Transcapillary fluid shifts in tissues of the head and neck during and after simulated microgravity

[ENT in zero g: the cosmic challenges of otolaryngology]

Manned spaceflight places special demands on the human body, including the organs in the ENT region. These organs play a critical role in maintaining the health and safety of astronauts in space. In this paper, we review common ENT problems of spaceflight, including upper airway edema, middle ear and mastoid effusions, hearing loss, and dizziness with nausea. We discuss the underlying mechanisms contributing to these complaints, their clinical manifestations, and potential treatment strategies. In addition, we examine the potential impact of these conditions on astronaut health and mission outcomes. Finally, we emphasize the importance of preventive measures and future research in this area to optimize astronaut health and safety on future missions.

Gravitational effects on carotid and jugular characteristics in graded head-up and head-down tilt

Altered gravity affects hemodynamics and blood flow in the neck. At least one incidence of jugular venous thrombosis has been reported in an astronaut on the International Space Station. This investigation explores the impact of changes in the direction of the gravitational vector on the characteristics of the neck arteries and veins. Twelve subjects underwent graded tilt from 45° head-up to 45° head-down in 15° increments in both supine and prone positions. At each angle, the cross-sectional area of the left and right common carotid arteries (ACCA) and internal jugular veins (AIJV) were measured by ultrasound. Internal jugular venous pressure (IJVP) was also measured by compression sonography. Gravitational dose-response curves were generated from experimental data. ACCA did not show any gravitational dependence. Conversely, both AIJV and IJVP increased in a nonlinear fashion with head-down tilt. AIJV was significantly larger on the right side than the left side at all tilt angles. In addition, IJVP was significantly elevated in the prone position compared with the supine position, most likely because of raised intrathoracic pressure while prone. Dose-response curves were compared with existing experimental data from parabolic flight and spaceflight studies, showing good agreement on an acute timescale. The quantification of jugular hemodynamics as a function of changes in the gravitational vector presented here provides a terrestrial model to reference spaceflight-induced changes, contributes to the assessment of the pathogenesis of spaceflight venous thromboembolism events, and informs the development of countermeasures.NEW & NOTEWORTHY Flow stasis and thrombosis have been identified in the jugular vein during spaceflight. We measured the area and pressure of the internal jugular vein and the area of the common carotid artery in graded head-up and head-down tilt. Experimental data are used to generate gravitational dose-response curves for the measured variables, demonstrating that jugular vein area and pressure exhibit a nonlinear response to altered gravity. Gravitational dose-response curves show good agreement with spaceflight and parabolic flight studies.

Reduced Regional Cerebral Blood Flow Measured by 99mTc-Hexamethyl Propylene Amine Oxime Single-Photon Emission Computed Tomography in Microgravity Simulated by 5-Day Dry Immersion

Microgravity induces a cephalad fluid shift that is responsible for cephalic venous stasis that may increase intracranial pressure (ICP) in astronauts. However, the effects of microgravity on regional cerebral blood flow (rCBF) are not known. We therefore investigated changes in rCBF in a 5-day dry immersion (DI) model. Moreover, we tested thigh cuffs as a countermeasure to prevent potential microgravity-induced modifications in rCBF. Around 18 healthy male participants underwent 5-day DI with or without a thigh cuffs countermeasure. They were randomly allocated to a control (n=9) or cuffs (n=9) group. rCBF was measured 4days before DI and at the end of the fifth day of DI (DI5), using single-photon emission computed tomography (SPECT) with radiopharmaceutical 99mTc-hexamethyl propylene amine oxime (99mTc-HMPAO). SPECT images were processed using statistical parametric mapping (SPM12) software. At DI5, we observed a significant decrease in rCBF in 32 cortical and subcortical regions, with greater hypoperfusion in basal ganglia (right putamen peak level: z=4.71, p uncorr<0.001), bilateral occipital regions (left superior occipital peak level: z=4.51, p uncorr<0.001), bilateral insula (right insula peak level: 4.10, p uncorr<0.001), and bilateral inferior temporal (right inferior temporal peak level: 4.07, p uncorr<0.001). No significant difference was found between the control and cuffs groups on change in rCBF after 5days of DI. After a 5-day DI, we found a decrease in rCBF in cortical and subcortical regions. However, thigh cuffs countermeasure failed to prevent hypoperfusion. To date, this is the first study measuring rCBF in DI. Further investigations are needed in order to better understand the underlying mechanisms in cerebral blood flow (CBF) changes after exposure to microgravity.

Microgravity × Radiation: A Space Mechanobiology Approach Toward Cardiovascular Function and Disease

In recent years, there has been an increasing interest in space exploration, supported by the accelerated technological advancements in the field. This has led to a new potential environment that humans could be exposed to in the very near future, and therefore an increasing request to evaluate the impact this may have on our body, including health risks associated with this endeavor. A critical component in regulating the human pathophysiology is represented by the cardiovascular system, which may be heavily affected in these extreme environments of microgravity and radiation. This mini review aims to identify the impact of microgravity and radiation on the cardiovascular system. Being able to understand the effect that comes with deep space explorations, including that of microgravity and space radiation, may also allow us to get a deeper understanding of the heart and ultimately our own basic physiological processes. This information may unlock new factors to consider with space exploration whilst simultaneously increasing our knowledge of the cardiovascular system and potentially associated diseases.

Ocular Outcomes in Healthy Subjects Undergoing a Short-Term Head-Down Tilt Test

PURPOSE: This study aimed to examine the effect of head-down tilt (HDT) on vascular autoregulation in different age groups and determine its effects on intraocular pressure (IOP) and central corneal thickness (CCT).METHODS: Included were 43 eyes of 23 men. The optic nerve head and parafoveal vascular densities were measured by optical coherence tomography angiography before and after 20 min 10 HDT. Also, the study comprised an examination of the IOP and CCT in a subset of 8 participants (14 eyes) in the sitting position and during 15 min of 10 HDT.RESULTS: Grid-based inside disc all-vessel density (GBID) was statistically significantly lower after the HDT test in subjects under 30 yr (1.26). Whole image and peripapillary capillary vessel density (WICVD, PCVD), and whole image and peripapillary all-vessel density (WIAVD, PAVD) were significantly higher after the HDT test in subjects ages 30-39 yr (1.34, 2.16, 1.05, 1.72, respectively). Inside disc capillary, all-vessel density (IDCVD, IDAVD) and GBID were significantly higher after HDT in subjects over 40 yr (2.48, 2.15, 1.52, respectively). In a subset of eight participants, IOP was significantly higher (3.7 mmHg) and CCT was unchanged after 15 min of HDT.CONCLUSION: Our study showed that simulated microgravity induced optic nerve head vessel density at the inside disc area, especially in persons over 40 years. In addition, IOP was increased by HDT, although no change in CCT was observed.Özelbaykal B, Öğretmenoğlu G, Tunçez I.H. Ocular outcomes in healthy subjects undergoing a short-term head-down tilt test. Aerosp Med Hum Perform. 2021; 92(8):619-626.

Head-Down Tilt Bed Rest Studies as a Terrestrial Analog for Spaceflight Associated Neuro-Ocular Syndrome

Astronauts who undergo prolonged periods of spaceflight may develop a unique constellation of neuro-ocular findings termed Spaceflight Associated Neuro-Ocular Syndrome (SANS). SANS is a disorder that is unique to spaceflight and has no terrestrial equivalent. The prevalence of SANS increases with increasing spaceflight duration and although there have been residual, structural, ocular changes noted, no irreversible or permanent visual loss has occurred after SANS, with the longest spaceflight to date being 14 months. These microgravity-induced findings are being actively investigated by the United States' National Aeronautics Space Administration (NASA) and SANS is a potential obstacle to future longer duration, manned, deep space flight missions. The pathophysiology of SANS remains incompletely understood but continues to be a subject of intense study by NASA and others. The study of SANS is of course partially limited by the small sample size of humans undergoing spaceflight. Therefore, identifying a terrestrial experimental model of SANS is imperative to facilitate its study and for testing of preventative measures and treatments. Head-down tilt bed rest (HDTBR) on Earth has emerged as one promising possibility. In this paper, we review the HDTBR as an analog for SANS pathogenesis; the clinical and imaging overlap between SANS and HDTBR studies; and potential SANS countermeasures that have been or could be tested with HDTBR.

