The effect of +Gz offset rate on recovery from acceleration-induced loss of consciousness

The incapacitation resulting from +Gz-induced loss of consciousness (G-LOC) depends on the magnitude of the ischemic/hypoxic insult to the central nervous system (CNS). This magnitude is defined by the rate of onset of the +Gz, the +Gz level, the length of time at +Gz, the offset rate, and individual tolerance to +Gz. Offset rates have rarely been emphasized or even reported in +Gz research. This study compared the incapacitation between two groups of asymptomatic men resulting from generally similar rapid onset (greater than 3G/s) +Gz exposures to induce G-LOC but with different +Gz offset rates. For one group (N = 90) of G-LOC exposures the offset rate was 0.97 G/s and for the other group (N = 17) the offset rate was 2.75 G/s. The incapacitation following G-LOC with slower offset resulted in an absolute incapacitation period of 10.47 +/- 3 s, a relative incapacitation of 14.40 +/- 10.05 s, and a total incapacitation period of 25.04 +/- 10.13 s. With the more rapid offset G-LOC exposures the absolute incapacitation was 7.59 (+/- 3.14)s, relative incapacitation 5.40 +/- 3.38 s, and the total incapacitation was 13.20 +/- 4.36 s. The rate of +Gz offset also affects the time following G-LOC before onset of myoclonic convulsions. Rapid offset G-LOC exposures had a shorter period from the onset of unconsciousness to the onset of convulsions as compared to slower offset rates. The convulsion period, however, remained essentially the same. The results strongly favor an ischemic/hypoxic mechanism for G-LOC. The results also document the importance of offset rates in determining the magnitude of the ischemic-hypoxic insult to the CNS.(ABSTRACT TRUNCATED AT 250 WORDS)     

一种新型囊式抗荷系统的+Gz防护作用

目的在离心机上,研究新型囊式抗荷系统的抗荷性能,评估其抗荷作用.方法 6名男性被试者采用本囊式抗荷系统不做抗荷动作(AGSM),暴露于10～15 s的梯形+Gz曲线后,再暴露于+5～9 Gz 模拟空战动作曲线(SACM) ,并伴随腿部适度收紧.结果得到了该先进囊式抗荷系统的+Gz防护作用和被试者的+Gz时间耐力.本抗荷系统的抗荷效果为5.33±0.26 G,均顺利通过了+5～9 Gz SACM,且用力程度和疲劳程度均较低.结论本抗荷系统+Gz防护效果良好,其配套方案可行.     

Hemodynamic responses to seated and supine lower body negative pressure: comparison with +Gz acceleration.

Hemodynamic changes between upright and supine lower body negative pressure (LBNP) to levels of -70 mm Hg were compared in 8 subjects (5 males, 3 females) and correlated with their findings during simulated Shuttle reentry acceleration with a slow onset rate of 0.002 G/s (1,020 s to peak +2 Gz) and during gradual onset exposures (0.03 G/s) to +3 Gz and +4 Gz. Six of the 8 subjects were able to tolerate 2 min at peak +2 Gz, 2-5 min at +3 Gz, and 1-2 min at +4 Gz. Heart rate (HR) at any given level of upright LBNP regularly exceeded supine levels. HR change at -50 mm Hg in upright subjects (+47.7 bpm from 74.1 +/- 1.9 (M +/- S.E.) bpm, control) was 2.6 times greater than in supine subjects (+18.3 bpm from 64.8 +/- 2.8 bpm, control). HR values at -40 mm Hg supine (73.7 +/- 2.6) matched seated upright pre-LBNP control levels (74.1 +/- 1.9 bpm), while values at -70 mm Hg supine (102.5 +/- 4.4 bpm) were not significantly different from those at -40 mm Hg upright (103.1 +/- 4.0 bpm). Peak HR during +3 Gz (145.8 +/- 7.7 bpm) and +4 Gz (152.3 +/- 6.5 bpm) significantly exceeded recorded supine and upright LBNP levels, whereas values at +2 Gz (104.8 +/- 5.5 bpm) closely matched those at -40 mm Hg upright (103.1 +/- 4.0 bpm) and -70 mm Hg supine (102.5 +/- 4.4 bpm). Supine LBNP HR changes in this relatively small group of subjects closely matched those previously reported in the literature.(ABSTRACT TRUNCATED AT 250 WORDS)     

