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。     

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     

下体负压旋转床模拟航空推拉效应对心血管功能的影响

目的探讨下体负压旋转床模拟航空推拉效应的效果 ,观察推拉动作对心血管功能的影响。方法 8名被试者在下体负压旋转床上进行“直立位 (HUT ,+1Gz) -倒立位 -直立位 +下体负压 (LBNP ,- 5 0mmHg) 1 0min”的模拟推拉效应试验及单纯直立位 +下体负压 (- 5 0mmHg) 1 0min的对照试验 ,用阻抗法测量了试验过程中心率 (HR)、血压 (BP)、基础阻抗 (Z0 )、每搏心输出量 (SV)、心输出量 (CO)及总外周阻力 (TPR)等心血管功能指标的变化。结果在模拟推拉效应试验中 ,有 3名被试者没有完成直立位 +LB NP作用 1 0min的试验 ,出现晕厥前症状 ,8名被试者平均耐受时间为 8.99± 1 .47min。而对照试验时 ,被试者均完成了 1 0min的直立 +LBNP试验。模拟推拉效应试验时 ,在倒立位 ,被试者HR、Z0 较直立位对照值降低 ,SV和CO较直立位对照值升高 ,均有显著性意义 ;直立位 +LBNP过程中 ,HR、Z0 、TPR较对照和倒立位值显著增高 ,SV和CO较对照和倒立位值显著降低 ,SBP在HUT即刻较对照值显著性增高 ,在HUT +LBNP过程中显著性降低。在对照试验时 ,上述指标呈现出相同的变化 ,但增高或降低的百分比 (% )低于模拟推拉效应试验 (HR除外 )。结论倒立位后 ,再直立 +LBNP作用 ,心血管功能下降程度大于单纯直立 +LBNP作用 ,下体负压旋转床     

立位-下体负压时心血管指标的变化

目的了解立位 -下体负压 (HUT +LBNP)期间心血管系统的变化 ,确定心律变异和脉图两种方法在评价心血管调节功能中的作用。方法观察 1 6名被试者在 75°头高位倾斜加 - 4kPa下体负压 2 0min期间血压、脉图、心律变异和脑血氧饱和度的变化。结果 ( 1 )HUT +LBNP可引起被试者出现明显的心率、血压、规 -化低频峰功率 (LFn)、LFn/HFn增加 ,脑血氧饱和度、心电T波和规 -化高频峰功率(HFn)下降 ;( 2 )晕厥前出现明显的血压、心率和脑血氧饱和度下降 ;( 3)低耐力组在HUT +LBNP初期的心率明显高于高耐力组 ;( 4 )HUT +LBNP时脉搏波波形发生很大变化 ,无法进行分析。结论HUT +LBNP是一种负荷量较高的立位耐力检查方法 ,可充分暴露被检者在立位中的心血管调节功能 ;脉图检测方法不能作为评价被试者HUT +LBNP耐力的方法。     

下身负压时诱发的外周血管反射

应用模拟超重和引起血液倾注到下肢的下身负压(LBNP)方法导致一些恢复和维持中心血容量的代偿性心血管反射。某些研究者观察到在LBNP 时     

航天活动各阶段动脉血压的变化

人体动脉血压（BP）是4大生命体征之一,动脉血压某种程度上可反映心血管功能状态。国外载人航天活动各阶段动脉血压总是作为一种重要的心血管参数。虽然航天中有关血压的报道不尽一致,航天后立位应激时低血压普遍存在,航天后立位耐力降低的机理实质上是航天后立位低血压的机理。本文对国外载人航天活动各阶段有关动脉血压的报道和航天后立位低血压的机理研究作一简要概述。     

20天卧床前后冷加压和下身负压时(LBNP)的心血管反应

失重后血量、每搏量、心输出量的减少及植物神经功能的失调是引起立位耐力下降的原因。冷加压实验时,不引起血量的改变,可用它来评价植物神经反应。因此本实验观察了20d 卧床前后,被试者在冷加压时的心血管反应,并与LBNP 时的反应相比较。     

