OA Guide to High Altitude: Acclimatization and Illnesses (2024)

by Rick Curtis, Director, Outdoor Action Program

Last Page update 07/07/1999

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Traveling at high altitude can be hazardous. The information provided here is designed for educational use only and is not a substitute for specific training or experience. Princeton University and the author assume no liability for any individual's use of or reliance upon any material contained or referenced herein. This paper is prepared to provide basic information about altitude illnesses for the lay person. Medical research on high altitude illnesses is always expanding our knowledge of the causes and treatment. When going to altitude it is your responsibility to learn the latest information. The material contained in this article may not be the most current. Copyright © 1995 Rick Curtis, Outdoor Action Program, Princeton University.

High altitude-we all enjoy that tremendous view from a high summit, but there are risksin going to high altitude, and it's important to understand these risks. Here is a classicscenario for developing a high altitude illness. You fly from New York City to a Denver at5,000 feet (1,525 meters). That afternoon you rent a car and drive up to the trailhead at8,000 feet (2,438 meters). You hike up to your first camp at 9,000 feet (2,745 meters).The next day you hike up to 10,500 feet (3,048 meters). You begin to have a severeheadache and feel nauseous and weak. If your condition worsens, you may begin to havedifficulty hiking. Scenarios like this are not uncommon, so it's essential that youunderstand the physiological effects of high altitude.

What is High Altitude?

Altitude is defined on the following scale High (8,000 - 12,000 feet [2,438 - 3,658meters]), Very High (12,000 - 18,000 feet [3,658 - 5,487 meters]), and Extremely High(18,000+ feet [5,500+ meters]). Since few people have been to such altitudes, it is hardto know who may be affected. There are no specific factors such as age, sex, orphysical condition that correlate with susceptibility to altitude sickness. Some peopleget it and some people don't, and some people are more susceptible than others. Mostpeople can go up to 8,000 feet (2,438 meters) with minimal effect. If you haven't been tohigh altitude before, it's important to be cautious. If you have been at that altitudebefore with no problem, you can probably return to that altitude without problems as longas you are properly acclimatized.

What Causes Altitude Illnesses

The concentration of oxygen at sea level is about 21% and the barometric pressureaverages 760 mmHg. As altitude increases, the concentration remains the same but thenumber of oxygen molecules per breath is reduced. At 12,000 feet (3,658 meters) thebarometric pressure is only 483 mmHg, so there are roughly 40% fewer oxygen molecules perbreath. In order to properly oxygenate the body, your breathing rate (even while at rest)has to increase. This extra ventilation increases the oxygen content in the blood, but notto sea level concentrations. Since the amount of oxygen required for activity is the same,the body must adjust to having less oxygen. In addition, for reasons not entirelyunderstood, high altitude and lower air pressure causes fluid to leak from the capillarieswhich can cause fluid build-up in both the lungs and the brain. Continuing to higheraltitudes without proper acclimatization can lead to potentially serious, evenlife-threatening illnesses.

Acclimatization

The major cause of altitude illnesses is going too high too fast. Given time, your bodycan adapt to the decrease in oxygen molecules at a specific altitude. This process isknown as acclimatization and generally takes 1-3 days at that altitude. For example, ifyou hike to 10,000 feet (3,048 meters), and spend several days at that altitude, your bodyacclimatizes to 10,000 feet (3,048 meters). If you climb to 12,000 feet (3,658 meters),your body has to acclimatize once again. A number of changes take place in the body toallow it to operate with decreased oxygen.

• The depth of respiration increases.
• Pressure in pulmonary arteries is increased, "forcing" blood into portions of the lung which are normally not used during sea level breathing.
• The body produces more red blood cells to carry oxygen,
• The body produces more of a particular enzyme that facilitates
• the release of oxygen from hemoglobin to the body tissues.

Prevention of Altitude Illnesses

Prevention of altitude illnesses falls into two categories, proper acclimatization andpreventive medications. Below are a few basic guidelines for proper acclimatization.

