Illness at altitude
Acute Mountain Sickness / High Altitude Pulmonary Edema / High Altitude Cerebral Edema
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Early August, 2017, a group of Boy Scouts were trekking through the Uinta Mountain Range in Utah. A seventeen year old Scout complained of bad headache with nausea. Over the next day he continued to degrade. The decision was made to evacuate the teen the next morning. As the teen went to bed he was still complaining of the headache and displayed a loss of muscle control. The Scouts awoke early to effect their evacuation while a Scout Master hiked 6-10 miles to get into cellular phone range and called fo
r a helicopter medical evacuation. Upon arrival, the medics aboard found the Scout dead. Reports from the Scouts was that he managed to walk 50 yards, collapsed unresponsive and CPR was attempted. The Scout was a healthy and fit teenager with no significant medical history. The elevation in the crater where he passed was between 10,000’ in the valley to 13,500’. If a healthy teenaged Boy Scout can succumb to altitude sickness at these altitudes, common through the Rocky Mountains, then we must become very familiar with the signs and symptoms of altitude sickness and understand what treatments we have, and most importantly, when the disease has progressed to a point of evacuation.
One of the most common misconceptions about altitude sickness is that the disease is caused by a low amount of oxygen in the breathing air. The atmosphere is comprised of about 21% oxygen up to an altitude of about 21,000` (900` higher than the summit of Denali, 9,000` less than the summit of Everest) but the partial pressure of this oxygen in the lungs, specifically in the capillary beds where gas exchanges, can be significantly lower. This partial pressure is required to “push” oxygen into the bloodstream. The concept of air-pressure can be hard to understand but when we think of air as a fluid it makes more sense. When we are at sea level we have a huge amount of “fluid” above us, being pulled down by gravity, increasing the pressure of these gasses in the lungs. As we ascend we have less “fluid” above us and the pressure decreases. This decrease in pressure is the root cause of what is known as “Acute Mountain Sickness”. The oxygen-carrying component in blood is unable to take on the oxygen molecules leading to overall hypoxia (low oxygen) at the organs.
Elevation can be broken down into three primary categories; high altitude (4,900` to 11,500`), Very high altitude (11,500` to 18,000`) and lastly, EXTREME high altitude (over 18,000`). At about 7,000` the partial pressure of oxygen drops enough that the percentage of oxygen-saturated hemoglobin drops enough that it is detectable with the use of pulse oximetry, or “pulse ox”. Most people at this elevation will have a “pulse ox” of >90%. Typically the people at these elevations that display mountain sickness have rapidly ascended from sea level to altitude. Above 11,500` people will display pulse ox less than 90% and their symptoms will be exacerbated by exertion or attempting sleep. The majority of altitude illness occurs in this range. Elevations over 18,000` are, for most people, unsustainable for anything but short periods. Pronounced hypoxia (low oxygen) causes progressive deterioration of physiological functions and travel to these elevations without acclimatization can be fatal.
Now we know that less pressure of oxygen in the lungs means that the blood gets less oxygen and the end organs get less oxygen. But that’s still not the total picture that we should observe with assessing our patients. One of the biggest predictors of altitude sickness isn’t the altitude per se but the speed at which this altitude was ascended to. Trekkers that take time to appropriately acclimatize and follow tried and tested axioms (“sleep low, climb high”) do the best with mountain sickness and those that have rapid change in elevations (think of the skier travelling from New York, sea level, to Breckenridge Colorado, 12,998` at top of resort, in one day) typically have the most pronounced symptoms.
Higher altitudes not only are frequently colder than lower altitudes but the air is much drier. People at altitude will breathe faster to compensate for the hypoxia and urinary production is increased to help deal with these metabolic byproducts. Combining these frequently leads to dehydration which can be compounded, especially in ski and tourist destinations, by use of alcohol. This is why we stress proper hydration in the back country and especially at elevation.
To sum up everything so far; altitude sickness is primarily caused by a decreased pressure partial of oxygen in the lungs. This decrease in pressure leads to fewer oxygen molecules crossing into the blood through the capillary beds. Dehydration and rapid ascent can compound these processes. These disease processes can manifest in predictable manners and it is our responsibility to recognize the signs and symptoms for early treatment and to understand when the patient's situation is critical and an immediate evacuation must begin.
In the early 90’s a conference was held at Lake Louise called the International Hypoxia Conference. The physicians there developed a scoring system to determine the presence and severity of acute mountain sickness. There is extensive literature about this and a full discussion would take up dozens of pages so it’s mentioned here so that interested individuals can look more into it. We use the Lake Louise Consensus on Altitude Illness which defines the three identifiable conditions caused by altitude; Acute Mountain Sickness (AMS), High Altitude Pulmonary Edema (HAPE) and High Altitude Cerebral Edema (HACE).
In the presence of a recent gain of altitude:
Acute Mountain Sickness (AMS)
Headache plus one of the following;
Loss of appetite, nausea, vomiting
Fatigue or weakness
Dizziness or lightheadedness
High Altitude Cerebral Edema (HACE)
High Altitude Pulmonary Ed