Ethical approval not obtained yet as questioning the danger of providing 100% oxygen to parturients.

Pregnancy is associated with physiological changes affecting the cardiorespiratory system as a consequence of an increase in both cardiac output and intra-abdominal pressure. The aim of this prospective observational study is to examine the perioperative changes in ventilation inhomogeneity and respiratory function measured by the non-invasive nitrogen multiple breath washout and forced oscillation techniques.

Pregnancy is associated with physiological changes affecting the cardiorespiratory system as a consequence of an increase in both cardiac output and intra-abdominal pressure. These alterations lead to a ventilation/perfusion mismatch which is potentiated by a decrease in functional residual capacity (FRC). These effects explain why pregnant women are more prone to the occurrence of hypoxemia, particularly in the third trimester of their pregnancy. The importance of the ventilation inhomogeneity can be estimated from the lung clearance index (LCI) measured by the non-invasive nitrogen multiple breath washout (N2 MBW) technique. Moreover the loss in lung volume is associated with reduction in respiratory system compliance, which can also be assessed non-invasively by the forced oscillation technique (FOT).

To our knowledge, there is no existing data on LCI or FRC using the aforementioned techniques in pregnant women. Furthermore, existing data on respiratory function in pregnant women is largely restricted to spirometric and body plethysmographic measurements taken primarily in the 1970s-1980s. As such, the important roles of lung ventilation inhomogeneity as well as the potential changes following birth after caesarean section have yet to be completely characterised.

Inclusion Criteria:

• Written informed consent
• Pregnant woman in the third trimester (37-40 weeks gestational age)
• Scheduled for elective caesarean section under regional (spinal or combined spinal-epidural) anaesthesia

Exclusion Criteria:

• Pregnant women outside 37-40 weeks gestational age
• Non-singleton pregnancy
• Previous history of ≥2 caesarean sections
• History or clinical signs of cardiopulmonary disease in the last 12 months (chronic hypertension, gestational hypertension, preeclampsia, asthma, acute or chronic bronchitis, others)
• Positive current smoking status
• Pre-pregnant body mass index (BMI) >30 kg/m2 (based on booking records)
• Respiratory infection <2 weeks prior to surgery
• Inability to perform the respiratory function tests

Pregnant women scheduled for elective caesarean section in gestational weeks 37-40