Air University Review, July-August 1968
Present-Day flight research activities, including manned space light and testing of advanced aircraft, impose new and increasing demands on man’s biologic tolerance. Not all apparently healthy individuals posses the physical and mental stamina needed to cope with the exacting mission requirements. As a consequence, the Air Force has developed a medical evaluation program to identify those candidates who have the biologic capacity to complete such specialized missions safety.
The primary goal of the examination continues to be the detection of significant disorders or disease processes, but emphasis is placed also on the measurement of an individual’s response to various forms of stress and to those procedures that might provide information of a predictive nature regarding his future health.
The foundation of any medical evaluation consists of a comprehensive history and a detailed physical examination. Many health disorders, some not suspected by the individual himself, are detected by these means. As some significant abnormalities may not be uncovered by these techniques, additional laboratory, X-ray, and other diagnostic tools must be utilized. Finally, despite all these procedures, some disease states may remain hidden when the subject is examined in the basal or resting state. Only when he is evaluated under conditions that tax the functions of the various organs do the defects appear. Thus, medical evaluation during dynamic states characterizes the Air Force program and distinguishes it from the more conventional and traditional approaches of medicine.
One of the most prevalent disorders that we watch for during the Air Force regimen is coronary heart disease. This entity is not limited to the aged but is encountered frequently in the fifth, fourth, or even in the third decade of life. It is of particular significance in aerospace medicine in that it may cause sudden incapacitation or death. Not, uncommonly, warning symptoms that may be present for variable periods of time are misinterpreted by the individual and passed off as merely as pains, indigestion, muscle soreness, etc. A skilled examiner is alerted once he elicits such a history, but then he must obtain confirmation that these complaints are really due to significant underlying heart disease rather than to innocent bodily changes. In some instances, there may be no symptoms whatsoever preceding the catastrophic event. It is obvious, then, that one cannot rely on history alone to detect susceptible individuals. Physical examination usually is not revealing in these cases. The electrocardiogram will show diagnostic changes if permanent damage has been done to the heart muscle damage is the end result of coronary heart disease. Significant abnormality can exist in the coronary arteries for long periods of time before segments of heart muscle are altered in any way.
Years ago, it was noted that characteristic electrocardiographic changes occurred in individuals with coronary heart disease during their episodes of pain. After the pain subsided, these changes disappeared and the tracing returned to completely normal limits. Since pain in people afflicted with this disease is usually brought on by physical exertion, a diagnostic test was developed utilizing the electrocardiogram in association with exercise. This procedure, known as the Master 2-step test, is one of the earliest (1940) stress tests still in use today. Equipment and techniques were not available at that time for recording the electrocardiogram successfully while the subject was exercising, so tracings were taken immediately after, two minutes after, and five minutes after a standardized work load: a given number of trips up and down two steps in a given period of time, depending on sex, age, and weight. Because excessive physical exertion in individuals with latent coronary heart disease may induce not only pain but also other very serious consequences, great care was taken to limit the work load on the heart in the Master test to safe levels.
As was hoped, this test did indeed produce characteristic findings in 50 to 70 percent of patients in whom the diagnosis of coronary heart disease could be established by other criteria as well. Apparently healthy groups have been tested with the Master technique, and abnormal responses have been obtained in a very small percentage of them. When followed over a period of years (up to ten), healthy individuals who demonstrated an abnormal response to this test have developed overt signs of coronary heart disease twenty times more frequently than control groups of healthy individuals who had a negative response to the test originally. This procedure, then, is useful in evaluating those who complain of chest discomfort, indigestion, gas pains, etc., as well as providing data of a predictive nature in individuals with no complaints at all.
More recently, with the advent of devices that permit reliable electrocardiographic monitoring during physical activity, subjects can be placed on a treadmill and exercised to any degree of exhaustion. If electrocardiographic changes appear indicating potential hazard, the activity can be stopped at any time, before pain or other untoward effects appear. Maximal exercise testing has increased considerably the yield of diagnostic responses in patients with known coronary heart disease, providing an even more sensitive method for identifying high-risk individuals who are otherwise apparently healthy.
