GUIDE TO ACCLIMATIZATION
Normal Acclimatization
Acclimatization is the process of the body adjusting to the decreased availability of oxygen at high altitudes. It is a slow process, taking place over a period of days to weeks.
High altitude is defined as:
- High Altitude: 1500 - 3500 m (5000 - 11500 ft)
- Very High Altitude: 3500 - 5500 m (11500 - 18000 ft)
- Extreme Altitude: above 5500 m
Practically speaking, however, we generally don't worry much about elevations below about 2500 m (8000 ft) since altitude illness rarely occurs lower than this.
Certain normal physiologic changes occur in every person who goes to altitude:
- Hyperventilation (breathing faster, deeper, or both)
- Shortness of breath during exertion
- Changed breathing pattern at night
- Awakening frequently at night
- Increased urination
As one ascends through the atmosphere, barometric pressure decreases (though the air still contains 21% oxygen) and thus every breath contains fewer and fewer molecules of oxygen. One must work harder to obtain oxygen, by breathing faster and deeper. This is particularly noticeable with exertion, such as walking uphill. Being out of breath with exertion is normal, as long as the sensation of shortness of breath resolves rapidly with rest. The increase in breathing is critical. It is therefore important to avoid anything that will decrease breathing, e.g. alcohol and certain drugs. Despite the increased breathing, attaining normal blood levels of oxygen is not possible at high altitude.
Persistent increased breathing results in reduction of carbon dioxide in the blood, a metabolic waste product that is removed by the lungs. The build-up of carbon dioxide in the blood is the key signal to the brain that it is time to breathe, so if it is low, the drive to breathe is blunted (the lack of oxygen is a much weaker signal, and acts as an ultimate safety valve). As long as you are awake it isn't much trouble to consciously breathe, but at night an odd breathing pattern develops due to a back-and-forth balancing act between these two respiratory triggers. Periodic breathing consists of cycles of normal breathing which gradually slows, breath-holding, and a brief recovery period of accelerated breathing. The breath-holding may last up to 10-15 seconds. This is not altitude sickness. It may improve slightly with acclimatization, but does not usually resolve until descent. Periodic breathing can cause a lot of anxiety:
- In the person who wakes up during the breath-holding phase and knows he has stopped breathing.
- In the person who wakes up in the post-breath-holding hyperventilation (recovery) phase and thinks he's short of breath and has High Altitude Pulmonary Edema (HAPE).
- In the person who wakes up and realizes his neighbor has stopped breathing.
In the first two cases waiting a few moments will establish a normal breathing pattern. In the final case, the sleeping neighbor will eventually take a breath, though periodic breathing cycles will likely continue until he or she is awake. If periodic breathing symptoms are troublesome, a medication called acetazolamide may be helpful.
Dramatic changes take place in the body's chemistry and fluid balance during acclimatization. The osmotic center, which detects the "concentration" of the blood, gets reset so that the blood is more concentrated. This results in an altitude diuresis as the kidneys excrete more fluid. The reason for this reset is not understood, though it has the effect of increasing the hematocrit (concentration of red blood cells) and perhaps improving the blood's oxygen-carrying ability somewhat; it also counteracts the tendency for edema formation. It is normal at altitude to be urinating more than usual. If you are not, you may be dehydrated, or you may not be acclimatizing well.
Acute Mountain Sickness
Acute Mountain Sickness (AMS) is a constellation of symptoms that represents your body not being acclimatized to it's current altitude.
As you ascend, your body acclimatizes to the decreasing oxygen (hypoxia). At any moment, there is an "ideal" altitude where your body is in balance; most likely this is the last elevation at which you slept. Extending above this is an indefinite gray zone where your body can tolerate the lower oxygen levels, but to which you are not quite acclimatized. If you get above the upper limit of this zone, there is not enough oxygen for your body to function properly, and symptoms of hypoxic distress occur - this is AMS. Go too high above what you are prepared for, and you get sick.
This "zone of tolerance" moves up with you as you acclimatize. Each day, as you ascend, you are acclimatizing to a higher elevation, and thus your zone of tolerance extends that much higher up the mountain. The trick is to limit your daily upward travel to stay within that tolerance zone.
The exact mechanisms of AMS are not completely understood, but the symptoms are thought to be due to mild swelling of brain tissue in response to the hypoxic stress. If this swelling progresses far enough, significant brain dysfunction occurs (See next section, on HACE). This brain tissue distress causes a number of symptoms; universally present is a headache, along with a variety of other symptoms.
