RESUMO
Nine unanesthetized, chronically instrumented, female miniature swine (MS) (avg wt, 39.7 kg) were exposed to head-to-tail inertial load (+Gz) levels of +3, +5, and +7 Gz for 60 s, with and without anti-G-suit inflation. Venous flow (VF) was measured by an electromagnetic flow sensor around the inferior thoracic vena cava at the diaphragm. Central venous pressure (CVP), abdominal venous pressure (AVP), eye-level blood pressure (ELBP), and esophageal pressure (EP) were also measured before, during, and after +Gz. There was a progressive significant decrease from control of both ELBP (P less than 0.001) and VF (P less than 0.05) during the three +Gz exposures, both with and without G-suit inflation. Without G-suit inflation, most of the MS were unable to tolerate +5 and +7 Gz. Although VF was significantly (P less than 0.02) improved by G-suit inflation during +Gz there was no significant difference in VF between the three +Gz levels, with or without G-suit inflation. The MS does a spontaneous straining maneuver (cyclic Valsalva) during +Gz with G-suit support. Using EP as a trigger, the data were grouped as strain or no strain (relaxation). A continuous AVP-to-CVP gradient existed during G-suit inflation, which increased dramatically during no strain with increasing +Gz, and was associated with an increase in VF. Thus, the majority of VF occurred during relaxation between strains, even though relaxation time was shortened as +Gz increased. Although ELBP is obviously dependent on cardiac output and venous return, the progressive reduction in ELBP with increased +Gz loads was not significantly related to changes in VF at the diaphragm which was maintained, although at a reduced rate, by the AVP-to-CVP gradient during G-suit inflation.
Assuntos
Aceleração/efeitos adversos , Gravitação , Trajes Gravitacionais , Hemodinâmica , Animais , Pressão Sanguínea , Pressão Venosa Central , Circulação Cerebrovascular , Esôfago/fisiologia , Feminino , Hipotensão/etiologia , Pressão , Suínos , Porco MiniaturaRESUMO
This study was conducted to determine if preinflation of a standard five-bladder anti-g suit 10 minutes before exposure to a centrifuge simulation of a Space Shuttle reentry would provide significantly better protection against orthostasis than the standard symptomatic inflation regimen. This study differed significantly from prior studies: The rate of g onset was slower, peak g was lower, duration of exposure was longer, and the subjects were dehydrated to mimic conditions observed in astronauts immediately postflight. Preinflation demonstrated physiological advantages as determined by arterial blood pressure and heart rate changes in seven healthy male, experienced centrifuge subjects.
Assuntos
Desidratação/fisiopatologia , Gravitação , Trajes Gravitacionais , Voo Espacial , Aceleração , Adulto , Água Corporal/fisiologia , Centrifugação , Diurese/fisiologia , Frequência Cardíaca/fisiologia , Humanos , Masculino , Fatores de TempoRESUMO
The operation, development, and use of high-performance centrifuges with high G-onset rates, similar to those found in modern fighter aircraft, are discussed. Their performance characteristics are examined, and the USAFSAM centrifuge at Brooks AFB in Texas is then described. Centrifuges for space research and large-radius track centrifuges are considered. The benefits of the latter are discussed.
RESUMO
Five subjects wearing standard USAF anti-G suits and seated at a 65 degrees back angle were exposed to a simulated aerial combat maneuver (SACM) which was repeated 5 times with 4-min 1-G rests between each SACM exposure. The SACM was 122 s in duration with 10 s acceleration peaks of 10, 8, and 6 G; between these peaks, the subject was at 4 G for 15 s. This series of repeated SACM exposure fatigued four of the five subjects. Heart rate and rhythm, arterial oxygen saturation, expired gases, lactate, pyruvate, glucose, CPK enzymes and isoenzymes, blood volume, subjective fatigue measurements, and subject performance were examined relative to the development of fatigue, the energy cost of the M-1, and tolerance to the SACM. All physiologic-metabolic parameters were significantly affected by the repeated SACMs. However, only heart rate changes appeared to be correlated with developing fatigue. A significant amount of energy is required to perform the M-1. Subjects whose energy-metabolic and cardiovascular states are least disturbed by high G exposure are those persons who will perform best and become least fatigued during repeated aerial combat maneuvers.
