Introduction
High-altitude mountaineering presents a physiological problem that fitness, determination and technical ability cannot eliminate.
As altitude increases, atmospheric pressure falls. Although the proportion of oxygen in the atmosphere remains essentially unchanged, its partial pressure falls with altitude. Less oxygen is therefore available to move from the lungs into the blood.
The resulting hypobaric hypoxia requires the human body to adapt.
That adaptation takes time.
A mountaineer can be exceptionally fit, technically accomplished and psychologically determined and still develop serious altitude illness. Indeed, physical fitness does not provide immunity from acute mountain sickness.
The fundamental problem is not weakness.
It is ascent faster than an individual’s physiology can adequately accommodate.
The consequences range from relatively benign Acute Mountain Sickness (AMS) to two potentially fatal conditions:
High-Altitude Cerebral Oedema (HACE)
and
High-Altitude Pulmonary Oedema (HAPE).
Understanding the progression, recognising the warning signs and—most importantly—being prepared to stop ascending or descend can make the difference between an uncomfortable expedition and a fatal one.
What Happens to the Body at Altitude?
At sea level, atmospheric pressure allows relatively efficient transfer of oxygen from the lungs into the circulation.
As altitude increases, barometric pressure decreases.
The percentage of atmospheric oxygen remains approximately 21%, but the partial pressure of oxygen falls.
The result is progressive hypoxaemia.
At approximately 3,050 metres, for example, inspired oxygen partial pressure is only around 69% of that at sea level, and arterial oxygen saturation in a newly exposed person can fall into approximately the high-80s to low-90s.
The body responds almost immediately.
Ventilation increases.
Heart rate may rise.
Pulmonary vascular responses change.
Renal mechanisms begin modifying the acid-base consequences of increased ventilation.
Over subsequent days, ventilation and oxygenation progressively improve.
This is acclimatisation.
The crucial early acclimatisation process takes approximately three to five days, although physiological adaptation continues considerably longer.
The mistake is assuming that because someone can physically continue climbing, they have physiologically acclimatised.
The two are not the same.
Acute Mountain Sickness
Acute Mountain Sickness is the commonest manifestation of failure to acclimatise adequately.
It usually occurs after relatively rapid ascent, particularly when sleeping altitude increases significantly.
Typical symptoms include:
- headache
- nausea
- loss of appetite
- dizziness
- fatigue or unusual lethargy
- disturbed sleep
- sometimes vomiting.
The important point is context.
A headache after ascending rapidly to 3,500 metres is not automatically AMS, but altitude illness should be considered.
Dehydration, exhaustion, migraine, alcohol, infection, carbon monoxide exposure, hyponatraemia and other conditions can produce similar symptoms.
But there is a fundamental mountaineering rule:
A climber who develops symptoms of altitude illness should not continue ascending to a higher sleeping altitude.
Mild AMS frequently improves with rest and acclimatisation at the same altitude.
If symptoms deteriorate despite remaining at the same altitude, descent becomes necessary.
A descent of approximately 300 metres or more can produce significant improvement.
HACE: When Altitude Affects the Brain
High-Altitude Cerebral Oedema is a neurological emergency.
It can be regarded clinically as the severe end of the AMS spectrum, involving cerebral dysfunction associated with altitude-related hypoxia and brain swelling.
The transition from “someone feeling rough at altitude” to HACE is critically important.
The warning signs include:
ataxia
confusion
altered behaviour
marked drowsiness
altered consciousness.
A climber may appear intoxicated.
They may stumble.
They may become irrational.
They may stop cooperating with the group.
Their judgement can deteriorate.
Perhaps most dangerously, they may lose insight into their own condition and insist that nothing is wrong.
The Heel-to-Toe Test
One of the simplest neurological assessments available on a mountain requires no equipment.
Ask the climber to walk heel-to-toe along a straight line.
Significant unexplained ataxia at altitude should immediately raise concern for HACE.
This is particularly valuable because deterioration may initially be attributed to exhaustion.
A climber staggering around camp after a long summit attempt might simply be exhausted.
But they might not be.
Ataxia associated with altitude illness should be treated extremely seriously.
Untreated HACE can progress to coma and death.
This is not a condition to “sleep off”.
Treating HACE
The definitive field intervention is:
DESCEND
Descent should begin as soon as practically possible.
