立位耐力监测系统重复性评价

目的 评价自行设计的立位耐力监测系统的稳定性和重复性。方法 应用自行设计的立位耐力监测系统对28例健康歼击机飞行员进行立位耐力监测,间隔一周时间分别在相同条件下做相同的试验。结果 在两次立位耐力试验中平卧位、立位即刻、1 min、2 min、3 min、4 min、5 min、10 min、15 min和20 min各时间心率、收缩压、舒张压和平均动脉压比较差异均无显著性意义(P>0.05)。结论 自行设计的立位耐力监测系统稳定、可靠。     

中国歼击机飞行员立位耐力试验正常参考值

目的为了评价中国歼击机飞行员立位耐力水平及建立中国歼击机飞行员立位耐力试验正常值.方法应用自行研制的立位耐力监测系统,对177例中国歼击机飞行员进行立位耐力试验(OTT).该系统是由计算机控制倾斜床和自动记录多项生理指标系统组成.结果从立位即刻到持续20min,受试者最高心率平均为(84.8±11.9)次/min,最低平均收缩压(SP)、舒张压(DP)和平均动脉压(MAP)分别是(119.0±11.3)mmHg、(74.4±10.3)mmHg和(89.7±10.1)mmHg.结论按95%可信度,建议中国歼击机飞行员立位耐力试验正常参考值可以用104次/min作为OTT心率上限,而100mmHg、57mmHg和73mmHg分别为SP、DP和MAP的下限,越限者应视为OTT阳性.     

立位耐力试验对空中晕厥的诊断价值

目的评价立位耐力试验对空中晕厥诊断的特异性、敏感性和诊断价值。方法应用自行研制的立位耐力监测系统，对31例空中晕厥病例和61例健康飞行员进行立位耐力试验。结果立位耐力试验阳性晕厥组为12例（38.71%），对照组为5例(8.20%)。立位耐力试验敏感性38.71%,特异性91.80%,诊断价值73.91%。结论立位耐力试验对空中晕厥的敏感性低，特异性高，可作为一种诊断空中晕厥的客观方法。     

立位耐力不良飞行员模拟飞行中心理生理反应与心率变？…

目的 探讨立位耐力不良飞行员模拟空中环境下心脏自主神经功能调节状态和心理生理反应特点，为空中晕厥的诊断提供客观的评价方法。方法 对２２名立位耐力不良（不良组）飞行员进行了连续动态心电记录和模拟飞行条件下的心理生理参数测定与心率变异性（ＨＲＶ）分析，并与１５名立位耐力正常的健康飞行员（健康组）做了对比。结果 不良组飞行员在模拟仪表飞行中心理生理储备能力明显降低，表现为完成两项任务的质量和处理信息速度     

立位耐力监测仪的可行性研究

立位耐力监测仪的可行性研究陈同欣，纪桂英，王铃航空病中心主题词被动立位耐力监测仪，立位耐力试验目前，立位耐力试验已成为对不明原因晕厥、植物神经功能失调患者的诊断和疗效评价的客观辅助方法［１～５］，尤其在航空航天医学领域更是应用频繁［６～９］...     

立位耐力不良飞行员的血液动力学指标与心率变异性的改变

目的 探讨易发血管神经性晕厥飞行员的评价方法方法对３０例有晕厥史的立位耐力不良飞行员（阳性组）２０例有晕厥史的立位耐力正常飞行员（阴性组）和１５例健康飞行员（对照组），通过立位耐力结合心率变异性（ＨＲＶ）分析进行血压、心率与时域、频域指标的比较，按照国际常用标准进行判断分析。结果 阳性组立位耐力试验前平均心率（ＨＲ）、收缩压（ＳＢＰ）１、舒张压（ＤＢＰ）与阴性组、对照组比较，无显著性差异。试验后，     

