Effects of 18 days of bed rest on leg and arm venous properties

Venous Thromboembolism in Exploration Class Human Spaceflight

INTRODUCTION: A recent finding of a deep venous thrombosis during spaceflight has prompted the need to clarify mechanisms and risks of venous thromboembolism (VTE). In turn, mitigation countermeasures, diagnostic modalities, and treatment options must be explored. The objective of this review was to synthesize current evidence on VTE in spaceflight.METHODS: A literature review was performed from inception to April 2023 pertaining to VTE in the context of spaceflight or ground-based analogs with human participants. PubMed was searched for papers written in English using the terms "spaceflight" or "weightlessness" and "thrombotic" or "embolism" or "thromboembolism" in "venous" or "veins". Papers using cellular or animal models were excluded.RESULTS: There were 63 papers captured; 7 original scientific studies, 3 narrative reviews, 2 systematic reviews, and 3 commentaries discussed VTE in spaceflight. Reference lists were screened. Important themes included: altered venous hemodynamics, increased fibrinogen and coagulation markers, hypoalbuminemia, and immune dysfunction. Additional risk factors may be seen in women, such as the use of oral contraceptives.DISCUSSION: Venous stasis and decreased shear stress secondary to fluid shifts may induce inflammatory changes in the venous system, resulting in endothelial damage and upregulation of the coagulation cascade. Additionally, women in space are subject to physiological factors increasing their VTE risk, such as the use of oral contraceptives, inducing increased blood viscosity and hypoalbuminemia. Efforts should also be placed in optimizing sensitivity and specificity of imaging markers, payload, and training ability, notably the use of vector flow imaging, and improving point-of-testing biomarkers, such as albumin and p-selectin.Levasseur S, Purvis N, Trozzo S, Chung SH, Ades M, Drudi LM. Venous thromboembolism in exploration class human spaceflight. Aerosp Med Hum Perform. 2024; 95(1):45-53.

The effect of microgravity on the human venous system and blood coagulation: a systematic review

NEW FINDINGS

What is the central question of this study? Recently, an internal jugular venous thrombus was identified during spaceflight: does microgravity induce venous and/or coagulation pathophysiology, and thus an increased risk of venous thromboembolism (VTE)? What is the main finding and its importance? Whilst data are limited, this systematic review suggests that microgravity and its analogues may induce an enhanced coagulation state due to venous changes most prominent in the cephalad venous system, as a consequence of changes in venous flow, distension, pressures, endothelial damage and possibly hypercoagulability in microgravity and its analogues. However, whether such changes precipitate an increased VTE risk in spaceflight remains to be determined.

ABSTRACT

Recently, an internal jugular venous thrombus was identified during spaceflight, but whether microgravity induces venous and/or coagulation pathophysiology, and thus, an increased risk of venous thromboembolism (VTE) is unclear. Therefore, a systematic (Cochrane compliant) review was performed of venous system or coagulation parameters in actual spaceflight (microgravity) or ground-based analogues in PubMed, MEDLINE, Ovid EMBASE, Cochrane Library, European Space Agency, National Aeronautics and Space Administration, and Deutsches Zentrum für Luft-und Raumfahrt databases. Seven-hundred and eight articles were retrieved, of which 26 were included for evaluation with 21 evaluating venous, and five coagulation parameters. Nine articles contained spaceflight data, whereas the rest reported ground-based analogue data. There is substantial variability in study design, objectives and outcomes. Yet, data suggested cephalad venous system dilatation, increased venous pressures and decreased/reversed flow in microgravity. Increased fibrinogen levels, presence of thrombin generation markers and endothelial damage were also reported. Limited human venous and coagulation system data exist in spaceflight, or its analogues. Nevertheless, data suggest spaceflight may induce an enhanced coagulation state in the cephalad venous system, as a consequence of changes in venous flow, distension, pressures, endothelial damage and possibly hypercoagulability. Whether such changes precipitate an increased VTE risk in spaceflight remains to be determined.