Optimum sampling times for maximum blood lactate levels after exposures to sustained +Gz

Six male subjects wearing anti-G suits were exposed to +4.5 Gz and +4.5-7 Gz simulated aerial combat maneuvers (SACM), sustained until terminated because of fatigue. Before and after each G exposure, venous blood was withdrawn (using the finger-prick method) and analyzed for lactate concentration. Five samples were taken serially, at 1, 3, 6, 10, and 20 min after each G exposure to determine the maximum (peak) level of blood lactate. Individual lactate levels varied, with peaks at 1, 3, and 6 min; however, group levels were maximum at 3 min after the G exposure. Maximum lactate levels (mean +/- S.D.) of 27.8 +/- 11.3 mg% and 42.7 +/- 19.4 mg% were found for the 4.5-G and SACM exposures, respectively. Lactate recoveries were a simple exponential function, with a half life of approximately 10 min. The relationship between these data, following G exposure, and those measured after aerobic physical activities, is discussed.     

Performance and physiological effects of acceleration-induced (+ Gz) loss of consciousness

Loss of consciousness (LOC) was intentionally induced by exposing eight volunteers to individually-titrated levels of head-to-foot acceleration (+ Gz) using 2- and 4-s onset rates (mean = 6.1 + Gz required to induce LOC) and a gradual, .067 G X s-1 onset rate (mean = 7.2 Gz required). Subjects were trained over a prior 2-week period on a multitask battery comprising three simultaneously executed tasks representative of those required in piloting, and then centrifuged to LOC at each of the three onset rates on alternate days. Performance was assessed for 5 min prior and 7 min after each LOC. Primary results indicated: a) significant and substantial impairment in the two discrete response secondary tasks (choice reaction time and arithmetic computation), with mean recovery to pre-LOC levels within 3 min on each task, b) no group mean impairment for the primary, compensatory tracking task, c) substantial individual variation in physiologically and behaviorally defined recovery from LOC, d) a negative influence of aerobic fitness on G tolerance and LOC recoverability, and e) that recovery effects were not generally dependent upon onset rate. Mean absolute incapacitation (head dropped) for the rapid onset rates was 12.1 s. For the gradual onset rate, mean absolute incapacitation was 16.6 s. Mean relative incapacitation (head erect, no voluntary task engagement) for the rapid onset rates was 11.6 s; for the gradual onset rates, mean relative incapacitation was 15.7 s. Evidence for retrograde amnesia effects was equivocal.     

不同压力制度抗荷正压呼吸的+Gz防护作用

目的探讨抗荷正压呼吸(PBG)系统中最佳的PBG压力制度. 方法 5名被试者穿GZ-2抗荷服及中囊代偿背心,在快增长率(3.0 G/s)离心机+Gz暴露时分别进行了1.6、2.4及 3.2 kPa/G 3种压力制度PBG实验.分别测定了3种压力制度PBG的抗荷性能,并对主观感觉进行评分.结果 1.6、2.4及 3.2 kPa/G 3种压力制度PBG的抗荷性能分别为2.00±0.31、2.54±0.32及2.44±0.31G;主观感觉评分表明,2.4 kPa/G压力制度PBG优于其它两者. 结论 PBG系统的压力制度以采用2.4 kPa/G为最佳.这样,既可保证PBG的抗荷性能,又可减轻PBG的副作用.     

Concurrent loss of consciousness and sino-atrial block during +Gz stress.

