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.     

加速度暴露下的心律失常及其航空医学意义

目的 探讨正加速度（＋Gz)作用下心血管自主神经调节与心律失常及G耐力的关系。方法 对36名立位耐力不良和62名健康飞行员＋Gz作用下的ECG及动态心电图进行对比分析。结果 ＋Gz作用下,立位耐力不良者心律失常发生率高,＋Gz耐力低。结论 ＋Gz作用下的心律失常可以作为评价心血管代偿功能及G致意识丧失（G－LOG）的预警指标。     

Fludrocortisone does not prevent orthostatic hypotension in astronauts after spaceflight

BACKGROUND: During stand/tilt tests after spaceflight, 20% of astronauts experience orthostatic hypotension and presyncope. Spaceflight-induced hypovolemia is a contributing factor. Fludrocortisone, a synthetic mineralocorticoid, has been shown to increase plasma volume and orthostatic tolerance in Earth-bound patients. The efficacy of fludrocortisone as a treatment for postflight hypovolemia and orthostatic hypotension in astronauts has not been studied. Our purpose was to test the hypothesis that astronauts who ingest fludrocortisone prior to landing would have less loss of plasma volume and greater orthostatic tolerance than astronauts who do not ingest fludrocortisone. METHODS: There were 25 male astronauts who were randomized into 2 groups: placebo (n = 18) and fludrocortisone (n = 7), and participated in stand tests 10 d before launch and 2-4 h after landing. Subjects took either 0.3 mg fludrocortisone or placebo orally 7 h prior to landing. Supine plasma and red cell volumes, supine and standing HR, arterial pressure, aortic outflow, and plasma norepinephrine and epinephrine were measured. RESULTS: On landing day, 2 of 18 in the placebo group and 1 of 7 in the fludrocortisone group became presyncopal (chi2 = 0.015, p = 0.90). Plasma volumes were significantly decreased after flight in the placebo group, but not in the fludrocortisone group. During postflight stand tests, standing plasma norepinephrine was significantly less in the fludrocortisone group compared with the placebo group. CONCLUSIONS: Treatment with a single dose of fludrocortisone results in protection of plasma volume but no protection of orthostatic tolerance. Fludrocortisone is not recommended as a countermeasure for spaceflight-induced orthostatic intolerance.     

Improvement of head-pelvis axis (+Gz) tolerance by increasing body hydration

Experiments were performed to determine tolerance to head-to-feet (+Gz) acceleration in 62 test subjects aged 23 to 33 years. They were rotated in a human centrifuge before and after they consumed water and water-salt supplements under the conditions of normal activity or dry immersion simulating microgravity effects. During the centrifugation experiments the following parameters were recorded: stroke volume and cardiac output, arterial pressure by means of Korotkov sounds, electrolytes, total protein and hematocrit. Water and water-salt supplements were found to produce a beneficial effect on acceleration tolerance: tolerance threshold increased, stability of cardio-respiratory functions grew, cardiac arrhythmias developed less frequently. The efficacy of the methods increasing the hydration level was related to the amount of water consumed and retained in the body. It is recommended to use a differential approach to the development of procedures for increasing body hydration to be employed in aerospace medicine.     

Short-arm (1.9 m) +2.2 Gz acceleration: isotonic exercise load-O2 uptake relationship