The patient should ideally be assisted or carried because exertion increases oxygen requirements and a severely ataxic patient may be incapable of safe independent descent.
Supplemental oxygen should be administered when available.
Dexamethasone is the principal emergency medication.
Current guidance for adults recommends:
8 mg initially, followed by 4 mg every six hours.
It can be given orally, intramuscularly or intravenously depending upon circumstances and available expertise.
Dexamethasone can produce dramatic clinical improvement.
But that creates another potential danger.
Improvement after dexamethasone does not mean that the mountain has suddenly become safe.
The drug is buying time.
It is not providing acclimatisation.
The casualty still needs descent.
Portable Hyperbaric Chambers
Remote expeditions may carry portable pressure chambers, commonly associated with names such as the Gamow bag.
The patient is placed inside the chamber and the pressure is increased manually.
This effectively creates a temporary reduction in physiological altitude without physically moving the casualty down the mountain.
Such equipment can be lifesaving when immediate descent is impossible because of:
weather,
terrain,
darkness,
avalanche danger,
or the patient’s condition.
But again the principle is important.
A hyperbaric bag is not a substitute for evacuation when evacuation becomes possible.
It is a means of buying time.
HAPE: When Altitude Affects the Lungs
High-Altitude Pulmonary Oedema is different.
HAPE is a form of non-cardiogenic pulmonary oedema associated with an abnormal response of the pulmonary circulation to hypoxia.
Hypoxia causes pulmonary vasoconstriction.
This response is uneven across the lungs. In susceptible individuals, pulmonary arterial pressures rise substantially and uneven blood flow contributes to leakage of fluid into the alveolar spaces.
The lungs progressively become less effective at transferring oxygen.
That produces more hypoxia.
Which worsens pulmonary vasoconstriction.
Which can worsen the pulmonary oedema.
A dangerous cycle develops.
The First Warning Sign of HAPE May Be Performance
One of the most useful observations in mountain medicine is not necessarily obtained from a medical device.
It comes from watching the climber.
A person who was previously keeping pace begins falling behind.
They repeatedly stop.
They appear disproportionately breathless.
Their performance deteriorates much more than everyone else’s.
That unexplained deterioration in exercise capacity is an important early warning sign of HAPE.
It should not automatically be dismissed as poor fitness or lack of determination.
Signs and Symptoms of HAPE
Early HAPE may produce:
- disproportionate breathlessness during exertion
- significantly reduced exercise performance
- unusual fatigue
- cough
- chest tightness or congestion.
As the condition progresses:
breathlessness occurs with minimal exertion
and eventually:
breathlessness occurs at rest.
Other findings may include:
- tachypnoea
- tachycardia
- crackles or bubbling breath sounds
- marked hypoxaemia
- cyanosis
- worsening cough.
Severe disease can produce pink or blood-stained frothy sputum.
By this stage the patient is critically ill.
The Pulse Oximeter: Useful but Not an Oracle
Pulse oximetry is useful at altitude, particularly when assessing trends and comparing an unwell climber with others at the same elevation.
HAPE can produce oxygen saturations dramatically below those expected for that altitude.
But pulse oximetry must be interpreted carefully.
Cold fingers, poor peripheral perfusion, movement and technical limitations can produce misleading readings.
Furthermore, “normal” oxygen saturation at 5,000 metres is obviously not the same as normal saturation at sea level.
A number should therefore never replace clinical assessment.
The important question is not merely:
What is their SpO₂?
It is:
What is their SpO₂ compared with what would reasonably be expected here, and what is happening to the patient clinically?
Treating HAPE
Again, the most important treatment is straightforward:
DESCEND
Current guidance regards descent as urgent in most field circumstances.
Exertion should be minimised.
The casualty should preferably be carried or otherwise assisted where circumstances permit.
Supplemental oxygen is extremely valuable.
A portable hyperbaric chamber can again provide temporary simulated descent when actual descent is impossible.
Medication can assist but should never distract from the fundamental treatment.
Nifedipine
Nifedipine reduces pulmonary arterial pressure and has an established role in HAPE prevention in susceptible individuals and as an adjunct in field treatment where appropriate.
Current adult guidance includes sustained-release nifedipine:
30 mg every 12 hours
or
20 mg every eight hours.