The diagnosis of AMS is made when a headache, with any one or more of the following symptoms is present after a recent ascent above 2500 meters (8000 feet):
- Loss of appetite, nausea, or vomiting
- Fatigue or weakness
- Dizziness or light-headedness
- Difficulty sleeping
All of these symptoms may vary from mild to severe. A scoring system has been developed based on the Lake Louise criteria; look at the AMS questionnaire for a simple method to evaluate an individual's AMS severity.
AMS has been likened to a bad hangover, or worse. However, because the symptoms of mild AMS can be somewhat vague, a useful rule-of-thumb is: if you feel unwell at altitude, it is altitude sickness unless there is another obvious explanation (such as diarrhea).
Anyone who goes to altitude can get AMS. It is primarily related to individual physiology (genetics) and the rate of ascent; there is no significant effect of age, gender, physical fitness, or previous altitude experience. Some people acclimatize quickly, and can ascend rapidly; others acclimatize slowly and have trouble staying well even on a slow ascent. There are factors that we don't understand; the same person may get AMS on one trip and not another despite an identical ascent itinerary. Unfortunately, no way has been found to predict who is likely to get sick at altitude.
It is remarkable how many people mistakenly believe that a headache at altitude is "normal"; it is not. Denial is also common - be willing to admit that you have altitude illness, that's the first step to staying out of trouble.
It is OK to get altitude illness, it can happen to anyone. It is not OK to die from it. With the information in this tutorial, you should be able to avoid the severe, life-threatening forms of altitude illness.
High Altitude Cerebral Edema
(HACE)
AMS is a spectrum of illness, from mild to life-threatening. At the "severely ill" end of this spectrum is High Altitude Cerebral Edema; this is when the brain swells and ceases to function properly. HACE can progress rapidly, and can be fatal in a matter of a few hours to one or two days. Persons with this illness are often confused, and may not recognize that they are ill.
The hallmark of HACE is a change in mentation, or the ability to think. There may be confusion, changes in behavior, or lethargy. There is also a characteristic loss of coordination that is called ataxia. This is a staggering walk that is similar to the way a person walks when very intoxicated on alcohol. This loss of coordination may be subtle, and must be specifically tested for. Have the sick person do a straight line walk (the "tandem gait test"). Draw a straight line on the ground, and have them walk along the line, placing one foot immediately in front of the other, so that the heel of the forward foot is right in front of the toes behind. Try this yourself. You should be able to do it without difficulty. If they struggle to stay on the line (the high-wire balancing act), can't stay on it, fall down, or can't even stand up without assistance, they fail the test and should be presumed to have HACE. (The formal diagnostic definition is here)
Immediate descent is the best treatment for HACE. This is of the utmost urgency, and cannot wait until morning (unfortunately, HACE often strikes at night). Delay may be fatal. The moment HACE is recognized is the moment to start organizing flashlights, helpers, porters, whatever is necessary to get this person down. Descent should be to the last elevation at which they woke up feeling well. Bearing in mind that the vast majority of cases of HACE occur in persons who ascend with symptoms of AMS, this is likely to be the elevation at which the person slept two nights previously. If you are uncertain, a 500-1000 meter descent is a good starting point. Other treatments include oxygen, hyperbaric bag, and dexamethasone. These are usually used as temporizing measures until descent can be effected (see physician section for more details).
People with HACE usually survive if they descend soon enough and far enough, and usually recover completely. The staggering gait may persist for days after descent. Once recovery has been complete, and there are no symptoms, cautious re-ascent is acceptable.
High Altitude Pulmonary Edema
(HAPE)
Another form of severe altitude illness is High Altitude Pulmonary Edema, or fluid in the lungs. Though it often occurs with AMS, it is not felt to be related and the classic signs of AMS may be absent. Signs and symptoms of HAPE include any of the following:
- Extreme fatigue
- Breathlessness at rest
- Fast, shallow breathing
- Cough, possibly productive of frothy or pink sputum
- Gurgling or rattling breaths
- Chest tightness, fullness, or congestion
- Blue or gray lips or fingernails
- Drowsiness
HAPE usually occurs on the second night after an ascent, and is more frequent in young, fit climbers or trekkers.
