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

Segmental Tissue Resistance of Healthy Young Adults during Four Hours of 6-Degree Head-Down-Tilt Positioning

(1) Background: One effect of microgravity on the human body is fluid redistribution due to the removal of the hydrostatic gravitational gradient. These fluid shifts are expected to be the source of severe medical risks and it is critical to advance methods to monitor them in real-time. One technique to monitor fluid shifts captures the electrical impedance of segmental tissues, but limited research is available to evaluate if fluid shifts in response to microgravity are symmetrical due to the bilateral symmetry of the body. This study aims to evaluate this fluid shift symmetry. (2) Methods: Segmental tissue resistance at 10 kHz and 100 kHz was collected at 30 min intervals from the left/right arm, leg, and trunk of 12 healthy adults over 4 h of 6° head-down-tilt body positioning. (3) Results: Statistically significant increases were observed in the segmental leg resistances, first observed at 120 min and 90 min for 10 kHz and 100 kHz measurements, respectively. Median increases were approximately 11% to 12% for the 10 kHz resistance and 9% for the 100 kHz resistance. No statistically significant changes in the segmental arm or trunk resistance. Comparing the left and right segmental leg resistance, there were no statistically significant differences in the resistance changes based on the side of the body. (4) Conclusions: The fluid shifts induced by the 6° body position resulted in similar changes in both left and right body segments (that had statistically significant changes in this work). These findings support that future wearable systems to monitor microgravity-induced fluid shifts may only require monitoring of one side of body segments (reducing the hardware needed for the system).

Physiological adaptations affecting drug pharmacokinetics in space: what do we really know? A critical review of the literature

As human spaceflight progresses with extended mission durations, the demand for effective and safe drugs will necessarily increase. To date, the accepted medications used during missions (for space motion sickness, sleep disturbances, allergies, pain, and sinus congestion) are administered under the assumption that they act as safely and efficaciously as on Earth. However, physiological changes have been documented in human subjects in spaceflight involving fluid shifts, muscle and bone loss, immune system dysregulation, and adjustments in the gastrointestinal tract and metabolism. These alterations may change the pharmacokinetics (PK) and pharmacodynamics of commonly used medications. Frustratingly, the information gained from bed rest studies and from in-flight observations is incomplete and also demonstrates a high variability in drug PK. Therefore, the objectives of this review are to report (i) the impact of the space environmental stressors on human physiology in relation to PK; (ii) the state-of-the-art on experimental data in space and/or in ground-based models; (iii) the validation of ground-based models for PK studies; and (iv) the identification of research gaps.

Neuro-consequences of the spaceflight environment

As human space exploration advances to establish a permanent presence beyond the Low Earth Orbit (LEO) with NASA's Artemis mission, researchers are striving to understand and address the health challenges of living and working in the spaceflight environment. Exposure to ionizing radiation, microgravity, isolation and other spaceflight hazards pose significant risks to astronauts. Determining neurobiological and neurobehavioral responses, understanding physiological responses under Central Nervous System (CNS) control, and identifying putative mechanisms to inform countermeasure development are critically important to ensuring brain and behavioral health of crew on long duration missions. Here we provide a detailed and comprehensive review of the effects of spaceflight and of ground-based spaceflight analogs, including simulated weightlessness, social isolation, and ionizing radiation on humans and animals. Further, we discuss dietary and non-dietary countermeasures including artificial gravity and antioxidants, among others. Significant future work is needed to ensure that neural, sensorimotor, cognitive and other physiological functions are maintained during extended deep space missions to avoid potentially catastrophic health and safety outcomes.

Short-Term Segmental Bioimpedance Alterations During 6° Head-Down Tilt

As missions in space increase in duration and distance from Earth it is critical to understand the impact that exposure to microgravity has on the health and potential performance of crews. Segmental bioimpedance measurements can track resistances changes in tissues that result from fluid redistribution and could be a tool for continuous fluid shift monitoring in microgravity. In this work, the range of segmental (legs, arms, torso, and neck) 10 kHz and 100 kHz resistances and their relative changes during 4 hours of 6° head down tilt are reported as well as the observed resistance differences between left/right body segments throughout the protocol.