But the word adjunct matters.
Nifedipine is not a pharmacological alternative to getting a seriously ill mountaineer down the mountain.
Descent and oxygen are substantially more important.
Pulmonary vasodilators can also produce systemic hypotension, particularly in an already compromised casualty, so their use requires appropriate clinical judgement.
Sildenafil and Tadalafil
Phosphodiesterase-5 inhibitors reduce pulmonary arterial pressure and have also been investigated in HAPE.
Tadalafil has evidence supporting prophylaxis in HAPE-susceptible individuals who cannot use nifedipine.
Sildenafil also reduces pulmonary vascular resistance.
However, these drugs should not encourage an expedition mentality in which multiple medications are combined in an attempt to pharmacologically defeat altitude.
Combining pulmonary vasodilators can increase adverse effects, including hypotension.
Current guidance does not recommend routinely stacking these drugs together.
Again:
Medication supports altitude management. It does not replace it.
Acetazolamide: Probably the Most Important Altitude Drug to Understand
Acetazolamide—Diamox—is frequently misunderstood.
It is not simply a drug that masks altitude sickness.
It actually accelerates acclimatisation.
At altitude, increased ventilation produces respiratory alkalosis.
Acetazolamide causes renal bicarbonate loss and a mild metabolic acidosis. This stimulates ventilation and improves oxygenation, particularly during sleep.
The result is faster ventilatory acclimatisation.
For prevention of AMS, a commonly recommended adult regimen is:
125 mg twice daily, beginning the day before ascent and continuing during the initial period at altitude while ascent continues.
Higher dosing may be appropriate in particular circumstances.
For treatment of established AMS, current CDC guidance lists:
250 mg twice daily.
But acetazolamide illustrates another important principle.
It assists acclimatisation.
It does not make acclimatisation unnecessary.
It should never become a pharmacological licence to ignore a badly designed ascent profile.
Nor should acetazolamide be relied upon as HAPE prophylaxis in people with a previous history of HAPE.
Dexamethasone: Effective but Fundamentally Different
Dexamethasone and acetazolamide should not be thought of as interchangeable drugs.
Acetazolamide facilitates acclimatisation.
Dexamethasone suppresses manifestations of altitude illness but does not produce the same acclimatisation process.
Dexamethasone is highly valuable in treating moderate-to-severe AMS and particularly HACE.
But if it is being used to keep a deteriorating climber ascending, something has gone seriously wrong with expedition decision-making.
A climber who requires emergency medication to remain functional at altitude should prompt consideration of descent—not discussion about how quickly they can reach the summit.
The Case for Proper Acclimatisation
All of this leads to the most important treatment of altitude illness.
Prevention.
The Wilderness Medical Society recommends gradual increases in sleeping altitude once above approximately 3,000 metres.
A useful practical principle is:
Increase sleeping altitude by no more than approximately 500 metres per day above 3,000 metres and include an additional acclimatisation night for approximately every 1,000 metres gained.
Some mountain-medicine guidance is even more conservative, commonly recommending increases of approximately 300–500 metres in sleeping altitude.
Individual susceptibility varies.
These figures should therefore be regarded as planning guidance rather than physiological guarantees.
Climb High, Sleep Low
The traditional mountaineering principle remains extremely useful:
Climb high, sleep low.
A climber can ascend higher during the day, expose the body to greater altitude and then return to a lower camp to sleep.
The important variable is therefore not simply the highest altitude reached during the day.
Sleeping altitude matters enormously.
This is why expedition itineraries traditionally contain rotations rather than simply progressing relentlessly upward.
The apparent inefficiency is precisely the point.
The expedition is deliberately allowing physiology to catch up with ambition.
Why Rest Days Are Not Wasted Days
Commercial expedition schedules can create psychological pressure around acclimatisation days.
Clients have paid substantial sums.
Weather windows are limited.
Holiday time is finite.
Everyone wants to progress.
A rest or acclimatisation day can therefore feel like a day in which nothing is happening.
Physiologically, the opposite is true.
Ventilatory adaptation is occurring.
Renal compensation is occurring.
Sleep physiology is adapting.
The body is becoming better able to function in the hypoxic environment.
Acclimatisation days are not interruptions to the expedition. They are part of the ascent.
