High-Altitude Medicine

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High-Altitude Medicine

• Alicia Bond MD

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High altitude

• Moderate altitude
• 5,000 10,000 feet above sea level
• Highest U.S. ski resorts
• High altitude
• 10,000 18,000 feet above sea level
• High peaks in the lower 48, Europe
• Extreme altitude
• Greater that 18,000 feet above sea level
• Denali, Himalaya, Karakoram, Andes

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Epidemiology

• Most cases of high-altitude illness take place in
  people rapidly ascending to altitudes between
  8,000 and 12,000 feet
• Can affect people who live at low altitude as
  well as people who live at high altitude and
  return from travel to lower altitude (re-entry)
• Millions at risk each year roughly 20-40
  affected by some type of altitude illness
• 30 million Western states visitors
• 12,000 Mt. Everest trekkers
• 1,200 Denali climbers
• 1 million visitors to extreme high ranges
  worldwide

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High-altitude environments

• Decreased barometric pressure logarithmically
  lower partial pressure of oxygen (PO2) in
  inspired air
• Higher latitudes have lower barometric pressure
  at equivalent altitudes
• Weather systems can significantly lower
  barometric pressure transiently
• Cold, dry conditions may be contribute to
  high-altitude illness

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Factors affecting risk

• Rate of ascent
• Recent high-altitude exposure
• Genetic variability
• Sleeping altitude
• Maximum altitude reached

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Acclimatization

• Series of physiologic adaptations to maintain
  tissue oxygenation
• Ability to acclimatize varies genetically
• Hours Hypoxic ventilatory response (HVR), fluid
  shift to increase hematocrit, increase in cardiac
  output
• Days Increased erythropoiesis, return of cardiac
  function to baseline, increase in 2,3-DPG
• Weeks Increased plasma volume and red blood cell
  mass

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Hypoxic ventilatory response

• Most important component of acclimatization
• Affected by genetics, ethanol, sleep medications,
  caffeine, cocoa, progesterone
• PaO2 PiO2 (PaCO2/R)
• Hyperventilation decreases the partial pressure
  of CO2 in the alveoli, thereby increasing the
  partial pressure of oxygen in the alveoli to
  facilitate oxygenation
• Resulting metabolic alkalosis slows HVR, and
  ventilation slowly increases over several days as
  kidneys excrete bicarb
• Can be facilitated by acetazolamide
• People with low HVR at higher risk for illness

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Cardiovascular

• Initial increase in resting HR, which normalizes
  with acclimatization
• Decrease in maximal heart rate
• Decrease in plasma volume -gt lower stroke volume,
  increase in hematocrit
• Shift to extracellular space
• Diuresis from bicarbonate excretion
• Decrease in max HR and SV are cardioprotective
  myocardial ischemia is rare

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Hematopoietic response

• Initial increase in hematocrit due to fluid shift
  and diuresis
• Erythropoietin stimulated early, resulting in new
  RBCs within 4-5 days
• Over weeks to months, red cell and total
  circulating volume expand to meet demand

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Oxygen-hemoglobin curve

• Above 10,000 feet (PO2 60), small changes in
  PO2 cause large changes in SaO2
• Initial increase in 2,3-diphosphoglycerate (DPG)
  promotes O2 release to tissues
• Opposed by respiratory alkalosis, which shifts
  curve left, favoring oxygen uptake in the lung
  and higher SaO2

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Sleep and periodic breathing

• Disturbed sleep with less deep sleep and
  significant arousals common
• Periodic breathing common
• Hyperpnea and respiratory alkalosis cause apnea
• CO2 builds during apnea, causing hyperpnea
• Not usually associated with significant hypoxemia
  or high-altitude illness
• Decreases with acclimatization
• People with low HVR may have overall regular
  breathing pattern with periods of more
  significant apnea and hypoxemia, which are
  associated with high-altitude illness

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Acute high-altitude illness

• Spectrum of disease with intertwining
  pathophysiology
• Acute mountain sickness (AMS)
• High altitude cerebral edema (HACE)
• High altitude pulmonary edema (HAPE)
• All correct rapidly with descent

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Prevention of high-altitude illness

• Avoid ascent to greater than 8,000 feet in one
  day
• Spend 2-3 nights at 8,000-9,000 feet before
  further ascent
• Dont ascend sleeping altitude more than 1500
  feet per day
• Limit exertion, alcohol, and sedative-hypnotics
  during first days at altitude
• Day trips to higher altitude while maintaining
  sleeping altitude can speed acclimatization
• Acetazolamide 125-250 mg BID

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Acute mountain sickness

• Most common with rapid ascent from below 3,000
  feet to above 8,000 feet
• Develops within hours of ascent
• Headache plus at least one of
• Gastrointestinal discomfort
• Sleep disturbance
• Generalized weakness or fatigue
• Dizziness or lightheadedness
• Headache is usually throbbing, bitemporal, worse
  at night and with Valsalva