Postural orthostatic tachycardia syndrome (POTS): State of the science and clinical care from a 2019 National Institutes of Health Expert Consensus Meeting - Part 1

Postural orthostatic tachycardia syndrome (POTS) is a chronic and often disabling disorder characterized by orthostatic intolerance with excessive heart rate increase without hypotension during upright posture. Patients often experience a constellation of other typical symptoms including fatigue, exercise intolerance and gastrointestinal distress. A typical patient with POTS is a female of child-bearing age, who often first displays symptoms in adolescence. The onset of POTS may be precipitated by immunological stressors such as a viral infection. A variety of pathophysiologies are involved in the abnormal postural tachycardia response; however, the pathophysiology of the syndrome is incompletely understood and undoubtedly multifaceted. Clinicians and researchers focused on POTS convened at the National Institutes of Health in July 2019 to discuss the current state of understanding of the pathophysiology of POTS and to identify priorities for POTS research. This article, the first of two articles summarizing the information discussed at this meeting, summarizes the current understanding of this disorder and best practices for clinical care. The evaluation of a patient with suspected POTS should seek to establish the diagnosis, identify co-morbid conditions, and exclude conditions that could cause or mimic the syndrome. Once diagnosed, management typically begins with patient education and non-pharmacologic treatment options. Various medications are often used to address specific symptoms, but there are currently no FDA-approved medications for the treatment of POTS, and evidence for many of the medications used to treat POTS is not robust.

Segmental Volume and Circulatory Changes that Occur in Humans and Rhesus Monkeys During 4 Hour, -6 Degree Head Down Tilt

Nonhuman primates are often used to investigate physiologic processes that occur in man during aerospace/cardiovascular orthostatic research. Few studies have compared nonhuman primates and man under identical test conditions to assess the degree of similarity between the two species. Impedance plethysmography was used to measure calf, thigh, pelvic, thoracic, upper arm, and lower arm volume changes in eight rhesus (Macacca Mulatta) monkeys and twelve human subjects during four hour exposures to -6 degree head down tilt (HDT).

Gravitational Transitions Increase Posterior Cerebral Perfusion and Systemic Oxidative-nitrosative Stress: Implications for Neurovascular Unit Integrity

The present study examined if repeated bouts of micro- and hypergravity during parabolic flight (PF) alter structural integrity of the neurovascular unit (NVU) subsequent to free radical-mediated changes in regional cerebral perfusion. Six participants (5♂, 1♀) aged 29 ± 11 years were examined before, during and after a 3 h PF and compared to six sex and age-matched (27 ± 6 years) normogravity controls. Blood flow was measured in the anterior (middle cerebral artery, MCA; internal carotid artery, ICA) and posterior (vertebral artery, VA) circulation (duplex ultrasound) in-flight over the course of 15 parabolas. Venous blood was assayed for free radicals (electron paramagnetic resonance spectroscopy), nitric oxide (NO, ozone-based chemiluminescence) and NVU integrity (chemiluminescence/ELISA) in normogravity before and after exposure to 31 parabolas. While MCA velocity did not change (P > 0.05), a selective increase in VA flow was observed during the most marked gravitational transition from micro- to hypergravity (P < 0.05). Increased oxidative-nitrosative stress defined by a free radical-mediated reduction in NO and elevations in glio-vascular GFAP and S100ß were observed after PF (P < 0.05), the latter proportional to the increase in VA flow (r = 0.908, P < 0.05). In contrast, biomarkers of neuronal-axonal damage (neuron-specific enolase, neurofilament light-chain, ubiquitin carboxy-terminal hydrolase L1 and tau) did not change (P > 0.05). Collectively, these findings suggest that the cumulative effects of repeated gravitational transitions may promote minor blood-brain barrier disruption, potentially related to the combined effects of haemodynamic (posterior cerebral hyperperfusion) and molecular (systemic oxidative-nitrosative) stress.

