Altitude sickness: AMS, HAPE, and HACE¶

High-altitude illness strikes hikers, skiers, mountaineers, and travelers who ascend above 2,500 meters (8,200 feet) faster than their bodies can acclimatize to lower barometric pressure. While mild Acute Mountain Sickness (AMS) causes an uncomfortable headache and nausea that resolves with rest at the same elevation, its two life-threatening progressions, High Altitude Pulmonary Edema (HAPE) and High Altitude Cerebral Edema (HACE), fill the lungs with fluid or cause fatal brain swelling within hours. Following the Three Golden Rules of Altitude (recognize symptoms early, never ascend with symptoms, and descend immediately if symptoms worsen or involve the lungs or brain) is the single most effective way to prevent death in high mountains.

Do this now¶

If you or a companion at high altitude above 2,500 meters (8,200 feet) develops a throbbing headache, extreme fatigue, breathlessness, or clumsiness:

1. Perform the 10-second "Heel-to-Toe Walking Test" (Tandem Gait Test) immediately: Draw a straight line in the dirt or snow and ask the person to walk heel-to-toe along the line like a police sobriety test. If they stagger off the line, wave their arms wildly for balance, or cannot stand straight with their eyes closed (ataxia), assume High Altitude Cerebral Edema (HACE) is developing and start an immediate assisted descent.
2. Check breathing after 10 minutes of complete sitting rest: It is normal to breathe hard while hiking uphill at altitude. However, if a person is still panting, gasping, or has a fast resting heart rate (over 110 beats per minute) and crackling chest sounds after sitting completely still for 10 minutes, assume High Altitude Pulmonary Edema (HAPE) and descend immediately.
3. Stop ascending immediately for any mild symptoms: If the person passes the balance test and breathes comfortably at rest, but has a headache alongside nausea, dizziness, or unusual fatigue, diagnose Acute Mountain Sickness (AMS). Halt all upward travel immediately; do not climb even 100 meters higher to reach a camp or pass.
4. Execute an immediate assisted descent of 500 to 1,000 meters (1,600 to 3,300 feet) for HAPE/HACE: Take the sick person's backpack, support their weight between two companions (or carry them on a litter/horse), and head down to the lowest possible elevation. Physical exertion increases pressure in the lungs and brain, so the patient must work as little as possible during descent.
5. Administer high-flow oxygen or a Gamow bag if descent is delayed: If bottled medical oxygen is available, deliver 4 to 6 liters per minute via mask or nasal cannula. If trapped by a blizzard or cliff terrain, seal the patient inside a portable hyperbaric chamber (Gamow bag) and pump continuously until weather allows descent.
6. Activate satellite rescue and keep the patient warm: Trigger an SOS on a satellite communicator (Garmin inReach, ZOLEO, or smartphone satellite SOS) or call your local emergency numbers if cellular coverage exists. Keep the patient warmly insulated; shivering and cold exposure constrict lung arteries and rapidly worsen HAPE.

flowchart TD
    Symptom["Altitude Symptoms Above 2,500 m (8,200 ft): Headache, Fatigue, or Breathlessness"] --> Balance{"1. Heel-to-Toe Walk Test: Does Patient Stagger or Show Confusion?"}
    Balance -->|Yes: Ataxia or Altered Mental State| HACE["HACE (Brain Swelling): Immediate Assisted Descent 500 to 1,000 m"]
    Balance -->|No: Normal Balance and Alert| RestBreath{"2. Short of Breath After 10 Minutes of Complete Sitting Rest?"}
    RestBreath -->|Yes: Breathless at Rest, Cough, or Blue Lips| HAPE["HAPE (Fluid in Lungs): Sit Upright, Keep Warm, Descend 500 to 1,000 m Now"]
    RestBreath -->|No: Comfortable at Rest, Headache + Nausea/Fatigue| AMS["Mild to Moderate AMS: Stop Ascending, Rest 24 to 48 Hours, Hydrate"]
    AMS --> Worsen{"3. Do AMS Symptoms Worsen or Fail to Improve in 24 Hours?"}
    Worsen -->|Yes| Descend["Descend at Least 500 m (1,600 ft) Until Symptoms Resolve"]
    Worsen -->|No: Fully Resolved| Resume["Resume Gradual Ascent: Max 300 to 500 m Sleeping Gain per Day"]
    HACE --> Rescue["Administer Oxygen (4 to 6 L/min) or Gamow Bag + Activate Mountain Rescue SOS"]
    HAPE --> Rescue

Clinical comparison: AMS vs HAPE vs HACE¶

Understanding why thin air causes fluid to leak out of blood vessels helps you distinguish benign high-altitude adaptation from a medical emergency. At 3,600 meters (12,000 feet), each breath contains roughly 40 percent fewer oxygen molecules than at sea level; at 5,500 meters (18,000 feet), each breath contains half the oxygen of sea level.

