航天中各阶段心脏动力学变化

人体正常动脉血压的维持依赖于心功能正常。国外载人航天活动各阶段心脏动力学参数是检测的重点之一。虽然航天中有关心脏动力学参数的报道不尽一致，但一般都观察到航天后立位应激时心脏动力学参数有较大变化，立位耐力降低的机理尚不清楚，心脏动力学变化对立位耐力的影响也有待探讨。本文对国外载人航天活动各阶段有关心脏动力学的变化及变化机理作一简要概述。     

失重／模拟失重对心肌结构与功能的影响

自开展载人航天活动以来,已发现在失重环境停留可引起航天员运动能力及立位耐力下降.后者主要表现为:在失重条件下受到下体负压作用时,或返回地面IG条件下取直立体位时,航天员感到心慌、头晕,甚至出现晕厥,其体征则为心率过分增快,维持正常动脉血压的能力明显减弱;一般需数日至数周始能恢复.目前,将此类症状与体征称为“心血管脱适应(cardiovascular deconditioning)”,或者称为心血管机能障碍(cardiovascular dysfunct-ion).虽经几十年研究,对这种心血管机能障碍的机理仍然不甚清楚,特别是关于长期失重所致的心血管机能障碍的机理则了解更少.虽然为期一年的载人航天已经实现,但所用对抗措施并不理想,亦不能完全克服心血管机能障碍所带来的各种问题.为保证未来长时间航天活动,也急需发展新的对抗措施.     

航天活动各阶段心电图和心动时相的变化及其机理分析

心电图（ＥＣＧ）仍是国外载人航天活动各阶段中连续医学监测的唯一技术。我国载人航天计划中ＥＣＧ也作为重要的医监项目。采用ＥＣＧ,可动态观察人体４大生命体征之一的脉搏（或心率）和心功能,与ＥＣＧ有关的心动进相是反映心功能的一项重要指标,系统了解国外载人航天活动中ＥＣＧ和心动时相的变化及其机理对我国载人航天紧急措施的制定具有重要的参考价值。本文对此作了较系统的回顾与分析。     

防止心血管失调的对抗措施

防止心血管失调的对抗措施航天员飞行后立位实验时，除出现明显的心跳加快外，约有３０％～４０％的航天员出现立位性低血压和晕厥，说明微重力可损伤航天员的心血管调节功能。因此，为了保证航天员的健康，在飞行中需要采用一些防止心血管失调的措施。本文主要介绍在航天...     

载人航天训练模拟器的回顾与分析

从载人航天活动一开始,人们便认识到,为保证航天员安全高效地完成飞行任务,必须在地面利用飞行训练模拟器对航天员进行充分的专业技术方面的训练和包括各飞行阶段故障与应急处理的全任务综合程序训练。载人航天训练模拟器的作用与重要性已被国外(尤其是美国     

前庭系统适应性变化对心血管调节和立位耐力的影响

空间飞行初期人体感受到的空间适应综合征,主要由前庭传入的改变和体液头向分布引起,这种适应性的逆向变化同样发生在人体由微重力返回正常1G重力时,造成航天员返回后出现步态不稳及立位耐力下降。动物模型的证据和人体实验的部分数据表明,前庭系统通过自主神经系统影响心血管调节,其中耳石感受器的传入起着重要作用。由于心血管调节异常引起的立位耐力下降是载人航天的重要医学问题。在心血管功能和立位耐力下降过程中前庭系统功能变化有重要影响。     

模拟微重力或微重力时血容量减少对立位耐力影响的研究进展

航天后立位耐力降低普遍存在,其机理仍不清楚。一般认为血容量减少是航天后立位耐力降低的一个重要因素。现有对抗航天后一耐力降低的措施均与血容量有关。微重力（μＧ）和模拟μＧ时研究表明：μＧ与现有模拟μＧ模型时人体生理学变化的主要差异表现在低压区循环和体液、电解质代谢。细胞外体液主要受心肺反射和适当的模拟μＧ模型研究低压区循环、心肺反射变化的时间过程,对认为立位耐力降低机理和制定更有效的对抗立位耐力降低     

