Since the first sustained flight of a powered aircraft at Kitty Hawk, North Carolina (United States), in 1903, aviation has become a major international activity. It is estimated that from 1960 to 1989, the annual number of air passengers of regularly scheduled flights increased from 20 million to over 900 million (Poitrast and deTreville 1994). Military aircraft have become indispensable weapons systems for the armed forces of many nations. Advances in aviation technology, in particular the design of life support systems, have contributed to the rapid development of space programmes with human crews. Orbital space flights occur relatively frequently, and astronauts and cosmonauts work in space vehicles and space stations for extended periods of time.

In the aerospace environment, physical stressors that may affect the health of aircrew, passengers and astronauts to some degree include reduced concentrations of oxygen in the air, decreased barometric pressure, thermal stress, acceleration, weightlessness and a variety of other potential hazards (DeHart 1992). This article describes aeromedical implications of exposure to gravity and acceleration during flight in the atmosphere and the effects of microgravity experienced in space.

Gravity and Acceleration

The combination of gravity and acceleration encountered during flight in the atmosphere produces a variety of physiological effects experienced by aircrew and passengers. At the surface of the earth, the forces of gravity affect virtually all forms of human physical activity. The weight of a person corresponds to the force exerted upon the mass of the human body by the earth’s gravitational field. The symbol used to express the magnitude of the acceleration of an object in free fall when it is dropped near the earth’s surface is referred to as g, which corresponds to an acceleration of approximately 9.8 m/s² (Glaister 1988a; Leverett and Whinnery 1985).

Acceleration occurs whenever an object in motion increases its velocity. Velocity describes the rate of movement (speed) and direction of motion of an object. Deceleration refers to acceleration that involves a reduction in established velocity. Acceleration (as well as deceleration) is a vector quantity (it has magnitude and direction). There are three types of acceleration: linear acceleration, a change of speed without change in direction; radial acceleration, a change in direction without a change of speed; and angular acceleration, a change in speed and direction. During flight, aircraft are capable of manoeuvring in all three directions, and crew and passengers may experience linear, radial and angular accelerations. In aviation, applied accelerations are commonly expressed as multiples of the acceleration due to gravity. By convention, G is the unit expressing the ratio of an applied acceleration to the gravitational constant (Glaister 1988a; Leverett and Whinnery 1985).

Biodynamics

Biodynamics is the science dealing with the force or energy of living matter and is a major area of interest within the field of aerospace medicine. Modern aircraft are highly manoeuvrable and capable of flying at very high speeds, causing accelerative forces upon the occupants. The influence of acceleration upon the human body depends upon the intensity, rate of onset and direction of acceleration. The direction of acceleration is generally described by the use of a three-axis coordinate system (x, y, z) in which the vertical (z) axis is parallel to the long axis of the body, the x axis is oriented from front to back, and the y axis oriented side to side (Glaister 1988a). These accelerations can be categorized into two general types: sustained and transitory.

Sustained acceleration

The occupants of aircraft (and spacecraft operating in the atmosphere under the influence of gravity during launch and re-entry) commonly experience accelerations in response to aerodynamic forces of flight. Prolonged changes in velocity involving accelerations lasting longer than 2 seconds may result from changes in an aircraft’s speed or direction of flight. The physiological effects of sustained acceleration result from the sustained distortion of tissues and organs of the body and changes in the flow of blood and distribution of body fluids (Glaister 1988a).

Positive or headward acceleration along the z axis (+G_z) represents the major physiological concern. In civil air transportation, G_z accelerations are infrequent, but may occasionally occur to a mild degree during some take-offs and landings, and while flying in conditions of air turbulence. Passengers may experience brief sensations of weightlessness when subject to sudden drops (negative G_z accelerations), if unrestrained in their seats. An unexpected abrupt acceleration may cause unrestrained aircrew or passengers to be thrown against internal surfaces of the aircraft cabin, resulting in injuries.

In contrast to civil transport aviation, the operation of high- performance military aircraft and stunt and aerial spray planes may generate significantly higher linear, radial and angular accelerations. Substantial positive accelerations may be generated as a high-performance aircraft changes its flight path during a turn or a pull-up manoeuvre from a steep dive. The +G_z performance characteristics of current combat aircraft may expose occupants to positive accelerations of 5 to 7 G for 10 to 40 seconds (Glaister 1988a). Aircrew may experience an increase in the weight of tissues and of the extremities at relatively low levels of acceleration of only +2 G_z. As an example, a pilot weighing 70 kg who performed an aircraft manoeuvre which generated +2 G_z would experience an increase of body weight from 70 kg to 140 kg.

