WBR0864

Author	[[PageAuthor::Serge Korjian M.D. (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Pulmonology
Prompt	[[Prompt::A 28-year-old man visits his primary care physician for a regular check-up. During his visit, the patient tells the physician that he will be visiting Tibet in a month and was wondering if he was fit enough to go mountain climbing in the Himalayas. The physician assures the patient that he is in good health and that he can proceed with his plans as long as he does it safely. Which of the following physiologic reactions would be expected if the patient reaches the highest point in the Himalayan strip?]]
Answer A	AnswerA::Increased oxygen consumption
Answer A Explanation	AnswerAExp::Increased oxygen consumption is seen in exercise rather than at high altitudes.
Answer B	AnswerB::Increased carbon dioxide production
Answer B Explanation	AnswerBExp::Increased carbon dioxide production is also seen in exercise rather than at high altitudes.
Answer C	AnswerC::Increased erythropoietin production
Answer C Explanation	AnswerCExp::Increased erythropoietin production is usually seen in patients with chronic exposures to high altitude, not in acute settings.
Answer D	AnswerD::Increased renal excretion of bicarbonate
Answer D Explanation	AnswerDExp::With acute exposures to high altitudes increased renal excretion of bicarbonate balances the intial respiratory alkalosis due to hyperventilation.
Answer E	AnswerE::Increased residual volume
Answer E Explanation	AnswerEExp::Increased residual volume is seen in patients with obstructive lung diseases not at high altitudes.
Right Answer	RightAnswer::D
Explanation	[[Explanation::The normal physiological response to high altitude comes secondary to a decrease in the inspired PaO2. The PaO2 drops from approximately 150 mmHg to as low as 40 mmHg at around 2000 to 2400 meters(7000 to 8000 feet). As PaO2 decreases hyperventilation ensues, leading acutely to respiratory alkalosis. Within a few hours, renal compensation kicks in to adjust the metabolic disturbance. The kidney increases bicarbonate loss to lower the pH. With chronic exposure to high altitude, processes related to chronic hypoxia begin to appear including increased EPO synthesis, polycythemia, and pulmonary vasoconstriction. Educational Objective: With acute exposure to high altitude, initial respiratory alkalosis followed renal compensation via bicarbonate excretion is typical. References: West JB. The physiologic basis of high-altitude diseases. Ann Intern Med. 2004;141(10):789-800.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Response to high altitude, WBRKeyword::high altitude, WBRKeyword::renal bicarbonate excretion, WBRKeyword::AMS, WBRKeyword::Acute mountain sickness, WBRKeyword::Altitude sickness
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