血容量减少对立位应激反应影响的仿真研究

目的 研究不同程度的血容量减少对心血管系统立位应激反应的影响,探讨血容量降低在航天失重后心血管失调和立位耐力降低机理中的意义。方法 在仿真下体负压（LBNP）暴露时心血管系统反应模型的血液重新分配子模型中引入血容量减少因素,仿真血容量减少0－25％后LBNP时心率（HR）和血压BP变化,结果 血容量减少低于总血量的5％条件下,心血管系统可以通过压力反射调节作用维持LBNP时政党的HR和BP;血容量减少超过约15％,在安静仰卧位时,HR和BP正常,但LBNP时BP迅速降低,系统可失去稳定性。结论 血容量减少将导致心血管系统对立位应激反应的改变。     

用综合反射系数检测体位改变对人体心血管自主神经调节功能的影响

目的通过研究人体体位改变时心率变异性(HRV)、血压变异性(BPV)和综合反射系数(SEC)评价心血管自主神经调节功能的敏感性和可靠性,为航天医学研究探索一种能简单、无创反映心血管自主神经调节功能的新方法。方法10名健康男性志愿者,坐位(Z)5 min,平卧位(P1)10 min,头低位(HDT)15°10 min,后转平卧位(P2)10 min,接着+75°被动立位(HUT)10 min,采用LXJ12立柱式水银血压计测血压、SZ-Ⅱ型多用途无创心功能检测分析系统和Cardio-Win心电工作站同步检测心血管功能。结果从Z转P1、HDT和P2期间HRV值增大,动脉收缩压变异性(SBPV)和SEC值减小;HUT期间HRV值显著减小,SBPV和SEC值显著增大。HRV、SBPV和SEC间有很好的相关性。结论HRV、SBPV和SEC均能反映人体体位改变时心血管自主神经调节功能状态,SEC作为一种简单、无创和可靠的检测指标,在航天医学研究中心血管自主神经调节功能评价方面有一定的应用前景。     

心肌收缩功能降低对下体负压效应影响的仿真研究

目的：研究不同程度心肌收缩功能降低对心血管系统立位应激反应的影响，探讨心肌收缩功能改变在航天失重心后心血管失调和立位耐力降低机理中的意义。方法：以我们原有仿真血量减少后下体负压（LBNP）暴露时心血管系统反应的模型为基础，对其心脏工作子模型中最大弹性系数（maximum elastance)乘以一个反映心肌收缩功能改变的系数。然后逐渐降低收缩功能改变系，仿真血容量减少12％后，心肌收缩功能降低0－305后LBNP时心率（HR）、血压（BP）和心输出量（CO）的变化。结果：心肌收缩功能降低将使LBNP时，HR加快，BP降低和CO减小。结论：心肌收缩功能降低导致心血管系统对立位应激的调节能力降低。     

Presyncopal/non-presyncopal outcomes of post spaceflight stand tests are consistent from flight to flight

INTRODUCTION: The overall prevalence of orthostatic hypotension after short duration (6-18 d) spaceflight is 20% with existing countermeasures. However, it is not known if the outcomes of stand tests for orthostatic tolerance are consistent within individuals on subsequent flights, or if first time fliers are more (or less) likely to experience orthostatic hypotension and presyncope than are veteran astronauts. METHODS: There were 50 astronauts who were studied retrospectively. Stand test data, which had been collected before and after spaceflight, were compared from at least two flights for each astronaut. For 25 of these astronauts, their first flight in this database was also their first spaceflight. For the remaining 25, their first flight in this database was their second, third, or fourth flight, as data were available. RESULTS: No subject became presyncopal during preflight testing. Of the 50 subjects, 45 (90%) had the same outcome on their first and second flights of this study. Of 14 subjects on whom we had data from a third mission, 12 had the same stand test outcome on all 3 flights (86% same outcome across 3 flights). There was no correlation between flight duration and orthostatic tolerance (r = 0.39). DISCUSSION: These data support the idea that astronauts are predisposed to orthostatic tolerance/intolerance after spaceflight and that this predisposition is not altered by subsequent flights. Flight durations within this data set did not alter the likelihood of orthostatic intolerance and rookie fliers were no more likely to experience orthostatic intolerance than were veteran astronauts.     

