics as a system of triage for those with acute low back pain. In March 2001, a back pain screening clinic was established in the Adelaide and Meath Hospitals incorporating the National Children's hospital. Six hundred and sixty-five patients were seen in the first year of this clinic. Of these, only ninety-six required referral to the orthopaedic spinal clinic. Just twenty-nine of these patients required surgical intervention. The introduction of this service resulted in a reduction in the waiting periods for the specialised spine clinic. We conclude that back pain screening clinics result in a clinical and economical improvement in the care of those suffering from acute low back pain. This is achieved by the systematic and efficient assessment, treatment and referral of patients afflicted with acute low back pain.     

Body volume changes during simulated microgravity: Auditory changes,segmental fluid redistribution,and regional hemodynamics

Space adaptation syndrome (SAS), manifested by cephalad fluid shifts, spacial disorientation, nausea, and vomiting, is of varied expression and uncertain etiology. One theory is that fluid shift to the upper body alters the function of the vestibular apparatus to create an entity similar to Meniere's disease. Since clinical vestibular dysfunction syndromes are mirored by altered cochlear function, this experiment was undertaken to study the relation between fluid redistribution and the auditory effects of initial antiorthostatic bed rest. Manual and bone audiometry, impedance tympanometry, and brain-stem evoked potentials were used to monitor auditory changes prior to, during, and following short term exposure to−6° head down tilt. Impedance plethysmography was performed to assess the segmental and intracranial fluid redistribution and hemodynamic changes during short-term head down tilt simulated microgravity. Even though significant cephalad fluid shift produced marked intracranial congestion and the subjects exhibited SAS symptoms, no clinically significant changes in the auditory system could be detected.     

Hemodynamic responses to simulated weightlessness of 24-h head-down bed rest and KAATSU blood flow restriction

The KAATSU training is a unique method of muscle training with restricting venous blood flow, which might be applied to prevent muscle atrophy during space flight, but the effects of KAATSU in microgravity remain unknown. We investigated the hemodynamic responses to KAATSU during actually simulated weightlessness (6° head-down tilt for 24 h, n = 8), and compared those to KAATSU in the seated position before bed rest. KAATSU was applied to the proximal ends of both the thighs. In the seated position before bed rest, sequential incrementing of KAATSU cuff pressure and altering the level of blood flow restriction resulted in a decrease in stroke volume (SV) with an increase in heart rate (HR). KAATSU (150–200 mmHg) decreased SV comparable to standing. Following 24-h bed rest, body mass, blood volume (BV), plasma volume (PV), and diameter of the inferior vena cava (IVC) were significantly reduced. Norepinephrine (NOR), vasopressin (ADH), and plasma renin activity (PRA) tend to be reduced. A decrease in SV and CO induced by KAATSU during the simulated weightlessness was larger than that in the seated position before bed rest, and one of eight subjects developed presyncope due to hypotension during 100 mmHg KAATSU. High-frequency power (HF_RR) decreased during KAATSU and standing, while low-frequency/high-frequency power (LF_RR/HF_RR) increased significantly. NOR, ADH and PRA also increased during KAATSU. These results indicate that KAATSU blood flow restriction reproduces the effects of standing on HR, SV, NOR, ADH, PRA, etc., thus stimulating a gravity-like stress during simulated weightlessness. However, syncope due to lower extremity blood pooling and subsequent reduction of venous return may be induced during KAATSU in microgravity as reported in cases of lower-body negative pressure.     

