CASE Heart rate tracing suggests fetal distress
Ms. M. presents for elective induction of labor at 39 weeks’ gestation. During the course of her labor, a Category II fetal heart rate (FHR) tracing is noted, and maternal oxygen is administered as part of the intrauterine resuscitative efforts. Her infant ultimately was delivered vaginally with an arterial cord blood pH of 7.1 and Apgar scores of 5 and 7.
Should intrauterine resuscitation include maternal oxygen administration?
It is a common sight on labor and delivery: An FHR monitoring strip is noted to be a Category II tracing. There may be fetal tachycardia, late decelerations, or perhaps decreased variability. The nurse or physician goes to the laboring mother’s room, checks cervical dilation, changes the patient’s position, and puts an oxygen mask over her face.
The American College of Obstetricians and Gynecologists (ACOG) lists maternal oxygen administration, most commonly at 10 L/min via a nonrebreather face mask, as an intrauterine resuscitative measure for Category II or Category III FHR tracings.1 Maternal oxygen is used to treat abnormal FHR tracings in approximately half of all births in the United States.2 Despite these recommendations and the frequency of its use, however, evidence is limited that maternal oxygenation improves neonatal outcome. In fact, there is emerging evidence of potential harm.
Why use oxygen?
The use of maternal oxygen supplementation intuitively makes sense. We know that certain abnormalities in FHR tracings can signal fetal hypoxia. Left untreated, the hypoxia could lead to fetal acidemia and associated neonatal sequelae. Theoretically, the administration of maternal oxygen should lead to improved fetal oxygenation and improved fetal outcome. This is supported by studies from the 1960s that demonstrate improved FHR tracings after maternal oxygen administration.3
This idea was further supported by studies that demonstrated an increase in fetal oxygen levels when maternal oxygen is administered. Haydon and colleagues evaluated the administration of maternal oxygen in women with nonreassuring FHR tracings.4 Their data showed that maternal oxygen administration increased fetal oxygen as measured by fetal pulse oximetry. The lower the initial fetal oxygen levels prior to oxygen administration, the greater the increase.
Despite these findings, evidence for improved neonatal outcomes is lacking.5 While heart rate tracings and fetal oxygen saturation may be improved with maternal oxygen supplementation, neonatal morbidity appears to remain unchanged (FIGURE). In fact, newer research suggests potential harm. Although an improved FHR tracing may be comforting to the clinician, the end result may be less so. Given these findings on maternal oxygen supplementation, it is time to break this practice habit.
Maternal cardiovascular effects
Most of the literature on maternal hyperoxygenation focuses on fetal response. Before examining the effects on the fetus, however, we must consider the effect on the mother. Cardiovascular changes occur during and after maternal oxygen administration that should be taken into account.
McHugh and colleagues measured the hemodynamic changes in 46 pregnant and 20 nonpregnant women before, immediately, and 10 minutes after a 30-minute period of high-flow oxygen administration.6 While there were no changes in the nonpregnant women’s parameters, in the pregnant women heart rate and stroke volume were decreased after oxygen administration. Additionally, systemic vascular resistance increased and did not return to baseline by 10 minutes postadministration.
Since the purpose of the maternal oxygen administration is to increase oxygen to the fetus, this decrease in cardiac output and increase in systemic vascular resistance is concerning. These results may negate the intended effect of increased oxygen delivery to the fetus.
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