立位耐力不良飞行员模拟飞行中心理生理反应与心率变异性分析

目的探讨立位耐力不良飞行员模拟空中环境下心脏自主神经功能调节状态和心理生理反应特点，为空中晕厥的诊断提供客观的评价方法。方法对２２名立位耐力不良（不良组）飞行员进行了连续动态心电记录和模拟飞行条件下的心理生理参数测定与心率变异性（ＨＲＶ）分析，并与１５名立位耐力正常的健康飞行员（健康组）做了对比。结果不良组飞行员在模拟仪表飞行中心理生理储备能力明显降低，表现为完成两项任务的质量和处理信息速度的能力较健康组低（Ｐ＜０．０１）；心血管自主神经功能严重失调；心理生理储备值与ＨＲＶ多数指标之间存在明显的相关性。结论结合模拟仪表飞行条件下的心理生理负荷评定和ＨＲＶ分析能较客观地反映立位耐力不良飞行员心血管自主神经功能障碍的发生机制和调节规律，为空中晕厥的诊断提供量化的指标。     

飞行人员立位耐力不良与颈椎小关节不稳

为探讨飞行人员立位耐力不良的病因，我们对立位耐力正常及不良者进行了Ⅹ线颈椎片、血压及心率对比观察。     

加速度晕厥飞行人员立位耐力试验的医学鉴定及护理作用

对67例加速度晕厥飞行人员进行常规立位耐力试验,说明该试验可以客观地评价飞行人员在低G值下的耐力情况,同时阐明了进行该试验的护理作用,提出在进行该试验时对护士的几点要求。     

重力再适应相关立位耐力评估与预测

目的研发适宜于返回后立位耐力早期评估的安全检测方法,探索构建重力再适应相关立位耐力预测模型。方法 15名健康男性进行被动立位耐力测试,按原有评分标准,对被动立位测试数据中的3min、5 min、10 min数据分别计分;采用ROC法重新界定阈值进行分级,留一交叉验证,与原分级结果比对,验证一致性。利用15名健康男性头低位卧床模拟失重数据,基于贝叶斯原理,构建立位耐力预测模型,留一交叉验证后,选取4例飞行数据进一步验证。结果 3 min测试的ROC曲线下面积为0.944,5min和10 min的均为1.00,与原立位耐力测试分级结果一致性好。基于头低位模拟失重数据确立了5个立位耐力预测特征量及其似然比,留一交叉验证结果显示,ROC曲线下面积为0.75;4名参加飞行航天员验证结果显示,ROC曲线下面积为1,重力再适应立位耐力得到有效预测。结论采用新的方法对被动立位耐力3 min、5 min、10 min测试数据进行评估与20 min分级结果一致性好,具有应用前景。建立的重力再适应相关立位耐力预测模型具有较好预测能力。     

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.     

Inspiratory CO2 increases orthostatic tolerance during repeated tilt.

INTRODUCTION: The partial pressure of end tidal CO2 (PetCO2) is known to decrease with head-up tilt. Decreases in arterial CO2 reduce cerebral blood flow (CBF) and may increase the incidence of presyncope. We measured cerebral and central cardiovascular responses to repeated tilt where: 1) PetCO2 was allowed to change with tilt (eucapnic): and 2) PetCO2 was clamped at supine levels (isocapnic). METHODS: In eight healthy subjects breath-by-breath measurements were made of ventilation (VE) and PetCO2 along with beat-by-beat measurements of blood pressure (BP), heart rate (HR) and middle cerebral artery mean flow velocities (MFV). Following 30-min in the supine position, a series of six 10-min 90 degrees head-up tilts were performed, with 30-s of supine between each. Presyncopal subjects were returned immediately to the supine position. RESULTS: Statistical comparisons were made between the supine, and the first and last minute of the first tilt. BP, HR responses were not different between the eu- and isocapnic conditions; however, by the end of the first tilt VE was significantly higher than supine. MFV and BP at brain level decreased and HR increased from supine to tilt. MFV was higher in the isocapnic compared with the eucapnic condition but decreased from the beginning to the end of the first tilt in both conditions (i.e., tilt #1: eucap. 49.4 to 46.7; isocap. 65.0 to 59.6 cm s(-1); p < 0.05) while the BP remained constant. Five subjects were presyncopal in the study. With isocapnic tilt, presyncopal time was not reduced but was extended in four of the five subjects (2.2, 5.5, 6.3 and 31 min) yet at presyncope the values for MFV, BP and HR were the same in both conditions. CONCLUSIONS: Inspiratory CO2 contributed to increased MFV at the beginning of tilt and increased orthostatic tolerance.     

