Significant Achievements in Space Bioscience 1958-1964 Part 13

Localized or diffuse collapse of alveoli in the lungs may, if the condition persists, lead to arterial hypoxia which may be extremely undesirable under the stresses of s.p.a.ce flight. The alveoli are probably unstable when pure oxygen is breathed; they tend to collapse if there is blockage of the airways, especially at low pressures. This collapse occurs because each of the gases present in the alveoli (oxygen, water vapor, and carbon dioxide) is subject to prompt and complete absorption from the alveoli by the blood.

The alveoli are normally stabilized against collapse by the presence of inert and relatively insoluble gas (nitrogen) and an internal coating of lipoprotein substances with low surface tension.

Theoretical and experimental results strongly suggest the desirability of using oxygen-inert gas atmospheres for long missions to avoid atelectasis and other gas absorption phenomena, such as retraction of the eardrum. However, further experimental evidence is required both to confirm this point and to establish its upper limit of suitability of pure oxygen atmospheres.

At Ohio State University in 1962, scientists studied the effect on young rats exposed for 27 days to 100 percent oxygen (with no nitrogen), at a reduced barometric pressure equivalent to 33 000 feet alt.i.tude. The rats showed no difference in growth rate, oxygen consumption, food and water intake, or behavior from control rats in air at 1 atm.

Oxygen Toxicity

It has long been known that breathing pure oxygen at normal atmospheric pressure often produces pulmonary irritation and other toxic effects both in man and animals. This knowledge has occasioned concern over the use of pure oxygen atmospheres in s.p.a.cecraft.

The effect of 100 percent oxygen at a simulated alt.i.tude of 26 000 feet for 6 weeks was studied using white rats at Oklahoma City University under a NASA grant. Radioactive carbon techniques revealed a 15-percent reduction of metabolism in the 100-percent oxygen-exposed rats, compared with rats in air at 1 atmosphere. There was a 20-percent decrease in lipid metabolism in the liver compared with controls, but no decrease in heart metabolism. There was no gross change in body weight.

The White Leghorn chick between 2 and 7 weeks old is markedly resistant to the toxic effects of 1 atm of O2. Continuous exposure (Ohio State University) for as long as 4 weeks did not cause deaths, obvious morbidity, or any signs of pulmonary damage on gross autopsy.

Nevertheless, the hyperoxia had some adverse effects, primarily reducing the growth rate to between three-fourths to one-fourth of normal; reducing feed intake per unit body weight to three-fourths of normal; slowing respiratory rate by 30 percent; decreasing erythrocytes, hemoglobin, and hematocrit by 9 to 12 percent; and causing reversible histological changes in the lungs. Arterial O2 tensions were elevated over 300-mm Hg, but arterial pCO2 and blood pH were unaffected. No residual effects were noted upon return to air breathing. It is possible that the anatomical peculiarities of the avian lung play some role in the chicks' resistance to hyperoxia, but it is also possible that this resistance is a function of age, similar to the tolerance shown by the young rat but not the adult.

Carbon Dioxide Tolerance

Studies of CO2 tolerance in submarine crews indicate that no loss of performance is involved if the concentration in air at normal pressure does not exceed 1.5 percent with exposures of 30 to 40 days. However, biochemical adaptive changes were observed at this concentration.

Inert-Gas Components

If other investigations establish the need for an inert gas in manned s.p.a.cecraft atmospheres, gases other than nitrogen may be considered.

Compared with nitrogen, the physical properties or helium and neon offer advantages with respect to solubility in body fluids, storage weight, and thermal properties.

Studies at Ohio State University in 1964, under a NASA grant, showed that helium subst.i.tuted for nitrogen in a closed container causes humans to feel "cold" at a normally comfortable temperature. Studies with animals have shown that in a helium atmosphere there is greater heat loss due to the increased conducting capacity and probably greater evaporative capacity. In 6 days at 21 percent oxygen and 79 percent helium at 1-atmosphere pressure, young rats grew at the same rate as controls, but drank more water, excreted more urine, and had a higher rate of food and oxygen consumption than controls in air at 1 atmosphere. Men are being tested on a bicycle ergometer in saturated and low relative humidity helium atmospheres to study heat balance.