The cardiovascular system is the most important organ system for determining the overall tolerance and response to +G_z stress (Glaister 1988a). The effects of positive acceleration on vision and mental performance are due to decreases in blood flow and delivery of oxygen to eye and brain. The capability of the heart to pump blood to the eyes and brain is dependent upon its capability to exceed the hydrostatic pressure of blood at any point along the circulatory system and the inertial forces generated by the positive G_z acceleration. The situation may be likened to that of pulling upward a balloon partially full of water and observing the downward distension of the balloon because of the resultant inertial force acting upon the mass of water. Exposure to positive accelerations may cause temporary loss of peripheral vision or complete loss of consciousness. Military pilots of high- performance aircraft may risk development of G-induced blackouts when exposed to rapid onset or extended periods of positive acceleration in the +G_z axis. Benign cardiac arrhythmias frequently occur following exposure to high sustained levels of +G_z acceleration, but usually are of minimal clinical significance unless pre-existing disease is present; –G_z acceleration seldom occurs because of limitations in aircraft design and performance, but may occur during inverted flight, outside loops and spins and other similar manoeuvres. The physiological effects associated with exposure to –G_z acceleration primarily involve increased vascular pressures in the upper body, head and neck (Glaister 1988a).

Accelerations of sustained duration which act at right angles to the long axis of the body are termed transverse accelerations and are relatively uncommon in most aviation situations, with the exception of catapult and jet- or rocket-assisted take-offs from aircraft carriers, and during launch of rocket systems such as the space shuttle. The accelerations encountered in such military operations are relatively small, and usually do not affect the body in a major fashion because the inertial forces act at right angles to the long axis of the body. In general, the effects are less pronounced than in G_z accelerations. Lateral acceleration in ±G_y axis are uncommon, except with experimental aircraft.

Transitory acceleration

The physiological responses of individuals to transitory accelerations of short duration are a major consideration in the science of aircraft accident prevention and crew and passenger protection. Transitory accelerations are of such brief duration (considerably less than 1 second) that the body is unable to attain a steady-state status. The most common cause of injury in aircraft accidents results from the abrupt deceleration that occurs when an aircraft impacts the ground or water (Anton 1988).

When an aircraft impacts the ground, a tremendous amount of kinetic energy applies damaging forces to the aircraft and its occupants. The human body responds to these applied forces by a combination of acceleration and strain. Injuries result from deformation of tissues and organs and trauma to anatomic parts caused by collision with structural components of the aircraft cockpit and/or cabin.

Human tolerance to abrupt deceleration is variable. The nature of injuries will depend on the nature of the applied force (whether it primarily involves penetrating or blunt impact). At impact, the forces which are generated are dependent on the longitudinal and horizontal decelerations which are generally applied to an occupant. Abrupt decelerative forces are often categorized into tolerable, injurious and fatal. Tolerable forces produce traumatic injuries such as abrasions and bruises; injurious forces produce moderate to severe trauma which may not be incapacitating. It is estimated that an acceleration pulse of approximately 25 G maintained for 0.1 second is the limit of tolerability along the +G_z axis, and that about 15 G for 0.1 sec is the limit for the –G_z axis (Anton 1988).

Multiple factors affect human tolerance to short-duration acceleration. These factors include the magnitude and duration of the applied force, the rate of onset of the applied force, its direction and the site of application. It should be noted that people can withstand much greater forces perpendicular to the long axis of the body.

Protective Countermeasures

Physical screening of crew members to identify serious pre- existing diseases which might put them at increased risk in the aerospace environment is a key function of aeromedical programmes. In addition, countermeasures are available to crew of high-performance aircraft to protect against the adverse effects of extreme accelerations during flight. Crew members must be trained to recognize that multiple physiological factors may decrease their tolerance to G stress. These risk factors include fatigue, dehydration, heat stress, hypoglycemia and hypoxia (Glaister 1988b).

