High Altitude Sickness

Aka: High Altitude Sickness, Acute Mountain Sickness, High Altitude Illness

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II. Background: Altitudes

  1. See FIO2 and PiO2 at Altitude
  2. Low Altitude: <1500 meters (<4900 feet)
  3. High Altitude: 1500 to 3500 meters (4900 to 11500 feet)
    1. Most common elevations for Acute Mountain Sickness: 2500 to 3500 meters (8200 to 11482 feet)
    2. Affects the greatest number of people exposed who ascend quickly (e.g. travel to alpine city)
      1. Colorado Ski Resorts: Affects 25% of travelers
      2. Himalayas: Affects 50% of travelers
  4. Very High Altitude: 3500 to 5500 meters (11500 to 18000 feet)
    1. Blood Oxygen Saturation <90% even in health persons
    2. Associated with severe Acute Mountain Sickness (as well as HACE and HAPE)
    3. Affects 25% who climb to this altitude
  5. Extreme Altitude: >5500 meters (>18000 feet)
    1. Significant Hypoxia and hypercarbia
    2. Affects 50% who climb above 6000 meters

III. Pathophysiology: Mechanism

  1. See FIO2 and PiO2 at Altitude
  2. Acclimitization to altitude occurs over days to weeks and decreases the risk of High Altitude Illness
    1. Sleep and Exercise tolerance improve after acclimitization
    2. Initial response to altitude is hypoxic Ventilatory response
      1. Increased Respiratory Rate and Minute Ventilation
      2. Effect is tempered by carbon dioxide levels that fall, resulting in Respiratory Alkalosis
        1. Respiratory Alkalosis results in decreased respiratory drive
        2. Respiratory Alkalosis is compensated over 48 hours by increased renal bicarbonate
    3. Cardiovascular response (Sympathetic Nervous System)
      1. Increased Heart Rate, venous tone and Cardiac Output
    4. Pulmonary Hypertension results from Hypoxemia response
      1. Risk of High Altitude Pulmonary Edema (HAPE) in severe Pulmonary Hypertension
    5. Cerebral Blood Flow results from Hypoxemia response
      1. Risk of High Altitude Cerebral Edema (HACE) in disordered autoregulation of cerebral Hypertension
    6. Erythropoietin released from Kidney as Hypoxemia response
      1. Increases Red Blood Cell production and oxygen carrying capacity
  3. Hypobaric Hypoxemia results in paradoxical and maladaptive physiologic changes at altitude (>1500 meters)
    1. Hypoxic stress due to lower barometric pressure and less available oxygen (decreased PiO2 and FIOO2)
    2. Symptom onset may begin within 6-12 hours of ascent
    3. Fluid retention
      1. Contrast with non-affected persons at altitude who experience diuresis
  4. Changes at altitude that may exacerbate comorbid illness
    1. Low Partial Pressure of oxygen
    2. Increased sympathetic tone
      1. Pulmonary artery Vasoconstriction
      2. Increased Systemic Vascular Resistance

IV. Types: High Altitude Illness

  1. Acute Mountain Sickness
  2. High Altitude Cerebral Edema (HACE)
  3. High Altitude Pulmonary Edema (HAPE)

V. Risk Factors

  1. Rapid ascent (as opposed to gradual acclimatization)
  2. Very high altitude
  3. Significant physical exertion
  4. Prior history of altitude sickness
  5. Traveling from low altitude
  6. Prolonged time at altitude
  7. Genetic susceptibility
  8. Younger age
    1. Aside from comorbidity, older adults may be less affected by altitude
  9. Substances increasing High Altitude Illness risk (decrease hypoxic Ventilatory response)
    1. Alcohol use
    2. Sleep Aids

VI. Symptoms

  1. See High Altitude Cerebral Edema (HACE)
  2. See High Altitude Pulmonary Edema (HAPE)
  3. Common Symptoms
    1. Headache
    2. Malaise
    3. Anorexia
    4. Generalized Weakness
  4. Other Symptoms
    1. Fatigue
    2. Nausea or Vomiting
    3. Insomnia
    4. Dyspnea
    5. Dyspnea on Exertion
    6. Dry cough
    7. Irritable
    8. Decreased Urine Output

VII. Course

  1. Onset: 6-12 hours following high altitude ascent

VIII. Diagnosis

  1. Headache and
  2. One or more of the following
    1. Fatigue or weakness
    2. Dizziness or light headedness
    3. Gastrointestinal distress (Nausea, Vomiting, Anorexia)
    4. Sleep disturbance

