HSS 486 Altitude & Microgravity

-baric

Pressure (e.g., hypobaric = low pressure; hyperbaric = high pressure)

Partial pressure

Pressure exerted by a single gas within a mixture of gases

Hypoxemia

Low blood oxygen levels

Hypoxia

Low oxygen levels in the tissues of the body, often a result of hypoxemia

Erythropoietin (EPO)

Hormone that signals for the production of RBCs

Barometric pressure

Aka atmospheric pressure; the pressure exerted by the atmosphere upon oneself (~760 mmHg at sea pressure)

• Higher altitude = lower pressure

• Lower altitude = higher pressure

Partial pressure of oxygen (PO2)

Portion of barometric pressure exerted by oxygen; reduced PO2 at altitude limits performance (less O2 available in atmosphere for uptake)

What level of elevation is considered “altitude”?

Greater than/equal to 1,500 m

Low altitude

500 to 2,000 m

• Does not affect well being

• Performance may decrease at higher end (above 1,500 m)

Moderate altitude

2,000 to 3,000 m

• Affects well-being in unacclimated people

• Performance & aerobic capacity decrease (can be restored by acclimation)

High altitude

3,000 - 5,500 m

• Acute mountain sickness often occurs

• Performance decreases and is not restored by acclimation

Extreme high altitude

Over 5,500 m

• Results in severe hypoxic effects

Air temperature at altitude

Temperature decreases 1°C per 150 m ascent; contributes to risk of cold-related disorders

Humidity at altitude

Cold air holds very little water → decreased humidity at altitude; dry air → quick dehydration

Solar radiation at altitude

Solar radiation increases at high altitude as UV rays travel through less atmosphere, low water vapor is unable to absorb solar radiation, & snow amplifies/reflects solar radiation; increased risk of sunburn

Respiratory response to altitude

• Pulmonary ventilation increases immediately at rest & during submaximal exercise due to decreased PO2 stimulating chemoreceptors

• Respiratory alkalosis (high blood pH) possible consequence of increased ventilation — hyperventilation increases CO2 loss & oxyhemoglobin curve shifts left

Kidney response to altitude

Excrete more bicarbonate; potentially reverses alkalosis (result of increased ventilation) and blood pH decreases to normal

• May result in increased urination

Effect of altitude on pulmonary diffusion

Low arterial blood PO2 occurs as a direct reflection of low alveolar PO2 (which reflects low oxygen content in atmosphere) → decreased diffusion gradient and lesser diffusion of O2 into the tissues; decreased (a-v)O2 diff

• Decreased gas exchange at muscles

• Decreased PO2 gradient at muscle → decreased exercise capacity

• O2 diffusion into muscle significantly reduced at high altitude

CV response to altitude

• Short-term decrease in plasma volume within a few hours due to respiratory water loss & increased urine production; causes short-term increase in hematocrit & O2 density (good)

• RBC count increases after weeks/months due to EPO release from the kidneys triggered by hypoxemia; results in long-term increase in hematocrit

• Cardiac output increases (despite decreased plasma volume) due to increased SNS activity driving increased HR; inefficient, short-term adaptation (6-10 days)

• After a few days. muscles extract more O2 → increased (a-v)O2 difference

**Despite short-term adaptations, performance is still limited until long-term acclimation occurs

Metabolic response to altitude

• Increased basal metabolic rate possibly due to increased thyroxine & catecholamine secretion; results in increased foot intake required to maintain body mass

• Other nutritional considerations:

  • Faster dehydration = more fluids needed

  • Appetite & thirst may be dysregulated

  • Iron intake needed to support increased hematocrit

• Hypoxic conditions = increased anaerobic metabolism → increased lactic acid

Affect of altitude on VO2max

VO2 max decreases linearly as altitude increases part 1,500 m due to decreased arterial PO2 & Qmax

• Mt. Everest study found that sea level VO2max below 50 mg/kg/min → climbing not possible without supplemental oxygen

