Tourists know the symptoms well — headaches, shortness of breath, the occassional bloody nose.

Visiting the Colorado Rockies can be taxing on lowlanders, but they can rest assured that their altitude sickness will disappear when they return to sea level.

What happens to those who live full time at elevation? Surprisingly, much remains unsettled and unknown as to its long-term effects on the human body.

That’s why two Summit County doctors are embarking on an effort to launch a research study on life at high altitude.

Dr. Warren Johnson, M.D., and Dr. Peter Lemis, M.D., both cardiologists based in Frisco, intend to investigate if a relationship exists between the lower oxygen levels found at elevations between 8,000 and 10,500 feet and abnormal sleep patterns and, perhaps, even depression.

“Summit County is a unique place in the United States in that very few areas in the world have such a high altitude with a sizable population,” said Lemis, noting Nepal and Tibet, Ethiopia and the Andes Mountains in South America as the others. “A study like this has not been done at high altitude.”

As heart specialists, he said their office sees a number of referrals for what they call nocturnal hypoxia, or oxygen deficiency during nightly sleep. What additional health issues that may lead to down the road is what the good docs are after.

Full-time residents of Breckenridge, Silverthorne, Dillon and Frisco, for instance, tend to develop breathing problems over time because of the decreased amount of oxygen at elevation compared to those who live at sea level. This often leads to a narrowing of the blood vessels and increased blood pressure in the lungs, also known as pulmonary hypertension, eventually stressing the right side of the heart. After some years, that has the possibility of resulting in heart failure.

“It’s a very prevalent problem up here,” Johnson said of pulmonary hypertension. “Some people tolerate it fine, at least for some time, and then there’s others who can’t even be up her for the first year. Some people just can’t acclimate.”

He started his inquiry into the issue not long after he moved to the area about seven years ago and began noticing he was seeing a number of members of ski patrol and other patients who live near Eisenhower Tunnel (about 11,000 feet) with the common symptom of loss of breath. After testing the overnight oxygen levels of these individuals, he found many were in the 70s, whereas the high-90s are normal at lower elevations. They were going through cyclic breathing with prolonged periods where respiration even ceased temporarily.

“If we all go up high enough, we all will have sleep apnea,” said Johnson, pointing to problems usually starting somewhere around 7,000 feet. “We don’t know why some are better or worse than others, but it’s probably associated with our genes, and, as we get older, it seems to get a little worse.”

Whether there is then any correlation between oxygen deprivation and aforementioned maladies is at the moment unknown. And that’s the crux of the proposed study.

“These are only theories right now,” acknowledged Lemis. “But depression is a problem with chemical balances in the brain, and serotonin production might be impaired due to low oxygen levels. And when we have low oxygen, we sometimes don’t stay in a deep level of sleep. Then sleep is not refreshing, and restful sleep is not sustained, meaning people feel fatigued.”

To explore the relationships between these health issues, Johnson and Lemis envision enrolling 200 adult patients at random who are without heart or lung disease and have lived in Summit County year-round for five or more years. They would begin with sleep questionnaires, followed by analyzing each participant’s oxygen levels while sleeping as well as the pressure in their lungs with an echocardiogram.

From there, the two doctors would be able to assess the percentage of the local population that suffers from this oxygen deficiency. This would help them better understand how to treat the problem and prevent future issues and the more harmful life-threatening illnesses.

“What we need to know is what is the prevalence, so we can start to evaluate individuals for medications or some kind of therapy,” said Johnson. “If we knew that it was 10 percent of the population, then that would be helpful, but it could also be 50 percent, I don’t know. So that’s what we’re trying to figure out up here.”

He and Lemis are only in the early stages of securing funding for the screening study and are just into the planning stages of the prospective research. They hope to begin locating participants for the noninvasive, anonymous study sometime in 2016.

“What we don’t want to do is scare the population,” said Johnson. “For those people who are symptomatic, we need to help them. We have more and better medications than several years ago, and we’re doing that now.”