艾司洛尔预防气管插管时心血管反应的临床观察

全麻诱导后喉镜窥视和气管插管可引起心率（HR）加快,血压（BP）升高等心血管反应。如何控制和降低气管插管引起的心血管反应是临床医师关注的问题之一。我们观察了诱导前预先静脉注射艾司洛尔后插管期间BP和HR的变化,现报告如下。     

下体负压在航空航天医学中的应用

下体负压（LBNP）作为心血管系统强有效的应激因素，类似于重力的刺激，一直为航空航天医学领域所关注。本文简要介绍了LBNP的实验装置、实验方法及其产生的生理学效应，重点阐述了LBNP在航空航天医学领域中应用，并提出了今后的研究方向。     

正常受试者下体负压差别效应的生理学和超声心动图评价

前言 下体负压(LBNP)是一种常用的评价立位耐力的方法。然而尚未对腿部和腹盆腔(AP)血管系统的作用进行过评价。我们观察腿和腹盆腔血管系统在分离和不分离状态下心血管系统对LBNP的反应。方法 头晚休息和标准化水合作用后,15名正常健康志愿者(8名男性)在下列三种情况中穿着医学抗休克裤(MAST)经受阶梯式的(5 min一阶,每阶增加-10mmHg)LBNP,直到-60mmHg。着医学抗休克裤的三种情况分别为:①非充气状态;②腿充气;③腿和腹盆腔充气。进行生理学多普勒超声心动图检查。结果     

Model of depressed myocardium shows orthostatic intolerance with or without reduced blood volume

INTRODUCTION: The development of orthostatic hypotension (OH) is complex and multi-factorial. Previous simulation work indicates that myocardial contractility depression (MCD) may increase OH when there is a total blood volume decrease. This paper hypothesized that MCD increased OH in both humans with and without decrease in blood volume. METHODS: A model, which was previously used to reproduce cardiovascular response to lower body negative pressure (LBNP), hypovolemia, and MCD, was modified by incorporating the physiologic mechanism of plasma filtration into the interstitium during LBNP. The model was evaluated by human experimental results. Using the model, HR and BP response to LBNP were simulated at conditions of 10%, 20%, and 30% MCD. Additionally, HR and BP response to LBNP were simulated at conditions of 10% and 20% MCD with a 12% decrease in blood volume. RESULTS: Simulation results indicate that the increments of HR and decrements of systolic BP (SBP) and mean arterial pressure (MAP) rise with the increases of MCD. Specifically, simulation results indicate that about 30% MCD would cause OH (HR: 117 bpm; SBP: 92 mmHg; MAP: 78 mmHg). It also indicates that about 20% MCD would cause OH (HR: 134 bpm, SBP: 84 mmHg, MAP: 73 mmHg) with a 12% decrease in total blood volume. CONCLUSION: It is suggested that MCD increases OH whether or not there is a total blood decrease, and further suggested that MCD induced by both spaceflight and heart disease may increase OH.     

Effect of a central redistribution of fluid volume on response to lower-body negative pressure

We studied cardiovascular responses to lower-body negative pressure (LBNP) following 1 hour (h) of 6 degrees head-down tilt to determine whether a redistribution of blood volume toward the central circulation modifies the subsequent response to orthostatic stress. Responses of 12 men, ages 30-39 years, were evaluated by electrocardiography, impedance cardiography, sphygmomanometry, and measurement of calf circumference. During the LBNP that followed head-down tilt--as compared with control LBNP (no preceding head-down tilt)--subjects had smaller stroke volume and cardiac output, greater total peripheral resistance, and less calf enlargement. These differences reflect differences in the variables immediately preceding LBNP. Magnitudes of the responses from pre-LBNP to each pressure stage of the LBNP procedure did not differ between protocols. Mean and diastolic arterial pressures were slightly elevated after LBNP-control, but they fell slightly during LBNP post-tilt. These cardiovascular responses to simulated gravitational stress following head-down tilt may reflect the manner in which adaptation to microgravity affects subsequent responses to orthostatic stress on return to Earth.     

