Problem
35-year-old John Day is planning on a hiking and climbing expedition in Colorado's Gore Mountain range. He plans to climb to one of its highest peaks in August. He is in excellent health and has frequently hiked and climbed mountains of lower altitude. In preparation he had a complete physical exam in which he was found to be in top physical condition. On John's physical exam his ABGs' were pH 7.399 units, PaCO2 39 mmHg, PaO2 95 mmHg (sea level), HCO3- 24 mmHg.
The peak in the Gore range which John plans to climb is 12,000 feet above sea level. At 12,000 feet the barometric pressure (Pb) is ~ 480 mmHg (Pb @ sea level is 760 mmHg).
• Calculate the fractional concentration of oxygen (PO2) in the atmosphere at 12,000 feet (@ the mountain top) vs the calculated the partial pressure of oxygen (PO2) of humidified air at sea level?
When John is at high altitude on the Gore range PO2 in the atmosphere decreases (as calculated in Q3.) Consequently, his PaO2 decreases to ~50 mmHg as he inhales mountain air into his alveoli. The high mountain air with a lower PO2 mixes with water vapor (PH2O) & dead space gas (CO2) in his airway per the calculation below*, assuming H2O vapor pressure at body temp is 47 mmHg and end tidal alveolar CO2 is 40 mmHg.
*PAO2 (alveolar oxygen) = 0.21 (480 - 47) = 90.9 then air is inhaled into alveoli to mix with end tidal gas (PACO2 ~ 40 mmHg): John's approximate PaO2 = 90.9 - 40 = 50.9 mmHg at 12,000 feet.