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At least 811 records · Page 45Linked to original sources
PubMed2000

Cerebral blood flow velocity by transcranial Doppler during a vertical-rotating table simulation of the push-pull effect.

BACKGROUND: The push-pull effect (PPE) has been suspected of causing many aircraft accidents. The perfusion and then withdrawal of cerebral blood during the PPE may change the state of the cerebral blood vessel. HYPOTHESIS: During head-down tilt (HDT) cerebral vasoconstriction occurs in response to the elevated perfusion pressure to maintain cerebral blood flow, and during subsequent head-up tilt (HUT) the increased resistance of the cerebral blood vessel recovers slowly. METHODS: Ten healthy male non-pilots were exposed to the following protocol using a rotating-table to simulate the push-pull maneuver: HUT (+1 Gz) for 1 min followed by transition to HDT (-1 Gz) 10 s followed by transition to HUT (+1 Gz) 1 min. Cerebral blood flow velocity and pulsatility indices in the left middle cerebral artery were continually measured with a transcranial Doppler (TCD) instrument. RESULTS: Mean blood flow velocity (Vm) increased significantly by 10%, during the first 5 s of HDT, recovered to baseline during HDT 5 10 s, and remained unchanged during subsequent HUT. Systolic blood flow velocity (Vs) increased by 9% during HDT 5-10 s and 11% during HUT 0-5 s. Diastolic blood flow velocity (Vd) decreased by -9% during HDT 5-10 s, and -22% during HUT 0-5 s. Vs-Vd increased by 26% during HDT 5 10 s, and 41%, during HUT 0-5 s. Pulsatile indices (PI) and resistance index (RI) increased by 26%) and 15% during HDT 5-10 s, and by 40% and 27% during HUT 0-5 s, respectively. Vs, Vs-Vd, PI, and RI remained at the higher level, and Vd remained at the lower level to HDT 15-20 s. CONCLUSIONS: The results indicate that cerebral vasoconstriction occurred to prevent brain over-perfusion during HDT. During HUT, the elevated resistance of the cerebral vessel remained at the higher level for about 20 s, and may have worsened the cerebral perfusion from exposure to +Gz. This may be one of the mechanisms of PPE.

Adaptation, Physiological↗
PubMed2000

Further support for the concept of a G-LOC syndrome: a survey of military high-performance aviators.

METHODS: Some 329 military high-performance pilots were anonymously surveyed to determine the occurrence rates for a symptom complex of acceleration-induced neurologic manifestations. The premise for this symptom complex is the theory that acceleration-induced neurologic effects are not always an all-or-none phenomenon with G-LOC as the operational endpoint. RESULTS: A significant number of aircrew in selected types of aircraft reported symptoms such as euphoria, apathy, displacement, depersonalization, poor response to auditory stimuli, immediate memory difficulties, sensory abnormalities, motor abnormalities, confusion, and dream-like state without loss of consciousness. CONCLUSION: These findings may signal a need for alterations in G-awareness training.

Adult↗
PubMed2000

Pressurized sleeves and gloves for protection against acceleration-induced arm pain.

BACKGROUND: Acceleration (or G) induced arm pain may develop in centrifuge runs and in flight with low arm position and assisted pressure breathing during G (PBG) in combination with an extended coverage anti-G suit. To decrease this arm pain, pressurized sleeves and gloves were developed. METHODS: Eight subjects who earlier exhibited G-induced arm pain were tested on the centrifuge. The G-exposures consisted of a gradual onset run up to a maximum of +9 G2, rapid onset runs to +3, +4, +5, +6, +7, +8, and +9 Gz and a simulated aerial combat maneuver (SACM) with peaks up to +9 Gz. On separate days, the subjects were tested without the sleeves and gloves, and with the sleeves and gloves pressurized to a maximum of 40, 60, or 80 mmHg at +9 Gz. The subjects reported their left and right arm pain on a subjective rating scale. RESULTS: G-induced arm pain, usually starting above +6 Gz, was often the reason for termination of the G-exposure without the pressurized sleeves and gloves. The pressurized sleeves and gloves significantly (p < 0.001) decreased arm pain, put no significant difference was found among the different pressures used. Heart rate was not different with and without the pressurized sleeves and gloves. CONCLUSIONS: The pressurized sleeves and gloves are an effective method to alleviate and sometimes eliminate G-induced arm pain.

