What are the types of altitude sickness?
While there are several altitude-related illnesses, the most common and most clinically important ones are Acute Mountain Sickness (AMS), High-Altitude Cerebral Edema (HACE) and High-Altitude Pulmonary Edema (HAPE).
Can you talk a bit more about each one, beginning with AMS?
AMS is the most common of these three, and is the one usually referred to as altitude sickness. It has an estimated prevalence of around 25% among visitors to Summit County from out-of-state. The prevalence of AMS seems to be about half of that when looking only at travelers from Denver staying in Summit County. This is thought to be caused by new exposure to the lower oxygen content in the air at high elevations.
While the makeup of air is always 21%, no matter what elevation it is measured, the lower pressure (thinner air) results in less available oxygen per breath at altitude. The brain senses this and subconsciously increases the body’s respiratory rate to make up for the lower oxygen content. This then causes the lungs to breathe off more CO2 than usual, leading to alkalinization of the blood.
The lower blood CO2 level leads to constriction of the blood vessels in the brain and swelling of the brain tissue. The kidneys compensate for the alkalinization of blood by excreting bicarbonate into the urine. While this can lead to dehydration, it is an important part of the acclimatization process. Anything that disrupts this process, such as impaired respiratory compensation, alcohol consumption, or dehydration, can increase the likelihood of developing AMS.
And what about HACE?
This is a much rarer condition. It typically starts after a traveler has been at high elevation (above 8,000 feet) for at least three days, and has not taken the time to acclimatize, and often develops after AMS symptoms have started. The mortality of this illness is very high unless patients can be rapidly evacuated to a lower elevation.
Finally, what is HAPE?
This is also a rare condition but has a different underlying mechanism from AMS and HACE. HAPE is caused by the pulmonary artery constricting in response to lower oxygen content in the environment. This causes increased pressures in the pulmonary artery, and subsequently leakage of fluid into the lungs. This fluid and swelling causes cough, difficulty breathing and chest pain. The mortality of this illness is around 50% if a patient is not rapidly evacuated to lower elevation.
At what elevation can altitude sickness set in, and how long does it normally last?
AMS usually only occurs at elevations above 8,000 feet, although rarely, some individuals may experience symptoms at just 5,000 feet. Symptoms typically develop within two to 12 hours and last around one to two days, with the mean symptom duration at 10,000 feet being around 15 hours. The more quickly someone ascends to high elevation, and the higher the elevation they ascend to is, the more likely that they will develop AMS.
What are the symptoms of AMS?
The most common symptom is headache, followed by fatigue, dizziness, nausea, and insomnia. In extreme cases where a traveler continues to ascend despite their symptoms, they can develop HACE, a life-threatening condition. These symptoms can progress to confusion, difficulty walking and coma.
Are some people more likely to get altitude sickness than others?
Any illness which interferes with the body’s acclimatization to higher elevations will put a person at increased risk of developing AMS. This includes illnesses such as asthma, chronic obstructive pulmonary disease (COPD) and viral illnesses. Other risk factors are alcohol consumption, dehydration and obesity. Previous altitude exposure, physical fitness level, age, sex and smoking do not appear to have any effect on the likelihood of developing altitude sickness.
Why can altitude sickness happen in anyone, no matter how physically fit they are?
Physical fitness can help improve the efficiency of oxygen utilization, and therefore exercise tolerance at altitude, but it doesn't speed up the acclimatization process. Even the most fit athletes are at a relatively similar risk of experiencing AMS symptoms as those who do not exercise regularly. The reasons for this are unclear, but this may be because the improved oxygen utilization of fit individuals is minimally involved in the acclimatization process. No matter how physically fit a person is, their body will still need time to adjust to the lower oxygen content of air at higher elevations.
If you find yourself/friends in the middle of the backcountry with no immediate service, what emergency measures should you take?
The most important thing to do if someone is experiencing symptoms of AMS is to immediately halt the ascent to higher elevations. In fact, a small descent of as little as 1,000 feet can significantly improve the severity of symptoms or resolve them altogether. Fortunately, if the affected person does not ascend any further, symptoms will usually resolve within one to two days without descent and are unlikely to progress to become life-threatening. If oxygen or medications such as acetazolamide or dexamethasone are available, most patients experience significant symptomatic relief. Both ibuprofen and acetaminophen (Tylenol) have also been effective in relieving AMS symptoms. It is still important to stop ascent, even if symptoms improve with medications.
Why is allowing time for altitude adjustment so important for out-of-state visitors?
AMS symptoms can be severe enough to cause major disruptions to your itinerary and enjoyment of your trip. It is never a bad idea to adjust to the slightly higher elevation in Denver for one night before heading up to the mountains. While most travelers to Colorado rarely spend significant amounts of time above 10,000 feet, it is usually considered safe to ascend 1,500 feet per day when traveling above 8,000 feet. Additionally, travelers should consume plenty of fluids and avoid excessive caffeine and alcohol intake.
Anything you want to add?
Here at the CU Anschutz Medical Campus, we are actively involved in further understanding this illness through multiple research projects. We still have so much to learn about the cause, treatment and prevention of acute mountain sickness.
April 25, 2024