Baroreflex control of heart rate during heating in subjects with low orthostatic tolerance

BACKGROUND: Heat stress induces a reduction of orthostatic tolerance. The cardiovascular responses, including the cardiac baroreflex response to heat stress, were examined to test the hypothesis that subjects with orthostatically low tolerance demonstrate an impaired baroreflex control of heart rate (HR) during heat stress. METHODS: There were 44 healthy young volunteers who underwent whole body heat stress produced by a hot-water-perfused suit during supine rest for 45 min and 75 degrees head-up tilt (HUT) for 6 min. Esophageal temperature, HR, arterial pressure, and skin blood flow in the forearm and palm were measured continuously throughout the experiment. The sensitivity of the arterial baroreflex control of HR was calculated from the spontaneous changes in beat-to-beat arterial pressure and HR. RESULTS: The HUT was uneventful for 22 volunteers (higher tolerance group), but 22 volunteers (lower tolerance group) reached presyncope after 195 +/- 19 s. Esophageal temperature, HR, arterial pressure, and skin blood flow changed similarly in the two groups during heating. In the preheating condition, the sensitivity of the baroreflex control of HR did not differ significantly between the two groups. Heating did not alter the sensitivity of baroreflex control of HR in the higher tolerance group, but decreased it significantly in the lower tolerance group. Heating increased the number of heartbeats used for analysis of the baroreflex sensitivity in the higher tolerance group, but did not change it in the lower tolerance group. CONCLUSIONS: These results suggest that the impairment of vagal baroreflex control of HR during heat exposure aggravates the orthostatic intolerance in heat-stressed humans.     

Centrifuge training increases presyncopal orthostatic tolerance in ambulatory men

INTRODUCTION: Exposure to spaceflight or simulations of microgravity reduce human postflight orthostatic tolerance. Exercise training and volume loading can reduce associated losses of plasma volume and muscle strength, but are not successful in maintaining postflight orthostatic tolerance. A preliminary study (16) indicated that short bouts of artificial gravity (AG) training on a centrifuge could increase orthostatic tolerance in healthy, ambulatory volunteers. We tested the same AG protocol for its tolerance effect on 14 men who underwent a 3-wk exposure to Gz acceleration training on NASA-Ames' (Moffet Field, CA) human-powered centrifuge. METHODS: Subjects trained supine (head near the center of rotation) and in pairs (one subject rode passively while the other provided power to operate the 1.9-m centrifuge). The acceleration profile consisted of 7 min at 1 Gz before alternating between 1 and 2.5 Gz at 2-min intervals for 28 min. Each subject's presyncopal orthostatic tolerance limit (to a combination of 70 degrees head-up tilt and increasing lower body negative pressure) was determined before and after training. RESULTS: There were no significant differences between training groups, but presyncopal orthostatic tolerance time was improved 17 +/- 10% (p < 0.05) for the combined groups. Mechanisms associated with increased tolerance included: increased cardiac output (p < 0.04), stroke volume (p < 0.01) and low-frequency spectral power of arterial pressure (p < 0.006), and decreased arterial pressure (p < 0.05) and vascular resistance (p < 0.04). Artificial gravity training in this group of men appears to increase orthostatic tolerance through a combination of decreased vascular resistance and enhanced cardiac function.     

