Lower body negative pressure: the second manned Skylab mission.

Results of orthostatic evaluations of the crew of Skylab 3 with lower body negative pressure (LNBP) stress tests during their 59-d mission are reported. The test protocol was identical to that used in the first manned Skylab mission and the latter Apollo flights. Except for an inflight increase (rather than a decrease) in resting heart rates, results were essentially parallel to those observed in crewmen of the shorter Skylab 2 mission. Exaggerated elevations in heart rate and decreases in pulse pressure during LBNP stress inflight and immediately postflight corresponded to lowered orthostatic tolerance. Large decrements in resting calf size inflight and in total leg volume postflight indicated significant headward fluid shifts as had already been seen in the Skylab 2 crewmen. In addition, decreases in calf circumference gave no certain indication of a plateau over the 59 d inflight. On the other hand, percentage volume increase in calf size during LBNP stress inflight was greater than those in either preflight or postflight tests. Hypotheses elaborated after the Skylab 2 mission seem to have been substantiated, but several enigmas await data from the last and longer mission for clarification.     

提踵动作对下体负压耐力的影响

目的验证提踵动作对提高下体负压耐力的有效性,并探讨其可能的作用机制。方法健康男性志愿者10名,在下体负压(LBNP)条件下分别随机进行2次头高位倾斜检查(HUT)测定其下体负压耐力。一次为志愿者的基础下体负压耐力(对照组),另外一次要求志愿者在LBNP条件下做提踵动作,直至耐力终点(提踵组)。结果提踵组下体负压耐受时间(DNP)和累计应激指数(CSI)均显著增加(P<0.05)。在下体负压过程中,提踵组平均动脉压呈升高趋势,心率增加呈降低趋势,每搏量增加,且均在-40 mmHg及-50 mmHg时达到显著水平(P<0.05)。结论提踵动作可以提高下体负压耐力,可望作为航天员在长期太空飞行后引起立位耐力不良时的应对措施,也可作为血管迷走性晕厥患者的预防措施和治疗手段。     

Relationship between the hemodynamic disorders in cosmonauts on short-term space flights and orthostatic stability]

Comparative investigation of the hemodynamic responses of cosmonauts at rest, during occlusive leg plethysmography, and LBNP in relatively short (less than a month) space flights were performed to tie up these measurements with post-landing orthostatic stability (OS). The decreased LBNP tolerance in flight that correlated later on with impaired reactions to both active and passive orthostatic testing are attributed, among other reasons, to deficient vasoconstriction of the main arteries and over-extendibility of veins in the lower extremities. Results of ultrasonic investigations of hemodynamics during LBNP tests before and after space flight and data of leg venous extension in SF can and should be used in predicting cosmonaut's orthostatic status post flight.     

预测下体负压耐力方法初探

为探讨预测下体负压耐力的方法，对１１名被试者进行了－４．００ｋＰａ，－５．３３ｋＰａ，－６．６７ｋＰａ各５ｍｉｎ的阶梯式坐位下体负压耐力检查，以晕厥前症状或持续１５ｍｉｎ为终止指征。检查结果将被试者分为耐力良好组（Ａ组，ｎ＝５）和耐力不良组（Ｂ组，ｎ＝６）；组间比较时发现负压作用第１１ｍｉｎ时的ＭＡＰ、△ＳｒＯ２和△ＣＯ（％）判别显著（Ｐ〈０．０５）；用这一时刻的ＭＡＰ和△ＳｒＯ２建立的回归方程对     

Aerobic fitness does not contribute to prediction of orthostatic intolerance

Several investigations have suggested that orthostatic tolerance may be inversely related to aerobic fitness (VO2max). To test this hypothesis, 18 males (age 29 to 51 yr) underwent both treadmill VO2max determination and graded lower body negative pressures (LBNP) exposure to tolerance. VO2max was measured during the last minute of a Bruce treadmill protocol. LBNP was terminated based on pre-syncopal symptoms, and LBNP tolerance (peak LBNP) was expressed as the cumulative product of LBNP and time (torr-min). Changes in heart rate, stroke volume, cardiac output, blood pressure, and impedance rheographic indices of mid-thigh-leg fluid accumulation were measured at rest and during the final minute of LBNP. For all 18 subjects, mean (+/- SE) fluid accumulation index and leg venous compliance index at peak LBNP were 139 +/- 22 ml and 3.9 +/- 0.4 ml . 100 ml . torr-min-2 x 10(3), respectively. Pearson product-moment correlations and step-wise linear regression were used to investigate relationships with peak LBNP. Variables associated with endurance training, such as VO2max and percent body fat, were not found to correlate significantly (P less than 0.05) with peak LBNP and did not add sufficiently to the prediction of peak LBNP to be included in the step-wise regression model. The step-wise regression model included only fluid accumulation index, leg venous compliance index, and blood volume, and resulted in a squared multiple correlation coefficient of 0.978. These data do not support the hypothesis that orthostatic tolerance as measured by LBNP is lower in individuals with high aerobic fitness.     

