The Air Is Thinner Up There — What Does That Mean?!

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June 20, 2014

Each year in the United States 30 million people travel to altitudes above 6500 feet to ski, hike, marvel at the scenery or just relax. Nature’s beauty at that altitude is amazing, but warrants precautions before and during the visit.

HIGH ALTITUDE PHYSIOLOGY – What do we mean by, “The air is thinner up there?”

Whether you’re at sea level or on the highest peaks, the air you breathe is 21% oxygen. What differs is that it is 21% of the barometric pressure minus 47 mmHg for water vapor. The barometric pressure decreases with altitude and further decreases with colder temperatures. If the oxygen in each breath is lower, the level of oxygen in our blood will be lower.

Our bodies “acclimate” in several ways. Earliest and most importantly, we breathe more in response to the lower oxygen level. The extent of this response is genetically determined and variable. It is decreased by factors, such as alcohol, sedatives and fragmented sleep. Caffeine may actually improve it. Increased breathing per minute decreases carbon dioxide and increases our bicarbonate. The kidneys then compensate by excreting more bicarbonate and getting our acid-base balance back to normal. More about this in the treatment section. The majority of this response occurs in the first seven days.

The kidney compensation and other changes can decrease fluids in our circulation by 12%, making us dehydrated. As a result, our heart rate increases, so we can feel like we’re running a marathon, even with minimal exertion.

In the brain, there are opposing reactions, both increasing blood flow and decreasing it, so that overall good perfusion of the brain can usually be maintained even when there is a modest fall in blood oxygen saturation.  When we exceed the limits, however, the symptoms of acute mountain sickness and cerebral edema ensue.

The hemoglobin in your red blood cells is responsible for carrying oxygen to your tissues. At altitude, It increases over 10-14 days to improve oxygen delivery to the cells that need it. If you are anemic and start out with a low red blood count, you’re likely to have more symptoms at altitude. In addition, there are multiple other changes that occur in our bodies that enhance oxygen delivery. All of our organs need oxygen to function optimally, but especially the heart, lungs and brain.

High Altitude Illness (HAI) includes:
1) acute mountain sickness (AMS) and
2) high altitude cerebral edema (HACE) which affect the brain, and
3) high altitude pulmonary edema (HAPE) which affects the lungs.

All are due to the hypoxic or low oxygen stress of high altitude. Many of the symptoms are caused by fluid accumulation outside the blood vessels in the brain or lungs, respectively. All of these situations generally respond to descent of 300 feet or so in altitude and to oxygen therapy.

Acute Mountain Sickness – AMS – is the mildest and most common form of HAI. It usually presents as lightheadedness, headache, fatigue, decreased appetite and disturbed sleep. To help make sense of what follows, remember that 1 meter is equal to approximately 3.3 feet. Although there are many factors involved, symptoms of AMS are uncommon below 2000 meters (6500 ft). However, the incidence is about 25% for those at sleeping elevations closer to 10,000 feet, where most ski resorts in the U.S. are. People often ascend quickly and are anxious to ski as soon as possible, but must wait a day or two to allow the body to acclimate as noted above. Symptoms can occur from a few to 24 hours after arrival and generally resolve within a day if there is no further ascent. It should not recur at the same altitude, but may if you go to higher altitudes during your trip.

Cerebral edema – HACE – is most likely an exaggerated form of AMS and is life-threatening, so it must be treated promptly. Symptoms include unsteady gait, severe fatigue or weakness, confusion, irritability and possibly coma. It can develop over hours or days. The person must descend and seek medical attention immediately while still able to cooperate. Interestingly, one theory is that the risk associated with the fluid accumulation around the brain is not related to the amount of fluid that accumulates, but how it is tolerated – i.e. the ‘tight fit’ hypothesis states that it depends on the amount of space available for fluid before it presses on the brain. HACE is the most common cause of death with respect to altitude sickness.

Pulmonary edema -HAPE – is uncommon, but also can be life-threatening. It usually occurs 2-4 days after ascent above 2500 meters (8000 feet). Symptoms include shortness of breath, usually even at rest. Immediate descent is essential.

