What is the primary mechanism responsible for hypoxic hypoxia in unpressurized flight?

• A. Reduced inspired oxygen partial pressure leading to lower arterial oxygen content.
• B. Decreased diffusion of oxygen from alveoli to blood.
• C. Increased hemoglobin affinity for oxygen reducing release to tissues.
• D. Hyperventilation-induced respiratory alkalosis.

Answer: (A)

Hypoxic hypoxia during unpressurized flight happens because the amount of oxygen available to the lungs is reduced as you ascend. When the cabin is unpressurized, the barometric pressure—and thus the partial pressure of inspired oxygen—falls. Breathing air with less oxygen lowers the alveolar PO2 and, in turn, the arterial PO2, so there is less oxygen bound to hemoglobin and delivered to tissues. This drop in arterial oxygen content is the defining feature of hypoxic hypoxia.

Diffusion limitations would require a problem with gas transfer at the alveolar-capillary membrane, which isn’t the primary issue here in healthy flight conditions. If hemoglobin had a higher affinity for oxygen, it would hinder release to tissues (leftward shift), but altitude hypoxia is driven by reduced oxygen availability, not altered Hb-O2 affinity. Hyperventilation-induced respiratory alkalosis changes acid-base balance and can cause symptoms, but it doesn’t by itself reduce arterial oxygen content—the key problem is the lowered inspired oxygen partial pressure.