21天头低位(-6°)卧床前后下体负压和头高位倾斜暴露时的心血管反应

目的探讨２１天头低位（ＨＤＴ）卧床对人体立位应激下心血管反应的影响,以及比较下体负压（ＬＢＮＰ）和头高位倾斜（ＨＵＴ）两种立位应激下心血管反应的差异。方法６名受试者在２１天ＨＤＴ前、后分别进行ＬＢＮＰ（－４．００ｋＰａ／３ｍｉｎ、－６．６７ｋＰａ／３ｍｉｎ及－９．３３ｋＰａ／３ｍｉｎ）和ＨＵＴ试验（３０°／３ｍｉｎ、４５°／３ｍｉｎ、６０°／３ｍｉｎ及７５°／３ｍｉｎ）,以比较两种检测方法的血压（ＢＰ）和心率（ＨＲ）的变化情况。结果与ＬＢＮＰ（或ＨＵＴ）前相比：①ＬＢＮＰ（ＨＵＴ）时ＨＲ显著增加（Ｐ＜０．０１）,ＳＢＰ显著降低（Ｐ＜０．０５）;卧床后相应的变化量增加。②ＬＢＮＰ时ＤＢＰ降低（卧床后达显著,Ｐ＜０．０５）;而ＨＵＴ时ＤＢＰ增加（卧床后达显著,Ｐ＜０．０５）。③ＬＢＮＰ时ＭＡＰ均显著降低（Ｐ＜０．０５）;而ＨＵＴ时在卧床前无变化（Ｐ＞０．０５）,在卧床后显著增加（Ｐ＜０．０５）。结论ＬＢＮＰ和ＨＵＴ引起的ＣＶＳ反应并不相同。笔者认为,ＨＵＴ更能促进心血管系统对立位应激的调节作用。     

Characteristics of the reactions to active orthostatic and water loading tests in persons with varying resistance to +Gz loads

Thirty-seven healthy male test subjects, aged 19-21, with different +Gz acceleration tolerance were examined. Their blood pressure (BP) and heart rate (HR) during 5-min tilt tests and 2% water loading tests were measured 2-3 weeks prior to centrifugation. Quantitative evaluation of orthostatic tolerance using an orthostatic index and BP and HR responses to tilt tests before and after water loading revealed specific features of cardiovascular regulation in the subjects with high and low +Gz acceleration tolerance. The negative predictive indicators include: decreased BP, HR and cardiac index in the supine position in combination with high orthostatic tolerance, as well as decreased orthostatic tolerance in combination with a lower function of vasoconstrictor mechanisms in the upright position and a lower sensitivity of carotid sinus reflexes to blood volume changes during tilt and water loading tests. When examining test subjects with high +Gz tolerance, preference should be given to those who can well tolerate tilt tests and show moderately high BP and HR in the supine position, as well as to those who exhibit a noticeable increment of diastolic BP during 5-min tilt tests.     

The effect of low-level normobaric hypoxia on orthostatic responses

BACKGROUND: Altitude-induced hypoxia is believed to decrease tolerance to orthostasis. This preliminary investigation assesses the interaction between low-level normobaric hypoxia and the cardiovascular responses to orthostasis. HYPOTHESIS: We hypothesize that low-level hypoxia at simulated altitudes up to 3350 m adversely affects cardiovascular responses to orthostasis. METHODS: Six women and eight men underwent + 75 degrees head-up tilt (HUT) at 3 simulated altitudes (115 m (377 ft), 1680 m (5500 ft) and 3350 m (11,000 ft)). Minute ventilation (VE), tidal volume (VT), breathing frequency (f), heart rate (HR), and arterial blood pressure (BP) were measured non-invasively. Ventilatory parameters (VE, VT, and f) were measured during the control period to assess the respiratory response to hypoxia. Cardiovascular variables (HR and BP) during HUT were compared and analyzed by calculating the deviation from control (pre-tilt) values. RESULTS: Respiratory data showed no significant hypoxic ventilatory response. The average HR deviation significantly decreased (p < 0.05) with HUT from 13 +/- 2 bpm (115 m) to 9 +/- 2 bpm (3350 m). The pattern of mean arterial BP deviation with HUT was similar across the three altitudes (-6.6 +/- 2.9 mm Hg at 115 m to -6.8 +/- 2.4 mm Hg at 3350 m), with no significant differences. CONCLUSIONS: Changes in HR and arterial BP are fundamental aspects of the cardiovascular response to orthostatic stress. Although there were no significant changes in the BP response to orthostasis with hypoxia, the HR changes seen in this study suggest that the ability to modulate HR under orthostatic stress is reduced with exposure to low-level normobaric hypoxia.     

