Hyperventilation results in a reduction in arterial CO 2 respiratory alkalosis that limits the increase in ventilation. Within several days, the kidneys compensate for the respiratory alkalosis by increasing excretion of bicarbonate.
Resting ventilation continues to rise slowly, reaching a maximum in about 1 week. With continued ascent, the process is repeated to a smaller degree with further increases in ventilation. Increased ventilation is the principal characteristic of acclimatization the adaptation to altitude exposure and is the primary mechanism responsible for improving the oxygen availability at the cellular level. Figure illustrates the relative changes in ventilation, end-tidal CO 2 partial pressure, and blood oxygen saturation over the course of 20 days at the summit of Pikes Peak 4, m [14, ft].
The increase observed in blood oxygen saturation is not entirely independent of ventilatory acclimatization. Some of the increase can be attributed to changes in lung parenchyma, circulation, erythrocytes, and metabolism Bender et al. Figure shows that the initial increase in cardiac output is caused principally by the increase in heart rate with little or no change in stroke volume. SaO 2 , arterial oxygen saturation; PetCO 2 , end tidal partial pressure of carbon dioxide. FIGURE Relationship between cardiac output, heart rate, and stroke volume during 14 days of acclimatization to 4, m 14, ft.
With continued exposure, stroke volume falls as a result of the reduced plasma volume and decreased venous return. After 1 week of exposure, cardiac outputs are essentially normal or only slightly reduced Hannon and Vogel, Stimulation of the sympathetic nervous system, principally secretion of norepinephrine, as a response to altitude exposure has been confirmed in many studies Brooks et al. The typical response of an increase in norepinephrine without a corresponding increase in epinephrine is depicted in Figure This increase is probably responsible for many of the cardiovascular and metabolic changes seen with acclimatization, such as augmentation of metabolic rate, lactate accumulation during exercise, and increased heart rate.
Norepinephrine increases may also be correlated with alterations in myocardial contractility, atrial natriuretic factor, angiotensin, and aldosterone Reeves et al. Exact indices of acclimatization are difficult to determine and are altitude-and time dependent. Some of the physiological variables used to delineate successful acclimatization include the plateauing of the rise in ventilation, the.
The degree of adaptability of the human organism to very high terrestrial elevations is best exemplified by the natives of the Andes mountains.
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From present-day observations and historical records of South American Indians, there is evidence that oxygen concentrations as low as High-altitude natives can apparently maintain a routine of working in mines at 5, m 18, ft and sleeping at lower elevations for extended periods of time without marked deterioration. For sojourners to high altitudes without any acclimatization, this level of exposure would result in marked deterioration and illness. The ability for newcomers to acclimatize adequately to a moderate altitude is dependent on the rate of ascent and the time allowed for acclimatization.
Figure illustrates the relative time, magnitude, and direction of changes at the respiratory, circulatory, and cellular levels with respect to their relationship to general health and performance at 4, to 4, m 14, to 15, ft. Several illnesses can occur with ascent to altitude.
Of these, only two are of real concern for the purposes of this book. A brief discussion of these illnesses is warranted because they affect nutrition. FIGURE Approximate size, direction, and temporal changes that occur during acclimatization to 4, to 4, m 14, to 15, ft.
AMS is the most common manifestation of the altitude illnesses. It is associated with rapid exposure of the unacclimatized individual to altitudes above 3, m 10, ft. Symptoms usually start after several hours of exposure. The illness is self-limiting, remitting over the course of 3 to 7 days, depending on staging temporary residence at an intermediate altitude and any changes in elevation.
Prominent manifestations of the disorder are listed in Table Sleep disorders characterized by periodic breathing are not uncommon. A factor not. The problem obviously persists with continued ascent due to the continued reduction in barometric pressure. The incidence of HACE is extremely low but requires immediate attention and medical treatment. From a nutritional standpoint, it should not be a concern for troops at high terrestrial elevations. Altitude exposure usually leads to significant weight losses in nonacclimatized individuals.
Table summarizes the weight losses of subjects from several selected studies conducted on high-altitude expeditions, in high-altitude field laboratories, and in hypobaric chambers. This weight loss has been attributed to the novelty of the environment, the effects of altitude-related illnesses, and to the factors shown in Figure These factors may or may not be related to altitude-induced illness. Weight loss appears to occur in those whose travel commences at sea level and stabilizes with acclimatization.
It does not occur to any appreciable extent in high-altitude natives. The degree of weight loss is apparently dependent on the altitude and the duration of stay at high altitudes. The direct relationship between the severity of hypoxia and body weight loss has not been demonstrated decisively for humans, but has been demonstrated in animals Schnakenberg et al.
With acclimatization, weight loss may not be observed unless the altitude is extreme, that is, above 5, m 16, ft.righvitotab.tk
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In fact, Pugh observed an increase in body weight in one subject on the — Silver Hut Expedition when that subject had a respite of 20 days at 4, m 14, ft after descent from 5, m 19, ft. Even in mountain laboratories at moderate altitudes where diet was controlled, there may not be evidence of an obligatory loss of weight Butterfield et al.
