Physiologic Effects of Altitude
Moderate, high, very high, and extreme high altitude are defined as follows:
Moderate altitude = 5000-8000 ft (1524-2438 m) above sea level
High altitude = 8000-14,000 ft (2438-4267 m)
Very high altitude = 14,000-18,000 ft (4267-5486 m)
Extreme high altitude = 18,000-29,028 ft (5486-8847 m)
In America, migration to the Western Mountain states has increased the number of children living at moderate-to-high altitudes. In isolated geographic areas of the world, adaptation to altitude has occurred over many generations. However, the population living in the United States is genetically mixed and has varied responses to the added stress of altitude-induced hypoxia.
As the altitude increases, barometric pressure decreases. This decrease in barometric pressure affects the partial pressure of alveolar oxygen (PAO2). The percentage of oxygen remains stable at about 21%. At sea level, the partial pressure of oxygen available in the environment is equal to 0.21 times the barometric minus the water vapor pressure (ie, (760 – 47 mm Hg)* 0.21, or 149 mm Hg). PAO2 is 103 mm Hg.
PAO2 is calculated by using the alveolar gas equation, as follows:
PAO2 = FiO2 (PB – PH2 O) – PACO2 [FiO2 + (1 – FiO2/R)],
In this equation, FiO2 is the fraction of inspired oxygen, PB is the ambient barometric pressure, PH2 O is the pressure exerted by water vapor at body temperature, PACO2 is the alveolar partial pressure of carbon dioxide, and R is the respiratory exchange quotient. The decrease in barometric pressure with increasing altitude reduces PAO2. PAO2 decreases from 103 mm Hg at sea level to 81 mm Hg in Denver, Colorado (5280 ft [1609 m]) and to 48 mm Hg at the top of Pikes Peak (14,110 ft [4301 m]). In mountain areas popular with vacationers, such as Leadville, Colorado (10,200 ft [3109 m]), the PAO2 is 61 mm Hg.
Pneumonia, asthma, bronchiolitis, neonatal lung disease, pulmonary edema and various other pulmonary diseases impair the efficiency of oxygen transfer from the alveolus to the pulmonary capillaries through ventilation-perfusion (V/Q) mismatch. Therefore, infants and children with pulmonary disease may have lower partial pressure of arterial oxygen (PaO2).
Further decrements in PAO2 due to altitude result in proportionate decreases in the PaO2. Thus, infants and children with pulmonary disease may have a PaO2 on the steep slope of the oxygen dissociation curve. As a result, small changes in the PaO2 cause large changes in arterial oxygen saturation (SaO2). In infants and children with pulmonary disease who live at moderate altitudes, changes in oxygen saturation can be observed, even as the barometric pressure falls with passing storm systems.
Newborns living at moderate altitudes have remarkably similar oxygen saturations during the first 24-48 hours of life. In Denver, Colorado, newborns younger than 48 hours have saturations of 85-97%; in Leadville, Colorado, saturations during the first 24 hours are 85-93%. Afterwards, the range widens. This change probably reflects a variable adaptive response to the transition from a fetal circulation to an adult circulation.
In Leadville, saturations in 1-week-old newborns are 83-93% during wakefulness and decrease to 75-86% during quiet sleep. By age 4 months, these values increase to 89-93% and 81-91% during waking and sleeping periods, respectively. Oxygen saturation values for healthy awake infants younger than 2 years are 89-94% in Colorado’s Summit County ski area (9000 ft [2743 m]) and 90-99% in Denver.
Newborns living at moderate altitudes are often sent home from the hospital with low-flow oxygen (25-50 mL/min given by nasal cannula) for 2-6 weeks to keep their oxygen saturations at an arbitrary level (>90%) for more than 90% of the time. This treatment may be unnecessary, but it is given to mimic sea-level oxygenation and to promote the transition from fetal to adult physiology.
Physicians who care for infants and children with borderline oxygen saturations at their local altitude must consider these changes when they advise parents about travel to a high elevation.
Pregnant women may also benefit from discussing travel to high altitude with their physician. Acetazolamide should generally be avoided and travel may be contraindicated in situations such as preeclampsia. While the low number of studies done in this population make specific recommendations difficult, a recent review by Jean et al provides an excellent overview of the available data.
Physicians who practice at altitude should be aware of the normal for their population. When practicing medicine at altitude with limited supplies of oxygen, children with saturations of < 85% may be those who benefit most from limited resources.