BACKGROUND: The deconditioning syndrome from prolonged bed rest (BR) or spaceflight includes decreases in maximal oxygen uptake (VO2max), muscular strength and endurance, and orthostatic tolerance. In addition to exercise training as a countermeasure, +Gz (head-to-foot) acceleration training on 1.8-2.0 m centrifuges can ameliorate the orthostatic and acceleration intolerances induced by BR and immersion deconditioning. PURPOSE: Study A was designed to determine the magnitude and linearity of the heart rate (HR) response to human-powered centrifuge (HPC) acceleration with supine exercise vs. passive (no exercise) acceleration. Study B was designed to test the hypothesis that moderate +Gz acceleration during exercise will not affect the respective normal linear relationships between exercise load and VO2max, HR, and pulmonary ventilation (VEBTPS). Study C: To determine if these physiological responses from the HPC runs (exercise + on-platform acceleration) will be similar to those from the exercise + off-platform acceleration responses. METHODS: In Study A, four men and two women (31-62 yr) were tested supine during exercise + acceleration and only passive acceleration at 100% [maximal acceleration (rpm) = Amax] and at 25%, 50%, and 75% of Amax. In Studies B and C, seven men (33+/-SD 7 yr) exercised supine on the HPC that has two opposing on-platform exercise stations. A VO2max test and submaximal exercise runs occurred under three conditions: (EX) exercise (on-platform cycle at 42%, 61%, 89% and 100% VO2max) with no acceleration; (HPC) exercise + acceleration via the chain drive at 25%,50%, and 100% Gzmax (35%, 72% and 100% VO2max); and (EXA) exercise (on-platform cycle at 42%, 61%, 89%, and 100% VO2max) with acceleration performed via the off-platform cycle operator at +2.2+/-0.2 Gz [50% of max (rpm) G]. RESULTS: Study A: Mean (+/-SE) Amax was 43.7+/-1.3 rpm (mean = +3.9+/-0.2, range = 3.3 to 4.9 Gz). Amax run time for exercise +acceleration was 50-70 s, and 40-70 s for passive acceleration. Regression of X HR on Gz levels indicated explained variances (r2) of 0.88 (exercise) and 0.96 (passive). The mean exercise HR of 107+/-4 (25%), to 189+/-13 (100%) bpm were 43-50 bpm higher (p < 0.05) than comparable passive HR of 64+/-2 to 142+/-22 bpm, respectively. Study B: There were no significant differences in VO2, HR or VEBTPS at the submaximal or maximal levels between the EX and EXA runs. Mean (+/-SE) VO2max for EX was 2.86+/-0.12 L x min(-1)(35+/-2 ml x min(-1) x kg(-1)) and for EXA was 3.09+/-0.14 L x min(-1) (37+/-2 ml-min(-1) x kg(-1)). Study C: There were no significant differences in the essentially linear relationships between the HPC and EXA data for VO2 (p = 0.45), HR (p < 0.08), VEBTPS (p = 0.28), or the RE (p = 0.15) when the exercise load was % VO2max. CONCLUSION: Addition of + 2.2 Gz acceleration does not significantly influence levels of oxygen uptake, heart rate, or pulmonary ventilation during submaximal or maximal cycle ergometer leg exercise on a short-arm centrifuge.     

Evaluation of cardiovascular risks of spaceflight does not support the NASA bioastronautics critical path roadmap

INTRODUCTION: Occurrence of serious cardiac dysrhythmias and diminished cardiac and vascular function are the primary cardiovascular risks of spaceflight identified in the 2005 NASA Bioastronautics Critical Path Roadmap. METHODS: A review of the literature was conducted on experimental results and observational data obtained from spaceflight and relevant ground simulation studies that addressed occurrence of cardiac dysrhythmias, cardiac contractile and vascular function, manifestation of asymptomatic cardiovascular disease, orthostatic intolerance, and response to exercise stress. RESULTS: Based on data from astronauts who have flown in space, there is no compelling experimental evidence to support significant occurrence of cardiac dysrhythmias, manifestation of asymptomatic cardiovascular disease, or reduction in myocardial contractile function. Although there are post-spaceflight data that demonstrate lower peripheral resistance in astronauts who become presyncopal compared with non-presyncopal astronauts, it is not clear that these differences are the result of decreased vascular function. However, the evidence of postflight orthostatic intolerance and reduced exercise capacity is well substantiated by both spaceflight and ground experiments. Although attenuation of baroreflex function(s) may contribute to postflight orthostatic instability, a primary mechanism of orthostatic intolerance and reduced exercise capacity is reduced end-diastolic and stroke volume associated with lower blood volumes and consequent cardiac remodeling. CONCLUSION: Data from the literature on the current population of astronauts support the notion that the primary cardiovascular risks of spaceflight are compromised hemodynamic responses to central hypovolemia resulting in reduced orthostatic tolerance and exercise capacity rather than occurrence of cardiac dysrhythmias, reduced cardiac contractile and vascular function, or manifestation of asymptomatic cardiovascular disease. These observations warrant a critical review and revision of the 2005 Bioastronautics Critical Path Roadmap.     