All of the factors shown in Figure can contribute to an overall loss in body weight in deployed soldiers who may not have the benefit of time for acclimatization that is afforded to mountain trekkers and climbers. Hypoxia-induced diuresis and subliminal thirst hypodipsia are usual occurrences in healthy subjects acutely exposed to altitudes similar to the summit of Pikes Peak 4, m [14, ft]. With individuals having symptoms of AMS, fluid retention that is independent of fluid intake has been observed.
These individuals are characterized by a gain in weight.
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Headache, the most prominent symptom of AMS, can be sufficiently severe to have a negative impact on food intake. It is estimated that food intakes are decreased by as much as 25 to 50 percent during the first 3 days of acute exposures to 4, m 14, ft , the period when illness symptoms are at their peak Consolazio et al.
Dysbarism is a symptom complex resulting from exposure to decreased or changing barometric pressure unaffected by hypoxia. Although decreased energy intake per se can be associated with gastrointestinal upset, gas expansion above 3, m 11, ft intensifies the degree of distress. The expansion of intraluminal gas, which can be caused by both gas expansion and a high-fiber mountain diet, may be very disconcerting and even painful Hultgren, If diet and constantly changing altitudes are continued for any length of time, there can be adverse effects on food consumption. In addition to a possible direct effect on brain centers controlling appetite, changes in menu inherent in military mountain field studies usually result in reduced dietary nutrient intake.
Although palatability of food may have been a factor with early dietary studies, a caloric deficit secondary to appetite suppression anorexia may still occur when the best possible food is available. Edwards et al. They concluded that despite the availability of ample, well-cooked food, soldiers with high rates of energy expenditure failed to maintain their body weights and lost approximately 1.
The effect of solid carbohydrate supplementation was not significant. Even when the diet may not be as palatable as in the Edwards study, Butterfield et al.
In a short review of nutrition and altitude exposure, Kayser et al. The implication from the latter two studies is that there is no dysfunction of nutrient absorption at altitudes up to 5, m 16, ft ; rather, energy intakes are voluntarily reduced for some unexplained reason. The result is that with continued residence at a constant, moderate altitude such as 4, m 14, ft and no illness symptomatology, there may be progressive loss of body weight in a military population performing tasks requiring a high level of exertion.
When military operations require ''living off the land," the problems associated with ingestion of indigenous foods are common and well known. Often the basic principles of sanitary food preparation are dismissed when time is limited and basic supplies are in short supply or rationed. Proper sterilization of water is also often neglected when the supply is thought to be clean because of remoteness.
This type of negligence usually results in a wide range of gastrointestinal problems such as parasitic infection, anorexia, and diarrhea. The discordance in energy intake and expenditure is evident in military field situations irrespective of the terrestrial elevation. This expenditure could produce energy deficits as high as 31 percent Moore et al.
In two different studies of soldiers on field training exercises at moderate terrestrial elevations 2, to 3, m [7, to 10, ft] , Hoyt et al. This level of energy expenditure resulted in body weight losses of 2. It appears that body weight losses, especially at altitude, are inevitable in field studies involving high levels of strenuous exertion, regardless of the availability of food. A more definitive discussion of energy expenditure is provided by Hoyt and Honig see Chapter 20 in this volume.
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Losses in sensible and insensible body water due to working in a high, dry environment can have serious consequences on the ability to function and survive. Insensible water losses are discussed in the section on water. Losses via both avenues, however, will occur at altitude under conditions that are high-hot and high-cold and that result in voluntary dehydration—the difference between water intake and water losses.
Especially when exertion is involved, voluntary dehydration will be a universal phenomenon at altitude. The condition occurs soon after exposure and probably lasts for an indefinite period of time. The loss of total body water combined with fluid shifts within the extravascular compartment can interfere with the standard measurements of energy expenditure and body composition Fulco et al. Relative hypodypsia—decreased water intake—can be as much as one-half of the fluid loss with active periods in a hot, dry environment Adolph et al.
It is anticipated that it could be higher under high-altitude conditions. Some studies have suggested a malabsorption of nutrients from the gastrointestinal tract at altitude.
Most of these studies have involved mountaineering expeditions and altitudes in excess of 5, m 16, ft. For example, Pugh found visual evidence of steatorrhea excess fat in the stools at 4, m 14, ft , and Boyer and Blume found increased fecal fat content at 5, m 17, ft.
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However, the consensus of findings is that fat digestibility is normal up to altitudes of 5, m 16, ft. The same conclusion has been drawn for carbohydrate absorption and protein Chesner et al.
Both Butterfield et al. When analyzing the results of nutrition studies at altitude, it may be difficult to compare and interpret the conclusions. Boyer and Blume noted that weight loss on expeditions may be due to the fact that climbers will "bulk up" before expeditions in anticipation of increased energy expenditure and that different genetic and cultural factors may be operative.
Sherpas, members of the Tibetan people living on the high southern slopes of the Himalayas, do not undergo the same body composition changes as do Caucasians above 5, m 17, ft. Statistical problems are also encountered when subject numbers are so low that including or excluding one aberrant subject is sufficient to place the significance of changes in doubt.
In conclusion, at moderate altitudes there appears to be no problem in the digestion and absorption of nutrients from the gastrointestinal tract.