Acceleration-induced loss of consciousness. A review of 500 episodes.

Unconsciousness resulting from exposure to increased levels of head-to-foot (+Gz) acceleration stress (501 unconsciousness episodes) on a centrifuge in asymptomatic, healthy human subjects was investigated. A method for quantitatively measuring the kinetics of the unconsciousness and associated phenomenon was developed. In addition, a theoretical framework for describing the central nervous system (CNS) alteration resulting from acute reduction of blood flow was formulated to allow a method for defining unconsciousness phenomenon. The length of unconsciousness and the associated incapacitation was found to be dependent on the magnitude of the CNS insult resulting from reduced blood flow. The magnitude of the insult was determined by the onset and offset rates of the +Gz-stress and the length of time at increased +Gz. The incapacitation resulting from +Gz-stress included 11.9 seconds of absolute incapacitation (unconsciousness) and 16 seconds of relative incapacitation (confusion/disorientation) for 28 seconds of total incapacitation (period of time for lack of purposeful movement). Myoclonic convulsions were observed in approximately 70% of the unconsciousness episodes. The convulsions lasted 4 seconds and occurred following the return of CNS blood flow. The convulsions occurred after 8 seconds of unconsciousness and ended coincident with the return of consciousness. They occurred when the CNS insult was of greater magnitude. Memorable dreams occurred and were considered to occur during the terminal portion of the convulsion period. The dreams occurred with exposures having longer unconsciousness. The length of unconsciousness and incapacitation was affected by the wearing of an anti-G suit, with unconsciousness and incapacitation being reduced if the suit were worn. Performance of an anti-G straining maneuver resulted in an increased length of incapacitation by allowing the subject to get to higher levels of +Gz-stress and to sustain a greater amount of acceleration exposure. The results of this 11-year study of human unconsciousness provide a quantitative kinetic description of the phenomenon in healthy humans that is completely documented on videotape. These results should be of interest to neuropsychophysiologists investigating unconsciousness, convulsive activity, and dream phenomenon. They also provide the basis for future research aimed at solving +Gz-induced loss-of-consciousness problems in fighter-aircraft aviation.

Granulomatous interstitial pneumonia in a miniature swine associated with repeated intravenous injections of Tc-99m human serum albumin: concise communication.

Albumin lung-scanning agents have a proven high degree of safety, with the only contraindication to their use being allergic hypersensitivity. We have used these agents to investigate the physiologic effects of high Gz acceleratory forces on pulmonary perfusion using the miniature swine. Multiple doses of human macroaggregated albumin and human-albumin microspheres were given to a miniature swine at various levels of centrifugal acceleration over a 6-wk period. The dosages given were the same per kilogram as those used for routine clinical human studies. The animal subsequently died from a severe granulomatous interstitial pneumonia. The granulomatous lesions suggest that the pathogenesis may have involved a cell-mediated delayed hypersensitivity. This interstitial pneumonia may represent the end point in a chronic hypersensitivity response to the human-albumin lung-scanning agents.

The electrocardiographic response to maximal treadmill exercise of asymptomatic men with right bundle branch block.

This study presents the results of maximal treadmill testing and cardiac catheterization in 40 asymptomatic and apparently healthy men with acquired right bundle-branch block. Eight of the men had significant angiographic coronary artery disease, and six of the eight only had single-vessel disease. The 40 men had normal maximal oxygen consumptions, normal maximal heart rates, and normal maximal blood pressure responses; none of the men had abnormal ST-segment changes in response to maximal treadmill testing. Thus, the sensitivity of exercise testing for coronary artery disease in men with right bundle branch block is uncertain. However, the apparently high specificity of exercise testing demonstrated by this study necessitates further evaluation for coronary artery disease in men with right bundle branch block who develop abnormal ST-segment depression in response to exercise testing.

Contribution of skeletal muscle activity to the natural history of acceleration-induced loss of consciousness (G-LOC).