21d头低位卧床期间第一周和最后一周下体负压锻炼对立位耐力和心功能的影响

目的观察在 2 1d头低位卧床的第一周和最后一周进行下体负压锻炼对立位耐力和心功能的影响。方法 1 2名健康男性志愿者 ,随机分为对照组和LBNP组 ,每组 6人 ,均参加 - 6°头低位倾斜卧床实验。对照组在卧床期间不做任何处理 ,LBNP组在卧床第一周和最后一周 ,每天进行 1h、- 30mmHg的下体负压锻炼。卧床前和卧床第 1 0、2 1天在倾斜床上进行 3次立位耐力检查 ,卧床期间测量心脏泵血和收缩功能等指标。结果卧床前两组 1 2名被试者均顺利通过立位耐力检查。卧床第 1 0和 2 1天 ,对照组所有被试者均不能通过立位耐力检查 ,而下体负压组仅有 1人未通过 ,卧床第 2 1天下体负压组( 1 9.7± 0 .9min)平均耐受时间较对照组 ( 1 5 .0± 3.2min)显著延长 (P <0 .0 5 )。对照组每搏输出量(SV)及心输出量 (CO)在卧床第 3、1 0天较卧床前显著降低 (P <0 .0 5 ) ,而LBNP组SV及CO在卧床期间均无显著性变化 ;两组的射血前期 (PEP) /左室射血时间 (LVET)在卧床第 3、1 4天均显著升高 (P <0 .0 5 )。两组间比较 ,LBNP组PEP/LVET在卧床第 3天较对照组显著降低 (P <0 .0 5 ) ,LVET在卧床第 3、7和 1 4天较对照组显著升高 (P <0 .0 5 )。结论 2 1d头低位卧床可引起立位耐力、心脏泵血和收缩功能显著降低 ;在 2 1d头低位卧床的     

Physical fitness and hemodynamic response of women to lower body negative pressure

Aerobic fitness as assessed by maximal aerobic capacity (VO2max) has been shown to be associated with an attenuated baroreflex function during lower body negative pressure (LBNP) in men. Sixteen women (mean age = 24.7 yrs) were evaluated during progressive LBNP to -50 torr. Each subject's VO2max was determined using indirect calorimetry during the Bruce protocol exercise test. Eight subjects [mean VO2max = 56.8 (ml O2 X min-1) X kg-1] were designated as trained, and eight subjects [mean VO2max = 39.4 (ml O2 X min-1) X kg-1] were designated as untrained. During LBNP, heart rate, blood pressure, cardiac index, forearm blood flow, and leg circumference were measured. All subjects completed the LBNP protocol without clinical symptoms of pre-syncope. The over-all hemodynamic responses of both groups to LBNP were qualitatively similar to previous findings reported for males. However, no significant differences in response of hemodynamic variables were observed between trained and untrained subjects during LBNP to -50 torr (P greater than 0.05) except for vascular resistance and diastolic blood pressure at -50 torr where the untrained value was greater than the trained value. This would suggest that a fitness-related difference may have been present at higher levels of LBNP. Furthermore, in contrast to previous reports in males, the index of baroreflex responsiveness (delta heart rate/delta systolic blood pressure) was similar for both groups (P greater than 0.05). When these data were compared with a similar subject pool of males, the females displayed a significantly greater (P less than 0.05) tolerance of LBNP to -50 torr regardless of fitness.(ABSTRACT TRUNCATED AT 250 WORDS)     

Study of the cardiovascular system in long-term space flights

During the Salyut-6 space flights cardiac bioelectrical and mechanical activity, blood content and tone of cerebral and limb vessels, systemic arterial and venous pressure were measured in 10 primary crewmembers. The measurements were taken at rest and during LBNP and ergometry tests. The study has shown that the circulation changes are adaptive reactions of the body to a new environment, particularly weightlessness. Cardiovascular responses to the provocative tests were changes and more marked, probably, due to blood redistribution.     

