Maternal oxygen (O ₂) administration for Category II electronic fetal monitoring (EFM) is a widely used intrauterine resuscitation technique, despite a paucity of evidence on its ability to improve EFM patterns. We investigated the effect of intrapartum O ₂ administration on Category II EFM patterns.

This is a secondary analysis of a randomized trial conducted from 2016-2017, in which patients ≥ 37 weeks in active labor with Category II EFM were assigned to 10 L/min O ₂ by facemask or room air (RA) until delivery. Trained obstetric research nurses blinded to allocation extracted EFM data. The primary outcome was a composite of high-risk Category II features including recurrent variable decelerations, recurrent late decelerations, prolonged decelerations, tachycardia or minimal variability 60 minutes after randomization to RA or O ₂. Secondary outcomes included individual components of the composite, resolution of recurrent decelerations within 60 minutes of randomization and total deceleration area. Outcomes were compared between RA and O ₂ groups using univariable statistics. Time-to-event analysis was used to compare time to resolution of recurrent decelerations between groups. Paired analysis was used to compare pre- and post- randomization outcomes within each group.

All 114 randomized patients (57 RA, 57 O ₂) were included in this analysis. There was no difference in resolution of recurrent decelerations within 60 minutes between O ₂ and RA groups (75.4% vs 86.0%, p 0.15). RA and O ₂ groups had similar rates of composite high risk Category II features including recurrent variables, recurrent lates, prolonged decelerations, tachycardia, and minimal variability 60 minutes after randomization. Time to resolution of recurrent decelerations and total deceleration area were similar between RA and O ₂ groups. Among patients who received O ₂, there was no difference in EFM features pre- and post- randomization. Similar findings were observed in pre- and post- EFM features in RA patients.

Intrapartum maternal O ₂ administration for Category II EFM did not resolve high risk Category II features or hasten the resolution of recurrent decelerations. These results suggest that O ₂ administration has no impact on improving Category II EFM patterns.