The Lake Louise AMS Self-Assessment Score¶

According to the International Consensus Lake Louise Scoring System and the Wilderness Medical Society (WMS), Acute Mountain Sickness (AMS) is diagnosed when a person who has recently gained altitude above 2,500 meters (8,200 feet) has a Headache (at least 1 point) plus at least one other symptom for a total score of 3 or more:

1. Headache:
2. 0 = None at all
3. 1 = Mild headache
4. 2 = Moderate headache
5. 3 = Severe, incapacitating headache
6. Gastrointestinal Symptoms:
7. 0 = Good appetite
8. 1 = Poor appetite or mild nausea
9. 2 = Moderate nausea or vomiting
10. 3 = Severe, incapacitating nausea and vomiting
11. Fatigue and Weakness:
12. 0 = Not tired or weak
13. 1 = Mild fatigue or weakness
14. 2 = Moderate fatigue or weakness
15. 3 = Severe, incapacitating fatigue or weakness (reluctant to leave tent for meals)
16. Dizziness or Lightheadedness:
17. 0 = Not dizzy
18. 1 = Mild dizziness
19. 2 = Moderate dizziness

20. 3 = Severe, incapacitating dizziness

21. Score 3 to 5 (Mild AMS): Stop ascending, rest, hydrate sensibly, and monitor every 4 hours.

22. Score 6 or higher (Moderate to Severe AMS): Treat as high risk for progression to HACE; descend 300 to 500 meters (1,000 to 1,600 feet) if symptoms do not rapidly improve.

Step-by-step field management for HAPE and HACE¶

When a team member fails the heel-to-toe balance test (HACE) or becomes breathless at rest (HAPE), time is critical. Remember that HAPE is the #1 cause of death from altitude illness, and roughly 50 percent of people with severe HAPE also develop HACE simultaneously because their fluid-filled lungs cannot deliver oxygen to their brain.

1. Execute immediate, low-exertion descent¶

• Why descent works like a miracle: Descending even 300 to 500 meters (1,000 to 1,600 feet) increases atmospheric barometric pressure enough to raise arterial oxygen tension immediately. A descent of 1,000 meters (3,300 feet) is often life-saving within two hours.
• Do not let a HAPE patient carry weight or hike vigorously: Physical exertion increases cardiac output and spikes pulmonary artery pressure, forcing even more fluid through the fragile lung capillaries into the air sacs. Strip the patient of their backpack, use trekking poles or two teammates to support them on gradual downhill trails, or evacuate via horse, yak, vehicle, or stretcher.
• Positioning: Keep a HAPE patient sitting upright or semi-upright whenever resting or inside a tent. Lying flat on the back causes lung fluid to pool across the entire lung surface, causing an immediate feeling of drowning.

2. Protect strictly against cold and hypothermia¶

Cold exposure triggers automatic vasoconstriction throughout the body, shunting blood from the skin into the core and sharply elevating blood pressure inside the lungs. A climber who gets chilled or develops hypothermia at altitude is far more likely to develop acute HAPE.

• Wrap the patient immediately in a down parka, windproof shell, and dry sleeping bag, with an insulated foam pad underneath them to block ground cold.
• If available, place warm water bottles in their armpits and groin, and shield their face with a balaclava or neck gaiter to warm the air they inhale.

3. Operating a portable hyperbaric chamber (Gamow Bag / PAC)¶

High-altitude expeditions and mountain huts frequently carry an inflatable nylon hyperbaric bag (such as a Gamow bag, Certec bag, or Portable Altitude Chamber). It is a life-saving bridge when a midnight blizzard, avalanche hazard, or vertical cliff makes immediate walking descent impossible:

1. Pre-check the patient: Have the patient urinate before entering the bag. Check that they do not experience claustrophobia panic, and remind them to swallow, yawn, or pinch their nose and blow gently (Valsalva maneuver) as pressure rises so their eardrums do not hurt. If the patient has HAPE and cannot lie completely flat without gasping, prop the head end of the entire inflated bag up on a 30-to-45-degree snow slope or boulders.
2. Zip and pressurize: Place the patient inside with a warm sleeping bag and a water bottle, zip the airtight pressure zipper closed completely, and pump the foot pump rapidly until the built-in pressure release valve begins hissing (usually at 105 to 140 millibars / 1.5 to 2.0 psi above outside air pressure). This pressurization simulates an instant descent of 1,500 to 2,500 meters (5,000 to 8,000 feet).
3. Pump continuously to flush carbon dioxide: Once inflated, a rescuer must continue pumping 10 to 15 strokes every minute without stopping. Continuous pumping is not just to maintain pressure; it flushes the patient's exhaled carbon dioxide (\(\text{CO}_2\)) out of the bag and brings fresh oxygen in. Stopping pumping while a patient is zipped inside causes lethal carbon dioxide suffocation.
4. Descend as soon as the bag session improves symptoms: After 2 to 4 hours inside the chamber, HACE and HAPE patients often regain enough mental clarity and breathing comfort to walk down the mountain with assistance. Descend immediately while that window lasts, as fluid buildup rebounds within hours if the patient stays at high camp.