Assuntos
Medicina Aeroespacial , Fadiga/fisiopatologia , Gravitação , Glicemia/análise , Eletroencefalografia , Metabolismo Energético , Frequência Cardíaca , Humanos , Lactatos/sangue , Consumo de Oxigênio , Piruvatos/sangueRESUMO
G-induced loss of consciousness (G-LOC) is defined as "a state of altered perception wherein (one's) awareness of reality is absent as a result of sudden, critical reduction of cerebral blood circulation caused by increased G force." This phenomenon was first identified in Great Britain in World War I (circa 1918-1919) as "fainting in the air." In the United States G-LOC was first encountered in 1922 during the Pulitzer Trophy Air Race. Although recognized during World War II as an operational hazard for fighter aircraft, the invention of the pneumatic anti-G suit reduced concern about G-LOC at that time. A 1984 survey of pilots of high performance aircraft has shown G-LOC to be an operational problem--probably one that has caused aircraft mishaps for several years. The concern of this panel was to focus on various approaches in reducing the G-LOC hazard.
Assuntos
Transtornos Cognitivos/etiologia , Transtornos da Consciência/etiologia , Gravitação , HumanosRESUMO
Relaxed +Gz rapid-onset tolerances were measured on the human-use centrifuge at the USAF School of Aerospace Medicine, using male research subjects with and without inflated anti-G suits. Tolerance at 6 G.s-1 onset rates were 0.2-0.3 G lower than those measured with 1 G.s-1 onset rates, thus suggesting the existence of another relaxed G tolerance measurement called very high onset G. Baroceptor effect was considered the reason for this difference. Delays of a mean of 3.3 s in inflating the anti-G suit did not change relaxed G tolerances at 6 G.s-1 onset rates; however, with a 4.2-s mean delay, light-loss tolerance criteria occurred sooner during the G exposure. Tolerances to 7 G with 6 G.s-1 onset rates, during which the subjects had to perform the anti-G straining maneuver (AGSM), required a mean delay of 2.8 s in six subjects before a noticeable change in light-loss criteria occurred--a mean delay of 2.0 s resulted in no change in light-loss criteria from zero delay control inflation rates. These results clearly indicate that the inflation of the anti-G suit can be delayed by at least 1 s without compromising anti-G suit protection.
Assuntos
Medicina Aeroespacial , Gravitação , Trajes Gravitacionais , Aceleração , Adulto , Humanos , Masculino , Fatores de TempoRESUMO
The Armstrong Laboratory and its predecessors have conducted responsible animal research in support of USAF operations for nearly three decades. The use of animal models, is essential in research that requires a complex living system, but would be too hazardous to humans. The Laboratory also has aggressively pursued alternatives to the use of animals and improved methods in conducting animal research. Thus far, fewer animals are used currently in some areas of research. Since the human is the focus for Armstrong Laboratory research, human test subjects are used as frequently as possible. As improved noninvasive physiologic monitoring methods become available, humans will be used more extensively.
Assuntos
Medicina Aeroespacial , Alternativas aos Testes com Animais , Medicina Militar , Humanos , Toxicologia , Estados UnidosRESUMO
A static model based on eye-heart vertical distance has been developed which predicts group mean G tolerances relative to the application of any of the following anti-G methods and/or physiologic responses: a) anti-G suit, b) reclined seat, c) anti-G straining maneuver (AGSM), d) positive pressure breathing (PPB), e) gradual onset of G, f) isometric muscular contraction, and g) leg elevation. This model was validated with published data. A variation of this model (derived equation) predicts the amount of AGSM (in mm Hg) required, in combination with any of the anti-G methods/responses at any G level. This calculated effort of AGSM can be equated to level of fatigue and performance decrements. A level of 50 mm Hg or an increase of 2 G in the upright seat was the maximum AGSM recommended for routine use as an anti-G method for operational fighter pilots.