Whole muscle contractile parameters and thickness loss during 35-day bed rest

Extended exposure to microgravity leads to significant musculoskeletal adaptations. Contractile parameters of four skeletal muscles (biceps brachii-BB, vastus medialis-VM, biceps femoris-BF and gastrocnemius medialis-GM) were measured in ten healthy males (aged 22.3 +/- 2.2 years) during 35 days of horizontal bed rest by a mechanomyography-based method termed 'tensiomyography' (TMG). Two contractile parameters: contraction time (Tc) and maximal displacement (Dm) were individually measured from electrically evoked maximal single twitch TMG response of all four muscles before and after bed rest. Significant changes in Tc were found after bed rest, as shown by an increase in GM muscle Tc by 18% (p < 0.01). Dm values significantly increased (p < 0.01) after bed rest, by 24, 26 and 30% in the VM, BF and GM muscles, respectively. In the GM, the change in Dm significantly correlated with the decrease in muscle thickness (r = -0.70, p < 0.01). In conclusion, bed rest induced changes in both Dm and Tc of the TMG signal; changes in Dm being inversely related to those of muscle thickness. Amongst the investigated muscles, most affected, in terms of atrophy and mechanical alterations, were those of the lower limbs. The observed increase in Dm may be attributed to a decrease in muscle, as well as tendon stiffness, causing larger muscle fibre and non-contractile tissue oscillations following contraction.     

Doctor-patient concordance and patient initiative during episodes of low back pain.

Doctor-patient concordance and patient initiative were examined in a prospective network interview study, with telephone follow-up, of a cohort of 100 patients presenting with low back pain to their family physician. The average overall rate of concordance was 60% (95% CI = 53 to 66), with the highest rates for radiographic imaging studies and sick leave. No correlation was found between concordance and patient parameters. Subjects initiated an average of two (95% CI = 1.7 to 2.3) diagnostic or therapeutic procedures, the most common of which were for medications (40%), followed by bed rest (26%) and back school (22%). One out of every six subjects initiated a referral to a complementary therapist. Positive correlation was found between patient initiatives and pain severity (P = 0.022) and disability (P = 0.02). There was a negative correlation between the subjects' initiatives and their belief that the physician understood the cause of their pain and its influence on their life (P = 0.02). Overall, those patients who described more pain or disability sought more types of diagnostic and therapeutic measures, while those who felt they had been understood sought less.     

Plasma vasopressin,neurophysin, renin and aldosterone during a 4-day head-down bed rest with and without exercise

Summary The purpose of this study was to investigate the main renal and hormonal responses to head-down bed rest, which is curently considered a reliable experimental model for the simulation of weightlessness. Urinary output and electrolytes, plasma renin activity (PRA), aldosterone (PA), antidiuretic hormone (ADH) and immunoreactive neurophysin-I (Np) were measured in eight adult volunteers submitted to a 4-day head-down bed rest (–6) after a 24-h control period in the horizontal position (day 0). Four of the eight subjects were submitted to two 1-h periods of controlled muscular exercise (50% ) from day 1 to day 4. Throughout the head-down bed rest period, urinary output remained stable, although lower than in the control period (day 0), but the urinary Na/K ratio decreased. Plasma electrolytes and osmolality, and creatinine clearance remained unchanged. There was no significant difference between exercising and non-exercising subjects. At the hormonal level, PRA and PA increased during the head-down bed rest. This increase was more pronounced in the group with exercise. At the end of the tilt period, PRA and PA were about 3 times higher than on day 1. No significant changes could be observed for ADH and Np. It is concluded that a 4-day head-down bed rest results in no apparent changes in neurohypophyseal secretory activity, and in a progressive secondary hyperaldosteronism.Presented in part at the 35^th Congress of the International Astronautical Federation, Lausanne, October 1984     

Circadian rhythm of peripheral perfusion during 10-day hypoxic confinement and bed rest

Introduction

Future planetary habitats will be hypobaric and hypoxic to reduce the risk of decompression sickness during preparation for extra-vehicular activities. This study was part of a research programme investigating the combined effects of hypoxia and microgravity on physiological systems.

Purpose

We tested the hypothesis that hypoxia-induced peripheral vasoconstriction persists at night and is aggravated by bed rest. Since sleep onset has been causally linked to nocturnal vasodilatation, we reasoned that hypoxia-induced vasoconstriction at night may explain sleep disturbances at altitude. Peripheral perfusion alterations as a consequence of bed rest may explain poor sleep quality reported during sojourns on the International Space Station.