G-LOC and the ensuing incapacitation represents an ever present threat to modern fighter aircraft and aviators. Of the techniques developed to combat gravitational induced stress, particularly such stress acting along the head to foot axis (+Gz), a variety of straining maneuvers have assumed prominence. All the latter techniques involve voluntary skeletal muscle tensing to a varying degree. It is hypothesized that the basis for the importance of skeletal muscle activity in improving G tolerance predominantly lies in the increased muscle spindle afferent activity to the reticular activating system. It is also hypothesized that the natural history of G-LOC can be significantly improved by supplementing muscle spindle activity via active and passive skeletal muscle activity.

Theoretical analysis of acceleration-induced central nervous system ischemia.

The mechanism responsible for the +G(z)-induced loss of consciousness (G-LOC) experienced by fighter pilots is investigated. The nature of a G-LOC episode is described formcases of rapid onset. Evidence suggests that the G-LOC mechanism might be closely related to the mechanism for sleep. A theory of G-LOC that treats it as a protective physiological mechanism is proposed.

Converging research on +Gz-induced loss of consciousness.

The G-induced loss of consciousness (G-LOC) hazard can be reduced either by preventing its occurrence or shortening the period of incapacitation. The latter requires an understanding of this period of incapacitation. Two types of G-LOC occur: Type I is short duration and without convulsive type movements; and Type II is longer and with convulsions. Psychological suppression (denial) by pilots that G-LOC had occurred appears to be an important problem in reporting surveys and flying safety. Auditory and visual types of sensory stimuli to reduce the period of incapacitation are discussed. Recognition by the pilot that G-LOC has occurred appears to decrease incapacitation times and should be considered part of G training. Methods of developing an aircraft recovery system after G-LOC has occurred in pilots is considered a viable approach and is examined. Converging on the G-LOC problem by both, reducing its incidence as well as its duration appears to offer an additional dimension in the approach towards solving this important operational problem.

Technique for simulating G-induced tunnel vision.

A simple technique is described which helps subjects gain an understanding of the endpoint used to terminate exposure to centrifuge stress during training procedures on the human centrifuge. The technique involves manual induction of increased intraocular pressure which, in turn, causes a decrease in retinal perfusion followed by greyout and tunnel vision.

Motivational analysis of human volunteers for centrifuge acceleration research.

Motivational analysis of a representative group of individuals who volunteer to participate in centrifuge acceleration research at the USAF School of Aerospace Medicine was performed. Monetary reward and curiosity were the main reasons for volunteering to participate in the research program. Fear and monetary reward not worth the risk were the main reasons for not volunteering to participate. The major reason for stopping participation in the acceleration research program was not because of volunteer subject-related reasons, but because of being asked to stop for medical reasons by the medical members of the research staff. This type of periodic motivational analysis serves a number of constructive purpose, including self review of human-use research practices and volunteer subject feedback which enhance methods for recruitment and retention of high quality volunteer subjects.

G-tolerance enhancement: straining ability comparison of aircrewmen, nonaircrewmen, and trained centrifuge subjects.

G tolerance measurements, both relaxed and using a protective straining maneuver, were made on aircrewmen, nonaircrewmen, and trained centrifuge subjects at the USAF School of Aerospace Medicine over the past 3 years. Results indicated that currently trained pilots (including fighter-type pilots) are not as proficient at performing a G protective straining maneuver as fully trained centrifuge panel members. Pilots should be able to gain at least 2 G over relaxed G tolerance with adequate training. Evidence exists to indicate an optimum time for G training would be during the period of undergraduate pilot training.

Post-acceleration chaotic atrial rhythm.

Chaotic atrial rhythm has traditionally been a dysrhythmia of the seriously ill elderly patient and commonly associated with pulmonary disease. It has been reported less frequently in young individuals with and without pulmonary disease. An apparently healthy asymptomatic centrifuge subject had reproducible episodes of chaotic atrial rhythm only in the recovery period after exhaustive +Gz simulated aerial combat maneuvering. The underlying mechanism responsible for initiation of chaotic atrial rhythm is unknown, but may be related to distension of atrial tissue. In lung disease with attendant pulmonary hypertension or post +Gz stress with the sudden increase in venous return, the necessary distension of the right atrium can be induced. The prognosis of individuals with chaotic atrial rhythm depends on the severity of the underlying illness and is probably benign in apparently healthy asymptomatic individuals with normal cardiovascular evaluation.