Carotid sinus pressure changes during push-pull maneuvers

INTRODUCTION: The push-pull maneuver (PPM) is defined as a reduction in G-tolerance when positive acceleration (+Gz) immediately follows negative acceleration (-Gz) exposure, with the carotid baroreceptors presumably playing a dominant role in the ensuing BP (SBP) responses. The objective of this study was to determine whether application of neck pressure (NP) during the preceding -Gz phase maintains +Gz tolerance during subsequent +Gz. METHODS: There were 10 experienced men who were exposed to 3 centrifuge run types using a multi-axis centrifuge: a relaxed control run from +1.4 baseline to visual tolerance; a relaxed control PPM run (PPM-C) consisting of 5 s of -1 Gz followed by 15 s of +Gz to visual tolerance; and an experimental PPM run performed with pressurized neck (PPM-NP) consisting of -1 Gz for 5 s followed by 15 s of +Gz at the previous PPM-C G-tolerance level. RESULTS: Relaxed control G tolerance (3.6 _ 0.26 Gz) was greater vs. the PPM-C (3.0 +/- 0.21 Gz) and PPM-NP (3.1 +/- 0.20 Gz) conditions, but the two PPM conditions did not differ significantly. During -Gz, mean R-R interval for PPM-NP was significantly shorter than in the PPM-C from second 1 to second 3. During the +Gz phase, however, R-R interval responses between PPM-C and PPM-NP differed only at seconds 8 and 9. There were no differences in carotid sinus SBP between PPM-C and PPM-NP during -Gr. During +Gz, carotid sinus SBP was significantly depressed in PPM-NP and PPM-C conditions vs. Control. DISCUSSION: Application of NP during the -Gz phase, despite altering R-R interval, did not ameliorate SBP responses or reductions in G tolerance during subsequent +Gz exposure. Despite neck compression counteracting increased carotid hydrostatic pressure during -Gz, the carotid baroreceptor response is likely opposed by the aortic or other baroreceptors.     

离心机训练矫治+Gz耐力不良的高性能战斗机飞行员

目的 矫治两名＋Ｇｚ耐力不良的高性能战斗机飞行员。方法在离心机上高＋ＧＺ负荷下训练抗荷动作（ＡＧＳＭ）和加压呼吸（ＰＢＧ）动作。结果 ＡＧＳＭ的抗荷效果提高３．０－３．２５Ｇ,加压呼吸了系统的抗荷效果提高２．７５－３．０Ｇ,综合耐力比基础耐力高４．２５－４．５Ｇ,两人均顺利通过８Ｇ１０Ｓ的ＳＡＣＭ＋ＧＺ曲线,达到训练标准。     

On medical screening of civilian candidates for cosmonauts by exposure to the head-to-pelvis acceleration profiles]

The subject of analysis was the data on +3 and +5 Gz tolerance of 130 civilian non-pilot applicants for cosmonauts (men and women, aged 23 to 55) gathered over the past 30 years. Length of the centrifuge arm was 7.25 meters and the total number of primary centrifuge runs was 309. For nearly every second of the applicants (46.7%) acceleration at +5 Gz was an ordeal causing distinct vascular or coronary decompensation. Thus, 29.7% exhibited various combinations of brief visual disturbances, tachycardia, tachypnea, and systolic arterial pressure in the shoulders; in 17%, visual disturbances and/or their precursors were combined with exaggerated cardio-vascular functional parameters, arrhythmia, and serious vegetative disorders. Most of those who had failed to endure the first centrifugation were unable to improve G tolerance during next runs; indeed, they showed negative G-tolerance dynamics. G intolerance grew in significance or was exacerbated by new disorders and their combinations. These results testify against exposure of non-pilot applicants for cosmonauts to +5 G, during the primary medical screening.     