Assessing lymphatic route of CSF outflow and peripheral lymphatic contractile activity during head-down tilt using near-infrared fluorescence imaging

Evidence overwhelmingly suggests that the lymphatics play a critical role in the clearance of cerebrospinal fluid (CSF) from the cranial space. Impairment of CSF outflow into the lymphatics is associated with a number of pathological conditions including spaceflight‐associated neuro‐ocular syndrome (SANS), a problem that limits long‐duration spaceflight. We used near‐infrared fluorescence lymphatic imaging (NIRFLI) to dynamically visualize the deep lymphatic drainage pathways shared by CSF outflow and disrupted during head‐down tilt (HDT), a method used to mimic the cephalad fluid shift that occurs in microgravity. After validating CSF clearance into the lymph nodes of the neck in swine, a pilot study was conducted in human volunteers to evaluate the effect of gravity on the flow of lymph through these deep cervical lymphatics. Injected into the palatine tonsils, ICG was imaged draining into deep jugular lymphatic vessels and subsequent cervical lymph nodes. NIRFLI was performed under HDT, sitting, and supine positions. NIRFLI shows that lymphatic drainage through pathways shared by CSF outflow are dependent upon gravity and are impaired under short‐term HDT. In addition, lymphatic contractile rates were evaluated from NIRFLI following intradermal ICG injections of the lower extremities. Lymphatic contractile activity in the legs was slowed in the gravity neutral, supine position, but increased under the influence of gravity regardless of whether its force direction opposed (sitting) or favored (HDT) lymphatic flow toward the heart. These studies evidence the role of a lymphatic contribution in SANS.

Neurosurgery and Manned Spaceflight

There has been a renewed interest in manned spaceflight due to endeavors by private and government agencies. Publicized goals include manned trips to or colonization of Mars. These missions will likely be of long duration, exceeding existing records for human exposure to extra-terrestrial conditions. Participants will be exposed to microgravity, temperature extremes, and radiation, all of which may adversely affect their physiology. Moreover, pathological mechanisms may differ from those of a terrestrial nature. Known central nervous system (CNS) changes occurring in space include rises in intracranial pressure and spinal unloading. Intracranial pressure increases are thought to occur due to cephalad re-distribution of body fluids secondary to microgravity exposure. Spinal unloading in microgravity results in potential degenerative changes to the bony vertebrae, intervertebral discs, and supportive musculature. These phenomena are poorly understood. Trauma is of highest concern due to its potential to seriously incapacitate crewmembers and compromise missions. Traumatic pathology may also be exacerbated in the setting of altered CNS physiology. Though there are no documented instances of CNS pathologies arising in space, existing diagnostic and treatment capabilities will be limited relative to those on Earth. In instances where neurosurgical intervention is required in space, it is not known whether open or endoscopic approaches are feasible. It is obvious that prevention of trauma and CNS pathology should be emphasized. Further research into neurosurgical pathology, its diagnosis, and treatment in space are required should exploratory or colonization missions be attempted.

Association of Exercise and Swimming Goggles With Modulation of Cerebro-ocular Hemodynamics and Pressures in a Model of Spaceflight-Associated Neuro-ocular Syndrome

Importance

Astronauts on International Space Station missions demonstrate adverse neuro-ocular changes. Reversing a negative translaminar pressure gradient (TLPG) by modulating cerebral blood flow, decreasing intracranial pressure, or increasing intraocular pressure (IOP) has been proposed as potential intervention for spaceflight-associated neuro-ocular syndrome (SANS).

Objective

To examine whether exercise (resistance, moderate-intensity aerobic, and high-intensity aerobic) or artificially increasing IOP is associated with modulated cerebro-ocular hemodynamic and pressure changes during head-down tilt (HDT), an analogue of spaceflight, in healthy adults.

Design, Setting, and Participants

A single-center investigation was conducted at Johnson Space Center, Houston, Texas, from January 1, 2014, to December 31, 2016, in 20 healthy men.

Exposure

On 3 separate days, participants rested supine, were tilted to -15° HDT, and then completed 1 of 3 experimental exercise conditions (moderate-intensity aerobic, resistance, or high-intensity interval aerobic). A subset of 10 participants wore swimming goggles on all days.

Main Outcomes and Measures

Applanation rebound tonometry was used to noninvasively assess IOP, and compression sonography was used to assess internal jugular venous pressure (IJVP). Estimated TLPG was calculated as the difference between IOP and IJVP. Cerebral inflow and outflow were measured in extracranial arteries using color-coded duplex ultrasonography.

Results

Twenty men participated in the study (mean [SD] age, 36 [9] years). Compared with supine IOP (mean [SD], 19.3 [3.7] mm Hg), IJVP (mean [SD], 21.4 [6.0] mm Hg), and estimated TLPG (mean [SD], -2.1 [7.0] mm Hg), -15° HDT was associated with increased IOP (mean difference, 2.3 mm Hg; 95% CI, 1.4-3.3 mm Hg; P < .001) and IJVP (mean difference, 10.5 mm Hg; 95% CI, 8.9-12.2 mm Hg; P < .001) and with decreased TLPG (mean difference, -8.2 mm Hg; 95% CI, -10.1 to -6.3 mm Hg; P < .001). Exercise (regardless of modality) at -15° HDT was associated with decreased IOP (mean difference, -1.6 mm Hg; 95% CI, -2.6 to -0.6 mm Hg; P = .002) and TLPG (mean difference, -3.5 mm Hg; 95% CI, -6.2 to -0.7 mm Hg; P = .01) compared with rest. Both IOP (mean difference, 2.9 mm Hg; 95% CI, 0.7-5.1 mm Hg; P = .01) and TLPG (mean difference, 5.1 mm Hg; 95% CI, 0.8-9.4 mm Hg; P = .02) were higher in participants who wore swimming goggles compared with those not wearing goggles.

Conclusions and Relevance

In this study, exercise was associated with decreased IOP and estimated translaminar pressure gradient in a spaceflight analogue of HDT. The addition of swimming goggles was associated with increased IOP and TLPG in HDT. Further evaluation in spaceflight may be warranted to determine whether modestly increasing IOP is an effective SANS countermeasure.

Biofluid modeling of the coupled eye-brain system and insights into simulated microgravity conditions

This work aims at investigating the interactions between the flow of fluids in the eyes and the brain and their potential implications in structural and functional changes in the eyes of astronauts, a condition also known as spaceflight associated neuro-ocular syndrome (SANS). To this end, we propose a reduced (0-dimensional) mathematical model of fluid flow in the eyes and brain, which is embedded into a simplified whole-body circulation model. In particular, the model accounts for: (i) the flows of blood and aqueous humor in the eyes; (ii) the flows of blood, cerebrospinal fluid and interstitial fluid in the brain; and (iii) their interactions. The model is used to simulate variations in intraocular pressure, intracranial pressure and blood flow due to microgravity conditions, which are thought to be critical factors in SANS. Specifically, the model predicts that both intracranial and intraocular pressures increase in microgravity, even though their respective trends may be different. In such conditions, ocular blood flow is predicted to decrease in the choroid and ciliary body circulations, whereas retinal circulation is found to be less susceptible to microgravity-induced alterations, owing to a purely mechanical component in perfusion control associated with the venous segments. These findings indicate that the particular anatomical architecture of venous drainage in the retina may be one of the reasons why most of the SANS alterations are not observed in the retina but, rather, in other vascular beds, particularly the choroid. Thus, clinical assessment of ocular venous function may be considered as a determinant SANS factor, for which astronauts could be screened on earth and in-flight.

