As altitude increases above sea level, atmospheric (or barometric) pressure drops with a parallel decrease in the amount of oxygen available at the blood/air interface in the lung alveolus. Hypoxia (a low blood oxygen level) occurs and results in a decrease in the amount of oxygen delivered to the cell to do physical work. Although the heart rate (and thus the cardiac output) increases to deliver more blood (with less oxygen per ml) to the cell, complete compensation does not occur and
the maximal aerobic ability (VO2 max.) is reduced by approximately 1% for every 100 meters (~ 300 feet) above 4500 feet in recreational athletes and can be detected in highly trained athletes at altitudes as low as 1500 feet above sea level.
Other adaptive changes (acclimatization) include a higher ventilation (respiratory or breathing) rate and a higher blood lactate level for any level of submaximal exercise, both of which increase the sensation of dyspnea (shortness of breath) and fatigue. Some acclimatization responses occur immediately while others may take 4 to 6 weeks.
In addition to decreases in maximal aerobic capacity, acute mountain sickness (AMS) affects, to varying degrees, all travelers to high altitudes (elevations greater than 5280 feet). In a small percentage of patients, AMS can lead to high-altitude pulmonary edema (HAPE) or high-altitude cerebral edema (HACE). Symptoms of AMS range from a combination of headache, insomnia, anorexia, nausea, and dizziness,to more serious manifestations, such as vomiting, dyspnea, muscle weakness, oliguria, peripheral edema, and retinal hemorrhage.
Although the primary cause of these symptoms is related to the reduced oxygen content and humidity of the ambient air at high altitudes, the physiologic pathway relating hypoxemia to AMS and its sequelae remains unclear. Tips on self-diagnosis and symptom recognition are critical elements to be included in educating patients who are contemplating a trip to high altitudes.
Short term physiologic responses to altitude
The most immediate response to altitude is the hyperventilation that occurs in response to a decrease in arterial oxygen levels above 2000 meters. And this increased respiratory rate can remain elevated for up to a year at altitude. The hyperventilation response varies from individual to individual. Those with a strong hypoxic drive will perform exercise tasks better at altitude than those with a blunted ventilatory response.
There is also an increase in the resting heart rate and cardiac output. The increase in blood flow compensates for the decreased blood oxygen concentration and leaves the total amount of oxygen delivered to the muscles unchanged. However, the fact that there is always less oxygen available means that even with the compensatory increase in heart rate and blood flow, the level of exercise at which oxygen demands are unmet and metabolism becomes anaerobic (VO2 max.) will always be less than at sea level.
Long term adjustments to altitude
Hyperventilation and the increased cardiac output provide an immediate response to limit the effects of altitude on physical performance. With time, a change in the body’s acid-base balance counters the effects of a chronically lower blood CO2 from hyperventilation (respiratory alkalosis), but does not affect physical performance to any significant degree.
An increase in the blood hemoglobin (hematocrit) level increases the oxygen carrying capacity of the blood and is the most important performance adaptation to altitude. The result is that every milliliter of blood that moves through the muscle capillaries will be able to deliver an increased amount of oxygen compared to the same volume of blood with a sea level hematocrit.
Finally, there are cellular changes that favor oxygen delivery to the muscle cell. The capillary concentration in skeletal muscle is increased in animals living at altitude compared to those at sea level, and muscle biopsies in acclimatized men have demonstrated an increase in myoglobin, mitochondria, and metabolic enzymes necessary for aerobic energy transfer. These changes should improve the efficiency of oxygen delivery and extraction at the muscle cell level.
Together these adaptations are sufficient to restore exercise capacity to NEAR sea level values at altitudes up to 2500 meters (7500 feet). At higher elevations, acclimatization is not sufficient to restore VO2 max. to normal.
But not all the changes that occur with acclimatization are favorable to improve athletic performance in the face of a decrease in available oxygen. One notable negative is the loss of lean body mass and body fat that occurs with long term exposure to high altitudes. The result is a decreased maximum potential for athletic performance because of decreased muscle mass.
The time course of acclimitization
As mentioned, the ventilatory response begins immediately upon climbing to altitude from sea level and continue over several days at altitude. Hyperventilation changes the blood acid base balance (with a respiratory alkalosis) which in turn stimulates the kidneys to excrete bicarbonate to compensate. This renal compensatory response takes about a week.
The sympathetic nervous system is activated almost immediately with an increase in both sympathetic nerve activity and an increase in blood epinephrine levels - resulting in an increase in heart rate and cardiac output to maintain tissue oxygen delivery at near sea level values. By two to three weeks, blood flow returns toward sea level values as oxygenation improves as a result of the other compensatory mechanisms.
The hematocrit level increases within 24 to 48 hours because of a reduction in plasma volume, not an increase in red cell mass. Erythropoietin levels increase within hours, peak at about 48 hours, and remain elevated for 1 to 2 weeks. The red cell mass increases slowly and may take several years to reach levels equal to natives living permanently at these altitudes.
The vast majority of these metabolic changes are complete by 3 to 4 weeks at altitude, but the structural changes (capillary density, mitochondrial number) take weeks to months to complete.
Does hypoxic exercise at altitude provide a training benefit? This is controversial, but controlled studies in trained athletes have not been confirmed any benefit for hypoxic exercise WITHOUT CONCOMITANT ACCLIMATIZATION.
And the direct effects of interval training to stress and improve an athlete's maximum aerobic capacity (VO2 max.) definitely deteriorate with training at elevation as a result of the inability to maintain a VO2 max. comparable to sea level when training in a hypoxic environment. During interval work outs, speed, oxygen uptake, heart rate, and lactate levels are all lower than those from lower altitudes suggesting that interval training is best performed as near sea level as possible.