Segmental Volume Changes that Occur in Nonhuman Primates During Short Term Head Up (HUT) and Head Down (HDT) Tilt

Nonhuman primates are often used in biomedical research and to investigate physiologic processes that occur in man. Impedance plethysmography was used to measure calf, thigh, pelvic, abdominal, and thoracic volume changes in ten Rhesus and eight squirrel monkeys during five-minute exposures to HUT and HDT at angles of 5, 10, and 20 degrees. Calf, rump and tail measurements were made in three squirrel monkeys at 10 and 20 degrees of HUT and HDT. Fluid volume changes in all segments of the Rhesus monkeys were found to change during HUT an HDT in direct relation to the angle of tilt used. However, the volume changes that occurred in the squirrel monkeys were found to be quite different. Their calf, thigh, and pelvic segments lost volume during both HUT and HDT while their abdominal and thoracic segments responded similarly to those of the Rhesus monkeys. These results and those of the calf/tail measurements of the squirrel monkeys suggest that they may utilize their tails as a compensatory reservoir during postural changes and therefore, may not be an appropriate animal model for man under some orthostatic test conditions.

Mechanisms of tilt-induced vasovagal syncope in healthy volunteers and postural tachycardia syndrome patients without past history of syncope

Upright tilt table testing has been used to test for vasovagal syncope (VVS) but can result in "false positives" in which tilt-induced fainting (tilt+) occurs in the absence of real-world fainting. Tilt+ occurs in healthy volunteers and in patients with postural tachycardia syndrome (POTS) and show enhanced susceptibility to orthostatic hypotension. We hypothesized that the mechanisms for hypotensive susceptibility differs between tilt+ healthy volunteers (Control-Faint (N = 12)), tilt+ POTS patients (POTS-Faint (N = 12)) and a non-fainter control group of (Control-noFaint) (N = 10). Subjects were studied supine and during 70° upright tilt while blood pressure (BP), cardiac output (CO), and systemic vascular resistance (SVR), were measured continuously. Impedance plethysmography estimated regional blood volumes, flows, and vascular resistance. Heart rate was increased while central blood volume was decreased in both Faint groups. CO increased in Control-Faint because of reduced splanchnic vascular resistance; splanchnic pooling was similar to Control-noFaint. Splanchnic blood flow in POTS-Faint decreased and resistance increased similar to Control-noFaint but splanchnic blood volume was markedly increased. Decreased SVR and splanchnic arterial vasoconstriction is the mechanism for faint in Control-Faint. Decreased CO caused by enhanced splanchnic pooling is the mechanism for faint in POTS-Faint. We propose that intrahepatic resistance is increased in POTS-Faint resulting in pooling and that both intrahepatic resistance and splanchnic arterial vasoconstriction are reduced in Control-Faint resulting in increased splanchnic blood flow and reduced splanchnic resistance.

Development and evaluation of a novel system for inducing orthostatic challenge by tilt tests and lower body negative pressure

Lower body negative pressure (LBNP) is a method derived from space medicine, which in recent years has been increasingly used by clinicians to assess the efficiency of the cardiovascular regulatory mechanisms. LBNP with combined tilt testing is considered as an effective form of training to prevent orthostatic intolerance. We have developed a prototype system comprising a tilt table and LBNP chamber, and tested it in the context of the feasibility of the device for assessing the pilots' efficiency. The table allows for controlled tilting in the range from -45 to +80° at the maximum change rate of 45°/s. The LBNP value can smoothly be adjusted down to -100 mmHg at up to 20 mmHg/s. 17 subjects took part in the pilot study. A 24-minute scenario included -100 mmHg supine LBNP, head up tilt (HUT) and -60 mmHg LBNP associated with HUT, separated by resting phases. The most noticeable changes were observed in stroke volume (SV). During supine LBNP, HUT and the combined stimulus, a decrease of the SV value by 20%, 40% and below 50%, respectively, were detected. The proposed system can map any pre-programed tilt and LBNP profiles, and the pilot study confirmed the efficiency of performing experimental procedures.

