Alterations in Cerebral Hemodynamics During Microgravity: A Literature Review

Thrombotic triad in microgravity

Thrombotic disease may be an underdiagnosed condition of prolonged exposure to microgravity and yet the underlying factors remain poorly defined. Recently, an internal jugular vein thrombosis was diagnosed in a low-risk female astronaut after an approximately 7-week space mission. Six of the additional 10 crew members demonstrated jugular venous flow risk factors, such as suspicious stagnation or retroversion. Fortunately, all were asymptomatic. Observations in space as well as clinical and in vitro microgravity studies on Earth, where experiments are designed to recapitulate the conditions of space, suggest effects on blood flow stasis, coagulation, and vascular function. In this article, the related literature on thrombotic disease in space is reviewed, with consideration of these elements of Virchow's triad.

Retinal venular vessel diameters are smaller during ten days of bed rest

Older individuals experience cardiovascular dysfunction during extended bedridden hospital or care home stays. Bed rest is also used as a model to simulate accelerated vascular deconditioning occurring during spaceflight. This study investigates changes in retinal microcirculation during a ten-day bed rest protocol. Ten healthy young males (22.9 ± 4.7 years; body mass index: 23.6 ± 2.5 kg·m-2) participated in a strictly controlled repeated-measures bed rest study lasting ten days. High-resolution images were obtained using a hand-held fundus camera at baseline, daily during the 10 days of bed rest, and 1 day after re-ambulation. Retinal vessel analysis was performed using a semi-automated software system to obtain metrics for retinal arteriolar and venular diameters, central retinal artery equivalent and central retinal vein equivalent, respectively. Data analysis employed a mixed linear model. At the end of the bed rest period, a significant decrease in retinal venular diameter was observed, indicated by a significantly lower central retinal vein equivalent (from 226.1 µm, CI 8.90, to 211.4 µm, CI 8.28, p = .026), while no significant changes in central retinal artery equivalent were noted. Prolonged bed rest confinement resulted in a significant (up to 6.5%) reduction in retinal venular diameter. These findings suggest that the changes in retinal venular diameter during bedrest may be attributed to plasma volume losses and reflect overall (cardio)-vascular deconditioning.

Cerebral Circulation and Brain Temperature during an Ultra-Short Session of Dry Immersion in Young Subjects

The primary aim of the study was to assess cerebral circulation in healthy young subjects during an ultra-short (45 min) session of ground-based microgravity modeled by "dry" immersion (DI), with the help of a multifunctional Laser Doppler Flowmetry (LDF) analyzer. In addition, we tested a hypothesis that cerebral temperature would grow during a DI session. The supraorbital area of the forehead and forearm area were tested before, within, and after a DI session. Average perfusion, five oscillation ranges of the LDF spectrum, and brain temperature were assessed. Within a DI session, in the supraorbital area most of LDF parameters remained unchanged except for a 30% increase in respiratory associated (venular) rhythm. The temperature of the supraorbital area increased by up to 38.5 °C within the DI session. In the forearm area, the average value of perfusion and its nutritive component increased, presumably due to thermoregulation. In conclusion, the results suggest that a 45 min DI session does not exert a substantial effect on cerebral blood perfusion and systemic hemodynamics in young healthy subjects. Moderate signs of venous stasis were observed, and brain temperature increased during a DI session. These findings must be thoroughly validated in future studies because elevated brain temperature during a DI session can contribute to some reactions to DI.

[Potential use of thermoneutral «dry immersion» in oncological rehabilitation]

OBJECTIVE

To conduct an analytical review of the available literature data on thermoneutral «dry immersion» (TSI) - a method that simulates the state of weightlessness/microgravity.

MATERIAL AND METHODS

The review included data from electronic databases: Scopus, Web of Science, MedLine, Wiley, World Health Organization, The Cochrane Central Register of Controlled Trials, ScienceDirect, PubMed, elibrary, CyberLeninka, disserCat.

RESULTS

The extensive database of in vitro studies contains information on the reduction of cell proliferation, invasion, migration and increased apoptosis of thyroid, breast, lung, stomach, colon cancer cells, Hodgkin's lymphoma, glioblastoma, leukemia, melanoma, osteosarcoma of a human under the influence of microgravity. The vast majority of works are devoted to experiments on healthy people to finding out the mechanisms of action of long-term continuous microgravity. The study of the therapeutic effect of TSI as a physiotherapeutic procedure of one or repeated sessions was carried out by individual authors. Positive results of a short stay in the unsupported model were obtained in the treatment of children with perinatal disorders, cerebral palsy, patients with hypertension in a state of hypertensive crisis, Parkinson's disease, skin burn II gr. The results of the analytical review provide an opportunity to begin scientific research on the effectiveness and safety of thermoneutral «dry immersion» in the complex rehabilitation of cancer patients.

