High altitude medicine: Difference between revisions

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==Physiology of Altitude Acclimatization==
==Background==
===Ventilation===
[[File:Altitude and air pressure & Everest.jpg|thumb|Relationship between total atmospheric pressure and altitude above sea level.]]
#Increased elevation -> decreased partial pressure of O2 -> decreased PaO2
===Altitude Stages===
##Hypoxic ventilatory response results in incr ventilation to maintain PaO2
{| class="wikitable"
##Vigor of this inborn response relates to successful acclimatization
| align="center" style="background:#f0f0f0;"|'''Stage'''
#Initial hyperventilation is attenuated by respiratory alkalosis
| align="center" style="background:#f0f0f0;"|'''Altitude'''
##As renal excretion of bicarb compensates for resp alkalosis, pH returns toward normal
| align="center" style="background:#f0f0f0;"|'''Physiology'''
###At this point ventilation continues to increase
|-
##Process of maximizing ventilation culminates 4-7d at a given altitude
| Intermediate Altitude ||5,000 - 8,000 ft
###With continuing ascent the central chemoreceptors reset to ever lower values of PaCO2
:(1,524 - 2,438 meters)
###Completeness of acclimatization can be gauged by partial pressure of arterial CO2
||
###Acetazolamide, which results in bicarb diuresis, can facilitate this process
*Decreased exercise performance without major impairment in SaO2<ref>Richalet JP, Hermand E, Lhuissier FJ. Cardiovascular physiology and pathophysiology at high altitude. Nat Rev Cardiol. 2024 Feb;21(2):75-88. PMID 37783743</ref>
===Blood===
|-
#Erythropoietin level begins to rise within 2d of ascent to altitude
| High Altitude||8,000 - 12,000 ft
#Takes days to weeks to significantly increase red cell mass
:(2,438 - 3,658 meters)
##This adaptation is not important for the initial initial acclimatization process
||
===Fluid Balance===
*Decreased SaO2 with marked impairment during exercise and sleep
#Peripheral venoconstriction on ascent to altitude causes increase in central blood vol
|-
##This leads to decreased ADH -> diuresis
| Very High Altitude ||12,000-18,000 ft
##This diuresis, along with bicarb diuresis, is considered a healthy response to altitude
:(3,658 - 5,487 meters)
###One of the hallmarks of AMS is antidiuresis
||
===Cardiovascular System===
*Abrupt ascent can be dangerous; acclimatization is required to prevent illness
#SV decreases initially while HR increases to maintain CO
|-
#Cardiac muscle in healthy pts can withstand extreme hypoxemia w/o ischemic events
| Extreme Altitude ||>18,000 ft
#Pulmonary circulation constricts w/ exposure to hypoxia
:(>5,500 meters)
##Degree of pulm HTN varies and a hyperreactive resopnse is a/w HAPE
||
==Altitude Stages==
*Only experienced by mountain climbers; accompanied by severe hypoxemia and hypocapnia  
#Hypoxemia is maximal during sleep; the altitude in which you sleep is most important
*Sustained human habitation is impossible  
#Intermediate Altitude (5000-8000ft)
*RV strain, intestinal malabsorption, impaired renal function, polycythemia
##Decreased exercise performance without major impairment in SaO2
|}
#High Altitude (8000-14,000ft)
##Decreased SaO2 with marked impairment during exercise and sleep
#Very High Altitude (14,000-18,000ft)
##Abrupt ascent can be dangerous; acclimatization is required to prevent illness
#Extreme Altitude (>18,000ft)
##Only experienced by mountain climbers; accompanied by severe hypoxemia and hypocapnia
##Sustained human habitation is impossible
###RV strain, intestinal malabsorption, impaired renal function, polycythemia


==High Altitude Syndromes==
Height of Mount Everest (tallest in world): 29,035 feet (8,850 meters)
#All caused by hypoxia, seen in rapid ascent in unacclimatized pts, respond to O2/descent


