G-induced loss of consciousness: retrospective survey results from 2259 military aircrew

INTRODUCTION: Prevalence of G-induced loss of consciousness (G-LOC) in the United Kingdom Royal Air Force (RAF) was found to be 19.3% in 1987. With the introduction of the Typhoon, a fourth generation aircraft, the prevalence of G-LOC has been re-assessed to determine the effectiveness of current G tolerance training. METHOD: A survey was sent to 4018 RAF aircrew, irrespective of their current role. Information was requested on G-LOC, role and aircraft type, experience, and attitudes toward G-LOC prevention. RESULTS: Responses were received from 2259 (56.2%) individuals, 882 (39%) of whom were current fast jet aircrew. At least one episode of G-LOC was reported by 20.1% of all respondents. In front line aircraft, prevalence of G-LOC among the 882 fast jet aircrew who responded was 6%. In the whole group, G-LOC was reported most commonly in aircrew under training (70.9%), and was most prevalent in training aircraft (77.4% of G-LOC events). At the time of the G-LOC, 64% of aircrew had less than 100 h total flying time. G-LOC was reported most frequently between +5 to +5.9 Gz, and "push-pull" maneuvers were associated with 31.3% of G-LOC events. Pulling G was not considered a problem by 50.6% of respondents, although over 80% recognized the value of flying currency, use of an anti-G suit, and physical fitness, and 55.6% felt that centrifuge training would be valuable. DISCUSSION: The prevalence of G-LOC in the RAF has changed little since 1987, and there remains considerable scope for aircrew education, particularly with the introduction of the Typhoon.     

G-induced visual and cognitive disturbances in a survey of 65 operational fighter pilots

INTRODUCTION: Only one previous study has assessed almost loss of consciousness (A-LOC) in operational fighter pilots, reporting an incidence rate of 14%. Research also indicates that 8-13% of pilots have experienced G-induced loss of consciousness (G-LOC). A-LOC can be as insidious as G-LOC due to the associated altered state of awareness and relative incapacitation time, making it a significant risk factor in the high +Gz environment. Royal Australian Air Force (RAAF) pilots currently fly the F/A-18 and Hawk 127, producing +Gz accelerations up to +7.5 Gz, which places these pilots at risk of both A-LOC and G-LOC. METHODS: A survey was administered to 100 active RAAF fighter pilots requesting information on G-induced visual and cognitive disturbances, A-LOC symptoms, and G-LOC. Details regarding type of aircraft, flying maneuvers performed and mission outcome were also sought. RESULTS: There were 65 RAAF fighter pilots who completed the survey (age 20-53 yr, height 168-193 cm, weight 64-110 kg, jet hours 30-5700 h). Of these pilots, 98% indicated they had experienced at least one visual or cognitive disturbance in the high G environment: gray-out 98%; black-out 29%; and A-LOC symptoms 52%, including abnormal sensation in limbs, disorientation, and confusion. There were 9% who indicated they had experienced G-LOC (50% were the pilot flying the aircraft). DISCUSSION: These findings indicate that RAAF fighter pilots are experiencing a similar rate of visual disturbances and G-LOC when compared with other air forces. However, RAAF pilots reported a much higher incidence of A-LOC compared with the only other study of operational fighter pilots.     

Enhancing aircrew centrifuge high-G training using on-line videotape documentation

Many air forces are training fighter aircrew to improve their tolerance to the high-G environment by using human centrifuges. It is important to enhance and upgrade the methods and techniques employed in this aircrew training. At the Naval Air Development Center (NADC) we have developed several techniques to consolidate as much of the information as possible on the videotape recording of the centrifuge training. This includes a continuous electrocardiographic display, anti-G suit pressure tracing, heart rate parameters, +Gz parameter, and, if +Gz. induced loss of consciousness (G-LOC) occurs, the duration of incapacitation resulting from G-LOC. The techniques and information provided have been well received by Navy and Marine aviators participating in high-G training at NADC.     

Visual symptoms and G-LOC in the operational environment and during centrifuge training of Turkish jet pilots.

