Altitude training

Our previous discussions of the psychological responses to exercise have all been based on the conditions that exist at or near sea level, where the barometric(air) pressure(P_b) averages about 760mmHg. Barometric pressure is a measure of the total pressure that all of the gases comprising the atmosphere exert on the body(and everything else). Regardless of the P_b, oxygen molecules always make up 20.93% of the air. The partial pressure of oxygen(PO₂) is that portion of P_b exerted only by the oxygen molecules in the air. At sea level, PO₂ is therefore 0.2093 times 760mmHg, or 159mmHg. Partial pressure is an important concept in understanding altitude physiology, because it is primarily the low PO₂ at altitude that limits exercise performance. Although the human body tolerates small fluctuations in pressure, large variations pose special problems. This is evident when mountain climbers ascend to higher altitudes where reduced pressures can substantially impair physical performance and can even jeopardize life.

The reduced barometric pressure at altitude is referred to as a hypobaric environment or simply hypobaria(low atmospheric pressure). The lower atmospheric pressure also means a lower PO₂, which limits pulmonary diffusion of oxygen from the lungs and oxygen transport to the tissues. When oxygen delivery to the body tissues is compromised, the result is cellular hypoxia(oxygen deficiency).

Hypobaric environments: conditions at altitude

Clinical problems associated with altitude were reported as early as 400BC. However, most of the early concerns about ascent to high altitudes focused on the cold conditions at altitude rather than the limitations imposed by low air pressure. The initial landmark discoveries that led to our current understanding of the reduced P_b and PO₂ at altitude can be credited primarily to three 15^th and 16^th century scientists. Torricelli(ca. 1644) developed the mercury barometer, an instrument that permits the accurate measurement of atmospheric pressure. Only a few years later(1648), Pascal demonstrated a reduction in barometric pressure at high altitudes. Nearly 130 years after that(1777), Lavoisier described oxygen and the other gases that contribute to the total barometric pressure.

The deleterious effects of high altitude on humans that are caused by low PO₂(hypoxia) were recognized in the late 1800s. More recently, a team of scientists led by the late John Sutton performed an intricate series of laboratory studies in the hypobaric chamber at the U.S. Army Institute of Environmental Medicine. These experiments, known collectively as Operation Everest II, have significantly added to our understanding of exercise at altitude. For our discussion, the term altitude refers to elevations above 1,500m(4,921ft), because few negative physiological effects on performance are seen below that altitude.

While the major impact of altitude on exercise physiology is attributable to the low PO₂ that ultimatively limits oxygen availability to the tissues, the atmosphere at altitude also differs in other ways from sea-level conditions.

Atmospheric pressure at altitude

Air has weight. The barometric pressure at any place on Earth is related to the weight of the air in the atmosphere above that point. At sea level, for example, the air extending to the outermost reaches of the earth’s atmosphere(approximately 38.6km, or 24mi) exerts a pressure equal to 760mmHg. At the summit of Mount Everest, the highest point on Earth(8,848m, or 29,028ft), the pressure exerted by the air above is only about 250mmHg. These and related altitude differences are depicted in the picture below.

The barometric pressure on Earth does not remain constant. Rather, it varies with changes in climatic conditions, time of year, and the specific site at which the measurement is taken. On Mount Everest, for example, the mean barometric pressure varies from 243mmHg in January to nearly 255mmHg in June and July. Also, the earth’s atmosphere bulges outward slightly at the equator, which increases the barometric pressure there by a few millimeters of mercury above standard pressure. These variations are of little interest to people living near sea level but are of considerable physiological importance for a climber attempting to ascend Mount Everest without supplemental oxygen.

Although barometric pressure varies, the percentages of gases in the air that we breathe remain unchanged from sea level to high altitude. At any elevation, the air always contains 20.93% oxygen, 0.03% carbon dioxide, and 79.04% nitrogen. Only the partial pressures change. As shown in the figure above, the pressure that oxygen molecules in the air exert at various altitudes drops proportionally with decreases in the barometric pressure. The consequent changes in PO2 have significant effects on the partial pressure of oxygen that reaches the lungs, as well as the gradients between the alveoli of the lungs and the blood(where oxygen is loaded) and between the blood and the tissues(where oxygen is unloaded).

Air temperature at altitude

Clearly, the low PO₂ at altitude has the greatest impact on exercise physiology. However, other environmental factors contribute as well. For example, air temperature decreases at a rate of about 1°C(1.8°F) for every 150m(about 490ft) of ascent. The average temperature near the summit of Mount Everest is estimated to be about -40°C(-40°F), whereas the sea level the temperature would be about 15°C(59°F). The combination of low temperatures, low absolute humidity, and high winds at altitude poses a serious risk of cold-related disorders, such as hypothermia and windchill injuries.

Because of the cold temperatures at altitude, the absolute humidity is extremely low. Cold air holds very little water. Thus, even if air is fully saturated with water(100% relative humidity), the actual amount of water contained in the air is small. The partial pressure of water, also known as the water vapor pressure(PH₂O), at 20°C(68°F) is about 17mmHg. But at -20°C(-4°F), this pressure decreases to only about 1mmHg. The very low PH₂O at high altitude promotes evaporation of moisture from the skin(or clothing) surface, because of the high gradient between skin and air, and can lead quickly to dehydration. In addition, a large volume of water is lost through respiratory evaporation attributable to the dry air and increased respiration rate experienced at altitude.

Solar radiation at altitude

The intensity of solar radiation increases at high altitude for two reasons. First, at high altitudes, light travels through less of the atmosphere before reaching the earth. For this reason, less of the sun’s radiation, especially ultraviolet rays, is absorbed by the atmosphere at higher altitudes. Second, because atmospheric water normally absorbs a substantial amount of the sun’s radiation, the low water vapor in the air at altitude also increases radiant exposure. Solar radiation may be further amplified by reflective light from snow, which is usually found at higher elevations.