It has been estimated that 2 of 3 women in labor in the US receive oxygen supplementation for nonreassuring fetal heart rate patterns at some point during labor.1 During the course of labor, maternal-fetal oxygen delivery is interrupted by uterine contractions and ceases when the contractions exceed 40 mm Hg in intensity, a level typically exceeded many times before delivery. This hypoxic stress results in the development of significant metabolic acidemia even under normal conditions, with the umbilical artery pH decreasing from a prelabor mean of approximately 7.38 to a mean pH of 7.25 at the time of delivery.2 These changes occur despite numerous mechanisms that serve to mitigate the degree of tissue hypoxia and associated metabolic acidemia resulting from the process of labor, including a relatively high fetal hemoglobin level, enhanced oxygen-carrying capacity and tissue delivery by hemoglobin F, and high levels of fetal 2,3-disphosphoglycerate.
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Pammi M, Clark SL, Shamshirsaz AA. Intrapartum Maternal Oxygen Supplementation—Friend or Foe? JAMA Pediatr. 2021;175(3):236–237. doi:10.1001/jamapediatrics.2020.5363
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