Venous collapse regulates intracranial pressure in upright body positions

Recent interest in intracranial pressure (ICP) in the upright posture has revealed that the mechanisms regulating postural changes in ICP are not fully understood. We have suggested an explanatory model where the postural changes in ICP depend on well-established hydrostatic effects in the venous system and where these effects are interrupted by collapse of the internal jugular veins (IJVs) in more upright positions. The aim of this study was to investigate this relationship by simultaneous invasive measurements of ICP, venous pressure, and IJV collapse in healthy volunteers. ICP (monitored via the lumbar route), central venous pressure (peripherally inserted central catheter line), and IJV cross-sectional area (ultrasound) were measured in 11 healthy volunteers (47 ± 10 yr, mean ± SD) in 7 positions, from supine to sitting (0–69°). Venous pressure and anatomical distances were used to predict ICP in accordance with the explanatory model, and IJV area was used to assess IJV collapse. The hypothesis was tested by comparing measured ICP with predicted ICP. Our model accurately described the general behavior of the observed postural ICP changes (mean difference, −0.03 ± 2.7 mmHg). No difference was found between predicted and measured ICP for any tilt angle ( P values, 0.65–0.94). The results support the hypothesis that postural ICP changes are governed by hydrostatic effects in the venous system and IJV collapse. This improved understanding of postural ICP regulation may have important implications for the development of better treatments for neurological and neurosurgical conditions affecting ICP.

Spaceflight-Induced Intracranial Hypertension and Visual Impairment: Pathophysiology and Countermeasures

Visual impairment intracranial pressure (VIIP) syndrome is considered an unexplained major risk for future long-duration spaceflight. NASA recently redefined this syndrome as Spaceflight-Associated Neuro-ocular Syndrome (SANS). Evidence thus reviewed supports that chronic, mildly elevated intracranial pressure (ICP) in space (as opposed to more variable ICP with posture and activity on Earth) is largely accounted for by loss of hydrostatic pressures and altered hemodynamics in the intracranial circulation and the cerebrospinal fluid system. In space, an elevated pressure gradient across the lamina cribrosa, caused by a chronic but mildly elevated ICP, likely elicits adaptations of multiple structures and fluid systems in the eye which manifest themselves as the VIIP syndrome. A chronic mismatch between ICP and intraocular pressure (IOP) in space may acclimate the optic nerve head, lamina cribrosa, and optic nerve subarachnoid space to a condition that is maladaptive to Earth, all contributing to the pathogenesis of space VIIP syndrome. Relevant findings help to evaluate whether artificial gravity is an appropriate countermeasure to prevent this seemingly adverse effect of long-duration spaceflight.

Bone microvascular flow differs from skin microvascular flow in response to head-down tilt

Loss of hydrostatic pressures in microgravity may alter skin and bone microvascular flows in the lower extremities and potentially reduce wound healing and bone fracture repair. The purpose of this study was to determine the rate at which skin and bone microvascular flows respond to head-down tilt (HDT). We hypothesized that microvascular flows in tibial bone and overlying skin would increase at different rates during HDT. Tibial bone and skin microvascular flows were measured simultaneously using photoplethysmography (PPG) in a total of 17 subjects during sitting (control posture), supine, 6° HDT, 15° HDT, and 30° HDT postures in random order. With greater angles of HDT, bone microvascular flow increased significantly, but skin microvascular flow did not change. Tibial bone microvascular flow increased from the sitting control posture (0.77 ± 0.41 V) to supine (1.95 ± 1.01 V, P = 0.001) and from supine posture to 15° HDT (3.74 ± 2.43 V, P = 0.004) and 30° HDT (3.91 ± 2.68 V, P = 0.006). Skin microvascular flow increased from sitting (0.703 ± 0.75 V) to supine (2.19 ± 1.72 V, P = 0.02) but did not change from supine posture to HDT (P = 1.0). We show for the first time that microcirculatory flows in skin and bone of the leg respond to simulated microgravity at different rates. These altered levels of blood perfusion may affect rates of wound and bone fracture healing in spaceflight.NEW & NOTEWORTHY Our data show that bone microvascular flow increases more than cutaneous blood flow with greater degrees of head-down tilt. A higher level of perfusion in bone may give insight into the bone mineral density loss in lower extremities of astronauts and why similar tissue degradation is not observed in the skin of the same areas.

Thirty days of spaceflight does not alter murine calvariae structure despite increased Sost expression

Previously our laboratory documented increases in calvaria bone volume and thickness in mice exposed to 15 days of spaceflight aboard the NASA Shuttle mission STS-131. However, the tissues were not processed for gene expression studies to determine what bone formation pathways might contribute to these structural adaptations. Therefore, this study was designed to investigate both the structural and molecular changes in mice calvariae after a longer duration of spaceflight. The primary purpose was to determine the calvaria bone volume and thickness of mice exposed to 30 days of spaceflight using micro-computed tomography for comparison with our previous findings. Because sclerostin, the secreted glycoprotein of the Sost gene, is a potent inhibitor of bone formation, our second aim was to quantify Sost mRNA expression using quantitative PCR. Calvariae were obtained from six mice aboard the Russian 30-day Bion-M1 biosatellite and seven ground controls. In mice exposed to 30 days of spaceflight, calvaria bone structure was not significantly different from that of their controls (bone volume was about 5% lower in spaceflight mice, p = 0.534). However, Sost mRNA expression was 16-fold (16.4 ± 0.4, p < 0.001) greater in the spaceflight group than that in the ground control group. Therefore, bone formation may have been suppressed in mice exposed to 30 days of spaceflight. Genetic responsiveness (e.g. sex or strain of animals) or in-flight environmental conditions other than microgravity (e.g. pCO₂ levels) may have elicited different bone adaptations in STS-131 and Bion-M1 mice. Although structural results were not significant, this study provides biochemical evidence that calvaria mechanotransduction pathways may be altered during spaceflight, which could reflect vascular and interstitial fluid adaptations in non-weight bearing bones. Future studies are warranted to elucidate the processes that mediate these effects and the factors responsible for discordant calvaria bone adaptations between STS-131 and Bion-M1 mice.

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Previously, 15 days of spaceflight augmented bone volume in mice calvariae.

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In this study, calvaria bone structure was not altered after 30 days of spaceflight.

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Sost mRNA expression was higher in murine calvariae after 30 days of spaceflight.

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Longer duration, or other spaceflight factors, may negate short-term calvarial growth.

Human Pathophysiological Adaptations to the Space Environment

Space is an extreme environment for the human body, where during long-term missions microgravity and high radiation levels represent major threats to crew health. Intriguingly, space flight (SF) imposes on the body of highly selected, well-trained, and healthy individuals (astronauts and cosmonauts) pathophysiological adaptive changes akin to an accelerated aging process and to some diseases. Such effects, becoming manifest over a time span of weeks (i.e., cardiovascular deconditioning) to months (i.e., loss of bone density and muscle atrophy) of exposure to weightlessness, can be reduced through proper countermeasures during SF and in due time are mostly reversible after landing. Based on these considerations, it is increasingly accepted that SF might provide a mechanistic insight into certain pathophysiological processes, a concept of interest to pre-nosological medicine. In this article, we will review the main stress factors encountered in space and their impact on the human body and will also discuss the possible lessons learned with space exploration in reference to human health on Earth. In fact, this is a productive, cross-fertilized, endeavor in which studies performed on Earth yield countermeasures for protection of space crew health, and space research is translated into health measures for Earth-bound population.

Human jugular vein collapse in the upright posture: implications for postural intracranial pressure regulation

Background

Intracranial pressure (ICP) is directly related to cranial dural venous pressure (P_dural). In the upright posture, P_dural is affected by the collapse of the internal jugular veins (IJVs) but this regulation of the venous pressure has not been fully understood. A potential biomechanical description of this regulation involves a transmission of surrounding atmospheric pressure to the internal venous pressure of the collapsed IJVs. This can be accomplished if hydrostatic effects are cancelled by the viscous losses in these collapsed veins, resulting in specific IJV cross-sectional areas that can be predicted from flow velocity and vessel inclination.

Methods

We evaluated this potential mechanism in vivo by comparing predicted area to measured IJV area in healthy subjects. Seventeen healthy volunteers (age 45 ± 9 years) were examined using ultrasound to assess IJV area and flow velocity. Ultrasound measurements were performed in supine and sitting positions.

Results

IJV area was 94.5 mm² in supine and decreased to 6.5 ± 5.1 mm² in sitting position, which agreed with the predicted IJV area of 8.7 ± 5.2 mm² (equivalence limit ±5 mm², one-sided t tests, p = 0.03, 33 IJVs).

