High flow humidified nasal Oxygen to PrEvent desaturation FOR labouring people using remifentanil PCA (The HOPE-for-REMI study)

The study period will be two hours from the commencement of Remifentanil PCA. A member of the investigator team will be present in-person with the participant for the entire duration. All study interventions (Remifentanil PCA, high flow nasal oxygen/standard care nasal oxygen) will cease at the end of the two hours. Adherence to protocols will be assessed and recorded by direct observation by investigator. Remifentanil PCA administration protocol Remifentanil will be drawn up by an investigator and double checked with a qualified member of staff (eg. anaesthetist, anaesthetic accredited fellow/registrar, registered nurse, midwife). Two milligrams of remifentanil will be diluted into 50 ml of 0.9% normal saline to final concentration of 40 microgram/ml. The drug syringe will then be loaded into a pre-programmed PCA pump which will be set to a remifentanil PCA protocol which will deliver 40 microgram (or 1 ml) when triggered with a two minute lockout. This administration protocol is the same one used in the RESPITE trial. We think it is important to mirror the practice of the most comprehensive remifentanil PCA trial to date so as to be able to compare secondary outcomes and maximise potential clinical applicability. If the participant already has a peripheral intravenous cannula (20 gauge or larger) inserted, the cannula placement and patency will be checked with a manual hand flush of 10 ml of 0.9% normal saline and a check of good flow of intravenous crystalloid fluid through the line will be confirmed. If the participant does not have a peripheral intravenous cannula (20 gauge or larger) inserted, one will be placed and checked in the same manner as above post-insertion. The intravenous fluid will be set to a constant flow rate as a baseline to prevent drug accumulation in the line. The remifentanil syringe will be connected via extension tubing to the anti-reflux injection port closest to the participant. The PCA trigger button will be given to the participant with instructions to press the button at the earliest point during a contraction. The participant will be monitored continuously by a study investigator and a desaturation episode will be declared if her oxygen saturation falls to 94% or less. If this occurs, the attending investigator will rouse the participant verbally and ask them to take a breath. The remifentanil PCA will be removed from the participant’s reach. If the participant remains apnoeic, the attending investigator will attempt to rouse the participant physically by trapezius squeeze. If the participant remains apnoeic and oxygen saturation levels continue to fall, rescue breaths will be administered via a Laerdal bag valve mask running at 15 litres/min of supplemental oxygen by trained investigator. After safe oxygen saturation levels have been re-established, any desaturation episode will trigger the commencement of the corresponding oxygen protocol. Intervention group High flow humidified nasal oxygen group protocol High flow humidified nasal oxygen will be applied via the Airvo 2 (Fisher & Paykel, New Zealand) machine. It will be commenced at a flow of 30 litres/min and an inspired fraction of oxygen of 60%. The comfort scale questionnaire will be asked at this flow rate. The flow rate will then be increased to 40 litres/min for 5 minutes and then 50 litres/min with the comfort scale questionnaire asked after each adjustment. The flow rate will be decreased by 10 litres/min if the participant complains of discomfort to a minimum of 30 litres/min after which the study protocol will be abandoned. The inspired fraction of oxygen of 60% has be chosen because this is the concentration that was studied by Khaw et al when investigating the effects of maternal supplemental oxygen on the fetus. In their prospective double-blinded trial, they showed no difference in neonatal outcomes and levels of lipid peroxidation between groups of mothers receiving supplemental oxygen versus room air during caesarean section. This provides us with a starting point which we can be assured is safe. We expect that the average required inspired fraction of oxygen required to prevent desaturation will be lower than this.