View source for Altitude sickness

{{Short description|Medical condition due to rapid exposure to low oxygen at high altitude}}
{{Use British English|date=April 2012}}
{{Use dmy dates|date=February 2023}}
{{Infobox medical condition (new)
| name            = Altitude sickness
| image           = Altitude Sickness Warning.jpg
| caption         = Altitude sickness warning – [[Indian Army]]
| pronounce       =
| field           = [[Emergency medicine]]
| alt             = Sign displays "Caution! You are at 17586&nbsp;ft (5360&nbsp;m)"
| synonyms        = High-altitude sickness,<ref name=Fer2016/> altitude illness,<ref name=Fer2016/> hypobaropathy, altitude bends, soroche
| symptoms        = [[Headache]], [[vomiting]], [[Fatigue|feeling tired]], [[Insomnia|trouble sleeping]], [[dizziness]]<ref name=Fer2016/>
| complications   = [[High-altitude pulmonary edema]] (HAPE),<br>[[high-altitude cerebral edema]] (HACE)<ref name=Fer2016/>
| onset           = Within 24 hours<ref name=Fer2016/>
| duration        =
| types           = Acute mountain sickness, [[high-altitude pulmonary edema]], [[high-altitude cerebral edema]], [[chronic mountain sickness]]<ref name=Mer2018Pro/>
| causes          = Low [[Breathing gas#Partial pressure of oxygen|amounts of oxygen]] at [[high altitude|high elevation]]<ref name=Fer2016/><ref name=Mer2018Pro/>
| risks           = Prior episode, high degree of activity, rapid increase in elevation<ref name=Mer2018Pro/>
| diagnosis       = Based on symptoms<ref name=Mer2018Pro/>
| differential    = [[Fatigue|Exhaustion]], viral infection, [[hangover]], [[dehydration]], [[carbon monoxide poisoning]]<ref name=Fer2016/>
| prevention      = Gradual ascent<ref name=Fer2016/>
| treatment       = Descent to lower altitude, sufficient fluids<ref name=Fer2016/><ref name=Mer2018Pro/>
| medication      = [[Ibuprofen]], [[acetazolamide]], [[dexamethasone]], [[oxygen therapy]]<ref name=Mer2018Pro/>
| prognosis       =
| frequency       = 20% at {{convert|2500|m|ft|sigfig=1}}<br>40% at {{convert|3000|m|ft|sigfig=1}}<ref name=Fer2016/><ref name=Mer2018Pro/>
| deaths          =
}}
<!-- Definition and symptoms -->
'''Altitude sickness''', the mildest form being '''acute mountain sickness''' ('''AMS'''), is a harmful [[effects of high altitude on humans|effect of high altitude]], caused by rapid exposure to low [[Breathing gas#Partial pressure of oxygen|amounts of oxygen]] at [[high altitude|high elevation]].<ref name=Fer2016/><ref name=Mer2018Pro>{{cite web |title=Altitude Diseases – Injuries; Poisoning |url=https://www.merckmanuals.com/professional/injuries-poisoning/altitude-diseases/altitude-diseases |website=Merck Manuals Professional Edition |access-date=3 August 2018 |date=May 2018}}</ref><ref>{{cite book |last1=Rose |first1=Stuart R. |chapter=CHAPTER 15 – Altitude Illness |date=1 January 2006 |chapter-url=https://www.sciencedirect.com/science/article/pii/B9780323040501500201 |title=International Travel Health Guide 2006-2007 |pages=216–229 |editor-last=Rose |editor-first=Stuart R. |place=Philadelphia |publisher=Mosby |language=en |isbn=978-0-323-04050-1|last2=Keystone |first2=Jay S. |last3=Connor |first3=Bradley A. |last4=Hackett |first4=Peter |last5=Kozarsky |first5=Phyllis E. |last6=Quarry |first6=Doug |editor2-last=Keystone |editor2-first=Jay S. |editor3-last=Connor |editor3-first=Bradley A. |editor4-last=Hackett |editor4-first=Peter |edition=Thirteenth |archive-date=11 October 2022 |archive-url=https://web.archive.org/web/20221011143411/https://www.sciencedirect.com/science/article/pii/B9780323040501500201 |url-status=live}}</ref> People's bodies can respond to high altitude in different ways. Symptoms of altitude sickness may include [[headache]]s, vomiting, tiredness, confusion, trouble sleeping, and [[dizziness]].<ref name=Fer2016/> Acute mountain sickness can progress to [[high-altitude pulmonary edema]] (HAPE) with associated [[shortness of breath]] or [[high-altitude cerebral edema]] (HACE) with associated confusion.<ref name=Fer2016/><ref name=Mer2018Pro/> [[Chronic mountain sickness]] may occur after long-term exposure to high altitude.<ref name=Mer2018Pro/>

<!-- Cause and diagnosis -->
Altitude sickness typically occurs only above {{convert|2500|m|ft|sigfig=1}}, though some people are affected at lower altitudes.<ref name=Mer2018Pro/><ref name=Sim2018/> Risk factors include a prior episode of altitude sickness, a high degree of activity, and a rapid increase in elevation.<ref name=Mer2018Pro/> Being physically fit does not decrease the risk.<ref name=Mer2018Pro/> Diagnosis is based on symptoms and is supported for those who have more than a minor reduction in activities.<ref name=Mer2018Pro/><ref name="Clinical Examination Systematic Review 2017">{{cite journal |vauthors=Meier D, Collet TH, Locatelli I, Cornuz J, Kayser B, Simel DL, Sartori C |title=Does This Patient Have Acute Mountain Sickness?: The Rational Clinical Examination Systematic Review |journal=[[JAMA]] |volume=318 |issue=18 |pages=1810–1819 |date=November 2017 |pmid=29136449 |doi=10.1001/jama.2017.16192 |url=https://serval.unil.ch/notice/serval:BIB_78F586DBF354 |s2cid=205087288}}</ref> It is recommended that at high altitude any symptoms of headache, nausea, shortness of breath, or vomiting be assumed to be altitude sickness.<ref>{{cite book |title=A Comprehensive Guide to Wilderness & Travel Medicine |edition=3rd |vauthors=Weiss E |date=2005 |isbn=978-0-9659768-1-7 |chapter=Altitude Illness |pages=137–141 |publisher=Adventure Medical Kits |quote=The Golden Rules of Altitude Illness 1) Above 8,000 feet, headache, nausea, shortness of breath, and vomiting should be considered to be altitude illness until proven otherwise. 2) No one with mild symptoms of altitude illness should ascend any higher until symptoms have resolved. 3) Anyone with worsening symptoms or severe symptoms of altitude illness should descend immediately to a lower altitude.}}</ref>

