Pulmonary function at altitude.
Pulmonary function studies are also performed at higher altitudes such as 15,000 feet as in the case of some cities in Peru and in the Himalayas. The effects on pulmonary physiology and pulmonary function at altitudes greater than 12,000 feet on persons born at sea level, persons visiting altitude, and on persons born and raised at high altitude or altitude natives, will be reviewed.
People born at high altitudes exhibit physiological differences from people who are recent arrivals or even long-term visitors. Diffusing capacity is a pulmonary function measurement that attracts the most attention in altitude studies. But keep in mind that the challenges of accurate and consistent measurement, especially when testing devices are temporarily carted up to obscure places.
Studies have shown that sea-level natives, or lowlanders, exhibit no change in diffusing capacity after living four weeks at high altitude (12,000 feet). Natives of that altitude, or highlanders, had significantly higher diffusing capacities. Non-smoking, healthy, young lowlanders living more than six months at altitude gain an average 5-percent increase in diffusing capacity when tested by single breath carbon monoxide, but it is lost within two week of return to sea level.
It has been suggested that the alveolar capillary membrane surface area in these individuals has been increased with long-term residence at altitude but is lost after returning to lower elevations. One could infer from that fact that a healthy, non-smoking lowlander living at high altitude has the capability to grow new alveoli. Interestingly, it has been shown that natives of 16,000 feet have significantly higher diffusing capacity than natives of 12,000 feet.
Diffusing capacity predicted values, as well as results, are slightly different between sea level and at altitudes of a little over 5,000 such as the case in Denver and Salt Lake City. Diffusing capacity will increase 0.35 percent for each 1 mm Hg drop in elevation and the increase in diffusing capacity that would be expected as the FI02 is decreased from 0.21 to 0.17 is 9 percent. Therefore, pulmonary function laboratories even at moderate altitudes should either use tank mixtures with the 02 component equal to 0.21.
Better yet, use predicted values based on diffusion capacities performed on at least 200 standard subjects at the particular altitude in question in order to obtain diffusing capacity predicted values appropriate for that particular altitude using the ambient FI02. Lung volumes, airway resistance and other pulmonary function measurements are not significantly different from those found at sea level, measured at comparative altitudes of 10,000 feet or less.
Another example of the advantages held by natives of altitude is demonstrated by the fact that lowlander Peruvian medical students increased their total hemoglobin by 20 percent after living six weeks at 16,000 feet, while highlanders there already had hemoglobin up to 65 percent higher than normal. There was also found to be a right shift in the hemoglobin dissociation curve in the highlander, which suggests that there may be a variation in the form of a long-term adaptation in the structure of the hemoglobin molecule. This structural change must be long-term adaptation, which is specific to that population and has become a component of the population's genome.
Non-genome adaptations for persons born at altitude beyond the example described above probably begin in utero. Studies show that puppies born at altitude show large volumes of blood in the lung tissue during gestation to the extent that scan data was difficult to accurately obtain due to the blood density shadows. This data corresponds with other studies showing that natives of 16,000 feet had approximately 60-percent larger blood volumes than from individuals observed at sea level. This increase in blood volume correlates with an increased pulmonary vascularization found in highlanders.
With lowlanders having acclimatized to altitude, as well as with the case of highlanders, the minute ventilation is increased. At very high altitudes, the work of ventilation or breathing becomes the limiting factor in maintaining a reasonable Pa02. The lower air density should lead to a decrease in airway resistance and decreased work of breathing together with increased gas flow within the bronchial tree.
On the other hand, the increased pulmonary blood volumes result in a decrease in static lung volumes due to the volume compression. To summarize a large body of research on persons from sea level acclimating to altitude, some studies at altitude demonstrate increases in VC, FRC and TLC. It should be pointed out that pulmonary function testing results demonstrate a significant degree of inter laboratory variation and factors to consider for accuracy, including testing device manufacturer, testing personnel and patient cooperation.
For natives of altitude from 12,000 feet and higher, studies demonstrate RV of 38-percent higher than normal while the VC remains the same in both groups resulting in an increased TLC for the highlanders. Other studies have shown a larger VC for high-landers, which was significantly higher than the VC of acclimated individuals.
From multiple studies performed in the Andes, it can be inferred that increased lung volumes of highlanders is an adaption as the result of hypoxia, which is not innate but develops during growth. On the other hand, persons having grown up at lower altitudes do not see changes in VC or TLC even after having lived at altitude for extended periods of time. In contrast, children born at lower altitudes that moved to higher elevation are found to share in larger lung volumes similar to highlanders.
Dynamic volumes and maximal flows are also found to be increased at altitudes of more than 12,000 feet. There is a general decrease in air resistance as a result of the lower density of air and also from the effects of bronchodilitation caused by hypoxia. The above effects result in an observed increased MVV found in the general high altitude population. These increases in MVV were found by numerous researchers to be has high as 24 percent.
There is a loss of elastic recoil of the lung during short-term exposure to altitude, and this may explain the increase in RV. The loss of elastic recoil at altitude would be expected to cause elastic recoil equilibrium of the respiratory system and, therefore, an increase in FRC.
In another words, the lung would be expanded during heavy breathing and would not recover back at each breath to the innate FRC and RV. Counteracting this effect is the increase in thoracic blood volume, which also tends to decrease thoracic volume, but this is not observed in highlanders. In conclusion, lung volume changes at any altitude in either highlander or visiting populations are probably not significant in either direction.
Barometric pressure should always be accurately entered into the testing device. To check on your machine's accurate measurement of barometric pressure, go online to a search engine and type in the name of your nearest local airport. There, you will see local current barometric pressures reported on the airport's Web site.
Acclimatization and its effects on pulmonary function results include the following adaptations for both the visitor and native at altitude. Hypoxia through a chemoreceptor mechanism increases the depth of alveolar ventilation, which can be increased by as much as 60 percent at higher elevations. This is an immediate response to oxygen debt.
A longer term adaptation for a lowlander is to increase the capacity for alveolar ventilation by up to at least 200 percent. This increased capacity is mostly achieved by an increase in minute ventilation mostly through increases in respiratory rate, as anyone who has climbed to elevation can attest to.
A related adaptation is the increased pressure in pulmonary arteries, forcing blood into portions of the lung that are normally not used while breathing at sea level. This adaptation along with the increased blood volume increases the A-a oxygen gradient, thus supplying more oxygen to the tissues.
Another adaptation, as previously mentioned, is that the body produces more red blood cells in the bone marrow along with an increase in blood volume in order to carry more oxygen. For a person raised at sea level, this process takes three to four weeks for maximum levels. Highlanders can have blood volumes up to 60 percent greater than a person at sea level. This increase in blood volume is possible because of a large increase in vascularization. Also at altitude the body produces more of the enzyme, 2,3-biphosphoglycerate, which facilitates the release of oxygen from hemoglobin to the body tissues.
In conclusion, there are not significant differences in lung volumes and flow rates between those at sea level, long-term visitors to altitude, and altitude natives. The main differences include having huge increases in blood volume, capillary diffusion and tidal volume, which result in slightly higher diffusing capacity and increased oxygen delivered to the tissues.
by Jim Harvey MS, RPFT, RCP
Jim Harvey MS, RPFT, RCP works in the Pulmonary Function Laboratory at Stanford Hospital and Clinics in Palo Alto, and teaches Pulmonary Function at Skyline College in San Bruno, California.
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|Title Annotation:||PULMONARY FUNCTION TESTING|
|Publication:||FOCUS: Journal for Respiratory Care & Sleep Medicine|
|Date:||Nov 1, 2009|
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