'In the United States, more than one million skiers annually travel from near sea level
to high altitude in one day. As a result of this increased exposure, the incidence of
high-altitude illness has risen'.
LIVING AT sea level, I don't get to see any patients acutely ill with altitude sickness, but I am often asked to advise patients who are planning to visit high altitudes or fly in airplanes.
Over the years, high-altitude travel has become increasingly common. Hundreds of mountain climbers visit the Mount Everest Base Camp, located at an altitude 17,500 feet (3.3 miles) each year, while thousands of tourists annually visit high elevations throughout the world, such as Mount Rainier at 14,410 feet (2.7 miles), Mount Blanc at 16,500 feet (3.12 miles) and Mount Kilimanjaro at 19,340 feet (3.66 miles).
In the United States, more than one million skiers annually travel from near sea level to high altitude in one day. As a result of this increased exposure, the incidence of high-altitude illness has risen.
HIGH ALTITUDE CAN BE DIVIDED INTO FOUR CATEGORIES:
1. Moderate altitude (5,000 to 8,000 feet), including many popular tourist venues that are well-inhabited regions in the western United States and world-wide. Patients with cardiac or pulmonary disease who travel to these altitudes may experience increased symptoms. Some susceptible individuals may experience a mild form of acute mountain sickness, but serious altitude-related illness rarely occurs at these heights. Commercial airliners are pressurised to this level.
2. High altitude, (8,000 to 14,000 feet) is the threshold at which altitude-related illness usually begins. Up to this level, the arterial oxygen saturation falls gradually. Above this altitude, the arterial oxygen saturation decreases more steeply; by 14,000 feet, the arterial oxygen saturation is quite low.
Skiers visiting the Lake Tahoe area in California (below 7,000 feet) rarely experience high-altitude sickness, while such is not the case slightly to the south, in the Mammoth ski area (9,000 feet) and in many Rocky Mountain ski resorts located above 8,000 feet.
The highest altitude in the continental United States is about 14,000 feet, and not many large cities in the world are located at an altitude higher than this.
3. Very high altitude (14,000 to 18,000 feet) is now a common temporary destination for hikers and climbers. Rapid ascent to these levels results in a high incidence of severe medical problems, and most altitude-related fatalities occur at this level.
Prolonged stays above 18,000 feet results in deterioration, not continued altitude acclimatisation. No permanent inhabitants live above this altitude.
4. Extreme altitude (18,000 to 29,028 feet) is tolerated by only experienced and well-acclimatised mountaineers. Physiologic effects are severe at this elevation, but falls, avalanches, hypothermia and frostbite are the greatest risks for those who travel these heights.
It is important to note that the altitude at which one sleeps is a more important determinant of high-altitude illness than the altitude attained during a day of activity. Mountaineers have long recognised this phenomenon, thus their saying, "Sleep low and climb high."
At sea level, blood oxygen saturation is normally 97 per cent; at 8,000 feet it falls to 92 per cent; at 15,000 feet, 80 per cent; at 18,000 feet, 71 per cent; and at 20,000 feet, 65 per cent (33 per cent less than sea level).
This decrease in blood's oxygen-carrying capacity requires increased breathing to maintain even this lowered oxygen saturation. The increased breathing expels carbon dioxide from the blood stream, decreasing the body's acidity and requiring the kidneys to react to maintain the body's chemical balance.
Further, the decreased oxygen saturation stimulates the bone marrow to make more, and smaller, red blood cells in order to transport the available oxygen more efficiently.
Within a few hours of being at altitude, fluid moves from the blood stream to the body tissues. This alteration relatively increases the haemoglobin level in the circulation because the haemoglobin stays within the confines of the bloodstream. This shift of fluid can cause tissue swelling, including the brain (cerebral edema) and/or congestion of the lungs (pulmonary edema).
For a few days after being at altitude, adrenaline-like activity increases, raising the heart rate, blood pressure and metabolic rate. Gradually, after two weeks or so, physiologic acclimatisation occurs and these levels return to those at sea level.
Because commercial airliners are pressurised equivalent to 8,000 feet, or less, it is rare for a flight in a commercial airplane to precipitate major problems for patients with mild to modest heart problems. However, for the heart patient who is very symptomatic at sea level, even the 5,000 to 8,000 feet pressurisation may present a serious health hazard.
The combination of physiologic and chemical responses to altitude lead to decreased exercise capacity. For those people with heart and lung conditions, limiting exertion tolerance when not at altitude, the exposure to altitude can increase symptoms or precipitate medical emergencies. Be careful up there.
- Marshall Franklin, M.D., Copley News Service, www.copleynews.com