Hemodynamic reactions to modified orthostatic test in persons with different +Gz tolerance

Hemodynamic responses to the passive and modified orthostatic tests were investigated in 27 low and 13 high +Gz susceptibility individuals. The modified orthostatic test included 5 min head-down tilt (-30 degrees) and head-up tilt (78 degrees) with occlusion cuffs (50 mm Hg and 100 mm Hg) applied to the upper third of the hip. Analysis of typological reactions in low and high +Gz susceptibility individuals revealed no significant differences in their tolerance to the above tests. It was found that during the 1st minute of the modified orthostatic test with 100 mm Hg pressure in the occlusion cuffs heart rate was lower and systolic blood volume was higher (p less than 0.05) than during the passive test. Differences in these parameters disappeared by the 10th minute of exposure. During the modified orthostatic test with 50 mm Hg pressure in the occlusion cuffs these parameters did not show statistically significant differences.     

Blood pressure variability of the individual in orthostatic testing.

Within-subject blood pressure (BP) variability was studied in the context of orthostatic testing. Nineteen healthy men volunteered for biweekly orthostatic testing. BP's were taken on alternate minutes during 5 min of supine rest and a subsequent 5 min of quiet standing. Within-subject variance was computed for systole and diastole by protocol condition; respective variances were than pooled (i.e. averaged) across subjects. The within-subject estimate of variance for a single BP reading in either position was approximately 6 torr (i.e. 1 S.D.). This variance estimate was reduced by averaging additional BP readings from the same visit, and more so by averaging BP readings from multiple visits. These findings have use in the design of orthostatic protocols and in the interpretation of BP data derived therefrom.     

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.     

Maximal exercise as a countermeasure to orthostatic intolerance after spaceflight

Previous investigators have suggested that maximal exercise performed 24 h before the end of bed rest, a spaceflight analog, restores prebed rest plasma volume, baroreflex responses, and orthostatic tolerance. PURPOSE: In this case report, we examined the effect of a similar exercise protocol 24 h before a Shuttle landing on the orthostatic responses of four crewmembers (EX) after spaceflights of 8-14 d. Four additional crewmembers (CON) served as controls and did not perform exercise during the final day of the flight. METHODS: Each crewmember performed a 10-min stand test approximately 10 d before launch (L-10) and within 1-2 h of landing (R+0). Cardiac stroke volume was measured (Doppler ultrasound) supine and during each min of standing for three EX and three CON subjects. RESULTS: Preflight, all crewmembers completed the stand test and each group had similar heart rate and blood pressure responses. Postflight, all subjects also completed the 10-min stand test. Each group had similarly elevated supine and standing heart rates, elevated diastolic and mean arterial blood pressures, and reduced pulse pressures compared to L-10. However, postflight cardiac output, mean +/- SEM, (EX: 4.5+/-0.6 L x min(-1); CON: 3.1+/-0.3 L x min(-1)) and stroke volume (EX: 43+/-7 mL x beat; CON: 30+/-6 mL x beat) were higher after 10 min standing in the EX subjects compared to CON subjects. CONCLUSIONS: For these four crewmembers, maximal exercise performed 24 h before landing may have helped maintain stroke volume but did not maintain heart rate and blood pressure responses during standing compared to preflight.     

前庭刺激对心血管自主神经调节及立位耐力的影响

目的观察前庭刺激前后被动直立位耐力试验心率、血压和压力感受器敏感性等指标的变化,了解前庭刺激对心血管自主神经调节功能及直立位耐力的影响。方法对15名男性健康志愿者在转椅刺激前、后进行被动直立位耐力检查。被动直立位耐力实验全程监测心电图和左手中指逐跳血压,在试验的第20,21,23,25,30,35,40,41,45 min测量常规血压、心率和压力感受器反射敏感性指标(BRS)。结果前庭刺激前直立位耐力检查结果均正常,刺激后有两人发生晕厥前症状;常规血压(SBP、DBP)检查在大部分实验时间无统计学差异,心率在检测的大部分时间点有统计学差异;BRS指标仅在直立位25～min段、30～min段有统计学差异,其余检测时间段没有统计学差异。结论前庭刺激可能会削弱机体心血管压力感受器反射调节功能的稳定性,造成部分志愿者直立位耐力的降低。     

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.