Assuntos
Aeronaves , Gravitação , Pressão Sanguínea , Trajes Gravitacionais , Humanos , Contração Isométrica , Modelos Teóricos , Postura , RespiraçãoRESUMO
A summary of human +Gz tolerance and physical conditioning to simulated aerial combat maneuvering is reviewed as a response to the preceding Science News Note by Dr. Bulbulian.
Assuntos
Medicina Aeroespacial , Gravitação , Educação Física e Treinamento , Humanos , Músculos/fisiologia , Consumo de Oxigênio , Aptidão Física , PosturaRESUMO
Over the last decade, G-induced loss of consciousness (G-LOC) has been recognized as a significant operational problem for pilots of high performance aircraft in both the U.S. Air Force and U.S. Navy. Consequently, government laboratories have initiated research studies to learn more about the G-LOC phenomenon in an attempt to reduce its hazards. Many of these studies require the occurrence of LOC during their conduct. For this reason, animal models have been developed to use in these studies. However, even though sophisticated animal models such as baboons can be taught to perform tasks before and after G-LOC has occurred, these models have deficiencies that can be overcome only by the use of human volunteers who willingly submit to G-LOC studies on the centrifuge. Such human G-LOC studies began in World War II and continue today. These studies are presently conducted without guidelines regarding subject selection, numbers of G-LOCs allowed per subject per unit time, restraint systems, or specific medical examinations required before, during, and after each episode of LOC, or after each completed study. This panel will discuss what is known about the pathophysiology of G-LOC, the limitations of animal models in these types of studies, the possible medical and psychological sequelae, and the legal implications of conducting deliberate G-LOC research. We hope that the information developed by this panel will be useful to laboratory human-use review committees in determining the requirements and the nature of guidelines for conducting such studies.
Assuntos
Medicina Aeroespacial , Gravitação , Inconsciência/etiologia , Aeronaves , Humanos , Pesquisa , Estados UnidosRESUMO
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.
Assuntos
Hipergravidade , Modelos Biológicos , Resistência Física , Medicina Aeroespacial , Trajes Gravitacionais , Humanos , Análise de Regressão , Reprodutibilidade dos Testes , Fatores de TempoRESUMO
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.
Assuntos
Adaptação Fisiológica , Hipergravidade , Modelos Teóricos , Medicina Aeroespacial , Aviação , HumanosRESUMO
Loss of consciousness (LOC) during exposure to +Gz occurs in aircrew flying high-performance aircraft. This phenomenon is responsible for several USAF aircraft losses with accompanying loss of life. It has been recognized as a potential flying problem since 1938. Acceleration-induced LOC results in 15-to 20-s periods of aircrew total incapacitation, amnesia, and clonic spasms. It can occur at relatively low G levels and without symptoms of loss of light (blackout). Operational anti-G equipment and methods are not completely effective in preventing LOC. Considering the present status of G protection, a deficiency in the anti-G straining maneuver is considered to be the primary cause of G-induced LOC. This deficiency usually can be corrected with proper centrifuge training. Our present understanding of G protection suggests that the elimination of LOC as a serious problem for USAF high-performance aircraft operations must include the supination of aircrew to a minimum seat back angle of 60 degrees to 65 degrees.
Assuntos
Gravitação , Inconsciência/fisiopatologia , Aceleração , Circulação Cerebrovascular , Humanos , Hipóxia Encefálica/fisiopatologia , Orientação/fisiologia , Desempenho Psicomotor/fisiologia , Voo EspacialRESUMO
+Gz-induced loss of consciousness (G-LOC) continues to be a threat to aircrew flying high-performance fighter aircraft. All avenues to prevent G-LOC, and to reduce the resulting incapacitation should G-LOC occur, must be explored. Research has begun to accurately quantify all aspects of the G-LOC phenomenon. The emerging pattern from this research indicates that, theoretically, G-LOC incapacitation could be significantly reduced. Comparison of G-LOC with LOC induced by acute arrest of cerebral circulation reveals that the G-LOC incapacitation could be reduced by as much as 17 s. Results also indicate that the relative incapacitation period (confusion and disorientation) following unconsciousness could be reduced by at least 9 s for an individual who has previously experienced G-LOC. This suggests that exposure to G-LOC during centrifuge training could provide this orientation to G-LOC and potentially reduce the incapacitation period should it occur inflight. This exposure may be likened to the current altitude-hypoxia training requirement for aircrew. Experience to date supports the contention that such training may be accomplished with an acceptable safety margin.