Interaction between graviception and carotid baroreflex function in humans during parabolic flight-induced microgravity

The aim of the present study was to assess carotid baroreflex (CBR) function during acute changes in otolithic activity in humans. To address this question, we designed a set of experiments to identify the modulatory effects of microgravity on CBR function at a tilt angle of -2°, which was identified to minimize changes in central blood volume during parabolic flight. During parabolic flight at 0 and 1 g, CBR function curves were modeled from the heart rate (HR) and mean arterial pressure (MAP) responses to rapid pulse trains of neck pressure and neck suction ranging from +40 to -80 Torr; CBR control of HR (carotid-HR) and MAP (carotid-MAP) function curves, respectively. The maximal gain of both carotid-HR and carotid-MAP baroreflex function curves were augmented during microgravity compared with 1 g (carotid-HR, -0.53 to -0.80 beats·min^-1·mmHg^-1, P < 0.05; carotid-MAP, -0.24 to -0.30 mmHg/mmHg, P < 0.05). These findings suggest that parabolic flight-induced acute change of otolithic activity may modify CBR function and identifies that the vestibular system contributes to blood pressure regulation under fluctuations in gravitational forces. NEW & NOTEWORTHY The effect of acute changes in vestibular activity on arterial baroreflex function remains unclear. In the present study, we assessed carotid baroreflex function without changes in central blood volume during parabolic flight, which causes acute changes in otolithic activity. The sensitivity of both carotid heart rate and carotid mean arterial pressure baroreflex function was augmented in microgravity compared with 1 g, suggesting that the vestibular system contributes to blood pressure regulation in humans on Earth.

Mechanisms of Vasovagal Syncope in the Young: Reduced Systemic Vascular Resistance Versus Reduced Cardiac Output

BACKGROUND

Syncope is a sudden transient loss of consciousness and postural tone caused by cerebral hypoperfusion. The most common form is vasovagal syncope (VVS). Presyncopal progressive early hypotension in older VVS patients is caused by reduced cardiac output (CO); younger patients have reduced systemic vascular resistance (SVR). Using a priori criteria for reduced CO (↓CO) and SVR (↓SVR), we studied 48 recurrent young fainters comparing subgroups of VVS with VVS-↓CO, VVS-↓SVR, and both VVS-↓CO&↓SVR.

METHODS AND RESULTS

Subjects were studied supine and during 70-degrere upright tilt with a Finometer to continuously measure blood pressure, CO, and SVR and impedance plethysmography to estimate thoracic, splanchnic, pelvic, and calf blood volumes, blood flows, and vascular resistances and electrocardiogram to measure heart rate and rhythm. Central blood volume was decreased in all VVS compared to control. VVS-↓CO was associated with decreased splanchnic blood flow and increased splanchnic blood pooling compared to control. Seventy-five percent of VVS patients had reduced SVR, including 23% who also had reduced CO. Many VVS-↓SVR increased CO during tilt, with no difference in splanchnic pooling, caused by significant increases in splanchnic blood flow and reduced splanchnic resistance. VVS-↓CO&↓SVR patients had splanchnic pooling comparable to VVS-↓CO patients, but SVR comparable to VVS-↓SVR. Splanchnic vasodilation was reduced, compared to VVS-↓SVR, and venomotor properties were similar to control. Combined splanchnic pooling and reduced SVR produced the earliest faints among the VVS groups.

CONCLUSIONS

Both ↓CO and ↓SVR occur in young VVS patients. ↓SVR is predominant in VVS and is caused by impaired splanchnic vasoconstriction.

Rapid adaptation of multisensory integration in vestibular pathways

Sensing gravity is vital for our perception of spatial orientation, the control of upright posture, and generation of our everyday activities. When an astronaut transitions to microgravity or returns to earth, the vestibular input arising from self-motion will not match the brain's expectation. Our recent neurophysiological studies have provided insight into how the nervous system rapidly reorganizes when vestibular input becomes unreliable by both (1) updating its internal model of the sensory consequences of motion and (2) up-weighting more reliable extra-vestibular information. These neural strategies, in turn, are linked to improvements in sensorimotor performance (e.g., gaze and postural stability, locomotion, orienting) and perception characterized by similar time courses. We suggest that furthering our understanding of the neural mechanisms that underlie sensorimotor adaptation will have important implications for optimizing training programs for astronauts before and after space exploration missions and for the design of goal-oriented rehabilitation for patients.