• If possible, don't fly or drive to high altitude. Start below 10,000 feet (3,048 meters) and walk up.
• If you do fly or drive, do not over-exert yourself or move higher for the first 24 hours.
• If you go above 10,000 feet (3,048 meters), only increase your altitude by 1,000 feet (305 meters) per day and for every 3,000 feet (915 meters) of elevation gained, take a rest day.
• "Climb High and sleep low." This is the maxim used by climbers. You can climb more than 1,000 feet (305 meters) in a day as long as you come back down and sleep at a lower altitude.
• If you begin to show symptoms of moderate altitude illness, don't go higher until symptoms decrease (&quotDon't go up until symptoms go down").
• If symptoms increase, go down, down, down!
• Keep in mind that different people will acclimatize at different rates. Make sure all of your party is properly acclimatized before going higher.
• Stay properly hydrated. Acclimatization is often accompanied by fluid loss, so you need to drink lots of fluids to remain properly hydrated (at least 3-4 quarts per day). Urine output should be copious and clear.
• Take it easy; don't over-exert yourself when you first get up to altitude. Light activity during the day is better than sleeping because respiration decreases during sleep, exacerbating the symptoms.
• Avoid tobacco and alcohol and other depressant drugs including, barbiturates, tranquilizers, and sleeping pills. These depressants further decrease the respiratory drive during sleep resulting in a worsening of the symptoms.
• Eat a high carbohydrate diet (more than 70% of your calories from carbohydrates) while at altitude.
• The acclimatization process is inhibited by dehydration, over-exertion, and alcohol and other depressant drugs.

Preventive Medications

• Diamox (Acetazolamide) allows you to breathe faster so that you metabolize more oxygen, thereby minimizing the symptoms caused by poor oxygenation. This is especially helpful at night when respiratory drive is decreased. Since it takes a while for Diamox to have an effect, it is advisable to start taking it 24 hours before you go to altitude and continue for at least five days at higher altitude. The recommendation of the Himalayan Rescue Association Medical Clinic is 125 mg. twice a day (morning and night). (The standard dose was 250 mg., but their research showed no difference for most people with the lower dose, although some individuals may need 250 mg.) Possible side effects include tingling of the lips and finger tips, blurring of vision, and alteration of taste. These side effects may be reduced with the 125 mg. dose. Side effects subside when the drug is stopped. Contact your physician for a prescription. Since Diamox is a sulfonamide drug, people who are allergic to sulfa drugs should not take Diamox. Diamox has also been known to cause severe allergic reactions to people with no previous history of Diamox or sulfa allergies. Frank Hubbell of SOLO recommends a trial course of the drug before going to a remote location where a severe allergic reaction could prove difficult to treat.
• Dexamethasone (a steroid) is a prescription drug that decreases brain and other swelling reversing the effects of AMS. Dosage is typically 4 mg twice a day for a few days starting with the ascent. This prevents most symptoms of altitude illness. It should be used with caution and only on the advice of a physician because of possible serious side effects. It may be combined with Diamox. No other medications have been proven valuable for preventing AMS.

Acute Mountain Sickness (AMS)

AMS is common at high altitudes. At elevations over 10,000 feet (3,048 meters), 75% ofpeople will have mild symptoms. The occurrence of AMS is dependent upon the elevation, therate of ascent, and individual susceptibility. Many people will experience mild AMS duringthe acclimatization process. Symptoms usually start 12-24 hours after arrival at altitudeand begin to decrease in severity about the third day. The symptoms of Mild AMS areheadache, dizziness, fatigue, shortness of breath, loss of appetite, nausea, disturbedsleep, and a general feeling of malaise. Symptoms tend to be worse at night and whenrespiratory drive is decreased. Mild AMS does not interfere with normal activityand symptoms generally subside within 2-4 days as the body acclimatizes. As long assymptoms are mild, and only a nuisance, ascent can continue at a moderate rate. Whenhiking, it is essential that you communicate any symptoms of illness immediately to otherson your trip. AMS is considered to be a neurological problem caused by changes in thecentral nervous system. It is basically a mild form of High Altitude Cerebral Edema (seebelow).