<!-- Prevention and treatment -->
Sickness is prevented by gradually increasing elevation by no more than {{convert|300|m|ft|sigfig=1}} per day.<ref name=Fer2016/>  Generally, descent and sufficient fluid intake can treat symptoms.<ref name=Fer2016/><ref name=Mer2018Pro/> Mild cases may be helped by [[ibuprofen]], [[acetazolamide]], or [[dexamethasone]].<ref name=Mer2018Pro/> Severe cases may benefit from [[oxygen therapy]] and a [[portable hyperbaric bag]] may be used if descent is not possible.<ref name=Fer2016>{{cite book |vauthors=Ferri FF |title=Ferri's Clinical Advisor 2017 E-Book: 5 Books in 1 |date=2016 |publisher=Elsevier Health Sciences |isbn=9780323448383 |page=590 |url=https://books.google.com/books?id=rRhCDAAAQBAJ&pg=PA590 |language=en}}</ref> The only definite and reliable treatment for severe AMS, HACE, and HAPE is to descend immediately until symptoms resolve. Other treatment efforts have not been well studied.<ref name=Sim2018>{{cite journal |vauthors=Simancas-Racines D, Arevalo-Rodriguez I, Osorio D, Franco JV, Xu Y, Hidalgo R |title=Interventions for treating acute high altitude illness |journal=The Cochrane Database of Systematic Reviews |volume=6 |pages=CD009567 |date=June 2018 |issue=12 |pmid=29959871 |pmc=6513207 |doi=10.1002/14651858.CD009567.pub2}}</ref>

<!-- Epidemiology and history -->
AMS occurs in about 20% of people after rapidly going to {{convert|2500|m|ft|sigfig=1}} and in 40% of people after going to {{convert|3000|m|ft|sigfig=1}}.<ref name=Fer2016/><ref name=Mer2018Pro/> While AMS and HACE occurs equally frequently in males and females, HAPE occurs more often in males.<ref name=Fer2016/> The earliest description of altitude sickness is attributed to a Chinese text from around 30 BCE that describes "Big Headache Mountains", possibly referring to the [[Karakoram Mountains]] around [[Kilik Pass]].<ref>{{cite book |vauthors=West JB |title=High Life: A History of High-Altitude Physiology and Medicine |date=2013 |publisher=Springer |isbn=9781461475736 |pages=2–7 |url=https://books.google.com/books?id=o0rhBwAAQBAJ&pg=PA2 |language=en}}</ref>

== Signs and symptoms ==
[[File:Peripheral edema of a woman's face, before and after; Annapurna Base Camp, 2015.JPG|thumb|upright=1.3|Left: A woman at normal altitude. Right: The same woman with a swollen face while trekking at high altitude ([[Annapurna]] Base Camp, Nepal; {{convert|4130|m|ft|abbr=on}}).]]
People have different susceptibilities to altitude sickness; for some otherwise healthy people, acute altitude sickness can begin to appear at around {{convert|2000|m}} above sea level, such as at many mountain ski resorts, equivalent to a pressure of {{convert|80|kPa|atm|lk=on}}.<ref>{{Cite web |first1=K. |last1=Baillie |first2=A. |last2=Simpson |title=Acute mountain sickness |url=http://www.altitude.org/high_altitude.php |publisher=Apex (Altitude Physiology Expeditions) |access-date=8 August 2007 |archive-url=https://web.archive.org/web/20100201060010/http://www.altitude.org/high_altitude.php |archive-date=1 February 2010 |url-status=dead}}&nbsp;— High altitude information for laypeople</ref> This is the most frequent type of altitude sickness encountered. Symptoms often manifest within ten hours of ascent and generally subside within two days, though they occasionally develop into the more serious conditions. Symptoms include headache, confusion, fatigue, stomach illness, dizziness, and sleep disturbance.<ref name=Thompson>{{Cite web |first=A. A. R. |last=Thompson |title=Altitude Sickness |url=http://www.altitude.org/altitude_sickness.php |publisher=Apex |access-date=8 May 2007}}</ref> Exertion may aggravate the symptoms.{{citation needed|date=September 2022}}

Those individuals with the lowest initial [[Capnography|partial pressure of end-tidal pCO<sub>2</sub>]] (the lowest concentration of [[carbon dioxide]] at the end of the respiratory cycle, a measure of a higher alveolar ventilation) and corresponding high oxygen saturation levels tend to have a lower incidence of acute mountain sickness than those with high end-tidal pCO<sub>2</sub> and low oxygen saturation levels.<ref>{{cite journal |vauthors=Douglas DJ, Schoene RB |title=End-tidal partial pressure of carbon dioxide and acute mountain sickness in the first 24 hours upon ascent to Cusco Peru (3326 meters) |journal=[[Wilderness & Environmental Medicine]] |volume=21 |issue=2 |pages=109–13 |date=June 2010 |pmid=20591371 |doi=10.1016/j.wem.2010.01.003 |doi-access=free}}</ref>

=== Primary symptoms ===
[[Headache]]s are the primary symptom used to diagnose altitude sickness, although a headache is also a symptom of [[dehydration]].{{citation needed|date=September 2022}} A headache occurring at an altitude above {{convert|2400|m}}{{snd}}a pressure of {{convert|76|kPa|atm}}{{snd}}combined with any one or more of the following symptoms, may indicate altitude sickness:
{|
|- style="vertical-align:bottom;text-align:left;"
! Disordered system
! Symptoms
|- style="vertical-align:top;"
| Gastrointestinal
| Loss of appetite, [[nausea]], vomiting, [[high-altitude flatus expulsion|excessive flatulation]]<ref name="Auerbach 2007" />
|- style="vertical-align:top;"
| Nervous
| [[Fatigue]] or weakness, headache with or without [[dizziness]] or [[lightheadedness]], [[insomnia]], [[Paraesthesia|"pins and needles" sensation]]
|- style="vertical-align:top;"
| Locomotory
| [[Peripheral edema]] (swelling of hands, feet, and face)
|- style="vertical-align:top;"
| Respiratory
| Nose bleeding, shortness of breath upon exertion
|- style="vertical-align:top;"
| Cardiovascular
| Persistent rapid pulse
|- style="vertical-align:top;"
| Other
| General [[malaise]]
|}