Conclusions

The agreement between predicted and measured IJV area in sitting supports the occurrence of a hydrostatic-viscous pressure balance in the IJVs, which would result in a constant pressure segment in these collapsed veins, corresponding to a zero transmural pressure. This balance could thus serve as the mechanism by which collapse of the IJVs regulates P_dural and consequently ICP in the upright posture.

Electronic supplementary material

The online version of this article (doi:10.1186/s12987-017-0065-2) contains supplementary material, which is available to authorized users.

Lower-body negative pressure decreases noninvasively measured intracranial pressure and internal jugular vein cross-sectional area during head-down tilt

Long-term spaceflight induces a near visual acuity change in ~50% of astronauts. In some crew members, postflight cerebrospinal fluid (CSF) opening pressures by lumbar puncture are as high as 20.9 mmHg; these members demonstrated optic disc edema. CSF communicates through the cochlear aqueduct to affect perilymphatic pressure and tympanic membrane motion. We hypothesized that 50 mmHg of lower-body negative pressure (LBNP) during 15° head-down tilt (HDT) would mitigate elevations in internal jugular vein cross-sectional area (IJV CSA) and intracranial pressure (ICP). Fifteen healthy adult volunteers were positioned in sitting (5 min), supine (5 min), 15° HDT (5 min), and 15° HDT with LBNP (10 min) postures for data collection. Evoked tympanic membrane displacements (TMD) quantified ICP noninvasively. IJV CSA was measured using standard ultrasound techniques. ICP and IJV CSA increased significantly from the seated upright to the 15° HDT posture (P < 0.05), and LBNP mitigated these increases. LBNP at 25 mmHg reduced ICP during HDT (TMD of 322.13 ± 419.17 nl) to 232.38 ± 445.85 nl, and at 50 mmHg ICP was reduced further to TMD of 199.76 ± 429.69 nl. In addition, 50 mmHg LBNP significantly reduced IJV CSA (1.50 ± 0.33 cm²) during 15° HDT to 0.83 ± 0.42 cm² LBNP counteracts the headward fluid shift elevation of ICP and IJV CSA experienced during microgravity as simulated by15° HDT. These data provide quantitative evidence that LBNP shifts cephalic fluid to the lower body, reducing IJV CSA and ICP.NEW & NOTEWORTHY The current study provides new evidence that 25 or 50 mmHg of lower body negative pressure reduces jugular venous pooling and intracranial pressure during simulated microgravity. Therefore, spaceflight countermeasures that sequester fluid to the lower body may mitigate cephalic venous congestion and vision impairment.

Long-duration bed rest as an analog to microgravity

Long-duration bed rest is widely employed to simulate the effects of microgravity on various physiological systems, especially for studies of bone, muscle, and the cardiovascular system. This microgravity analog is also extensively used to develop and test countermeasures to microgravity-altered adaptations to Earth gravity. Initial investigations of bone loss used horizontal bed rest with the view that this model represented the closest approximation to inactivity and minimization of hydrostatic effects, but all Earth-based analogs must contend with the constant force of gravity by adjustment of the G vector. Later concerns about the lack of similarity between headward fluid shifts in space and those with horizontal bed rest encouraged the use of 6 degree head-down tilt (HDT) bed rest as pioneered by Russian investigators. Headward fluid shifts in space may redistribute bone from the legs to the head. At present, HDT bed rest with normal volunteers is the most common analog for microgravity simulation and to test countermeasures for bone loss, muscle and cardiac atrophy, orthostatic intolerance, and reduced muscle strength/exercise capacity. Also, current physiologic countermeasures are focused on long-duration missions such as Mars, so in this review we emphasize HDT bed rest studies with durations of 30 days and longer. However, recent results suggest that the HDT bed rest analog is less representative as an analog for other important physiological problems of long-duration space flight such as fluid shifts, spinal dysfunction and radiation hazards.

Evaluation of a fiber-optic technique for recording intramuscular pressure in the human leg

To evaluate a forward-sensing fiber-optic pressure technique for recording of intramuscular pressure (IMP) in the human leg and investigate factors that may influence IMP measurements used in diagnosing compartment syndromes. IMP in the tibialis anterior muscle was recorded simultaneously by a fiber-optic technique and needle-injection technique in 12 legs of 7 healthy subjects. Both measurement catheters were placed in parallel with the muscle fibers to the same depth, as verified by sonography. IMP recordings were performed at rest before, during and after applying a model of abnormally elevated IMP (simulated compartment syndrome). IMP was elevated by venous obstruction induced by a thigh tourniquet of a casted leg. IMP was also measured during injections of 0.1 ml of saline into the muscle through the catheters. IMP at baseline was 5.1 (SD = 2.6) mmHg measured with the fiber-optic technique and 7.1 (SD = 2.5) mmHg with the needle-injection technique (p < 0.001). It increased to 48.5 (SD = 6.9) mmHg and 47.6 (SD = 6.6) mmHg respectively, during simulated compartment syndrome. IMP increased significantly following injection of 0.1 ml of saline, measured by both techniques. It remained increased 1 min after injection. The fiber-optic technique was able to record pulse-synchronous IMP oscillations. The fiber-optic technique may be used for IMP measurements in a muscle with both normal and abnormally elevated IMP. It has good dynamic properties allowing for measurement of IMP oscillations. Saline injection used with needle-injection systems to ensure catheter patency compromises IMP readings at least one minute after injection.

Fibrin, γ'-fibrinogen, and transclot pressure gradient control hemostatic clot growth during human blood flow over a collagen/tissue factor wound

OBJECTIVE

Biological and physical factors interact to modulate blood response in a wounded vessel, resulting in a hemostatic clot or an occlusive thrombus. Flow and pressure differential (ΔP) across the wound from the lumen to the extravascular compartment may impact hemostasis and the observed core/shell architecture. We examined physical and biological factors responsible for regulating thrombin-mediated clot growth.

APPROACH AND RESULTS

Using factor XIIa-inhibited human whole blood perfused in a microfluidic device over collagen/tissue factor at controlled wall shear rate and ΔP, we found thrombin to be highly localized in the P-selectin(+) core of hemostatic clots. Increasing ΔP from 9 to 29 mm Hg (wall shear rate=400 s(-1)) reduced P-selectin(+) core size and total clot size because of enhanced extravasation of thrombin. Blockade of fibrin polymerization with 5 mmol/L Gly-Pro-Arg-Pro dysregulated hemostasis by enhancing both P-selectin(+) core size and clot size at 400 s(-1) (20 mm Hg). For whole-blood flow (no Gly-Pro-Arg-Pro), the thickness of the P-selectin-negative shell was reduced under arterial conditions (2000 s(-1), 20 mm Hg). Consistent with the antithrombin-1 activity of fibrin implicated with Gly-Pro-Arg-Pro, anti-γ'-fibrinogen antibody enhanced core-localized thrombin, core size, and overall clot size, especially at venous (100 s(-1)) but not arterial wall shear rates (2000 s(-1)). Pathological shear (15 000 s(-1)) and Gly-Pro-Arg-Pro synergized to exacerbate clot growth.

CONCLUSIONS

Hemostatic clotting was dependent on core-localized thrombin that (1) triggered platelet P-selectin display and (2) was highly regulated by fibrin and the transclot ΔP. Also, γ'-fibrinogen had a role in venous but not arterial conditions.

Microgravity-induced fluid shift and ophthalmic changes

Although changes to visual acuity in spaceflight have been observed in some astronauts since the early days of the space program, the impact to the crew was considered minor. Since that time, missions to the International Space Station have extended the typical duration of time spent in microgravity from a few days or weeks to many months. This has been accompanied by the emergence of a variety of ophthalmic pathologies in a significant proportion of long-duration crewmembers, including globe flattening, choroidal folding, optic disc edema, and optic nerve kinking, among others. The clinical findings of affected astronauts are reminiscent of terrestrial pathologies such as idiopathic intracranial hypertension that are characterized by high intracranial pressure. As a result, NASA has placed an emphasis on determining the relevant factors and their interactions that are responsible for detrimental ophthalmic response to space. This article will describe the Visual Impairment and Intracranial Pressure syndrome, link it to key factors in physiological adaptation to the microgravity environment, particularly a cephalad shifting of bodily fluids, and discuss the implications for ocular biomechanics and physiological function in long-duration spaceflight.