Current USAF fighter aircraft easily exceed human physiologic limits with their rapid onset of head-to-foot acceleration forces (+Gz). Sudden in-flight incapacitation caused by these increased +Gz forces could be disastrous with loss of materiel and human life. The physiologic mechanisms responsible for loss of consciousness (LOC) secondary to high +Gz must be fully understood so that maximum protection against it can be provided. An interesting case of an episode of LOC with concurrent sino-atrial block occurring during a relaxed rapid onset (1 G/s) centrifuge run is presented. The patient was undergoing flight medical evaluation for an episode of syncope, etiology unknown. An unusual characteristic of the patient was his high level of endurance training. The possibility of an excessive increase in vagal tone, developed by endurance training, is discussed as a probable etiology for this patient's prolonged time of incapacitation evidenced after +Gz-induced loss of consciousness.     

Endurance and performance during multiple intense high +Gz exposures with effective anti-G protection

BACKGROUND: In many high-intensity wartime scenarios, pilots may be required to fly multiple, strenuous missions during the same day. HYpothesis: New anti-G protection allows fighter pilots to endure multiple high +G, exposures during several sorties in a limited time interval. METHODS: Nine well-trained centrifuge subjects were exposed to tactical aerial combat maneuvers using balanced pressure breathing during G (PBG) and an extended coverage anti-G suit. The centrifuge exposures consisted of five simulated sorties during a 4-h period, each consisting of four engagements with rapid onset cycles (6 G x s(-1)) varying between +4 G, and -9 GC. The subjects executed a tracking task before, during, and after each engagement and verbally indicated their effort level. Neck muscle strength was measured before and after the test. RESULTS: Seven of the nine subjects could endure all five sorties during the 4-h period. On a scale from 0 (no effort) to 11 (maximum possible effort), highest effort level during runs varied from 5.5 to 11 units (mean 8.7). Maximal heart rate varied from 140-173 bpm (mean 159) and minimum finger oxygen saturation from 75-93% (mean 88). Maximal peripheral and central light-loss varied from 0 to 100% (mean 71 and 40, respeclively). Three G-LOCs (two in the same subject) and four cases of near loss of consciousness occurred. The general fatigue recovery time varied from 9 to 48 h (mean 21). The tracking tests showed that performance deteriorated significantly during all G exposures; the neck muscle contraction was impaired by 12% (p = 0.035) after the C exposures. CONCLUSION: It is possible to train subjects to withstand five simulated flight sorties during a 4-h period with a total of up to 80 peaks to +9 Gz and 80 peaks to +8 Gz using PBCG and an extended coverage anti-G suit.     

Protection to +12 Gz

BACKGROUND: The U.S. Air Force has developed +Gz-protective equipment that will provide most pilots protection to +9 Gz with minimal-to-no straining. This equipment includes a pressure breathing system called COMBAT EDGE (CE), which is currently operational, and the Advanced Technology Anti-G Suit (ATAGS), which is not yet operational. For future high-performance aircraft design it is important to know the upper limit of various protective equipment and techniques. METHODS: Six subjects were randomly exposed to a 12-cell matrix composed of +Gz and the following combinations of protective equipment at three seat-back angles (13 degrees, 30 degrees and 55 degrees from the vertical): 1) the standard CSU-13B/P anti-G suit (STD); 2) the STD suit with CE; 3) the ATAGS; and 4) the ATAGS with CE. Relaxed, followed by straining +Gz tolerance was determined using 15-s rapid onset runs to a maximum of +12 Gz. A comprehensive battery of baseline and post-exposure medical surveillance studies was performed to evaluate the medical consequences of these high +Gz exposures. RESULTS: All 6 subjects were able to achieve +12 Gz with various combinations of +Gz-protective equipment, seat-back angle, and various amounts of straining, from none to maximum. When the data were collapsed over all protective equipment there was a significant (p < 0.05) seat effect. Relaxed tolerance to ROR increased with seat-back angle from 13 degrees to 30 degrees to 55 degrees. There was also a significant protective equipment effect when the data were collapsed over all seat-back angles. CONCLUSIONS: These data confirm that effortless protection to +9 Gz is available using ATAGS/CE with the 13 degree and 30 degree seat-back angle (F-15, F-16 and F-22) and to +10.5 Gz with a 55 degree seat-back angle. Moreover, with ATAGS/CE, and a moderate degree of straining, +12 Gz is definitely achievable at 55 degrees, even with reduced anti-G suit pressure at 55 degrees. With additional straining +12 GC is also achievable at the 13 degree and 30 degree seat-back angles.     

