A 34-year-old man travels to Peru to participate in a mountain-climbing expedition. He has no prior medical problems and takes no medications. The patient does not use tobacco, alcohol, or illicit drugs. During his trip, he stays in a high-altitude camp in the mountains at over 4,000 m (13,000 ft) above sea level. After 2 days at the camp, the patient has blood drawn by the camp's physician while breathing ambient air. Which of the following sets of arterial blood gas values is most likely to be seen in this patient?

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11ec160b_3cdd_9a78_9249_5d8a1017f215_MD0001_00 The partial pressure of inspired oxygen (PiO₂) is decreased at high altitude, meaning that fewer oxygen molecules are inspired with each breath. This leads to hypoxemia with a marked reduction in the arterial partial pressure of O₂ (PaO₂). Hypoxemia triggers chemoreceptors in the carotid bodies to stimulate ventilation, and with hyperventilation, there is increased exhalation of CO₂, leading to respiratory alkalosis. Over several days, the kidneys compensate by increasing bicarbonate (HCO₃⁻) excretion to decrease serum HCO₃⁻ and help normalize pH. Hypoxemia persists because the low PiO₂ allows hyperventilation to increase PaO₂ only slightly. Accordingly, the body makes several adjustments that improve O₂ delivery and O₂-carrying capacity to optimize O₂ use in the setting of persistently low PaO₂. These adjustments include: Therefore, in this individual who spent 2 days at high altitude, the expected arterial blood gas findings reflect partially compensated respiratory alkalosis (slightly elevated pH, low partial pressure of arterial CO₂ [PaCO₂], somewhat low HCO₃⁻) and persistent hypoxemia (PaO₂ <75 mm Hg). (Choice A) Low pH with high PaCO₂ indicates respiratory acidosis. Near-normal HCO₃⁻ is consistent with acute respiratory acidosis because there has been little time for renal metabolic compensation. Acute respiratory acidosis with hypoxemia can occur due to hypoventilation (eg, from opioid overdose). (Choice B) Low pH with low HCO₃⁻ indicates metabolic acidosis. The low PaCO₂ reflects respiratory compensation via hyperventilation. (Choice C) These values are close to approximate normal values: pH of 7.4, PaCO₂ of 40 mm Hg, and HCO₃⁻ of 24 mEq/L. This reflects an absence of acid-base disturbance. (Choice D) Slightly elevated pH with low PaCO₂ and low HCO₃⁻ is consistent with chronic respiratory alkalosis with appropriate metabolic compensation. These acid-base findings are expected after >72 hours at high altitude (adequate time for full renal compensation); however, PaO₂ does not normalize. (Choice F) High pH with high HCO₃⁻ indicates metabolic alkalosis. The elevated PaCO₂ represents respiratory compensation via hypoventilation. Such findings can occur with excessive vomiting or overuse of diuretics. Educational objective: At high altitude, the low partial pressure of inspired oxygen (PiO₂) leads to hypoxemia that triggers hyperventilation with increased exhalation of CO₂ and resulting respiratory alkalosis. The kidneys compensate by increasing bicarbonate (HCO₃⁻) excretion to decrease serum HCO₃⁻ and help normalize pH. Hypoxemia persists, but physiologic adjustments take place to improve O₂ use. __________ References: The physiological effects of hypobaric hypoxia versus normobaric hypoxia: a systematic review of crossover trials. (http://www.ncbi.nlm.nih.gov/pubmed/25722851)

A 34-Year-Old Man Travels to Peru to Participate in a Mountain-Climbing