=== Severe symptoms ===
Symptoms that may indicate life-threatening altitude sickness include:
; [[Pulmonary edema]] (fluid in the lungs)
: Symptoms similar to [[bronchitis]]
: Persistent dry cough
: Fever
: Shortness of breath even when resting

; [[Cerebral edema]] (swelling of the brain)
: Headache that does not respond to analgesics
: Unsteady gait
: Gradual loss of consciousness
: Increased nausea and vomiting
: Retinal hemorrhage

The most serious symptoms of altitude sickness arise from [[edema]] (fluid accumulation in the tissues of the body). At very high altitude, humans can get either [[high-altitude pulmonary edema]] (HAPE), or [[high-altitude cerebral edema]] (HACE). The physiological cause of altitude-induced edema is not conclusively established. It is currently believed, however, that HACE is caused by local vasodilation of cerebral blood vessels in response to [[hypoxia (medical)|hypoxia]], resulting in greater blood flow and, consequently, greater capillary pressures. On the other hand, HAPE may be due to general vasoconstriction in the pulmonary circulation (normally a response to regional ventilation-perfusion mismatches) which, with constant or increased cardiac output, also leads to increases in capillary pressures. For those with HACE, [[dexamethasone]] may provide temporary relief from symptoms in order to keep descending under their own power.{{citation needed|date=September 2022}}

HAPE can progress rapidly and is often fatal. Symptoms include fatigue, severe [[dyspnea]] at rest, and cough that is initially dry but may progress to produce pink, frothy [[sputum]]. Descent to lower altitudes alleviates the symptoms of HAPE.

HACE is a life-threatening condition that can lead to coma or death. Symptoms include headache, fatigue, visual impairment, bladder dysfunction, bowel dysfunction, loss of coordination, paralysis on one side of the body, and confusion. Descent to lower altitudes may save those affected by HACE.

==Cause==
{{See also|Breathing#Breathing at altitude|Effects of high altitude on humans}}
[[File:Mount Everest as seen from Drukair2 PLW edit.jpg|thumb|Climbers on [[Mount Everest]] often experience altitude sickness.]]
Altitude sickness can first occur at {{convert|1500|m|ft}}, with the effects becoming severe at extreme altitudes (greater than {{convert|5500|m|ft}}). Only brief trips above {{convert|6000|m|ft}} are possible and supplemental oxygen is needed to avert sickness.

As altitude increases, the available amount of oxygen to sustain mental and physical alertness decreases with the overall air pressure, though the relative percentage of oxygen in air, at about 21%, remains practically unchanged up to {{convert|21000|m|ft}}.<ref>{{Cite journal |author=FSF Editorial Staff |url=http://flightsafety.org/hf/hf_may-jun97.pdf |title=Wheel-well Stowaways Risk Lethal Levels of Hypoxia and Hypothermia |journal=[[Human Factors and Aviation Medicine]] |page=2 |volume=44 |issue=3 |date=May–June 1997 |publisher=[[Flight Safety Foundation]] |access-date=28 October 2010 |quote=The relative amount of oxygen in the air (21 percent) does not vary appreciably at altitudes up to 21,350 meters (70,000 feet). |archive-url=https://web.archive.org/web/20101128161251/http://flightsafety.org/hf/hf_may-jun97.pdf |archive-date=28 November 2010 |url-status=live}}</ref> The [[Root mean square speed|RMS velocities]] of diatomic nitrogen and oxygen are very similar and thus no change occurs in the ratio of oxygen to nitrogen until stratospheric heights.

[[Dehydration]] due to the higher rate of water vapor lost from the lungs at higher altitudes may contribute to the symptoms of altitude sickness.<ref>{{cite journal |vauthors=Hackett PH, Roach RC |title=High-altitude illness |journal=[[The New England Journal of Medicine]] |volume=345 |issue=2 |pages=107–14 |date=July 2001 |pmid=11450659 |doi=10.1056/NEJM200107123450206}}</ref>

The rate of ascent, altitude attained, amount of physical activity at high altitude, as well as individual susceptibility, are contributing factors to the onset and severity of high-altitude illness.

Altitude sickness usually occurs following a rapid ascent and can usually be prevented by ascending slowly.<ref name=Thompson/> In most of these cases, the symptoms are temporary and usually abate as altitude acclimatization occurs. However, in extreme cases, altitude sickness can be fatal.

High altitude illness can be classified according to the altitude: high ({{convert|1500|–|3500|m|ft}}), very high ({{convert|3500|–|5500|m|ft}}) and extreme (above {{convert|5500|m|ft}}).<ref name=":1">{{cite journal |vauthors=Molano Franco D, Nieto Estrada VH, Gonzalez Garay AG, Martí-Carvajal AJ, Arevalo-Rodriguez I |title=Interventions for preventing high altitude illness: Part 3. Miscellaneous and non-pharmacological interventions |journal=The Cochrane Database of Systematic Reviews |volume=4 |issue=4 |pages=CD013315 |date=April 2019 |pmid=31012483 |pmc=6477878 |doi=10.1002/14651858.CD013315}}</ref>

=== High altitude ===
At high altitude, {{convert|1500|to|3500|m|ft}}, the onset of physiological effects of diminished inspiratory oxygen pressure (PiO<sub>2</sub>) includes decreased exercise performance and increased ventilation (lower arterial [[partial pressure of carbon dioxide]]: PCO<sub>2</sub>). While arterial oxygen transport may be only slightly impaired the [[Oxygen saturation (medicine)|arterial oxygen saturation]] (SaO<sub>2</sub>) generally stays above 90%. Altitude sickness is common between {{convert|2400|and|4000|m|ft}} because of the large number of people who ascend rapidly to these altitudes.<ref name="Auerbach 2007">{{cite book |vauthors=Auerbach P |author-link=Paul Auerbach |title=Wilderness Medicine |publisher=Mosby Elsevier |edition=5th |year=2007 |isbn=978-0-323-03228-5 |ref=Auer07}}</ref>