Does exercise training at altitude improve sea level performance?
Many scientists, athletes, and coaches have been intrigued by the similarities of altitude acclimatization and training effects. Does living and training at altitude (with the associated changes in red cell mass and cellular changes in mitochondria, etc.) lead to an increase in the maximal aerobic exercise capacity (VO2 max.) upon return to sea level? The answer is "it depends". It is the net balance between the benefits of the acclimatization effects and the negatives of a reduction in training intensity and deconditioning from hypoxia that are the ultimate determinate of the outcome of altitude training in endurance athletes. Controlled studies have NOT shown any advantage of TRAINING at altitude compared to a similar TRAINING program (the same absolute VO2 max. being achieved at both altitudes) at sea level.
Are there any strategies that can use altitude to benefit a training program?
The answer to this question is YES. But it requires balancing the acclimatization benefits of an increased red cell mass from living at altitude (one must be at altitude for more than 12 hours a day to maintain an increase erythropoietin level) while maintaining a VO2 max. in training equivalent to that possible at sea level.
How high must one live to maximize acclimatization? An altitude of 2500 to 2800 meters maintains a balance between stimulating erythropoietin and minimizing the effects of acute mountain sickness that occur with increasing frequency at higher elevations.
How long should one live at altitude to maximize benefits?? At least 3 to 4 weeks.
How long will the acclimatization effects last? Based on actual performance studies, 2 to 3 weeks at most before they begin to reverse.
And the optimal training altitude? Although this should be individualized as some athletes do quite well maintaining a high VO2 max training at high altitudes, the general rule is to train as close to sea level as possible, preferably below 1500 meters.
So it is the balance between acclimatization and deconditioning that gives the personalized answer for each individual athlete. A few can maintain a high training VO2 max. even while training at altitude enabling them to live at altitude and train there as well. But the vast majority need to descend to train several times a week or face a competitive disadvantage from deconditioning.
THE BOTTOM LINE
Altitude can be used to improve sea level performance. But it needs to be used correctly. Its advantages are related to acclimatization effects i.e. an increase in the red cell mass from 2 to 3 weeks at altitude. The same benefits could be gained from using injections of erythropoietin if it were not a banned substances (and one with some health risks as well from overzealous use and exceedingly high hematocrits). Blood doping has the same effects. And it has been suggested that living (or sleeping for more than 12 hours a day) in a high altitude chamber or using nitrogen houses as the Scandinavians have proposed (and utilized) may have the same beneficial effect.
But to maximize the benefits of the altitude effect, training (i.e. absolute VO2 max.) needs to be maintained at sea level values. Some athletes can train at altitude and pull this off, but the majority need will need to do interval training at least twice a week at sea level oxygen levels to avoid the offsetting disadvantages of deconditioning.
Altitude effects on performance are a complex issue, but are best summarized in the simple phrase:
Is there any way to avoid the hassles of traveling to a lower elevation to train - gaining the advantages of the hypoxia of altitude to acclimatize during the majority of your day (and while sleeping at night) while maintaining a high level training program?
The scandinavians reportedly live in a "nitrogen" house which lowers the ambient oxygen level during sleep and the portion of the day they spend there (and training is as easy as stepping out the door), while others have suggested sleeping in an altitude chamber. Another option that seemed to make sense to the author was living at altitude and using supplemental oxygen while training to raise the amount of oxygen available to the alveoli in the lung. This question was addressed to Dr. Ben Levine who has done the majority of the work leading up to the high-low theory of training.
His response:
Dear Dr. Rafoth,
Thanks for your note. You are absolutely right that an alternative to travel for high-low is training high with supplemental O2. In fact, this is exactly the tack taken by US Cycling and US Swimming at Colorado Springs. It is a bit cumbersome, but as long as the workouts can be reproduced, will work fine.
Ben Levine
What should an athlete do to prepare for competiton at altitude ?
For endurance events, adequate time should be allowed to complete acclimatization - 2 to 3 weeks. The longer one waits, the more deconditioning of the VO2 max. that occurs. Returning to sea level to do interval training several times a week would be a definite advantage but is usually impractical.
For sprints (400 meters or less) most of the energy for muscular activity is oxygen independent and acclimatization will not be of any benefit. And the lower air resistance at altitude will increase race times - that is why the 400 meter events were very fast in Mexico City in 1968 but the longer 1500 meter results were slower than at sea level.
Preventive strategies include allowing 2 days of acclimatization before engaging in strenuous exercise at high altitudes, avoiding alcohol, and increasing fluid intake. A high-carbohydrate, low-fat, low-salt diet can also aid in preventing the onset of AMS.
Although slow ascent is the preferred approach to avoiding AMS, there are times when this is impractical (plane connections to the start of a ride, emergency situations). In those cases, there are medications available that can decrease the chances of developing AMS. Acetazolamide (250 mg twice daily or 500 mg slow release once daily), taken before and during, ascent is recommended by many physicians although dexamethasone (4 mg, 4 times daily) has been shown to be of equal effectiveness. And in one study, those on acetazolamide actually had more symptoms of nausea at low altitudes (where AMS was not an issue) than a placebo group.Nausea was not a problem for those using dexamethasone, and indeed a mild euphoria was often reported. The usual recommendation for both medications is to start 24 hours before going to altitude and then continuing for 48 hours after starting the ascent. By that time, normal adaptive mechanisms should have had time to take over.
As dexamethasone is faster acting than acetazolamide, some authorities suggest taking the dexamethasone along, but starting it only when and if symptoms develop. As severe AMS is uncommon, this eliminates the inconvenience (and possible drug allergy or intolerance) of a medication that might not be needed.