Effects of exercise-heat acclimation on fluid, electrolyte, and endocrine responses during tilt and +Gz acceleration in women and men

Plasma fluid, electrolyte, protein, renin, and vasoactive hormone (epinephrine, norepinephrine, vasopressin) responses were measured in six women (21-23 yr) and four men (21-38 yr) before and immediately following an orthostatic tolerance test (70 degrees head-up tilt) and a +Gz (head-to-foot) acceleration tolerance test (0.5 G X min-1 linear ramp to grayout). These tests were conducted before and after 12 consecutive days of exercise-heat acclimation when the subjects exercised on a cycle ergometer at a relative oxygen uptake of 44% to 49% peak oxygen uptake in a hot environment (Ta = 40 degrees C, 42% rh). During acclimation plasma volume increased by 10.6% (p less than 0.05) in the women and by 11.9% (p less than 0.05) in the men; in both groups exercise heart rate decreased significantly. After acclimation, acceleration tolerance was unchanged in both groups (range 3.1 to 3.4 G); the women's tilt tolerance was unchanged (range 33.6 to 39.5 min), but the men's tilt tolerance increased from 30.4 min before to 58.3 min (delta = 91%, p less than 0.05) after acclimation. Since the pattern of fluid, electrolyte, and protein shifts and acceleration tolerances in the women and men were virtually the same, the hormone responses were highly variable, and the men's tilt tolerance increased significantly after acclimation, it is clear that responses to tilting cannot be used to predict responses to acceleration. Analysis of data from the present study and the literature suggests that current exercise training regimes should be unrestricted for astronauts who have not previously been highly endurance trained.(ABSTRACT TRUNCATED AT 250 WORDS)     

The electrocardiographic response of females to centrifuge +Gz stress

Women continue to expand their participation in all areas of aviation, including flying high performance fighter aircraft. Acceleration (+Gz) stress is unique to fighter aviation, therefore it is important to thoroughly understand the electrocardiographic (ECG) response to +Gz stress since it reflects a portion of the cardiovascular +Gz tolerance. A comparison of the ECG response to centrifuge +Gz stress between 685 men and 94 women was made from data existing within a centrifuge data repository. The frequency of occurrence of specific types of atrial, ventricular, and the other most frequently observed ECG changes to +Gz stress were compared for females and males. Females had less atrial ectopy; essentially equivalent premature ventricular contractions (PVC's), multiformed PVC's, paired PVC's; less frequent ventricular and supraventricular tachycardia; and more frequent PVC's in a bigeminal pattern and QRS on T PVC's. Sinus arrhythmia, sinus bradycardia, and increased T-waves post +Gz stress were more frequent in males, with ectopic atrial rhythm and atrioventricular dissociation essentially equivalent in males and females. Although few women have participated in either simulated aerial combat maneuver type centrifuge profiles or centrifuge high-G training, they have shown similar ECG changes including conduction and rhythm disturbances infrequently seen in males, such as +Gz-induced right bundle branch block and high-G bradycardia. Based on the currently available ECG response data, women have no demonstrated unique susceptibility to +Gz-induced ECG changes. Therefore, no contraindication exists to initiating additional acceleration research to fully evaluate women's tolerance to the more stressful, higher levels of +Gz stress.     

Acceleration in other axes affects +Gz tolerance: dynamic centrifuge simulation of agile flight

BACKGROUND: Modern, thrust-vectored jet aircraft have the capability of developing multi-axis accelerations, especially during the performance of "supermaneuvers." These "agile" aircraft are capable of unconventional flight. The human consequences of this agile flight environment are unknown. METHODS: This multi-axis acceleration environment was studied on the Dynamic Environment Simulator gimbaled centrifuge. There were nine relaxed, unprotected subjects who were exposed to either lateral (+/- 1, +/- 2 Gy), transverse chest-to-back (+1, 2.5, or 4 Gx), or back-to-chest (-1 Gx) sustained acceleration. Positive C (+Gz) acceleration was then added beginning at 1.0 Gz by gradual onset (0.1 Gz x s(-1)) until the subjects lost nearly all of their vision. Baseline +Gz-only relaxed tolerances were measured before and after all combined Gy/Gz and Gx/Gz exposures. Heart rate, percent cerebral oxygen saturation, and cerebral blood volumes were collected during each exposure. RESULTS: Adding moderate transverse (+Gx) acceleration significantly reduced +Gz tolerance. Relaxed, unprotected +Gz tolerance was reduced approximately 0.25 G when 1.0 or 2.5 Gx was added to the increasing +Gz exposure. Adding moderate lateral Gy significantly increased +Gz tolerance. Relaxed, unprotected +Gz tolerance was increased approximately 0.5 G when +2 Gy or -2 Gy was added to the -Gz exposure. The decrease in cerebral blood volume was significantly less when +Gz was added to -1 Gx compared with the addition of +Gz to +Gx. CONCLUSIONS: Multi-axis sustained accelerations, such as those experienced during thrust-vectored aircraft maneuvers, can either enhance or reduce the +Gz tolerance of the pilot depending on the direction of the net gravitoinertial force. Gy acceleration in conjunction with Gz acceleration can enhance G tolerance. Gx acceleration in addition to Gz acceleration can reduce G tolerance.     