Recognizing +Gz-induced loss of consciousness and subject recovery from unconsciousness on a human centrifuge

Human exposure to +Gz-induced loss of consciousness (G-LOC) remains of some concern relative to the well-being of the individuals experiencing the unconscious episodes. Detailed kinetic analysis of over 500 G-LOC episodes on a human centrifuge allowed an evaluation of the time for subjective recognition by observers of the onset of G-LOC and subsequent recovery to normal baseline conditions. The characteristics of early, coincident, and late recognition of the onset of G-LOC were evaluated. Earlier recognition of G-LOC was observed to occur when the rate of onset of the +Gz-stress was gradual (less than 0.6 G/s). Rapid onset rate (greater than 0.6 G/s) exposures were more likely to result in late recognition of G-LOC. The duration of the resulting period of unconsciousness (absolute incapacitation) was very sensitive to the time for recognition of G-LOC and most rapid return to a normal (+1 Gz) environment. The absolute incapacitation increased significantly from early (10.7 s) to coincident (11.4 s) to late (13.2 s) recognition of G-LOC which differed by a total of only 4.6 s. The results allow development of an initial standard of care envelope for apparently safe exposure of human subjects to centrifuge G-LOC since no adverse effects were observed with any of the exposures. The results also demonstrate the extreme sensitivity of the central nervous system to small changes in exposure to +Gz-stress which can be accurately measured.     

新型侧管式抗荷装置与抗荷动作的综合防护性能

目的 对新型侧管式抗荷装备与抗荷动作的综合防护性能进行研究,探讨采用二者联合的方法对８Ｇ１０ｓ（增长率为２Ｇ／ｓ）高Ｇ进行防护的可行性。方法 在离心机上,＋Ｇｚ增长率为２Ｇ／ｓ,行测量９名飞行学员的基础＋Ｇｚ耐力,其中５名飞行学员采用新型侧管式抗荷服（ＮＫＨ）＋新型抗调器（ＮＫＴ）＋Ｌ－１动作（Ｌ－１）、另４名飞行学员采用新型侧管式代偿服务（ＤＮＣ）＋ＮＫＴ＋Ｌ－１,测定出其最大＋Ｇｚ耐力。结果５     

Brief exposure to -Gz reduces cerebral perfusion pressure during subsequent +Gz stress in rats

INTRODUCTION: In humans, +Gz exposure immediately preceded by exposure to zero or -Gz can result in unexpected incapacitation ("push-pull" effect). Our goals were to establish whether this phenomenon exists in rats and to evaluate the importance of varying the duration of -Gz exposure on magnitude of the push-pull effect on cerebral perfusion pressure. METHODS: Eight conscious male rats were studied in the transition from +5 Gz to +10 Gz imposed by centrifugation. This was done with (push-pull) or without (control) 2 s exposure to -5 Gz applied using a counterbalanced design. Seven isoflurane anesthetized rats were studied in the transition from 0Gz (+1Gy) to + 1Gz imposed by tilting. This was done with (push-pull) or without (control) 0.5, 1, 3, or 9 s exposure to -1Gz imposed immediately prior to the transition applied using a counterbalanced designed. RESULTS: Exposure to 2 s of -5 Gz significantly (p < 0.01) reduced carotid artery pressure in the 4th through 8th s of exposure to +10 Gz by an average of 15 mmHg compared with control. In the tilt experiments, a push-pull effect was found with mild Gz exposure (+/-1Gz) with as little as 0.5 s -Gz exposure. Varying the head-down dwell time did not alter the magnitude of the exaggerated hypotension induced by "push-pull" (p = 0.90). CONCLUSIONS: We conclude that rats express a "push-pull" effect similar to that observed in humans but that altering the duration of exposure to -Gz does not influence the magnitude of the "push-pull" effect.     