===Acute Mountain Sickness (AMS)===
Height of Mount Whitney (tallest in contiguous US): 14,505 feet (4,421 meters)
====Background====
#Usually only occurs with altitude >7000-8000ft
##May occur at lower altitudes in pts who are particularly susceptible (COPD, CHF)
#Associated w/ rate of ascent, sleeping altitude, strength of hypoxic vent response
##NOT associated with physical fitness, age, sex
#Pts tend to have recurrence of sx whenever they return to the symptomatic altitude


====Clinical Features====
Conversion: 1 meter = ~3.28 feet  [https://www.metric-conversions.org/length/meters-to-feet.htm (calculator)]
#Symptoms usually develop 1-6hr after arrival at elevation
##May be delayed for 1-2d (esp after a night's sleep)
#Average duration of symptoms at 10,000ft = 15hr
##At higher elevations symptoms may last weeks / more likely to progress to HACE
#Diagnosis requires headache + one or more of the following:
##Nausea/vomiting
##Fatigue/weakness
##Dizzy/light-headedness
##Difficulty sleeping
#Onset of ataxia and ALOC heralds onset of HACE
#Fluid retention with facial/peripheral edema is physical hallmark of AMS


====DDX====
==Physiology of Acclimatization==
#Hypothermia
===Ventilation===
#CO poisoning
*Increased elevation → decreased partial pressure of O2 → decreased PaO2
#Pulmonary or CNS infection
**Hypoxic ventilatory response results in ↑ ventilation to maintain PaO2
#Dehydration
**Vigor of this inborn response relates to successful acclimatization
#Migraine
*Initial hyperventilation is attenuated by respiratory alkalosis
##Whereas supp O2 dissipates HA due to AMS in 10-15min, O2 has no effect on migraines
**As renal excretion of bicarb compensates for respiratory alkalosis, pH returns toward normal
#Exhaustion
*Process of maximizing ventilation culminates within 4-7 days at a given altitude
**With continuing ascent the central chemoreceptors reset to ever lower values of PaCO2
**Completeness of acclimatization can be gauged by partial pressure of arterial CO2
**[[Acetazolamide]], which results in bicarb diuresis, can facilitate this process<ref>Luks AM, Swenson ER, Bärtsch P. Acute high-altitude sickness. Eur Respir Rev. 2017 Jan;26(143). PMID 28143879</ref>


====Treatment====
===Blood===
#Mild AMS
*Erythropoietin level begins to rise within 2 days of ascent to altitude  
##Terminate ascent
*Takes days to weeks to significantly increase red cell mass
###Either descend to lower altitude (by 1000-3000ft) or acclimatize at same altitude
**This adaptation is not important for the initial acclimatization process
##Acetazolamide
###Mechanism: speeds acclimatization by promoting bicarb diuresis
###Indications:
####History of altitude illness
####Abrupt ascent to >9800ft
####AMS requiring treatment
####Bothersome periodic breathing during sleep
###125-250mg PO BID until symptoms resolve
###Side-effects
####Allergic reaction (if pt allergic to sulfa), paresthesias, polyuria
##Symptomatic treatment as necessary w/ analgesics and antiemetics
##Sleep-agents
###Benzos are only safe if given in conjunction with acetazolamide
###Nonbenzos are safe (zolpidem, diphenhydramine)
#Moderate AMS
##Immediate descent for worsening symptoms
##Low-flow 0.5-1 L/min O2 if available (esp nocturnal administration)
##Acetazolamide 250mg PO BID
##Dexamethasone 4mg PO q6hr
###Symptom-improvement only; unlike acetazolamide does not aid acclimatization
##Hyperbaric therapy