Adult↗
PubMed2000

Mathematical models for predicting G-level tolerances.

The mathematical models developed in this article predict the following human G-level tolerances: 1) rapid onset relaxed (ROR); 2) gradual onset relaxed (GOR); and, 3) straining-rapid onset. Included in the model are specific functions of: 1) anti-G suit; 2) positive pressure breathing (PBG); 3) baroreceptor reflex; 4) handgrip reflex; 5) anti-G straining maneuver (AGSM) increasing intrathoracic pressures (Pi); 6) leg elevation; and, 7) reclining seatback angles < or = 55 degrees. These functions are based on sound physiologic principles. Also discussed in the development of this model, but not included in the models, were: 1) isometric muscle contraction reflex; 2) Qigong (Q-G) maneuver; and, 3) straining GOR tolerances. The straining GOR tolerance profile was calculated to be a measure of G-duration tolerance and not G-level tolerance. A maximum P of 125 mm Hg from the AGSM was used in these models that could be augmented with PBG to 185 mm Hg. G-level tolerance predictions using this model were validated with published data.

Adaptation, Physiological↗
PubMed2000

A mathematical model of cerebral perfusion subjected to Gz acceleration.

BACKGROUND: When the human body is exposed to a high gravitational load, the blood supply to the brain is reduced and loss of consciousness may occur. Our goal is to identify the principal mechanical causes of reduced blood supply to the brain during high +Gz. METHODS: We have developed a mathematical model to investigate the influence of Gz on the cerebral circulation. Blood flow is modeled using a one-dimensional flow approximation, in which the cross-sectional area of elastic vessels is determined as a non-linear function of the transmural (blood minus external) pressure. The intracranial vessels are subjected to cerebrospinal fluid pressure (PCSF) which is determined from the condition that the cranial volume is conserved. RESULTS: For a constant pressure difference of 100 mm Hg applied to the arterial and venous ends of the model, blood flow is diminished for +Gz. At approximately +5 G, the blood flow predicted by the model is insufficient to maintain normal functioning of the brain. PCSF is approximately equal to the blood pressure in the large intracranial veins for all values of Gz. Extracranial arteries and the intracranial vessels do not collapse, even when Gz is substantially higher than normal. However, the extracranial veins are collapsed even for moderate +Gz. CONCLUSIONS: Even if cardiac output is maintained at normal levels, cerebral perfusion will fall because of the increasing resistance of the cerebral flow circuit. This increase is largely due to the collapse of the extracranial veins, which begins at moderate Gz and becomes dominant at a Gz of approximately 4.5.

Adaptation, Physiological↗
PubMed2000

The experience of nausea during sustained hyper-gravity flight with negligible angular velocity.

BACKGROUND: The purpose of this study was to investigate nausea and vomiting during hyper-gravity flight at a slow rate of turn. During head-movements under these conditions, the semi-circular canals of the vestibular system function normally whereas the otoliths experience a G-excess effect, displacing further and moving faster than in a 1 G field. HYPOTHESIS: Nausea and vomiting are greater during hyper-gravity flight compared with historical data collected in a 1 G field at a similar rate of turn. METHODS: There were 27 subjects who were exposed to 1.8 G (hyper-G) during a slow rate turn on three NASA KC-135 flights. Subjects participated in one of three experimental periods, each period consisting of two 5.5-min hyper-G runs: 1) rest, with only incidental head movements; 2) active roll and pitch head movements; or 3) passive roll head movements. Subjective symptom data were collected pre-flight, following each experimental period and post-flight using a standardized nausea questionnaire. Electrogastrograms were obtained from eight subjects. RESULTS: Seven subjects (26%) vomited during the flight. Nausea initially increased but was then stable throughout the flight. Nausea appeared highest during active head movements. In subjects who vomited, greater gastrointestinal distress and somatic distress were reported. CONCLUSIONS: The levels of nausea and vomiting observed during hyper-G cannot be explained by Coriolis cross coupling and are likely due to the G-excess effect on the otoliths. The nausea profiles observed in individuals who vomit during hyper-G appear similar to those previously observed during nausea produced by an optokinetic drum stimulus in a 1-G field.

