At an altitude of 4500 m, the barometric pressure is approximately 430 mmHg. The inspired PO2 (PiO2) after humidification is approximately (assuming water vapour pressure = 47 mmHg):

• A 81 mmHg — calculated as (430 − 47) × 0.2093 ≈ 80 mmHg ✓
• B 90 mmHg — barometric pressure × FiO2
• C 64 mmHg — derived from PAO2 formula assuming normal PACO2
• D 105 mmHg — sea-level PiO2 reduced linearly by altitude

Explanation

Inspired PO2 (PiO2) = (Barometric pressure − Water vapour pressure) × FiO2 = (430 − 47) × 0.2093 = 383 × 0.2093 ≈ 80 mmHg. At sea level: (760 − 47) × 0.2093 = 713 × 0.2093 ≈ 149 mmHg. So at 4500 m, PiO2 is approximately 80 mmHg, roughly half the sea-level value. Alveolar PO2 (PAO2) would be even lower after accounting for CO2 (PAO2 = PiO2 − PaCO2/0.8). At altitude, hyperventilation reduces PaCO2, partially restoring PAO2. Chronic adaptations include polycythaemia (increased EPO), increased 2,3-DPG (right-shifting ODC to enhance O2 unloading), and pulmonary hypertension from hypoxic vasoconstriction. Option D describes sea-level PiO2 incorrectly; others give wrong calculations.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.