=== Very high altitude ===
At very high altitude, {{convert|3500|to|5500|m|ft}}, maximum SaO<sub>2</sub> falls below 90% as the arterial PO<sub>2</sub> falls below 60mmHg. Extreme [[hypoxemia]] may occur during exercise, during sleep, and in the presence of high altitude pulmonary edema or other acute lung conditions. Severe altitude illness occurs most commonly in this range.<ref name="Auerbach 2007"/>

=== Extreme altitude ===
Above {{convert|5500|m|ft}}, marked hypoxemia, [[hypocapnia]], and [[alkalosis]] are characteristic of extreme altitudes. Progressive deterioration of physiologic function eventually outstrips acclimatization. As a result, no permanent human habitation occurs above {{convert|6000|m}}. A period of acclimatization is necessary when ascending to extreme altitude; abrupt ascent without supplemental oxygen for other than brief exposures invites severe altitude sickness.<ref name="Auerbach 2007"/>

== Mechanism ==
The [[Breathing#Air pressure|physiology of altitude sickness]] centres around the [[alveolar gas equation]]; the atmospheric pressure is low, but there is still 20.9% oxygen. Water vapour still occupies the same pressure too—this means that there is less oxygen pressure available in the lungs and blood. Compare these two equations comparing the amount of oxygen in blood at altitude:<ref name=":0">{{cite journal |vauthors=Brown JP, Grocott MP |date=2013-02-01 |title=Humans at altitude: physiology and pathophysiology |url=https://academic.oup.com/bjaed/article/13/1/17/281180/Humans-at-altitude-physiology-and-pathophysiology |journal=Continuing Education in Anaesthesia, Critical Care & Pain |volume=13 |issue=1 |pages=17–22 |doi=10.1093/bjaceaccp/mks047 |issn=1743-1816 |doi-access=free}}</ref>

<div class="overflowbugx" style="overflow-x:auto;">
{| class="wikitable"
! Type
!At Sea Level
!At 8400 m (The Balcony of [[Mount Everest|Everest]])
!Formula
|-
|[[Alveolar gas equation|Pressure of oxygen in the alveolus]]
|<math display="inline">21\% \times(101.3\text{ kPa}-6.3\text{ kPa}) - \left (\frac{5.3\text{ kPa}}{0.8} \right ) = 13.3 \text{ kPa O}_2</math>
|<math display="inline">21\% \times(36.3\text{ kPa}-6.3\text{ kPa}) - \left (\frac{1.8\text{ kPa}}{0.74} \right ) = 3.9 \text{ kPa O}_2</math>
|<math display="inline">F_I \text{O}_2 \times(P_\text{B}-P_{\text{H}_2\text{O}}) - \left (\frac{P_{\text{CO}_2}}{\text{RQ}} \right ) </math>
|-
|Oxygen Carriage in the blood
|<math display="inline">\left(0.98 \times 1.34 \times 14\frac{\text{g}}{\text{dL}}\right) + (0.023\times 12\text{ kPa}) = \frac{17.3 \text{ mL O}_2}{100 \text{ mL blood}}</math>
|<math display="inline">\left(0.54 \times 1.34 \times 19.3\frac{\text{g}}{\text{dL}}\right) + (0.023\times 3.3\text{ kPa}) = \frac{14.0 \text{ mL O}_2}{100 \text{ mL blood}}</math>
|<math display="inline">(\text{Sa}_{\text{O}_2}\times 1.34\tfrac{\text{mL}}{\text{g Hb}} \times \text{Hb}) +
(\text{O}_2\text{ carriage in blood} \times \text{Pa}_{\text{O}_2})</math>
|}
</div>

The hypoxia leads to an increase in minute ventilation (hence both low {{chem2|CO2}}, and subsequently bicarbonate), Hb increases through haemoconcentration and erythrogenesis. Alkalosis shifts the haemoglobin dissociation constant to the left, 2,3-BPG increases to counter this. Cardiac output increases through an increase in heart rate.<ref name=":0" />

The body's response to high altitude includes the following:<ref name=":0" />
* ↑ Erythropoietin → ↑ hematocrit and haemoglobin
* ↑ [[2,3-BPG]] (allows ↑ release of {{chem2|O2}} and a right shift on the Hb-{{chem2|O2}} disassociation curve)
* ↑ kidney excretion of bicarbonate (use of acetazolamide can augment for treatment)
* Chronic hypoxic pulmonary vasoconstriction (can cause right ventricular hypertrophy)
People with high-altitude sickness generally have reduced hyperventilator response, impaired gas exchange, fluid retention or increased sympathetic drive. There is thought to be an increase in cerebral venous volume because of an increase in cerebral blood flow and hypocapnic cerebral vasoconstriction causing oedema.<ref name=":0" />

== Diagnosis ==
Altitude sickness is typically self-diagnosed since symptoms are consistent: nausea, vomiting, headache, and can generally be deduced from a rapid change in altitude or oxygen levels. However, some symptoms may be confused with [[dehydration]]. Some severe cases may require professional diagnosis which can be assisted with multiple different methods such as using an [[Magnetic resonance imaging|MRI]] or [[CT scan|CT]] scan to check for abnormal buildup of fluids in the lung or brain.<ref name="Clinical Examination Systematic Review 2017"/><ref>{{Cite web |title=What Is Altitude Sickness? |url=https://www.webmd.com/a-to-z-guides/altitude-sickness |access-date=2021-07-02 |website=[[WebMD]] |language=en}}</ref>

== Prevention ==
Ascending slowly is the best way to avoid altitude sickness.<ref name=Thompson/> Avoiding strenuous activity such as skiing, hiking, etc. in the first 24 hours at high altitude may reduce the symptoms of AMS. Alcohol and sleeping pills are respiratory depressants, and thus slow down the acclimatization process and should be avoided. Alcohol also tends to cause dehydration and exacerbates AMS. Thus, avoiding alcohol consumption in the first 24–48 hours at a higher altitude is optimal.

