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

Group 0: Simulated preoxygenation with face mask oxygen then with high flow humidified nasal oxygen Group 1: Simulated preoxygenation with high flow humidified nasal oxygen then with face mask 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 Troop™ 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 (Optiflow™ 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 percentage of 0%, 25%, 50%, 75% or 100%. If the maximum (70 l.min-1) flow rate is not tolerated, it will be reduced to 60 l.min-1 and then to 50 l.min-1 before aborting the protocol. At the end of three minutes the woman will be asked to hold her breath in inspiration while the nasal cannulae are quickly removed and the tightly fitting face mask (connected to 10 l.min-1 FiO2 100%) will be applied. The woman will then be asked to exhale normally and breathe normally and the first four etO2 concentration values measured by the end tidal gas analyzer on the anaesthetic machine will be recorded. Other variables including end-tidal carbon dioxide concentration, peak oxygen saturation values and tidal volumes will be recorded. Face mask oxygenation 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. 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 measured by real time oxygen analysis on the anaesthetic machine. After this has been achieved, the tightly fitting face mask will be applied on the woman by a trained investigator for three minutes observing the capnography trace to ensure a good seal is maintained throughout this time. Each woman will be instructed to breathe normally. The woman will then be asked to exhale normally and breathe normally and the first four etO2 concentration values measured by the end tidal gas analyzer on the anaesthetic machine will be recorded. Other variables including end-tidal carbon dioxide concentration, peak oxygen saturation values and tidal volumes will be recorded. Between each protocol, the woman will be asked to breathe room air for a minimum of five minutes in order for her etO2 to return to within 10% of her baseline level as assessed by re-application of the face mask (with 10 l.min-1 of room air in the circuit) and oxygen analysis on the anaesthetic machine. This method has been utilized in previous preoxygenation comparison studies in pregnant women to minimize the possibility of a carryover effect between protocols. If the participant’s etO2 has not returned to within 10% of her baseline level at five minutes, she will be asked to continue to breathe room air and her etO2 will be rechecked every two minutes until this criteria has been met. Furthermore, if there is any unaccounted residual carryover effect, our randomized crossover design should evenly spread this across groups. All data will be recorded on paper forms at time of experiment. Video recordings will be taken of the anaesthetic machine monitor screens (not of the study participants) throughout the procedure. The principal investigator will review each video in full on the day of experiment and any identifying details mentioned in the sound recording will be deleted before storage. 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. Comfort level of the standard face mask and the high flow nasal cannulae will be assessed using a three-stage questioning approach. Firstly, the investigator will specify that he/she is interested in comfort related to the specific modality (HFNO or face mask) and not global comfort. Secondly, the participant will be asked their preference (relative comfort) between the two devices. Thirdly, participants will rate both devices using a 5 point word associated scale for comfort levels. This approach encompasses most of the recommendations in a literature review investigating the assessment of comfort in a clinical setting. The women will not undergo general anaesthesia. Study participants will undergo repeat randomization and the corresponding order of experimental protocols at 6 months from the birth of their baby.