In some persons, the hypoxia of high altitude causes constriction of some of the blood vessels in the lungs, shunting all of the blood through a limited number of vessels that are not constricted. This dramatically elevates the blood pressure in these vessels and results in a high-pressure leak of fluid from the blood vessels into the lungs. Exertion and cold exposure can also raise the pulmonary blood pressure and may contribute to either the onset or worsening of HAPE.
Immediate descent is the treatment of choice for HAPE; unless oxygen is available delay may be fatal. Descend to the last elevation where the victim felt well upon awakening. Descent may be complicated by extreme fatigue and possibly also by confusion (due to inability to get enough oxygen to the brain); HAPE frequently occurs at night, and may worsen with exertion. These victims often need to be carried.
It is common for persons with severe HAPE to also develop HACE, presumably due to the extremely low levels of oxygen in their blood (equivalent to a continued rapid ascent).
HAPE resolves rapidly with descent, and one or two days of rest at a lower elevation may be adequate for complete recovery. Once the symptoms have fully resolved, cautious re-ascent is acceptable.
HAPE can be confused with a number of other respiratory conditions:
High Altitude Cough and Bronchitis are both characterized by a persistent cough with or without sputum production. There is no shortness of breath at rest, no severe fatigue. Normal oxygen saturations (for the altitude) will be measured if a pulse oximeter is available.
Pneumonia can be difficult to distinguish from HAPE. Fever is common with HAPE and does not prove the patient has pneumonia. Coughing up green or yellow sputum may occur with HAPE, and both can cause low blood levels of oxygen. The diagnostic test (and treatment) is descent - HAPE will improve rapidly. If the patient does not improve with descent, then consider antibiotics. HAPE is much more common at altitude than pneumonia, and more dangerous; many climbers have died of HAPE when they were mistakenly treated for pneumonia.
Asthma might also be confused with HAPE. Fortunately, asthmatics seem to do better at altitude than at sea-level. If you think it's asthma, try asthma medications, but if the person does not improve fairly quickly assume it is HAPE and treat it accordingly.
Treating Acute Mountain Sickness
The mainstay of treatment of AMS is rest, fluids, and mild analgesics: acetaminophen (paracetamol), aspirin, or ibuprofen. These medications will not cover up worsening symptoms. The natural progression for AMS is to get better, and often simply resting at the altitude at which you became ill is adequate treatment. Improvement usually occurs in one or two days, but may take as long as three or four days. Descent is also an option, and recovery will be quite rapid.
A frequent question is how to tell if a headache is due to altitude. See Golden Rule I. Altitude headaches are usually nasty, persistent, and frequently there are other symptoms of AMS; they tend to be frontal (but may be anywhere), and may worsen with bending over. However, there are other causes of headaches, and you can try a simple diagnostic/therapeutic test. Dehydration is a common cause of headache at altitude. Drink one liter of fluid, and take some acetaminophen or one of the other analgesics listed above. If the headache resolves quickly and totally (and you have no other symptoms of AMS) it is very unlikely to have been due to AMS.
Acetazolamide
Acetazolamide (Diamox®) is a medication that forces the kidneys to excrete bicarbonate, the base form of carbon dioxide; this re-acidifies the blood, balancing the effects of the hyperventilation that occurs at altitude in an attempt to get oxygen. This re-acidification acts as a respiratory stimulant, particularly at night, reducing or eliminating the periodic breathing pattern common at altitude. Its net effect is to accelerate acclimatization. Acetazolamide isn't a magic bullet, cure of AMS is not immediate. It makes a process that might normally take about 24-48 hours speed up to about 12-24 hours.
Acetazolamide is a sulfonamide medication, and persons allergic to sulfa medicines should not take it. Common side effects include numbness, tingling, or vibrating sensations in hands, feet, and lips. Also, taste alterations, and ringing in the ears. These go away when the medicine is stopped. Since acetazolamide works by forcing a bicarbonate diuresis, you will urinate more on this medication. Uncommon side effects include nausea and headache. A few trekkers have had extreme visual blurring after taking only one or two doses of acetazolamide; fortunately they recovered their normal vision in several days once the medicine was discontinued.
Acetazolamide Use & Dosage:
For treatment of AMS
We recommend a dosage of 250 mg every 12 hours. The medicine can be discontinued once symptoms resolve. Children may take 2.5 mg/kg body weight every 12 hours.
For Periodic Breathing
125 mg about an hour before bedtime. The medicine should be continued until you are below the altitude where symptoms became bothersome.
