Affects of Altitude on the Human Body.

The information posted on this page about the affects of altitude is supplemental material to first aid and CPR training. This information will not be covered in this immense detail in any of the comprehensive first aid courses. This material is provided to give candidates an understanding on the demands of the body as altitude increases and the body’s responses. This data can then be used to help in any cardiopulmonary or first aid scenario at increased altitudes. Additional topics such as pulmonary edema and high altitude cerebral edema are not covered in any significant detail in Red Cross first aid training.


Medium altitude – 5000 – 10,000 feet – in relation to athletics, we are concerned with this altitude range
High altitude – greater than 10,000 feet. More than 40 million people live and work between 10,000 ft. (3048 meters) and 18,000 ft. (5486 meters).
Barometric (air) pressure decreases as altitude increases (ie) as the weight of the column of air above the point of measurement decreases. However, the chemical composition of the atmosphere is uniform up to 20,000 meters.
PO2 in dry ambient air at sea level = .209 X 760 mm Hg = 160 mmHg
PO2 in dry ambient air at 3048 meters (10,000 ft.) = .209 X 510 mm Hg = 107 mm Hg.
PO2 in dry ambient air at summit of Mt Everest – 8848 meters (29,028 ft.) = .209 X 250 mm
Hg = 52 mm Hg.
Oxyhemoglobin dissociation curve – only a small change in percent saturation of hemoglobin is observed with decreasing PO2 until an altitude of about 10,000 ft. Measurable negative effects on VO2 max. have been noted at altitudes as low as 4000 ft. The critical alveolar PO2 at which an unacclimatized person loses consciousness within a few minutes during acute exposure to hypoxia occurs at an altitude of 23,000 ft.

Decreased density of air –> decreased external air resistance –> external work is decreased ataltitude in sprint type activities where high velocities are involved. There will also be less air resistance encountered by projectiles. Air temperature decreases linearly by 6.5o C per 1000 meters of altitude or 2oC (3o F) per 1000 ft. Air becomes increasingly dry with increasing altitude –> water loss via respiratory tract is
higher at high altitude.

Solar radiation – UV radiation is more intense at high altitude –> sunburn, snow blindness. Force of gravity is decreased with distance from the earth’s center –> higher altitudes should have a favourable effect on jumping and throwing events.

A. Cardiovascular System
VO2 = (HR X SV) X (CaO2 – CvO2)
With increasing altitude, CaO2 progressively decreases. To compensate, cardiac output initially increases during rest and submaximum exercise due to an increase in heart rate. Over the first week at altitude, cardiac output falls to or below sea level values for the same VO2 and there is a progressive increase in O2 extraction –> more efficient method of delivering O2. The most important long-term adaptation to altitude is an increase in the blood’s oxygen carrying capacity. Hemoglobin concentration starts to increase during the first two days at altitude due to a decrease in plasma volume and an increase in RBC production by bone marrow. These hematological changes during acclimatization are dependent on an adequate iron intake. In some high altitude natives and well-acclimatized sojourners, hemoglobin concentration may be increased 40 – 50% above normal.

Concentration of 2,3 DPG within RBC increases –> shift O2 dissociation curve to right –> unload more O2 at the tissues for a given capillary PO2. Even after several months of acclimatization, VO2 max. still remains significantly below sea level values.

B. Pulmonary System
Decreased alveolar PO2 –> decreased arterial PO2 –> stimulation of aortic and carotid
chemoreceptors –> increase in ventilation –> increase in PAO2 and PaO2
Hyperventilation –> decreased PACO2 and PaCO2 –> increase in blood pH (respiratory alkalosis) –> plasma bicarbonate levels decrease during first two days because the kidneys excrete excess HCO3 to compensate pH.
After the acid-base balance is corrected, hyperventilation persists during acclimatization. Within a week at high altitude, a new level for VE is attained – 40 to 100% above sea level values.

C. Sensory and Mental Function
Research studies have shown the following decrements in performance at altitude:
• At 3048 meters (10,000 ft.) – a 30% decrease in visual acuity, a 25% decrease in light sensitivity, and a 25% decrease in attention span.
• At 4500-5500 meters (14,800-18,000 ft.) – a 15-20% decrease in cognition and recall. A 25% decrease in pursuit tracking ability
• At 6100 meters (20,000 ft.) – a 25% decrease in reaction time.

