High altitude illness encompasses a spectrum of clinical entities to include: acute mountain sickness, high altitude cerebral edema, and high altitude pulmonary edema. These illnesses occur as a result of a hypobaric hypoxic environment. Although a mild case of acute mountain sickness may be self-limited, high altitude cerebral edema and high altitude pulmonary edema represent critical emergencies that require timely intervention. This article reviews recent advances in the prevention and treatment of high altitude illness, including new pharmacologic strategies for prophylaxis and revised treatment guidelines.
Key points
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Acetazolamide remains the best choice for prevention of acute mountain sickness (AMS).
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The best treatment for all high altitude illness is descent or oxygen, or both.
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Dexamethasone is excellent for treating moderate to severe AMS, and for high altitude cerebral edema (HACE).
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Supplemental oxygen is first-line therapy for high altitude pulmonary edema; descent is primary therapy if oxygen is not available.
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Descent is the definitive treatment for HACE and should not be delayed. Dexamethasone and supplemental oxygen are important adjunctive treatments for HACE until descent can be facilitated.
Introduction
High altitude illness (HAI) comprises a spectrum of conditions that occur at elevation as a result of hypoxia, and includes acute mountain sickness (AMS), high altitude cerebral edema (HACE), and high altitude pulmonary edema (HAPE). Whereas AMS is self-limited, HACE and HAPE represent true emergencies that require timely intervention and stabilization. This review focuses on recent advances in the prevention and treatment of these conditions.
Background
The concentration of oxygen in air remains constant at 21% regardless of the altitude. However, the partial pressure of oxygen decreases with increasing altitude, resulting in alveolar hypoxia, hypoxemia and eventual tissue hypoxia. In the lower Rocky Mountain resorts of Colorado (2500 m/8000 ft), there is one-quarter less available oxygen than at sea level. At Everest Base Camp (5300 m/17,500 feet) there is one-half the available oxygen, and on the summit of Mount Everest there is only one-third. Although a given elevation primarily determines oxygen availability, barometric pressure also decreases with increasing latitude, the winter season, and with low-pressure storm fronts. Accordingly, these effects may combine to raise the effective altitude by hundreds of meters, resulting in an increased risk of HAI.
The high altitude environment is roughly organized into stages according to physiologic stress and resultant pathology.
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Intermediate altitude (1520–2440 m/5000–8000 ft): Increased compensatory ventilation occurs along with a decrease in exercise performance. However, blood oxygen saturation is typically preserved at greater than 90%. For most susceptible individuals, AMS will occur above 2100 m.
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High altitude (2440–4270 m/8000–14,000 ft): Most HAI occurs in this range owing to the easy availability of overnight tourist facilities at these elevations. In this altitude range, oxygen saturation can be less than 90%, and hypoxemia worsens during exercise and sleep.
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Very high altitude (4270–5490 m/14,000–18,000 ft): Abrupt ascent is dangerous. A period of acclimatization is required to prevent HAI. Rates of HAPE and HACE are increased.
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Extreme altitude (>5490 m/18,000 ft): Marked hypoxemia and hypocapnia are present. Hypoxic stress leads to progressive physiologic deterioration that eventually overwhelms the body’s ability to acclimatize. Long-term human habitation is, therefore, impossible.
Table 1 summarizes the effect of increasing altitude on barometric pressure, blood oxygen saturation, and arterial concentration of P o 2 and C o 2 .
| Altitude | Equivalent | Pb (mm Hg) | Estimated Pa o 2 (mm Hg) | Estimated Sa o 2 (%) | Pa co 2 (mm Hg) |
|---|---|---|---|---|---|
| Sea level | — | 760 | 90–100 | 97–99 | 38–42 |
| 5280 ft (1610 m) | Denver | 623 | 65–80 | 93–97 | 32–42 |
| 8000 ft (2440 m) | Machu Pichu | 564 | 45–70 | 88–95 | 31–36 |
| 12,000 ft (3660 m) | La Paz, Bolivia | 483 | 42–53 | 80–89 | 24–34 |
| 17,500 ft (5330 m) | Everest Basecamp | 388 | 38–50 | 65–81 | 22–30 |
| 29,000 ft (8840 m) | Everest Summit | 253 | 28–32 | 54–62 | 10–14 |
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