Assuntos
Medicina Aeroespacial , Gravitação , Educação Física e Treinamento , Inconsciência/fisiopatologia , Adolescente , Adulto , Atitude , Circulação Cerebrovascular , Humanos , Masculino , Inconsciência/psicologiaRESUMO
A short-arm centrifuge (SAC) of 5-6 ft (1.5-1.8 m) radius may be useful in space to measure tolerances to acceleration (G) and to stimulate the cardiovascular system, thereby reducing cardiovascular decompensation that occurs in weightlessness. Relaxed rapid (1G/s onset rate, ROR) and gradual (0.1 G/s onset rate, GOR) G tolerances were measured on seven men using a 5-ft (1.5 m) radius centrifuge and compared with their G tolerances obtained on the 20-ft (6.1 m) radius human-use centrifuge at the Armstrong Laboratory, Brooks AFB, TX. Since the subjects were required to flex their legs to assume a squatting position on the SAC, a similar position was used on the 20 ft (6.1 m) centrifuge called feet up (FU), and compared with normal-seated +Gz tolerances (controls). The subjects tolerated the SAC exposures without any problems. ROR and GOR tolerances were as follows: control, 3.6 G and 4.2 G; FU, 4.5 G and 5.6 G; and SAC, 4.6 G and 6.4 G. We concluded that a 5-ft radius centrifuge can be used to measure G tolerances. The increases in the SAC GOR tolerances over ROR tolerances indicate that the baroreceptors were stimulated by the G, and the SAC exposure would be useful in preventing cardiovascular decompensation in microgravity.
Assuntos
Gravitação , Voo Espacial , Adulto , Estudos de Avaliação como Assunto , Frequência Cardíaca , Humanos , Masculino , Rotação , Ausência de PesoRESUMO
Adult miniature swine were exposed to various levels and durations of +Gz. After exposure, all swine were euthanized and necropsied. Gross, histologic, and electronmicroscopic observations were made on the heart tissue. Subendocaridal hemorrhage (SEH) was commonly found in the left ventricle, rarely in the right ventricle, and its severity was directly related to : a) level and duration of G exposure, b) heart rate, and c) catecholamine activity. SEH was made more severe with i.v. atropine 4 mg, and prevented with i.v. propranolol 20 mg. Heart hemorrhage was usually limited to the immediate subendocardial region and frequently surrounded Purkinje's fibers. In severe cases, however, hemorrhages penetrated several millimeters into the heart muscle and sometimes penetrated Purkinje's fibers. Restraint of unanesthetized swine in the centrifuge couch, low G-levels, and/or i.v. injections of atropine or epinephrine produced minimal SEH lesions.
Assuntos
Cardiomiopatias/patologia , Gravitação , Hemorragia/patologia , Miocárdio/patologia , Animais , Atropina/farmacologia , Feminino , Propranolol/farmacologia , Voo Espacial , Suínos , Fatores de TempoRESUMO
Discrete, finite, Fourier transforms of input +Gz stress and output arterial oxygen saturation (Sao2) were used to obtain transfer functions between G and Sao2 in 12 subjects. Ensemble averaging of transfer functions based on responses to variable G stress provided reasonable capability for predicting Sao2 responses to sustained 6-G stress. Prediction of responses to variable G stress from transfer functions based on responses to sustained G stress was unsuccessful, however. A synthetic transfer function with physiologic explicability and substantial predictive ability was obtained from an impulse response fabricated from simple mathematical functions.