Study protocol to examine the effects of spaceflight and a spaceflight analog on neurocognitive performance: extent, longevity, and neural bases

Background

Long duration spaceflight (i.e., 22 days or longer) has been associated with changes in sensorimotor systems, resulting in difficulties that astronauts experience with posture control, locomotion, and manual control. The microgravity environment is an important causal factor for spaceflight induced sensorimotor changes. Whether spaceflight also affects other central nervous system functions such as cognition is yet largely unknown, but of importance in consideration of the health and performance of crewmembers both in- and post-flight. We are therefore conducting a controlled prospective longitudinal study to investigate the effects of spaceflight on the extent, longevity and neural bases of sensorimotor and cognitive performance changes. Here we present the protocol of our study.

Methods/design

This study includes three groups (astronauts, bed rest subjects, ground-based control subjects) for which each the design is single group with repeated measures. The effects of spaceflight on the brain will be investigated in astronauts who will be assessed at two time points pre-, at three time points during-, and at four time points following a spaceflight mission of six months. To parse out the effect of microgravity from the overall effects of spaceflight, we investigate the effects of seventy days head-down tilted bed rest. Bed rest subjects will be assessed at two time points before-, two time points during-, and three time points post-bed rest. A third group of ground based controls will be measured at four time points to assess reliability of our measures over time. For all participants and at all time points, except in flight, measures of neurocognitive performance, fine motor control, gait, balance, structural MRI (T1, DTI), task fMRI, and functional connectivity MRI will be obtained. In flight, astronauts will complete some of the tasks that they complete pre- and post flight, including tasks measuring spatial working memory, sensorimotor adaptation, and fine motor performance. Potential changes over time and associations between cognition, motor-behavior, and brain structure and function will be analyzed.

Discussion

This study explores how spaceflight induced brain changes impact functional performance. This understanding could aid in the design of targeted countermeasures to mitigate the negative effects of long-duration spaceflight.

Ascorbate improves circulation in postural tachycardia syndrome

Low flow postural tachycardia syndrome (LFP) is associated with vasoconstriction, reduced cardiac output, increased plasma angiotensin II, reduced bioavailable nitric oxide (NO), and oxidative stress. We tested whether ascorbate would improve cutaneous NO and reduce vasoconstriction when delivered systemically. We used local cutaneous heating to 42°C and laser Doppler flowmetry to assess NO-dependent conductance (%CVC(max)) to sodium ascorbate and the systemic hemodynamic response to ascorbic acid in 11 LFP patients and in 8 control subjects (aged 23 ± 2 yr). We perfused intradermal microdialysis catheters with sodium ascorbate (10 mM) or Ringer solution. Predrug heat response was reduced in LFP, particularly the NO-dependent plateau phase (56 ± 6 vs. 88 ± 7%CVC(max)). Ascorbate increased baseline skin flow in LFP and control subjects and increased the LFP plateau response (82 ± 6 vs. 92 ± 6 control). Systemic infusion experiments used Finometer and ModelFlow to estimate relative cardiac index (CI) and forearm and calf venous occlusion plethysmography to estimate blood flows, peripheral arterial and venous resistances, and capacitance before and after infusing ascorbic acid. CI increased 40% after ascorbate as did peripheral flows. Peripheral resistances were increased (nearly double control) and decreased by nearly 50% after ascorbate. Calf capacitance and venous resistance were decreased compared with control but normalized with ascorbate. These data provide experimental support for the concept that oxidative stress and reduced NO possibly contribute to vasoconstriction and venoconstriction of LFP.

