Oxygen is routinely administered either
prophylactically during second stage or in response to a
non-reassuring fetal heart rate (FHR) tracings. The current
evidence on benefits and risks of supplementary oxygen
questions this practice and latest guidelines have
recommendations on indications for maternal oxygen therapy.
This review article focuses on the latest evidence and
guidelines.
Introduction
Estimates show that up to two third of women in labour are
given oxygen in response to a category II fetal heart rate
(FHR) tracing.[1] This practice came into vogue after a few
studies done several decades back showed an improvement in
fetal oxygen saturation (FSpO
2).[2,3] But the
recent evidence challenges this dogma and fails to show
improved fetal outcomes with random oxygen supplementation.
Reviewing the physiological principles of oxygenation in a
normal mother and fetus makes us understand the futility of
this practise. Critical evaluation of the recent trials
further throws light on the lack of benefit and plausible
adverse effects of maternal oxygen administration.[4]
Physiology of oxygenation in mother and fetus
A normal adult is already operating on the plateau portion of
oxygen dissociation curve (ODC) and arterial partial pressure
of oxygen (PaO
2) can be minimally increased with
further oxygen administration. The fetal haemoglobin (HbF) has
much higher affinity for oxygen as compared to adult
haemoglobin. Fetal oxygen supply is much higher than the
demand and compromise does not occur unless the supply drops
by 50%. Maternal hyperoxia shifts the carbon dioxide
dissociation curve downward reducing the ability to transport
carbon dioxide. Though an increase in maternal PaO
2
can increase umbilical PaO
2, because of the
significant shunting at placental level and high oxygen
consumption of placenta, fetal PaO
2 fails to
increase proportionally.[5]
Does maternal oxygen supplementation increase fetal
saturation?
The studies done forty years back had shown a beneficial
effect on FSpO
2 with oxygen administration, but
these studies had several limitations of having small sample
size, being nonrandomised and including mothers with hypoxia.
Recent studies show alternate results. A Cochrane systematic
review concluded that there was insufficient evidence to
support the use of oxygen as prophylaxis or for fetal distress
in laboring women.[6] Raghuraman et al,[7] conducted a
noninferiority trial to compare oxygen and room air for
resuscitation in mothers who developed Category II FHR
tracings and measured the umbilical artery lactate levels at
birth. They concluded that room air was noninferior to oxygen
for resuscitation in Category II FHR tracings. A secondary
analysis of the same study monitored the FHR tracings 60
minutes prior to and after oxygen or room air administration.
The primary outcome was a composite of high-risk category II
features. The time to resolution of deceleration and total
time spent in deceleration was also measured. They concluded
that intrapartum maternal oxygen administration was not
associated with an improvement in high-risk category II FHR
tracings or resolution of recurrent decelerations as compared
to room air.[8]
Does maternal oxygen supplementation increase fetal brain
oxygenation?
Improving the fetal brain oxygenation is the ultimate aim of
maternal oxygenation. Though oxygen supplementation does
increase the fetal PaO
2, does it really improve
oxygen supply to the brain. Blood oxygen level dependent
magnetic resonance imaging (BOLD MRI) can show increase in
signals in proportion with the oxygenation level of each
tissue. Sorenson et al,[9] studied the fetal BOLD MRI in
sheeps given oxygen and found that increased oxygenation was
observed in fetal side of placenta and several other organs,
but brain oxygenation did not increase. These findings were
confirmed by the study done in humans by Huen et al,[10] and
these results can help explain why fetal pulse oximetry has
been unreliable at predicting fetal well being and outcomes.
Does maternal oxygen supplementation improve fetal
acidosis?
The effect of maternal oxygenation on fetal acidosis is a
controversial matter with studies showing mixed results. Some
studies have shown no effect and some have shown a worsening
of acidosis with oxygen. The time dependent placental
vasoconstriction induced by oxygen might explain the worsening
of acidosis observed in some studies.[11,12]
Does maternal oxygen supplementation decrease rate of
caesarean / operative vaginal deliveries?
A meta-analysis included 5 studies which looked into the
effect of intrapartum oxygen on rate of caesarean sections
(CS). They concluded that when compared to room air
intrapartum oxygen was not associated with a reduction in
rates of caesarean for non-reassuring FHR, operative vaginal
delivery, APGAR score <7, Newborn Intensive Care Unit
admissions and cord blood acid base values.[13]
Does maternal oxygen supplementation cause any adverse
maternal effects?
McHugh et al,[14] measured the hemodynamic parameters before,
at completion and 10 minutes after cessation of
hyperoxygenation. They included both pregnant and nonpregnant
females. They observed a fall in cardiac index and rise in
systemic vascular resistance which did not recover even after
oxygen was stopped. Concomitantly there was a dip in stroke
volume and rise in heart rate which recovered once oxygen was
stopped. These changes were not observed in nonpregnant
patients. These adverse hemodynamic changes could negate the
beneficial effect contributed by the small increase in fetal
PaO
2.
Does maternal oxygen supplementation cause oxidative stress
to fetus?
One major adverse effect of supplemental oxygen is the
generation of reactive oxygen species which can cause maternal
and fetal cellular damage. The antioxidant mechanisms are not
fully developed in fetuses which makes them more vulnerable to
the oxidative damage. Khaw et al,[15] observed an increase in
lipid hydroperoxides both in maternal and fetal blood in
patients given oxygen at the time of CS.
Does deimplementing the practice of maternal oxygen
administration worsen outcomes?
From the era of studies looking into the benefits and harms of
supplemental oxygen, we are now moving to studies focused at
the deimplementation of this practice. In a recent study, a
change in the institutional practice was implemented to reduce
the use of oxygen in mothers. They found no difference in
maternal and fetal outcome despite a major reduction in the
use of oxygen for mothers.[12]
What do the latest guidelines say?
The guidelines published by the American College of
Obstetrician and Gynecologists and International Federation
and the International Federation of Gynecology and Obstetrics
recommend against the routine use of oxygen supplementation in
individuals with normal oxygen saturation and is not
recommended for fetal intrauterine resuscitation.[16,17]
Oxygen should be reserved for situations with Category II FHR
tracing with high risk features, maternal hypoxia and category
III FHR tracing while preparing for CS.
Conclusion
This review shows that supplemental oxygen cannot increase
fetal PaO
2 significantly as long as mother is
normoxic and normal physiology is maintained. The recent
studies on the impact of oxygen administration for laboring
women have shown worrying results with doubtful benefits. The
guidelines published after 2022 recommend against routine use
of oxygen for intrauterine resuscitation. Maternal oxygen
should be reserved for hypoxic mothers, Category III and
high-risk category II features on FHR tracings.
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