=== Pre-acclimatization ===
Pre-acclimatization is when the body develops tolerance to low oxygen concentrations before ascending to an altitude. It significantly reduces risk because less time has to be spent at altitude to acclimatize in the traditional way. Additionally, because less time has to be spent on the mountain, less food and supplies have to be taken up. Several commercial systems exist that use [[altitude tent]]s, so called because they mimic altitude by reducing the percentage of oxygen in the air while keeping air pressure constant to the surroundings. Examples of pre-acclimation measures include remote [[Ischemic preconditioning|ischaemic preconditioning]], using [[Hypobaric chamber|hypobaric]] air breathing in order to simulate altitude, and [[positive end-expiratory pressure]].<ref name=":1" />

=== Altitude acclimatization ===<!-- This section is linked from [[Mount Kilimanjaro]] -->
Altitude acclimatization is the process of adjusting to decreasing [[oxygen]] levels at higher elevations, in order to avoid altitude sickness.<ref name=Acclimatization>{{Cite journal |last1=Muza |first1=S.R. |last2=Fulco |first2=C.S. |last3=Cymerman |first3=A. |title=Altitude Acclimatization Guide |journal=U.S. Army Research Inst. Of Environmental Medicine Thermal and Mountain Medicine Division Technical Report |issue=USARIEM–TN–04–05 |year=2004 |url=http://archive.rubicon-foundation.org/7616 |access-date=5 March 2009 |archive-url=https://web.archive.org/web/20090423042451/http://archive.rubicon-foundation.org/7616 |archive-date=23 April 2009 |url-status=usurped}}</ref> Once above approximately {{convert|3000|m|-3}}{{snd}}a pressure of {{convert|70|kPa|atm}}{{snd}}most climbers and high-altitude trekkers take the "climb-high, sleep-low" approach. For high-altitude climbers, a typical acclimatization regimen might be to stay a few days at a [[base camp]], climb up to a higher camp (slowly), and then return to base camp. A subsequent climb to the higher camp then includes an overnight stay. This process is then repeated a few times, each time extending the time spent at higher altitudes to let the body adjust to the oxygen level there, a process that involves the production of additional [[Homeostasis#Blood oxygen content|red blood cells]].<ref name=tortora>{{cite book |vauthors=Tortora GJ, Anagnostakos NP |title=Principles of anatomy and physiology |url=https://archive.org/details/principlesofanat05tort |url-access=registration |pages=[https://archive.org/details/principlesofanat05tort/page/444 444]–445|edition= Fifth |location=New York |publisher=Harper & Row, Publishers |date=1987 |isbn=978-0-06-350729-6}}</ref> Once the climber has acclimatized to a given altitude, the process is repeated with camps placed at progressively higher elevations. The rule of thumb is to ascend no more than {{convert|300|m|-2|abbr=on}} per day to sleep. That is, one can climb from {{convert|3000|m|abbr=on}} ({{convert|70|kPa|atm|abbr=on|disp=or}}) to {{convert|4500|m|-3|abbr=on}} ({{convert|58|kPa|atm|abbr=on|disp=or}}) in one day, but one should then descend back to {{convert|3300|m|abbr=on}} ({{convert|67.5|kPa|atm|abbr=on|disp=or}}) to sleep. This process cannot safely be rushed, and this is why climbers need to spend days (or even weeks at times) acclimatizing before attempting to climb a high peak. Simulated altitude equipment such as [[altitude tent]]s provide hypoxic (reduced oxygen) air, and are designed to allow partial pre-acclimation to high altitude, reducing the total time required on the mountain itself.

Altitude acclimatization is necessary for some people who move rapidly from lower altitudes to higher altitudes.<ref>{{Cite web |url=http://www.altitudemedicine.org/altitude-illness/ |title=Altitude Illness |publisher=[[Institute for Altitude Medicine]] |location=Telluride, Colorado}}</ref>

