A 24-year-old man who lives at an elevation near sea-level goes hiking in the Colorado mountains and ascends to an altitude of 4,000 m (13,123 ft) . He stays on the mountain overnight and develops diffuse headache, nausea, and difficulty sleeping. The patient also notes mild fatigue and slight dizziness with changes in position. He has no medical conditions and currently takes no medications. He does not use alcohol, tobacco, or illicit drugs. Physical examination is within normal limits. Which of the following changes in this patient's acid-base physiology is most likely to be seen?

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11ec160b_3cf5_ddb7_9249_a165d4aae3f6_MD0001_00 At high altitude, the low barometric pressure decreases the partial pressure of inspired oxygen (PiO₂) and leads to hypoxemia, which in turn triggers a chemoreceptor-mediated increase in respiratory drive with resulting hyperventilation. An unwanted effect of the hyperventilation is excessive expiration of CO₂, leading to respiratory alkalosis and increased blood pH. The hypoxemia and alkalemia can cause altitude sickness, which presents with headache, fatigue, lightheadedness, nausea, and insomnia. Over time, the body makes physiologic adjustments to better tolerate high altitude: On initial exposure to high altitude, the administration of a carbonic anhydrase inhibitor (eg, acetazolamide) can accelerate the decrease in HCO₃^- reabsorption by the kidneys to help relieve the alkalemia and treat altitude sickness. Educational objective: At high altitude, the low partial pressure of inspired oxygen (PiO₂) leads to hypoxemia with consequent hyperventilation and respiratory alkalosis. The hypoxemia and alkalemia can cause symptoms of altitude sickness (eg, headache, fatigue, lightheadedness). The kidneys respond by creating a compensatory metabolic acidosis and by increasing erythropoietin secretion. __________ References: Physiology in medicine: a physiologic approach to prevention and treatment of acute high-altitude illnesses. (http://www.ncbi.nlm.nih.gov/pubmed/25539941) High-altitude headache and acute mountain sickness. (http://www.ncbi.nlm.nih.gov/pubmed/22703629)

A 24-Year-Old Man Who Lives at an Elevation near Sea-Level