High altitude adaptation

Immediate Response - Thin Air, Fast Breaths

• Primary Insult: ↓ Barometric pressure (P_B) at altitude → ↓ Partial pressure of inspired O₂ ($P_iO_₂$).
• Resulting State: Arterial hypoxemia (↓ PaO₂).

• Key Driver: Hypoxemia is sensed by peripheral chemoreceptors, driving the immediate ↑ in ventilation.
• Gas Exchange: Hyperventilation increases alveolar ventilation, partially correcting $PAO_₂$ but causing hypocapnia (↓ PaCO₂).
• Acid-Base: Acute respiratory alkalosis (↑ pH) develops, causing a left-shift in the O₂-hemoglobin curve.

⭐ The ventilatory response to high altitude is driven exclusively by peripheral chemoreceptors sensing ↓ PaO₂. Central chemoreceptors are initially inhibited by the resulting hypocapnia and CSF alkalosis.

Acclimatization - The Body's Comeback

• Immediate (Hours to Days):
  • Hyperventilation: ↓ PaO₂ stimulates peripheral chemoreceptors, driving ↑ ventilation.
  • Respiratory Alkalosis: Resulting from blowing off CO₂, leading to ↑ blood pH.
• Renal Compensation (2-3 days):
  • Kidneys increase excretion of bicarbonate ($HCO_3^−$) to help normalize pH.
• Hematologic & Cellular (Days to Weeks):
  • ↑ Erythropoietin (EPO): Kidney releases EPO → ↑ RBC production (erythropoiesis) → ↑ hematocrit and hemoglobin.
  • ↑ 2,3-BPG: Binds to hemoglobin, causing a rightward shift of the oxygen-hemoglobin dissociation curve, which facilitates O₂ unloading to tissues.
  • Cellular Changes: ↑ mitochondria, myoglobin, and angiogenesis.

⭐ A rightward shift in the oxygen-hemoglobin curve, mediated by increased 2,3-BPG, is a critical adaptation that improves peripheral oxygen delivery despite lower arterial oxygen saturation.

📌 Mnemonic for factors that shift the curve to the right: "CADET, face Right!" (↑ CO₂, Acid, 2,3-DPG, Exercise, Temperature).

Pathophysiology - Mountain Maladies

• Acute Mountain Sickness (AMS): Headache plus fatigue, dizziness, or GI upset. Develops 6-12 hrs after ascent. Caused by cerebral vasodilation & mild vasogenic edema from hypoxia.
• High-Altitude Cerebral Edema (HACE): Severe AMS progression. Key signs are ataxia & altered consciousness. Can be fatal within 24 hrs.
• High-Altitude Pulmonary Edema (HAPE): Most lethal. Non-cardiogenic edema from uneven hypoxic pulmonary vasoconstriction → pressure overload & capillary leak.

⭐ Ataxia is the most reliable clinical sign for HACE, distinguishing it from severe AMS. It indicates an emergency requiring immediate descent.

• Hypoxic ventilation response (HVR) triggers immediate hyperventilation, causing respiratory alkalosis.
• Renal compensation follows, with increased bicarbonate (HCO₃⁻) excretion to normalize blood pH.
• Erythropoietin (EPO) levels rise within hours, stimulating erythropoiesis and leading to polycythemia.
• Increased 2,3-BPG shifts the O₂-hemoglobin curve to the right, enhancing O₂ unloading to tissues.
• Chronic hypoxia induces pulmonary vasoconstriction, risking pulmonary hypertension and right heart strain.