BACKGROUND: High-performance fighter aircraft produce high-sustained +Gz forces with rapid onset rates. Because of this G-producing capability, military jet pilots are subjected to physiological stress, which may lead to visual disturbances and G-induced loss of consciousness (G-LOC). Although visual disturbances are very common in jet flights, G-LOC is relatively rare but more dangerous. The frequency and causes of G-LOC need to be determined in the interest of flight safety. METHODS: Part I. A survey was conducted on Turkish jet pilots to reveal the incidence of symptoms due to +Gz acceleration. Anonymous questionnaires were given to F-16, F-4, and F-5 pilots. They consisted of inquiries about the occurrence of visual symptoms and/or G-LOC during +Gz acceleration in the operational environment. Part II. During the years 1992-1996, 486 F-16, 801 F-4, and 256 F-5 fighter pilots underwent high "G" training at Turkish Aerospace Medical Center and they were assessed in terms of G-LOC rates. RESULTS: Part I. A total of 325 pilots who flew T-37 in undergraduate pilot training (UPT) answered the questionnaire. The pilots were divided into 3 groups according to the types of aircraft, which they fly now: 116 F-16, 182 F-4, and 27 F-5 pilots. A total of 311 pilots (95.7%) reported having experienced greyouts and/or blackouts. With 25 pilots (7.7%) experiencing G-LOC, the G-LOC frequency according to the type of aircraft was: 5.2% (T-37) [in UPT]; 4.3% (F-16), 1.6% (F-4), and 0% (F-5). Part II. In centrifuge training, the incidence of G-LOC in pilots of the various types of aircraft were: 12% (F-16), 6.4% (F-4), and 8.6% (F-5). CONCLUSIONS: Centrifuge training reduces G-LOC rates of subsequent centrifuge training; and it is hoped might reduce the G-LOC rate in the operational environment. Almost all jet pilots reported having experienced +Gz related visual symptoms, but G-LOC seems to be a more common problem for pilots who fly rapid onset rate aircraft than pilots who fly high "G" capable but lower G onset rate aircraft.     

The electrocardiographic response to high +Gz centrifuge training

The electrocardiographic (ECG) responses of 59 asymptomatic, healthy flight surgeons to the acceleration profiles included in current U.S. Air Force and U.S. Navy high-G centrifuge training programs are documented. ECG dysrhythmias were frequently observed during exposure to both gradual and rapid onset training profiles. Short self-limited episodes of ventricular tachycardia occurred in 5 subjects. Advanced Lown grade ventricular ectopy occurred in 13 subjects. The type of cardiac ectopy and the frequency of occurrence for each of the training profiles is described. The results suggest that significant ectopy frequently occurs during exposure of healthy, asymptomatic individuals to centrifuge training profiles. Since aircrew are expected to undergo high +Gz as part of their usual flying duties, ECG monitoring during high-G centrifuge training has not universally been a required part of the training exposures. Aircrew have not always accepted ECG monitoring during centrifuge training, fearing detection of certain cardiac dysrhythmias, which current aeromedical standards consider disqualifying for continued flying duties without clinical aeromedical evaluation. Based on the results of this study, and previous documentation of the occurrence of significant +Gz-induced cardiac dysrhythmias (both in flight and on the centrifuge), ECG monitoring might be considered appropriate to ensure optimum medical safety during high-G centrifuge training. The current inconsistency between 1) not monitoring ECG because of the aeromedical standards for flying qualification relating to the ECG response to +Gz stress, and 2) the need to monitor ECG to assure optimum safety during centrifuge training, deserves resolution.     

+Gz-induced loss of consciousness: a case for training exposure to unconsciousness

+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.     

Contact lenses and corrective flying spectacles in military aircrew--implications for flight safety

