Mammalian cardiovascular morphology and the maintenance of cerebral function at high Gz.

The persistence of consciousness at low cerebral perfusion pressures during exposure to high sustained G force in a head-to-foot direction (+Gz), is the rule. Concomitant changes in cerebrospinal fluid and cerebral venous dynamics together with cerebral autoregulation have been proposed to explain this preservation of cerebral function at high Gz. The non-uniform distensibility of the mammalian aortic distributing system plays a fundamental role in maintaining optimal cardiac energy exchange. Morphological evidence is presented suggesting that such a non-uniformity probably extends cranially as well as caudally in man. One consequence of such a cranial extension would be the amplification of the arterial pressure waves to the brain, with the potential for significant pulsatile flow at relatively low mean arterial blood pressures. Such phenomena would result from fundamental design properties of the mammalian cardiovascular system, would not be detected by conventional invasive pressure monitoring, and would serve as a mechanism to maintain optimal cerebral perfusion under condition of increasing +Gz.

G-LOC recovery with and without G-suit inflation.

During acceleration (+Gz) training in the human centrifuge, the anti-G suit (AGS) is usually deflated as acceleration decreases upon termination of the exposure, regardless of the reason for termination, including +Gz-induced loss of consciousness (G-LOC). This is when the trainee most needs the support provided by the AGS. A method to reduce the time of incapacitation resulting from G-LOC was evaluated. The standard CSU15-P suit worn by 30 aircrew while undergoing +Gz tolerance training was inflated to 10 psi immediately upon G-LOC (GS group). Incapacitation times and flailing activity were recorded and compared with 51 aircrew whose AGS was not abruptly inflated upon G-LOC (NGS group). Absolute incapacitation was significantly different between both groups (p = 0.024). The GS group exhibited flailing behavior for a longer period of time during relative incapacitation than the NGS group (p = 0.0003). Total incapacitation remained unaffected. A brief period of confusion occasionally accompanied by mimic or myoclonic convulsions was observed more often in the GS group. Inflation of the AGS upon G-LOC seems to reduce absolute incapacitation by approximately 2 s, thereby causing the trainee to be aware of his environment and G-LOC more quickly, even though his motor function has not yet been fully restored.

Effect of pyridostigmine bromide on acceleration tolerance and performance.

Pyridostigmine Bromide (PB) is used as a pre-exposure antidote for the prevention of potentially lethal effects of certain chemical warfare nerve agents by reversibly inhibiting acetylcholinesterase (AChE). This study was designed to determine whether PB has any deleterious effects on acceleration tolerance (+Gz) or performance. Double-blind placebo trials were conducted to evaluate the effects of PB (90 mg) per day on +Gz tolerances and performance. Three types of exposures were used: 1) gradual onset rate (GOR) exposures of 0.1 G/s; 2) a series of rapid onset rate (ROR) exposures of 6.0 G/s; and 3) a simulated aerial combat maneuver (SACM) of 4.5 to 9.0 +Gz. Performance tasks included the Unified Tri-Service Cognitive Performance Assessment Battery (UTC-PAB). The subjects were not able to correlate their symptoms with PB, placebo, or the acceleration exposure itself. Plasma PB individual levels ranged between 6 and 31 ng/ml and AChE levels of inhibition had a range of 12 to 45%. There were no significant effects on +Gz tolerance or performance related to PB. Based on the results of this study, PB does not significantly alter +Gz tolerance or performance. Therefore, we do not expect aircrew taking prophylactic doses of PB to be adversely affected during aerial combat operations.

The opticogravic nerve: eye-level anatomic relationships within the central nervous system.

The anatomic relationships between arterial blood supply and key structures within the central nervous system (CNS) are important in comprehending the neurophysiological effects of acceleration (+Gz) stress, including +Gz-induced loss of consciousness (G-LOC). An accurate understanding of the location of the vascular and neurologic structures at eye-level is vital, since it is possible to determine precisely when visual symptoms occur. Cerebral perfusion is supported by the pressure-equalizing effects provided by the cerebrospinal fluid. While brain and brain stem are protected by this pressure compensation, the eye is not. Decreased visual function results when retinal perfusion is compromised. G-LOC generally occurs following loss of visual function. The exact location(s) of altered perfusion within the CNS that results in G-LOC is currently unknown. To thoroughly understand G-LOC, it will be necessary to understand which CNS structures must be affected by +Gz-induced ischemia to cause G-LOC. Based on previous theoretical considerations of loss of consciousness as a protective mechanism, it is possible to consider the eye (visual system) as a dual sensor for both vision and gravitational (acceleration) stress. Due to this expanded definition of the sensory functions of the second cranial nerve, it would therefore be more appropriate to describe it as the "opticogravic" nerve. This manuscript discusses some of these considerations and the eye-level neuroanatomic relationships of vital importance for the acceleration medical subspecialist.

Techniques to enhance safety in acceleration research and fighter aircrew training.