Acceleration-induced atrioventricular dissociation: hemodynamic consequences.

A rest, compensatory mechanisms are able to make up for the loss of a properly timed atrial systole. At maximal stress or in a severely compromised patient, however, proper atrial function may be crucial for maintaining optimum cardiac output. We report a case of an apparently healthy male subject who developed A-V dissociation during +Gz stress and subsequently could not maintain vision at the higher (+7 Gz) levels. Anatomic, geometric, and physiologic considerations suggest that the loss of atrial function is probably associated with the loss of roughly 30 torr in arterial pressure during high +Gz stress. Since the subject was an avid jogger, the etiology of the A-V dissociation was considered to be due to physiologically enhanced vagal tone. This case, therefore, suggests that special attention should be given to the physical fitness programs designed for individuals in a unique profession, such as piloting single-seat high-performance fighter aircraft.

Acceleration stress-induced Wolff-Parkinson-White Syndrome with marked ST-segment depression.

Exercise can affect preexcitation in several ways. The possible presence of catecholamine-sensitive bypass, stimulated during periods of high stress, was recently reported. In addition to a direct effect on the preexcitation, when the preexcitation pattern exists, exercise-induced ST-segment changes may occur which preclude stress testing for coronary artery disease detection. Current high-performance fighter pilots, flying new generation aircraft, are under severe stress during aerial combat maneuvering when they are exposed to high sustained +Gz (head-to-foot) acceleration stress. We report the occurrence of a +Gz acceleration-induced episode of preexcitation with marked ST-segment depression in a healthy asymptomatic aircrewman. Autonomic imbalance, with high catecholamine levels developed during +Gz stress, may be the etiology of this preexcitation episode.

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.

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.

Medical considerations for human exposure to acceleration-induced loss of consciousness.

Unconsciousness in humans has probably been occurring since before recorded history. Acceleration-induced loss of consciousness (G-LOC) in flight has been occurring since 1919. Loss of consciousness and syncope are common occurrences in clinical medicine with G-LOC, occurring in a large number of aircrew and research subjects during centrifuge exposures. Although the major risk to humans exposed to centrifuge-induced G-LOC is related directly to the central nervous, cardiac, and musculoskeletal (neck and back) systems, other risks are also present. Human exposure to G-LOC is required to help solve the G-LOC problem in aviators. To perform such human research, the benefits must clearly outweigh the risks to the human. Even if the risk-benefit ratio is considered favorably balanced, continued monitoring of individuals exposed to G-LOC is mandatory. To facilitate monitoring of humans exposed to G-LOC, a central nervous system (CNS) insult classification system would be of significant value. A suggested classification scheme which considers the type of CNS insult, the history of exposure to G-LOC, and the temporal evolution of potential CNS insult is developed. To date there is no indication that G-LOC episodes have any associated long term or persistent psychophysiological sequelae. Improved acute and long term evaluation of humans exposed to G-LOC are, however, important aspects of conducting G-LOC research with humans. Such research and careful monitoring are necessary to understand and eventually solve the G-LOC problem in aviators.

Acceleration tolerance of asymptomatic aircrew with mitral valve prolapse.

Using the USAF School of Aerospace Medicine human centrifuge, 78 asymptomatic USAF aircrewmen with auscultatory and/or echocardiographic evidence of mitral valve prolapse (MVP) were evaluated for tolerance to +Gz stress. Both rapid (1G X s-1) and gradual (1G X 15 s-1) onset acceleration profiles were used to determine +Gz tolerance. There were no unusual characteristics, among the clinical parameters measured, which allowed separation of individuals with MVP into subgroups with altered +Gz tolerance. The MVP group was found to have a normal response to gradual onset +Gz stress, both while relaxed and when performing a protective straining maneuver. A small but statistically significant decrease in tolerance to rapid onset +Gz stress was found. There was no indication that asymptomatic individuals with MVP who have undergone complete aeromedical evaluation have any operationally significant intolerance to +Gz stress.

