exercise in difference environment

6. Exercise in Hypobaric & Hyperbaric Environments

EXERCISE IN HYPOBARIC & HYPERBARIC ENVIRONMENTS

Hypobaric environment Low atmospheric pressure. Barometric pressure is reduced at altitude. (e.g.: mountain) Lower atmospheric pressure also means lower partial pressure of oxygen

(PO2), which limits pulmonary diffusion and low O2 transport to tissue. This reduces O2 delivery to the body tissue, resulting in hypoxia (O2

deficiency).

Hyperbaric environment High atmospheric pressure. (e.g.: underwater world) High partial pressure of certain gases can lead to life-threatening

complication.

Microgravity Low gravitational force. (e.g.: environment in the outer space)

A. Hypobaric Environment: Exercising at Altitude

The term altitude refers to elevations above 1500 m (4,921 ft).

Altitude presents a hypobaric environment which the atmospheric pressure is reduced. Altitudes of 1500 m or above have a physiological impact on the human body.

Hypobaric Environment – an environment, such as that at high altitude, involving low atmospheric pressure.

1. Conditions at Altitude

i. Atmospheric pressure

At Sea level, the barometric pressure (Pb) averages = 760 mmHgSummit of Mount Everest, barometric pressure (Pb) = 250 mmHgTherefore, barometric pressure is low at altitude.

The % of gases in the air remain unchanged at altitude (O2 = 20.93%, CO2 = 0.03%, & N2 = 79.04%).

The partial pressures of each gas, is reduced in direct proportion to the increase in altitude.

The reduced partial pressure of O2 leads to decreased performance at altitude due to a reduced pressure gradient that hinder O2 transport to the tissue.

Differences in atmospheric conditions at sea level & at an altitude

At sea level At 8,900m1


Atmospheric Pressure 760 mmHg 250 mmHgPartial Pressure of O2 159.2 mmHg 48.4 mmHgAmbient temperature 15oC -40oC

*Mount Everest (8,848m = 29,028 ft)

ii. Air Temperature Air temperature decreases as altitude increases. Air temperature drops about

1oC for every 150m (490 ft) of ascent (Mt. Everest = -40oC).

Cold air holds very little water, so the air at altitude is dry. humidity of the air is low

The dry air increased evaporative water loss through sweating during exercise at altitude.

The cold & dry air can lead to cold-related disorders & dehydration through increased insensible water loss.

iii. Solar Radiation

The intensity of solar radiation increases at high altitude because the atmosphere is thinner & drier.

2. Physiological Responses to Altitude

The hypoxic conditions (diminished O2 supply) at altitude alter the body’s normal physiological responses.

i. Respiratory Responses

Pulmonary ventilation (breathing) increases at higher altitudes, when at rest & during exercise.

Because the number of O2 molecules in air is less, more air must be inspired to supply as much 02 during normal breathing at sea level.

Ventilation increases to bring in a larger volume of air.

People ventilate greater volumes of air at altitude because air is less dense.

Increased ventilation resulting in hyperventilation state in which too much CO2 can be cleared & allows blood pH to increase, leading to respiratory

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alkalosis. In response, the kidneys excrete more bicarbonate ion, so less acid can be buffered.

Pulmonary diffusion is not hinder by altitude, but O2 transport is slightly impaired because hemoglobin saturation at altitude is reduced, although by only a small amount.

The diffusion gradient that allows O2 exchange between the blood & active tissue is substantially reduced at elevation, thus O2 uptake is impaired. This is partially compensated for by a decrease in plasma volume, concentrating the RBCs & allowing more O2 to be transported per unit of blood.

Maximal O2 consumption decreases along atmospheric pressure. As the partial pressure of O2 decreases, VO2max (Maximal O2 uptake) decreases at a progressively greater rate.

Maximum O2 uptake decreases since PO2 decreases. At sea level VO2max = 50ml/kg/min but, at Mt. Everest peak, VO2max can be as low as 5ml/kg/min.

ii. Cardiovascular Responses

During submaximal work at altitude, the body increases its cardiac output, by increasing the heart rate, to compensate for the decrease in the pressure gradient that drives O2 exchange.

During maximal work, stoke volume & heart rate are both lower, resulting in a reduced cardiac output. This combined with the decreased pressure gradient severely impairs O2 delivery & uptake.

iii. Metabolic Responses

Because O2 delivery is restricted at altitude, oxidative capacity is decreased. More anaerobic energy production must occur, as evidenced by increased blood lactate levels for a given submaximal work rate. However, at maximal work rate, lactate levels are lower, perhaps because the body must work at a rate that cannot fully stress the energy systems.

*lactate = A salt formed from lactic acid.

3. Performance at Altitude

Endurance activities are severely affected because oxidative energy production is limited.

Anaerobic sprint activities (<1 min) are generally not impaired at moderate altitude. This is because thinner air imposes less resistance to movement. This explains the amazing performances of sprinters & long jumpers at the 1968 Olympics in Mexico City.

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Exhaustive activities – at altitude, less lactate is produced (actually should increase) due to decrease in muscle in muscle enzyme activities and total work.

