A 24-year-old professional athlete is advised to train in the mountains to enhance his performance. After 5 months of training at an altitude of 1.5 km (5,000 feet), he is able to increase his running pace while competing at sea-level venues. Which of the following changes would produce the same effect on the oxygen-hemoglobin dissociation curve as this athlete's training did?

Decreased 2,3-bisphosphoglycerate

Increased carbon monoxide inhalation

Increased partial pressure of oxygen

A healthy 20-year-old male college student attempts to climb Mount Everest and travels to the Tibetan plateau by plane. Upon landing, he feels increasingly dizzy and fatigued. He notices that he is breathing faster than usual. What is the initial stimulus for the most likely acid-base disorder?

Decreased partial pressure of alveolar oxygen

Undiagnosed atrial septal defect

Increasing arterial partial pressure of carbon dioxide

Worsened diffusion limitation of oxygen

A 27-year-old man presents to the emergency department after being hit by a car while riding his bike. The patient was brought in with his airway intact, vitals stable, and with a C-collar on. Physical exam is notable for bruising over the patient’s head and a confused man with a Glasgow coma scale of 11. It is noticed that the patient has a very irregular pattern of breathing. Repeat vitals demonstrate his temperature is 97.5°F (36.4°C), blood pressure is 172/102 mmHg, pulse is 55/min, respirations are 22/min and irregular, and oxygen saturation is 94% on room air. Which of the following interventions are most likely to improve this patient's vital signs?

Head elevation, sedation, mannitol, hyperventilation

Head elevation, sedation, hypertonic saline, hypoventilation

Lower head, sedation, hypertonic saline, hypoventilation

Head elevation, norepinephrine, mannitol, hyperventilation

Lower head, sedation, hypertonic saline, hyperventilation

A 25-year-old man is in the middle of an ascent up a mountain, at an elevation of about 4,500 meters. This is the 4th day of his expedition. His friend notices that in the last few hours, he has been coughing frequently and appears to be short of breath. He has used his albuterol inhaler twice in the past 4 hours, but it does not seem to help. Within the past hour, he has coughed up some frothy, slightly pink sputum and is now complaining of nausea and headache. Other than his asthma, which has been well-controlled on a steroid inhaler, he is healthy. Which of the following is the most likely cause of this man’s symptoms?

Non-cardiogenic pulmonary edema

An investigator is conducting a study on hematological factors that affect the affinity of hemoglobin for oxygen. An illustration of two graphs (A and B) that represent the affinity of hemoglobin for oxygen is shown. Which of the following best explains a shift from A to B?

Decreased serum 2,3-bisphosphoglycerate concentration

Increased hemoglobin γ-chain synthesis

Altitude physiology for USMLE Step 2 CK: High-Yield Physiology Lessons

Altitude Physiology - The Thin Air Threat

Primary Insult: Hypobaric hypoxia due to ↓ barometric pressure ($P_B$) at high altitude.
Pathophysiology: Lower $P_B$ directly reduces the partial pressure of inspired oxygen ($P_iO_2$), leading to alveolar hypoxia, as described by the Alveolar Gas Equation:
- $P_A O_2 = F_i O_2 (P_B - P_{H_2O}) - (P_a CO_2 / R)$

Atmospheric Pressure and PAO2 vs. Altitude

Immediate Physiological Response:

⭐ The primary, immediate ventilatory response to high altitude is driven by hypoxemia stimulating the peripheral chemoreceptors, not by central chemoreceptors or $CO_2$ levels.

Acclimatization - Adapting to the Apex

Immediate (Hours-Days):
- Hyperventilation: ↓ PaCO₂ → respiratory alkalosis.
- Renal Compensation: ↑ Bicarbonate diuresis (via carbonic anhydrase) to normalize pH. Acetazolamide can induce this.
- Cardiovascular: ↑ Heart rate & cardiac output.
Chronic (Days-Weeks):
- Erythropoiesis: ↑ Erythropoietin (EPO) from kidneys boosts RBC production.

-   **Oxygen Unloading:** ↑ **2,3-BPG** shifts oxyhemoglobin curve right, enhancing $O_2$ tissue delivery.
    > ⭐ Synthesis of 2,3-BPG in erythrocytes increases with chronic hypoxia, causing a rightward shift of the oxygen-hemoglobin curve, facilitating $O_2$ unloading in tissues.
    ![Oxyhemoglobin curve: 2,3-BPG at altitude](https://ylbwdadhbcjolwylidja.supabase.co/storage/v1/object/public/notes/L1/Physiology_Gas_exchange_Altitude_physiology/3d1fba6a-0098-442e-812c-92bbdb64b95c.jpg)
-   **Cellular:** ↑ Angiogenesis (VEGF), ↑ mitochondrial density.

📌 Mnemonic: 'High Altitude Body Compensation': H-A-B-C → Hyperventilation, Acid-base change, BPG (2,3-) increase, Creation of RBCs (EPO).

Mountain Maladies - When Physiology Fails

Failure to acclimatize can lead to life-threatening conditions. Prophylaxis with Acetazolamide 125mg BID can prevent illness.

Condition	Key Symptoms	Core Pathophysiology	First-line Treatment
AMS	Headache, fatigue, nausea	Mild cerebral edema	Halt ascent, Acetazolamide
HACE	Gait ataxia, confusion, worsening AMS	Vasogenic cerebral edema	Descend immediately, Dexamethasone
HAPE	Dyspnea at rest, cough (pink, frothy sputum)	Exaggerated hypoxic pulmonary vasoconstriction	Descend immediately, O₂, Nifedipine

High‑Yield Points - ⚡ Biggest Takeaways

Hypobaric hypoxia is the primary trigger, causing ↓ alveolar (PAO2) and arterial (PaO2) oxygen.

Hyperventilation is the immediate response, causing respiratory alkalosis, which is later corrected by renal HCO3− excretion.

Chronic changes include ↑ erythropoietin (EPO) causing polycythemia and ↑ 2,3-BPG shifting the O2-dissociation curve to the right.

Hypoxic pulmonary vasoconstriction is a key physiologic response that can lead to pulmonary hypertension.

Acetazolamide prevents/treats Acute Mountain Sickness (AMS) by promoting bicarbonate diuresis.

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