Blood pressure regulation IV: adaptive responses to weightlessness

During weightlessness, blood and fluids are immediately shifted from the lower to the upper body segments, and within the initial 2 weeks of spaceflight, brachial diastolic arterial pressure is reduced by 5 mmHg and even more so by some 10 mmHg from the first to the sixth month of flight. Blood pressure thus adapts in space to a level very similar to that of being supine on the ground. At the same time, stroke volume and cardiac output are increased and systemic vascular resistance decreased, whereas sympathetic nerve activity is kept surprisingly high and similar to when ground-based upright seated. This was not predicted from simulation models and indicates that dilatation of the arteriolar resistance vessels is caused by mechanisms other than a baroreflex-induced decrease in sympathetic nervous activity. Results of baroreflex studies in space indicate that compared to being ground-based supine, the carotid (vagal)-cardiac interaction is reduced and sympathetic nerve activity, heart rate and systemic vascular resistance response more pronounced during baroreflex inhibition by lower body negative pressure. The future challenge is to identify which spaceflight mechanism induces peripheral arteriolar dilatation, which could explain the decrease in blood pressure, the high sympathetic nerve activity and associated cardiovascular changes. It is also a challenge to determine the cardiovascular risk profile of astronauts during future long-duration deep space missions.

Side view thrombosis microfluidic device with controllable wall shear rate and transthrombus pressure gradient

Hemodynamic conditions vary throughout the vasculature, creating diverse environments in which platelets must respond. To stop bleeding, a growing platelet deposit must be assembled in the presence of fluid wall shear stress (τw) and a transthrombus pressure gradient (ΔP) that drives bleeding. We designed a microfluidic device capable of pulsing a fluorescent solute through a developing thrombus forming on collagen ± tissue factor (TF), while independently controlling ΔP and τw. Computer control allowed step changes in ΔP with a rapid response time of 0.26 mm Hg s(-1) at either venous (5.2 dynes cm(-2)) or arterial (33.9 dynes cm(-2)) wall shear stresses. Side view visualization of thrombosis with transthrombus permeation allowed for quantification of clot structure, height, and composition at various ΔP. Clot height was reduced 20% on collagen/TF and 28% on collagen alone when ΔP was increased from 20.8 to 23.4 mm Hg at constant arterial shear stress. When visualized with a platelet-targeting thrombin sensor, intrathrombus thrombin levels decreased by 62% as ΔP was increased from 0 to 23.4 mm Hg across the thrombus-collagen/TF barrier, consistent with convective removal of thrombogenic solutes due to pressure-driven permeation. Independent of ΔP, the platelet deposit on collagen had a permeability of 5.45 × 10(-14) cm(2), while the platelet/fibrin thrombus on collagen/TF had a permeability of 2.71 × 10(-14) cm(2) (comparable to that of an intact endothelium). This microfluidic design allows investigation of the coupled processes of platelet deposition and thrombin/fibrin generation in the presence of controlled transthrombus permeation and wall shear stress.

A rapid rotation to an inverted seated posture inhibits muscle force, activation, heart rate and blood pressure

Although previous studies have demonstrated neuromuscular and cardiovascular changes with slow inversion rates, emergencies, such as overturned vehicles and helicopters can occur rapidly. The purpose of this study was to investigate changes in neuromuscular and cardiovascular responses with rapid (1 s) and slower (3 s) transitions from upright to inverted seated positions. Twenty-two subjects performed separate and concurrent unilateral elbow flexion and leg extension maximal voluntary contractions (MVCs) for 6 s in an upright seated position and when inverted with 1 and 3 s rotations. Elbow flexion and leg extension force; biceps, triceps, quadriceps and hamstrings electromyographic (EMG) activity, heart rate (HR), systolic blood pressure (SBP) and diastolic blood pressure (DBP) were measured. Whether the elbow flexion or leg extension contractions occurred concurrently or individually, significant (p < 0.05) decreases in MVC force and EMG activity were found when inverted within 1 and 3 s rotations as compared to upright. Triceps and hamstrings EMG activity (p < 0.05) decreased when inverted within 1 s rotation as compared to upright. Following rotation, the maintenance of the inverted position (3-6 s timepoint) resulted in a significant (p < 0.05) increase in leg extension MVC as compared to the initial second of rotation to inversion. HR, SBP and DBP demonstrated (p < 0.001) decreases when inverted within 1 and 3 s rotations as compared to upright. In conclusion, this is the first study to show that irrespective of rotation speed, inversion inhibited neuromuscular and cardiovascular responses, similar to the more deliberate, slower rotation of previous inversion studies.

Space physiology IV: mathematical modeling of the cardiovascular system in space exploration

Mathematical modeling represents an important tool for analyzing cardiovascular function during spaceflight. This review describes how modeling of the cardiovascular system can contribute to space life science research and illustrates this process via modeling efforts to study postflight orthostatic intolerance (POI), a key issue for spaceflight. Examining this application also provides a context for considering broader applications of modeling techniques to the challenges of bioastronautics. POI, which affects a large fraction of astronauts in stand tests upon return to Earth, presents as dizziness, fainting and other symptoms, which can diminish crew performance and cause safety hazards. POI on the Moon or Mars could be more critical. In the field of bioastronautics, POI has been the dominant application of cardiovascular modeling for more than a decade, and a number of mechanisms for POI have been investigated. Modeling approaches include computational models with a range of incorporated factors and hemodynamic sophistication, and also physical models tested in parabolic and orbital flight. Mathematical methods such as parameter sensitivity analysis can help identify key system mechanisms. In the case of POI, this could lead to more effective countermeasures. Validation is a persistent issue in modeling efforts, and key considerations and needs for experimental data to synergistically improve understanding of cardiovascular responses are outlined. Future directions in cardiovascular modeling include subject-specific assessment of system status, as well as research on integrated physiological responses, leading, for instance, to assessment of subject-specific susceptibility to POI or effects of cardiovascular alterations on muscular, vision and cognitive function.

Computational simulation to understand vision changes during prolonged weightlessness

A mathematical model of whole body and cerebral hemodynamics is a useful tool for investigating visual impairment and intracranial pressure (VIIP), a recently described condition associated with space flight. VIIP involves loss of visual acuity, anatomical changes to the eye, and, usually, elevated cerebrospinal fluid pressure. Loss of visual acuity is a significant threat to astronaut health and performance. It is therefore important to understand the pathogenesis of VIIP. Some of the experimental measurements that could lead to better understanding of the pathophysiology are impossible or infeasible on orbit. A computational implementation of a mathematical model of hypothetical pathophysiological processes is therefore valuable. Such a model is developed, and is used to investigate how changes in vascular compliance or pressure can influence intraocular or intracranial pressure.

Platelet-targeting sensor reveals thrombin gradients within blood clots forming in microfluidic assays and in mouse

BACKGROUND

Thrombin undergoes convective and diffusive transport, making it difficult to visualize during thrombosis. We developed the first sensor capable of revealing inner clot thrombin dynamics.

METHODS AND RESULTS

An N-terminal-azido thrombin-sensitive fluorescent peptide (ThS-P) with a thrombin-releasable quencher was linked to anti-CD41 using click chemistry to generate a thrombin-sensitive platelet binding sensor (ThS-Ab). Rapid thrombin cleavage of ThS-P (K(m) = 40.3 μm, k(cat) = 1.5 s(-1) ) allowed thrombin monitoring by ThS-P or ThS-Ab in blood treated with 2-25 pm tissue factor (TF). Individual platelets had > 20-fold more ThS-Ab fluorescence after clotting. In a microfluidic assay of whole blood perfusion over collagen ± linked TF (wall shear rate = 100 s(-1) ), ThS-Ab fluorescence increased between 90 and 450 s for 0.1-1 molecule-TF μm(-2) and co-localized with platelets near fibrin. Without TF, neither thrombin nor fibrin was detected on the platelet deposits by 450 s. Using a microfluidic device to control the pressure drop across a thrombus forming on a porous collagen/TF plug (521 s(-1) ), thrombin and fibrin were detected at the thrombus-collagen interface at a zero pressure drop, whereas 80% less thrombin was detected at 3200 Pa in concert with fibrin polymerizing within the collagen. With anti-mouse CD41 ThS-Ab deployed in a mouse laser injury model, the highest levels of thrombin arose between 40 and 160 s nearest the injury site where fibrin co-localized and where the thrombus was most mechanically stable.

