Acclimatization

The Himalayan Mountain Range contains the world’s tallest peaks. Mount Everest is 29,031.7 feet (8,848.86 m) tall, while Annapurna I rises to 26,545 feet (8,091 m). Trekking around these peaks brings you to extremely high elevations. Feeling the effects of altitude is therefore very common, and everyone trekking in Nepal should be familiar with the symptoms of altitude sickness.

Acute Mountain Sickness (AMS)

The percentage of oxygen in the atmosphere at sea level is about 21%. As altitude increases, that percentage remains approximately the same, but lower atmospheric pressure means that less oxygen is available with each breath. The body therefore has to adjust to functioning with less available oxygen.

AMS occurs when the body does not adapt quickly enough to increased altitude. Altitude sickness can occur at elevations as low as 8,000 feet (2,400 m), although the risk increases as elevation and the rate of ascent increase. It is difficult to determine exactly who will be affected, since age, sex and physical condition alone do not reliably predict susceptibility.

The primary factors associated with developing AMS include:

• High altitude
• Fast rate of ascent
• High levels of exertion
• Individual susceptibility or a previous history of altitude illness

How Acclimatization Works

The main cause of altitude sickness is going too high too quickly. Given enough time, the body begins to adapt to the reduced availability of oxygen at a specific altitude. This process is known as acclimatization.

Several changes take place in the body as it acclimatizes. Breathing becomes deeper and more frequent, which helps increase oxygen intake. Over time, the body also produces more red blood cells and makes other physiological adjustments that improve the delivery and use of oxygen.

Mild AMS

AMS is common at high altitude, particularly after a rapid increase in elevation. Symptoms often begin within several hours of arriving at altitude and may include:

• Headache
• Nausea or dizziness
• Loss of appetite
• Fatigue or weakness
• Disturbed sleep
• General feeling of malaise

Moderate to Severe Altitude Illness

Signs and symptoms indicating that altitude illness is becoming more serious can include:

• Severe or worsening headache
• Nausea and vomiting
• Increasing weakness and fatigue
• Shortness of breath
• Decreased coordination or difficulty walking
• Confusion or changes in mental status

Normal activity becomes increasingly difficult as altitude illness worsens. A person with worsening symptoms should not continue ascending. Descent is the most important treatment when significant altitude illness develops, and even a relatively modest descent can result in improvement.

Two particularly serious conditions associated with altitude illness are High Altitude Pulmonary Edema (HAPE) and High Altitude Cerebral Edema (HACE). Both can be life-threatening and require immediate action.

High Altitude Pulmonary Edema (HAPE)

HAPE results from fluid building up in the lungs. This fluid interferes with effective oxygen exchange. As the condition becomes more severe, the amount of oxygen reaching the bloodstream decreases. HAPE can progress rapidly and is a medical emergency.

Symptoms of HAPE can include:

• Shortness of breath at rest
• Tightness or congestion in the chest
• Persistent cough
• Marked fatigue and weakness
• Reduced ability to walk or exercise
• Confusion or irrational behavior in severe cases

Anyone suspected of having HAPE should stop ascending and descend as quickly and safely as possible. Supplemental oxygen can be used when available, but it should not delay descent or evacuation for appropriate medical treatment.

High Altitude Cerebral Edema (HACE)

HACE is a severe form of altitude illness involving swelling of the brain. Symptoms of HACE can include:

• Severe headache
• Weakness
• Disorientation or confusion
• Loss of coordination
• Difficulty walking
• Decreasing levels of consciousness
• Unusual or irrational behavior
• Coma in advanced cases

This condition can be rapidly fatal without treatment. Anyone suspected of having HACE requires immediate descent whenever possible and should be evacuated for appropriate medical treatment.

Proper Acclimatization Guidelines

The following practices are recommended to help the body acclimatize:

• Ascend slowly. Because acclimatization takes time, your rate of ascent should be gradual. A slower ascent increases the likelihood of feeling well and enjoying the experience.
• Allow time to acclimatize. When possible, spend time at moderate altitude before traveling significantly higher. Once at high altitude, acclimatization days should be incorporated into the itinerary when appropriate.
• Do not overexert yourself. Light activity is generally appropriate, but strenuous exercise early in the acclimatization process can make altitude illness more likely.
• Climb high, sleep low. When practical, travel to a higher altitude during the day and return to a lower altitude to sleep. Additional acclimatization hikes can also be incorporated into an itinerary.
• Stay properly hydrated. Drink enough fluids to maintain normal hydration. There is no advantage to forcing excessive amounts of water.
• Eat adequately. Appetite often decreases at altitude, but sufficient food and calories remain important.
• Avoid alcohol during initial acclimatization. Alcohol can interfere with sleep and breathing at altitude. Use caution with medications that depress breathing and discuss prescription medications with your healthcare provider before your trip.
• Do not ascend while experiencing altitude illness. If symptoms worsen while remaining at the same altitude, descend.

Experienced guides play an important role in identifying altitude sickness and monitoring the well-being of trekkers. It is important to be open and honest with your guide. If you do not feel well, do not try to pretend that you are fine or mask your symptoms. Your guide needs accurate information to make appropriate decisions about your safety.

There is always a possibility that a trekker may have to turn around. If a guide determines that continuing higher would jeopardize a person’s health, the decision to descend should be respected. The goal is always for trekkers to complete their journey successfully, but safety must come first.

Altitude Safety Tools & Medications

Pulse Oximeter

A pulse oximeter is a medical device that estimates the oxygen saturation of the blood and measures pulse rate. The oximeter is placed on a person’s fingertip and uses light passing through the small blood vessels and capillaries in the finger to estimate oxygen saturation.

At sea level, normal blood oxygen saturation is commonly around 95–100%. Oxygen saturation normally decreases as elevation increases. Pulse oximeter readings can be useful when considered together with a person’s symptoms and overall condition, but a reading alone should not be used to determine whether someone has altitude illness.

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Diamox (Acetazolamide)

Diamox, the brand name for acetazolamide, is commonly used for the prevention and treatment of Acute Mountain Sickness. The medication helps accelerate acclimatization by stimulating ventilation.

Current medical guidance commonly uses 125 mg every 12 hours for prevention in most adults, beginning before ascent and continuing during the initial period at altitude. Different doses may be appropriate depending on body weight, treatment needs and individual circumstances.

Common side effects can include tingling or numbness in the fingers, toes or face, increased urination and changes in taste. Acetazolamide is a prescription medication, so trekkers should discuss its use with a qualified healthcare professional before traveling to altitude.

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Ibuprofen

Ibuprofen can be used to relieve altitude-induced headaches. Treating a headache, however, does not make it safe to continue ascending when other symptoms of altitude illness are present.

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Bottled Oxygen

Supplemental oxygen is an important emergency tool for serious altitude illness. It can improve oxygen levels and help stabilize an affected trekker while descent or evacuation is arranged.

Oxygen should not be used simply to allow someone who has not adequately acclimatized to continue climbing higher. For worsening AMS, HAPE or HACE, the priority is to stop ascending and descend when necessary.