D. Responses To Exercise
VO2 max. decreases 3 – 3.5% per 1000 ft. above 5000 ft. At 14,000 ft. VO2 max. is decreased approximately 30%. This is due to:
a) decreased oxygen content of arterial blood –> decreased a-vO2 difference in maximal exercise
b) after acclimatization – decrease in maximal cardiac output due to a decrease in maximum heart rate and stroke volume. The decrease in maximal stroke volume is most likely due to the reduction in venous return which is caused by the decreased blood volume – Starling mechanism
The percentage reduction in VO2 max. is equal in both trained and untrained individuals. Oxygen uptake is the same at altitude as at sea level for the same submaximal workload. However, heart rate and minute ventilation will be greater. During heavy exercise, muscle and blood lactate levels are higher at altitude for any given workload for two reasons:

a) Since the VO2 max. is reduced, any given workload now requires a higher percentage of the VO2 max. to perform.
b) There is a reduced blood buffering capacity due excretion of certain amount of bicarbonate via the kidneys. Higher level of perceived exertion for any workload.

E. Acclimatization Limits
The highest permanent settlement is located at 17,000 ft. in the Andes. It has generally been observed that acclimatization stops and physical condition & mental function begin to deteriorate at altitudes above 17,000 ft. (5200 meters).

F. Time Required for Acclimatization
The longer you stay at altitude, the better you perform in aerobic events but it never reaches sea level values. The number of days needed to acclimatize depends on the altitude:
9000 ft. –> 7 – 10 days
12,000 ft. –> 15 – 21 days
15,000 ft. –> 21 – 25 days
The length of time required depends to a large extent on the individual. A few people will never acclimatize and will continue to suffer mountain sickness. Acclimatization to one altitude ensures only partial acclimatization to a higher altitude. High altitude exposure for periods longer than two weeks results in a significant reduction is both body fat and lean body mass due mostly to appetite depression. The benefits of acclimatization are probably lost within 2 or 3 weeks after returning to sea level.


A. Altitude Training

In order to attain top achievement at altitudes of 6500 feet or higher, endurance athletes should acclimatize for 2-3 weeks before their major competition. Non-endurance athletes may arrive close to the time of competition.
During the first few days at altitude, athletes may experience acute mountain sickness, which may hinder their training. Since VO2 max. is decreased at altitude, intensity of training must be decreased. However, this problem can be solved by living at altitude, but going down to lower altitudes for few hours per day to train – “sleep high, train low”. It is not necessary to train at a higher altitude than the actual place of competition. Some athletes consistently fail at altitude –> complicates team selection.

B. Performance After Return To Sea Level

It is clear that altitude acclimatization improves one’s capacity to work at altitude. Several studies have reported no increase in VO2 max. or performance in running events at seal level after several weeks of training at altitudes ranging from 7500 to 13,000ft. compared with pre-altitude performance. There is no consistent scientific evidence to support training at altitude to improve sea level performance.

Adaptations to altitude which should increase VO2 max. on return to sea level:
a) increased hemoglobin concentration
b) local muscle adaptations – increased number of mitochondria, oxidative enzymes, etc.

Adaptations which hinder performance on return to sea level:
a) decreased maximum stroke volume and maximum heart rate which persists for a few weeks
b) increased VE at a given workload –> extra oxygen goes to respiratory muscles during exercise
c) decreased buffering capacity of blood for lactic acid
d) specificity of training – while at altitude, the athlete isn’t able to train at close to sea level race pace. One must lower the absolute workload to perform aerobic exercise at the same relative intensity at altitude as at sea level.

C. Live High – Train Low Altitude Training

Athletes who use this method of training live and/or sleep at moderate altitude (6500-9000 feet), while going to a lower elevation (less than 4000 feet), in reasonable proximity, for a few hours daily to train. The purpose of this procedure is to get the beneficial physiological altitude adaptations, while maintaining sea level training intensity. This method of training results in better sea level performance than is obtained by living and training at altitude for a number of weeks and then returning to sea level to perform.


Altitude illnesses are on a continuum and are not separate illnesses. Most altitude illness is preventable. Proper management requires early diagnosis and prompt intervention.

A. Prevention of Altitude Illnesses
The following measures will reduce the incidence and severity of high altitude illness.
1. Staged ascent – slow ascent to altitude while climbing. Once travellers reach 8000 ft., they should not ascend more than 1000 to 2000 ft. per day.
– if possible, don’t fly or drive to high altitude. Start below 10,000 feet (3048 meters) and walk up. If you do fly or drive don’t over-exert yourself or move higher for the first 24 hours.
– acclimatize to lower altitudes before going to high altitudes
– “work high but sleep low”
– if you begin to show symptoms of moderate altitude illness, don’t go higher until symptoms decrease. Don’t go up until symptoms go down! If symptoms increase, go down, down, down!
– in the presence of extreme cold or heat, or severe exertion, it may be necessary to ascend even more slowly than the above recommendations.