Nutritional interventions during bed rest and spaceflight: prevention of muscle mass and strength loss, bone resorption, glucose intolerance, and cardiovascular problems

Bed rest is necessary for many medical conditions but also used as a ground-based model for space flight (along with head-down tilt to simulate fluid shifts in microgravity). The purpose of this review is to examine nutritional interventions during bed rest and spaceflight for prevention of muscle and strength loss, glucose intolerance, bone resorption, and cardiovascular problems. Increased dietary protein intake and supplementation with amino acids, β-hydroxy-β-methylbutyrate, or cofactors with antioxidant properties are effective for ameliorating bed rest-induced loss of muscle mass and strength. Previous literature involving bed rest with dietary protein/amino acid supplementation had mixed findings, likely due to differences in dosage. Although high protein intake in some studies prevents bed rest-induced muscle loss, it also increases bone resorption. High calcium intake and vitamin D supplementation are not beneficial for preventing bone degradation during bed rest or spaceflight. Very few studies investigated countermeasures to prevent glucose intolerance and cardiovascular risks during bed rest/spaceflight. Low-glycemic index diets might be beneficial for the prevention of bed rest-induced glucose intolerance and cardiovascular problems. The present evidence warrants additional studies on the exact threshold of protein/amino acid intake to prevent the loss of muscle mass and strength during bed rest/spaceflight specifically to maintain the beneficial effects of proteins on muscle mass and function without increasing bone resorption. Furthermore, it is suggested to study the effects of vitamin K supplementation on bone health during bed rest/spaceflight and determine the role of long-term low-glycemic index diets on glucose regulation and cardiovascular health during extended bed rest.

Physical Exercise in the Oldest Old

Societies are progressively aging, with the oldest old (i.e., those aged >80-85 years) being the most rapidly expanding population segment. However, advanced aging comes at a price, as it is associated with an increased incidence of the so-called age-related conditions, including a greater risk for loss of functional independence. How to combat sarcopenia, frailty, and overall intrinsic capacity decline in the elderly is a major challenge for modern medicine, and exercise appears to be a potential solution. In this article, we first summarize the physiological mechanisms underlying the age-related deterioration in intrinsic capacity, particularly regarding those phenotypes related to functional decline. The main methods available for the physical assessment of the oldest old are then described, and finally the multisystem benefits that exercise (or "exercise mimetics" in those situations in which volitional exercise is not feasible) can provide to this population segment are reviewed. In summary, lifetime physical exercise can help to attenuate the loss of many of the properties affected by aging, especially when the latter is accompanied by an inactive lifestyle and benefits can also be obtained in frail individuals who start exercising at an advanced age. Multicomponent programs combining mainly aerobic and resistance training should be included in the oldest old, particularly during disuse situations such as hospitalization. However, evidence is still needed to support the effectiveness of passive physical strategies including neuromuscular electrical stimulation or vibration for the prevention of disuse-induced negative adaptations in those oldest old people who are unable to do physical exercise. © 2019 American Physiological Society. Compr Physiol 9:1281-1304, 2019.

Physical strategies to prevent disuse-induced functional decline in the elderly

Disuse situations can have serious adverse health consequences in the elderly, including mainly functional impairment with subsequent increase in the risk of falls or morbimortality. The present review provides clinicians and care givers with detailed and practical information on the feasibility and effectiveness of physical strategies that are currently available to prevent or attenuate the functional decline that occurs secondarily to disuse situations in the elderly, notably in the hospital setting. In this context, active approaches such as resistance exercises and maximal voluntary contractions, which can be performed both isometrically and dynamically, are feasible during most immobilization situations including in hospitalized old people and represent powerful tools for the prevention of muscle atrophy. Aerobic exercise should also be prescribed whenever possible to reduce the loss of cardiovascular capacity associated with disuse periods. Other feasible strategies for patients who are unwilling or unable to perform volitional exercise comprise neuromuscular electrical stimulation, vibration, and blood flow restriction. However, they should ideally be applied synchronously with voluntary exercise to obtain synergistic benefits.