Mice were exposed to 80 percent argon and 20 percent oxygen continuously at 1-atmosphere pressure for 35 days at Oklahoma City University. Carbon 14 studies of metabolism showed a slight slowing and a twofold to threefold increase in fat deposition.

Bends

Decompression, whether accidental (due to damage of the s.p.a.cecraft) or intentional (as in the use of the pressure suit outside the capsule), carries the risk of bends if the inert gases dissolved in the tissues and body fluids come out of solution. The magnitude of this risk is determined to a very considerable extent by-

(1) Individual susceptibility (2) The extent to which the nitrogen (or other inert gas) concentrations of tissues and body fluids have been reduced (3) The magnitude and rate of the inert-gas, partial pressure change on decompression

The probability of getting bends is reduced by-

(1) Selection of bends-resistant individuals (2) Thorough denitrogenation before flight (3) Limitation of decompressive pressure changes by appropriate choice of cabin atmosphere pressure and composition (4) s.p.a.ce-suit pressure setting

In some cases, further improvements might be obtained by using, in the cabin atmosphere, an inert-gas component which has a lower solubility in tissue and body fluids or less tendency than nitrogen to form bubbles.

Fire Hazard

Experience indicates that fires in pure oxygen atmospheres, even at low pressures (e.g., 1/3 atm), are extremely difficult to extinguish. While this phenomenon has nothing to do with respiratory physiology, the risk on flights of long duration may be so serious as to demand special measures. Unless effective countermeasures can be devised, this risk may argue very strongly against the use of such atmospheres in the future.

Further experimental investigation is required.

Acceleration Effects on the Lungs and Pulmonary Circulation

Forces produced by high acceleration overdistend one part and compress another part of the lungs. Blood flow diminishes in some parts of the lungs and increases in others. Fluid leaks from the blood into the tissues and into the air sacs in parts of the lungs. These effects cause difficulty in breathing, low arterial oxygen saturation, and impaired consciousness during high sustained acceleration and, to a lesser extent, after its cessation. They must be considered when selecting the best gas to be breathed, since a high partial pressure of oxygen is favorable for consciousness, but a low inert-gas concentration during acceleration is unfavorable for rapid lung recovery afterward.

PHYSIOLOGICAL PROBLEMS

A study of the manned s.p.a.ce flights and laboratory observations to date suggests that during long periods of weightlessness, some physiological difficulties may arise which may produce serious effects on human performance. Although recent experience gives no grounds for expecting insuperable difficulties, neither the quant.i.ty nor quality of the available observations permits the conclusion that long-term exposure to weightlessness will _not_ have serious consequences. The critical role to be played by the astronaut demands that every effort be made to identify in advance those phenomena which may affect performance, and to study their qualitative and quant.i.tative relationships so that proper precautions can be taken.

Lawton ([ref.197]), in reviewing the literature on prolonged weightlessness, found few instances in which physiological function was truly gravity dependent. He stated that the physiological systems likely to be most affected by weightlessness were the musculoskeletal system, the cardiovascular system, and the equilibrium senses. Subsequent experience proved this to be the case. McCally and Lawton ([ref.198]) a.n.a.lyzed the data from experiments since 1961 and concluded that much more basic laboratory work is necessary. Studies using immobilization, immersion, and cabin-confinement techniques were recommended approaches toward simulating weightlessness.

Much of the difficulty in obtaining precise information of antic.i.p.ated problems arises from a lack of knowledge of normal mammalian physiology.

Many of these deficiencies can be remedied in the laboratory. In s.p.a.ce-flight development, however, two distinct investigational approaches can be adopted. The first of these may be characterized as empirical and incremental; that is, the capabilities of the astronaut are explored in successive flights involving relatively modest increases in difficulty or severity of the environmental conditions. In this way it is hoped to ascertain the human limitations without running too great a risk. The second approach can be described as fundamental: determining by a series of controlled experiments the effects of exposure to s.p.a.ce-flight conditions upon comparative mammalian physiology, with emphasis on man. A fundamental understanding of the observed effects would be sought so that predictions for new situations and possible ways to control them could be made with confidence.

