Effect of Breathing Pure Oxygen on Alveolar PO2 at Different Altitudes
When a person breathes pure oxygen instead of air, most of the space in the alveoli formerly occupied by nitrogen becomes occupied by oxygen. At 30,000 feet, an aviator could have an alveolar PO2 as high as 139 mm Hg instead of the 18 mm Hg when breathing air (see Table 43–1).
The red curve of Figure 43–1 shows arterial blood hemoglobin oxygen saturation at different altitudes when one is breathing pure oxygen. Note that the sat-uration remains above 90 per cent until the aviator ascends to about 39,000 feet; then it falls rapidly to about 50 per cent at about 47,000 feet.
The “Ceiling” When Breathing Air and When Breathing Oxygen in an Unpressurized Airplane
Comparing the two arterial blood oxygen satura-tion curves in Figure 43–1, one notes that an aviator breathing pure oxygen in an unpressurized airplane can ascend to far higher altitudes than one breathing air. For instance, the arterial saturation at 47,000 feet when one is breathing oxygen is about 50 per cent and is equivalent to the arterial oxygen saturation at 23,000 feet when one is breathing air. In addition, because an unacclimatized person usually can remain conscious until the arterial oxygen saturation falls to 50 per cent, for short exposure times the ceiling for an aviator in an unpressurized airplane when breathing air is about 23,000 feet and when breathing pure oxygen is about 47,000 feet, provided the oxygen-supplying equipment operates perfectly
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