Part 13 (1/2)

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.