Basic Treatment of AMS

The only cure is either acclimatization or descent. Symptoms of Mild AMS can betreated with pain medications for headache and Diamox. Both help to reduce the severity ofthe symptoms, but remember, reducing the symptoms is not curing the problem. Diamox allowsyou to breathe faster so that you metabolize more oxygen, thereby minimizing the symptomscaused by poor oxygenation. This is especially helpful at night when respiratory drive isdecreased. Since it takes a while for Diamox to have an effect, it is advisable to starttaking it 24 hours before you go to altitude and continue for at least five days at higheraltitude. The recommendation of the Himalayan Rescue Association Medical Clinic is 125 mg.twice a day (morning and night). (The standard dose was 250 mg., but their research showedno difference for most people with the lower dose, although some individuals mayneed 250 mg.) Possible side effects include tingling of the lips and finger tips, blurringof vision, and alteration of taste. These side effects may be reduced with the 125 mg.dose. Side effects subside when the drug is stopped. Contact your physician for aprescription. Since Diamox is a sulfonamide drug, people who are allergic to sulfa drugsshould not take Diamox. Diamox has also been known to cause severeallergic reactions to people with no previous history of Diamox or sulfa allergies.Frank Hubbell of SOLO in New Hampshire recommends a trial course of the drug beforegoing to a remote location where a severe allergic reaction could prove difficult totreat.

Moderate AMS

Moderate AMS includes severe headache that is not relieved by medication,nausea and vomiting, increasing weakness and fatigue, shortness of breath, and decreasedcoordination (ataxia). Normal activity is difficult, although the person may still be ableto walk on their own. At this stage, only advanced medications or descent can reverse theproblem. Descending even a few hundred feet (70-100 meters) may help and definiteimprovement will be seen in descents of 1,000-2,000 feet (305-610 meters). Twenty-fourhours at the lower altitude will result in significant improvements. The person shouldremain at lower altitude until symptoms have subsided (up to 3 days). At this point, theperson has become acclimatized to that altitude and can begin ascending again. The besttest for moderate AMS is to have the person "walk a straight line" heel to toe.Just like a sobriety test, a person with ataxia will be unable to walk a straight line.This is a clear indication that immediate descent is required. It isimportant to get the person to descend before the ataxia reaches the pointwhere they cannot walk on their own (which would necessitate a litter evacuation).

Severe AMS

Severe AMS presents as an increase in the severity of the aforementioned symptoms,including shortness of breath at rest, inability to walk, decreasing mentalstatus, and fluid buildup in the lungs. Severe AMS requires immediatedescent to lower altitudes (2,000 - 4,000 feet [610-1,220 meters]).

There are two other severe forms of altitude illness, High Altitude Cerebral Edema(HACE) and High Altitude Pulmonary Edema (HAPE). Both of these happen less frequently,especially to those who are properly acclimatized. When they do occur, it is usually withpeople going too high too fast or going very high and staying there. The lack of oxygenresults in leakage of fluid through the capillary walls into either the lungs or thebrain.

High Altitude Pulmonary Edema (HAPE)

HAPE results from fluid buildup in the lungs. The fluid in the lungs prevents effectiveoxygen exchange. As the condition becomes more severe, the level of oxygen in thebloodstream decreases, and this can lead to cyanosis, impaired cerebral function, anddeath. Symptoms include shortness of breath even at rest, "tightness in thechest," marked fatigue, a feeling of impending suffocation at night, weakness, and apersistent productive cough bringing up white, watery, or frothy fluid. Confusion, andirrational behavior are signs that insufficient oxygen is reaching the brain. One of themethods for testing yourself for HAPE is to check your recovery time after exertion. Ifyour heart and breathing rates normally slow down in X seconds after exercise, but ataltitude your recovery time is much greater, it may mean fluid is building up in thelungs. In cases of HAPE, immediate descent is a necessary life-savingmeasure (2,000 - 4,000 feet [610-1,220 meters]). Anyone suffering from HAPE mustbe evacuated to a medical facility for proper follow-up treatment.

