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At least 937 records · Page 52Linked to original sources
PubMed1998

[Effect of +Gz-induced cerebral ischemia on the distribution of nitric oxide synthase in rabbit brain].

In order to study the effect of repeated short termed + Gz -induced cerebral ischemia on rabbit brain, rabbits were exposed to +4.0 Gz for 3 times (with 30 min interval in between) which caused the arterial pressure at eye level dropped to 0 kPa and lasted for 30 s. Changes of distribution of the nitric oxide synthase(NOS) positive neurons in rabbit brain were observed using the NADPH-diaphorase histochemical reaction. The results showed that NOS positive neurons in the parietal cortex, piriform cortex, hippocampus and dentate gyrus increased by 167%, 126%, 321%, 172% (P < 0.01) respectively, immediately after repeated + Gz exposures, as compared with those of controls. It still remained significantly more than that in controls (P < 0.01) 1 h and 6 h after exposure. It suggests that NO may have some effect on the neurons during cerebral ischemia.

Acceleration↗
PubMed1998

[Bispectral analysis of EEG signal during focal cerebral ischemia].

Early noninvasive detection of cerebral ischemia is important for fighter pilots as well as in clinical diagnosis. In this paper, a model of focal cerebral ischemia was presented. Bispectrum of EEG signals recorded from ischemic region and normal region after 30 min ischemia was analyzed using the third-order recursion method, which possesses higher frequency resolving ability. It can resolve two closely spaced peaks in the bispectrum and detect the presence or absence of quadratic phase coupling at frequency pairs that are very close to each other. It was found that some characteristics of such bispectrum are very sensitive to focal cerebral ischemia. The maximum bispectrum magnitude of EEG and the weighted center of bispectrum (WCOB) changed sensitively to the different ischemic regions and ischemic stages. The ischemic and normal regions as well as the degree of ischemia can be clearly distinguished by analyzing their bispectrum of EEG. It demonstrates that this method can be used as a quantitative method for the detection of focal cerebral ischemia in the early phase.

Acceleration↗
PubMed1998

A simulation study on +Gz protection afforded by extended coverage anti-G suits.

Computer simulation studies using our newly developed mathematical model of human cardiovascular (CV) system were performed to investigate the effect of bladder coverage and inflation schedule of anti-G suit on G protection and CV response. The simulation results agree well with centrifuge experiments. The study demonstrates that additional +Gz protection provided by the extension of the bladder coverage depends mainly upon the pressure transmission efficiency to the underlying tissues and vessels. Although G tolerance increases clearly with the increase of suit inflation schedule, the tendency of increase seems slower when inflation schedule becomes higher than 10 kPa/G.

Acceleration↗
PubMed1998

Launch conditions might affect the formation of blood vessels in the quail chorioallantoic membrane.

As a part of the first joint USA-Russian MIR/Shuttle program, fertilized quail eggs were flown on the MIR 18 mission. Post-flight examination indicated impaired survival of both the embryos in space and also of control embryo exposed to vibrational and g-forces simulating the condition experienced during the launch of Progress 227. We hypothesized that excess mechanical forces and/or other conditions during the launch might cause abnormal development or the blood supply in the chorioallantoic membrane (CAM) leading to the impaired survival of the embryos. The CAM, a highly vascularized extraembryonic organ, provides for the oxygen exchange across the egg shell and is thus pivotal for proper embryonic development. To test our hypothesis, we compared angiogenesis in CAMs of eggs which were either exposed to the vibration and g-force profile simulating the conditions at launch of Progress 227 (synchronous controls), or kept under routine conditions in a laboratory incubator (laboratory controls). At various time points during incubation, the eggs were fixed in paraformaldehyde for subsequent dissection. At the time of dissection, the CAM was carefully lifted from the egg shell and examined as whole mounts by bright-field and fluorescent microscopy. The development of the vasculature (angiogenesis) was assessed from the density of blood vessels per viewing field and evaluated by computer aided image analysis. We observed a significant decrease in blood-vessel density in the synchronous controls versus "normal" laboratory controls beginning from day 10 of incubation. The decrease in vascular density was restricted to the smallest vessels only, suggesting that conditions during the launch and/or during the subsequent incubation of the eggs may affect the normal progress of angiogenesis in the CAM. Abnormal angiogensis in the CAM might contribute to the impaired survival of the embryos observed in synchronous controls as well as in space.