Reflex heart rate response to variable onset +Gz

Rapid onset high sustained +Gz is a frequent requirement in air combat maneuvering. The cardiovascular response is inadequate to fully compensate for this rapid +Gz change. The rate of change in heart rate (HR) during gradual (0.1 G.s-1, GOR), rapid (1.0 G.s-1, ROR), and very high (6.0 G.s-1, VHOG) onset acceleration exposures to sustained (15 s) +7Gz, +8Gz, and +9Gz levels was measured in 81 healthy male subjects in a human centrifuge. The time (s) to reach maximum heart rate (T7) was measured as the time for the preacceleration exposure resting heart rate (RHR) to reach maximum heart rate (MHR). The change in heart rate upon reaching maximum +Gz level (delta HRA) from rest was calculated along with the change in HR from rest to the maximum heart rate achieved before maximum +Gz level was attained. During the ROR and VHOG runs, MHR was not achieved until after maximum +Gz level was attained. The change in heart rate from resting HR (immediately prior to acceleration) to the heart rate achieved at the onset of maximum +Gz level (delta HRA), decreased by 50% as the onset rate increased from GOR to ROR and VHOG. The delta HRB for very high onset rates exposures was significantly greater than that for ROR and GOR exposures. Acceleration exposure to levels of +7Gz and above (+7Gz, +8Gz and +9Gz) exhibited similar HR responses. VHOG to sustained +Gz stress levels of +7 to +9Gz for 15 s did not provide a sufficient length of time to allow maximum cardiovascular response.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Determination of a pressure breathing schedule for improving +Gz tolerance.

A base of empirical data for developing optimal pressure breathing during +Gz (PBG) schedules is lacking. Relaxed +Gz-intensity tolerance with PBG was measured during gradual +Gz-onset rate centrifuge profiles using standard lightbar criteria. Constant PBG levels ranging from 18-73 mm Hg were randomly assigned. G-suit pressure followed the standard or an increased inflation schedule. Nine subjects wore a jerkin, CSU-15/P G-suit, and TLSS helmet and mask. With mean mask cavity pressures of 0, 18, 38, 60, and 73 mm Hg, corresponding +Gz-tolerances (mean +/- S.E.M.) were: 5.3 +/- 0.2, 5.8 +/- 0.1, 6.6 +/- 0.2, 7.3 +/- 0.3, and 7.5 +/- 0.3 Gz (linear correlation, r = 0.994). Increased G-suit pressure did not change the +Gz-tolerance improvement with PBG. The inverse of individual subject regression slopes ranged from 22.6 to 58.1 mm Hg/+Gz. Considering additional factors and adequate +Gz protection for all subjects while relaxed, the proposed schedule would apply 42 mm Hg PBG/+Gz beginning at +3.3 Gz with a maximum pressure of at least 73 mm Hg.     