+Gz-induced loss of consciousness in undergraduate pilot training.

Class C physiologic incident reports during undergraduate pilot training (UPT) revealed that 1.7 episodes of G-induced loss of consciousness (LOC) occurred monthly in 1980-1984. The mean +Gz for the LOC episodes was +3.8 Gz; the minimum, +2.0 Gz. The mean time of total incapacitation was 12.0 s (maximum, 180.0 s) as subjectively estimated by the aircrew. Improper performance of the anti-G straining maneuver was the most common etiology of the LOC episodes. Symptoms associated with the LOC included complete unawareness of the episode, disorientation, confusion, tingling in the extremities, and flail-type movements of the extremities. The most common aerobatic maneuver causing LOC (30% of the episodes) was the split-S. Safety within the UPT program could potentially be enhanced by ensuring that instructor pilots know how to most efficiently perform the protective anti-G straining maneuvers and recognize the symptoms associated with G-induced LOC.

Observations on the neurophysiologic theory of acceleration (+Gz) induced loss of consciousness.

The characteristics of centrifuge +Gz-induced loss of consciousness (G-LOC) have revealed a specific kinetic pattern of resulting physiologic events. The relationship of these events provides an initial basis for describing the possible neurophysiologic mechanism of G-LOC. A description of G-LOC is developed which divides the G-LOC episode into specific periods based on the psychophysiologic events that occur. Emphasis is placed on the type of acceleration profile which would be most likely to occur during inflight aerial combat maneuvering in fighter aircraft. The symptoms of myoclonic flail movements and memorable dreams which are observed in association with G-LOC may provide key information for unraveling the neurophysiologic mechanism of G-LOC and subsequent recovery. A detailed understanding of the kinetics and mechanism of G-LOC is paramount to eventually solving this critical fighter aviation related problem.

Methods for describing and quantifying +Gz-induced loss of consciousness.

A thorough understanding of +Gz-induced loss of consciousness (G-LOC) is enhanced by defining all psychophysiologic phenomena associated with G-LOC. Defining the mechanism of G-LOC and investigating methods to reduce the resulting incapacitation are facilitated by determining the kinetics of G-LOC induction, incapacitation, and recovery. Permanent video recording of G-LOC episodes is required to accurately define G-LOC events. A method of measuring the time course of G-LOC events using video recording is developed. The resulting calculated and experimental data allow an accurate description of acceleration exposure, resulting incapacitation, and myoclonic convulsions. Although acute physiologic recovery is included in this quantitative framework, longer psychophysiologic recovery remains to be fully evaluated. As such, the techniques developed are established as an initial step in fulfilling the requirement to completely define G-LOC phenomena.

Defining risk in aerospace medical unconsciousness research.

The maneuverability envelopes of current and future fighter/attack aircraft exceed unprotected human tolerance to environmental stress. Human exposure to unconsciousness therefore can result not only inflight, but in research and training laboratories which endeavor to provide methods of enhanced protection for aircrew. Solving the problem of unconsciousness requires a thorough understanding of the phenomenon itself. This can only be accomplished by defining the psychophysiologic aspects of unconsciousness and techniques to prevent its occurrence or enhance recovery, should it occur. The safety of experimental human exposure to G-LOC however, remains of some concern. A framework for discussing the relative insult to the central nervous system may be constructed from what is currently known about G-LOC. The results of animal experimentation allow an estimation of the central nervous system tolerance to hypoxia without permanent alteration of tissue integrity. Clinical medicine documentation of syncope and fainting episodes, coupled with a long history of uncomplicated G-LOC episodes suggests that a certain window of safe exposure exists. Utilization of G-LOC as an endpoint included exposure of very large numbers of humans to unconsciousness without significant complication. Animal experimentation suggests that 180 s of central nervous system hypoxia is associated with uncomplicated recovery. Human exposure as long as 100 s has also been safely accomplished. Centrifuge G-LOC exposure typically results in only 15-20 s of central nervous system hypoxia. As long as G-LOC experimentation using humans is performed within well defined limits, it may be accomplished within an acceptable risk envelope.(ABSTRACT TRUNCATED AT 250 WORDS)