4. Acclimatization: Prolonged Exposure to Altitude

i. Blood adaptations

Hypoxic conditions stimulate the release of erythropoietin (EPO), which increases erythrocyte (RBC) production. More RBC means more hemoglobin. These adaptations improve the oxygen-carrying capacity of the blood.

Although plasma volume decreases initially (within a few hours of arrival at altitude as a result of fluid shifts and respiratory water loss), which also concentrates the erythrocytes (RBC) - hemoglobin, this changes also increasing the blood’s oxygen-carrying capacity.

ii. Muscle adaptations

Muscle fiber areas decrease when at altitude, thus decreasing total muscle area.

Total muscle mass & total body weight decrease. Part of this is from dehydration and appetite suppression, which leads to protein breakdown in the muscles.

Metabolic enzyme activities in the muscle also decreased. Capillary density in the muscle increased, which allow more blood &

oxygen to be delivered to the muscle.

iii. Cardiorespiratory adaptations

Increased in pulmonary ventilation both at rest and during exercise. Ventilation is stimulated by the decreased O2 content of the inspired air.

Decreased in VO2 max with initial exposure to altitude does not improve much (or improve very little) during several weeks of exposure.

5. Physical Training & Performance

Most studies show that training at altitude leads to no significant improvement of sea-level performance. The physiological changes that do occur, such as increased RBC production, are transient but could offer an advantage during the first few days after returning to sea level. This is still an area of debate.

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Athletes who must perform at altitude should do so within the first 24 hours of arrival while the detrimental changes that occur have not yet become too great.

Alternatively, athletes who must perform at altitude could train at an altitude of 1500 m to 3000 m for at least 2 weeks prior to performing. This allows the body time to adapt to hypoxic & other environmental conditions at altitude.

6. Clinical Problems of Acute Exposure to Altitude

Acute altitude sickness typically causes symptoms such as headaches, nausea, vomiting, dyspnea, & insomnia. These usually appear in 6 to 96 hr after arrival at altitude.

The exact cause of acute altitude sickness is not known, but many researchers suspect the symptoms may result from carbon dioxide accumulation in the tissues.

Acute altitude sickness can usually be avoided by a gradual ascent to altitude; climbing not more than 300 m per day at elevations above 3000 m. Medications can also be used to reduce the symptoms.

High-altitude pulmonary edema (HAPE) & high-altitude cerebral edema (HACE), which involve accumulation of fluid in the lungs & cranial cavity, respectively, are life-threatening conditions. Both are treated by O2 administration & descent.

B. Hyperbaric Conditions: Exercising Underwater

1. Water Immersion & Gas Pressures

Submersion in water exposes the human body to a hyperbaric environment where external pressure is greater than at sea level.

Because volume decreases when pressure increases. Therefore, air that is in the body before it goes underwater is compressed when the body is submerged. Conversely, the air taken in at depth expands during ascent.

When the body is submerged, more molecules of gases are forced into solution, but with a rapid ascent, the molecule come out of solution and can form bubbles.

2. Cardiovascular Response to Water Immersion

Water reduces the stress on the CV system, reducing its work load. When the body is submerged, plasma volume also increases. Because of these factors, resting HR drops even when the body is only partially submerged. This effect is enhanced by cold water.

Hyperventilation is often practiced before breath-hold diving to increase how long you can hold your breath. But this can lead to dangerously low O2 levels, which can cause you to lose consciousness underwater.

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During breath-hold diving, the gases in the body can become pressurized even when swimming at a depth of only 1 to 2 m below the surface. At greater depths, the volume of air in the lungs can be reduced to the residual volume, but not smaller.

The depth limit to breath-hold diving is determined by the ratio of the total lung volume & the residual volume. Those with large TLV:RV ratios can safely dive deeper than those with smaller ratios.

Scuba diving can alleviate many of the problems faced during breath-hold diving because you breathe pressurized air while submerged.

3. Health Risks of Hyperbaric Conditions

Breathing gases under pressure can cause the body to accumulate gases in toxic levels, so precautions must be taken when diving with pressurized gases.

O2 poisoning occurs when PO2 values are above 318 mmHg. Less O2 will be removed from hemoglobin for use by tissues. This impairs the biding of CO2 to hemoglobin, so less CO2 is removed by this route. High PO2 also causes vasoconstriction in the cerebral vessels, which decreases blood flow to the brain.

Decompression sickness (the bends) results from ascending too rapidly. The nitrogen dissolved in the body cannot be removed by the lungs quickly enough, so it forms bubbles. The bubbles can form emboli, which can be fatal. To treat this, the diver must undergo recompression to force the nitrogen back to solution, then undergo gradual decompression at the rate that allows the nitrogen to be removed during normal breathing. Tables have been formulated that specify how much time must be allowed for ascension from various depths, 7 divers must adhere strictly to these.

Nitrogen narcosis (rapture of the deep) results from the narcotic effects of nitrogen when its partial pressure is high, such as during depth diving. The symptoms are similar to alcohol intoxication. Judgment is impaired, which can lead to fatal mistakes.

Spontaneous pneumothorax (rupture alveoli) & ruptured eardrum are other health risks associated with the changing pressure experienced when diving.

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