by Debbie Hum

A team of experts has made discoveries about high-altitude acclimatization that may have implications for the treatment of illnesses at sea level, including heart failure, diabetes and lung disease.

Professor Robert Boushel, who joined Concordia’s Exercise Science Department last year, was part of a team that performed studies in the Andes in 1998. They presented their findings at an international meeting on high-altitude physiology and medicine held at the Danish Academy of Science in Copenhagen last November.

For more than 60 years, high-altitude physiologists have tried to explain the findings of pioneer Erik Hohwu-Christensen from his 1937 expedition to the Andes. His discoveries of dramatic reduction in heart rate and markedly reduced muscle lactate production during acclimatization to high altitudes were both striking and puzzling.

In 1998, the Chacaltaya Expedition, led by physiologist Bengt Saltin, set out to resolve the mechanisms underlying Christensen’s findings. An international team of 25 scientists and 16 study volunteers flew to La Paz, Bolivia, and set up a laboratory at the summit of Mount Chacaltaya, 17,000 feet above sea level.

Over a period of two months, subgroups carried out studies on the brain, the autonomic nervous system (the branch of the nervous system that works without conscious control), and cardiovascular and muscle metabolic function.

Boushel’s research involved examining the parasympathetic nervous system’s role in lowering heart rate and cardiac output at high altitude. (In lay terms, the parasympathetic nervous system slows the heart’s pumping rate and exerts cardio-protection by limiting the work of the heart and maintaining safe rhythm conduction.)

The subjects, who were volunteer medical students from Denmark, exercised to maximum on stationary bikes at the mountaintop laboratory. The same studies conducted at sea level yielded a maximal heart rate of 181 beats per minute.

After two months at altitude, the maximal heart rate was reduced to 140. The volunteers were then administered glycopyrrolate (similar to belladonna), which blocks the receptors in the heart that are activated by the parasympathetic neurotransmitter acetylcholine.

“The maximal heart rate was completely restored, so we were able to show that parasympathetic activity is markedly enhanced to suppress heart rate (and heart work) at high altitude,” Boushel reported.

Boushel’s is the first high-altitude study to directly measure the output of blood from the heart under parasympathetic blockade.

“We found that even though the heart rate declined, the total cardiac output was unaffected,” Boushel said. “That led us to the conclusion that cardiac output is likely regulated by a sensor of oxygen content in the blood.

The researchers conclude that higher parasympathetic neural activity is what lowers the heart rate in chronic hypoxia, a condition that generally occurs with diminished availability of oxygen to the body tissues.

“What we don’t know as of yet is to what extent these changes are attributed to more cardiac receptors, greater sensitivity, or more nerve activity itself. The finding is meaningful not only for mountain climbers who may risk hypoxic altitude illness, but also for the diagnosis and treatment of hypoxia in sea-level residents with pulmonary disorders, heart failure or circulatory deficiencies.

The scientists are now attracting attention as they begin to publish their findings. Boushel, who joined Concordia last July, was previously at the Copenhagen Muscle Research Center and has worked with the U.S. Army Institute of Environmental Medicine’s Altitude Medicine and Physiology Division.