The Effect of Submaximal Exercise on Jugular Venous Pulse Assessed by a Wearable Cervical Plethysmography System

The jugular venous pulse (JVP) is a one of the crucial parameters of efficient cardiovascular function. Nowadays, limited data are available regarding the response of JVP to exercise because of its complex and/or invasive assessment procedure. The aim of the present work is to test the feasibility of a non-invasive JVP plethysmography system to monitor different submaximal exercise condition. Twenty (20) healthy subjects (13M/7F mean age 25 ± 3, BMI 21 ± 2) underwent cervical strain-gauge plethysmography, acquired synchronously with the electrocardiogram, while they were carrying out different activities: stand supine, upright, and during the execution of aerobic exercise (2 km walking test) and leg-press machine exercise (submaximal 6 RM test). Peaks a and x of the JVP waveform were investigated since they reflect the volume of cardiac filling. To this aim, the Δax parameter was introduced, representing the amplitude differences between a and x peaks. Significant differences in the values of a, x, and Δax were found between static and exercise conditions (p < 0.0001, p < 0.0001, p < 0.0001), respectively. Particularly, the Δax value for the leg press was approximately three times higher than the supine, and during walking was even nine times higher. The exercise monitoring by means of the novel JVP plethysmography system is feasible during submaximal exercise, and it provides additional parameters on cardiac filling and cerebral venous drainage to the widely used heartbeat rate value.

Spaceflight-Associated Vascular Remodeling and Gene Expression in Mouse Calvaria

Astronauts suffer from a loss of bone mass at a rate of 1.5% per month from lower regions of the body during the course of long-duration (>30 days) spaceflight, a phenomenon that poses important risks for returning crew. Conversely, a gain in bone mass may occur in non-load bearing regions of the body as related to microgravity-induced cephalad fluid shift. Representing non-load bearing regions with mouse calvaria and leveraging the STS-131 (15-day) and BION-M1 (30-day) flights, we examined spatial and temporal calvarial vascular remodeling and gene expression related to microgravity exposure compared between spaceflight (SF) and ground control (GC) cohorts. We examined parasagittal capillary numbers and structures in calvaria from 16 to 23 week-old C57BL/6 female mice (GC, n = 4; SF, n = 5) from STS-131 and 19–20 week-old C57BL/6 male mice (GC, n = 6; SF, n = 6) from BION-M1 using a robust isolectin-IB4 vessel marker. We found that the vessel diameter reduces significantly in mice exposed to 15 days of spaceflight relative to control. Capillarization increases by 30% (SF vs. GC, p = 0.054) in SF mice compared to GC mice. The vessel numbers and diameter remain unchanged in BION-M1 mice calvarial section. We next analyzed the parietal pro-angiogenic (VEGFA) and pro-osteogenic gene (BMP-2, DMP1, RUNX2 and OCN) expression in BION-M1 mice using quantitative RT-PCR. VEGFA gene expression increased 15-fold while BMP-2 gene expression increased 11-fold in flight mice compared to GC. The linkage between vascular morphology and gene expression in the SF conditions suggests that angiogenesis may be important in the regulation of pathological bone growth in non-weight bearing regions of the body. Short-duration microgravity-mediated bone restructuring has implications in planning effective countermeasures for long-duration flights and extraterrestrial human habitation.

Emerging Role of Exosomal Long Non-coding RNAs in Spaceflight-Associated Risks in Astronauts

During spaceflight, astronauts are exposed to multiple unique environmental factors, particularly microgravity and ionizing radiation, that can cause a range of harmful health consequences. Over the past decades, increasing evidence demonstrates that the space environment can induce changes in gene expression and RNA processing. Long non-coding RNA (lncRNA) represent an emerging area of focus in molecular biology as they modulate chromatin structure and function, the transcription of neighboring genes, and affect RNA splicing, stability, and translation. They have been implicated in cancer development and associated with diverse cardiovascular conditions and associated risk factors. However, their role on astronauts' health after spaceflight remains poorly understood. In this perspective article, we provide new insights into the potential role of exosomal lncRNA after spaceflight. We analyzed the transcriptional profile of exosomes isolated from peripheral blood plasma of three astronauts who flew on various Shuttle missions between 1998-2001 by RNA-sequencing. Computational analysis of the transcriptome of these exosomes identified 27 differentially expressed lncRNAs with a Log2 fold change, with molecular, cellular, and clinical implications.