Increased vasoconstriction predisposes to hyperpnea and postural faint

Our prior studies indicated that postural fainting relates to splanchnic hypervolemia and thoracic hypovolemia during orthostasis. We hypothesized that thoracic hypovolemia causes excessive sympathetic activation, increased respiratory tidal volume, and fainting involving the pulmonary stretch reflex. We studied 18 patients 13-21 yr old, 11 who fainted within 10 min of upright tilt (fainters) and 7 healthy control subjects. We measured continuous blood pressure and heart rate, respiration by inductance plethysmography, end-tidal carbon dioxide (ET(CO(2))) by capnography, and regional blood flows and blood volumes using impedance plethysmography, and we calculated arterial resistance with patients supine and during 70 degrees upright tilt. Splanchnic resistance decreased until faint in fainters (44 +/- 8 to 21 +/- 2 mmHg.l(-1).min(-1)) but increased in control subjects (47 +/- 5 to 53 +/- 4 mmHg.l(-1).min(-1)). Percent change in splanchnic blood volume increased (7.5 +/- 1.0 vs. 3.0 +/- 11.5%, P < 0.05) after the onset of tilt. Upright tilt initially significantly increased thoracic, pelvic, and leg resistance in fainters, which subsequently decreased until faint. In fainters but not control subjects, normalized tidal volume (1 +/- 0.1 to 2.6 +/- 0.2, P < 0.05) and normalized minute ventilation increased throughout tilt (1 +/- 0.2 to 2.1 +/- 0.5, P < 0.05), whereas respiratory rate decreased (19 +/- 1 to 15 +/- 1 breaths/min, P < 0.05). Maximum tidal volume occurred just before fainting. The increase in minute ventilation was inversely proportionate to the decrease in ET(CO(2)). Our data suggest that excessive splanchnic pooling and thoracic hypovolemia result in increased peripheral resistance and hyperpnea in simple postural faint. Hyperpnea and pulmonary stretch may contribute to the sympathoinhibition that occurs at the time of faint.

Space motion sickness

Motion sickness remains a persistent problem in spaceflight. The present review summarizes available knowledge concerning the incidence and onset of space motion sickness and aspects of the physiology of motion sickness. Proposed etiological factors in the elicitation of space motion sickness are evaluated including fluid shifts, head movements, visual orientation illusions, Coriolis cross-coupling stimulation, and otolith asymmetries. Current modes of treating space motion sickness are described. Theoretical models and proposed ground-based paradigms for understanding and studying space motion sickness are critically analyzed. Prediction tests and questionnaires for assessing susceptibility to space motion sickness and their limitations are discussed. We conclude that space motion sickness does represent a form of motion sickness and that it does not represent a unique diagnostic entity. Motion sickness arises when movements are made during exposure to unusual force backgrounds both higher and lower in magnitude than 1 g earth gravity.

Regional blood volume and peripheral blood flow in postural tachycardia syndrome

Variants of postural tachycardia syndrome (POTS) are associated with increased ["high-flow" POTS (HFP)], decreased ["low-flow" POTS (LFP)], and normal ["normal-flow" POTS (NFP)] blood flow measured in the lower extremities while subjects were in the supine position. We propose that postural tachycardia is related to thoracic hypovolemia during orthostasis but that the patterns of peripheral blood flow relate to different mechanisms for thoracic hypovolemia. We studied 37 POTS patients aged 14-21 yr: 14 LFP, 15 NFP, and 8 HFP patients and 12 healthy control subjects. Peripheral blood flow was measured in the supine position by venous occlusion strain-gauge plethysmography of the forearm and calf to subgroup patients. Using indocyanine green techniques, we showed decreased cardiac index (CI) and increased total peripheral resistance (TPR) in LFP, increased CI and decreased TPR in HFP, and unchanged CI and TPR in NFP while subjects were supine compared with control subjects. Blood volume tended to be decreased in LFP compared with control subjects. We used impedance plethysmography to assess regional blood volume redistribution during upright tilt. Thoracic blood volume decreased, whereas splanchnic, pelvic, and leg blood volumes increased, for all subjects during orthostasis but were markedly lower than control for all POTS groups. Splanchnic volume was increased in NFP and LFP. Pelvic blood volume was increased in HFP only. Calf volume was increased above control in HFP and LFP. The results support the hypothesis of (at least) three pathophysiologic variants of POTS distinguished by peripheral blood flow related to characteristic changes in regional circulations. The data demonstrate enhanced thoracic hypovolemia during upright tilt and confirm that POTS is related to inadequate cardiac venous return during orthostasis.