IX. Differential Diagnosis

  1. Viral illness
  2. Alcohol-related Hangover
  3. Heat Exhaustion
  4. Dehydration
  5. Hypothermia
  6. Hypoglycemia
  7. Hyponatremia
  8. Medication: Sedative or hypnotic
  9. Carbon Monoxide Poisoning (e.g. cooking in tent)

X. Management

  1. Very mild symptoms may resolve spontaneously with acclimitization
  2. Immediate descent (at least 1000 feet or 300 meters) is most critical for moderate to severe symptoms
  3. Other measures for moderate to severe symptoms where descent is not immediately possible
    1. Supplemental Oxygen to keep Oxygen Saturation >90%
    2. Acetazolamide 250 mg orally twice daily
    3. Dexamethasone 4 mgPO/IV/IM every 6 hours
    4. Gamow Bag (Portable Hyperbaric Chamber)
  4. Other measures
    1. Antiemetics (e.g. Zofran) for Nausea, Vomiting
    2. Acetaminophen or Ibuprofen for Headache

XI. Complications (0.1 to 4 percent Incidence)

  1. See Pathophysiology above
  2. Altitudes above 11,400 feet (3500 meters) are associated with a more complicated course
  3. High Altitude Pulmonary Edema (HAPE)
  4. High Altitude Cerebral Edema (HACE)

XII. Prevention

  1. Medication Prophylaxis
    1. Indications
      1. Travel to 11,000 feet in one day (or over 9,000 feet if history of prior altitude sickness)
    2. Acetazolamide (Diamox)
      1. Adults: 125 mg every 12 hours (FDA approved)
        1. Up to 250 mg twice daily may be used (but 125 mg is typically sufficient)
      2. Children: 2.5 mg/kg up to 125 mg every 12 hours (off-label)
      3. Start 1 day or more before ascent
      4. Continue until acclimitization to the higheest sleeping altitude
    3. Dexamethasone
      1. Dose: 4 mg orally every 12 hours (not FDA approved)
      2. Alternative, in those who cannot take Acetazolamide
      3. Some Wilderness Medicine experts recommend limiting Dexamethasone for treatment (not prophylaxis)
      4. Risk of rebound mountain sickness when discontinued
      5. May require taper with prolonged use (risk of adrenal suppression)
      6. Does not speed acclimitization, but does reduce symptoms
    4. Additional symptom management (and may have a prophylactic role)
      1. Acetaminophen
      2. NSAIDs
  2. General Pointers
    1. Gradual ascent to allow for acclimitization is the most important single preventive factor
    2. Recognize the symptoms of Acute Mountain Sickness
    3. Never ascend to sleep higher if you have symptoms
    4. Descend if symptoms do not resolve or worsen
    5. Never leave a person with altitude sickness alone
    6. Maintain hydration
    7. Avoid overexertion
    8. Avoid Alcohol and Sedatives
    9. For altitudes above 9800 feet (3000 meters)
      1. Recommended ascent rate <1000 feet/day (300 meter/day)
      2. Spend an additional rest day if ascent over 2000 feet (600 meters)
      3. Do not sleep >2000 feet (600 meters) higher than the night before
  3. Comorbid Conditions
    1. See Air Travel Restriction
    2. Patients with asymptomatic cardiopulmonary disease may ascend safely to at least 8200 feet (2500 meters)
    3. Conditions which absolutely contraindicate high altitude travel
      1. Severe Chronic Obstructive Pulmonary Disease (COPD)
      2. Uncontrolled Congestive Heart Failure (CHF)
    4. Conditions for which caution should be Exercised due to risk of exascerbation (emphasize acclimitization)
      1. Arrhythmias
      2. Coronary Artery Disease
      3. Hypertension
      4. Sickle Cell Anemia (splenic infarct risk increases above 4900 feet (1500 meters)
        1. Keep Supplemental Oxygen available

XIII. References

  1. (2018) Presc Lett 25(2)
  2. Candy and Contant in Herbert (2020) EM:Rap 20(3): 3-4
  3. Comp and Rogich (2021) Crit Dec Emerg Med 35(4): 3-8
  4. Basnyat (2003) Lancet 361(9373): 1967-74 [PubMed]
  5. Fiore (2010) Am Fam Physician 82(9): 1103-10 [PubMed]
  6. Hackett (2001) N Engl J Med 345(2): 107-14 [PubMed]

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