Allantois↗
PubMed1999

[An analysis of the cardiovascular responses under hyper- and hypo-gravity environments using a mathematical model].

Gravity affects cardiovascular control system remarkably. Internal control mechanism responsible for such cardiovascular changes under hypo- and hyper-gravity have not yet been fully understood, although many biological and physiological measurements as to cardiovascular system have been conducted since man's first exploration to space. One reason for this arises from the difficulty in continuous and simultaneous measurements of hemodynamics of many parts of the body. To overcome this difficulty, a mathematical model was constructed based on animal and human physiological evidence in our previous study. In the present study, the model is used for explaining hemodynamics during hyper- and hypo-gravity environments obtained during parabolic flight. The parabolic flight experiment was conducted by a small rear-jet MU300. Three university male students volunteered as subjects. Five to eleven parabolic flights per day were performed for 6 days. The subjects sat on a chair either in an upright position or a 45 degree reclining position. Electrocardiogram and finger blood pressure were measured continuously during the flights. Variable parameters of the model were adjusted so that heart rate and blood pressure of the model fit to those of the experiment. It was shown that the model can quantitatively reproduce and predict experimental heart rate and blood pressure during a parabolic flight. Analysis of internal property of the model revealed hemodynamics of the human cardiovascular system during a parabolic flight which explains the mechanisms of cardiovascular responses under hyper- and hypo-gravitational environments.

Blood Pressure↗
PubMed1999

Effects of daily 2-Gz load on human cardiovascular function during weightlessness simulation using 4-day head-down bed rest.

An onboard short arm human centrifuge has been proposed as a countermeasure against physiological problems during long exposure to weightlessness in space and during extra planetary exploration. However, there are few studies on the effects of intermittent application of a Gz load via centrifuge during weightlessness. The present study evaluated the effects of a daily 2-Gz load on cardiovascular function during simulated weightlessness using a 4-day head-down bed rest (HDBR) period. Twelve young male subjects were exposed a HDBR period. Eight of them were exposed to a Gz load for up to 30 min twice per day (the Gz group). The remaining 4 were not exposed to a Gz load; they served as controls (the no-Gz group). Compared with the pre-HDBR period, the no-Gz group showed percent changes in the RR interval, the standard deviation (SD) of the RR interval, parasympathetic nervous activity, and baroreflex sensitivity (BRS) that progressively decreased and reached significance at the end of the HDBR period (-5.96 +/- 2.60%, -33.82 +/- 9.60%, -46.3 +/- 12.7% and -30.9 +/- 7.2%, respectively). In the Gz group, however, the percent changes in the RR interval, the SD of the RR interval, parasympathetic nervous activity, and BRS showed no significant changes throughout the HDBR period. At the end of the HDBR period, these indexes were 2.22 +/- 2.21%, -2.31 +/- 12.28%, 5.08 +/- 14.82% and 10.6 +/- 12.5%, respectively, and significantly greater than those of the no-Gz group. Sympathovagal balance indicators showed no significant change in the Gz and no-Gz groups (5.17 +/- 12.85% and 18.5 +/- 10.7%, respectively). These results indicate that a daily load of 2-Gz eliminates reduction of the RR interval, the SD of the RR interval, parasympathetic nervous activity, and BRS, and that it can maintain autonomic cardiovascular function in short-term weightlessness.

Adult↗
PubMed1996

Centrifuge man-rating of a conceptual internal abdominal bladder restraint in an extended coverage anti-G suit.

