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PubMed1993

Physiological considerations concerning positive pressure breathing (PBG) during +Gz.

The ability to tolerate +Gz radial acceleration depends primarily on the maintenance of sufficient head level arterial pressure and cerebral blood flow to prevent the occurrence of blackout and G-induced loss of consciousness (G-LOC). Because of the hydrostatic effect on the heart-to-head blood column during +Gz acceleration, if exposures to higher +Gz levels are to be tolerated, either the column must be shortened or arterial pressure at heart level must be elevated. This paper is an overview of the effect and concomitant side effects of positive pressure breathing (PBG) as a means to increase arterial pressure at the heart, and, indirectly, at the cerebral level. However, before doing that, it is necessary to summarise the different ways for increasing arterial pressure to obtain tolerance to increasing +Gz loads.

Acceleration↗
PubMed1993

Some genetic aspects of adaptation to hyperdynamic environments.

Reports by many investigators have described a wide variety of biological changes in man and animals exposed to gravitational fields, some of these having a beneficial effect but others are detrimental. These changes are considered to result from the alteration in the gravitational field since (a) they appear when the field is changed, (b) the degree of change tends to be proportional to the change in acceleration strength, and (c) these changes disappear, and at varying rates, when the gravitational field is returned to normal gravity. However, little information has been developed concerning the reception and transduction of the gravitational stimulus, or its lack, to produce these biological changes.

Acceleration↗
PubMed1993

Modeling and numerical simulation of the cerebral blood flow under acceleration.

When fighter aircrafts change directions, the pilots are subject to a centrifuge force. This force can be enough to induce the pilot loss of consciousness (LOC). The physiological cause of this LOC is a brain hypoxia which results from the increase of the hydrostatic pressure. Under gradual onset rate + Gz acceleration (GOR+Gz), the brain hypoxia occurs with visual prodromes familiar to fighter pilots. In this case, they feel a reduction of the peripheral vision (Grey-out), followed by a loss of the central vision (Black-out). The useful time of consciousness can be prolonged by using anti-G suit. Under rapid onset rate +Gz acceleration, a new symptomatology of the LOC, characterized by the absence of a grey or black-out, has been observed. The objective of this theoretical study is a better understanding of mechanisms which cause fighter pilot inflight LOC. A physical model has been developed. It allows to take into account the influence of the rate of change in acceleration on the cerebrovascular system behavior.

Acceleration↗
PubMed1993

Guidelines for a research and development (R&D) program for high sustained G.

This article is a contribution to the workshop on "Operational Requirements in the Prevention of G-Induced Loss of Consciousness (G-LOC) in High Performance Aircraft"; it focuses on the "operational" side of the requirements to prevent G-LOC. There are two types of requirements for prevention of G-LOC; a) pre-G-LOC detection devices that monitor physiologic changes of the pilot before G-LOC occurs, and b) the more generic personal G protection systems that increase G tolerance. Recently there have been major advances in G-protection research and development (R&D) systems (soon to become operational) that significantly improve G-level and G-duration tolerances. We do not know the extent of the impact of these new systems on G-LOC, but it could be substantial. These near-term operational anti-G systems are: 1. Positive pressure breathing (PPB) systems assisted by chest counterpressure that are activated by an increase in G levels; i.e., the higher G level, the greater the pressures applied to the aircrew member (Morgan et al., 1992). This PPB system is known as PBG. PBG has been flight tested with the standard operational anti-G suit. This test program known as Combat Edge significantly reduces pilot fatigue and extends G-duration tolerance. In limited operational testing of F-16 and F-15 aircraft, PBG has received substantial pilot acceptance. 2. An improved anti-G suit that provides uniform pressure of the lower body that includes the operational anti-G suit abdominal bladder. This new anti-G suit concept increases both G-level and G-duration tolerances (Krutz et al., 1990; Morgan et al., 1992). Flight tested as the Advanced Tactical Anti-G Suit (ATAGS), it is comfortable and preferred by pilots over the standard operational anti-G suit. 3. The combination of Combat Edge and ATAGS, the most advanced anti-G system in the final stages of development (Morgan et al., 1992). This anti-G equipment has been tested in the laboratory on the centrifuge. Experimental subjects are able to tolerate 8 to 9G "relaxed" or with a minimal anti-G straining maneuver (AGSM) (Morgan, 1992). Accleration scientists believe that once these systems become operational on high performance aircraft that the incidence of G-LOC will be significantly reduced, particularly G-LOC associated with fatigue.