Assuntos
Medicina Aeroespacial , Gravitação , Oxigênio/sangue , Artérias , Análise de Fourier , Humanos , Modelos BiológicosRESUMO
The pathology of +Gz acceleration was examined using unanesthetized adult miniature and immature "farm-type" swine, with and without anti-G suit inflation. Following single exposures of +8 or 9 Gz for 45 to 90 s--acceleration exposures that have been shown "tolerable" to man--swine were sacrificed and a detailed necropsy performed. Considering only the adult miniature swine, the endocardial area of the left ventricles showed evidence grossly of recent hemorrhage of varying severity involving both the wall and papillary muscles. The degree and location of the subendocardial hemorrhage were quantitated by grading the area of ventricle involved--1 (slight) to 4 (extensive). Of the 23 adult miniature pigs autopsied, the scores for papillary muscle hemorrhage, after one exposure to +9 Gz (45 to 90 s) ranged from a mean of 2.3 to 3.3 and the extent of ventricular wall involvement was 2.5 to 3.3. Histologically, heart hemorrhage was limited to the subendocardial area, primarily involving the space between heart muscle and the endocardium and was particularly evident surrounding Pukinje's fibers. Similar studies using immature farm-type swine (not miniature pigs) found these younger swine (4 to 5 months of age) to be less susceptible to such endocardial hemorrhage. Heart tissue recovery in these pigs following one exposure to +9 Gz for 45 s, required approximately 14 d. It appears that this lesion is similar, although less severe, than heart muscle lesions associated with loss blood volume (hemorrhagic shock) studies and may have similar physiologic bases. It was concluded that particular attention should be made of the endocardium of victims of high-performance aircraft accidents.
Assuntos
Gravitação , Cardiopatias/patologia , Miocárdio/patologia , Choque Hemorrágico/patologia , Fatores Etários , Animais , SuínosRESUMO
The effect of both assisted and unassisted positive pressure (PPB) breathing on +Gz tolerance was evaluated. A GOR (gradual onset rate = 0.1 G/s) acceleration profile was used to evaluate five different experimental conditions: 1) G-suit-only, 2) G suit + unassisted PPB, 3) G suit + PPB assisted by the English jerkin, 4) G suit + PPB assisted by the Canadian waistcoat, and 5) the Swedish counterpressure garment G suit combination. Acceleration tolerances for these experimental conditions (excluding the Swedish garment) with the subjects performing M-1 maneuvers were measured using an exhaustive ACM (Aerial Combat Maneuvers) acceleration profile. GOR tolerances were similar for both assisted and unassisted PPB with the pressure breathing experimental conditions providing a significant GOR tolerance increase over the G suit only condition. Assisted PPB increased ACM tolerances over both unassisted PPB and G suit only conditions. It appears that assisted PPB may offer a practical method for reducing the fatigue associated with exposure to high G.
Assuntos
Medicina Aeroespacial , Gravitação , Respiração com Pressão Positiva , Trajes Gravitacionais , Frequência Cardíaca , Humanos , Oxigênio/sangueRESUMO
An acceleration profile was developed on the USAF School of Aerospace Medicine's 6.1-m radius centrifuge to measure human tolerance to the Aerial Combat Maneuver (ACM). The ACM profile is a continuous repetitive, 4.5 G for 15 s to 7 G for 15 s, cyclic G exposure which is terminated by the subject at his fatigue endpoint. ACM tolerances using this type of G profile were determined for seven subjects at four different seatback angles; i.e. 13 degrees, 30 degrees, 55 degrees, and 65 degrees from the vertical. Group (mean +/- S.E.) tolerances for the ACM were 170 +/- 17 s at 13 degrees and 541 +/- 48 s at 65 degrees. These tolerances were not usually correlated with relaxed gradual onset G tolerances. The subjective fatigue endpoint was physiologically verified using heart rate, heart rhythm, and performance criteria at the four seatback angles. The potential value of the ACM profile is considered as a measure of the effectiveness of anti-G equipment and methods in the aerial combat environment.