BACKGROUND: Refractive devices used by aviators need to suit the aerospace environment or their failure can have serious implications. A relatively minor visual disability can result in loss of life and aircraft. We surveyed commonly occurring problems with the different types of refractive correction worn by Royal Air Force (RAF) aircrew over the previous 12 mo. We also asked if they had experienced any flight safety incidents (FSI) relating to their refractive correction. METHODS: A retrospective anonymous questionnaire survey was given to 700 active aircrew occupationally graded as requiring corrective flying spectacles (CFS) or contact lenses (CL) for flying. RESULTS: 63% (443) of the questionnaires were completed. CL were worn by 53% of aircrew; 71% of them used daily disposable CL. CFS were worn by the remaining 47% of aircrew, 14% of whom used multifocal lenses. Of CFS wearers, 83% reported problems including misting, moving, discomfort, and conflict with helmet-mounted devices (HMD). CL-related ocular symptoms were reported in 67% of wearers including cloudy vision, dry eye, photophobia, red eyes, excessive mucus formation, CL movement, itching, and grittiness. No CL-related FSI were reported over the previous 12 mo compared with 5% CFS-related FSI (p < 0.001). The graded performance of CL for vision, comfort, handling, convenience, and overall satisfaction was significantly higher than for CFS. CONCLUSION: CFS are associated with problems in terms of comfort and safety. CL are well tolerated by aircrew, and deliver improved visual performance.     

High +Gz centrifuge training: the electrocardiographic response to +Gz-induced loss of consciousness

Neural control of the heart involves complex interconnections within the central nervous system (CNS). Although various CNS abnormalities and processes (acute cerebrovascular accidents, cerebral ischemia, subarachnoid hemorrhages, and seizures) have been associated with alteration of cardiac electrophysiology, the effect of +Gz-induced loss of consciousness (G-LOC) on autonomic control of the heart is unknown. From a group of 59 healthy subjects undergoing centrifuge high +Gz training, 15 suffered G-LOC episodes. The +Gz training profiles included gradual (0.1 G/s) and rapid (approximately 6 G/s) exposures to levels as high as +9 Gz. Electrocardiographic rate and rhythm disturbances were evaluated during each of the +Gz training profiles. Rate and rhythm disturbances associated with the +Gz stress exposures were observed in 73% of the subjects. When considering only the period when the subjects were exposed to +Gz (During-G), 67% of the individuals had atrial or ventricular ectopy. When considering the period of unconsciousness (During-LOC), which lasted an average of 12.6 s, 33% of the individuals had atrial or ventricular ectopy. Electrocardiographic changes were related to +Gz stress and unrelated to the period of occurrence of G-LOC. Significant ectopy (ventricular tachycardia and supraventricular tachycardia) was observed only during +Gz stress and not related to the G-LOC period. The results of the study do not indicate that G-LOC alters the electrocardiographic response to +Gz stress.     

Positive pressure breathing for acceleration protection and its role in prevention of inflight G-induced loss of consciousness

That pressure breathing for G protection (PBG) can improve both tolerance and endurance to high sustained +Gz acceleration is now well established. It is not surprising, therefore, that the undoubted potential benefits of PBG in an operational environment have been greeted with enthusiasm by aircrew and their commanders. In some quarters, the use of positive pressure breathing (PPB) during periods of high sustained +Gz acceleration is being hailed as a potential cure for the problem of G-induced loss of consciousness (G-LOC). We believe, however, that confidence in the technique for this purpose in modern, highly agile fighter aircraft is misplaced. This article reviews PPB's background and present use as protection against +Gz acceleration, and summarizes the physiologic basis for its effectiveness, before relating it to its undoubted role in support of other anti-G strategies. From theoretical considerations supported by published evidence, we conclude that while PPB, if used correctly and when combined with other strategies, can enhance tolerance to +Gz acceleration, its principal influence on the occurrence of G-LOC will be by virtue of its ability to increase endurance by decreasing aircrew fatigue.     

+Gz-induced loss of consciousness and aircraft recovery

Aircrew incapacitation resulting from very high onset sustained +Gz stress has resulted in significant losses of aircraft and aircrew. Enhanced protection and training toward prevention of +Gz-induced loss of consciousness (G-LOC) will continue to be vital. Techniques for reduction of the time of incapacitation, should G-LOC occur, must also be explored and developed. Current capability of aircraft autorecovery as demonstrated by the Advanced Fighter Technology Integration F-16 (AFTI/F-16) promises to enhance safety from the acute incapacitation resulting from G-LOC (and spatial disorientation). Physiologic monitoring for determining G-LOC has certain advantages especially in the aerial combat arena. The optimum physiologic monitoring technique would be direct determination of failure of brain cell function at the cellular or subcellular level. Complete investigation of G-LOC is necessary to understand the phenomenon and to develop methods for enhancing recognition and recovery. This paper discusses aircraft autorecovery technology and potential methods for physiologic monitoring of G-LOC. Integration of physiologic monitoring techniques into aircraft autorecovery systems requires a broad approach for optimal development.     

