Acute Altitude Illnesses

Department of the Navy
Bureau of Medicine and Surgery

Acute mountain sickness (AMS) is associated with ascent to altitudes above 8000 ft, particularly with ascent rates greater than 1000 ft per day. Common symptoms include tachycardia, insomnia, headache, nausea, vomiting, anorexia, retinal hemorrhages, dizziness, and fatigue. It may be considered a mild form of cerebral edema. Symptoms are partially relieved by resting and are completely relieved by return to lower altitudes.

Cheyne-Stokes (periodic) breathing occurs in most persons with increasing altitude and is more pronounced during sleep. Several respiratory cycles of increasing depth and frequency are followed by cycles of decreasing depth and frequency until short periods of apnea result. After a few seconds, the cycle begins again. At higher elevations bizarre and colorful dreams may occur during sleep. While these symptoms may be disturbing and interfere with rest they occur commonly and are not harbingers of more serious disorders. Central nervous system (CNS) depressants or tranquilizers should not be used.

The primary stimulus for ventilation at sea level is carbon dioxide acting on the peripheral chemoreceptors (carotid) and the medullary respiratory center. The higher concentration of oxygen provides little drive to breathe. With ascent to altitude, there is a progressive decrease in inspired and arterial partial pressures of oxygen and in arterial oxygen saturation. This hypoxemia results in hypoxic stimulation of the peripheral chemoreceptors leading to increased minute ventilation which increases arterial oxygen saturation and proportionately reduces the arterial partial pressure of carbon dioxide and leads to alkalosis. These changes reduce the CO₂ dependent component of the respiratory drive, blunting the increased ventilation resulting from hypoxemia. Respiratory acclimatization to altitude occurs when the CSF re-establishes its usual pH and the medullary respiratory center ventilatory drive increases to levels associated with sea-level partial pressures of carbon dioxide in the arterial blood. This allows the full hypoxic stimulus to ventilation to manifest, further raising arterial oxygen saturation. The resetting of central carbon dioxide and pH sensitivity can be accelerated by the carbonic anhydrase activity of acetazolamide, reducing altitude symptoms and sleep disturbance. Peripheral chemoreceptor stimulation also results in redistribution of the cardiac output, reducing gut blood flow and increasing vital organ flows.

Barometric pressure and the partial pressure of oxygen are reduced as altitude increases. At a given altitude barometric pressure varies with latitude (high latitudes, lower pressures) and with season, weather, and temperature. As a result of the adiabatic lapse rate, so important in weather, temperature is reduced about 6.5° C per thousand meters of elevation. Cold and hypothermia are associated with higher elevations and should be considered for their contribution to altitude illness, as should dehydration, which results from increased respiratory and skin water losses if not adequately noted and replaced.

Respiratory and circulatory acclimitizations take place relatively quickly. Hormonal, hematopoietic and metabolic adaptations may take longer to maximize. The more rapid acclimatization responses protect against acute mountain sickness. Longer term acclimatization improves aerobic exercise capability. Excessive aerobic exercise at altitude may predispose to severe hypoxemia through reduction of the pulmonary capillary transit time of hemoglobin below that required for equilibration of hemoglobin saturation with alveolar partial pressure of oxygen. Strenuous exercise increases cardiac output and pulmonary blood flow velocity resulting in more severe hypoxemia, unlike sea level exercise, where hemoglobin saturation occurs in the higher alveolar partial pressures of oxygen. This may partially explain why the incidence of AMS is indifferent to the level of physical conditioning and is increased with more strenuous climbing, (i.e., carrying packs, increased pace, etc).

Hypoxemia may be worsened by carbon monoxide poisoning in poorly ventilated sleeping tents, sleep apnea, and high altitude pulmonary edema (HAPE).

Airlifting personnel to high altitudes increases the incidence of AMS, as does rapid rates of ascent on foot. Continued ascent in the face of symptoms of AMS results in increased incidence of high altitude cerebral edema (HACE) and high altitude pulmonary edema (HAPE). Graded ascent is the most certain prevention, particularly for sleeping altitudes. Prevent over-exertion and dehydration. Acetazolamide prophylaxis speeds ventilatory acclimatization, increases minute ventilation and hemoglobin saturation, counters the fluid retention of AMS, and decreases cerebral spinal fluid (CSF) production. It is particularly useful if rapid ascent cannot be avoided. A single 500 mg sustained-action capsule daily, beginning 24 hours before ascent and continued for 2-4 days, is probably the optimal dosage. Acetazolamide is a sulfonamide derivative and may cause allergic reactions in persons allergic to sulfa drugs. Paresthesias of the extremities and of the lips are common. Visual effects may result from its mild diuretic action by lowering the production of aqueous humor. Dexamethasone, 4 mg, followed 6 hours later with 4mg is frequently effective in improving AMS, in conjunction with acetazolamide.

1. Mild AMS: Rest, stop ascent and strenuous activities, acclimatize at same altitude, acetazolamide, 500 mg sustained release capsule daily, analgesics as needed, and antiemetics as needed. Or: descent 1000 ft or more.

2. Moderate AMS: Low-flow oxygen to relief or 90 percent saturation by pulse oximetry, acetazolamide as above plus dexamethasone, 4 mg, any route, every 6 hours, and hyperbaric therapy, or immediate descent to relief.

HACE is a potentially fatal altitude illness, which may develop in the presence of AMS or HAPE. HACE presents with altered mental status, progressing to ataxia, coma, and death. Less common symptoms include hallucinations and seizures and progression to fatal outcome may take from 12 hours to 1-3 days. Early focal signs suggest other neurologic problems since cerebral edema results in a generalized encephalopathy; but severe edema may produce focal deficits and particularly cranial nerve palsies. HACE usually occurs in persons with AMS, thus respiratory alkalosis and severe hypoxemia exist, and HAPE and/or cyanosis is common. Retinal hemorrhages and fever may be present.