Effects of thoracic blood volume on Valsalva maneuver

The Valsalva maneuver (VM) is frequently used to test autonomic function. However, the VM is also affected by changes in blood volume and blood volume redistribution. We hypothesized that even a standardized VM may produce a wide range of thoracic blood volume shifts. Larger blood volume shifts in some normovolemic individuals may be sufficient to induce decreases in blood pressure (BP) that preclude autonomic restoration of BP in phase II of the VM. To test this hypothesis, we studied 17 healthy volunteers aged 15-22 yr. All had similar vasoconstrictor responses when supine and upright and normal blood volume measurements. We assessed changes in thoracic blood volume by impedance plethysmography before and during the VM performed while subjects were supine. In some subjects, large decreases in BP were produced by thoracic hypovolemia. The maximum fractional decrease in BP correlated well (r(2) = 0.64; P < 0.001) with thoracic hypovolemia and with systolic BP at the end of phase II of the VM (r(2) = 0.67; P < 0.001). The BP overshoot in phase IV of the VM was uncorrelated to phase II changes, which suggests intact autonomic vasoconstriction. We conclude that the BP decrease during the VM is related to a variable decrease in thoracic blood volume that may be sufficient to preclude pressure recovery during phase II even with normal resting peripheral vasoconstriction. The VM depends on vascular as well as autonomic activation, which broadens its utility but complicates its analysis.

Renal and hormonal responses to isotonic saline infusion after 3 days' head-down tilt vs. supine and seated positions

AIM

The study aimed to determine whether prolonged exposure to simulated microgravity produces a level of thoracic volume receptor loading similar to that seen in the upright position or immediately after lying down.

METHODS

We used a cross-over design to compare responses to a saline infusion in eight healthy subjects during a 4-day, -6 degree head-down tilt (HDT) and in the acute seated and acute supine positions.

RESULTS

The first 24 h of HDT were associated with greater urinary excretion of water and sodium (UV, UNaV) than seated and acute supine [cumulative UV, 3035 +/- 219, 2311 +/- 156 (P < 0.05), and 2448 +/- 182 mL (P < 0.05), respectively; cumulative UNaV, 256 +/- 19, 180 +/- 11 (P < 0.05), and 189 +/- 15 mmol (P < 0.05), respectively]. Haemoglobin and haematocrit were increased after 24 h and plasma volume decreased after 48 h of HDT (P < 0.05). With prolongation of HDT, UV and UNaV returned near the baseline values, and plasma atrial natriuretic factor (ANF) and renin values returned to acute seated levels; in acute supine, ANF values were higher and renin lower than in the two other positions. After a 30-min infusion of 20 mL kg(-1) isotonic saline on the fourth HDT day or during acute seated or acute supine, sodium excretion within 4 h was similar during HDT and acute seated (83 +/- 6 and 84 +/- 9 mmol, respectively) and greater during supine (104 +/- 8 mmol, P < 0.05). The renin decrease was greater in HDT and seated than in supine. The plasma ANF increase was greater during HDT than during supine; during seated, plasma ANF was unchanged.

CONCLUSION

These data suggest that, after 4 days of HDT, thoracic volume receptor loading returns to the same level as in the seated position, leading to blunted responses to volume expansion as compared with the acute supine position.

[New methods for noninvasive and continuous determination of dynamic compliance of the carotid artery]

A new method for the noninvasive, continuous determination of the compliance of the carotid artery wall has been developed and, in an initial study, validated. Measurements of pulsatile changes in the diameter of the carotid artery are accomplished with the 4-electrode impedance method, and the intravascular blood pressure is measured using an applanation tonometer developed during this project. The method has been employed for measurements in 12 individuals with no vascular disease, and in one patient with carotid artery stenosis before, during and after successful dilatation. With the pressure-volume curves recorded during the cardiac cycle, it is possible to calculate dynamic compliance and the non-elastic deformation work. While initial results are very promising, further validation by a large-scale clinical study is required.