An extended coverage anti-G suit, has been demonstrated to improve +Gz tolerance substantially. In some pilots/subjects, however, the abdominal bladder of the anti-G suit may expand excessively upward and inward causing discomfort and pain. This man-rating was performed to evaluate the effects on +Gz protection of an internal abdominal bladder restraint in the Swedish Tactical Flight Combat Suit (TFCS) used in conjunction with pressure breathing during G (PBG). The tests were executed in the Armstrong Laboratory Centrifuge at Brooks AFB with four Swedish test fighter pilots. The centrifuge profiles included gradual onset runs (GOR, relaxed) and rapid onset runs (ROR, with straining), as well as simulated aerial combat maneuver (SACM) runs up to +9 Gz until subjects experienced light loss or fatigue or surpassed 228 s. All subjects withstood 60 s at +9 Gz during GOR and ROR runs with and without abdominal bladder restraint. No difference There was no difference in SACM duration times. In three of four subjects, abdominal pain or discomfort experienced without abdominal bladder restraint disappeared with the addition of a bladder restraint. Ratings of perceived exertion (after 5 peaks at +9 Gz in the SACM), subjective +Gz tolerance, overall comfort, fatigue, and heat stress demonstrated no relevant differences with and without abdominal bladder restraint. Therefore, to enhance comfort, it seems possible to modify the TFCS by adding an abdominal bladder internal restraint without compromising its operational +Gz protection.

Abdomen↗
PubMed1996

A survey of centrifuge subjects assessing the relationship between repeated +Gz exposures and chronic back pain.

Research using human centrifuges has been done for many years. Long-term follow-up of such subjects has not been carried out previously. To evaluate whether there is an increased incidence of back problems in former centrifuge subjects, we did a retrospective questionnaire survey of 20 subjects with significant centrifuge exposure histories to moderate G levels at least 5 to 10 years ago, and two USAF active duty, age-matched control groups (former hypobaric subjects and naive subjects). The results revealed no increase in back pain or injury in the study group. The only significant indicators for back pain or injury in our study population are a history of tobacco use and increasing height. Moderate G exposures were not a predictor of subsequent back pain. We recommend a larger prospective study controlling for intensity, profile and duration of G exposure.

Adult↗
PubMed1996

Implementation of a computer-controlled life support system in the acceleration research environment.

Electronic control of the G-valve and pressure breathing regulator is being implemented in some advanced life support systems used in aircrew protection. This technological improvement, however, has not reached its full potential in the research environment. A computer-controlled life support system interface providing programmable schedules for G-suit inflation and positive pressure breathing during +Gz (PBG) was developed. Output pressures from a G-valve and pressure breathing regulator (Carleton Technologies) were controlled by a Macintosh computer running LabVIEW software. Required pressures were determined as functions of single or multiple control inputs (i.e. +Gz level, a pressure signal, time, etc.). Subject safety was ensured via hardware limitations and status checks incorporated into the software. Experiments conducted at +1 Gz and at various +Gz levels evaluated the computer software-life support hardware interface. Open-loop algorithms allowed independent control of multiple regulators using simple (single input) and complex (multiple input with adaptive loop) control structures. The system provided accurate and reproducible G-suit and mask pressures. Time of inflation, peak pressure attained, and deflation rate were effectively controlled at all +Gz levels. The ability to alter the pressure schedules independent of +Gz or time allowed comprehensive control over all parameters necessary to conduct acceleration research involving advanced life support systems.

Aerospace Medicine↗
PubMed1999

Straining GOR tolerance determinations are a measure of G-duration not G-level tolerance.

Straining gradual G onset rate (GOR) tolerances are considered by physiologists as a measure of G-level tolerance. Using recently developed G-level and G-duration mathematical models, it was found that straining GOR tolerances may well be a measure of tolerance to G-duration. G-duration tolerance was determined to be limited with the onset of fatigue and not cardiovascular insufficiency. G-level tolerances that were predicted using a mathematical model were higher than determined using straining GOR tolerance measurements of subjects on a centrifuge. Also the G-duration tolerance mathematical model showed that those centrifuge subjects had not expended all of their "energy reserve" during their sustained G exposure most probably because of the onset of fatigue. Even if they were able to use all of their potential energy reserve, their G-duration tolerance would not have allowed them to reach the maximum G-level predicted with the G-level tolerance model. It is therefore concluded that the straining GOR tolerance profile, with G onset rates of 0.1G/s, is not a measure of G-level tolerance, as has been assumed, but is a measure of G-duration tolerance. These findings have significant safety implications world-wide since this straining GOR profile is commonly used as a G-level tolerance fighter-pilot-selection determination; i.e. pilot selection standards for G-level tolerance are not a measure of G-level tolerance. In testing equipment design changes, the proper G tolerance profiles must be used to correctly measure its impact on G tolerance.