High Altitude Cerebral Edema (HACE)

HACE is the result of swelling of brain tissue from fluid leakage. Symptoms can includeheadache, loss of coordination (ataxia), weakness, and decreasing levels of consciousnessincluding, disorientation, loss of memory, hallucinations, psychotic behavior, and coma.It generally occurs after a week or more at high altitude. Severe instances can lead todeath if not treated quickly. Immediate descent is a necessary life-savingmeasure (2,000 - 4,000 feet [610-1,220 meters]). There are some medications that may beprescribed for treatment in the field, but these require that you have proper training intheir use. Anyone suffering from HACE must be evacuated to a medicalfacility for proper follow-up treatment.

Other Medications for Altitude Illnesses

• Ibuprofen is effective at relieving altitude headache.
• Nifedipine rapidly decreases pulmonary artery pressure and relieves HAPE.
• Breathing oxygen reduces the effects of altitude illnesses.

Gamow Bag (pronounced ga´ mäf)

This clever invention has revolutionized field treatment of high altitude illnesses.The bag is basically a sealed chamber with a pump. The person is placed inside the bag andit is inflated. Pumping the bag full of air effectively increases the concentration ofoxygen molecules and therefore simulates a descent to lower altitude. In as little as 10minutes the bag can create an "atmosphere" that corresponds to that at 3,000 -5,000 feet (915 - 1,525 meters) lower. After a 1-2 hours in the bag, the person's bodychemistry will have "reset" to the lower altitude. This lasts for up to 12 hoursoutside of the bag which should be enough time to walk them down to a lower altitude andallow for further acclimatization. The bag and pump weigh about 14 pounds (6.3 kilos) andare now carried on most major high altitude expeditions. Bags can be rented for short termtrips such as treks or expeditions.

Cheyne-Stokes Respirations

Above 10,000 feet (3,000 meters) most people experience a periodic breathing duringsleep known as Cheyne-Stokes Respirations. The pattern begins with a few shallow breathsand increases to deep sighing respirations then falls off rapidly. Respirations may ceaseentirely for a few seconds and then the shallow breaths begin again. During the periodwhen breathing stops the person often becomes restless and may wake with a sudden feelingof suffocation. This can disturb sleeping patterns, exhausting the climber. Acetazolamideis helpful in relieving the periodic breathing. This type of breathing is notconsidered abnormal at high altitudes. However, if it occurs first during an illness(other than altitude illnesses) or after an injury (particularly a head injury) it may bea sign of a serious disorder.

Sources:

• Mountain Sickness, Peter Hackett, The Mountaineers, Seattle, 1980.
• High Altitude Illness, Frank Hubble, Wilderness Medicine Newsletter, March/April 1995.
• The Use of Diamox in the Prevention of Acute Mountain Sickness, Frank Hubble, Wilderness Medicine Newsletter, March/April 1995.
• The Outward Bound Wilderness First Aid Handbook, J. Isaac and P. Goth, Lyons & Burford, New York, 1991.
• Medicine for Mountaineering, Fourth Edition, James Wilkerson, Editor, The Mountaineers, Seattle, 1992.
• Gamow Bags - can be rented from Chinook Medical Gear, 34500 Hwy 6, Edwards, Colorado 81632, 970-926-9277. www.chinookmed.com
  

Additional Reading:

• Altitude Illness Prevention & Treatment, Steven Bezruchka, The Mountaineers, Seattle, 1994.
• Going Higher, Charles Houston, Little Brown, 1987.
• High Altitude Sickness and Wellness, Charles Houston, ICS Books, 1995.
• High Altitude Medicine and Physiology, Ward Milledge, West, Chapman and Hall, New York, 1995.

This article is written by Rick Curtis, Director, Outdoor Action Program. This material may be freely distributed for nonprofit educational use. However, if included in publications, written or electronic, attributions must be made to the author. Commercial use of this material is prohibited without express written permission from the author. Copyright © 1998 Rick Curtis, Outdoor Action Program, Princeton University.