FOUR miles into the wilderness, the lethargy hit me.
It's just laziness, I told myself, pushing farther along the High Sierra Trail in Sequoia National Park and trying to ignore the growing sensation that my limbs had become trapped in an invisible cocoon.
Finally, I could go no farther. I fell to my knees on the edge of a mountain -- blue peaks surrounding me -- fighting to keep my breakfast from bubbling up my throat. I lost. Once, twice, more than three times in the space of an hour. My companion and assorted passersby urged me on -- it was 11:30 a.m. and camp was more than seven miles away -- but my body refused. It told me to remain in the dirt and forest litter.
My body was right.
I had a classic case of acute mountain sickness, also known as altitude sickness. My symptoms had been relatively mild, but going any higher could have caused a pulmonary or cerebral edema, in which liquid pools in the lungs or skull. Left untreated, cerebral edema can cause coma or death. Pulmonary edema can cause oxygen loss, shock and death.
The symptoms are the body's response to the reduced air pressure and oxygen at higher altitudes. According to the U.S. National Library of Medicine, about 20% of people develop mild symptoms of acute mountain sickness at altitudes of 6,300 to 9,700 feet.
As more people venture to high altitudes -- for hiking, skiing or simply soaking up the view -- the cases of altitude sickness are rising, experts say.
Some are relatively mild, causing headache or nausea. Others are far more severe. In July, Colin Powell was hospitalized with altitude sickness while attending a dinner in Aspen, Colo. (After feelings of lightheadedness and some stumbling, he was taken by ambulance to a hospital and released a few days later.) The condition also may have contributed to the death of Englishman David Sharp near the summit of Mt. Everest in May.
"There's never been any change in human tolerance," said Dr. John B. West, editor of the medical journal High Altitude Medicine & Biology and a professor of medicine and physiology at the UC San Diego School of Medicine. "We just have more people going to higher altitudes having to deal with it."
Cases are difficult to quantify, because most sufferers get better after a day of acclimation. But the transitory nature of the mild illness can lead some people to take it less seriously than they should.
Dr. Thomas Dietz, an emergency physician at Providence Hood River Memorial Hospital and a prominent researcher in the field of altitude sickness, called the rise in cases the "once-in-a-lifetime" phenomenon. People with only two or three weeks for vacation are more than willing to take shortcuts rather than acclimate.
"At Mt. Kilimanjaro, the fee per day is high, so travelers are trying to do the minimum amount of days for significantly less cost," Dietz said.
So visitors end up climbing the 19,340-foot peak to their detriment. Of the 20,000 visitors a year, 10 people a year die -- mostly from high-altitude sickness, according to National Geographic Adventure.
"The same thing happens at Mt. Everest," Dietz said. "People want to take a monthlong trip in only two or three weeks of vacation. They miss out on the acclimatization on the way up."
Some research suggests obesity can be a factor for acute mountain sickness. A study in a 2003 issue of Annals of Internal Medicine found that obese men had much lower blood oxygen levels at a simulated altitude of 12,000 feet than nonobese men. "There were huge changes in lung function," said Tony G. Babb, now an associate professor of internal medicine with the University of Texas Southwestern Medical Center and the study's senior researcher.
And a report in the June issue of High Altitude Medicine & Biology suggests that susceptibility may be inherited, pointing out that some people living at high altitudes develop chronic cases of altitude sickness while others don't.
Dietz says predicting who will succumb to altitude sickness can be difficult. "You can't by looking at a person predict if they will become ill or not," he said. "Not by gender, physical fitness or obesity."
But, he said, abnormal circulation, emphysema, heart valve problems and pulmonary hypertension can be risk factors. Those conditions can interfere with the body's ability to oxygenate blood, Dietz said.
While rumors and folklore recommend potential remedies -- ginkgo biloba or high-carbohydrate diets -- only one prescription drug, acetazolamide, has shown consistently that it can speed up acclimation, Dietz said. The drug, which should be taken up to a day before the hike or climb, helps the kidneys excrete bicarbonate (a form of carbon dioxide) -- thereby counteracting the effects of hyperventilation.
Another drug, however -- Viagra -- has shown promise as well. It works at the pulmonary level by lowering blood pressure in the lungs and is aimed at those who are susceptible to high altitude pulmonary edema.
But the best way to prevent altitude sickness is to acclimate as much as possible before your trip. Stay the night before at an intermediate elevation, Dietz said. Once on the trail, pay attention to signals you may be getting sick. A persistent headache, vomiting and fatigue can be signs of acute mountain sickness, meaning you should head back down or at least go no higher, said Mark Fincher, a wilderness ranger for Yosemite National Park and a certified emergency medical technician.
I learned that the hard way. After recovering, I resolved to get in shape and condition myself to a higher altitude before my next hike. I began a cardio- and strength-training program, working up to 45 minutes of aerobic exercise and 30 minutes of weights and flexibility six times a week. I ate healthier, cut out fast food and junk food and dropped 15 pounds.
The night before my 11-mile hike -- the same journey, but three years later, I slept in a hotel higher than 5,000 feet and drank plenty of water. I ate a filling breakfast and started early on the trail.
My first mile was the hardest, when I panted embarrassingly up a hill. But eventually I adjusted -- to the elevation and exercise -- and my breathing grew easier, with no headache, no nausea and no collapse into the forest litter.