Adult↗
PubMed2000

Release from masking in virtual auditory space during sustained positive acceleration.

BACKGROUND: The use of a three-dimensional (3-D) auditory display to significantly lower subject detection level while maintaining comprehension under sustained positive G-stress was explored in this study. METHODS: Auditory threshold levels were measured for detecting a band limited pulsed signal in the presence of a broadband diotic masker at both + 1 Gz (rest position) and under sustained +3 Gz. The pulsed signal was presented diotically and was spatialized at one of four static azimuth positions on the horizontal plane. RESULTS: Results showed that auditory thresholds were not significantly affected by sustained +3 Gz stress. Compared with a diotic presentation, subjects reached an average of 6.8 dB lower auditory threshold at +1 Gz and under sustained +3 Gz when the pulsed signal was spatialized at a static position of 90 degrees azimuth on the horizontal plane. CONCLUSION: The implication of these results and suggestions for further research are discussed.

Adult↗
PubMed1999

Kinematic and dynamic analysis of running under conditions of variable gravity.

A new training system was designed and realised by the engineer G. Scuderi: a centrifugal track for runners. The principal advantage of this track is to increase the forces on athlete during the run with an effect very similar to that of an "artificial gravity", so the athlete can develop more muscle power. A multibody numerical model of a runner was developed to analyse the behaviour of the athlete on the centrifugal track. The multibody model was calibrated by experimental analysis; in fact, the joint angles were measured by a digital image processing system and introduced as input in the numerical simulation. The numerical results obtained are the values of many kinematic and dynamic variables, in particular the ground reaction force and the moments in the hip, knee, and ankle joints. A comparison between the normal running and that on the centrifugal track was carried out, demonstrating the validity of training on the latter. The results of the numerical simulation confirm that the power developed during running on the track, and the corresponding work performed by the joints, is considerably greater than that found during normal running.

Ankle Joint↗
PubMed2000

[A model system for the assessment of the stability of eukaryotic genes and expressed proteins in extended space flight].

Evidence was obtained that the changed gravity, tension profiles of the magnetic fields inside the orbital station and other spaceflight factors (SFF) substantially influence the cell genome and synthesis of recombinant proteins. The authors proposed a technique of fixation of possible alterations in genes and expressed recombinant proteins in strains-producers of human alpha-interferon: HuIFN-alpha 2b, HuIFN-alpha 8a, HuIFN-alpha 10a, and HuIFN-alpha 14a. Spaceflight factors were simulated by way of gamma-irradiation at 10 Gy and 50 Gy by a cobalt unit, and centrifugation of samples at 10 g and 50 g. Cultivation of the strains-producers during the SFF simulation yielded HuIFN-alpha proteins with altered functional characteristics. Following exposure to simulated SFF, strains-producers expressed HuIFN-alpha that preserved a high titre of antiviral activity (AVA) but suppressed the antiproliferative activity (APA) inferring some structural/functional shifts in the HuIFN-alpha molecule due to, presumably, mutations in the active center responsible for APA. Determination of species specificity of the HuIFN-alpha recombinant proteins following exposure to SFF revealed dissociation of cross-AVA in homologous and heterologous cell cultures which can be also attributed to the structural/functional shifts in the HuIFN-alpha molecule. These changes can be localized in the receptor cluster of molecule or consequent to modification of the center defining HuIFN-alpha species specificity. The proposed simulation system allows fixation of shifts in the HuIFN-alpha structural/functional characteristics and investigations of the stability of eucaryotic genes in long-term space flights.