The Dangerous Psychology of the Summit
Altitude illness also creates a decision-making problem.
The symptoms appear at precisely the point at which psychological investment in continuing is greatest.
Money has been spent.
Months of training have taken place.
The summit may be visible.
Weather may be deteriorating.
There may be only one summit opportunity.
The temptation becomes:
Just another 500 metres.
That is where objective expedition leadership becomes essential.
The mountain does not care how much someone has paid.
And cerebral oedema does not respond to determination.
Three Rules That Can Save a Life
A remarkably effective altitude strategy can be reduced to three principles:
1. Recognise the symptoms of altitude illness early.
2. Never ascend to a higher sleeping altitude with symptoms of altitude illness.
3. If symptoms deteriorate despite rest and treatment at the same altitude, descend.
If HACE or HAPE is suspected, the situation changes further.
The objective is no longer acclimatisation.
It is evacuation.
AMS, HACE and HAPE: The Critical Difference
It is useful to think of the three conditions differently.
AMS is a warning.
The body is failing to adapt adequately to the current ascent profile.
Stop ascending.
Allow acclimatisation.
Descend if deterioration occurs.
HACE is an emergency involving the brain.
Ataxia, confusion, abnormal behaviour or altered consciousness at altitude should trigger urgent action.
Dexamethasone and oxygen can buy time.
Descent saves lives.
HAPE is an emergency involving the lungs.
Unexplained performance deterioration and disproportionate breathlessness can be early warnings.
Breathlessness at rest is a major red flag.
Oxygen, appropriate adjunctive medication and portable hyperbaric treatment may buy time.
Again:
Descent saves lives.
Drugs Are Not Acclimatisation
Modern expedition medicine possesses an impressive pharmacological toolkit.
Acetazolamide.
Dexamethasone.
Nifedipine.
Selected pulmonary vasodilators.
Oxygen.
Portable hyperbaric systems.
These interventions have undoubtedly saved lives.
But they create a potential cultural problem.
Medication can produce the illusion that human physiology has become negotiable.
It has not.
Acetazolamide can accelerate acclimatisation.
Dexamethasone can temporarily reverse manifestations of serious cerebral altitude illness.
Nifedipine can reduce pulmonary vascular pressures.
Oxygen can temporarily correct hypoxaemia.
A hyperbaric bag can temporarily simulate lower altitude.
But none changes the fundamental geographical treatment available to a seriously deteriorating casualty.
Go down.
Evacuation Is Treatment
This may be the single most important principle in altitude medicine.
In many medical emergencies, evacuation takes the patient to treatment.
In severe altitude illness:
evacuation is itself part of the treatment.
Every metre of descent increases barometric pressure and therefore improves inspired oxygen partial pressure.
The mountain effectively contains its own treatment.
It lies downhill.
That makes delays particularly difficult to justify.
The question should not be:
Can this climber struggle on until tomorrow?
It should be:
What clinical benefit is gained by remaining here?
With suspected HACE or HAPE, there is rarely a convincing answer.
Conclusion: The Mountain Sets the Rules
Altitude illness is unusual because its fundamental cause and its most effective treatment are remarkably clear.
The casualty has ascended beyond their current capacity to acclimatise.
Reduce the altitude and the physiological insult decreases.
This is why adequate acclimatisation should never be regarded as wasted expedition time.
It is preventative medicine.
Modern drugs provide an important safety net.
Acetazolamide can facilitate acclimatisation and reduce AMS.
Dexamethasone can be lifesaving in HACE.
Nifedipine has an important role in HAPE, particularly in susceptible individuals and in circumstances where definitive measures are temporarily constrained.
Oxygen and portable hyperbaric chambers can buy invaluable time.
But none should be allowed to reverse the hierarchy of treatment.
Acclimatise properly.
Do not ascend while ill.
Recognise deterioration early.
Treat serious illness immediately.
And when HACE or HAPE develops, descend.
The strongest expedition is therefore not necessarily the team that reaches altitude fastest.
It is the team disciplined enough to progress slowly when everyone feels well, stop when someone becomes ill and abandon the summit when continuing would turn an avoidable physiological problem into an evacuation—or a fatality.
At altitude, ambition must remain subordinate to physiology.
The summit can wait. The brain and lungs cannot.