Adaptation, Physiological↗
PubMed1999

Assessment of two methods to reduce simulated +Gz-induced arm pain using a non-centrifuge model.

Complaints of +Gz-induced arm pain have been expressed by centrifuge subjects and, to a lesser degree, by high performance fighter pilots, usually when the control stick and throttle are positioned below heart level; the pain may be higher during pressure breathing for +Gz protection. Elevated transmural pressure and overdistension of the blood vessels in the arms have been suggested as causal factors. An earlier-developed non-centrifuge model was used to provoke arm pain similar to that induced by +Gz exposure. Eleven healthy subjects placed inside a hyperbaric chamber with an arm externalized through a sealed opening. They were exposed to chamber pressures at 75, 100 and 125 mmHg fitted with elastic bandage of the arm, or a localized pressure of 50 mmHg applied to the vessels of the upper arm or were provided no protection (control). Arm pain was estimated using a modified Borg scale. The sizes of the ulnar, radial and interosseous arteries, and corresponding veins, at the elbow level were measured using a sonographic imaging system. No statistically significant differences in pain were detected comparing control to proposed (counteracting) devices. There were no statistical differences in arm arterial or venous vessel size with the different devices at the different pressures. Thus, the devices tested did not show any protection against the provoked arm pain.

Adult↗
PubMed2000

The protection against +Gz afforded by pressure breathing with different pressure schedules.

OBJECTIVE: System of pressure breathing for +Gz (PBG) has been incorporated into service in the high performance fighter aircraft, but there were significant differences among PBG pressure schedules used in different countries. The purpose of this study was to define an optimal pressure schedule in PBG system. METHOD: Five male subjects wearing GZ-2 anti-G suit and medium-sized bladder vest, plus PBG with 1.6, 2.4, and 3.2 kPa/G pressure schedules, respectively, were exposed to rapid onset (3.0 G/s) centrifuge +Gz runs. +Gz protection of PBG with each of the three pressure schedules were measured and the subjective ratings were collected. RESULT: The +Gz protection afforded by PBG with 1.60, 2.40, and 3.20 kPa/G pressure schedules were 2.00 +/- 0.31, 2.54 +/- 0.32, and 2.44 +/- 0.31 G, respectively. Subjective ratings showed that the PBG with 2.40 kPa/G pressure schedule was better than the other two. CONCLUSION: Our data suggest that a PBG pressure schedule of 2.4 kPa/G in PBG system is optimal. It not only assures the anti-G performance of PBG, but also reduces its side effects.

Acceleration↗
PubMed2001

Effects of repeated +Gz exposures on lipid peroxidation of various organs in rats.

OBJECTIVE: To investigate the change of lipid peroxidation in various organs of rats after repeated +10 Gz stress. METHOD: Twenty male Wistar rats were randomly divided into control (+1 Gz) group and +10 Gz group (n = 10 for each group ). The two groups were exposed to +1 Gz or +10 Gz on an animal centrifuge respectively. The onset rate of +Gz was 0.5 G/s; the sustained time at peak +Gz was 30 s each time, with 5 times/d, 1 min intervals, 3 d/wk, 3 wk in total. The rats were decapitated in ice bath on the next day after the last +Gz exposure. The lung, heart, liver and kidney of rats were collected for the following determinations. These organs were made into homogenates and the mitochondria of heart and kidney were isolated from their homogenates. Then malondialdehyde (MDA) content , superoxide dismutase (SOD) activity and glutathion (GSH) concentration in these homogenates and mitochondria were determined individually. RESULT: Compared with the control, MDA concentration in myocardial mitochondria increased significantly (P < 0.01) after repeated +10 Gz exposures, and the SOD activity in liver homogenate and renal mitochondria decreased remarkably (P < 0. 01). But there was no significant difference between GSH concentrations of the two groups. CONCLUSION: Repeated +10 Gz stress induced lipid peroxidation injury of myocardial mitochondria and had some influences on the oxygen radical metabolism of liver and kidney in rats.

Animals↗
PubMed2001

[Counteraction effect of intermittent -Gx on changes of responsiveness of basilar artery in tail-suspended rats].