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.     

Effect of lower body negative pressure against orthostatic intolerance induced by 21 days head-down tilt bed rest

BACKGROUND: Exposure to actual or simulated weightlessness is known to induce orthostatic intolerance in humans. Many different methods have been suggested to counteract orthostatic hypotension. The repetitive or prolonged application of lower body negative pressure (LBNP) has shown beneficial effects to counter orthostatic intolerance, but devoting so much time to countermeasures is not compatible with space mission objectives or costs. The purpose of the present study was to assess the effects of brief LBNP sessions against orthostatic intolerance during a 21-d head-down tilt (HDT) bed rest. METHODS: There were 12 healthy male volunteers who were exposed to -6 degrees HDT bed rest for 21 d. Six subjects received -30 mm Hg LBNP sessions for 1 h x d(-1) from day 15 to day 21 of the HDT, and six others served as control. Orthostatic tolerance was assessed by means of standard tilt test. RESULTS: Before HDT, all the subjects in the two groups completed the tilt tests. After 21 d of HDT, five subjects of the control group and one subject of the LBNP group could not complete the tilt test due to presyncopal or syncopal symptoms. The mean upright time in the control group 13.0 +/- 4.0 min) was significantly shorter (p < 0.05) than that in the LBNP group (19.0 +/- 2.2 min). Body weight decreased significantly in the control group during HDT, while increasing significantly on day 21 of HDT in the LBNP group. Urine volume increased on days 15-21 of HDT in the control group, but remained unchanged throughout HDT in the LBNP group. A significant decrease in cardiac output and cardiac index, and a significant increase in total peripheral resistance, pre-ejection period, plasma renin activity, aldosterone, and prostaglandin 12 were observed during HDT in both groups. There were no significant differences in these parameters between the two groups. CONCLUSIONS: Brief daily LBNP sessions were effective in preventing orthostatic intolerance induced by 21 d HDT bed rest. However, it did not improve cardiac pump and systolic functions and did not preserve volume regulating hormones.     

Comparison of 70 degrees tilt, LBNP, and passive standing as measrues of orthostatic tolerance.

The present study was performed to assess the reliability of lower body negative pressure (LBNP) as a test of orthostatic tolerance. The need for this assessment arose from the prior observation in this laboratory that some subjects show wide day-to-day variation in heart rate responses to LBNP. The extent of these variations was so great as to a raise a serious question as to the value of LBNP as a measure of study-induced alterations (e.g. those produced by bedrest or weightlessness) in orthostatic tolerance. Five healthy volunteers were subjected to a series of tests, consisting of 70 degrees tilt, LBNP, and passive standing. on three occasions preceding and three occasions following a 2 week period of bedrest. Study results show that it is possible to subdivide the volunteers into subgroups which show either great or little day-to-day variability in any of the three tests. All three tests reaveled bedrest-induced alterations in orthostatic tolerande quite adequately. Of the three tests studied, LBNP most frequently resulted in the largest test-induced heart rate alterations, followed by quiet standing and, finally 70 degrees tilt     

下体负压反复暴露对下体负压耐力的影响

目的探讨反复下体负压暴露对人体下体负压（ＬＢＮＰ） 耐力的影响及其与心血管功能的关系。方法１０ 名男性健康被试者进行负压值为－ ５０ ｍ ｍ Ｈｇ 、持续８ ｍｉｎ 的下体负压锻炼,每天１ 次、连续８ ｄ ,记录心率及血压。锻炼前后进行ＬＢＮＰ 耐力检查,并测量心脏泵血功能。结果锻炼前、后负压最大耐受时间（ＤＮＰ） 及累积应激指数（ＣＳＩ） 分别为：１０５８ ±２１６ ｓ,１３５４ ±２３３ ｓ;４５８３３ ±１２５４６ ｓ·ｍ ｍ Ｈｇ 、６３４３３±１３４５８ ｓ·ｍ ｍ Ｈｇ ,锻炼前后差异显著（ Ｐ＜ ０ ．０５） ,心脏泵血功能呈增强趋势。结论下体负压反复暴露可提高人的下体负压耐力,其原因之一可能在于增强了心肌的收缩能力,降低外周血管的顺应性,心血管系统对低血压应激的代偿机制更加有效,从而提高了心水平动脉血压。     

