ALTITUDE AND COMMON MEDICAL CONDITIONS - Medicine for Mountaineering and Other Wilderness Activities - page 377

ary artery) is 10 to 15 mm Hg. Such low oxygen pressures are normal for a fetus and this
“low oxygen” environment is remarkably stable. The shape of the fetus' oxygen dissoci-
ation curve and the high affinity of its hemoglobin for oxygen minimize changes in mater-
nal arterial oxygenation.
The placenta acts as a buffer in several ways: It maintains a constant carbon dioxide
gradient (10 mm Hg), is relatively impermeable to bicarbonate ions (to protect the fetus
from maternal changes in acid-base balance), and maintains the fetal oxygen environment.
Additional changes that maintain fetal stability include increased ventilation by the mother
(even in high-altitude residents who normally are insensitive to chronic hypoxia) and in-
creased oxygen extraction by the fetus. As a result, oxygen consumption remains stable
evenwhenstressedwitha50percentreductionineitherplacentalbloodfloworbloodoxy-
gen content. (The fetus must avoid a high PO 2 because that triggers the drop in pulmon-
ary vascular resistance and closure of the ductus arteriosus that normally occurs following
birth.)
Given a stable fetal environment, what degree of maternal hypoxia can be detrimental,
and what evidence is there for compromise of the lowland fetus or mother upon ascent to
high altitude?
Studies of High-Altitude Residents: Implications for Pregnant Lowlanders
In native residents at an altitude of 6000 feet (1830 m) umbilical cord arterial and ven-
ous oxygen tensions are the same as at sea level. A slightly lower carbon dioxide pres-
sure reflects mild maternal hyperventilation. At altitudes over 10,000 feet (3000 m), fetal
response to hypoxia is evidenced by an increased hematocrit (2 to 3 percent higher), in-
creased fraction of fetal hemoglobin, and increased erythropoietin in the umbilical cord
blood.
Studiesfromalltheworld'shigh-altituderegionshaveidentifiedvariouseffectsofhigh-
altitude residence on pregnancy. The best documented is intrauterine growth retardation,
which leads to healthy, full-term infants that are small for gestational age. Where soph-
isticated medical care is accessible, intrauterine growth retardation is not associated with
increased morbidity or mortality unless the infants are also premature. Intrauterine growth
retardation, however, does indicate an apparent effect of hypoxia on the developing fetus.
Similar reductions in birth weight are seen with mothers whosmoke, but unlike the alti-
tude infants, the infants of mothers who smoke have higher perinatal mortality at every
birth weight. This important difference indicates that effects on fetal growth can occur in-
dependently of effects on mortality and also that smoking produces more abnormalities
that affect fetal growth and wellbeing than just reduced oxygen transport. Reduced size for
gestational age at sea level is also associated with pre-eclampsia, maternal hypoxic lung
disease, maternal cyanotic congenital heartdisease, andvariousanemias, allofwhichhave
diminishedfetaloxygen/nutrientdeliveryasacommonpathway.Unlikesomeoftheseoth-