There is a lot of Mythology About Acetazolamide
MYTH: acetazolamide hides symptoms
Acetazolamide accelerates acclimatization. As acclimatization occurs, symptoms resolve, directly reflecting improving health. Acetazolamide does not cover up anything - if you are still sick, you will still have symptoms. If you feel well, you are well.
MYTH: acetazolamide will prevent AMS from worsening during ascent
Acetazolamide DOES NOT PROTECT AGAINST WORSENING AMS WITH CONTINUED ASCENT. It does not change Golden Rule II. Plenty of people have developed HAPE and HACE who believed this myth.
MYTH: acetazolamide will prevent AMS during rapid ascent
This is actually not a myth, but rather a misused partial truth. Acetazolamide does lessen the risk of AMS, that's why we recommend it for people on forced ascents. This protection is not absolute, however, and it is foolish to believe that a rapid ascent on acetazolamide is without serious risk. Even on acetazolamide, it is still possible to ascend so rapidly that when illness strikes, it may be sudden, severe, and possibly fatal.
MYTH: If acetazolamide is stopped, symptoms will worsen
There is no rebound effect. If acetazolamide is stopped, acclimatization slows down to your own intrinsic rate. If AMS is still present, it will take somewhat longer to resolve; if not - well, you don't need to accelerate acclimatization if you ARE acclimatized. You won't become ill simply by stopping acetazolamide.
Dexamethasone
Dexamethasone (Decadron®) is a potent steroid used to treat brain edema. Whereas acetazolamide treats the problem (by accelerating acclimatization), dexamethasone treats the symptoms (the distress caused by hypoxia). Dexamethasone can completely remove the symptoms of AMS in a few hours, but it does not help you acclimatize. If you use dexamethasone to treat AMS you should not go higher until the next day, to be sure the medication has worn off and is not hiding a lack of acclimatization.
Side effects include euphoria in some people, trouble sleeping, and an increased blood sugar level in diabetics.
Dexamethasone Use & Dosage:
For treatment of AMS
Two doses of 4 mg, 6 hours apart. This can be given orally, or by an injection if the patient is vomiting. Children may be given 1 mg/kg of body weight, up to 4 mg maximum; a second dose is given in 6 hours. Do not ascend until at least 12 hours after the last dose, and then only if there are no symptoms of AMS.
Oxygen
AMS symptoms resolve very rapidly (minutes) on moderate-flow oxygen (2-4 liters per minute, by nasal cannula). There may be rebound symptoms if the duration of therapy is inadequate - several hours of treatment may be needed. In most high altitude enviroments, oxygen is a precious commodity, and as such is usually reserved for more serious cases of HACE and HAPE.
Hyperbaric Therapy
Treatment in a portable hyperbaric bag (see physician's section for details) is essentially equivalent to descent or treatment with oxygen; the person is inside a pressurized bag breathing an atmosphere equivalent to a much lower altitude. AMS symptoms rapidly resolve (minutes), but may recurr if treatment is too short - at least two hours are needed. Dexamethasone works as well, though not quite as fast, is much cheaper, and far less labor-intensive than hyperbaric therapy. Hyperbaric treatment is usually reserved for more serious cases such as HACE and HAPE.
A Review of the AMS Treatment Options
Descent
Pro Rapid recovery: trekkers generally improve during descent, recover totally within several hours.
Con Loss of "progress" toward trek goal; descent may be difficult in bad weather or at night; personnel needed to accompany patient.
Rest at same elevation
Pro Acclimatization to current altitude, no loss of upward progress.
Con It may take 24-48 hours to become symptom-free.
Rest plus acetazolamide
Pro As with rest alone, plus acclimatization is accelerated, recovery likely within 12-24 hours.
Con Recovery may take 12-24 hours; side effects of acetazolamide.
Rest plus dexamethasone
Pro Faster resolution of symptoms than with acetazolamide (usually in a few hours); minimal side effects; cheap.
Con Can hide symptoms & thus give a false sense of security to those who want to continue upwards. Does not accelerate acclimatization.
Rest plus dexamethasone & acetazolamide
Pro Fast resolution of symptoms from the dexamethasone, plus improved acclimatization from the acetazolamide.
Con Side effects of acetazolamide. Same cautions as above regarding ascent after taking dexamethasone.
Oxygen or Hyperbaric Therapy
Pro Very rapid relief of symptoms (minutes).