坐位下体负压下人体大脑中动脉血流速度及血氧饱和度的改变

目的探讨坐位下体负压对人体大脑中动脉血流速度、心率和血氧饱和度的影响。方法 15名健康青年男性在坐位下体负压 - 4.0 0kPa、 - 6.67kPa条件下 ,分别测试负压前、下体负压暴露 0 .5、1、2、3、4、5min和卸压后 1、3、5min的大脑中动脉血流速度、心率和血氧饱和度。结果在 - 4.0kPa下体负压作用 4、5min时 ,脑血流速度减慢 (P <0 .0 5 ) ;负压作用 3、4、5min时心率增快 (P <0 .0 5 ) ;血氧饱和度无明显改变。在 - 6.67kPa下体负压作用 2、3min时 ,脑血流速度减慢 (P <0 .0 5 ) ,在 4min和 5min时显著减慢 (P <0 .0 1) ,卸压后 1min尚未恢复 (P <0 .0 5 ) ,随后恢复至对照水平 ;负压作用后各时间点心率均显著增快 (P <0 .0 1) ;血氧饱和度在负压暴露 5min时显著下降 (P <0 .0 5 )。结论坐位下体负压造成血液在下肢淤积 ,大脑中动脉血流速度减慢 ,引起大脑血供减少 ,为空中晕厥和立位耐力不良的医学鉴定提供了人体实验依据     

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.     

Leg vascular responsiveness during acute orthostasis following simulated weightlessness

Ten men (35-49 years old) underwent lower body negative pressure (LBNP) exposures before and after 10 d of continuous 6 degrees head-down bedrest in order to predict the effect of weightlessness on the responsiveness of leg vasculature to an orthostatic stress. Heart rate (HR), mean arterial blood pressure (MAP), and impedance rheographic indices of arterial pulse volume (APV) of the legs were measured during rest and at 1 min of -30 mm Hg LBNP. Bedrest-induced deconditioning was manifested by decreases (p less than 0.05) in plasma volume (17%), peak oxygen uptake (16%), and LBNP tolerance (17%). Resting HR was unchanged after bedrest, but HR was higher (p less than 0.05) at 1 min of -30 mm Hg LBNP after, compared with before, bedrest. Responses of MAP to -30 mm Hg LBNP were not altered by bedrest. Resting APV was decreased (p less than 0.05) by simulated weightlessness. However, APV was reduced (p less than 0.05) from rest to 1 min -30 mm Hg LBNP by the same relative magnitude before and after bedrest (-21.4 +/- 3.4% and -20.5 +/- 2.7%, respectively). We conclude that peripheral arterial vasoconstriction, as indicated by reductions in APV during LBNP, was not affected by bedrest. These results suggest that there was no apparent alteration in responsiveness of the leg vasculature following simulated weightlessness. Therefore, it appears unlikely that control mechanisms of peripheral resistance contribute significantly to reduced orthostatic tolerance following spaceflight.     

模拟失重对心血管功能的影响及下体负压的对抗作用

总结了近年来本实验室有关模拟失重对心血管功能影响及下体负压对抗作用的研究，讨论了模拟失重致立位耐力不良的机理可能与心血管功能降低，脑血流降低及内分泌改变等有关，以及采用数学模型方法探讨失重致立位耐力降低机制的作用意义，重点论述了下体负压对抗方案问题。