新型载人离心机在军事航空医学中的应用

目的 探讨在新型载人离心机上进行飞行员抗荷耐力选拔与训练、-Gx体验、动态飞行模拟(dynamic flight simulation,DFS)训练以及抗荷装备生理鉴定实验的方法,探索新型载人离心机的应用方法. 方法 ①24名飞行员在六三型载人离心机上进行基础+Gz耐力检查,然后按同样方法,在新型载人离心机上进行了对照检查.②100名飞行员进行了离心机训练,其中的42名飞行员为被动控制训练,58名飞行员为闭环控制训练.③8名飞行员体验-1.5 Gx及-2.0 Gx载荷.④18名飞行员进行DFS飞行体验,特技飞行动作包括左盘旋、右盘旋、半滚倒转、斤斗、半斤斗翻转和桶滚.⑤6名志愿者和9名飞行员在新型载人离心机上,分别进行服装、PBG、服装结合PBG、服装结合PBG及抗荷收紧动作(anti-G straining manoeuvre,AGSM)的抗荷效果试验. 结果 ①在新、老离心机上检查出的+Gz耐力结果平均值分别为(4.25±0.34)G及(4.41±0.14)G(t=3.03,P<0.01),其差异与连续检查所致的疲劳及飞行员对重复检查的主观重视程度有关,排除二者影响,在新离心机上可采用现有抗荷耐力选拔方法.②在进行被动控制训练的飞行员中,25人完成7 G持续10 s,9人完成8 G持续10 s,仅有2人完成9 G持续10 s.在进行闭环控制训练的58名飞行员中,48人完成7 G持续10 s,19人完成8 G持续10 s,23人完成9 G持续10 s.③飞行员在进行-Gx载荷体验时,反映在座舱俯仰、滚转过程中头晕、恶心的感觉比较明显.④飞行员认为DFS体验与实际飞行接近,对提高机动空战能力具有实用价值.⑤抗荷装备的抗荷性能达到了飞机性能的要求.结论 新型载人离心机在高性能战斗机飞行员抗荷耐力选拔训练、-Gx体验、DFS体验及抗荷装备生理鉴定试验等工作中获得广泛应用,形成了相关应用方法,取得了较好的效果,为进一步开展相关工作奠定了基础. Abstract： Objective To investigate the approaches and the application effects of acceleration tolerance screening and training for fighter pilots,-Gx experience,dynamic flight simulation (DFS) experience and physiological evaluation of anti-G equipment on new human centrifuge. Methods ①Twenty-four pilots underwent the relaxed+Gz tolerance examination on a single degree of freedom centrifuge (Model 63) and on new human centrifuge for comparison.②One hundred high performance fighter pilots carried out centrifuge training.Among them,42 pilots were in passive control mode while the other 58 pilots were trained by close-loop mode.③Eight pilots experienced -1.5 Gx and-2.0 Gx on the new centrifuge.④Eighteen pilots performed aerobatic maneuvers,such as left spiral,right spiraI,split S loop,immelmann,and barrel roll in DFS awareness.⑤Six volunteers and 9 pilots served as subjects.The tests of anti-G capability evaluation for suits,suits combined with pressure breathing for G (PBG),and suits combined with PBG and anti-G strainingmaneuver (AGSM) were conducted respectively. Results ①The mean relaxed+Gz tolerance of pilots obtained on the new and old centrifuge was (4.25±0.34) G and (4.41±0.14) G respectively (t=3.03,P<0.01).The difference would be relative to fatigue that caused by continuous running and pilots' endurance to the reduplicate riding.So the current+Gz tolerance screening method was applicable on the new centrifuge regardless of the influence of the above mentioned factors.②In the training under passive control mode,25 of 42 pilots completed the training of 7 G for 10 s,9 pilots for 8 G for 10 s and 2 for 9 G for 10 s.In the close-loop control training,48 pilots passed training of 7 G for 10 s,19 pilots for 8 G for 10 s,and 23 for 9 G for 10 s.③During-Gx experience,the feelings of dizzy and disgust were evident during the roll and pitch movements.④Pilots reported that DFS training was similar to actual flight and possessed great practical value in improving aerial combat ability.⑤The performance of anti-G equipments reached the functional requirement. Conclusions New human centrifuge can be applied in acceleration tolerance screening and training for high performance fighter aircraft pilots,-Gx experience,DFS training,and physiological evaluation for anti-G equipments.Relevant approaches have been established,and the application of new human centrifuge has acquired a satisfactory result.which lays the foundation for further work.     