=== Medications ===
The drug [[acetazolamide]] (trade name Diamox) may help some people making a rapid ascent to sleeping altitude above {{convert|2700|m|ft|sigfig=1}}, and it may also be effective if started early in the course of AMS.<ref>{{Cite book |author=[[World Health Organization]] |title=International travel and health |chapter=Chapter 3: Environmental health risks |chapter-url=http://whqlibdoc.who.int/publications/2007/9789241580397_3_eng.pdf |page=31 |date=1 January 2007 |access-date=21 November 2009}}</ref> Acetazolamide can be taken before symptoms appear as a preventive measure at a dose of 125&nbsp;mg twice daily. The [[Everest Base Camp]] Medical Centre cautions against its routine use as a substitute for a reasonable ascent schedule, except where rapid ascent is forced by flying into high altitude locations or due to terrain considerations.<ref name="BaseCampMD_prophylaxis">{{Cite web |title=Prophylaxis |url=http://www.basecampmd.com/expguide/amspremed.shtml |publisher=Everest BC Clinic, BaseCampMD.com |access-date=21 November 2009}}</ref> The Centre suggests a dosage of 125&nbsp;mg twice daily for prophylaxis, starting from 24 hours before ascending until a few days at the highest altitude or on descending;<ref name="BaseCampMD_prophylaxis"/> with 250&nbsp;mg twice daily recommended for treatment of AMS.<ref>{{Cite web |title=Treating AMS |url=http://www.basecampmd.com/expguide/diamox.shtml |publisher=Everest BC Clinic, BaseCampMD.com |access-date=21 November 2009}}</ref> The [[Centers for Disease Control and Prevention]] (CDC) suggest the same dose for prevention of 125&nbsp;mg acetazolamide every 12 hours.<ref name="CDC_Yellow Book">{{Cite book |title=CDC Health Information for International Travel 2010 "The Yellow Book" |veditors=Turell D, Brunette G, Kozarsky P, Lefor A |chapter=Chapter 2 The Pre-Travel Consultation – Self-Treatable Diseases – Altitude Illness |vauthors=Hackett P, Shlim D |chapter-url=http://wwwnc.cdc.gov/travel/yellowbook/2010/chapter-2/altitude-illness.aspx |publisher=Mosby |location=St. Louis |year=2009 |isbn=978-0-7020-3481-7|url-access=registration |url=https://archive.org/details/cdchealthinforma0000unse_f7v6}}</ref> Acetazolamide, a mild diuretic, works by stimulating the kidneys to secrete more bicarbonate in the urine, thereby acidifying the blood. This change in pH stimulates the respiratory center to increase the depth and frequency of respiration, thus speeding the natural acclimatization process. An undesirable side-effect of acetazolamide is a reduction in aerobic endurance performance. Other minor side effects include a tingle-sensation in hands and feet. Although a [[sulfonamide]], acetazolamide is a non-antibiotic and has not been shown to cause life-threatening allergic cross-reactivity in those with a self-reported sulfonamide allergy.<ref>{{cite journal |vauthors=Platt D, Griggs RC |title=Use of acetazolamide in sulfonamide-allergic patients with neurologic channelopathies |journal=[[Archives of Neurology]] |volume=69 |issue=4 |pages=527–9 |date=April 2012 |pmid=22158718 |pmc=3785308 |doi=10.1001/archneurol.2011.2723}}</ref><ref>{{cite journal |vauthors=Kelly TE, Hackett PH |title=Acetazolamide and sulfonamide allergy: a not so simple story |journal=[[High Altitude Medicine & Biology]] |volume=11 |issue=4 |pages=319–23 |year=2010 |pmid=21190500 |doi=10.1089/ham.2010.1051}}</ref><ref>{{cite journal |vauthors=Lee AG, Anderson R, Kardon RH, Wall M | title = Presumed "sulfa allergy" in patients with intracranial hypertension treated with acetazolamide or furosemide: cross-reactivity, myth or reality? |journal=[[American Journal of Ophthalmology]] |volume=138 |issue=1 |pages=114–8 |date=July 2004 |pmid=15234289 |doi=10.1016/j.ajo.2004.02.019 |url=http://www.ajo.com/article/S0002-9394(04)00145-X/abstract}}</ref> Dosage of 1000&nbsp;mg/day will produce a 25% decrease in performance, on top of the reduction due to high-altitude exposure.<ref>{{cite web |url=http://www.usariem.army.mil/pages/download/altitudeacclimatizationguide.pdf |title=Altitude Acclimatization Guide |url-status=dead |archive-url=https://web.archive.org/web/20120324235011/http://www.usariem.army.mil/Pages/download/altitudeacclimatizationguide.pdf |archive-date=24 March 2012}}</ref> The CDC advises that [[Dexamethasone]] be reserved for treatment of severe AMS and HACE during descents, and notes that [[Nifedipine]] may prevent HAPE.<ref name="CDC_Yellow Book"/>

There is insufficient evidence to determine the safety of [[sumatriptan]] and if it may help prevent altitude sickness.<ref>{{Cite journal |last1=Gonzalez Garay |first1=Alejandro |last2=Molano Franco |first2=Daniel |last3=Nieto Estrada |first3=Víctor H. |last4=Martí-Carvajal |first4=Arturo J. |last5=Arevalo-Rodriguez |first5=Ingrid |date=March 2018 |title=Interventions for preventing high altitude illness: Part 2. Less commonly-used drugs |journal=The Cochrane Database of Systematic Reviews |volume=3 |issue=12 |pages=CD012983 |doi=10.1002/14651858.CD012983 |issn=1469-493X |pmc=6494375 |pmid=29529715}}</ref> Despite their popularity, [[antioxidant]] treatments have not been found to be effective medications for prevention of AMS.<ref name="pmid19273551">{{cite journal |vauthors=Baillie JK, Thompson AA, Irving JB, Bates MG, Sutherland AI, Macnee W, Maxwell SR, Webb DJ |display-authors=6 |title=Oral antioxidant supplementation does not prevent acute mountain sickness: double blind, randomized placebo-controlled trial |journal=[[QJM]] |volume=102 |issue=5 |pages=341–8 |date=May 2009 |pmid=19273551 |doi=10.1093/qjmed/hcp026 |doi-access=free}}</ref> Interest in phosphodiesterase inhibitors such as [[sildenafil]] has been limited by the possibility that these drugs might worsen the headache of mountain sickness.<ref name="pmid17408118">{{cite journal |vauthors=Bates MG, Thompson AA, Baillie JK |title=Phosphodiesterase type 5 inhibitors in the treatment and prevention of high altitude pulmonary edema |journal=Current Opinion in Investigational Drugs |volume=8 |issue=3 |pages=226–31 |date=March 2007 |pmid=17408118}}</ref> A promising possible preventive for altitude sickness is [[myo-inositol trispyrophosphate]] (ITPP), which increases the amount of oxygen released by hemoglobin.

Prior to the onset of altitude sickness, [[ibuprofen]] is a suggested non-steroidal anti-inflammatory and painkiller that can help alleviate both the headache and nausea associated with AMS. It has not been studied for the prevention of [[cerebral edema]] (swelling of the brain) associated with extreme symptoms of AMS.<ref>{{Cite web |url=http://med.stanford.edu/ism/2012/march/altitude.html |title=Ibuprofen decreases likelihood of altitude sickness, researchers find |first=John |last=Sanford |date=March 2012 |access-date=19 September 2012 |archive-url=https://web.archive.org/web/20120424180422/http://med.stanford.edu/ism/2012/march/altitude.html |archive-date=24 April 2012 |url-status=dead}}</ref>

=== Over-the-counter herbal supplements and traditional medicines ===
Herbal supplements and traditional medicines are sometimes suggested to prevent high altitude sickness including [[ginkgo biloba]], ''R crenulata'', minerals such as [[iron]], [[antacid]]s, and [[Hormone|hormonal]]-based supplements such as [[medroxyprogesterone]] and [[erythropoietin]].<ref name=":1" /> Medical evidence to support the effectiveness and safety of these approaches is often contradictory or lacking.<ref name=":1" /> [[Indigenous peoples of the Americas]], such as the [[Aymaras]] of the [[Altiplano]], have for centuries chewed [[coca]] leaves to try to alleviate the symptoms of mild altitude sickness. This therapy has not yet been proven effective in a clinical study.<ref>{{cite journal |vauthors=Bauer I |title=Erythroxylum – a comprehensive review |journal=[[Tropical Diseases, Travel Medicine and Vaccines]] |volume=5 |issue=1 |pages=20 |date=2019-11-26 |pmid=31798934 |pmc=6880514 |doi=10.1186/s40794-019-0095-7 |doi-access=free }}</ref> In Chinese and Tibetan traditional medicine, an extract of the root tissue of ''[[Rhodiola rosea|Radix rhodiola]]'' is often taken in order to prevent the symptoms of high altitude sickness, however, no clear medical studies have confirmed the effectiveness or safety of this extract.<ref>{{cite journal |vauthors=Wang J, Xiong X, Xing Y, Liu Z, Jiang W, Huang J, Feng B |title=Chinese herbal medicine for acute mountain sickness: a systematic review of randomized controlled trials |journal=[[Evidence-Based Complementary and Alternative Medicine]] |volume=2013 |pages=732562 |date=2013 |pmid=24454510 |pmc=3881533 |doi=10.1155/2013/732562 |doi-access=free}}</ref>