Operational G-induced loss of consciousness: something old; something new

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.     

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)     

Human tolerance to Gz acceleration loads generated in high-performance helicopters

BACKGROUND: As the Gz capabilities of tactical helicopters increase, the risk to unprotected helicopter aircrew resulting from the physiologic response to transitions from -1 Gz (push) to +4.5 Gz (pull) loads needs to be addressed. METHODS: There were 9 volunteers who participated in a study conducted at the Veridian Operations Centrifuge Facility in Warminster, PA. A 1-h mission scenario consisting of nine helicopter maneuvers, based on inflight G measurements (push-pull mission, PPM), simulated both current (CM: -0.2 to +3.5 Gz) and projected future platform capabilities (FM: -1 Gz to +4.5 Gz). Additional scenarios were run in which push transitions were limited to +1 Gz (GM). Measurements included blood pressure (BP), heart rate (HR), loss of vision, and subjective fatigue. RESULTS: Visual decrements were minimal during CM while muscular tensing was required to avoid blackout during FM. Light loss typically occurred during the transition from -Gz to +Gz. Within the scope of these tests, subjects tolerated the range of Gz stresses associated with current U.S. Navy rotary wing platforms. When subjected to FM G-loads (typical of current U.S. Army high-performance platforms), cardiovascular stress significantly increased, Gz tolerance dropped as much as 1.2 G, and HR increased as much as 67 bpm. Cardiovascular changes were significantly greater during FM PPM relative to GM. Four subjects reported Almost-Loss of Consciousness (A-LOC) symptoms during FM. CONCLUSIONS: While G-stress experienced by aircrew generated by current helicopters does not appear to present a high risk, G-awareness training is recommended to reduce risks to aircrew exposed to G-loads generated by more aggressive helicopters. Future studies are required to determine the impact of longer mission times and dehydration.     

Hypoxia symptoms reported during helicopter operations below 10,000 ft: a retrospective survey

INTRODUCTION: During routine aviation medicine training, rotary-wing aircrew are instructed that the impact of hypoxia on them from flying in unpressurized cabins up to 10,000 ft (3048 m) above mean sea level (AMSL) is relatively small and has few implications for aviation safety. Such reassurance is based on data derived from experiments conducted on resting subjects and may not reflect the true impact of hypoxia in aircrew engaged in operational tasks. METHOD: A survey listing common symptoms of hypoxia was distributed to Australian Army helicopter aircrew who had operated at altitudes up to 10,000 ft AMSL. RESULTS: There were 53 surveys that were returned (71% response), representing 25 loadmasters, 23 pilots, and 5 aircrewman technicians. All respondents were Australian Army aircrew. One or more symptoms consistent with hypoxia were reported by 86.6% of non-pilot aircrew and 60.9% of pilots. 60% of non-pilot aircrew reported four or more symptoms, compared with only 17% of pilots. The most commonly reported symptoms were difficulty with calculations (45%), feeling light-headed (38%), delayed reaction time (38%), and mental confusion (36%). Loadmasters reported more symptoms (mean 5.4) than pilots (mean 2.2) (p < 0.001). From the narratives provided (n = 21), aircrew experienced potentially operationally significant symptoms at a mean altitude of 8462 ft (2579 m). CONCLUSION: The helicopter aircrew surveyed reported symptoms consistent with hypoxia at altitudes within the so-called physiological zone; loadmasters reported more effects than pilots. It may be inappropriate to emphasize the benign nature of the physiological zone during aviation medicine training of a non-resting population such as helicopter aircrew.     

