37: Duration of maternal oxygen exposure and umbilical cord oxygen content


      Maternal oxygen (O2) administration is a commonly performed intrauterine resuscitation technique. O2 is frequently administered for several minutes, sometimes hours, in order to maximize fetal oxygenation. We tested the hypothesis that a longer duration of O2 exposure is associated with higher umbilical cord pO2.

      Study Design

      This is a planned secondary analysis of a randomized noninferiority trial comparing O2 to room air (RA) in laboring patients. Patients were randomized to 10 L/min O2 or RA at any point in active labor when they developed Category II EFM. Interventions were generally continued until delivery. The primary outcome for this analysis was umbilical vein (UV) pO2. A secondary outcome was umbilical artery (UA) pO2. These were compared between patients with short and long durations of O2 exposure, defined as < 75th and ≥75th percentile of the duration of exposure in O2 arm, respectively. Outcomes were also compared between RA, short O2, and long O2 groups.


      Of the 114 randomized patients, 99 patients (48 O2 and 51 RA) had paired cord gases and were included in this analysis. Among O2 patients, the median duration of exposure was 96 minutes (IQR 47, 176). The 75th percentile for duration of exposure was 176 minutes. UV pO2 was lower in patients who received long durations (≥176 minutes) of O2 compared to those who received shorter durations (< 176 minutes) (median [IQR] 25.5 [21.5,33] vs 32.5 [26.5, 37.5] mm Hg, p=0.03) (Figure 1). There was no difference in UA pO2 between short and long duration O2 groups (Figure 2). There was no difference in UA or UV pO2 between RA, short duration O2, and long duration O2 groups (Table 1).


      Long durations of O2 exposure are not associated with higher cord pO2. In fact, patients with longer O2 exposure had lower UV pO2, suggesting impaired placental O2 transfer, possibly mediated by hyperoxia-induced placental vasoconstriction.
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