离心机模拟连续推拉动作训练方法研究

目的建立贴近实战化训练模式的连续推拉动作(PPM)离心机训练方法。方法调研提炼飞行训练中典型的PPM载荷参数,编制离心机模拟连续PPM曲线。16名战斗机飞行员作为志愿者,A组6名,采用-1Gz/3s→+6Gz/10s→2Gz/10s→0Gz/3s→+4.5Gz/10s→2Gz/10s→0.5Gz/3s→+5 Gz/10s连续PPM曲线,B组10名,采用-1Gz/3s→+6Gz/10s→2Gz/10s→0Gz/3s→+4.5Gz/10s→2Gz/10s→-1Gz/3s→+7Gz/5s连续PPM曲线(根据A组建议对曲线进行了改进),进行了HP动作对抗连续PPM离心机训练。志愿者在相对-Gz(小于+1Gz)暴露时采用HP动作的呼吸方式,在向+Gz转换时开始做较用力的HP动作,记录分析+Gz耐力、心率(HR)等指标的变化。结果A组6名志愿者均完成了连续PPM离心机训练,对PPM曲线提出了改进建议。B组9名志愿者采用改进的连续PPM曲线完成离心机训练。两组在相对-Gz时的HR均显著高于安静状态(P<0.05),连续PPM暴露时的HR变化趋势说明心血管调节有持续效应,其适应负荷有滞后。结论建立了高性能战斗机飞行员连续PPM离心机训练方法,模拟连续PPM曲线中不同水平-Gz与+Gz交替作用的模式更能体现实战化训练中长时间反复空战的特点,将应用于后续飞行员离心机训练。     

Centrifuge training program with "push-pull" elements

INTRODUCTION: Pilots of fighter aircraft are often exposed to maneuvers that produce negative acceleration (-Gz) immediately followed by positive acceleration (+Gz). This sequence has been found to reduce tolerance to +Gz, a phenomenon known as the "push-pull" effect. We devised a centrifuge training program to demonstrate this phenomenon to pilots. METHODS: The centrifuge of the Military Institute of Aviation Medicine in Warsaw, Poland, was modified in 1996 to allow active positioning of the gondola during rotation. Head-down position of -6 degrees to -40 degrees were used to produce relative -Gz (r-Gz) in a range down to 0.2. As a side effect, this produces Gy acceleration between -1.3 Gy and -1.6 Gy. Pilots completed normal centrifuge training, including a relaxed, gradual-onset run and three rapid-onset runs. They were then exposed to a profile that included a series of push-pull exposures where r-Gz was followed by +Gz with stepwise increases in the latter from +2.5 to +5 Gz. The final profile was a simulated aerial combat maneuver with push-pull elements. RESULTS: The trainees expressed surprise at the push-pull effect, which forced them to begin an anti-G straining maneuver at lower levels than normal. They complained about the presence of the Gy, which rarely occurs in aircraft. DISCUSSION: This type of profile appears useful for training pilots about the push-pull phenomenon. After collection of additional data, the profiles may be refined.     

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.     