CONCLUSION

ThS-Ab reveals thrombin locality, which depends on surface TF, flow and intrathrombus pressure gradients.

Blood clots are rapidly assembled hemodynamic sensors: flow arrest triggers intraluminal thrombus contraction

OBJECTIVE

Blood clots form under flow during intravascular thrombosis or vessel leakage. Prevailing hemodynamics influence thrombus structure and may regulate contraction processes. A microfluidic device capable of flowing human blood over a side channel plugged with collagen (± tissue factor) was used to measure thrombus permeability (κ) and contraction at controlled transthrombus pressure drops.

METHODS AND RESULTS

The collagen (κ(collagen)=1.98 × 10(-11) cm(2)) supported formation of a 20-µm thick platelet layer, which unexpectedly underwent massive platelet retraction on flow arrest. This contraction resulted in a 5.34-fold increase in permeability because of collagen restructuring. Without stopping flow, platelet deposits (no fibrin) had a permeability of κ(platelet)=5.45 × 10(-14) cm(2) and platelet-fibrin thrombi had κ(thrombus)=2.71 × 10(-14) cm(2) for ΔP=20.7 to 23.4 mm Hg, the first ever measurements for clots formed under arterial flow (1130 s(-1) wall shear rate). Platelet sensing of flow cessation triggered a 4.6- to 6.5-fold (n=3, P<0.05) increase in contraction rate, which was also observed in a rigid, impermeable parallel-plate microfluidic device. This triggered contraction was blocked by the myosin IIA inhibitor blebbistatin and by inhibitors of thromboxane A2 (TXA(2)) and ADP signaling. In addition, flow arrest triggered platelet intracellular calcium mobilization, which was blocked by TXA(2)/ADP inhibitors. As clots become occlusive or blood pools following vessel leakage, the flow diminishes, consequently allowing full platelet retraction.

CONCLUSIONS

Flow dilution of ADP and thromboxane regulates platelet contractility with prevailing hemodynamics, a newly defined flow-sensing mechanism to regulate clot function.

Evaluation by an aeronautic dentist on the adverse effects of a six-week period of microgravity on the oral cavity

Objective. HDT bed rest condition is a simulated microgravity condition in which subject lies on bed inclined −6 degree feet up. To determine the influence of a simulated microgravity (HDT bed rest) on oral cavity, 10 healthy male volunteers were studied before, during, just after, and after 6 weeks of the simulated microgravity condition of −6° head-down-tilt (HDT) bed rest. Materials and Methods. Facial nerve function, facial sensation, chemosensory system, salivary biomarkers were measured. Results. Lactate dehydrogenase, MIP 1 alpha, malonaldehyde, 8-hydroxydeoxyguanosine, and thiocyanate were found to increase significantly, while flow rate, sodium, potassium, calcium, phosphate, protein, amylase activity, vitamin E and C, and mouth opening were decreased in simulation environments in contradiction to normal. The threshold for monosodium glutamate (MSG) and capsaicin increased during microgravity as compared to normal conditions. Moderate pain of teeth, facial oedema, mild pain, loss of sensation of pain and temperature, decreased tongue, and mandibular movement in simulation microgravity environments were observed. Conclusions. These results suggest that reversible effect of microgravity is oedema of face, change in taste, abnormal expression of face, teeth pain, and xerostomia. Further study will be required on large scale on long-term effects of microgravity on oral cavity to prevent the adverse effects.

Cardiovascular adaptations, fluid shifts, and countermeasures related to space flight

Significant progress has been made related to understanding cardiovascular adaptations to microgravity and development of countermeasures to improve crew re-adaptation to gravity. The primary ongoing issues are orthostatic intolerance after flight, reduced exercise capacity, the effect of vascular-smooth muscle loss on other physiologic systems, development of efficient and low-cost countermeasures to counteract these losses, and an understanding of fluid shift mechanisms. Previous animal studies of cardiovascular adaptations offer evidence that prolonged microgravity remodels walls of blood vessels, which in turn, is important for deconditioning of the cardiovascular system and other functions of the body. Over the past 10 years, our studies have documented that treadmill exercise within lower body negative pressure counteracts most physiologic decrements with bed rest in both women and men. Future studies should improve hardware and protocols to protect crew members during prolonged missions. Finally, it is proposed that transcapillary fluid shifts in microgravity may be related to the loss of tissue weight and external compression of blood vessels.

A simple nonmydriatic self-retinal imaging procedure using a Kowa Genesis-D hand-held digital fundus camera

Research on vascular adaptation to microgravity in the central nervous system requires a simple, noninvasive, direct imaging technique that can be performed with compact equipment. In this report we describe a practical, nonmydriatic, retinal self-imaging technique using a Kowa Genesis-D hand-held digital camera and a Black and Decker laser level. This simple technique will be useful to clinical physiologists conducting microgravity research, as well as for the studies of high-altitude medicine and aviation physiology.

An inverted seated posture decreases elbow flexion force and muscle activation

The purpose of this study was to determine if discrepancies exist between upright and inverted seated positions in isometric maximal voluntary contraction (MVC) elbow flexor force, MVC force produced in the first 100 ms (F100), MVC rate of force development, electromyographic (EMG) activity of the biceps and triceps as well as heart rate and blood pressure. The results showed significantly (p < 0.01) higher MVC force (543.6 +/- 29.6 vs. 486.5 +/- 23.0 N), F100 (328.3 +/- 94.5 vs. 274.6 +/- 101.8 N), rate of force development (p = 0.003) (1,851.9 +/- 742.2 vs. 1,591.0 +/- 719.6 N s(-1)) and biceps brachii EMG activity (48%, p < 0.01) in the upright versus inverted condition. There were relatively greater co-contractions with the inverted position (p < 0.01) due to the lack of change in triceps' EMG and the substantial decrease in biceps' EMG. There were no significant changes in trunk EMG activity. With inversion, there were significant decreases in heart rate (16.8%), systolic (11.6%) and diastolic (12.1%) blood pressures (p < 0.0001). These results illustrate decrements in neuromuscular performance with an inverted seated posture which may be related to an altered sympathetic response.

Head-down tilt posture elicits transient lymphocyte mobilization from the iliac, but not mesenteric, lymph nodes of rats

The effects of short-term simulated microgravity on the lymph dynamics of rat lymph nodes were investigated using a combination of Bollman's cage and head-down tilt (HDT). Efferent lymphatics of the iliac and mesenteric lymph nodes were cannulated for the collection of lymph. There was no significant difference in lymph flow rate from the iliac lymph nodes between non-HDT (control) and HDT rats. Lymph flow rate from the mesenteric lymph nodes in HDT rats was slightly higher than that obtained with the control. The cell count obtained from the iliac lymph nodes in HDT rats was significantly larger than those of the controls, while no significant difference in the number of cells from the mesenteric lymph nodes was observed between the control and HDT groups. The cells from the iliac lymph nodes in the control and HDT rats were mostly lymphocytes. The distribution of subsets of lymphocytes (CD3+, CD4+, CD8a+, and CD45R+) from the iliac lymph nodes in HDT rats was not significantly different from the subsets of lymphocytes in the control. Immunization did not affect the distribution of lymphocyte subsets from the iliac lymph nodes in the control and HDT groups. There was no significant difference in the concentrations of lymph albumin in iliac afferent or efferent lymphatics between the control and HDT groups. These findings suggest that HDT posture in Bollman's cage induces transient output of lymphocytes from the iliac lymph nodes of rats in vivo without changing the flow rate, lymphocyte subsets, or concentration of albumin.