Differentiating altitude sickness from dehydration, hypothermia, and carbon monoxide¶

In high mountain camps, three other life-threatening emergencies mimic the exact symptoms of AMS and HACE:

• Severe Dehydration: Dry, cold mountain air and rapid breathing cause climbers to lose 3 to 5 liters of water per day through respiration and sweat. Dehydration causes headache, dark yellow urine, weakness, and dizziness. Field test: If a climber has dark urine and a mild headache without lung/balance signs, have them rest and drink 1 liter (1 quart) of water with electrolytes over 60 minutes. If the headache vanishes completely after rehydration, it was dehydration; if it persists or worsens, treat as AMS. (Conversely, never force a climber with AMS to chug excessive gallons of plain water, which causes dangerous low blood sodium / hyponatremia that mimics HACE).
• Tent Carbon Monoxide Poisoning: Cooking with a camp stove inside a closed tent vestibule during a snowstorm produces lethal carbon monoxide gas, causing throbbing headache, nausea, ataxia, and coma identical to AMS and HACE. If multiple tentmates wake up with the exact same severe headache after running a stove, open the tent doors immediately and get into fresh air (see smoke and carbon monoxide first aid).
• Exhaustion and Hypothermia: Shivering, stumbling, apathy, and slurred speech occur in both moderate hypothermia and HACE. If you cannot tell whether a stumbling climber has hypothermia or HACE, treat for both simultaneously: insulate them warmly in dry layers and descend immediately.

Prevention and safe acclimatization rules¶

According to the Wilderness Medical Society (WMS) and the UIAA (International Climbing and Mountaineering Federation) Medical Commission, almost all cases of severe altitude illness are preventable by following a disciplined ascent profile:

• The 3,000-meter (10,000-foot) threshold: Above 3,000 meters (10,000 feet), never increase your sleeping elevation by more than 300 to 500 meters (1,000 to 1,650 feet) per night.
• Schedule mandatory rest days: Take one full acclimatization rest day (sleeping at the same camp for two consecutive nights) for every 1,000 meters (3,300 feet) of elevation gained (or every 3 to 4 days of climbing).
• Avoid flying or driving directly to high sleeping altitudes: Travelers flying directly from sea level to cities like Cusco (3,400 m / 11,150 ft), La Paz (3,640 m / 11,940 ft), Lhasa (3,650 m / 11,975 ft), or Rocky Mountain ski resorts (2,800 to 3,200 m / 9,200 to 10,500 ft) should spend their first 2 to 3 nights at the lowest available valley elevation and avoid strenuous exertion or alcohol during the first 48 hours.
• Physical fitness does not protect you: Elite marathon runners and triathletes get AMS, HAPE, and HACE at the exact same rate (or higher, because they push uphill faster) as average hikers. Prior susceptibility and ascent speed are the primary determinants.

Frequently asked questions¶

Does being in top physical shape prevent altitude sickness?¶

No, physical fitness offers zero protection against Acute Mountain Sickness, HAPE, or HACE; in fact, highly fit athletes frequently develop severe altitude illness because they ascend too quickly before their bodies can physiologically adapt to lower air pressure.

How do you test someone in the field for High Altitude Cerebral Edema (HACE)?¶

The fastest and most reliable field test for HACE is the heel-to-toe tandem walking test: ask the person to walk along a straight line in the dirt or snow placing the heel of one foot directly against the toe of the other; stumbling, swaying, or inability to stay on the line indicates brain swelling requiring immediate descent.

How far down do you need to descend to treat severe altitude sickness?¶

When HAPE or HACE is suspected, you should immediately assist the patient down at least 300 to 1,000 meters (1,000 to 3,300 feet) or continue descending until symptoms clearly improve, making sure to carry their pack so their physical exertion stays minimal.

Can you sleep off a mild altitude headache by climbing to the next camp?¶

Never ascend to sleep at a higher elevation while experiencing any symptoms of altitude sickness; ascending with a mild AMS headache prevents acclimatization and frequently triggers life-threatening HAPE or HACE during the night when breathing slows down.

Why is a cough at high altitude dangerous?¶

While dry mountain air can cause a mild throat tickle, a persistent cough accompanied by unusual fatigue, slow uphill pace, or shortness of breath while resting is the classic early warning sign of High Altitude Pulmonary Edema (HAPE), which turns into gurgling breathlessness and pink frothy sputum if you do not descend.

Sources checked¶

• Wilderness Medical Society (WMS): Clinical Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness (2019/2024 Update).
• Lake Louise Consensus Group: The 2018 Lake Louise Acute Mountain Sickness Score.
• International Climbing and Mountaineering Federation (UIAA Medical Commission): Standardized Field Recommendations for AMS, HAPE, and HACE.
• Centers for Disease Control and Prevention (CDC, United States): CDC Yellow Book: High-Elevation Travel and Altitude Illness.
• NHS (United Kingdom): Altitude Sickness Recognition, Acclimatisation, and Emergency Descent Guidance.

Related guides¶

• Wilderness hypothermia and cold exposure
• Hypothermia and frostbite first aid
• Lost while hiking or camping survival
• Rock climbing and mountaineering accidents
• Smoke inhalation and carbon monoxide poisoning first aid
• Emergency numbers by country