Analogs of microgravity: head-down tilt and water immersion

This article briefly reviews the fidelity of ground-based methods used to simulate human existence in weightlessness (spaceflight). These methods include horizontal bed rest (BR), head-down tilt bed rest (HDT), head-out water immersion (WI), and head-out dry immersion (DI; immersion with an impermeable elastic cloth barrier between subject and water). Among these, HDT has become by far the most commonly used method, especially for longer studies. DI is less common but well accepted for long-duration studies. Very few studies exist that attempt to validate a specific simulation mode against actual microgravity. Many fundamental physical, and thus physiological, differences exist between microgravity and our methods to simulate it, and between the different methods. Also, although weightlessness is the salient feature of spaceflight, several ancillary factors of space travel complicate Earth-based simulation. In spite of these discrepancies and complications, the analogs duplicate many responses to 0 G reasonably well. As we learn more about responses to microgravity and spaceflight, investigators will continue to fine-tune simulation methods to optimize accuracy and applicability.

Use of Accelerometers to Examine Sedentary Time on an Acute Stroke Unit

BACKGROUND AND PURPOSE

Observational studies demonstrate low levels of physical activity during inpatient stroke rehabilitation. There are no prior studies that have objectively measured sedentary time on the acute stroke unit and whether sedentary time is related to functional outcomes. The purpose of this study was to characterize sedentary time after acute stroke and determine whether there is a relationship to functional performance at discharge.

METHODS

Thirty-two individuals (18 men; 56.5 ± 12.7 years) with acute stroke were enrolled within 48 hours of hospital admission. An accelerometer was placed on the stroke-affected ankle to measure 24-hour activity and was worn for 4 days or until discharge from the hospital. Performance of activities of daily living, walking endurance, and functional mobility were assessed using the Physical Performance Test, Six-Minute Walk Test, and Timed Up and Go, respectively.

RESULTS

Mean percent time spent sedentary was 93.9 ± 4.1% and percent time in light activity was 5.1 ± 2.4%. When controlling for baseline performance, the mean time spent sedentary per day was significantly related to Physical Performance Test performance at discharge (r = -0.37; P = .05), but not the Six-Minute Walk Test or Timed Up and Go.

DISCUSSION AND CONCLUSIONS

Patients with acute stroke were sedentary most of their hospital stay. To minimize the potential negative effects of inactivity, our data suggest that there should be greater emphasis on increasing physical activity during the hospital stay.Video Abstract Available for more insights from the authors (Supplemental Digital Content 1, http://links.lww.com/JNPT/A101).

Vascular adaptations to low-load resistance training with and without blood flow restriction

PURPOSE

To examine the effects of low-load knee extensor training to fatigue with and without blood flow restriction (BFR) on calf vascular conductance, calf venous compliance, and peripheral arterial stiffness in middle-aged individuals.

METHODS

Eleven men (55 ± 8 years) and five post-menopausal women (57 ± 5 years) completed 6 weeks of unilateral knee extensor training with one limb exercising with BFR (BFR limb) and the contralateral limb exercising without BFR (free flow, FF limb). Before and after the training, femoral pulse wave velocity (PWV), calf blood flow (normalized as conductance), and calf venous compliance were measured in each limb.

RESULTS

PWV increased following training in both limbs (main effect of time, p = 0.036; BFR limb 8.9 ± 0.8 vs. 9.5 ± 0.9 m/s, FF limb 9.0 ± 1.2 vs. 9.0 ± 1.1; Pre vs. Post). Calf blood flow increased (p = 0.026) in the FF limb (25.0 ± 7.0 vs. 31.8 ± 12.0 flow/mmHg; Pre vs. Post) but did not change (p = 0.831) in the BFR limb (29.1 ± 11.3 vs. 28.7 ± 11.5 flow/mmHg; Pre vs. Post). Calf venous compliance did not change in either limb following training.