Inhibition of active lymph pump by simulated microgravity in rats

During spaceflight the normal head-to-foot hydrostatic pressure gradients are eliminated and body fluids shift toward the head, resulting in a diminished fluid volume in the legs and an increased fluid volume in the head, neck, and upper extremities. Lymphatic function is important in the maintenance of normal tissue fluid volume, but it is not clear how microgravity influences lymphatic pumping. We performed a detailed evaluation of the influence of simulated microgravity on lymphatic diameter, wall thickness, elastance, tone, and other measures of phasic contractility in isolated lymphatics. Head-down tail suspension (HDT) rats were used to simulate the effects of microgravity. Animals were exposed to HDT for 2 wk, after which data were collected and compared with the control non-HDT group. Lymphatics from four regional lymphatic beds (thoracic duct, cervical, mesenteric, and femoral lymphatics) were isolated, cannulated, and pressurized. Input and output pressures were adjusted to apply a range of transmural pressures and flows to the lymphatics. Simulated microgravity caused a potent inhibition of pressure/stretch-stimulated pumping in all four groups of lymphatics. The greatest inhibition was found in cervical lymphatics. These findings presumably are correlated to the cephalic fluid shifts that occur in HDT rats as well as those observed during spaceflight. Flow-dependent pump inhibition was increased after HDT, especially in the thoracic duct. Mesenteric lymphatics were less strongly influenced by HDT, which may support the idea that lymph hydrodynamic conditions in the mesenteric lymphatic during HDT are not dramatically altered.

Proteolytic N-terminal Truncation of Cardiac Troponin I Enhances Ventricular Diastolic Function

Besides the core structure conserved in all troponin I isoforms, cardiac troponin I (cTnI) has an N-terminal extension that contains phosphorylation sites for protein kinase A under beta-adrenergic regulation. A restricted cleavage of this N-terminal regulatory domain occurs in normal cardiac muscle and is up-regulated during hemodynamic adaptation (Z.-B. Yu, L.-F. Zhang, and J.-P. Jin (2001) J. Biol. Chem. 276, 15753-15760). In the present study, we developed transgenic mice overexpressing the N-terminal truncated cTnI (cTnI-ND) in the heart to examine its biochemical and physiological significance. Ca(2+)-activated actomyosin ATPase activity showed that cTnI-ND myofibrils had lower affinity for Ca(2+) than controls, similar to the effect of isoproterenol treatment. In vivo and isolated working heart experiments revealed that cTnI-ND hearts had a significantly faster rate of relaxation and lower left ventricular end diastolic pressure compared with controls. The higher baseline relaxation rate of cTnI-ND hearts was at a level similar to that of wild type mouse hearts under beta-adrenergic stimulation. The decrease in cardiac output due to lowered preload was significantly smaller for cTnI-ND hearts compared with controls. These findings indicate that removal of the N-terminal extension of cTnI via restricted proteolysis enhances cardiac function by increasing the rate of myocardial relaxation and lowering left ventricular end diastolic pressure to facilitate ventricular filling, thus resulting in better utilization of the Frank-Starling mechanism.

Acute hemodynamic responses in the head during microgravity induced by free drop in anesthetized rats

To examine acute hemodynamic responses to microgravity (μG) in the head, we measured carotid artery pressure (CAP) and jugular vein pressure (JVP) to calculate cephalic perfusion pressure (CPP = CAP − JVP) and recorded images of microvessels in the iris to evaluate capillary blood flow velocity (CBFV) and capillary diameter (CD) in anesthetized rats during 4.5 s of μG induced by free drop. Rats were placed in 30° head-up whole body-tilted (HU, n = 7) or horizontal (flat, n = 6) position. In the flat group, none of the measured variables was significantly affected by μG, whereas in the HU group, CAP, JVP, and CPP increased, respectively, by 23.4 ± 2.6, 1.3 ± 0.2, and 22.9 ± 3.1 mmHg, and CBFV and CD increased, respectively, by 33 ± 8 and 9 ± 3%, showing an increase in capillary blood flow. To further examine the mechanisms underlying these CAP and JVP increases, another experiment was performed in which CAP and JVP were measured in anesthetized rats ( n = 6) during a postural change from HU to flat. In these animals, the change in JVP was similar to that observed during actual μG, but no change in CAP was seen, indicating that the JVP increase during actual μG is caused by disappearance of the gravitational pressure gradient in the head-to-foot axis, whereas the CAP increase is not. In conclusion, actual μG elicits an increase in CPP due to a greater increase in CAP than JVP, resulting in increased capillary blood flow. Although the increase in JVP is explained by the disappearance of gravitational pressure gradient in the head-to-foot axis as a result of μG, the larger increase in CAP is not.

Cerebral Circulation during Acute Microgravity Induced by Free Drop in Anesthetized Rats

To evaluate changes in the cerebral circulation during acute microgravity (microG), we measured intracranial pressure (ICP), aortic pressure at the diaphragm level, and cerebral flow velocity (CFV) in anesthetized rats (n = 5) during 4.5 s of microG induced by free drop, then calculated arterial pressure at the eye level (AP(eye)) and cerebral perfusion pressure (CPP = AP(eye)-ICP), and estimated CPP-CFV relationship. The rats were placed in the flat and the 30 degrees head-up positions. In the head-up position, ICP, AP(eye), and CPP were significantly increased by 2.2 +/- 0.4, 12.3 +/- 2.0, and 10.1 +/- 1.7 mmHg respectively during microG, whereas the CFV did not change significantly. In the flat position, none of these variables were significantly affected by microG. The slope of the CPP-CFV relationship was decreased only in the head-up position, suggesting that the cerebrovascular resistance was increased by microG. These findings indicate that the change in gravitational (hydrostatic) pressure is a key factor in understanding the changes in cerebral circulation during acute microG.

Depression, mood state, and back pain during microgravity simulated by bed rest

OBJECTIVE

The objective of this study was to develop a ground-based model for spinal adaptation to microgravity and to study the effects of spinal adaptation on depression, mood state, and pain intensity.

METHODS

We investigated back pain, mood state, and depression in six subjects, all of whom were exposed to microgravity, simulated by two forms of bed rest, for 3 days. One form consisted of bed rest with 6 degrees of head-down tilt and balanced traction, and the other consisted of horizontal bed rest. Subjects had a 2-week period of recovery between the studies. The effects of bed rest on pain intensity in the lower back, depression, and mood state were investigated.

RESULTS

Subjects experienced significantly more intense lower back pain, lower hemisphere abdominal pain, headache, and leg pain during head-down tilt bed rest. They had higher scores on the Beck Depression Inventory (ie, were more depressed) and significantly lower scores on the activity scale of the Bond-Lader questionnaire.

CONCLUSIONS

Bed rest with 6 degrees of head-down tilt may be a better experimental model than horizontal bed rest for inducing the pain and psychosomatic reactions experienced in microgravity. Head-down tilt with balanced traction may be a useful method to induce low back pain, mood changes, and altered self-rated activity level in bed rest studies.

Fluid volume control during short-term space flight and implications for human performance

Space flight exerts substantial effects on fluid volume control in humans. Cardiac distension occurs during the first 1–2 days of space flight relative to supine and especially upright 1g conditions. Plasma volume contraction occurs quickly in microgravity, probably as a result of transcapillary fluid filtration into upper-body interstitial spaces. No natriuresis or diuresis has been observed in microgravity, such that diuresis cannot explain microgravity-induced hypovolemia. Reduction of fluid intake occurs irrespective of space motion sickness and leads to hypovolemia. The fourfold elevation of urinary antidiuretic hormone (ADH) levels on flight day 1 probably results from acceleration exposures and other stresses of launch. Nevertheless, it is fascinating that elevated ADH levels and reduced fluid intake occur simultaneously early in flight. Extracellular fluid volume decreases by 10–15% in microgravity, and intracellular fluid volume appears to increase. Total red blood cell mass decreases by approximately 10% within 1 week in space. Inflight Na+ and volume excretory responses to saline infusion are approximately half those seen in pre-flight supine conditions. Fluid volume acclimation to microgravity sets the central circulation to homeostatic conditions similar to those found in an upright sitting posture on Earth. Fluid loss in space contributes to reduced exercise performance upon return to 1g, although not necessarily in flight. In-flight exercise training may help prevent microgravity-induced losses of fluid and, therefore, preserve the capacity for upright exercise post-flight. Protection of orthostatic tolerance during space flight probably requires stimulation of orthostatic blood pressure control systems in addition to fluid maintenance or replacement.