In addition to these illnesses that are directly related to altitude, others may be exacerbated, such as heart disease and chronic lung disease. Frostbite can be an issue in winter.

It’s not clear why some individuals are more susceptible to AMS. Being physically fit doesn’t necessarily protect you. Factors that increase risk include:
– past history of high altitude illness
– rapid ascent
– colder temperatures
– vigorous exertion prior to adjustment
– alcohol use or dehydration
– factors limiting respiration or circulation – including chronic heart or lung disease, sleep apnea and muscular diseases.

It might surprise you that asthma may become less problematic at high altitude because airflow improves with the decreased air density. Also, young boys seem to be at higher risk – possibly because they tend to not limit their exertion at the critical time they need to acclimate.

– increase fluid intake starting several days prior to travel.
– avoid alcohol and sleeping pills until acclimated.
– avoid abrupt ascent from living at less than 1500 meters (5000′) to above 2800 meters (9200′).
– spend a night at an intermediate altitude if a significant ascent is necessary.
– follow the rule “climb high, sleep low” to allow acclimatization overnight.
– avoid vigorous exertion until you’re comfortable at altitude.
– try to take brief climbs to over 1500 meters (4900′) in the weeks prior to travel.
– those with a history of HAI should ascend more slowly than before and avoid exertion for longer.
– no one with symptoms of HAI should ascend; descent is mandatory if symptoms remain.
– mild exertion may actually assist acclimatization.
– providing oxygen at 2-4 liters per minute should help the headache and other AMS symptoms.

Of course, any person traveling to an area of high altitude should consult their personal physician for advice regarding possible health effects. Treatment should be individualized.

Generally prophylactic medications are reserved for those at highest risk, i.e., history of HAI who must ascend as quickly as before, or rescue workers who can’t alter their plans.
– Diamox (acetazolamide)is a diuretic and works in several ways to improve the body’s acid-base and fluid status and oxygen delivery. It should be started a day prior to ascent and stopped a few days after the highest level is achieved. This is not recommended for those with impaired kidney or liver function. The recommended dose varies but is usually 125 or 250 twice daily. Side effects include abnormal blood chemistry or cell counts, changes in taste, numbness and confusion, and vary with the dose. It is a sulfonamide medication, which does not necessarily cross-react with sulfa antibiotics, but should be used cautiously.
– dexamethasone (steroid) alleviates the symptoms of AMS but does not improve acclimatization. It can be used with or without diamox. The main side effect is increased sugar level, but prolonged use can also cause fluid retention, mental status changes and other symptoms. It is usually saved for treatment, not prevention, unless the patient is allergic to acetazolamide and is deemed to need medication.
– aspirin or ibuprofen can prevent headaches, but trials are insufficient to prove their usefulness regarding acute altitude sickness.

The same medications that are sometimes used for prevention are also used for treatment of acute mountain sickness. The most important thing is to avoid exertion and to descend if symptoms persist – with the help of others. Physicians in high altitude areas are very well versed in treating altitude sickness and should be consulted for these symptoms.

Cerebral edema and pulmonary edema are fortunately less common, but are much more serious, and medical help should be sought immediately for shortness of breath or agitation/ confusion.

HERE’S A BIT OF PULMONARY TRIVIA – we visited the Stanley Hotel while we were in Estes Park. It is the residence where F.O. Stanley (as in Stanley Steamer) recuperated from tuberculosis. Before we had anti-TB medications, patients used to be sent to the mountains to heal this infection. The tubercles causing the infection don’t survive well in a low-oxygen environment, so in fortunate patients, the infection slowly faded away and the host surfived. An additional benefit of sending patients to stay out in the fresh air in the mountains was that it took them out of their urban environments where they could infect others.

Happy Travels!

Gallagher, S., et al. High altitude illness: Physiology, risk factors and general prevention, April, 2014.
Gallagher, S., et al. Acute mountain sickness and high altitude cerebral edema, April, 2014.