Lower-body negative-pressure exercise and bed-rest-mediated orthostatic intolerance

PURPOSE: Supine, moderate exercise is ineffective in maintaining orthostatic tolerance after bed rest (BR). Our purpose was to test the hypothesis that adding an orthostatic stress during exercise would maintain orthostatic function after BR. METHODS: Seven healthy men completed duplicate 15-d 6 degrees head-down tilt BR using a crossover design. During one BR, subjects did not exercise (CON). During another BR, subjects exercised for 40 min.d(-1) on a supine treadmill against 50-60 mm Hg LBNP (EX). Exercise training consisted of an interval exercise protocol of 2- to 3-min intervals alternating between 41 and 65% (.)VO(2max). Before and after BR, an LBNP tolerance test was performed in which the LBNP chamber was decompressed in 10-mm Hg stages every 3 min until presyncope. RESULTS: LBNP tolerance, as assessed by the cumulative stress index (CSI) decreased after BR in both the CON (830 +/- 144, pre-BR vs 524 +/- 56 mm Hg.min, post-BR) and the EX (949 +/- 118 pre-BR vs 560 +/- 44 mm Hg.min, post-BR) conditions. However, subtolerance (0 to -50 mm Hg LBNP) heart rates were lower and systolic blood pressures were better maintained after BR in the EX condition compared with CON. CONCLUSION: Moderate exercise performed against LBNP simulating an upright 1-g environment failed to protect orthostatic tolerance after 15 d of BR.     

Gender differences in tolerance to lower body negative pressure

BACKGROUND: Previous studies suggest that women have lower tolerance than men do to lower body negative pressure (LBNP). The mechanism(s) responsible has not been determined. HYPOTHESES: Women would be less tolerant to presyncopal LBNP than men as determined by several indices of LBNP tolerance. Additionally, men and women, regardless of LBNP tolerance, would have similar cardiovascular responses to LBNP as presyncope was reached. METHODS: The subjects were 18 men and 18 women (average age 25) of similar fitness levels who volunteered for the study. A step-wise LBNP protocol to presyncope was employed. HR, stroke volume (SV), cardiac output (Q), BP, and systemic vascular resistance (SVR) were measured before and throughout the LBNP stress. Data from women were compared with those from all men, and to men with similar and higher LBNP tolerance. RESULTS: Women had significantly less LBNP tolerance than men regardless of index used: 30% less by duration of LBNP, 21% less by maximal LBNP tolerated, 44% less by a cumulative stress index, and 27% less by a linear tolerance index. Cardiovascular responses to LBNP were similar for women and men as presyncope was approached, whether the men were low-tolerant (LT) or high-tolerant (HT). In the 2 min pre-presyncope, HR increased by 80 +/- 6% in women, 72 +/- 7% in LT men and 96 +/- 14% in HT men; Q decreased by 47 +/- 3% in women, 52 +/- 6% in LT men and 55 +/- 2% in HT men. Similar comparisons occurred for the decline in BP and the rise in SVR. CONCLUSION: Women have lower LBNP tolerance than do men, although there is considerable gender overlap in tolerances. The cardiovascular response to LBNP is similar regardless of gender or tolerance level as presyncope is approached. Understanding the gender differences in LBNP tolerance may lie in determining how the LBNP stress is translated into a "trigger" for cardiovascular decompensation.     

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。     

Correlation of the cosmonauts' physiological reactions to +Gx loads during deorbit with the hemodynamic shifts in the period of short-term microgravity]

Physiological reactions of four cosmonauts to +Gx during the descent after short-term (less than 30 d) space flights were correlated with the hemodynamic shifts developed previously during the LBNP test and occlusive plethysmography in microgravity. The cosmonauts who had exhibited a sharp decrease in cerebral circulation during the in-flight LBNP tests showed signs of deteriorated cerebral blood flow (vertigo, weakness, hyperhydration) at the attempt to assume the vertical posture and to climb out of the landing module. Also, the cosmonauts found to have the arterial and venous tone significantly dropped in microgravity, suffered petechial hemorrhages in the back integument during the exposure to +Gx. These data suggest a correlation between the physiological reactions of cosmonauts to +Gx at the end of short-term space flight and the shifts in hemodynamics earlier in microgravity.     

Effect of aerobic training on orthostatic tolerance, circulatory response, and heart rate dynamics.