Validity of microgravity simulation models on earth

Many studies have used water immersion and head-down bed rest as experimental models to simulate responses to microgravity. However, some data collected during space missions are at variance or in contrast with observations collected from experimental models. These discrepancies could reflect incomplete knowledge of the characteristics inherent to each model. During water immersion, the hydrostatic pressure lowers the peripheral vascular capacity and causes increased thoracic blood volume and high vascular perfusion. In turn, these changes lead to high urinary flow, low vasomotor tone, and a high rate of water exchange between interstitium and plasma. In contrast, the increase in thoracic blood volume during a space mission is combined with stimulated orthosympathetic tone and lowered urine flow. During bed rest, body tissues are compressed by pressure from gravity, whereas microgravity causes a negative pressure around the body. The differences in renal function between space and experimental models appear to be explained by the physical forces affecting tissues and hemodynamics as well as by the changes secondary to these forces. These differences may help in selecting experimental models to study possible effects of microgravity.

Influence of body water distribution on skin thickness: measurements using high-frequency ultrasound

BACKGROUND

Although it is known that the skin acts as a water reservoir and participates in the fluid content of the whole body, no method has been established to quantify the fluid shifts in superficial tissue.

OBJECTIVES

The aim of this study was to investigate changes in dermal and subcutis thickness and echodensity at the forehead and lower leg by high-frequency (20 MHz) ultrasound under various physiological conditions influencing water balance.

METHODS

These parameters were measured in the skin of 20 healthy male volunteers at baseline and successively at 30 min after lying down, in a head-down position, after physical activity and after infusion of 10 mL kg-1 body weight of Ringer's solution.

RESULTS

Dermal thickness at the forehead showed a significant increase from baseline to a horizontal position and a further increase in the head-down position. Physical activity did not lead to further changes, whereas after fluid infusion the dermal thickness also increased markedly. The echodensity showed inverse changes, with decreasing values. The thickness of the subcutis increased slightly from baseline to a lying position and decreased in the head-down position and after fluid infusion. At the lower leg, skin thickness decreased slightly in the head-down position with elevated legs, and increased after fluid infusion.

CONCLUSIONS

Our results show that slight changes in the water distribution of the body influence the thickness and the echodensity of the dermis. Changes are more pronounced at the forehead than on the lower legs. Further, the fluid storage takes place mainly in the dermis and not in the subcutis. High-frequency ultrasound is able to quantify these effects and is a sensitive method for measuring fluid intake and balance during anaesthesia and therapy.

Intramuscular pressure and electromyographic responses of the vastus lateralis muscle during repeated maximal isokinetic knee extensions

The purpose of the study was to investigate changes in intramuscular pressure (IMP) (maximal during contraction - peak IMP, and between contraction, relaxation IMP - RxIMP) and surface electromyographic activity (EMG) parameters [mean frequency of the power spectrum (fmean), and signal amplitude, root mean square (RMS)] throughout 100 repetitive isokinetic contractions for six healthy subjects. Parameters were recorded simultaneously from the vastus lateralis muscle during maximal knee extension. Regression analyses revealed significant decreases for peak IMP and fmean, and an increase in RxIMP; RMS, however, did not change. All parameters demonstrated trends of change throughout the contractions that were non-linear. Details and inter relations for RxIMP and fmean were highlighted to express intramuscular fluid accumulation and fatigue development, respectively. Individual regression analyses for RxIMP revealed significant positive correlations for all subjects (range of r=0.62 to 0.89). At group level, mean RxIMP increased from 6.0 mmHg for the 1st contraction to 14.0 mmHg for the 100th contraction. For fmean, individual regressions were significantly negative for all subjects (r=-0.75 to -0.89). Fmean decreased from 89.2 Hz for the 1st contraction to 63.3 Hz for the 100th contraction. When the data were delineated between the fatigue (contractions 1-40) and endurance phases (41-100), the slopes of increase for RxIMP, and of decrease for fmean were significantly greater during the fatigue phase. RxIMP throughout the 100 contractions correlated negatively with fmean for each subject (r=-0.54 to -0.78); when delineated, the correlation between parameters was significantly greater for the fatigue as compared with the endurance phase. Relaxation IMP trends are mainly attributed to intramuscular water accumulations during repetitive contractions. In spite of consistent correlations between RxIMP and fmean a causal association could not be established. It may be suggested that a common factor occurring during the fatiguing process governs changes in RxIMP and fmean.

Does edema formation occur in the rabbit brain exposed to head-down tilt?

Earlier studies showed that exposure to microgravity caused cephalad fluid shift, increased capillary pressure in the head, and produced facial edema and nasal congestion. In the present study, edema formation in the brain was investigated in rabbits exposed to simulated microgravity, head-down tilt (HDT), by measuring water content and histological examinations. Water content in the brain tissues of rabbits exposed to 2 and 8 days of HDT did not increase significantly compared with that of control animals. Neither vital staining using Evans blue nor immunohistochemical examination demonstrated extravasation of plasma constituents in the brain tissues of the HDT rabbits. Although marked congestion was noted in the brain, hematoxylin and eosin staining did not show edematous changes, such as distension of the perivascular and pericellular spaces and vacuolar appearance, in the tissues obtained from HDT rabbits. Transmission electron microscopy revealed that tight junctions of the capillary endothelium were intact in the HDT rabbits. These results suggest that either HDT up to 8 days does not cause brain edema in rabbits or it induces only a slight brain edema which is hard to be demonstrated by measurement of water content or histological examinations.

Effects of hindlimb unloading on rat cerebral, splenic, and mesenteric resistance artery morphology

Hindlimb unloading (HU) of rats induces a cephalic shift in body fluids. We hypothesized that the putative increase in cranial fluid pressure and decrease in peripheral fluid pressure would alter the morphology of resistance arteries from 2-wk HU male Sprague-Dawley rats. To test this hypothesis, the cerebral basilar, mesenteric, and splenic arteries were removed from control (C) and HU animals. The vessels were cannulated, and luminal pressure was set to 60 cmH(2)O. The resistance arteries were then relaxed with 10(-4) M nitroprusside, fixed, and cut into transverse cross sections (5 microm thick). Media cross-sectional area (CSA), intraluminal CSA, media layer thickness, vessel outer perimeter, and media nuclei number were determined. In the basilar artery, both media CSA (HU 17, 893 +/- 2,539 microm(2); C 12,904 +/- 1,433 microm(2)) and thickness (HU 33.9 +/- 4.1 microm; C 22.3 +/- 3.2 microm) were increased with hindlimb unloading (P < 0.05), intraluminal CSA decreased (HU 7,816 +/- 3,045 microm(2); C 13,469 +/- 5,500 microm(2)) (P < 0.05), and vessel outer perimeter and media nuclei number were unaltered. There were no differences in mesenteric or splenic resistance artery morphology between HU and C rats. These findings suggest that hindlimb unloading-induced increases in cephalic arterial pressure and, correspondingly, increases in circumferential wall stress result in the hypertrophy of basilar artery smooth muscle cells.

Albumin infusion in humans does not model exercise induced hypervolaemia after 24 hours

We rapidly infused 234 +/- 3 mL of 5% human serum albumin in eight men while measuring haematocrit, haemoglobin concentration, plasma volume (PV), albumin concentration, total protein concentration, osmolality, sodium concentration, renin activity, aldosterone concentration, and atrial natriuretic peptide concentration to test the hypotheses that plasma volume expansion and plasma albumin content expansion will not persist for 24 h. Plasma volume and albumin content were expanded for the first 6 h after infusion (44.3 +/- 1.9-47.2 +/- 2.0 mL kg-1 and 1.9 +/- 0.1-2.1 +/- 0.1 g kg-1 at pre-infusion and 1 h, respectively, P < 0.05), but by 24 h plasma volume and albumin content decreased significantly from 1 h post-infusion and were not different from pre-infusion (44.8 +/- 1.9 mL kg-1 and 1.9 +/- 0.1 g kg-1, respectively). Plasma aldosterone concentration showed a significant effect of time over the 24 h after infusion (P < 0.05), and showed a trend to decrease at 2 h after infusion (167.6 +/- 32.5(-1) 06.2 +/- 13.4 pg mL-1, P = 0.07). These data demonstrate that a 6.8% expansion of plasma volume and 10.5% expansion of plasma albumin content by infusion does not remain in the vascular space for 24 h and suggest a redistribution occurs between the intravascular space and interstitial fluid space.