Effect of altitude on aerobic performance

Effected most by altitude

• VO2max decreases as percentage of sea level VO2max

• However, same absolute O2 requirement for a given task

Effect of altitude on anaerobic performance

Usually unaffected by altitude

• Primarily ATP-PCr & anaerobic glycolysis metabolism

• Minimal O2 requirements

Effect of altitude on power performance

Thinner air → less resistance

• Improved short distance & sprint times, jump distances

• Varied effects in throwing events

Acclimation to altitude — pulmonary adaptations

Increased ventilation at rest & during exercise

• Resting ventilation rate is 40% higher than at sea level

Acclimation to altitude — blood adaptations

• EPO releases stimulates polycythemia (increased RBCs/hematocrit)

  • Hemoglobin increases proportional to elevation

• Plasma volume decreases then increases

  • Early loss = increased hematocrit prior to polycythemia

  • Later increase = increased stroke volume & cardiac output

“Live high, train low” method

One strategy to optimize performance (best approach) is to have athletes live at altitude (~2,500 m) but train at sea level → increased oxygen carrying capacity of blood; maximizes both training AND adaptations

• Hypoxia during training at high altitude prevents high-intensity aerobic training

• Living and training high leads to dehydration, low blood volume, low muscle mass

Artificial altitude training

Attempts to gain benefits of hypoxia at sea level (e.g., altitude masks, altitude chambers); no evidence supports the idea that brief periods (1-2 hr/day) of hypoxia induce even a partial adaptation similar to that observed at altitude

• Living in hypoxic apartment but training normally not yet scientifically validated

How can performance be optimized for athletes that live at sea level but must compete at altitude?

Two strategies:

1. Compete ASAP after arriving at altitude — no benefits of acclimation, but too soon for adverse effects of altitude to significantly impact performance

2. Train at high altitude for 2-3 weeks before competing at altitude — past the worst adverse effects of altitude, however, aerobic training at altitude will not be as effective

Acute altitude/mountain sickness

Very common at altitude, generally not life-threatening but can progress; due to low ventilatory response to altitude → accumulation of CO2 & acidosis

• Symptoms: headache, nausea/vomiting, dyspnea, insomnia (usually begin 6 to 48 h after arrival; most severe days 2-3)

• Prevention/Treatment: gradual ascent to altitude, acetazolamine (carbonic anhydrase inhibitor), artificial oxygen, hyperbaric rescue bags

High-altitude pulmonary edema (HAPE)

Life-threatening condition caused by altitude; likely related to hypoxic pulmonary vasoconstriction & clot formation in pulmonary circulation

• Symptoms: shortness of breath, cough, tightness, fatigue, decreased blood O₂, cyanosis, confusion, unconsciousness

• Treatment: supplemental O2, hyperbaric bag, immediate descent to lower altitude

High-altitude cerebral edema (HACE)

Life-threatening condition caused by altitude; complication of HAPE with edemic pressure buildup in intracranial space

• Symptoms: confusion, lethargy, ataxia, unconsciousness, death

• Treatment: supplemental O2, hyperbaric bag, immediate descent to lower altitude

Hyperbaric environments

Environments in which atmospheric pressure is greater than at sea level (e.g., deep sea diving)

• Boyle’s law: volume and pressure are inversely related

• Because pressure increases below sea level, volume of the lungs decreases

CV response to water immersion

• Decreased blood pooling in legs and increased venous return

• Increased SV

• Decreased HR (~10-12 bpm lower than in air)

Decompression sickness (the bends)

Life-threatening health risk of hyperbaric environment like deep-sea diving; ascending too rapidly causes nitrogen to be trapped as bubbles in the blood/tissues

• Symptoms: aching in elbows, shoulders, and knees

• Prevention: ascent SLOWLY

• Treatment: placement in decompression chamber (forces nitrogen back into solution)

Nitrogen narcosis (rapture of the deep)

Life-threatening health risk of hyperbaric environment like deep-sea diving; nitrogen acts as anesthetic gas

• Divers can develop CNS symptoms similar to alcohol intoxication

• Worsens with depth & time at depth

• Encouraged to dive in groups/duos and watch for symptoms

Microgravity

Any condition where gravitation force is less than 1 g

Consequences of extended time in microgravity

• Muscle atrophy die to decreased protein synthesis

• Loss of muscle strength

• Reduced muscle fiber capillary density

• Decreased bone mineral density

Effects of exercise in microgravity

Acts as a countermeasure…

• Reduces losses in bone mass

• Reduces losses in cardiovascular capacity

• Lessens the declines in muscle strength