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 spacecraft.
The effect of 100 percent oxygen at a simulated altitude 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.