The specialty of aerospace medicine has had to develop new knowledge from experiences undergone in space flight, in order to accommodate man to meet the physical and mental requirements demanded of him. Some of the new phenomena encountered, e.g., in weightlessness, were found to have a useful basis in old and elementary medical knowledge. The state of consciousness is dependent upon adequate perfusion of the brain by oxygenated arterial blood. The contraction of the heart provides adequate force to propel oxygenated blood to the brain and the other organs of the body. This pressure head is dampened considerably by the passage of blood from arteries to the smaller arterioles, to the capillaries, and through the tissue spaces, so that once it reaches the venous side of the circulation for its trip back to the heart it is flowing under very little pressure. Since the greater part of the body lies below the heart with man in an upright posture and since venous flow must overcome the effect of gravity, blood would tend to pool, particularly in the lower extremities, if some other mechanisms were not present to add impetus to continuing blood circulation. If for any reason the amount of blood returning to the heart should be diminished, the amount it could expel to the lungs and subsequently to the remainder of the body would likewise be less. Brain perfusion would be compromised, and loss of consciousness would ensue. Compensatory bodily mechanisms do exist, however. The bellows-like action of normal breathing activity tends to “suck” blood up the veins. The large veins in the legs all lie in close proximity to large muscle groups. Muscle contraction tends to “milk” blood up the veins. Even when an individual stands still, the muscles are active, antagonistic groups alternately contracting and relaxing without conscious effort on his part. This activity is sufficient to help propel blood upward. Finally, neurovascular reflexes prevent excessive dilatation of the blood vessels and in this manner resist pooling.
It has been shown that prolonged exposure of normal man to weightlessness causes some loss of blood volume, wasting of muscle mass, and a temporary sluggishness of the neurovascular reflexes due to disuse. These changes become significant when man returns to earth’s environment and his blood must overcome the effects of gravity to return to the heart. Obviously, men whose cardiovascular system is not compromised even to a minor degree must be selected for such missions. Furthermore, various diseases that begin insidiously may affect either the neurovascular reflex components or the vessels themselves without any other manifestations at first. Under normal circumstances, the respiratory factors and muscle activity may well compensate for this deficiency, excessive pooling may be limited, and no symptoms may be recognizable. If such an individual is placed in an upright posture without permitting any weight bearing, the lower-extremity muscles could be relaxed completely. With both the neurovascular reflexes and the muscle activity gone, blood flow back to the heart would diminish considerably. Cardiac output and blood pressure then drop quickly, the brain receives inadequate circulation, and loss of consciousness follows.
The tilt-table orthostatic tolerance test provides these circumstances to identify loss of normal neurovascular responses. The subject lies on a table to which he is attached by a parachute harness. He is then tilted to an upright position and remains suspended so that no weight bearing is required. His pulse rate, blood pressure, electrocardiogram, and state of consciousness are monitored continuously. In healthy individuals, the normal breathing activity and neurovascular reflexes are sufficient to maintain adequate circulation for a good number of minutes despite the absence of muscle action.
Our pilots and astronauts frequently encounter circumstances for pooling of blood in the lower body during aerospace missions while “pulling” positive g-forces, All the compensatory mechanisms that have been described are required to maintain circulation, but these are eventually overcome even in normal individuals if the accelerative forces are maintained or increased enough, The range of tolerance varies. Individuals who are less able to cope with such forces, although they are physically normal, can be identified by their reactions in the human centrifuge, The accelerative forces of any flight profile, including lift-off and re-entry of space flight, can be accurately produced by this mechanism, The subject’s heart rate, heart rhythm, and electrocardiogram as well as his visual capacities can be monitored continuously during the procedure. The appearance of grayout can be determined quickly and the test concluded before actual loss of consciousness occurs. During the run, the subject is visually monitored continuously by television as well,
The strenuous medical evaluation of candidates for aerospace missions includes a thorough search for diabetes, another disorder that may develop insidiously. This entity is important in that its complications can progress significantly before the symptoms of diabetes itself can be recognized. Diabetics tend to develop atherosclerosis much earlier in life than others, and this degenerative process progresses at a rapid pace. If the arteries supplying the brain are involved primarily, sudden stroke may develop; if the small arteries of the eye are the targets, blindness may ensue; and if the arteries of the heart are implicated, coronary heart disease results, In diabetes, a common occurrence is a malfunction of the nerves supplying the arteries and providing the neurovascular reflexes so important in preventing the pooling of blood in the lower extremities.
