At high altitude, the fall in barometric pressure causes hypoxia despite the fraction of inspired oxygen (FiO2, 21%) remaining unchanged. Using P_iO_2 = FiO_2 times (P_ atm - P_ H_2O ), which parameter is primarily responsible?

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MBBS QUESTION #10250

Question 1

At high altitude, the fall in barometric pressure causes hypoxia despite the fraction of inspired oxygen (FiO2, 21%) remaining unchanged. Using $P_iO_2 = FiO_2 \times (P_{atm} - P_{H_2O})$, which parameter is primarily responsible?

The partial pressure of inspired oxygen (PiO2) falls because atmospheric pressure (Patm) falls✔️
Hemoglobin concentration falls immediately at altitude
Alveolar ventilation rate falls directly and independently of oxygen levels
Diffusion capacity of the lung membrane itself decreases

Correct Answer Explanation

Since FiO2 (21%) and water vapor pressure are essentially constant, a fall in total atmospheric pressure at altitude directly reduces the partial pressure of inspired oxygen (PiO2), reducing the driving gradient for oxygen diffusion into blood — the fundamental cause of high-altitude hypoxia.

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