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AMS Pathophysiology

• Pathophysiology incompletely understood
• Vasodilatory response to hypoxemia, fluid shift,
  inflammatory mediators, and alterations in
  cerebrospinal fluid buffering capacity are all
  implicated
• No evidence of cerebral edema in AMS, but some
  studies suggest transient ICP elevations with
  exertion and Valsalva
• At risk may be people with low HVR and people
  with smaller CSF capacity (tight fit)
• Hyperbaria contributes, but role unclear (AMS
  does not develop with hypoxia alone)

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AMS Management

• Usually resolves within 1-3 days if no additional
  ascent
• Mild Stop ascent, symptomatic treatment, may
  consider acetazolamide
• Moderate to severe Low-flow oxygen,
  acetazolamide /- dexamethasone 4 mg q 6 hours,
  hyperbarics, or descend
• Immediate descent if s/sx HAPE or HACE

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Acetazolamide

• Carbonic anhydrase inhibitor
• Promotes bicarbonate diuresis and metabolic
  acidosis, speeding acclimatization
• Decreases CSF production
• Maintains oxygenation during sleep
• Side effects polyuria and paresthesias
• 125-250 mg BID for treatment and prevention of AMS

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High-altitude cerebral edema

• Least common but most severe form of
  high-altitude illness
• Incidence 1-2 of ascents
• Usually develops above 12,000 feet
• Usually preceded by AMS and associated with HAPE
• Most commonly develops days 1-3 after ascent, but
  can develop later

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HACE Presentation

• Ataxia and altered mentation are hallmarks
  ataxia usually first symptom
• Focal neuro deficits may be present
• Seizures uncommon but reported
• Usually preceded by AMS symptoms
• Any ataxia or change in consciousness in a person
  at altitude should elicit immediate action!

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HACE Pathophysiology

• Vasogenic cerebral edema caused by same group of
  mechanisms as AMS (vasodilation, leakage of fluid
  from vessels) reversible
• Increased ICP causes decreased cerebral blood
  flow, resulting in cell death
• At advanced stages, cytotoxic edema and necrosis
  are present - not reversible

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HACE Management

• Immediate descent is key
• High-flow oxygen and dexamethasone 8 mg (IV, IM,
  PO) followed by 4 mg q 6 hours if available
• Hyperbarics may result in temporary improvement
  but may delay descent
• Intubation, hyperventilation if severely altered
• Can try mannitol or furosemide but caution due to
  dehydration common at altitude

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HACE Prognosis

• If descent initiated early, may be completely
  reversible over days to weeks without sequelae
• Reports of ataxia and other neuro deficits
  persisting months to years
• Mortality rate greater than 60 if progresses to
  coma

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High-altitude pulmonary edema

• Most common cause of altitude-related death
• Incidence up to 15 of ascents
• Usually greater than 10,000 feet, or greater than
  8,000 feet with heavy exertion
• Develops within 2-4 days of ascent, classically
  on the second night

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HAPE Presentation

• Early signs are severe dyspnea on exertion,
  fatigue with minimal activity, and dry cough
• Dyspnea at rest and clear, watery sputum develop
  as illness progresses
• Dyspnea at rest is red flag for HAPE and should
  prompt immediate action!
• Patchy infiltrates on CXR, worst right middle
  lobe

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HAPE Pathophysiology

• Hypoxic vasoconstriction causes pulmonary
  hypertension
• Uneven vasoconstriction (areas of extreme hypoxia
  or anatomic difference) causes hyperperfusion of
  some areas, leading to vascular leak and patchy
  edema
• Both hypoxia and pulmonary hypertension are
  exacerbated by exertion

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HAPE Management

• Symptoms resolve quickly upon descent of
  1500-3000 feet
• Mild cases may be treated with bedrest and O2 to
  maintain SaO2 gt 90
• Descent for severe symptoms, minimizing exertion
• High-flow oxygen
• Continuous positive airway pressure if available
• Air drops of O2 may be lifesaving if descent not
  possible
• Hyperbarics may help conserve O2 supply

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Hyperbarics

• Portable, lightweight,manually-pressurizedhyperb
  aric bags
• Raise atmospheric pressure 2 psi (103 mmHg)
• Simulates descent of 4,000-5,000 feet at
  moderate altitudes, more at higher altitudes
• Can be lifesaving in HAPE and HACE, relieving
  symptoms so that patients can descend without
  evacuation

Photo Rosens Emergency Medicine, Courtesy of
Thomas Dietz, MD
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Take-home

• Slow ascent and acetazolamide are effective in
  preventing illness
• Ataxia, altered mentation, and dyspnea at rest
  are red flags for serious illness
• Early recognition of HAPE and HACE with descent
  prevents morbidity and mortality
• Have fun up there!

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Key References

• Marx, JA, ed. Rosens Emergency Medicine, 7th Ed.
  Philadelphia Mosby Elsevier, 2010
• Auerbach, PS, ed. Wilderness Medicine, 6th Ed.
  Philadelphia Mosby Elsevier, 2012