High flow humidified nasal oxygen versus face mask oxygen for preoxygenation of pregnant women – a prospective randomised controlled crossover study

INTERVENTION: Group 1: Simulated preoxygenation with high flow humidified nasal oxygen then with face mask oxygen Group 2: Simulated preoxygenation with face mask oxygen then with high flow humidified nasal oxygen In order to simulate conditions in our operating theatres, each woman will be placed on an operating table or hospital bed in an optimally ramped position using the Troopâ„¢ elevation pillow and with a right lateral pelvic wedge to minimise aortocaval compression. Maternal vital signs (heart rate, blood pressure, respiratory rate and oxygen saturation) will be measured before, during and after each protocol. Fetal heart rate will be measured before and after each protocol. High flow humidified nasal oxygen protocol With 10 l.min‐1 of room air (FiO2 21%) running in the anaesthetic circuit, a tightly fitting face mask will be applied by a trained investigator (anaesthetist, anaesthetic registrar, medical student, or nurse) and a good seal and ideal fitting will be determined by observing the capnography trace as the woman breathes. (This step may be omitted if the woman undergoes the face mask oxygen protocol first as per her randomization). The face mask will then be removed. The oxygen concentration in the anaesthetic circuit will be increased to 100% (FiO2 100%) with flows remaining at 10 l.min‐1 until the oxygen concentration rises to 100% as determined by real time oxygen analysis on the anaesthetic machine. After this has been achieved, high flow humidified nasal cannulae (Optiflowâ„¢ by Fisher & Paykel Healthcare ‐ TGA approved) will be inserted into the woman’s nostrils by the trained investigator (anaesthetist, anaesthetic registrar, medical student, or nurse). The oxygen flow will be commenced (first 30 seconds at 30 l.min‐1, then next 150 seconds at 70 l.min‐1). Each woman will be instructed to breathe normally with her mouth closed as much as possible. The percentage of time mouth closure is achieved will be recorded to the closest per CONDITION: Anaesthesiology ‐ Anaesthetics Pregnancy;Preoxygenation; ; Pregnancy ; Preoxygenation Reproductive Health and Childbirth ‐ Childbirth and postnatal care Respiratory ‐ Normal development and function of the respiratory system PRIMARY OUTCOME: Expired end tidal oxygen concentration. Expired oxygen concentration and waveform data will be measured by the gas analyser on our SCIO Oxi Four Plus gas monitor (Drager, Lubeck, Germany) and recorded. First breath end tidal oxygen concentration will be determined from the display and completeness of breath capture determined from interrogation of the waveform.[First breath after respective protocols completed ; (i.e. end of high flow nasal oxygen protocol and end of face mask oxygen protocol). This will be attained at time of recruitment to study (woman in late pregnancy) and repeated at 6 months post‐birth. ] SECONDARY OUTCOME: Blood pressure (maternal) will be measured using the non‐invasive blood pressure apparatus on our anaesthetic machine ; [Before, during & after protocols (i.e. high flow nasal oxygen protocol and face mask oxygen protocol). This will be attained at time of recruitment to study (woman in late pregnancy) and repeated at 6 months post‐birth. ] Body mass index ; ‐ weight will be measured using an analog weighing scale (i.e. in late pregnancy and 6 months post birth) ; ; ; [First breath after respective protocols completed INCLUSION CRITERIA: Women in late pregnancy (more than or equal to 36 weeks gestation) ; ‐ height will be measured using a stadiometer in antenatal clinic prior to recruitment to study (this measurement will be used for both testing instances)[Weight will be measured just prior to commencement of protocols at both testing instances using an analog weighing scale (i.e. in late pregnancy and 6 months post birth) ; Height will be measured using a stadiometer in antenatal clinic prior to recruitment to study (this measurement will be used for both testing instances ‐ i.e. in late pregnancy and 6 months post birth)] Comfort scores will be measured using a 5 point word associated scale for comfort levels[Measured at the end of the protocol. This will be attained at at both testing instances (i.e. in late pregnancy and 6 months post birth)] End‐tidal carbon dioxide concentration (maternal) will be measured by the end‐tidal gas analyser on our anaesthetic machine ; (i.e. end of high flow nasal oxygen protocol and end of face mask oxygen protocol). This will be attained at time of recruitment to study (woman in late pregnancy) and repeated at 6 months post‐birth. ] Heart rate (fetal) will be measured using a doppler ultrasound.[Before & after protocols (i.e. high flow nasal oxygen protocol and face mask oxygen protocol). This will be attained at first testing instance (i.e. woman in late pregnancy). It cannot be repeated at post‐birth instance. ] Heart rate (maternal) will be measured using the pulse oximeter on our anaesthetic machine ; [Before, during & after protocols (i.e. high flow nasal oxygen protocol and face mask oxygen protocol). This will be attained at time of recruitment to study (woman in late pregnancy) and repeated at 6 months post‐birth. ] Oxygen saturation values (maternal) will be measured using the pulse oximeter on our anaesthetic machine ; [Before, during & after protocols (i.e. high flow nasal oxygen protocol and face mask oxygen protocol). This will be attained at time of recruitment to study (woman in late pregnancy) and repeated at 6 months post‐birth. ] Respiratory rate (maternal) will be measured using clinical assessment ; [Before, during & after protocols (i.e. high flow nasal oxygen protocol and face mask oxygen protocol). This will be attained at time of recruitment to study (woman in late pregnancy) and repeated at 6 months post‐birth. ] Tidal volume measured using volumeter on anaesthetic machine[During face mask oxygen protocol only. This will be attained at time of recruitment to study (woman in late pregnancy) and repeated at 6 months post‐birth. Tidal volume measurement during high flow nasal oxygen protocol impossible due to nature of apparatus.]