====Prevention====
===Fluid Balance===
#Graded ascent w/ adequate time for acclimatization is the best prevention
*Peripheral venoconstriction on ascent to altitude causes increase in central blood volume
#Acetazolamide prophylaxis
**This leads to decreased ADH → diuresis
##Indicated for pts w/ history of altitude illness or forced rapid ascent to altitude
**This diuresis, along with bicarb diuresis, is considered a healthy response to altitude  
##Start 24hr before ascent and continue for the first 2d at altitude
***One of the hallmarks of AMS is antidiuresis
##Can be restarted if illness develops
##Reduces symptoms of AMS by 75% in pts ascending rapidly to altitudes >8200ft
#Dexamethasone
##Start day of ascent and continue for first 2d at altitude
##4mg PO q12hr
##Prevents and treats cerebral edema
#Ginkgo biloba
##Controversial if effective; safe


===High Altitude Pulmonary Edema (HAPE)===
===Cardiovascular System===
#definition: two symptoms: dyspnea at rest, cough, weakness, chest tightness or congestion.
*SV decreases initially while HR increases to maintain CO
#And
*Cardiac muscle in healthy patients can withstand extreme hypoxemia without ischemic events
#two signs: central cyanosis, crackles or wheezes, tachypnea, tachycardia.
*Pulmonary circulation constricts with exposure to hypoxia
#most common medical cause of altitude related death.
**Degree of pulmonary hypertension varies; a hyper-reactive response is associated with [[High altitude pulmonary edema|HAPE]]
#>2500m, young males, usually second night of altitude or after 3- 4 days ascent.
#recent URI predisposes
#highest risk in mountain dweller who descends to sea level and then reascends- possibly due to pulm art muscle remodeling.
#is noncardiogenic pulmonary edema with pulm hypertension and inflammation of capillaries and transepithelial water and sodium transport. Caused by combination of both pulm hypertension and increased cap permeability.
#Nitric oxide (NO) inhalation decreases pulm art pressures and can improve oxygenation. Dz possibly due to NO deficiency?


===Prevention===
==Differential Diagnosis==
#limit exercise for first 1- 2 days. Also limit ascent when over 2500m to 300- 350m/day.
{{High altitude DDX}}
#Nifedipine 20mg TID or 30- 6- mg extended release qd- prevents HAPE but not pulm edema of exercise of AMS or HACE.


===Treatment===
==High Altitude Syndromes==
#descend, oxygen, nifedipine 10 mg po, CPAP mask, diuretics, GAMOW bag.
[[File:Altitude flow sheet.png|thumb|High altitude management algorithm.]]
#Can reascend in 2- 3days in needed but at increased risk for reoccurence.
*All caused by hypoxia
*All are seen in rapid ascent in unacclimatized patients
**Hypoxemia is maximal during sleep; the altitude in which you sleep is most important
**Above 10,000ft rule of thumb is to sleep no higher than 1,000 additional ft/day
*All respond to O2/descent


==High Altitude Cerebral Edema (HACE)==
{{Expected SpO2 at altitude}}
#Acute Mountain Sickness plus altered mental status or ataxia. Of if mountain sickness not present, is ataxia with mental status changes.
#occurs >4000m
#due to increased brain water, not just volume. Get increased intracranial pressure.
#initially get vasogenic edema- fluid and protein crosses BBB, Get reversible changes in white matter, especially corpus callosum.. Later get cytotoxic edema by toxins and ischemia. Mostly of gray matter and has poorer px.
#Theories: angiogenesis model- hypoxemia causes macrophages to release cytokines and vascular endothelium growth factor. Basement membranes of capillaries are dissolved causing leaks and petechial hemorrhages. Inhibited by dexamethasone.
#Other theory is due to unexpandable cranial vault. As brain volume increases buffering ability of CSF overcome and brain swells in closed nonexpanding space.
#Prevent as with AMS


===Treatment===
==See Also==
#descend, oxygen, dex 4- 8mg IV, then 4mg q6hr. If GAMOW bag available- 4-8 hr recompression may allow pt to walk down mountain (big help).
*[[Commercial in-flight medical emergencies]]


==Source==
==References==
Tintinalli
<references/>


[[Category:Environ]]
[[Category:Environmental]]

Latest revision as of 10:53, 22 March 2026

Background

Relationship between total atmospheric pressure and altitude above sea level.

