Researchers identify potential risk factors for severe altitude sickness

Measuring specific, exercise-related responses can help physicians determine who may be more at risk for severe high altitude illness (SHAI), according to a study conducted by researchers in France. The researchers also found that taking acetazolamide (ACZ), a drug frequently prescribed to prevent altitude illness, can reduce some of the risk factors associated with SHAI.

The findings were published online ahead of the print edition of the American Thoracic Society's American Journal of Respiratory and Critical Care Medicine.

The three exercise-related factors identified by the researchers include oxygen desaturation at exercise (Sae), hypoxic cardiac response at exercise (HCRe) and hypoxic ventilatory response at exercise (HVRe). Sae measures the amount of oxygen that is in the blood during exercise; HCRe measures the heart's response to exercise in a hypoxic, or low oxygen, setting, and HVRe refers to respiratory changes (notably rapid breathing) that occur during exercise in a hypoxic setting. The researchers measured these parameters in controlled, hypoxic conditions in a lab setting that mimicked high-altitude conditions.

"These results suggest that HCRe, HVRe and substantial decreases in Sae are independent risk factors of SHAI, and that decreases in Sae and HVRe can be used to accurately predict the risk of developing SHAI," said Jean-Paul Richalet, MD, PhD, a professor of physiology at Université Paris 13.

"To date, this is the largest epidemiological study of subjects exposed to high altitude-related illness, who were previously evaluated for their responses to hypoxia," he added.

The researchers collected data from 1,326 men and women who were seen prior to high-altitude excursions, which included at least 3 days above 4,000 meters with overnight sleeping above 3,500 meters. Study participants were asked to complete a questionnaire, providing information about their personal and family medical history, usual physical and mountaineering activity and other factors.

Next, study participants went through a routine hypoxic exercise test, which consisted of four, four-minute phases: rest at normal oxygen levels; rest at hypoxic levels; exercise in hypoxia; and exercise in normal levels of oxygen. During the exercise test, the researchers measured heart rate, breathing and blood oxygen levels.

What is a high altitude illness? A person with high altitude illness has symptoms that occur after a rapidly ascending to an altitude over 7,500 feet. Worsening high altitude illness can lead to high altitude pulmonary edema or high altitude cerebral edema. About 12% of people develop symptoms of high altitude illness at an altitude of 9,000 feet.

What are the symptoms of a high altitude illness?

Symptoms depend on whether the brain or lungs are affected. Initial symptoms of high altitude illness include weakness, fatigue, headache, and nausea. Symptoms of worsening high altitude illness include vomiting, difficulty with balance, lethargy, confusion, and seizures. Initial symptoms of high altitude illness may include dry cough, nausea, headache, fever, weakness, fatigue, and mild difficulty breathing that worsens at night and with exertion. Symptoms of severe high altitude illness include a bloody cough, severe difficulty breathing, confusion, cyanosis, and coma.