Effect of muscle tensing on cerebral oxygen status during sustained high +Gz

BACKGROUND: Successful monitoring of oxyhemoglobin during +Gz exposure was recently achieved using near-infrared spectroscopy (NIRS). To assess the effects of muscle tensing on sustained +Gz tolerance, we measured muscle activity and cerebral oxygen status (COS) during anti-G straining maneuvers at sustained high +Gz. METHOD: We exposed 21 male pilots wearing CSU-13/P anti-G suit to two different centrifuge profiles: 1) short-term repeated exposure (5 to 20 s) at 4, 5, 6, 5.5, or 7 Gz; 2) sustained exposure to a + 7Gz plateau for 30 s. During the Gz exposures, surface electromyographic (EMG) measurements were taken from the vastus medialis (VM) and rectus abdominis (RA) muscles. At the same time, the COS was recorded from the left forehead area using a commercial NIRS system. Mean muscular tensing for each muscle was calculated as a percentage of maximal voluntary contraction (% MVC). RESULTS: Oxyhemoglobin (O2Hb) and total hemoglobin (sum of O2Hb and deoxyhemoglobin) were decreased during both short-term and sustained +Gz exposure. RA muscle tensing was positively correlated with changes in the concentration of O2Hb during sustained + 7Gz exposure (r = 0.540, p < 0.05). RA tensing ranged from 6.2 to 36.8%MVC, and O2Hb ranged from -41.3 to -7.28 micromol x L(-1) during the exposures. No significant correlation was observed between VM tensing and O2Hb. CONCLUSION: NIRS measurements confirmed that a muscle straining maneuver increases G tolerance. Higher RA muscle tensing helps preserve brain blood volume during sustained high +Gz.     

Heart rate and blood pressure responses to +Gz following varied-duration -Gz

BACKGROUND: The push-pull effect has been defined previously as decreased +Gz tolerance caused by previous baseline zero or -Gz exposure. Earlier work indicates that the delay in BP (BP) recovery during +Gz is a function of time at -G7, and is due to the lengthened time-course of sympathetically mediated peripheral vasoconstriction. HYPOTHESIS: The purpose of this study was to retrospectively determine whether heart rate (HR) varies with BP as duration at preceding -Gz increased. METHODS: Continuous ECG R-R interval data from 15 s of +2.25Gz after preceding 2, 5, 10, or 15 s at 2Gz obtained from previous experiments were analyzed and compared with the previously reported BP data. Repeated measures ANOVA and regression analyses were used to compare +2.25Gz HR responses after the four -Gz conditions and one control +2.25Gz condition. RESULTS: An initial rapid rise in HR was observed for all conditions with a consistent steady-state plateau achieved after the first 7 s of +2.25Gz. However, there were significant differences in mean HR attained during the +2.25Gz plateau for preceding 15 s -2.0 Gz vs. the control, 2, 5, and 10s -Gz conditions (109+/-1.1 vs. 102+/-1.8, 100+/-2.0, 97+/-1.1 and 101+/-1.1, bpm, respectively; p<0.05). CONCLUSIONS: HR, unlike BP, increases briskly across all preceding -Gz time conditions, adapting within the initial baroreflex-compensatory time frame typically expected for +Gz exposures. These results suggest there may be a threshold effect for HR response. Consequently, vasoconstrictor response is a critical adaptive mechanism during +Gz when preceded by long (>10 s) -Gz exposures.     

Tolerance of domestic fowl to high sustained +Gz.

A system is described for the acceleration treatment of domestic fowl. A reasonable endpoint for acceleration tolerance is provided by a bradycardia which occurs fairly close to the lethal limit. In a group of 61 male Rhode Island Red chickens exposed to 6 Gz, the mean tolerance (+/- S.D.) was 11.1 +/- 10.6 min. Among individuals, the acceleration tolerance is inversely related to both body size and age, and positively to pre-treatment heart rate.     

飞行员在高+Gz暴露后的肺功能变化

目的 研究不同水平高＋Gz暴露后对人体肺功能的后效应.方法 95名现役男性歼击机飞行员,分别接受＋7 Gz和＋9 Gz离心机检查,于＋Gz暴露前1 d和暴露后12～65 min内测定肺容量、肺通气功能和弥散功能指标.结果 ＋7 Gz暴露后,飞行员肺弥散功能减退和轻度小气道功能异常,＋9 Gz暴露后程度加重,并出现肺活量降低.但＋7 Gz和＋9 Gz之间,各项指标无明显差异.结论 高G暴露可导致小气道功能和弥散功能减低的后效应,其持续时间和对飞行员肺功能有无长期影响有待进一步研究.