Cell Movement↗
PubMed2000

[The structural organization of the normal rat area postrema and under conditions of chronic exposure to gravitational loads].

Topography and structure of rat area postrema were specified using light optic and electron microscope. Disturbance of liquor-encephalic transport (hydrops of ependymocytes), sharp changes of cytoplasmic organelles in neurons, signs of intercellular desintergration (synapse degeneration after "clear" type), death of part of neurons, disorder in secretory processes in neuroendocrine cells were shown to occur following chronic effect of gravitation overloads, in its structural elements. These changes take place on the background of hemomicrocirculatory disturbances, appearance of connective tissue fibres in perivascular space of Virchow-Robin and are probably conditioned by a developing circulatory hypoxia.

Animals↗
PubMed2000

Mathematical models for predicting G-duration tolerances.

Mathematical models that predict fatigue-based G-duration tolerances for relaxed and straining subjects are developed and validated using published data. These models are based on regression analysis calculations using published G-duration tolerance data of relaxed subjects exposed to 3-5 G and subjects exposed to 6-9 G using an anti-G suit and performing the anti-G straining maneuver. These G-duration models are derived from published G-level tolerance models based on intravascular hydrostatic pressures and physiologic responses to maximum voluntary contractions (MVC%). Included in the validation of these models are the baroreceptor and muscle contraction cardiovascular reflexes that support arterial BP. A basic energy pool that supports a G-duration of 140 s for G exposures > 5 G is theorized. Because of the long duration of sustained G exposures in these models, the physiologic dynamics involved in predicting straining G-duration tolerances, are identified and validated using different time periods, i.e., Phases I and II. These models, based on sustained G exposures to a constant G level are also applicable to exposures of variable G levels known as simulated aerial combat maneuver (SACM) G-profile tolerances. G-duration tolerances > 9 G are predicted using these models for subjects using reclined-seat backs and positive pressure breathing.

Aerospace Medicine↗
PubMedDate not supplied

Influence of acceleration over the hydroelectrolytic homeostasis in audiogenic seizure-prone rats compared to normoexcitable rats.

This research concentrated on the hydroelectrolytic balance response after centrifugation (+5G/30 minutes, five times) in two lots of rats: one lot made up of animals with normal cerebral excitability, and another one of animals prone to audiogenic seizure. Both before and after centrifugation, we determined: water (ml/24 hrs.) and sodium chloride (mEq/24 hrs.) consumption; dependence of sodium, potassium and water elimination on the amount of sodium ingested (mEq/24 hrs.). We also determined the renal capacity of sodium concentration and the mineral-corticoid response based on the urine Na/K ratio and on the determination of plasma renin activity (PRA). Data obtained show that audiogenic seizure-prone rats consume less sodium and water after centrifugation, unlike normoexcitable rats in which there have been no differences in this respect. Exposure to hypergravitation induces in both lots an increase of sodium, potassium and water elimination. Renal elimination of water is greater in the normoexcitable animals than in the seizure-prone ones. By contrast, sodium elimination is greater in the audiogenic seizure-prone animals. Urine sodium concentration is lower in the seizure-prone animals, consistent with the amounts of water eliminated. Their mineral-corticoid response is intensely diminished after centrifugation in comparison to that of the control, normo-excitable animals. PRA is diminished in both lots. Our findings support the assumption that seizure-prone rats are unable to adaptatively respond to acceleration by preserving sodium through the intervention of cortico-surrenal mechanisms. As water is not preserved, it seems that in this category of animals, both during and after centrifugation, it is mostly the hypothalamo-retrohypophyseal mechanisms that come into play. This could be the outcome of disturbances occurring in the mechanisms of blood sodium transport that are deficient in the seizure-prone animals.

Acceleration↗
PubMed2000

[Gravitation sensitivity of human endothelium].