CONCLUSIONS

These results suggest low-load BFR resistance training to fatigue elicits small increases in peripheral arterial stiffness without eliciting concomitant changes in venous compliance. In addition, unlike low-load knee extensor training without BFR, training with BFR did not enhance calf blood flow.

Cardiovascular re-adjustments and baroreflex response during clinical reambulation procedure at the end of 35-day bed rest in humans

During the reambulation procedure after 35-day head-down tilt bed rest (HDTBR) for 9 men, we recorded for the first time heart rate (HR; with electrocardiogram) and arterial pressure profiles (fingertip plethysmography) for 5 min in HDTBR and horizontal (SUP) positions, followed by 12 min in standing position, during which 4 subjects fainted (intolerant, INT) and were laid horizontal again (Recovery). We computed: mean arterial pressure (P¯; pressure profiles integral mean), stroke volume (SV; obtained with Modelflow method), and cardiac output (Q̇; SV × HR). All cardiovascular data remained stable in HDTBR and SUP for both groups (EXP). Taking the upright posture, EXP showed a decrease in SV and an increase in HR, becoming significantly different from SUP within 1 min. Further evolution of these parameters kept Q̇ stable in both groups until the second minute of standing. Afterward, in INT, P̄ precipitated without further HR increases: SV stopped being corrected and Q̇ reached 2.9 ± 0.4 L·min−1 at the last 15 s of standing. Sudden drop in P̄ allowed identification of a low-pressure threshold in INT (70.7 ± 12.9 mm Hg), after which syncope occurred within 80 s. During Recovery, baroreflex curves showed a flat phase (P̄ increase, HR stable), followed by a steep phase (P̄ increased, HR decreased, starting when P̄ was 84.5 ± 12.5 mm Hg and Q̇ was 9.6 ± 1.5 L·min−1). INT, in contrast with tolerant subjects, did not sustain standing because HR was unable to correct for the P̄ drop. These results indicate a major role for impaired arterial baroreflexes in the onset of orthostatic intolerance.

Effects of walking with blood flow restriction on limb venous compliance in elderly subjects

Venous compliance declines with age and improves with chronic endurance exercise. KAATSU, an exercise combined with blood flow restriction (BFR), is a unique training method for promoting muscle hypertrophy and strength gains by using low-intensity resistance exercises or walking. This method also induces pooling of venous blood in the legs. Therefore, we hypothesized that slow walking with BFR may affect limb venous compliance and examined the influence of 6 weeks of walking with BFR on venous compliance in older women. Sixteen women aged 59-78 years were partially randomized into either a slow walking with BFR group (n = 9, BFR walk group) or a non-exercising control group (n = 7, control group). The BFR walk group performed 20-min treadmill slow walking (67 m min(-1) ), 5 days per week for 6 weeks. Before (pre) and after (post) those 6 weeks, venous properties were assessed using strain gauge venous occlusion plethysmography. After 6 weeks, leg venous compliance increased significantly in the BFR walk group (pre: 0·0518 ± 0·0084, post: 0·0619 ± 0·0150 ml 100 ml(-1)  mmHg(-1) , P<0·05), and maximal venous outflow (MVO) at 80 mmHg also increased significantly after the BFR walk group trained for 6 weeks (pre: 55·3 ± 15·6, post: 67·1 ± 18·9 ml 100 ml(-1)  min(-1) , P<0·01), but no significant differences were observed in venous compliance and MVO in the control group. In addition, there was no significant change in arm compliance in the BFR walk group. In conclusion, this study provides the first evidence that 6 weeks of walking exercise with BFR may improve limb venous compliance in untrained elder female subjects.