Methods

Eleven males underwent three 10-day interventions in a randomised order: (1) hypoxic ambulatory confinement; (2) hypoxic bed rest; (3) normoxic bed rest. During each intervention we conducted 22-h monitoring of peripheral perfusion, as reflected by the skin temperature gradient. Measurements were conducted on the first (D 1) and last day (D 10) of each intervention.

Results

All interventions resulted in a decrease in daytime toe perfusion from D 1 to D 10. There was no difference in the magnitude of the daytime reduction in toe perfusion between the three interventions. There was a significant vasodilatation of the toes in all interventions by 11 pm. The fingertips remained well perfused throughout.

Conclusions

Daytime vasoconstriction induced by hypoxia and/or bed rest is abolished at night, lending further support to the theory that changes in peripheral skin temperature may be functionally linked to sleep onset.     

Health-related fitness, physical activity, and history of back pain.

The relationship between history of back pain and measurements of both health-related fitness and physical activity participation was examined in 233 males and 287 females aged 15-69 years. Participants were divided by gender into those reporting no history (NH) or a history (H) of recurring back pain. Analysis of variance indicated that trunk flexion, back extensor endurance, and physical activity participation were significantly higher for NH and waist girth significantly lower for NH in both genders. In females, mean abdominal muscular endurance was significantly higher in NH. Forward stepwise discriminant function analyses indicated that the best discriminators between NH and H were back extensor endurance and physical activity participation in both genders and waist girth in females. These findings support using measurements of trunk flexion, abdominal muscular endurance, back extensor endurance, physical activity participation, and waist girth as indicators of back fitness in the evaluation of back health.     

Relations between pain, anxiety, mood and muscle tension in chronic pain patients. A correlation study

The relationship between psychological variables, particularly depressed mood and anxiety, and pain has received a good deal of attention. In this study, 16 chronic pain patients rated their mood, anxiety level, pain, muscle tension, and reclining time daily over a period of about 6 weeks. Correlations were calculated for each combination of variables for each individual patient. The results indicated a good deal of variability between patients. The majority of patients did have significant correlations for pain vs. mood, pain vs. anxiety, pain vs. reclining time, anxiety vs. muscle tension, anxiety vs. mood, and reclining time vs. mood. However, the size of the correlations was sometimes small, and some patients even occasionally had correlations in the opposite direction of that predicted. Overall, the data suggest that psychological variables are related to the experience of chronic pain, but that the size of the relationship may be smaller than previously thought. Furthermore, the high degree of individual variability needs to be stressed.     

Correlation dimension of EEG slow-wave activity during sleep in narcoleptic patients under bed rest conditions.

The calculation of the correlation dimension (D2) was applied to the study of the profiles of EEG slow-wave activity in nine narcoleptic subjects and nine sex- and age-matched control subjects who, following a baseline night recording, were maintained on 16 h of diurnal sleep deprivation and, thereafter, submitted to a 32-h bed rest protocol. The reversibility test allowed us to reject the null hypothesis that the time series considered in our study were generated by a static transformation of a linear Gaussian random process. Similarly, all profiles showed a positive largest Lyapunov exponent. Finally, the computation of D2 showed an average value of 5.27 (0.68 S.D.) in normal controls and 4.05 (1.49 S.D.) in narcoleptic patients (p = 0.067). Four of the narcoleptic patients showed values of D2 lower than 4, this was never observed in the normal controls (p = 0.0294). This study indicates that the mechanism of sleep-wake regulation in narcolepsy shows a somewhat lower degree of complexity as compared to normal controls. In particular, these data seem to confirm the already suggested different and simpler coupling between the homeostatic process of sleep regulation and the circadian and ultradian drives to sleep that occurs in bed rest condition in this disease.