Experiments on the effects of hypogravity (clinostatting) on growth and formation of monolayer of cultivated endothelium cells of the human umbilical vein demonstrated sensitivity of endothelium to the gravitational stimulus as it responded by a significant reduction of the proliferative activity of cells in culture. The most favorable conditions for cultivating endothelium cells under extended (15-d) hypogravity in vitro were determined.

Adaptation, Physiological↗
PubMed2000

[Technique for registration of auricular pulsogram in macaca mulatta monkeys].

An upgraded technique for registration of auricular pulsogram in monkeys (Macaca mulatta) has been proposed to study shifts in the common artery bed. The measurement is made with a photo-electrical sensor taped to the cartilage of the floor of the auricle and secured with a special cap and helmet of elastic net, and a system for data transfer from the sensor to a register enhanced by preamplifiers. Potentialities of the technique are exemplified by the investigation of Macaca mulatta's tolerance of +Gz loads.

Animals↗
PubMed2001

Dynamic strength capabilities of small-stature females to eject and support added head weight.

BACKGROUND: Naval Air Warfare Center Aircraft Division investigated the abilities of small-stature females (< or = 120 lb) to fly under G-stress using the Dynamic Flight Simulator (DFS) and its tactical fight/attack cockpit, displays and controls. OBJECTIVES: Determine ability to exert NACES ejection seat actuation pull force under static, acceleration and simulated flight conditions; support up to 5 lb of added head weight (AHW) under catapult, arrestment, and aerial combat maneuver G-loads; and reach all controls. METHODS: Seven female subjects (six small and one medium stature) participated. The AHW task included three helmet weights, 3.5 lb (standard configuration), 4.25 lb and 5 lb and subjects were tasked to accurately read cockpit displays. Muscular exertion and fatigue (arm, shoulder, neck) assessment used electromyography (EMG). Limits in overall reach and throttle and stick movements were measured. RESULTS: Subjects successfully ejected using a two-hand grip under G-stress. Subjects read all displays supporting 5 lb under +6 Gz. Most small-stature subjects could not fully support their heads wearing 3.5 lb helmet during flat spin conditions. CONCLUSIONS: Within the scope of these tests, small-stature subjects demonstrated the strength to safely initiate ejection during severe physically-taxing dynamic conditions but had difficulty supporting AHW under -Gx stress. Human factors deficiencies were noted in the areas of torso harness fit, inertia reel placement relative to shoulder width, and the ability maintain a full range of stick motion.

Adult↗
PubMed2001

Simulated shuttle egress: comparison of two Space Shuttle protective garments.

BACKGROUND: In a previous study from our laboratory, we observed carbon dioxide (CO2) accumulation in the helmet of the NASA Launch and Entry Suit (LES) during a simulated emergency egress from the Space Shuttle. Of 12 subjects, 8 were unable to complete the egress simulation with a G-suit inflation pressure of 1.5 psi. The purpose of this report was to compare CO2 accumulation and egress walking time in the new Advanced Crew Escape Suit (ACES) with that in the LES. METHODS: Four male subjects who previously were unable to complete the egress in the LES performed a simulated egress while wearing the ACES with the G-suit inflated to 1.5 psi. The egress simulation consisted of 6 min of seated rest, 2 min of standing, and 5 min of walking on a treadmill at 1.56 m x s(-1) (3.5 mph) and 0% grade. The helmet visor was closed with the subjects receiving 100% oxygen throughout the simulation. Inspired CO2 and walking time were measured. RESULTS: The rate of CO2 accumulation was significantly less (ACES: 0.53 +/- 0.03, LES: 1.07 +/- 0.15 %CO2 x min(-1); p = 0.05) and walk time was greater in the ACES (ACES: 5.0 +/- 0.0, LES: 2.7 +/- 0.2 min; p = 0.002). CONCLUSIONS: Changes in the design of the ACES from the LES resulted in a decreased rate of CO2 accumulation and an improved egress walking time compared with the LES.

Adult↗