=== Oxygen enrichment ===
In high-altitude conditions, oxygen enrichment can counteract the hypoxia related effects of altitude sickness. A small amount of supplemental oxygen reduces the equivalent altitude in climate-controlled rooms. At {{convert|3400|m}} ({{convert|67|kPa|atm|abbr=on|disp=or}}), raising the oxygen concentration level by 5% via an [[oxygen concentrator]] and an existing ventilation system provides an effective altitude of {{convert|3000|m|-3|abbr=on}} ({{convert|70|kPa|atm|abbr=on|disp=or}}), which is more tolerable for those unaccustomed to high altitudes.<ref name="pmid7777705">{{cite journal |vauthors=West JB |title=Oxygen enrichment of room air to relieve the hypoxia of high altitude |journal=Respiration Physiology |volume=99 |issue=2 |pages=225–32 |date=February 1995 |pmid=7777705 |doi=10.1016/0034-5687(94)00094-G}}</ref>

Oxygen from gas bottles or liquid containers can be applied directly via a nasal cannula or mask. Oxygen concentrators based upon [[pressure swing adsorption]] (PSA), VSA, or vacuum-pressure swing adsorption (VPSA) can be used to generate the oxygen if electricity is available. Stationary oxygen concentrators typically use PSA technology, which has performance degradations at the lower barometric pressures at high altitudes. One way to compensate for the performance degradation is to use a concentrator with more flow capacity. There are also portable oxygen concentrators that can be used on vehicular DC power or on internal batteries, and at least one system commercially available measures and compensates for the altitude effect on its performance up to {{convert|4000|m|abbr=on}}. The application of high-purity oxygen from one of these methods increases the partial pressure of oxygen by raising the [[FIO2|FiO<sub>2</sub>]] (fraction of inspired oxygen).

=== Other methods ===
Increased water intake may also help in acclimatization<ref name="dannen">{{Cite book |vauthors=Dannen K, Dannen D |title=Rocky Mountain National Park |publisher=Globe Pequot |year=2002 |pages=9 |url=https://books.google.com/books?id=txzevi145t8C&q=%22drinking+plenty+of+water%22+%22altitude+sickness%22&pg=PA9 |isbn=978-0-7627-2245-7 |quote=Visitors unaccustomed to high elevations may experience symptoms of Acute Mountain Sickness (AMS)[...s]uggestions for alleviating symptoms include drinking plenty of water[.] |via=[[Google Books]]}}</ref> to replace the fluids lost through heavier breathing in the thin, dry air found at altitude, although consuming excessive quantities ("over-hydration") has no benefits and may cause dangerous [[hyponatremia]].

== Treatment ==
The only definite and reliable treatment for severe AMS, HACE, and HAPE is to descend immediately until symptoms resolve.<ref name="JAMAClinicalGuidel">{{Cite journal |last1=Derstine |first1=Mia |last2=Small |first2=Elan |last3=Davis |first3=Andrew M. |date=2024-10-24 |title=Prevention, Diagnosis, and Treatment of Acute Altitude Illness |url=https://jamanetwork.com/journals/jama/fullarticle/2825331 |journal=JAMA |volume=332 |issue=19 |pages=1665–1666 |language=en |doi=10.1001/jama.2024.19562 |pmid=39446318 |issn=0098-7484}}</ref>

