TUESDAY, Feb. 13 (HealthDay News) -- A drug used to ease "mountain sickness" holds promise as a treatment for a potentially fatal condition that can strike high-altitude climbers, researchers report.

In tests that exposed human subjects to a simulation of high altitude, the drug acetazolamide appeared to boost the ability of the lungs to function properly while facing major stress.

"It may open up avenues to treat and prevent the more serious forms of altitude-related illnesses," said study co-author Marc Poulin, an associate professor of physiology at the University of Calgary, in Alberta, Canada.

However, the research only looked at a small group of subjects, and a doctor who specializes in climbers said the drug -- known by the brand name Diamox -- isn't yet ready for prime time.

Mountain sickness, also known as altitude sickness, occurs when people go beyond about 6,560 to 9,840 feet of altitude, Poulin said. Symptoms include insomnia, nausea, headaches, fatigue, lack of appetite, shortness of breath and lightheadedness.

"Rapid ascent is a risk factor, but the mechanism is not really well understood," Poulin said. "It's hard to predict who will be susceptible and who will not."

According to Poulin, about half of the trekkers in the Khumbu region of Nepal, the valley below Mount Everest, will experience symptoms of altitude sickness. In some cases, they can get better by going down -- quickly.

Even more dangerous conditions are high-altitude pulmonary edema and high-altitude cerebral edema, in which the lungs and brain become filled with fluid.

In the new study, researchers from Canada, Britain and the Netherlands wanted to see if acetazolamide -- now used to treat altitude sickness and other conditions like glaucoma -- would help with the other conditions.

Poulin and colleagues enlisted nine subjects, six men and three women, who each took 250 mg of acetazolamide every eight hours for three days. Then, they subjected them to a simulation of high-altitude to see how their bodies reacted.

The study is published in the February issue of the American Journal of Respiratory and Critical Care Medicine.

The team found that the drug reduced pressure within the lungs by relaxing muscles that line the lungs' capillaries. This increased oxygenation, Poulin said.

That matters, because pulmonary edema -- fluid in the lungs -- may be caused by "a mismatch between the amount of air that's taken up by the lungs and the blood that passes through the lungs," he said.

The findings suggest that the drug could be used to treat other conditions that cause a build-up in lung pressure, Poulin said.

However, one specialist in high-altitude medicine is skeptical of the findings. Dr. Ken Zafren, associate medical director of the Himalayan Rescue Association and clinical assistant professor of emergency medicine at Stanford University, called the study "speculative" and based on too little exposure to high-altitude conditions.

Zafren cautioned against relying on acetazolamide to treat altitude sickness until more research is available, including findings from a much larger Nepal study that he is working on with others.

According to Zafren, a heart drug called nifedipine remains the best-studied medication to treat high-altitude pulmonary edema. Nifedipine, brand named Adalat, "is safe and effective both as prophylaxis (a preventative) and as treatment."

The erectile dysfunction drugs sildenafil (Viagra) and tadalafil (Cialis) can also lower pulmonary artery pressure at high altitude, he said.

More information

The U.S. Library of Medicine can tell you more about acute mountain sickness.