For many years, testing urine for sugar has been one of the hallmarks of a standard medical evaluation and is a part of aerospace medical evaluation, too, Diabetes mellitus is a complex disease process in which the action of insulin, which is produced by the pancreas, is inadequate, hampering the steps by which the liver takes up excess sugar and stores it. The blood sugar levels become inordinately high, the kidneys’ threshold to preserve blood sugar may be exceeded, and then sugar spills into the urine. People with severe diabetes usually show a positive response for sugar when their urine is tested. In moderate diabetes, though, even if the blood sugar level is high, it may be negative. Unfortunately, the complications of diabetes are very common even in moderate or mild instances of this disease. Obviously, the measurement of sugar in the blood is a better test than urinalysis. In mild or latent cases, however, the blood sugar level with the patient in a fasting state may well be within normal limits. The liver-pancreas axis must be stressed to bring out the abnormality. Such is the rationale for the glucose tolerance test. A heavy sugar load (a specific amount of glucose in lemonade or some other vehicle) is ingested by the subject. Blood sugar determinations are checked one-half, one, one and one-half, and two hours later, and the blood sugar levels are plotted in a curve. The normal ranges are well known, and levels above them are indicative of diabetes.
Because of the complete incapacity produced by a convulsion, a susceptible individual cannot be considered qualified for flying missions. Epilepsy, brain tumor, blood-vessel disease, and brain scarring from previous injury or infection as well as other abnormalities can first manifest themselves by a seizure, Brain activity is associated with electrical phenomena that can be recorded by the electroencephalogram. When it discloses the specific disarray of the normal pattern which indicates such a possibility, little question remains as to the proper decision from the standpoint of flying safety. Even in the presence of such underlying abnormalities, however, the electroencephalogram may remain normal. Examining the brain after it has been stressed may uncover a diagnostic electroencephalographic record. Forms of stress include sleep deprivation, hypoxia (subject breathes a mixture of 10% oxygen and 90% nitrogen for a given period of time), and partial occlusion manually, one side at a time, of the carotid artery in the neck, which supplies oxygenated blood to the brain.
A normally functioning vestibular organ is essential to flying, particularly when visual orientation is not possible. The vestibular apparatus located in the inner ear is one of the organs involved in the maintenance of equilibrium and sense of balance in man. This organ perceives accelerative or decelerative changes and signals the direction of the gravitational attraction of the earth. A tilt of the head is registered as a change in gravitic relations, and appropriate reflexes are initiated to bring about certain changes in the neck and body musculature to maintain posture and produce adjustments for preservation of normal position of the eyes. There is also impartation to the brain of an awareness of the position of the head.
To determine the range of tolerance in normal individuals and to detect insidious abnormalities of the vestibular mechanism, this organ can be stressed in several ways and the response measured by recording and measuring the direction of the compensatory eye movements. In one technique, the subject is placed in a biaxial stimulator called a Coriolis chair, rotated in one plane, and then tilted on an additional axis.
Emotional stability and well-being as well as mental capacity are equally as important as physical health in the selection of astronauts and research test pilots. A detailed psychiatric interview is an integral part of the medical evaluation. The addition of psychometric testing to this facet of the examination also includes a form of stress, since some of the procedures require responses within a distinct period of time.
The cited examples of examination under stress are characteristic of the Air Force approach to medical evaluation in a dynamic state. Newer and more revealing techniques now under development will require validation before they can be utilized as diagnostic tools. We have much to learn yet as to what the aerospace mission will demand of man’s biologic tolerances. As our knowledge grows, the standards for acceptance of prospective test pilots and astronauts will become more specific and the details of the examination will become more refined.
Aerospace Medical Division, AFSC
Colonel Timothy N. Caris
(M.D.,
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