Most elective lower segment Caesarean sections in the United Kingdom are performed under regional blockade  and it is a common practice to give women oxygen through a face mask until the baby is delivered . If this has become an established practice, it lacks a clear rationale.
Marx and colleagues  showed the relationship between maternal and umbilical vein PaO2 during general anaesthesia for Caesarean section and demonstrated that increasing inspired maternal oxygen fraction elevated oxygen delivery to the fetus. Crawford  in his textbook recommended the administration of supplemental oxygen to the mother and helped establish the practice of oxygen therapy at Caesarean section. Conn and colleagues  have shown that spinal anaesthesia impaired maternal ventilatory performance. Subsequently, Kelly and colleagues  reported significant pulmonary function changes consistent with restrictive ventilatory defect, showing no fetal benefit from administration of 35% oxygen by face mask.
Some fetal benefits of maternal oxygen in patients receiving epidural anaesthesia were shown by Ramanathan and colleagues . Increasing inspired maternal oxygen fraction during epidural anaesthesia elevated umbilical venous oxygen tension (UVpO2) and improved oxygen delivery to the fetus. They did not find significant differences in the Apgar score and umbilical arterial pH (UApH) when comparing babies born to normoxic and hyperoxic mothers, but neonatal standardized base excess (SBE) improved in women given 74% oxygen fraction vs. air sufficiently to be statistically significant, but of debatable clinical significance .
Improved fetal biochemistry is not a universal finding. Babies born to women given 60% oxygen during spinal anaesthesia did not demonstrate improved biochemistry when compared to those breathing air . Even if there are doubts about its efficacy, oxygen was given to the mother on the basis that it could do no harm.
Recently, this has been challenged. Khaw and colleagues  have shown greater free radical activity in cord blood at delivery in mothers who breathed supplementary oxygen. In addition, neonatal condition has been shown to be better following resuscitation with air rather than oxygen .
The aim of this study was to determine if beneficial effects of maternal oxygen therapy on the fetus could be revealed using a neonatal behavioural scoring system.
Local Ethics (Research) Committee approval was obtained for this prospective, randomized, double-blind study. After written informed consent, women equivalent to ASA Physical Status I or II, who had a normal singleton pregnancy beyond 36 weeks gestation, undergoing elective Caesarean section under spinal anaesthesia were recruited.
All women received a standardized combined spinal–epidural anaesthetic. Acid prophylaxis with 30 mg lansoprazole was given on the morning of surgery followed by 30 mL of 0.3 M sodium citrate on arrival in the anaesthetic room. All were monitored with electrocardiogram, non-invasive blood pressure and pulse oximetry from the outset. An epidural catheter was placed in the second lumbar interspace in the sitting position, using an 18-G Tuohy needle and loss-of-resistance to saline. A 27-G Whitacre spinal needle was used for the intrathecal injection of a standard dose of 13 mg 0.5% w/v hyperbaric bupivacaine with 400 μg diamorphine in the third lumbar interspace. A free flow of clear cerebrospinal fluid before and after the intrathecal injection indicated a successful technique. A block height of T5 to light touch was required prior to surgery, and epidural boluses were used to achieve this, if necessary.
All patients received an intravenous infusion of saline 0.9% w/v 500 mL with ephedrine 30 mg to prevent hypotension. Further boluses of ephedrine were given at the discretion of the anaesthetist.
Women were allocated to two groups according to a computer generated random code. A face mask was immediately applied after the spinal anaesthetic was given and the pregnant women laid supine with left tilt. The patients in Group 1 received air and oxygen mixture through a Hudson style face mask, where the flows were adjusted to provide FiO2 of 0.21–0.25. The patients in Group 2 received FiO2 of 0.40–0.60 through an identical Hudson style face mask.
A gas sampling tube was held under the mask until the oxygen sensor registered a steady reading at the desired oxygen concentration. Flows were increased until negligible carbon dioxide levels were recorded, to ensure there was no rebreathing.
If SPO2 fell to less than 94% at any time preoperatively or intraoperatively, additional oxygen was given and the patient withdrawn from the study.
After delivery of the baby the cord was double clamped, the face mask removed, oxytocin 10 units given, and the placenta delivered. Heparinized uterine arterial and venous samples were taken from the cord by an experienced nurse, and analysed in a Radiometer ABL 500 blood gas analyser (Radiometer A/S, Brønshøj, Denmark). A neonatologist, unaware of the allocation, recorded Apgar scores at 1 and 5 min.
One anaesthetist was responsible for the care of the patient, performed the spinal anaesthetic and adjusted the gas flows in accordance with the protocol, but took no part in collection of neonatal data. One of the authors, blind to the allocation, after specific training and with no other role in the study, performed the Neurologic Adaptive Capacity Score (NACS)  on all the infants. Anticipating that oxygen bias might be transient, the first assessment was done within 5 min of birth. The second assessment was done between 10 and 24 h after the Caesarean delivery.
Age, height, weight, gestation, parity and ASA Class were recorded. The primary outcome was the NACS. The secondary outcomes included Apgar score, umbilical venous blood oxygen tension (UVpO2) and umbilical artery standardized base excess (UASBE).
The power calculation used NACS data from a similar study . A true clinical difference was taken as 2, and standard deviation (SD) was 3. For a power of 80% and P < 0.05, the sample size required for a significant difference was approximately 60 subjects.
Data were expressed as mean (SD), median [range] and count. These were analysed using χ2 for categorical data, and t-tests for comparison of means. Analyses were performed using SPSS v. 8.0 (SPSS Inc, Chicago, IL, USA) on a Sony Vaio computer (Sony Corporation, Tokyo, Japan).