Altitude Stages

Stage Altitude Physiology
Intermediate Altitude 5,000 - 8,000 ft
(1,524 - 2,438 meters)
  • Decreased exercise performance without major impairment in SaO2[1]
High Altitude 8,000 - 12,000 ft
(2,438 - 3,658 meters)
  • Decreased SaO2 with marked impairment during exercise and sleep
Very High Altitude 12,000-18,000 ft
(3,658 - 5,487 meters)
  • Abrupt ascent can be dangerous; acclimatization is required to prevent illness
Extreme Altitude >18,000 ft
(>5,500 meters)
  • Only experienced by mountain climbers; accompanied by severe hypoxemia and hypocapnia
  • Sustained human habitation is impossible
  • RV strain, intestinal malabsorption, impaired renal function, polycythemia

Height of Mount Everest (tallest in world): 29,035 feet (8,850 meters)

Height of Mount Whitney (tallest in contiguous US): 14,505 feet (4,421 meters)

Conversion: 1 meter = ~3.28 feet (calculator)

Physiology of Acclimatization

Ventilation

  • Increased elevation → decreased partial pressure of O2 → decreased PaO2
    • Hypoxic ventilatory response results in ↑ ventilation to maintain PaO2
    • Vigor of this inborn response relates to successful acclimatization
  • Initial hyperventilation is attenuated by respiratory alkalosis
    • As renal excretion of bicarb compensates for respiratory alkalosis, pH returns toward normal
  • Process of maximizing ventilation culminates within 4-7 days at a given altitude
    • With continuing ascent the central chemoreceptors reset to ever lower values of PaCO2
    • Completeness of acclimatization can be gauged by partial pressure of arterial CO2
    • Acetazolamide, which results in bicarb diuresis, can facilitate this process[2]

Blood

  • Erythropoietin level begins to rise within 2 days of ascent to altitude
  • Takes days to weeks to significantly increase red cell mass
    • This adaptation is not important for the initial acclimatization process

Fluid Balance

  • Peripheral venoconstriction on ascent to altitude causes increase in central blood volume
    • This leads to decreased ADH → diuresis
    • This diuresis, along with bicarb diuresis, is considered a healthy response to altitude
      • One of the hallmarks of AMS is antidiuresis

Cardiovascular System

  • SV decreases initially while HR increases to maintain CO
  • Cardiac muscle in healthy patients can withstand extreme hypoxemia without ischemic events
  • Pulmonary circulation constricts with exposure to hypoxia
    • Degree of pulmonary hypertension varies; a hyper-reactive response is associated with HAPE

Differential Diagnosis

High Altitude Illnesses

High Altitude Syndromes

High altitude management algorithm.
  • All caused by hypoxia
  • All are seen in rapid ascent in unacclimatized patients
    • Hypoxemia is maximal during sleep; the altitude in which you sleep is most important
    • Above 10,000ft rule of thumb is to sleep no higher than 1,000 additional ft/day
  • All respond to O2/descent

Expected SpO2 and PaO2 levels at altitude[3]

Altitude SpO2 PaO2 (mm Hg)
1,500 to 3,500 m (4,900 to 11,500 ft) about 90% 55-75
3,500 to 5,500 m (11,500 to 18,000 ft) 75-85% 40-60
5,500 to 8,850 m (18,000 to 29,000 ft) 58-75% 28-40

See Also

References

  1. ↑ Richalet JP, Hermand E, Lhuissier FJ. Cardiovascular physiology and pathophysiology at high altitude. Nat Rev Cardiol. 2024 Feb;21(2):75-88. PMID 37783743
  2. ↑ Luks AM, Swenson ER, Bärtsch P. Acute high-altitude sickness. Eur Respir Rev. 2017 Jan;26(143). PMID 28143879
  3. ↑ Gallagher, MD, Scott A.; Hackett, MD, Peter (August 28, 2018). "High altitude pulmonary edema". UpToDate. Retrieved May 2, 2019.
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