Aerospace Medicine↗
PubMed1994

Changes of pulmonary function in humans during exposure to +Gx acceleration after simulated and real microgravity.

An important goal of space medicine is preserving high tolerance and performance of cosmonauts an ring exposure to acceleration at the final flight stage given varying mission duration. Among physiological mechanisms limiting +Gx acceleration tolerance, an important role is played by disturbances of external respiration resulting from alterations of respiratory biomechanics, pulmonary gas exchange conditions, and arterial hypoxemia. However, at present data on external respiration changes during exposure to +Gx acceleration after simulated and real microgravity of varying duration, are extremely scanty.

Bed Rest↗
PubMed1994

Artificial gravity in space flight.

Clearly, physiologic adaptation to terrestrial life for all animals is assured only by frequent encounters with gravity. Indeed, upon exposure to weightlessness in space flight, losses of physiologic functions quickly begin. Some physiologic parameters change more rapidly than others, but the deconditioning process starts rapidly. The rates of functional losses for all affected parameters are interesting in that they appear to approach a limit; i.e., losses of these functions may not continue until indefinitely. The regulation of this functional asymptotic response to space is not known, but probably based on functional requirements of the body to life itself and perhaps genetic expression. The latter controlling mechanism (DNA) functions only on aquatic (weightless) animals on Earth--land animals must stimulate these physiologic functions as they relate to gravity on a regular frequent basis. This loss of regulation upon entering the weightless environment is fascinating since land-based animals including the humans have evolved from millions (perhaps billions) of years of terrestrially adapted ancestors. One would expect some DNA involvement in the regulation of its physiology, but it appears to be absent. Therefore, if the functional debilitation of space is to be denied, we must begin to understand the adaptation process of the sole basis for the control of our physiologic processes on land; i.e., how gravity regulates our biologic functions. To learn about this regulatory mechanism, some inquiry into how aquatic animals first adapted to living on land might be helpful.

Adaptation, Physiological↗
PubMed1994

Effect of acceleration on the distribution of ventilation in man.

The Symposium 'High G and the Lungs' looks at the physiological and potentially pathophysiological effects of the high-G aerospace environment on the lungs of human subjects and experimental animals. It is logical, therefore, to start off with a brief description of that environment and how sustained inertial forces affect pulmonary function.

Aerospace Medicine↗
PubMed1994

The influence of the AGSM, PBG, and anti-G suit inflation on thoracic hemodynamics during +Gz in swine.

With the advent of pressure breathing for +Gz (head-to-foot inertial loading) protection (PBG) and the development of improved extended coverage anti-G suits (ECGS) it has become important to expand our knowledge of the cardiopulmonary physiologic interrelationships of pressure breathing, anti-G suit protection, and the anti-G straining maneuver (AGSM). Although high levels of pressure breathing have been previously investigated, there was continuing concern within the aeromedical community regarding the introduction of COMBAT EDGE, a PBG system. Some of the concerns were: barotrauma, pneumothorax, air embolism, excessive transmural vascular pressures, possible cardiac valvular damage, and possible overdilation of the right ventricle from a surge in venous return following +Gz. This study describes the experimental preparation and results of a hemodynamic investigation to address some of these concerns using chronically instrumented miniature swine (MS).

Animals↗
PubMed1994

Changes in hypothalamic [correction of hypothalmic] staining for c-Fos following 2G exposure in rats.

The static gravitational field of the earth has been an important selective pressure that has shaped the evolution of biological organisms. This is illustrated by the evolution of tetrapods from a water environment where gravitational force was partially negated to a terrestrial environment where gravity is of greater consequence. Terrestrial invasion resulted in a series of new structural, physiological, and behavioral features. Therefore, it is not surprising that alterations in the gravitational field can cause widespread effects in many physiological systems and behaviors. Our previous studies have demonstrated that both exposure to hyperdynamic fields and the microgravity condition of space flight have significant effects on body temperature, heartrate, activity, feeding, drinking, and circadian rhythms. However, it has not been determined whether these physiological adaptations are associated with changes in neural activity within the hypothalamic nuclei that regulate these functions. This study examined the changes in body temperature, activity, body weight and food and water intake in rats caused by exposure to a hyperdynamic field. In addition, the immediate early gene activation marker, c-Fos, was used to examine potential protein synthesis changes in the hypothalamic nuclei that regulate these functions.

Animals↗