Three types of manoeuvres which crew members of high- performance aircraft employ to minimize adverse effects of sustained acceleration during flight are muscle tensing, forced expiration against a closed or partially closed glottis (back of the tongue) and positive-pressure breathing (Glaister 1988b; DeHart 1992). Forced muscle contractions exert increased pressure on blood vessels to decrease venous pooling and increase venous return and cardiac output, resulting in increased blood flow to the heart and upper body. While effective, the procedure requires extreme, active effort and may rapidly result in fatigue. Expiration against a closed glottis, termed the Valsalva manoeuver (or M-1 procedure) can increase pressure in the upper body and raise the intrathoracic pressure (inside the chest); however, the result is short lived and may be detrimental if prolonged, because it reduces venous blood return and cardiac output. Forcibly exhaling against a partially closed glottis is a more effective anti-G straining manoeuver. Breathing under positive pressure represents another method to increase intrathoracic pressure. Positive pressures are transmitted to the small artery system, resulting in increased blood flow to the eyes and brain. Positive-pressure breathing must be combined with the use of anti-G suits to prevent excessive pooling in the lower body and limbs.

Military aircrew practise a variety of training methods to enhance G tolerance. Crews frequently train in a centrifuge consisting of a gondola attached to a rotating arm which spins and generates +G_z acceleration. Aircrew become familiar with the spectrum of physiological symptoms which may develop and learn the proper procedures to control them. Physical fitness training, particularly whole-body strength training, also has been found to be effective. One of the most common mechanical devices used as protective equipment to reduce the effects of +G exposure consists of pneumatically inflated anti-G suits (Glaister 1988b). The typical trouser-like garment consists of bladders over the abdomen, thighs and calves which automatically inflate by means of an anti-G valve in the aircraft. The anti-G valve inflates in reaction to an applied acceleration upon the aircraft. Upon inflation, the anti-G suit produces a rise in the tissue pressures of the lower extremities. This maintains peripheral vascular resistance, reduces the pooling of blood in the abdomen and lower limbs and minimizes downward displacement of the diaphragm to prevent the increase in the vertical distance between the heart and brain that may be caused by positive acceleration (Glaister 1988b).

Surviving transitory accelerations associated with aircraft crashes is dependent on effective restraint systems and the maintenance of the cockpit/cabin integrity to minimize intrusion of damaged aircraft components into the living space (Anton 1988). The function of lap belts, harnesses and other types of restraint systems are to limit the movement of the aircrew or passengers and to attenuate the effects of sudden deceleration during impact. The effectiveness of the restraint system depends on how well it transmits loads between the body and the seat or vehicle structure. Energy-attenuating seating and rearward facing seats are other features in aircraft design which limit injury. Other accident-protection technology includes the design of airframe components to absorb energy and improvements in seat structures to reduce mechanical failure (DeHart 1992; DeHart and Beers 1985).

Microgravity

Since the 1960s, astronauts and cosmonauts have flown numerous missions into space, including 6 lunar landings by Americans. Mission duration has been from several days to a number of months, with a few Russian cosmonauts logging approximately 1-year flights. Subsequent to these space flights, a large body of literature has been written by physicians and scientists describing in-flight and post-flight physiological aberrations. For the most part, these aberrations have been attributed to exposure to weightlessness or microgravity. Although these changes are transient, with total recovery within several days to several months after returning to Earth, nobody can say with complete certitude whether astronauts would be so fortunate after missions lasting 2 to 3 years, as envisioned for a round trip to Mars. The major physiological aberrations (and countermeasures) can be categorized as cardiovascular, musculoskeletal, neurovestibular, haematological and endocrinological (Nicogossian, Huntoon and Pool 1994).

Cardiovascular hazards

Thus far, there have been no serious cardiac problems in space, such as heart attacks or heart failure, although several astronauts have developed abnormal heart rhythms of a transient nature, particularly during extra-vehicular activity (EVA). In one case, a Russian cosmonaut had to return to Earth earlier than planned, as a precautionary measure.

On the other hand, microgravity seems to induce a lability of blood pressure and pulse. Although this does not cause impaired health or crew performance during flight, approximately half of astronauts immediately post-flight do become extremely dizzy and giddy, with some experiencing fainting (syncope) or near fainting (pre-syncope). The cause of this intolerance to being vertical is thought to be a drop in blood pressure upon re-entering the earth’s gravitational field, combined with the dysfunction of the body’s compensatory mechanisms. Hence, a low blood pressure and decreasing pulse unopposed by the body’s normal response to such physiological aberrations results in these symptoms.