2. Avoid alcohol and other depressant drugs including barbituates, tranquilizers and sleeping pills. These depressants further decrease the respiratory drive during sleep resulting in a worsening of the symptoms.

3. High carbohydrate diet – a diet of at least 70% carbohydrates reduces symptoms of acute mountain sickness by about 30% and can be started one to two days prior to ascent.

4. Appropriate exercise level – until acclimatized, it is best to exercise moderately, avoiding excessive breathlessness and fatigue.

5. Drug prophylaxis – there are several drugs that can lessen the symptoms of high altitude illness. However their use is not recommended as a routine measure. The drug of choice is acetazolamide (Diamox).

6. Fluid ingestion – drink lots of water (but not much alcohol), enough to produce a very diluted urine. Dehydration is a common cause of headache at altitude.

7. Keep in mind that different will acclimatize at different rates. Make sure that everyone in your party is properly acclimatized before going higher.

B. Acute Mountain Sickness

Most common form of altitude illness. It can occur at altitudes over 6500 ft., but more common over 10,000 ft.

AMS occurs 12 – 36 hours after arriving at altitude and usually lasts 2 to 3 days.

The occurrence of AMS is dependent on the elevation, the rate of ascent, and individual susceptibility. Many people will experience mild AMS during the acclimatization process. This condition is exacerbated by exercise during the first few hours of altitude exposure.
Symptoms – headache, fatigue, irritability, loss of appetite, nausea, vomiting, dizziness, insomnia, generalized weakness. The syndrome resembles an alcohol hangover.
Treatment – the only cure is either acclimatization or descent. Symptoms of mild AMS can be treated with pain medications for headache and Diamox. Moderate or severe AMS requires descent to lower altitudes.

C. High-Altitude Pulmonary Edema (HAPE)

Pulmonary edema – accumulation of fluid in the alveoli –> decreased diffusing capacity for oxygen
Occurs above 10,000 ft. and takes 36 – 72 hrs. to become obvious. It can strike individuals of any age, particularly those who ascend rapidly. The occurence rate is 1% to 2% of trekkers at altitudes over 10,000 ft. Risk factors for HAPE include the altitude above sea level, the rate of ascent to that altitude, and individual susceptibility. The direct effect of hypoxia on systemic arterioles is vasodilation. In contrast, hypoxia in the
lung causes vasoconstriction –> increased pulmonary vascular resistance –> right ventricle must generate a higher pressure –> pulmonary hypertension –> greatly elevated pulmonary capillary pressure –> movement of fluid from the circulatory system to the pulmonary interstitial spaces and alveoli.

Symptoms – shortness of breath, severe fatigue, cough which sometimes produces a frothy and/or bloody sputum, tachycardia, severe headache, insomnia, chest tightness or congestion, blue or gray fingernails or lips, often a slight fever; may rapidly go on to unconsciousness and death. The symptoms often begin at night when shortness of breath at rest may occur.

The hypoventilation and associated hypoxemia that occur during sleep may further predispose persons to HAPE –> avoid sleeping medications, alcohol, and sedatives that further depress ventilation.

Treatment – descend to lower altitude immediately with or without oxygen
– diuretics are quite effective if and only if fluids are replaced as rapidly as they are excreted.
– Diamox is effective for prevention and treatment
– Dexamethasone will improve symptoms

D. High-Altitude Cerebral Edema (HACE)

Accumulation of excess fluid in the brain

Rare below 12,000 ft.

Symptoms – loss of coordination, confusion, hallucinations, severe headache, severe weakness and fatigue, coma
Treatment – same as for HAPE
Treatment of altitude illness is based on four principles:

(1) stop ascent in presence ofsymptoms;

(2) descend if no improvement or if condition worsens;

(3) descend immediately if HAPE, loss of coordination, or changes in consciousness are present;

(4) ill persons must not be left behind alone or sent down alone.

E. Other Medical Problems at Altitude

Retinal hemorrhage – occurs in 50% of people going above 17,500 ft. – reversible after return to sea level. However, irreversible visual defects can occur.
Low temperatures –> hypothermia, frostbite; Sunburn

The material posted in this blog on first aid and medical conditions in levels of high altitude is for information purposes only. To learn to recognize and treat major first aid emergencies take a first aid course near you. Certifications can last as long as 3 years and successful candidates receive certification cards upon completion of the course.