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INTRODUCTION

Altitude is an environmental state of elevation >760 m (∼2500 ft) above sea level and is a continuous physiologic stressor of hypobaric hypoxia (Fig. 91-1). Altitudes are subdivided into very high (3500–5500 m or ∼11,500–18,000 ft) and extreme (>5500 m or >18,000 ft). Eighty million people live at altitude around the world, and 40 million people per year travel to an altitude ≥2500 m (∼8000 ft).1 Many more work in mines, the military, or border operations at high altitude. Clinical encounters specific to altitude result from hypoxemia (FIO2 equivalent to ∼17% O2 at 2500 m, and ∼8% O2 at the summit of Everest), which leads to physiologic responses and illnesses. Such illnesses have been recognized and empirically treated in prehistoric native populations.2 Birthweights are generally lower, and the rates of small-for-gestational-age babies and congenital heart defects are higher than in lowland populations.3 With a hurried ascent, ∼80% of lowlanders will report a transient headache (high-altitude headache [HAH]), and some will develop an acute high-altitude illness: acute mountain sickness (AMS). Fewer will develop high-altitude cerebral edema (HACE) and high-altitude pulmonary edema (HAPE).3,4 HAH and AMS are annoying and interfere with activity and work; however, HACE and HAPE can be fatal, with mortality rates approaching 30%.5–7 Some residents at altitude exhibit chronic mountain sickness (CMS) and right ventricular hypertrophy, developing over months to years of residence at altitude. Cold air and extreme exercise also may contribute to illness. Other environmental features may include UV radiation, trauma, and infections (which are not covered in this chapter). Finally, managing illness in a remote altitude location can be challenging, as evacuation to a lower altitude as a primary treatment may be limited or not feasible.

Figure 91-1

Land topography of the surface of the Earth. (This topographic map is made from data collected from three sources: NASA’s Space Shuttle, Canada’s Radarsat satellite, and topographic maps made by the U.S. Geological Survey (NASA Earth Observations). Available at neo.sci.gsfc.nasa.gov/view.php?datasetId=SRTM_RAMP2_TOPO.)

CONSIDERATIONS OF EXPOSURE AND TIME COURSE

Individual responses to low-barometric-pressure hypoxia may be conceptualized along a time continuum of interrelated phases: acute (immediate to 3–5 days in which acute illnesses present), subacute (over weeks leading toward acclimatization), chronic (years), and lifelong residence (Fig. 91-2). The reduction in environmental oxygen as result of altitude exposure lowers the oxygen available for gas exchange in the lungs (PAO2), arterial oxygen (PaO2), and cellular oxidative phosphorylation for adenosine triphosphate (ATP) production.

Figure 91-2

Oxygen tension is lower along the axis of oxygen delivery at 4540 m or ∼15,000 ft (red) compared with sea level (blue).

Single-cell and multicellular organisms have been exposed to oxygen ...

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