航天员常见症状的用药与体位性低血压的关系

目的对航天员在航天飞行中与航天飞行后常见症状的用药与体位性低血压的关系进行综述。资料来源与选择该领域公开发表的相关研究论文、综述、报告汇编（包括美国国家航空航天局网站上的公开文献）和著作。资料引用引用公开发表的报刊文献32篇,报告汇编5篇,和著作12部。资料综合航天员在航天飞行中与航天飞行后最常出现的症状分别为头晕、呕吐（航天运动病）、头痛、背部疼痛、失眠和晕厥,对于这些症状的西药治疗有时会对心血管系统产生不良影响。止吐药异丙嗪伴有抑制去甲肾上腺素、肾素、醛固酮分泌的作用;安眠药替马西泮具有松弛骨骼肌张力的作用;升压药氟氢可的松具有抑制去甲肾上腺素分泌的作用。这些药物都会引起血管收缩下降,水钠储留减少,回心血量降低从而造成体位性低血压。结论航天员常见症状的西药疗法常会引起体位性低血压。中国可以运用中药针灸和少量的西药复合疗法来防治航天员的常见症状,从而减少西药对人体的不良作用。     

Cerebrovascular responses during lower body negative pressure-induced presyncope

BACKGROUND: Reduced orthostatic tolerance is commonly observed after spaceflight, occasionally causing presyncopal symptoms which may be due to low cerebral blood flow (CBF). It has been suggested that CBF decreases in early stages of exposure to orthostatic stress. The purpose of this study was to investigate cerebrovascular responses during presyncope induced by lower body negative pressure (LBNP). HYPOTHESIS: Although CBF decreases during LBNP exposure, blood pressure (BP) or heart rate (HR) contributes more to induce presyncopal conditions. METHODS: Eight healthy male volunteers were exposed to LBNP in steps of 10 mm Hg every 3 min until presyncopal symptoms were detected. Electrocardiogram (ECG) was monitored continuously and arterial BP was measured by arterial tonometry. CBF velocity at the middle cerebral artery was measured by transcranial Doppler sonography (TCD). Cerebral tissue oxygenation was detected using near-infrared spectroscopy (NIRS). We focused our investigation on the data obtained during the final 2 min before the presyncopal endpoint. RESULTS: BP gradually decreased from 2 min to 10 s before the endpoint, and fell more rapidly during the final 10 s. HR did not change significantly during presyncope. CBF velocity did not change significantly, while cerebral tissue oxygenation decreased prior to the presyncopal endpoint in concert with BP. Our results suggest that CBF is maintained in the middle cerebral artery during presyncope, while BP decreases rapidly. CONCLUSIONS: Cerebrovascular hemodynamics are relatively well maintained while arterial hypotension occurs just prior to syncope.     

Head-out immersion in the non-human primate: a model of cardiovascular deconditioning during microgravity.