BACKGROUND: This study was performed to investigate the effects of aerobic training on orthostatic tolerance and to quantify the post-training changes in cardiovascular response and heart rate variability (HRV). METHODS: Tolerance and circulatory responses to two types of lower body negative pressure (LBNP) were examined and compared in a group of healthy male students before and after 6 mo of aerobic training, and the results were further compared with a group of athletes (runners). Changes in HRV associated with training were analyzed by conventional and time-varying autoregressive spectral analysis, as well as by approximate entropy measurement (ApEn)--a statistic quantifying heart rate "complexity" derived from non-linear dynamics. RESULTS: After aerobic training, there was an initial transient hypotension during the supine -50 mmHg LBNP testing and a significant decrease in tolerance to upright graded LBNP in most of the student-subjects. Moreover, after training, there was a significant decrease in ApEn value of the HRV time series during both supine control and LBNP testing, and the rate of cardiac vagal withdrawal and sympathetic activation during the onset of LBNP was faster than that before training. CONCLUSIONS: The present study has provided further evidence that certain types of aerobic training may affect orthostatic tolerance and may be associated with a loss of complexity of HRV during supine resting and orthostatic stress.     

下体负压对抗21d头低位卧床后立位耐力不良的研究

目的观察ＬＢＮＰ对２１ｄＨＤＴ－６°卧床模拟失重所致立位耐力不良的对抗效果。方法１２名健康男性青年志愿者,进行２１ｄＨＤＴ－６°卧床实验。随机分为对照组和下体负压组,每组６人。与对照组不同,下体负压组在最后一周,每天进行１ｈ、－４．０ｋＰａ的下体负压锻炼。结果卧床前,１２名受试者顺利通过７５°、２０ｍｉｎ立位耐力检查。卧床第１０ｄ立位耐力检查时,对照组有５人、ＬＢＮＰ组有４人出现晕厥前或晕厥症状,两组平均耐受时间均低于卧床前（Ｐ＜０．０５）;第２１ｄ时,对照组有５人未通过,平均耐受时间较卧床前显著降低（Ｐ＜０．０５）;而ＬＢＮＰ组有１人未通过,平均耐受时间显著高于对照组（Ｐ＜０．０５）。结论２１ｄ头低位卧床后立位耐力显著降低。下体负压能够有效对抗头低位卧床导致的立位耐力降低。     

Central blood circulation in the healthy human being during 7 days' head-down tilt hypokinesia

The effect of 7-day head-down tilt (-15 degrees) and lower body negative pressure on circulation and oxidative metabolism was investigated on 13 healthy male test subjects. For 7-10 days they had Swan-Ganz catheters implanted in the pulmonary artery and a special cannula in the radial artery. The most marked changes were seen in the pulmonary artery pressure (PAP) and central venous pressure (CVP) that varied in a phase-like manner. By the 7th hour of bed rest the PAP increased significantly; this was followed by increases in the total lung resistance and the right ventricle function, as well as by a slight decrease of renin and aldosterone. Beginning with bed rest days 2 or 3 the PAP and CVP declined and remained lowered, as compared to the pretest level, till the end of bed rest. The responses to LBNP tests changed by bed rest day 2. Possible mechanisms of the above changes at rest and during LBNP tests are discussed.     

Cardiovascular dynamics associated with tolerance to lower body negative pressure

The purpose of this investigation was to identify cardiovascular responses associated with tolerance to lower body negative pressure (LBNP). In this study, 18 men, ages 29-51 years, were categorized as high (HT) or low (LT) LBNP tolerant based on a graded presyncopal-limited LBNP exposure criterion of -60 mm Hg relative to ambient pressure. Groups were matched for physical characteristics and preLBNP cardiovascular measurements, with the exceptions of greater (p less than 0.05) end-diastolic volume and cardiac output in the HT group. During peak LBNP, cardiac output was similar (NS) in both groups, although the HT group displayed a greater heart rate (p less than 0.05). In both groups, venous return appeared to limit cardiac output resulting in decreased arterial pressure. Tolerance to LBNP did not appear solely dependent on the absolute amount of blood pooled in the legs since the HT group demonstrated a greater (p less than 0.05) peak LBNP-induced increase in midthigh-leg volume. Greater tolerance to LBNP was associated with a larger preLBNP cardiac output reserve and higher compensatory increases in heart rate and peripheral resistance.