How does cabin altitude relate to the partial pressure of oxygen and the risk of hypoxemia during air transport?

How does cabin altitude relate to the partial pressure of oxygen and the risk of hypoxemia during air transport?

• A. Higher cabin altitude increases ambient pressure and raises PO2.
• B. Cabin altitude lowers ambient pressure, reducing inspired and arterial PO2, increasing risk of hypoxemia.
• C. Cabin altitude has no effect on PO2.
• D. Cabin altitude increases humidity, improving oxygenation.

Answer: (B)

The tendency being tested is how cabin pressure affects the amount of oxygen your body can use. In flight, the cabin is pressurized to a level equivalent to roughly 6,000–8,000 feet. At that lower ambient pressure, even though oxygen is still about 21% of the air, the partial pressure of oxygen in the air you breathe is reduced. That lowers the inspired PO2, which in turn lowers the alveolar PO2 and arterial PO2, making tissue oxygen delivery less efficient. The result is an increased risk of hypoxemia, especially in people with preexisting cardiopulmonary issues or limited oxygen reserves. Supplemental oxygen helps by increasing the inspired PO2, offsetting the drop from cabin altitude. Humidity doesn’t meaningfully improve PO2, so it doesn’t reduce hypoxemia risk.

The tendency being tested is how cabin pressure affects the amount of oxygen your body can use. In flight, the cabin is pressurized to a level equivalent to roughly 6,000–8,000 feet. At that lower ambient pressure, even though oxygen is still about 21% of the air, the partial pressure of oxygen in the air you breathe is reduced. That lowers the inspired PO2, which in turn lowers the alveolar PO2 and arterial PO2, making tissue oxygen delivery less efficient. The result is an increased risk of hypoxemia, especially in people with preexisting cardiopulmonary issues or limited oxygen reserves. Supplemental oxygen helps by increasing the inspired PO2, offsetting the drop from cabin altitude. Humidity doesn’t meaningfully improve PO2, so it doesn’t reduce hypoxemia risk.