The role of arterial elastance in ventricular-arterial coupling in normal gravity and altered acceleration environments

The role of physiological elastance (Ep) in maximizing external work (EW) transfer is not well understood and has not been investigated during microgravity and increased acceleration conditions. By better understanding this relationship, cardiovascular control mechanisms for meeting metabolic demands during normal gravity and altered acceleration stresses may be elucidated. Therefore, the objectives of this study were to determine the effect of Ep in maximizing EW of the left ventricle and to investigate this relationship during altered acceleration states. Ventricular and arterial parameters were estimated using established lumped parameter models from isolated beats of experimental data. These data were obtained during parabolic flight (0 and approximately 2 Gz) and centrifuge runs (approximately 1 to approximately 4 Gz) where acceleration was used to drive the cardiovascular system into a wide range of physiologic operating and coupling conditions. Parameter estimates at each Gz level were used in a series of computer simulations in which Ep was varied over a wide range to find the point of maximum EW for that coupling condition. Cardiac output and mean arterial pressure were maintained throughout the simulation process by adjusting heart rate. Results of the simulation showed that as arterial elastance decreased from its initially estimated (physiologic) value, external work increased slightly and as elastance increased, external work decreased. In particular, we found that the arterial elastance was set at a point near that which would produce maximal external work. In addition, it was found that altered Gz states may affect the Ep-EW relationship.     

抗荷正压呼吸对下体正压模拟推拉效应的防护作用

目的利用下体正压在单轴向离心机上模拟推拉动作,观察抗荷正压呼吸对推拉效应的防护效果。方法受试者5名,均参加过载人离心机试验并掌握了正确的抗荷正压呼吸方法。第1天测定受试者的基础 Gz耐力、穿KH-x抗荷服时的 Gz耐力和实施抗荷正压呼吸时的 Gz耐力;第2天依次测定受试者的基础耐力、下体正压作用后穿KH-x抗荷服 Gz耐力和抗荷正压呼吸时的 Gz耐力。结果与第1天的试验结果相比,下体正压作用后受试者穿着KH-x抗荷服的 Gz耐力显著降低(P<0.05);但是进行抗荷正压呼吸时的 Gz耐力没有变化。结论抗荷正压呼吸可以减轻推拉效应对 Gz耐力的影响。关于抗荷正压呼吸的压力制度及其训练方法还有待进一步优化。     

高+Gz负荷下犬心血管系统反应特性

目的探讨犬＋Ｇｚ负荷中、负荷后心电图、动脉血压反应特性。方法将压力传感器导管置于６只麻醉犬的升主动脉和髂总动脉,以测量犬暴露于＋１Ｇｚ、＋３Ｇｚ、＋５Ｇｚ、＋７Ｇｚ的血压,并采用Ａ导联持续监测心电图。结果（１）Ｐ波高度受＋Ｇｚ负荷的影响,负荷前、后１ｍｉｎ内差异显著（＋Ｇｚ负荷前Ｐ波２．３±０．２ｍＶ,＋３Ｇｚ后４．５±０．５ｍＶ,＋５Ｇｚ后４．８±０．３ｍＶ,＋７Ｇｚ后５．３±０．７ｍＶＰ＜０．０５）。（２）高＋Ｇｚ负荷时,髂总动脉血压表现为平均动脉压增高,脉压减低。（３）＋Ｇｚ终止后,升主动脉血压（ＡＰ）出现一过性增高（＋７Ｇｚ前ＡＰ１７．２９±５．５９／１１．３１±３．８６ｋＰａ,＋７Ｇｚ后第３０ｓ２７．５３±６．１２／２０．６２±１．８６ｋＰａ,Ｐ＜０．０１）。结论Ｐ波高度的变化反映心房移位,这可能是心律失常的原因。在高＋Ｇｚ负荷条件下,心血管功能不全,组织灌注压是一个重要的生理参数。＋Ｇｚ负荷终止后即刻,动脉血压极度增高,为血管性衰竭的表现     

Tolerance of +Gz acceleration in pilots with health problems

Fighter pilots with health abnormalities were examined for their tolerance to +Gz acceleration. It was found that pilots with psychoneurological disorders and cardiac arrhythmias showed a lower acceleration tolerance. Pilots with arterial hypertension displayed a markedly higher tolerance as compared to the average pilot population. Other health disturbances produced no effect on +Gz acceleration tolerance.