Although these pre-syncopal and syncopal episodes are transient and without sequelae, there remains great concern for several reasons. First, in the event that a returning space vehicle were to have an emergency, such as a fire, upon landing, it would be extremely difficult for astronauts to rapidly escape. Second, astronauts landing on the moon after periods of time in space would be prone to some extent to pre-fainting and fainting, even though the moon’s gravitational field is one-sixth that of Earth. And finally, these cardiovascular symptoms might be far worse or even lethal after very long missions.

It is for these reasons that there has been an aggressive search for countermeasures to prevent or at least ameliorate the microgravity effects upon the cardiovascular system. Although there are a number of countermeasures now being studied that show some promise, none so far has been proven truly effective. Research has focused on in-flight exercise utilizing a treadmill, bicycle ergometer and rowing machine. In addition, studies are also being conducted with lower body negative pressure (LBNP). There is some evidence that lowering the pressure around the lower body (using compact special equipment) will enhance the body’s ability to compensate (i.e., raise blood pressure and pulse when they fall too low). The LBNP countermeasure might be even more effective if the astronaut drinks moderate amounts of specially constituted salt water simultaneously.

If the cardiovascular problem is to be solved, not only is more work needed on these countermeasures, but also new ones must be found.

Musculoskeletal hazards

All astronauts returning from space have some degree of muscle wasting or atrophy, regardless of mission duration. Muscles at particular risk are those of the arms and legs, resulting in decreased size as well as strength, endurance and work capacity. Although the mechanism for these muscle changes is still ill-defined, a partial explanation is prolonged disuse; work, activity and movement in microgravity are almost effortless, since nothing has any weight. This may be a boon for astronauts working in space, but is clearly a liability when returning to a gravitational field, whether it be that of the moon or Earth. Not only could a weakened condition impede post-flight activities (including work on the lunar surface), it could also compromise rapid ground emergency escape, if required upon landing. Another factor is the possible requirement during EVA to do space vehicle repairs, which can be very strenuous. Countermeasures under study include in-flight exercises, electrical stimulation and anabolic medication (testosterone or testosterone-like steroids). Unfortunately, these modalities at best only retard muscle dysfunction.

In addition to muscle wasting, there is also a slow but inexorable loss of bone in space (about 300 mg per day, or 0.5% of total bone calcium per month) experienced by all astronauts. This has been documented by post-flight x rays of bones, particularly of those that bear weight (i.e., the axial skeleton). This is due to a slow but unremitting loss of calcium into the urine and faeces. Of great concern is the continuing loss of calcium, regardless of flight duration. Consequently, this calcium loss and bone erosion could be a limiting factor of flight, unless an effective countermeasure can be found. Although the precise mechanism of this very significant physiological aberration is not fully understood, it undoubtedly is due in part to the absence of gravitational forces on bone, as well as disuse, similar to muscle wasting. If bone loss were to continue indefinitely, particularly over long missions, bones would become so brittle that eventually there would be risk of fractures with even low levels of stress. Furthermore, with a constant flow of calcium into the urine via the kidneys, a possibility of renal stone formation exists, with accompanying severe pain, bleeding and infection. Clearly, any of these complications would be a very serious matter were they to occur in space.

Unfortunately, there are no known countermeasures that effectively prevent calcium loss during space flight. A number of modalities are being tested, including exercise (treadmill, bicycle ergometer and rowing machine), the theory being that such voluntary physical stresses would normalize bone metabolism, thereby preventing or at least ameliorating bone loss. Other countermeasures under investigation are calcium supplements, vitamins and various medications (such as diphosphonates—a class of medications that has been shown to prevent bone loss in patients with osteoporosis). If none of these simpler countermeasures prove to be effective, it is possible that the solution lies in artificial gravity that could be produced by continuous or intermittent rotation of the space vehicle. Although such motion could generate gravitational forces similar to that of the earth, it would represent an engineering “nightmare”, in addition to major add-on costs.