Acceleration (+Gz) research and aircrew training using human centrifuges involves considerable stress that can alter normal cardiovascular and neurologic function even in completely healthy individuals. It is clear that electrocardiographic rate, rhythm, and conduction disturbances are frequently associated with +Gz exposures. These cardiac changes can result in altered perfusion of the central nervous system (CNS) to an extent which exceeds that induced by the +Gz stress alone. Although centrifuge-based research and training have a proven record of overall safety, there is finite risk associated with such stressful exposures, and adverse events have been observed. It is, therefore, extremely important to continually develop improved avenues to enhance human safety during centrifuge exposure. We have implemented techniques that can be immediately employed by centrifuge medical personnel to reduce the potential for significant CNS embarrassment and possible injury. These include techniques to 1) reduce excessive parasympathetic tone that may result in marked bradycardia and transient asystole post +Gz stress, and 2) manually controlled inflation and pulsation of the anti-G suit to enhance CNS perfusion post +Gz stress.

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.

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.

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.

The electrocardiographic response of females to centrifuge +Gz stress.

Women continue to expand their participation in all areas of aviation, including flying high performance fighter aircraft. Acceleration (+Gz) stress is unique to fighter aviation, therefore it is important to thoroughly understand the electrocardiographic (ECG) response to +Gz stress since it reflects a portion of the cardiovascular +Gz tolerance. A comparison of the ECG response to centrifuge +Gz stress between 685 men and 94 women was made from data existing within a centrifuge data repository. The frequency of occurrence of specific types of atrial, ventricular, and the other most frequently observed ECG changes to +Gz stress were compared for females and males. Females had less atrial ectopy; essentially equivalent premature ventricular contractions (PVC's), multiformed PVC's, paired PVC's; less frequent ventricular and supraventricular tachycardia; and more frequent PVC's in a bigeminal pattern and QRS on T PVC's. Sinus arrhythmia, sinus bradycardia, and increased T-waves post +Gz stress were more frequent in males, with ectopic atrial rhythm and atrioventricular dissociation essentially equivalent in males and females. Although few women have participated in either simulated aerial combat maneuver type centrifuge profiles or centrifuge high-G training, they have shown similar ECG changes including conduction and rhythm disturbances infrequently seen in males, such as +Gz-induced right bundle branch block and high-G bradycardia. Based on the currently available ECG response data, women have no demonstrated unique susceptibility to +Gz-induced ECG changes. Therefore, no contraindication exists to initiating additional acceleration research to fully evaluate women's tolerance to the more stressful, higher levels of +Gz stress.

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)

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.

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.

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.

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.

Dynamic cardiovascular response to +Gz stress in aerobically trained individuals.

Very high onset sustained +Gz stress requires rapid cardiovascular response to support human tolerance. This study was conducted following a previous study concerning +Gz tolerance in aerobically trained individuals, and was initiated to determine if intense aerobic conditioning might affect cardiovascular +Gz tolerance through reduction in heart rate response to +Gz stress. The study compared heart rate response data on 22 aerobically trained runners and 13 less-conditioned individuals. All subjects were exposed to a standard medical evaluation protocol, which consisted of a gradual-onset (0.1 G/s) acceleration exposure (GOR1), followed by a series of rapid-onset (1.0 G/s) acceleration exposures (ROR), a second gradual-onset rate exposure (GOR2), and a third gradual-onset rate exposure with the subjects performing anti-G straining maneuvers (GORS). Aerobic conditioning was not found to be associated with a reduced heart rate response to +Gz stress, compared to the response of unconditioned subjects, when the following variables were considered; heart rate change from rest to maximum exposure heart rate, heart rate change from rest to the heart rate achieved at the onset of maximum G, and the rate of change in heart rate per unit +Gz. Although enhanced parasympathetic tone, induced by long-term aerobic conditioning (running) results in a reduced heart rate at rest and during +Gz stress, it does not alter the responsiveness of the heart rate to +Gz stress.

Recovery to +1Gz and +2Gz following +Gz-induced loss of consciousness: operational considerations.

With the development of aircraft autorecovery technology, the need to understand the effects of potential inflight recovery profiles on human physiology has become important. Eight male volunteer subjects were exposed to +7Gz with an onset rate of 6 G.s-1 until they were unconscious. The subjects did not wear anti-G suits and did not perform anti-G straining maneuvers. The subjects controlled the centrifuge utilizing an F-16A stick, thereby deliberately self-inducing their unconsciousness. Following +Gz-induced loss of consciousness (G-LOC), recovery to the usual +1Gz level was compared to recovery to a +2Gz level by comparing absolute, relative, and total incapacitation times. The mean (+/- S.D.) absolute incapacitation time (period of unconsciousness) was 11.9 +/- 2.9 s for recovery to a +1Gz level and 12.9 S (+/- 6.9 S.D.) for recovery to a +2Gz level. The mean relative incapacitation time (period of confusion/disorientation) was 3.6 +/- 2.3 s for recovery to a +1Gz level as compared to 2.9 +/- 0.8 s for recovery to a +2Gz level. The total incapacitation time (sum of the absolute and relative incapacitation) was 15.6 +/- 2.7 s for recovery to a +1Gz level and 16.0 s (+/- 6.8 S.D.) for recovery to a +2Gz level. No significant differences in any of the incapacitation times were found when comparing recovery to +1Gz and +2Gz. The mean time from the onset of +Gz-stress to the onset of unconsciousness was approximately 7 s.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.