Altitude sickness

Altitude sickness, the mildest form being acute mountain sickness (AMS), is a harmful

Altitude sickness typically occurs only above 2,500 metres (8,000 ft), though some are affected at lower altitudes.[2]^[4] Risk factors include a prior episode of altitude sickness, a high degree of activity, and a rapid increase in elevation.^[2] Diagnosis is based on symptoms and is supported for those who have more than a minor reduction in activities.^[2][5] It is recommended that at high altitude any symptoms of headache, nausea, shortness of breath, or vomiting be assumed to be altitude sickness.^[6]

Sickness is prevented by gradually increasing elevation by no more than 300 metres (1,000 ft) per day.^[1] Being physically fit does not decrease the risk.^[2] Generally, descent and sufficient fluid intake can treat symptoms.^[1][2] Mild cases may be helped by ibuprofen, acetazolamide, or dexamethasone.^[2] Severe cases may benefit from oxygen therapy and a portable hyperbaric bag may be used if descent is not possible.^[1] Treatment efforts, however, have not been well studied.^[4]

AMS occurs in about 20% of people after rapidly going to 2,500 metres (8,000 ft) and in 40% of people after going to 3,000 metres (10,000 ft).

People have different susceptibilities to altitude sickness; for some otherwise healthy people, acute altitude sickness can begin to appear at around 2,000 metres (6,600 ft) above sea level, such as at many mountain ski resorts, equivalent to a pressure of 80

Those individuals with the lowest initial partial pressure of end-tidal pCO₂ (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 pCO₂ and low oxygen saturation levels.^[10]

Primary symptoms

Headaches are the primary symptom used to diagnose altitude sickness, although a headache is also a symptom of dehydration.^{[citation needed]} A headache occurring at an altitude above 2,400 metres (7,900 ft) – a pressure of 76 kilopascals (0.75 atm) – combined with any one or more of the following symptoms, may indicate altitude sickness:

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

Altitude sickness can first occur at 1,500 metres (4,900 ft), with the effects becoming severe at extreme altitudes (greater than 5,500 metres (18,000 ft)). Only brief trips above 6,000 metres (20,000 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 21,000 metres (69,000 ft).

Dehydration due to the higher rate of water vapor lost from the lungs at higher altitudes may contribute to the symptoms of altitude sickness.^[13]

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.^[9] 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 (1,500–3,500 metres (4,900–11,500 ft)), very high (3,500–5,500 metres (11,500–18,000 ft)) and extreme (above 5,500 metres (18,000 ft)).^[14]

High altitude

At high altitude, 1,500 to 3,500 metres (4,900 to 11,500 ft), the onset of physiological effects of diminished inspiratory oxygen pressure (PiO₂) includes decreased exercise performance and increased ventilation (lower arterial

At very high altitude, 3,500 to 5,500 metres (11,500 to 18,000 ft), maximum SaO₂ falls below 90% as the arterial PO₂ 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.^[11]

Extreme altitude

Above 5,500 metres (18,000 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 6,000 metres (20,000 ft). 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.^[11]

Mechanism

The 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:^[15]

	At Sea Level	At 8400 m (The Balcony of Everest)	Formula
Pressure of oxygen in the alveolus	${\textstyle 21\%\times (101.3{\text{ kPa}}-6.3{\text{ kPa}})-\left({\frac {5.3{\text{ kPa}}}{0.8}}\right)=13.3{\text{ kPa O}}_{2}}$	${\textstyle 21\%\times (36.3{\text{ kPa}}-6.3{\text{ kPa}})-\left({\frac {1.8{\text{ kPa}}}{0.74}}\right)=3.9{\text{ kPa O}}_{2}}$	${\textstyle F_{I}{\text{O}}_{2}\times (P_{\text{B}}-P_{{\text{H}}_{2}{\text{O}}})-\left({\frac {P_{{\text{CO}}_{2}}}{\text{RQ}}}\right)}$
Oxygen Carriage in the blood	${\textstyle \left(0.98\times 1.34\times 14{\frac {\text{g}}{\text{dL}}}\right)+(0.023\times 12{\text{ kPa}})={\frac {17.3{\text{ mL O}}_{2}}{100{\text{ mL blood}}}}}$	${\textstyle \left(0.54\times 1.34\times 19.3{\frac {\text{g}}{\text{dL}}}\right)+(0.023\times 3.3{\text{ kPa}})={\frac {14.0{\text{ mL O}}_{2}}{100{\text{ mL blood}}}}}$	${\textstyle ({\text{Sa}}_{{\text{O}}_{2}}\times 1.34{\tfrac {\text{mL}}{\text{g Hb}}}\times {\text{Hb}})+({\text{O}}_{2}{\text{ carriage in blood}}\times {\text{Pa}}_{{\text{O}}_{2}})}$

The hypoxia leads to an increase in minute ventilation (hence both low 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.^[15]

The body's response to high altitude includes the following:^[15]

• ↑ Erythropoietin → ↑ hematocrit and haemoglobin
• ↑
  2,3-BPG
  (allows ↑ release of O₂ and a right shift on the Hb-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.^[15]

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 MRI or CT scan to check for abnormal buildup of fluids in the lung or brain.^[5][16]

Prevention

Ascending slowly is the best way to avoid altitude sickness.^[9] 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 tents, 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 ischaemic preconditioning, using hypobaric air breathing in order to simulate altitude, and positive end-expiratory pressure.^[14]

Altitude acclimatization

Altitude acclimatization is the process of adjusting to decreasing

Altitude acclimatization is necessary for some people who move rapidly from lower altitudes to higher altitudes.[19]

Medications

The drug

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.^[31]

Over-the-counter herbal supplements and traditional medicines

Herbal supplements and traditional medicines are sometimes suggested to prevent high altitude sickness including

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 3,400 metres (11,200 ft) (67 kPa or 0.66 atm), raising the oxygen concentration level by 5% via an oxygen concentrator and an existing ventilation system provides an effective altitude of 3,000 m (10,000 ft) (70 kPa or 0.69 atm), which is more tolerable for those unaccustomed to high altitudes.^[34]

Oxygen from gas bottles or liquid containers can be applied directly via a nasal cannula or mask. Oxygen concentrators based upon

Other methods

Increased water intake may also help in acclimatization^[35] 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 reliable treatment, and in many cases the only option available, is to descend. 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 3,700 metres (12,000 ft) and is commonly provided by physicians at mountain resorts. Symptoms abate in 12 to 36 hours without the need to descend.^{[citation needed]}
• For more serious cases of AMS, or where rapid descent is impractical, a
  Gamow bag, a portable plastic hyperbaric chamber
  inflated with a foot pump, can be used to reduce the effective altitude by as much as 1,500 m (5,000 ft). A Gamow bag is generally used only as an aid to evacuate severe AMS patients, not to treat them at altitude.
• Acetazolamide 250 mg twice daily dosing assists in AMS treatment by quickening altitude acclimatization.^[36] 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."^[37]
• The folk remedy for altitude sickness in Ecuador, Peru and Bolivia is a tea made from the coca plant. See 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.^[38][39]
• Paracetamol (acetaminophen) has also shown to be as good as ibuprofen for altitude sickness when tested on climbers ascending Everest.^[40]

See also

References

External links

Look up altitude sickness in Wiktionary, the free dictionary.

Wikivoyage has a travel guide for Altitude sickness.

• Travel at High Altitude: a free booklet about how to keep healthy at altitude. Available in many languages.
• Merck Manual entry on altitude sickness
• Altitude Illness Clinical Guide for Physicians Archived 18 October 2006 at the Wayback Machine