Neurovestibular hazards

More than half of the astronauts and cosmonauts suffer from space motion sickness (SMS). Although the symptoms vary somewhat from individual to individual, most suffer from stomach awareness, nausea, vomiting, headache and drowsiness. Often there is an exacerbation of symptoms with rapid head movement. If an astronaut develops SMS, it usually occurs within a few minutes to a few hours after launch, with complete remission within 72 hours. Interestingly, the symptoms sometimes recur after returning to the earth.

SMS, particularly vomiting, can not only be disconcerting to the crew members, it also has the potential to cause performance decrement in an astronaut who is ill. Furthermore, the risk of vomiting while in a pressure suit doing EVA cannot be ignored, as the vomitus could cause the life-support system to malfunction. It is for these reasons that no EVA activities are ever scheduled during the first 3 days of a space mission. If an EVA becomes necessary, for example, to do emergency repairs on the space vehicle, the crew would have to take that risk.

Much neurovestibular research has been directed toward finding a way to prevent as well as to treat SMS. Various modalities, including anti-motion-sickness pills and patches, as well as using pre-flight adaptation trainers such as rotating chairs to habituate astronauts, have been attempted with very limited success. However, in recent years it has been discovered that the antihistamine phenergan, given by injection, is an extremely effective treatment. Hence, it is carried onboard all flights and given as required. Its efficacy as a preventive has yet to be demonstrated.

Other neurovestibular symptoms reported by astronauts include dizziness, vertigo, dysequilibrium and illusions of self-motion and motion of the surrounding environment, sometimes making walking difficult for a short time post-flight. The mechanisms for these phenomena are very complex and are not completely understood. They could be problematical, particularly after a lunar landing following several days or weeks in space. As of now, there are no known effective countermeasures.

Neurovestibular phenomena are most likely caused by dysfunction of the inner ear (the semicircular canals and utricle-saccule), because of microgravity. Either erroneous signals are sent to the central nervous system or signals are misinterpreted. In any event, the results are the aforementioned symptoms. Once the mechanism is better understood, effective countermeasures can be identified.

Haematological hazards

Microgravity has an effect upon the body’s red and white blood cells. The former serve as a conveyor of oxygen to the tissues, and the latter as an immunological system to protect the body from invading organisms. Hence, any dysfunction could cause deleterious effects. For reasons not understood, astronauts lose approximately 7 to 17% of their red blood cell mass early in flight. This loss appears to plateau within a few months, returning to normal 4 to 8 weeks post-flight.

So far, this phenomenon has not been clinically significant, but, rather, a curious laboratory finding. However, there is clear potential for this loss of red blood cell mass to be a very serious aberration. Of concern is the possibility that with very long missions envisioned for the twenty-first century, red blood cells could be lost at an accelerated rate and in far greater quantities. If this were to occur, anaemia could develop to the point that an astronaut could become seriously ill. It is hoped that this will not be the case, and that the red blood cell loss will remain very small, regardless of mission duration.

In addition, several components of the white blood cell system are affected by microgravity. For example, there is an overall increase in the white blood cells, mainly neutrophils, but a decrease in lymphocytes. There is also evidence that some white blood cells do not function normally.

As of now, in spite of these changes, no illness has been attributed to these white blood cell changes. It is unknown whether or not a long mission will cause further decrease in numbers as well as further dysfunction. Should this occur, the body’s immune system would be compromised, making astronauts very susceptible to infectious disease, and possibly incapacitated by even minor illness that would otherwise easily be fended off by a normally functioning immunological system.

As with the red blood cell changes, the white blood cell changes, at least on missions of approximately one year, are not of clinical significance. Because of the potential risk of serious illness in-flight or post-flight, it is critical that research continue on the effects of microgravity on the haematological system.

Endocrinological hazards

During space flight, it has been noted that there are a number of fluid and mineral changes within the body due in part to changes in the endocrine system. In general, there is a loss of total body fluids, as well as calcium, potassium and calcium. A precise mechanism for these phenomena has eluded definition, although changes in various hormonal levels offer a partial explanation. To further confound matters, laboratory findings are often inconsistent among the astronauts who have been studied, making it impossible to discern a unitary hypothesis as to the cause of these physiological aberrations. In spite of this confusion, these changes have caused no known impairment of health of astronauts and no performance decrement in flight. What the significance of these endocrine changes are for very long flight, as well as the possibility that they may be harbingers of very serious sequelae, is unknown.

Acknowledgements: The authors would like to recognize the work of the Aerospace Medical Association in this area.

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