How does oxygen change at high altitudes and what are the health implications?

Oxygen Changes at High Altitude and Health Implications

At high altitudes, decreasing barometric pressure reduces oxygen partial pressure in inspired air, triggering physiological adaptations including increased respiratory rate, heart rate, and sympathetic nervous system activation, which can lead to altitude illness in susceptible individuals. 1, 2

Altitude Classification and Physiological Changes

Altitude is classified as:

• Low altitude: <1500m
• Moderate altitude: 1500-2500m
• High altitude: 2500-3500m
• Very high altitude: 3500-5800m
• Extreme altitude: >5800m 2

Immediate Physiological Responses

• Respiratory System:

  • Increased respiratory rate and tidal volume
  • Respiratory alkalosis
  • Hypoxic diuresis 1

• Cardiovascular System:

  • Increased sympathetic nervous system activity
  • Elevated heart rate and cardiac output
  • Pulmonary vasoconstriction leading to increased pulmonary artery pressure
  • Reduced maximum heart rate during exercise compared to sea level 1, 2

• Hematological System:

  • Increased erythropoietin production
  • Elevated red blood cell count (with prolonged exposure) 3

Health Implications and Altitude Illness

Acute Mountain Sickness (AMS)

• Symptoms: Headache, nausea, dizziness, fatigue, sleep disturbance
• Risk factors: Rapid ascent, previous history of AMS, poor physical fitness
• Onset: Typically within 6-12 hours of ascent, peaks at 24-48 hours 3, 4

High-Altitude Cerebral Edema (HACE)

• Symptoms: Severe headache, ataxia, altered mental status, confusion
• Pathophysiology: Vasogenic edema from disrupted blood-brain barrier
• Management: Immediate descent, supplemental oxygen, dexamethasone 5, 3

High-Altitude Pulmonary Edema (HAPE)

• Symptoms: Dyspnea at rest, cough, pink frothy sputum, cyanosis
• Pathophysiology: Excessive hypoxic pulmonary vasoconstriction leading to increased capillary pressure
• Management: Immediate descent, supplemental oxygen, nifedipine 5, 3

High-Altitude Retinopathy (HAR)

• Features: Retinal edema, hemorrhages, ischemia, optic disc swelling
• Occurrence: Common in trekkers ascending beyond 4900m
• Management: Usually self-resolving; supplemental oxygen and descent if symptomatic 1

Special Considerations for Medical Conditions

Heart Failure

• NYHA Class I-II: May safely reach altitudes up to 3500m with moderate activity restriction
• NYHA Class III: May reach altitudes up to 3000m with light activity restriction
• Unstable/NYHA IV: Should avoid high altitude exposure 1

Other Cardiovascular Conditions

• Patients with pre-existing pulmonary hypertension are at increased risk
• Remote monitoring may be useful for patients with cardiac implantable devices during travel 1

Sex Differences

• Women may have:
  • Higher risk of AMS
  • Lower vasoconstriction and greater hypoxia-induced vasodilation
  • Higher hypoxic ventilatory response during luteal phase
  • Less sensitivity to substrate shift (carbohydrate vs. fat metabolism) at altitude 1

Prevention Strategies

1. Gradual Ascent:

   • 300-500m/day when above 2500m
   • Include rest days during ascent 2

2. Medications:

   • Acetazolamide: 125-250mg twice daily, starting 1 day before ascent
   • Dexamethasone: 4mg every 6 hours (for prevention in high-risk individuals)
   • Nifedipine: 30mg extended-release for HAPE prevention in susceptible individuals 4, 6

3. Other Measures:

   • Proper hydration
   • Avoid alcohol and sedatives
   • Consider pre-acclimatization for those with access to altitude/hypoxic facilities 1, 2

Treatment Approach

• For AMS: Rest, hydration, acetazolamide, analgesics
• For HACE/HAPE: Immediate descent (most important), supplemental oxygen, hyperbaric therapy if descent not possible
• For all altitude illnesses: Recognize early, prioritize descent when severe, and administer supplemental oxygen when available 3, 6