Con Expensive; hyperbaric bags are very labor-intensive; rebound symptoms may occur if treatment is too short - several hours are needed.
Questioning Your Porters About AMS Symptoms
If you hire your own porters, please be aware that they are just as susceptible as you to the ravages of AMS. Porters may even be at increased risk of severe forms of altitude illness as they are unlikely to know anything about AMS, are more likely to have a communication barrier to telling you how they feel, and may actively hide their symptoms as they fear losing their job due to illness. For those of you who will be trekking in Nepal, please look at the AMS questionnaire with phonetic Nepali translations (based on the Lake Louise AMS scoring criteria). For more information, also look at the International Porter Protection Group's web site.
Preventing AMS
The key to avoiding AMS is a gradual ascent that gives your body time to acclimatize. People acclimatize at different rates, so no absolute statements are possible, but in general, the following recommendations will keep most people from getting AMS:
- If possible, you should spend at least one night at an intermediate elevation below 3000 meters.
- At altitudes above 3000 meters (10,000 feet), your sleeping elevation should not increase more than 300-500 meters (1000-1500 feet) per night.
- Every 1000 meters (3000 feet) you should spend a second night at the same elevation.
Remember, it's how high you sleep each night that really counts; climbers have understood this for years, and have a maxim "climb high, sleep low". The day hikes to higher elevations that you take on your "rest days" (when you spend a second night at the same altitude) help your acclimatization by exposing you to higher elevations, then you return to a lower (safer) elevation to sleep. This second night also ensures that you are fully acclimatized and ready for further ascent.
Things to Avoid
Respiratory depression (the slowing down of breathing) can be caused by various medications, and may be a problem at altitude. The following medications can do this, and should never be used by someone who has symptoms of altitude illness (these may be safe in persons who are not ill, although this remains controversial):
- Alcohol
- Sleeping pills (acetazolamide is the sleeping tablet of choice at altitude)
- Narcotic pain medications in more than modest doses
Prophylaxis
Under certain circumstances, prophylaxis with medication may be advisable.
- for persons on forced rapid ascents (such as flying into Lhasa, Tibet, or La Paz, Bolivia), for climbers who cannot avoid a big altitude gain due to terrain considerations, or for rescue personnel on a rapid ascent
- for persons who have repeatedly had AMS in the past
Acetazolamide
We do not recommend acetazolamide as a prophylactic medication, except under the specific limited conditions outlined above. Most people who have a reasonable ascent schedule will not need it, and in addition to some common minor but unpleasant side effects it carries the risk of any of the severe side effects that may occur with sulfonamides.
The dose of acetazolamide for prophylaxis is 125-250 mg twice a day starting 24 hours before ascent, and discontinuing after the second or third night at the maximum altitude (or with descent if that occurs earlier). Sustained release acetazolamide, 500 mg, is also available and may be taken once per day instead of the shorter acting form, though side effects will be more prominent with this dose.
Ginkgo Biloba Extract
Some early work with Ginkgo biloba extract was encouraging with regards to its use in preventing AMS, but some recent large, well-designed studies have shown no benefit.
AMS Prophylaxis
Acetazolamide (Diamox®)
125-250 mg (depending on body weight; persons over 100 kg (220 lbs) should take the higher dose) twice a day starting 24 hours before ascent, and discontinuing after the second or third night at the maximum altitude (or with descent if that occurs earlier). Children may take 2.5 mg/kg of body weight twice a day.
Preventing Severe AMS
This simply cannot be emphasized too much. If you have symptoms of AMS, DO NOT ASCEND ANY HIGHER. Violating this simple rule has resulted in many tragic deaths.
If you ascend with AMS you will get worse, and you might die. This is extremely important - even a day hike to a higher elevation is a great risk. In many cases of High Altitude Cerebral Edema, this rule was violated. Stay at the same altitude (or descend) until your symptoms completely go away. Once your symptoms are completely gone, you have acclimatized and then it is OK to continue ascending. It is always OK to descend, you will get better faster.
The Golden Rules
GOLDEN RULE I
If you feel unwell at altitude it is altitude illness until proven otherwise.
GOLDEN RULE II
Never ascend with symptoms of AMS.
GOLDEN RULE III
If you are getting worse (or have HACE or HAPE), go down at once.
Author: Thomas E. Dietz
Reviewed & Approved by the ISMM Publications Committee (pending)