Attempts to treat or stabilize the patient ''in situ'' (at altitude) are dangerous unless highly controlled and with good medical facilities. However, the following treatments have been used when the patient's location and circumstances permit:
* Oxygen may be used for mild to moderate AMS below {{convert|3700|m|-3}} and is commonly provided by physicians at mountain resorts. Symptoms abate in 12 to 36 hours without the need to descend.{{citation needed|date=December 2022}}
* For more serious cases of AMS, or where rapid descent is impractical, a [[Gamow bag]], a portable plastic [[hyperbaric medicine|hyperbaric chamber]] inflated with a foot pump, can be used to reduce the effective altitude by as much as {{convert|1500|m|-3|abbr=on}}. A Gamow bag is generally used only as an aid to evacuate severe AMS patients, not to treat them at altitude.
* [[Acetazolamide]] 250&nbsp;mg twice daily dosing assists in AMS treatment by quickening altitude acclimatization.<ref name="pmid5916650">{{cite journal |vauthors=Cain SM, Dunn JE |title=Low doses of acetazolamide to aid accommodation of men to altitude |journal=[[Journal of Applied Physiology]] |volume=21 |issue=4 |pages=1195–200 |date=July 1966 |pmid=5916650 |doi=10.1152/jappl.1966.21.4.1195}}</ref> A study by the Denali Medical Research Project concluded: "In established cases of acute mountain sickness, treatment with acetazolamide relieves symptoms, improves arterial oxygenation, and prevents further impairment of pulmonary gas exchange."<ref name="pmid1739236">{{cite journal |vauthors=Grissom CK, Roach RC, Sarnquist FH, Hackett PH |title=Acetazolamide in the treatment of acute mountain sickness: clinical efficacy and effect on gas exchange |journal=[[Annals of Internal Medicine]] |volume=116 |issue=6 |pages=461–5 |date=March 1992 |pmid=1739236 |doi=10.7326/0003-4819-116-6-461|s2cid=1731172 }}</ref>
* The folk remedy for altitude sickness in [[Ecuador]], [[Peru]] and [[Bolivia]] is a tea made from the coca plant. See [[Coca tea|mate de coca]].
* [[Steroids]] can be used to treat the symptoms of pulmonary or cerebral edema, but do not treat the underlying AMS.
* Two studies in 2012 showed that ibuprofen 600 milligrams three times daily was effective at decreasing the severity and incidence of AMS; it was not clear if HAPE or HACE was affected.<ref name="Lipman 2012">{{cite journal |vauthors=Lipman GS, Kanaan NC, Holck PS, Constance BB, Gertsch JH |title=Ibuprofen prevents altitude illness: a randomized controlled trial for prevention of altitude illness with nonsteroidal anti-inflammatories |journal=[[Annals of Emergency Medicine]] |volume=59 |issue=6 |pages=484–90 |date=June 2012 |pmid=22440488 |doi=10.1016/j.annemergmed.2012.01.019}}</ref><ref name="Gertsch 2012">{{cite journal |vauthors=Gertsch JH, Corbett B, Holck PS, Mulcahy A, Watts M, Stillwagon NT, Casto AM, Abramson CH, Vaughan CP, Macguire C, Farzan NN, Vo BN, Norvelle RJ, May K, Holly JE, Irons H, Stutz AM, Chapagain P, Yadav S, Pun M, Farrar J, Basnyat B |display-authors=6 |title=Altitude Sickness in Climbers and Efficacy of NSAIDs Trial (ASCENT): randomized, controlled trial of ibuprofen versus placebo for prevention of altitude illness |journal=Wilderness & Environmental Medicine |volume=23 |issue=4 |pages=307–15 |date=December 2012 |pmid=23098412 |doi=10.1016/j.wem.2012.08.001 |doi-access=free}}</ref>
* Paracetamol (acetaminophen) has also shown to be as good as ibuprofen for altitude sickness when tested on climbers ascending Everest.<ref>{{cite journal |vauthors=Kanaan NC, Peterson AL, Pun M, Holck PS, Starling J, Basyal B, Freeman TF, Gehner JR, Keyes L, Levin DR, O'Leary CJ, Stuart KE, Thapa GB, Tiwari A, Velgersdyk JL, Zafren K, Basnyat B |display-authors=6 |title=Prophylactic Acetaminophen or Ibuprofen Results in Equivalent Acute Mountain Sickness Incidence at High Altitude: A Prospective Randomized Trial |language=en |journal=Wilderness & Environmental Medicine |volume=28 |issue=2 |pages=72–78 |date=June 2017 |pmid=28479001 |doi=10.1016/j.wem.2016.12.011 |url=http://www.wemjournal.org/article/S1080-6032(17)30009-1/fulltext |doi-access=free}}</ref>

==Epidemiology==
{{expand section|date=December 2024}}
Tourists and mountain climbers are two groups of people who typically contract elevation sickness.<ref>{{Cite web |title=Acute mountain sickness: MedlinePlus Medical Encyclopedia |url=https://medlineplus.gov/ency/article/000133.htm |access-date=2025-02-09 |website=medlineplus.gov |language=en}}</ref>

===Mining===
Elevation sickness is a common workplace illness in [[mining]] operations in the [[Chile]]an [[Andes]] where workers dwelling in the lowlands have to perform work at the mines and associated facilities several thousand meters higher up. [[Chilean law]] demands for compatibility tests for employees performing work at altitude and prescribes those workers already working at altitude and deemed unfit for such work to be redeployed to lower elevations while retaining the prior salary.<ref name=Productividad/> A 2017 study found that contrary to Chile both the United States and Peru lacked legislation regarding altitude sickness in mining operations.<ref name=Productividad>{{cite book|last1=Ulloa Urrutia| first1=Alfie|last2=Contreras Biekert|first2=José Luis|last3=Gana Aravena|first3=Joaquín|last4=Miranda Toledo |first4=Rodrigo|last5=Chauveau Gerber|first5=Paul|first6=Fernando|last6=Villalobos Valenzuela|last7=Farías Soto|first7=Javiera|title=Productividad en la Gran Minería del Cobre|chapter=Aspectos Laborales y Seguridad Minera|year=2017|pages=260–261|publisher=Comisión Nacional de Productividad|language=Spanish|isbn=978-956-7725-08-3}}</ref> 

Little is known about the effects of the intermittent exposure to high elevation that is common among workers in Chilean mines high up in the Andes.<ref name=Productividad/>

== See also ==
{{col div}}
* {{annotated link|Altitude training}}
* {{annotated link|Cabin pressurization}}
* {{annotated link|Effects of high altitude on humans}}
* {{annotated link|Mountaineering}}
* Secondary {{annotated link|polycythemia}}
* {{annotated link|Hypoxic ventilatory response}}
* {{annotated link|Decompression sickness}}
{{colend}}

== References ==
{{Reflist}}

== External links ==
{{Medical resources
| DiseasesDB = 8375
| ICD10 = {{ICD10|T|70|2|t|66}}
| ICD9 = {{ICD9|E902.0}}
| ICDO =
| OMIM =
| MedlinePlus = 000133
| eMedicineSubj = med
| eMedicineTopic = 3225
| MeshID = D000532
| diseasesDB_mult = {{DiseasesDB2|29615}}
}}
{{Wiktionary|altitude sickness}}
{{Wikivoyage|Altitude sickness}}
* [https://web.archive.org/web/20120711093830/http://medex.org.uk/medex_book/about_book.php Travel at High Altitude: a free booklet about how to keep healthy at altitude. Available in many languages.]
* [http://www.merck.com/pubs/mmanual/section20/chapter281/281a.htm Merck Manual entry on altitude sickness]
* [http://www.high-altitude-medicine.com/AMS-medical.html Altitude Illness Clinical Guide for Physicians] {{Webarchive|url=https://web.archive.org/web/20061018063829/http://www.high-altitude-medicine.com/AMS-medical.html |date=18 October 2006 }}

{{Consequences of external causes}}
{{External causes of morbidity and mortality}}

{{Authority control}}

{{DEFAULTSORT:Altitude Sickness}}
[[Category:Effects of external causes]]
[[Category:Mountaineering and health]]
[[Category:Skiing]]
[[Category:Wikipedia medicine articles ready to translate]]
[[Category:Hazards of outdoor recreation]]