Recent findings in cardiovascular physiology with space travel

The cardiovascular system undergoes major changes in stress with space flight primarily related to the elimination of the head-to-foot gravitational force. A major observation has been that the central venous pressure is not elevated early in space flight yet stroke volume is increased at least early in flight. Recent observations demonstrate that heart rate remains lower during the normal daily activities of space flight compared to Earth-based conditions. Structural and functional adaptations occur in the vascular system that could result in impaired response with demands of physical exertion and return to Earth. Cardiac muscle mass is reduced after flight and contractile function may be altered. Regular and specific countermeasures are essential to maintain cardiovascular health during long-duration space flight.

WISE-2005: tibial and gastrocnemius vein and calf tissue response to LBNP after a 60-day bed rest with and without countermeasures

The objective of this study was to quantify by echography the changes in the intramuscular [gastrocnemius (Gast)] and nonintramuscular [posterior tibial (Tib)] calf veins cross-sectional area (CSA) and the superficial tissue thickness (STth) in response to lower body negative pressure (LBNP) after 60-day head-down bed rest (HDBR). Twenty-four healthy women (25–40 yr) were divided into three groups: control (Con), treadmill-LBNP and flywheel (Ex-Lb), nutrition (Nut; protein supplement). All underwent a LBNP (0 and −45 mmHg) before and on day 55 of HDBR. Subjects were identified as finisher (F) or nonfinisher (NF) of a 10-min tilt test after 60 days of HDBR. There were no differences in resting CSA of the Tib and Gast veins on HDBR day 55 compared with pre-HDBR for the Ex-Lb, Con and Nut, or the F groups; however, for NF both the Tib and Gast vein CSA at rest were significantly smaller after HDBR. At −45 mmHg LBNP, Tib and Gast CSAs were not significantly different from before HDBR in all groups (Ex-Lb, Con, Nut, F, NF). However, percent change in CSA of both veins from rest to −45 mmHg LBNP was significantly greater in the Con and Nut groups compared with Ex-Lb, and also NF compared with F. Similarly, the percent increase in STth on going from rest to −45 mmHg was higher after HDBR in the Con and Nut groups compared with Ex-Lb, as well as NF compared with F. These results showed that the Ex-Lb countermeasure minimized the bed rest effect on leg vein capacitance (CSA percent change) and STth increase during LBNP, whereas Nut had no effect and that higher leg vein and superficial tissue capacitance were associated with reduced orthostatic tolerance.

From space to Earth: advances in human physiology from 20 years of bed rest studies (1986-2006)

Bed rest studies of the past 20 years are reviewed. Head-down bed rest (HDBR) has proved its usefulness as a reliable simulation model for the most physiological effects of spaceflight. As well as continuing to search for better understanding of the physiological changes induced, these studies focused mostly on identifying effective countermeasures with encouraging but limited success. HDBR is characterised by immobilization, inactivity, confinement and elimination of Gz gravitational stimuli, such as posture change and direction, which affect body sensors and responses. These induce upward fluid shift, unloading the body's upright weight, absence of work against gravity, reduced energy requirements and reduction in overall sensory stimulation. The upward fluid shift by acting on central volume receptors induces a 10-15% reduction in plasma volume which leads to a now well-documented set of cardiovascular changes including changes in cardiac performance and baroreflex sensitivity that are identical to those in space. Calcium excretion is increased from the beginning of bed rest leading to a sustained negative calcium balance. Calcium absorption is reduced. Body weight, muscle mass, muscle strength is reduced, as is the resistance of muscle to insulin. Bone density, stiffness of bones of the lower limbs and spinal cord and bone architecture are altered. Circadian rhythms may shift and are dampened. Ways to improve the process of evaluating countermeasures--exercise (aerobic, resistive, vibration), nutritional and pharmacological--are proposed. Artificial gravity requires systematic evaluation. This review points to clinical applications of BR research revealing the crucial role of gravity to health.