OBJECTIVE: To investigate counteracting effects of intermittent -Gx exposure on changes of responsiveness of basilar artery in tall-suspended rats. METHOD: Rats tail-suspended for 3 wk were daily subjected to 1 h 1.5 G or 2.6 G centrifugation, simulating -Gx exposure of various G values. Responses of perfused isolated basilar arterial rings to KCl was examined. RESULT: Maximal isometric contractile response to KCl were significantly enhanced in arterial rings isolated from 3 wk tail-suspended rats as compared with that from simultaneous control rats (P < 0. 05). However, maximal isometric contractile responses to KCl of arterial rings isolated from 3 wk tail-suspended plus daily -Gx exposure rats were no difference as compared with that from simultaneous control rats, and were significantly decreased (P < 0.05) as compared with that from 3 wk tail-suspended rats. CONCLUSION: Daily intermittent -Gx exposure as a countermeasure to prevent the effects of simulated weightless on changes in responsiveness of basilar artery from tail-suspended rats was valid.

Animals↗
PubMed2001

[Determination of +Gz tolerance end-point by steady state visual evoked potential].

OBJECTIVE: To study the possibility of applying steady-state visual evoked potential (SSVEP) to detect the end-point of +Gz tolerance. METHOD: When a subject was exposed to +Gz acceleration on a human centrifuge, the SSVEP signals (evoked by LED lamps triggered by 8 Hz pulse train) were recorded. The characters of SSVEP in frequency domain were extracted by adaptive filtering, and a modified T2circ statistic was used to determine the visual obstruct objectively. RESULT: SSVEP had strong relationship with visual obstruct of the subjects during +Gz exposure. CONCLUSION: SSVEP might be used as an objective criterion for determination of end-point of +Gz tolerance and an index of the degree of symptoms as well. But, multi-parameter method seemed still necessary to get reliable estimations.

Acceleration↗
PubMed2001

[Cardiac arrhythmias in pilots under positive (+Gz) acceleration].

OBJECTIVE: To study the relationship between cardiac arrhythmias and autonomic nervous regulation in pilots under +Gz acceleration. METHOD: Dynamic ECG during +Gz exposures in 36 orthostatic intolerance pilots and 62 healthy pilots were analysed and compared. RESULT: The orthostatic intolerance pilots had obviously lower +Gz tolerance and more cardiac arrhythmias. The cardiac arrhythmias could affect +Gz tolerance. CONCLUSION: The cardiac arrhythmias under +Gz acceleration could be taken as an index of evaluating cardiovascular compensatory function and warning against acceleration (+Gz) induced loss of consciousness (G-LOC).

Acceleration↗
PubMed1999

Effects of tea polyphenols on cerebral lipid peroxidation, liver and renal functions in rats after repeated +Gz stress.

OBJECTIVE: To observe the effects of repeated +10 Gz stress on cerebral lipid peroxidation, liver and renal function in rats and the prophylactic effects of antioxidant tea polyphenols (TP). METHOD: Twenty-four male Wistar rats were randomly divided into three groups (n = 8, each): group A (control), group B (+10 Gz), and group C (TP). Group B and C were exposed to repeated +10 Gz stress (each for 30s, onset rate about 0.5 G/s, 3 times/d with +1 Gz 1 min intervals, 3 d/wk, 4 weeks in total), but group A was only submitted to +1 Gz. TP (200 mg/kg) was given orally in group C about 1 h prior to the +Gz experiment, while distilled water was given in group A and B. Lipid peroxidation in the brain, liver and renal functions and serum lipids were determined. RESULTS: As compared with the control, lipid peroxidation in rat cerebral homogenate, mitochondria and cytoplasm was significantly increased (P < 0. 05), and serum creatinine concentration was markedly elevated after repeated +10 Gz stress (P < 0. 01). But, TP had significant inhibitory effect on +10 Gz stress induced peroxidative injury in rat brain and reduced the serum creatinine level. There were no differences of serum triglyceride and total cholesterol concentrations and glutamic-pyruvic transaminase activity among the three groups. CONCLUSION: These results indicated that repeated high +Gz stress could bring about peroxidative injury in brain and harmful effect on renal function, and natural antioxidant TP had significant protective effects.

Acceleration↗