Five patients, three from the Group 1 and two from the Group 2, were withdrawn from the study because of protocol violation. Four of them received inappropriate inspired oxygen mixture before or during the Caesarean section. In one newborn the second assessment with NACS was performed earlier than the study protocol permitted. One mother was excluded because of the history of drug abuse.
Patient characteristics in terms of age, gestation, parity height and weight were similar in both groups (Table 1). NACS at birth in Groups 1 and 2 were 32.6 (SD 4.6) and 31.3 (SD 4.3), respectively. The second NACS were 36.0 (SD 3.0) and 36.5 (SD 1.9) in Groups 1 and 2, respectively. There were no significant differences between the groups for any of the recorded variables (P = 0.25 and P = 0.52, respectively). The distribution of NACS is given in the Table 2.The Apgar scores at 5 min expressed as median [range] were 10 [9-10] in Group 1 and 10 [8-10] in Group 2. The difference was not statistically significant (P = 0.17).
UVpO2 was 3.8 kPa (SD 1.1) in Group 1, compared to 4.1 KPa (SD 0.6) in Group 2 which was not statistically significant (P = 0.20). UASBE was not significantly different at −2.2 mmol L−1 (SD 3.3) and −3.1 mmol L−1 (SD 2.7) between the groups (P = 0.28).
Mean time intervals between skin incision and delivery of the baby were similar at 8.8 (SD 5.0) min in Group 1 and 8.7 (SD 4.4) min in Group 2. Also, mean time intervals between the application and removal of face mask were similar at 18.8 (SD 5.0) min in Group 1 and 18.8 (SD 4.8) min in Group 2, respectively. The inspired oxygen concentration was 0.23 [0.21–0.25] and 0.46 [0.41–0.60] in Groups 1 and 2, respectively. Mean dose of ephedrine used in prevention of hypotension was 31.6 mg (SD 5.58) in Group 1 and 35.5 mg (SD 8.56) in Group 2. No patient was excluded because of SPO2 less than 94%.
In this study we have demonstrated that doubling the inspired oxygen fraction of oxygen given to the awake mother undergoing an elective Caesarean section with spinal anaesthesia, had no effect on the NACS. Similarly there were no statistically significant differences in Apgar score, UASBE and UVpO2. Whilst there is some elevation of UVpO2 in the cord gases of babies subject to the higher of the two maternal oxygen fractions, this was small and insignificant. The improved oxygenation provided by a Hudson style face mask is insufficient to influence cord gas tension to a degree that is either clinically or statistically significant.
With hindsight, this might have been predicted. Ramanathan  required a higher inspired oxygen fraction to achieve change than given here (74%). Khaw , with spinal anaesthesia, achieved a significant elevation of UVpO2 with maternal FiO2 0.6. Kelly , with maternal FiO2 0.35, also failed to significantly improve fetal oxygen delivery. If improvements are sought, it appears that maternal FiO2 must be at least 0.6. To achieve FiO2 in this range, the standard version of a commercially available face mask that we and many others used was unsuitable. Clearly, a high oxygen fraction can be achieved with a tight fitting anaesthesia style breathing circuit, but this is not a comfortable arrangement for either the mother or her attendants. Khaw  used a high flow venturi style mask to achieve high FiO2, and there may be other devices with this capability. The best for Caesarean section has not been researched.
Another criticism of our study might be the use of NACS. This testing was originally proposed as a simple, non-invasive, quick neurobehavioral examination to assess subtle effect of drugs on neonates, distinguishing them from birth trauma, perinatal asphyxia or neurological disease. Since NACS was first described, their use has been widespread. More recently, their reliability and validity have been questioned [12-14]. Had there been an alternative score with better scientific foundation, then this would have been used. In the event, there was not a better tool to be found in the search for subtle neonatal effects. Understanding the drawbacks, it was decided to conduct the study using NACS. The initial NACS was performed as soon after birth as was practicable. Fetal physiology was still adjusting to breathing air, and the newborn was exposed to the noise and glare of the operating room. Initial NACS not surprisingly were outside the range of ‘normal’ values. The second score, the following day was carried out at the mother's bedside, at a constant temperature and between feeds, to ensure that all newborns were capable of an NACS in the ‘normal’ range. The difference between the first and the second scores was not important, since it was the between group differences that were under test.
In all respect the groups were managed identically and any source of bias would have affected both to the same extent. Moreover, one examiner after adequate training conducted all the scoring eliminating another possible source of bias. The group size also should have been sufficient to compensate the variations. In the design stage of this study we considered allowing our control group to breath air. If the control group was breathing normal air without face mask, than we would have been unable make the study double blind. If only a face mask was used without any additional adjustments of the inspiratory mixture, then we would risk rebreathing, which may impact on fetal acidosis, adding another possible source of bias. Therefore, we used an identical mask for both groups, adjusting the flows to provide an adequate gas mixture and monitored the desired oxygen concentration and etCO2.
Although it was considered while the study was in the design stage, we decided not measure maternal PO2. Our major interest was in the materno-placental unit, and maternal PO2 would only have provided us with partial information. Moreover, SPO2 gives sufficient information about the oxygenation of the mother, and UVpO2 about the oxygenation of the fetus. Finally, invasive sampling, that was not central to the aim of the study might have caused difficulties in obtaining Ethical Committee approval, and also with the recruitment of the patients, as it involved additional discomfort and risks, although the associated risks were minimal.
In conclusion, giving maternal oxygen using a standard commercial Hudson style face mask does not appear to significantly improve oxygen delivery to, nor does it influence acidosis or behavioural effects in, the normal neonate at elective Caesarean delivery with spinal anaesthesia. Unless a more specialized oxygen delivery device is used, breathing room air seems equally satisfactory.
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