The effects of acceleration forces on night vision

The effects of Gy and Gz acceleration forces on cone-type mesopic vision threshold values are examined. An experiment has been conducted on the Dynamic Environment Simulator, a three-axis human centrifuge, to reproduce an acceleration environment in a simulated night-flight combat situation. Acceleration environments studied were levels of +1 Gz, + 1Gy, +1.4 Gz, +2 Gz, +3 Gz, and +2 Gy in combination with +1 Gz. A visual task was performed which determined 20/50 visual acuity illumination threshold values. Physiological parameters recorded were arterial oxygen saturation (SaO2) by ear oximetry, heart rate, and visual acuity threshold values. There were 10 male subjects, all members of the United States Air Force. Their ages ranged from 25-39 years (mean +/- S.D., 29.1 +/- 4.3). Results were zero means obtained by self-pairing with +1 Gz controls. Analysis was done by self-pairing, two-tailed t test. Results showed no significant shift in luminance threshold values at +1 Gy or +1.4 Gz, and significant increases in luminance thresholds at the 0.01 level for +2 Gz, +3 Gz, and +2 Gy in combination with +1 Gz.     

Patterns of physical conditioning in Royal Australian Air Force F/A-18 pilots and the implications for +Gz tolerance.

BACKGROUND: The role of physical conditioning in tolerance to +Gz remains the subject of debate, particularly in relation to the relative merits of aerobic vs. anaerobic conditioning. The purpose of this study was to document the patterns of physical conditioning in Royal Australian Air Force (RAAF) fighter pilots and to relate these findings to the question of +Gz tolerance. METHODS: A questionnaire was used to determine the physical conditioning activities of RAAF F/A-18 pilots. Aerobic fitness levels (VO2max) were determined in a sample of eight pilots using a progressive cycle ergometer protocol. RESULTS: Of the 42 F/A-18 pilots who completed the questionnaire, 86% reported regular physical conditioning, mostly three times per week. Aerobic activities were the most common (83%), with running the most popular activity (55%). Anaerobic activities were reported by 26% of respondents. Most respondents reported doing more than one activity, with an average weekly training volume of 129+/-77 min. The ergometer test results revealed a mean VO2max of 50+/-6 ml O2 x kg x min(-1). CONCLUSION: The high rates of participation in regular physical activity suggest that physical fitness is perceived as important by the fighter pilot operating in the high +Gz environment. The fighter pilots in this study participated in aerobic activities at a much higher rate than anaerobic activities. The aerobic fitness levels measured suggest that the pilots have good but not exceptionally high levels of aerobic power that are unlikely to influence +Gz tolerance.     

The medical acceptability of soft contact lens wear by USAF tactical aircrews.

Seventy-two Tactical Air Command (TAC) aircrew members completed one full year of soft contact lens (SCL) wear. A daily-wear regimen, using extended-wear lenses, was used to minimize corneal stress. Baseline measurements of visual acuity with SCLs and with spectacles after SCL removal and ocular indicator gradings were compared to measurements at 5-d, 10-d, 1-month, 3-month, 6-month, and 12-month examinations. Visual acuity did not decrease during the test. No aircrew member developed corneal ulcers or other serious complications requiring elimination from the test. Two aircrew members lost a total of 9 "duties not to include flying" (DNIF) days: one flyer was grounded for 1 d with a corneal abrasion and another for 8 d with epithelial microcysts. The TAC SCL Test, as designed, was generally successful. The conservative approach to SCL wear during the test and the meticulous follow-up care by United States Air Force eye care professionals most likely contributed to the low ocular complication rate.     

Management of neck pain in Royal Australian Air Force fast jet aircrew

To examine the type and effectiveness of various strategies used by Royal Australian Air Force (RAAF) fast jet (FJ) aircrew in self-referral and management of flight-related neck pain, a 6-section, 18-question survey tool was distributed to 86 eligible RAAF aircrew. Selective results from the sections evaluating aircrew demographics, incidence of flight-related neck pain, and the self-referral strategies of aircrew to manage these injuries are presented here. Eighty-two RAAF FJ aircrew responded to the survey. Ninety-five percent of the respondents experienced flight-related neck pain. The most commonly sought treatment modalities were on-base medical and physiotherapy services. Many respondents reported that currently provided on-base treatment and ancillary services such as chiropractic therapy are the most effective in alleviating symptoms. Further investigation into the effectiveness and safety of these ancillary therapies needs to be performed to allow appropriate consideration of their place in the management of neck pain in FJ aircrew.