High altitude illness: Physiology, risk factors, and general prevention
- Scott A Gallagher, MD
Scott A Gallagher, MD
- Senior Clinical Instructor, Department of Surgery
- University of Colorado Denver School of Medicine
- Peter Hackett, MD
Peter Hackett, MD
- Clinical Professor of Emergency Medicine
- University of Colorado Denver School of Medicine
- Director, Institute for Altitude Medicine, Telluride, Colorado
- Jonathan M Rosen, MD
Jonathan M Rosen, MD
- Undergraduate Medical Education
- Associate Dean
- University of Vermont College of Medicine
- Western Connecticut Health Network
The beauty and recreational opportunities of the mountains attract millions of visitors from lowland elevations to high-altitude destinations worldwide. Resort towns in the Western United States alone attract over 30 million visitors annually, generally to sleeping elevations in the 2000 to 3000 m (6500 to 9800 feet) range. Many millions more visit cities at these elevations, including several large cities in South America and Asia situated above 3000 m . Most of these destinations can be reached within a day.
In addition, tens of thousands of climbers, trekkers, and skiers worldwide ascend to elevations in the 3000 to 5500 m (9800 to 18,000 feet) range, often at a rate that exceeds an individual's ability to acclimatize. A growing number of mountaineers seek the summits of peaks over 5500 m. Military, rescue, and other professional personnel may also be called upon to ascend to high altitudes with little or no time for acclimatization. Such rapid ascents place the unacclimatized traveler at risk for developing high altitude illness (HAI).
Clinicians working in or near mountainous areas must familiarize themselves with the presentation and management of HAI, while all health care workers who advise travelers need to understand the best prevention strategies and treatment options. The different types of HAI, their pathophysiology, and general methods for prevention will be reviewed here. The diagnosis, treatment, and prevention of specific types of HAI are discussed separately. (See "Acute mountain sickness and high altitude cerebral edema" and "High altitude pulmonary edema" and "High altitude, air travel, and heart disease".)
HIGH ALTITUDE PHYSIOLOGY
Hypobaric hypoxia — The partial pressure of oxygen (PO2) is the driving force for the diffusion of oxygen down the oxygen cascade. Oxygen moves from inspired air to the alveolar space via the airways and then diffuses across the alveoli into the blood (figure 1 and figure 2), where it is carried mainly bound to hemoglobin but also in dissolved form. At the level of the capillaries, oxygen diffuses across vessel walls, through the tissues and into cells, and ultimately into the mitochondria. (See "Oxygen delivery and consumption" and "Oxygenation and mechanisms of hypoxemia".)
The partial pressure of oxygen of inspired air (PIO2) is given by the equation: PIO2 = FIO2 x (Pb - 47 mmHg), where FIO2 is the fraction of oxygen in inspired air, Pb is the barometric pressure, and 47 mmHg is the vapor pressure of H2O at 37°C. Inspired gas is 100 percent humidified by the time it reaches the alveoli and water vapor pressure is affected by temperature but, unlike other gases, is not dependent on altitude. The proportion of air comprised by oxygen (FIO2, 20.94 percent) remains constant at the highest terrestrial elevations and even into the upper troposphere. Hence, the PIO2 and therefore, the oxygen cascade, are directly affected by barometric pressure.
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- HIGH ALTITUDE PHYSIOLOGY
- Hypobaric hypoxia
- - Overview
- - Ventilation and PaO2
- - Circulatory changes
- - Hematologic changes
- - Oxygen delivery and utilization
- High altitude illness
- Acute mountain sickness and high altitude cerebral edema
- High altitude pulmonary edema
- Other altitude-related illnesses
- RISK FACTORS
- RISK STRATIFICATION OF THE TRAVELER TO HIGH ALTITUDE
- Determining overall risk
- Assessing the proposed trip
- Diseases and medical conditions that increase risk
- Assessing the patient
- Prophylactic medication: Who needs it and what to give
- General approach
- Behavioral methods
- - Gradual ascent
- - Preacclimatization
- - Education
- - Alcohol and drug use
- - Diet and hydration
- - Exertion
- - Coca
- Pharmacologic prophylaxis
- INFORMATION FOR PATIENTS
- SUMMARY AND RECOMMENDATIONS