BACKGROUND: Orthostatic intolerance is a common complication associated with spaceflight. It has been speculated that this is due to changes in blood volume and alterations in cardiovascular reflexes. The objective of the current study was to develop a model that would allow us to study the cardiovascular system and the regulation of blood volume during short-term microgravity exposure in the primate with the intent of eventually being able to elucidate those factors responsible for the orthostatic intolerance. HYPOTHESIS: Head-out water immersion in the conscious non-human primate simulates the cardiovascular and volume regulatory responses observed in astronauts during exposure to microgravity. METHODS: Four monkeys were chronically instrumented for measuring BP and heart rate and then conditioned to the primate restraint chair. They were then subjected to 72 h of head-out water immersion (two immersions in three monkeys and one immersion in the fourth) in order to simulate the cardiovascular and renal effects of the microgravity environment. RESULTS: During the immersion, there was an increase in arterial BP (ABP) and central venous pressure (CVP) and a reflex decrease in heart rate (HR). Urine flow (UV) increased and water intake decreased, producing a negative water balance. This was not associated with an alteration in food intake. CVP and UV decreased following de-immersion. There was also resetting of the arterial baroreflex control of HR. Significant tachycardia occurred after the immersion that was associated with a decrease in ABP. CONCLUSION: These results are similar to those observed in astronauts during and after spaceflight, suggesting that head-out water immersion of the non-human primate provides a good model for studying cardiovascular and renal adaptations to spaceflight.     

Orthostatic response in rats after hindlimb unloading: effect of transcranial electrical stimulation

INTRODUCTION: Orthostatic hypotension is a commonly observed phenomenon after exposure to microgravity and in various forms of autonomic failure. It has been suggested that insufficient activation of supraspinal structures responsible for descending sympathetic drive could play a significant role in this disorder. We examined the effect of transcranial electrical stimulation (TES) of autonomic nuclei within the brain on the orthostatic hypotension induced by exposure to simulated microgravity using a hindlimb unloading model. METHODS: There were 20 male Wistar rats that were suspended by their tail with the angle of elevation between the cage floor and the rat's body approximately 40 degrees. There were 11 age-matched Wistar rats used as cage controls. Orthostatic stability was examined by using an orthostatic challenge test (450 head-up test for a period of 3 min). In 10 rats from the tail-suspended group, the orthostatic challenge test was applied during TES. RESULTS: In the rats exposed to simulated microgravity (tail suspension), the orthostatic challenge test caused a significant decrease in mean arterial blood pressure by 18.4 +/- 2.2%. TES attenuated this microgravity-induced orthostatic hypotension to 9.5 +/- 1.8% (P < 0.05), which was similar to the observed response to an orthostatic challenge in the control group (6.9 +/- 1.1%). DISCUSSION: Results of this study suggest that TES significantly reduces the changes in blood pressure during an orthostatic challenge test in animals exposed to simulated microgravity. Our observations support the notion that a reduction in descending sympathoexcitatory input from supraspinal structures could contribute to orthostatic hypotension and intolerance observed in astronauts following their return from spaceflight.     

Considerations in prescribing preflight aerobic exercise for astronauts

Many human responses to the weightless environment have been documented from actual spaceflights. These include physiological effects on the nervous system, cardiovascular system and fluid balance, and the musculoskeletal system, as well as psychological effects. Simulations on Earth have added to our knowledge about the physiology of weightlessness. Early data on orthostatic intolerance after real and simulated spaceflight led some scientists to discourage a high level of aerobic fitness for astronauts. They believed it was detrimental to orthostatic tolerance on return to Earth. However, most of the data available today do not support this contention. Furthermore, aerobic fitness is beneficial to cardiovascular function and mental performance. Therefore, it may be important in performing extra-vehicular activities during flight. Some astronauts claim exercise enhances their feeling of well-being and self image. And, although the cardiovascular system and exercise performance may recover more slowly after flight to preflight levels when fitness level prior to flight is high, the musculoskeletal system may recover more rapidly. Research is needed to determine optimal levels of aerobic training for performing tasks in flight, maintaining health and well-being during flight, and assuring satisfactory recovery on return to Earth.     

失重∕模拟失重与血管重塑

航天中航天员不可避免地处于失重环境中，重力的消失将引起人体生理系统的明显变化，血管重塑是失重或模拟失重下机体心血管系统所发生的重要变化之一。本文回顾近年来有关失重／模拟失重对机体血管重塑影响的研究进展，综述了血管重塑概念的产生、发展，扼要论述了近年来有关血管重塑的研究进展，重点阐述了失重／模拟失重对心血管系统重塑的影响及其机理，提出血管重塑对研究飞行后立位耐力的下降有重要意义。