Altitude Illnesses

Altitude Basics & Acclimatization - Thin Air Truths

• High Altitude: ↓ Barometric pressure ($P_B$) → ↓ $P_IO_2$ → hypobaric hypoxia.
• Acclimatization (Adaptation): Key physiological responses to chronic hypoxia.
  • Immediate (minutes-hours):
    • Hyperventilation (↑ rate/depth via peripheral chemoreceptors sensing ↓ $P_aO_2$) → respiratory alkalosis.
    • ↑ Sympathetic activity (↑ HR, ↑ CO).
  • Intermediate (days):
    • Renal $HCO_3^-$ excretion (corrects alkalosis, normalizes pH).
    • ↑ 2,3-DPG in RBCs (shifts ODC right, ↑ O₂ unloading).
  • Long-term (weeks):
    • ↑ Erythropoietin (EPO) → ↑ RBC mass, ↑ Hb.
    • ↑ Capillary density, myoglobin, mitochondria.

⭐ Hyperventilation, driven by carotid body hypoxemia sensing, is the primary and earliest acclimatization response to altitude.

Acute Mountain Sickness (AMS) - Summit Sickness

• Definition: Most common altitude illness; benign, self-limiting syndrome occurring with rapid ascent to >2500m without adequate acclimatization.

• Onset: Typically 6-12 hours after arrival at altitude; usually resolves in 24-72 hours if ascent is halted.

• Pathophysiology: Hypobaric hypoxia → cerebral vasodilation & impaired blood-brain barrier function → ↑ capillary permeability → mild vasogenic cerebral edema.

• Symptoms: Headache (cardinal symptom, often throbbing, worse at night/on waking) PLUS one or more of:

  • Gastrointestinal upset (anorexia, nausea, or vomiting)
  • Fatigue or weakness
  • Dizziness or lightheadedness
  • Difficulty sleeping (peripheral Cheyne-Stokes respirations common)

• Diagnosis: Lake Louise Score (LLS): Requires headache PLUS a total score of ≥3 from the other listed symptoms (each scored 0-3, except sleep 0-2).

• Prevention:

  • Gradual ascent: Limit increase in sleeping elevation to <500m per day above 3000m.
  • Acetazolamide: 125mg twice daily, starting 24 hours before ascent.

• Treatment:

• ⭐ > Descent is the most definitive and crucial treatment for moderate to severe AMS; failure to descend with worsening symptoms can lead to life-threatening HACE or HAPE.

HACE & HAPE - Brain & Lung Alarms

HACE (High Altitude Cerebral Edema):

• Severe AMS progression; life-threatening brain swelling.
• Patho: Vasogenic edema (↑ cerebral blood flow, ↑ capillary permeability).
• Symptoms: Ataxia (key! 📌 Altered Coordination Emergency), confusion, altered consciousness, severe lassitude.
• Rx: IMMEDIATE DESCENT (critical!), O2, Dexamethasone (8mg stat, then 4mg q6h).

HAPE (High Altitude Pulmonary Edema):

• Most lethal altitude illness; non-cardiogenic pulmonary edema.
• Patho: Uneven hypoxic pulmonary vasoconstriction → ↑ Pulmonary Artery Pressure (PAP) → capillary stress failure & leakage.
• Symptoms: Dyspnea at rest, cough (dry → pink frothy sputum 📌 Pulmonary Edema Problems), chest tightness, ↓SpO2, rales.
• Rx: IMMEDIATE DESCENT (critical!), O2, Nifedipine (30mg SR q12h or 10mg IR q4-6h).

⭐ Ataxia is the hallmark and earliest reliable sign of HACE, indicating an emergency requiring immediate descent.

Prevention & Other Issues - Altitude Safeguards

• Gradual Ascent: Crucial. Limit ascent to 300-500 m/day above 2500 m.
• Acclimatization: Rest day per 1000 m ascent or every 2-3 days.
• Avoid: Alcohol, sedatives. Hydrate well. High carbohydrate diet.
• Pharmacoprophylaxis (AMS):
  • Acetazolamide: 125-250 mg BD, start 24h prior.
  • Dexamethasone: If acetazolamide contraindicated/ineffective.
• Other: Periodic breathing (sleep), UV keratitis (sunglasses), peripheral edema.

⭐ Acetazolamide (carbonic anhydrase inhibitor) is the most common drug for AMS prophylaxis, aiding acclimatization by causing metabolic acidosis and stimulating ventilation.

High‑Yield Points - ⚡ Biggest Takeaways

• Acute Mountain Sickness (AMS) is most common; headache is a hallmark.
• High Altitude Cerebral Edema (HACE): severe AMS progression with ataxia, altered consciousness.
• High Altitude Pulmonary Edema (HAPE): most lethal, presents with dyspnea at rest, cough.
• Acclimatization via gradual ascent and prophylactic acetazolamide is key for prevention.
• Acetazolamide aids acclimatization by inducing metabolic acidosis, promoting bicarbonate diuresis.
• Treat HACE/HAPE: immediate descent, oxygen; dexamethasone for HACE, nifedipine for HAPE.