Administration of increased partial pressure of oxygen is critically important in survival, either by use of supplemental oxygen, immediate return to lower elevation, or use of a (portable) pressure chamber, such as the Gamow bag. Pending descent, dexamethasone, 8 mg, IV, IM or orally followed by 4 mg QID should be started. Oxygen at 4 liters per minute (lpm) or as required for 90 percent saturation by pulse oximetry, should be instituted. Rapid descent is critical. Patients should be hospitalized ASAP. MRI studies point to a reversible vasogenic etiology, most prominent in the corpus callosum with edema of white matter. If not corrected, cytotoxic neurologic damage follows with permanent neurologic deterioration and death.

Hospitalization is usually required for 1-2 weeks (return to normal neurologic exam) and full recovery by MRI may require several weeks to months.

High Altitude Pulmonary Edema (HAPE)

Personnel at higher altitudes have increased ventilation/perfusion inequalities and increased lung water. With AMS there is increased pulmonary interstitial edema and elevated pulmonary artery pressures. Hypoxemia probably leads to increased pulmonary vascular endothelium permeability which can progress to HAPE, the most common cause of death related to high altitude. The incidence of HAPE in troops may reach 15 percent, depending upon rate of ascent, final altitude, severity of cold weather, level of exertion, and use of depressant sleeping medications. HAPE is more common in young persons, particularly males. It most commonly occurs on the second night of exposure, manifested by fatigue, dyspnea on exertion, dry cough, and need for longer rest periods. Some peripheral edema is common. A close examination for HAPE and HACE is absolutely necessary. As the condition worsens, tachypnea, dypsnea at rest, and audible pulmonary rales may be present. Signs of HACE may also appear and may obscure symptomatology of HAPE (more than half show HACE on autopsy). Increases in pulmonary artery pressure and pulmonary vascular resistance are present.

Increasing proteinaceous interstitial exudate, sometimes resembling bronchopneumonia, develops as pulmonary edema worsens. The exudate contans vasoactive eicosanoids and complement proteins. Autopsy frequently shows thrombi, pulmonary hemorrhage, and infarcts. However, left-sided cardiac function is maintained. The hypoxic ventilatory response is thought to play a role in susceptibility to AMS, and the hypoxic pulmonary vascular resistance response probably also plays a role in susceptibility to HAPE.

Treatment consists of oxygen and immediate descent of at least 1000 meters, if possible. Bed rest and oxygen are helpful; exercise should be minimized until recovered. Hyperbaric treatment can substitute for low-flow oxygen. Oxygen rapidly reduces pulmonary artery pressure, respiratory rate, and other symptoms. In severe cases, if immediate descent is not possible, request an airdrop of oxygen rather than wait for rescue teams or helicopter evacuation. Nifedipine, 30 mg slow release capsule, one every 12-24 hours or 10 mg sublingually (remove the gel from the capsule) as needed is an effective adjunct in reducing pulmonary vascular resistance and pressure. Hospitalization is indicated until functionally recovered, but hypocapnic alkalosis and some hypoxemia persist for days. Oxygen saturation of 90 percent or higher is satisfactory for discharge, usually in several days. Subsequent ascents should be more gradual, but are not contraindicated. Prophylactic acetazolamide is effective in preventing repeat HAPE upon subsequent altitude exposure and both acetazolamide and nefedipine should be carried by the individual on subsequent ascents.

Other Altitude Related Illnesses

Altitudes conducive to AMS are frequently also cold. Thermal insulation is less effective at altitude because of the reduction in density of the insulating air. The metabolic generation of heat is also reduced because of hypoxia. Symptoms of hypothermia may be confused with AMS and HACE. Ultraviolet radiation exposure increases about 4 percent per 1,000 feet of altitude and reflection from snow and ice increases this exposure further. Snowblindness, sunburn, and hyperthermia during vigorous exercise may occur. The later contributes to dehydration, which is common at altitude, where greater respiratory and sensory losses occur. Alcohol and CNS depressant medications should be avoided. Breathing cold air may trigger asthma in some, but has not been reported and is not a contraindication to ascent. High altitude retinal hemorrhages usually resolve spontaneously in 7-14 days. Current data suggests normal pregnancy is not a contraindication to moderate ascents.

1. High Altitude Medicine Guide, (http://www.gorge.net/hamg/), (excellent, physician-oriented.)

2. Wilderness Medicine, Management of Wilderness and Environmental Emergencies, 3^rd Ed., Paul S. Auerbach, Ed., Mosby, St. Louis 1995, (excellent, every doc should own this)

3. Altitude Illness, Prevention and Treatment, http://www.mindspring.com/~treeline/altitu~1.htm,

4. Outdoor Action Guide to High Altitude: Acclimatization and Illness, http://www.princeton.edu/~oa/altitude.html

5. Hackett, PH et al, High-altitude cerebral edema evaluated with magnetic resonance imaging, clinical correlation and pathophysiology, JAMA 1998; 280(22):1920-1925

6. Cox, GR, Danger in the Mountains: Pathophysiology and treatment of altitude-related illnesses, Proc of 13^th Ann Emer Med Symposium, July, 1996, (excellent review by Navy Doc)

7. Murdoch, DR, Symptoms of infection and altitude illness among hikers in the Mount Everest region of Nepal, Aviat Space Environ Med 1995; 66:148-51

8. Cox, GR, NAMI Flight Surgeon Program Instructor Guide, Environmental Physiology 7: Altitude Related Illnesses

9. The Gamow Bag, http://spot.colorado.edu/~gamow/research/bag.html