It is not possible now to predict for flights of 30 days or more-

(1) The effects of sudden reimposition of reentry accelerations and terrestrial gravity (2) Changes in body fluid distribution and composition (3) The effects of violent physical effort on respiratory and cardiovascular systems in prolonged weightlessness (4) Central nervous system functions, especially coordination, skilled motor performance, judgment, and sleep-wakefulness cycles

NASA has emphasized that planning for manned s.p.a.ce programs involves a systematic extension from physiological observations in animals to man, and finally the establishment of man as part of the man-vehicle system design. Moreover, these studies require the evaluation of central nervous, cardiovascular, respiratory, gastrointestinal, and other systems as a matrix in mutual interdependence. There is particular interest in the effects of weightlessness on flights exceeding 30 days.

Mammalian flights of about 30 days also merit attention, including the development of the life-support systems which must precede such a program. Development of facilities for biological experiments may well be an important requirement for studies in antic.i.p.ation of manned flights of longer duration than Apollo. Unless the biological satellite programs of the type mentioned above are successful in providing the necessary data, a manned orbiting laboratory may also be important in studies of shorter range.

General Studies of Biological Rhythmicity

The effects of weightlessness on the organism as a whole may be manifested by important changes in certain integrated behavioral patterns having an inherently rhythmic character. Modifications in basic behavioral patterns and performance may occur as disruptions of rhythmic physiological phenomena, which are themselves the end product of interrelated functional activity in a number of physiological systems, such as the neuroendocrine, cardiovascular, and central nervous systems.

Measurements of interdependent components of biological rhythmicity are beginning to be a.n.a.lyzed by methods well established in physics-including correlation and spectral a.n.a.lyses, and phase modulation and variance in rhythmic processes. A wide variety of physiological functions can be treated as periodic variables in the a.n.a.lysis, including rhythmicities in cardiac output and blood pressure, respiration, brain waves, and the slower tides of appet.i.te, and sleep-wakefulness. The importance of such investigations argues for their inclusion in forthcoming flight programs. Their experimental simplicity is an additional advantage. Biorhythms have been discussed in more detail in the section on "Environmental Biology."

Effects of Weightlessness on the Cardiovascular System

Earlobe oximetry, indirect measurements of blood flow and of blood pressure by finger plethysmography or impedance plethysmography, and ballistocardiographic techniques have potential application to manned s.p.a.ce flight.

Adaptation to prolonged exposure to weightlessness or to lunar gravity may cause difficulties when the astronaut is exposed again to reentry forces and terrestrial gravity. It is possible that these adaptive changes may thus produce unacceptable effects on performance or cause risk to life. It is important to obtain experimental evidence on this subject.

It is common knowledge that following a stay in bed, dizziness, faintness, and weakness characterize arising, and that a feeling of general weakness may persist for several days. The phenomenon has been investigated in a number of laboratories. One approach has been to put healthy young subjects to bed, and even in extensive casts for periods of 2 or 3 weeks or more. Two major findings have emerged from these studies. First, a substantial adjustment in the blood circulatory system occurs, which is termed the "hypodynamic state." Second, there is a large decrease in the skeletal and muscle ma.s.s of the body.

There are two kinds of evidence for the hypodynamic state: measurement of parameters of circulatory function, and measurement of the response of the individuals to a quant.i.tatively imposed mild gravitational load.

After 3 weeks in bed, otherwise healthy persons exhibit an increase of more than 20 percent in heart rate; a reduction of 10 to 20 percent in total blood volume, primarily as a result of reduction of plasma volume; and a decrease in heart size of about 8 percent. Coupled with these cardiovascular changes is a reduction of 10 percent in the basal metabolic rate. It appears as though the circulation and metabolism are reset to a lower functional level commensurate with the reduced demands placed on the whole organism.

After 3 weeks of bed rest, all of the subjects tested showed p.r.o.nounced orthostatic hypotension. After tilting, the average heart rate increased by 37 beats per minute, the systolic blood pressure fell some 12-mm Hg, and some of the subjects fainted. The measurements were continued for 16 days after the bed-rest period, and it was round that recovery was not quite complete when the experiment was terminated.

There is little question that in prolonged exposures to the weightless state, there is a fair probability of extensive circulatory adjustments, the seriousness of which cannot yet be foretold. While it is likely that the astronauts will adapt successfully to long periods of weightlessness at some new circulatory functional level, the remote possibility exists that the circulatory changes may be progressive to the point of ultimate failure.