Ultrasound: a reproducible method to measure conduit vein compliance

Classical venous occlusion plethysmography (VOP) of the leg, often used to assess venous compliance, measures properties of the whole calf, including volume changes at the arterial side and the interstitial fluid accumulation that occurs as a result of the enhanced capillary pressure during venous occlusion. We present an ultrasound technique to measure the compliance of one major conduit vein in the leg. Ultrasound measurements of the popliteal vein were compared with classical VOP measurements, which were performed simultaneously in one subject. Six healthy individuals were measured on three occasions to assess short- and long-term reproducibility of the measurements. Six motor complete spinal cord-injured (SCI) individuals were included to compare venous compliance in subjects with known pathological changes of the venous system with controls. The ultrasound and VOP measurements of venous compliance correlated significantly ( r2= 0.39, P = 0.001). Ultrasound provides reproducible measurements with short- and long-term coefficients of variation ranging from 10 to 15% for popliteal vein compliance and from 2 to 9% for absolute diameters at the different venous pressure steps. In addition, by using ultrasound, we were able to detect an 80% reduction in the compliance of the popliteal vein in SCI individuals compared with controls ( P < 0.01). In conclusion, ultrasound is a suitable and reproducible method to measure conduit vein compliance and provides the possibility to specifically assess compliance of one vein instead of the whole calf.

Vascular adaptation to 4 wk of deconditioning by unilateral lower limb suspension

Physical inactivity or deconditioning is an independent risk factor for atherosclerosis and cardiovascular disease. In contrast to exercise, the vascular changes that occur as a result of deconditioning have not been characterized. We used 4 wk of unilateral lower limb suspension (ULLS) to study arterial and venous adaptations to deconditioning. In contrast to previous studies, this model is not confounded by denervation or microgravity. Seven healthy subjects participated in the study. Arterial and venous characteristics of the legs were assessed by echo Doppler ultrasound and venous occlusion plethysmography. The diameter of the common and superficial femoral artery decreased by 12% after 4 wk of ULLS. Baseline calf blood flow, as measured by plethysmography, decreased from 2.1 ± 0.2 to 1.6 ± 0.2 ml·min−1·dl tissue−1. Both arterial diameter and calf blood flow returned to baseline values after 4 wk of recovery. There was no indication of a decrease in flow-mediated dilation of the superficial femoral artery after ULLS deconditioning. This means that functional adaptations to inactivity are not simply the inverse of adaptations to exercise. The venous pressure-volume curve is shifted downward after ULLS, without any effect on compliance. In conclusion, deconditioning by 4 wk of ULLS causes significant changes in both the arterial and the venous system.

Age- and fitness-related differences in limb venous compliance do not affect tolerance to maximal lower body negative pressure in men and women

Aging and chronic exercise training influence leg venous compliance. Venous compliance affects responses to an orthostatic stress; its effect on tolerance to maximal lower body negative pressure (LBNP) in the elderly is unknown. The purpose of this study was to determine the influence of age and fitness, a surrogate measure of exercise training, on calf venous compliance and tolerance to maximal LBNP in men and women. Forty participants, 10 young fit (YF; age = 22.6 ± 0.5 yr, peak oxygen uptake = 57.1 ± 2.0 ml·kg−1·min−1), 10 young unfit (YU; 23.1 ± 1.0 yr, 41.1 ± 2.0 ml·kg−1·min−1), 10 older fit (OF; 73.9 ± 2.0 yr, 39.0 ± 2.0 ml·kg−1·min−1), and 10 older unfit (OU; 70.9 ± 1.6 yr, 27.1 ± 2.0 ml·kg−1·min−1), underwent graded LBNP to presyncope or 4 min at −100 mmHg. By utilizing venous occlusion plethysmography, calf venous compliance was determined by using the first derivative of the pressure-volume relation during cuff pressure reduction. We found that the more fit groups had greater venous compliance than their unfit peers ( P < 0.05) as did the young groups compared with their older peers ( P < 0.05) such that OU < YU = OF < YF. LBNP tolerance did not differ between groups. In conclusion, these data suggest that aging reduces, and chronic exercise increases, venous compliance. However, these data do not support a significant influence of venous compliance on LBNP tolerance.