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 a three-day head-down tilt on renal and hormonal responses to acute volume expansion

To clarify whether exposure to 6 degrees head-down tilt (HDT) leads to alterations in body fluid volumes and responses to a saline load similar to those observed during space flight we investigated eight healthy subjects during a 4-day, 6 degrees HDT and during a time-control ambulatory period with cross-over. Compared with the ambulatory period, HDT was associated with greater urinary excretion of water and sodium (UV, U(Na)V) from 0 to 12 h (cumulated UV 1,781 +/- 154 vs. 1,383 +/- 170 ml, P < 0.05; cumulated U(Na)V 156 +/- 14 vs. 117 +/- 9 mmol, P < 0.05), and with higher plasma atrial natriuretic factor (ANF) at 4 h. Hemoglobin and hematocrit increased over the first 24 h, and blood and plasma volumes were decreased after 48 h of HDT (P < 0.05). Plasma renin activity (PRA) and aldosterone did not differ between the two groups. With prolongation of HDT, UV and U(Na)V returned close to baseline values. On the fourth HDT day, a 30-min infusion of 20 ml/kg isotonic saline was performed, while a large oral water load maintained a high urine output. The ambulatory period experiment was done with the subjects in the acute supine posture. Sodium excreted within 4 h of loading was 123 +/- 8 mmol during HDT vs. 168 +/- 16 mmol during the ambulatory period (P < 0.05). The increase in plasma ANF and decrease in PRA were greater during HDT than during the ambulatory period (ANF 30 +/- 5 vs. 13 +/- 4 pg/ml, P < 0.05; PRA -1.4 +/- 0.4 vs. -0.5 +/- 0.2 ng. ml(-1). h(-1), P < 0.05). Our data suggest that after a 3-day HDT period, thoracic volume receptor loading returns to the level seen in the upright position, leading to blunted responses to volume expansion, compared with acute supine control.

Monitoring relative fluid balance alterations in haemodialysis of diabetic patients by electrical impedance

Measurements of total-body electrical impedance in the frequency range between 200 Hz and 300 kHz were performed on 37 diabetic patients undergoing chronic haemodialysis. Special attention was paid to the instrument design, where a self-balancing differential current source was used, reducing considerably the common-mode voltage at the amplifier input. The patient-instrument interface includes screened leads, separately driven by unity-gain buffers. The measurement error was < 1% for the impedance within the range of 20 to 1000 omega and < 0.3 degree (mean) for the phase angle. Impedance/phase and ultrafiltration measurements were carried out throughout the entire procedure. Total and extracellular water were computed and compared with extracted fluid volumes. The trends of change of the extracellular and intracellular fluid volumes during and immediately after dialysis corresponded to the respective clinical condition of the patients and enabled us to divide them into four groups. This approach is a step toward continuous monitoring and adaptive treatment, tailored to the individual patient needs.

A multifrequency multichannel electrical impedance data acquisition system for body fluid shift monitoring

This paper discusses some important issues for the design of electrical impedance measurement systems intended for body fluid shift monitoring, in particular during dialysis treatments. We have studied two common signal generation systems: digital synthesis and carrier recovery. We have found that in prolonged measurement applications, digital synthesis yields the best performance. On the demodulation side, we balance the demodulator errors between the real and imaginary parts by rotating the demodulation axes. We use segmental multifrequency impedance measurements to estimate the values of intracellular and extracellular impedance by adjusting the parameters of a Cole-Cole model for each segment measured. We stress the need to perform segmental measurements in order to accurately measure the segments of interest, in particular the trunk during dialysis treatments. Our results show that there is a sharp disequilibrium between the intracellular and extracellular compartments in the very first dialysis period. This fact generates the need to continuously measure segmental impedance instead of comparing initial and final values.

Errors in prolonged electrical impedance measurements due to electrode repositioning and postural changes

Long-term electrical impedance measurements are affected by specific errors. Electrode failure, changes in its impedance due to aging, and postural changes are among the most important. We analyse errors due to electrode replacement and body postural changes. Electrode replacement errors can cause impedance changes up to 5% of basal value. This is one of the most important factors in data reproducibility. Body postural changes also contribute to impedance variations. We have proposed the use of a reference position to carry out impedance measurements as the one that shows the smallest impedance sensitivity to postural changes. In general, we observed that this is achieved with arms and legs slightly separated from the body. We propose the use of a ratio of impedance at two different frequencies to discern the origin of impedance changes, whether from physiological phenomena or postural errors.