An investigation of Gas Flows during Preoxygenation with Nasal High Flow in volunteers

Healthy volunteer participants will be reclined on a hospital bed at 20 degrees in a laboratory setting. Two researchers will be present; one to operate the equipment and the other to monitor the participant. Flow, generated by a computer-controlled flow source (custom, Fisher&Paykel Healthcare, New Zealand) will pass through a humidifier (950, Fisher & Paykel Healthcare, New Zealand) before being administered to the participant through a nasal interface, either a high flow interface (AA001, Fisher & Paykel Healthcare, New Zealand) or a low flow cannula (Hudson standard nasal tip, Teleflex, Morrisville, USA). Tolerability Assessment: An initial test will be performed to check the participant's tolerance to flow and mouth open/closed combinations by increasing flow (10, 25, 40, 55, 70 L/min) until either the flow reaches 70 L/min or the participant indicates it is not tolerable. Measurements: The participant will be fitted with a modified CPAP mask (FitLife Total, Phillips, Netherlands). A mouthpiece extends from the participant’s mouth through the body of the mask to atmosphere. This mouthpiece is modified from a commercially available spirometry mouthpiece (EasyOne, ndd Medizintechnik AG, Switzerland). An additional port provides a path for gas from inside the mask to atmosphere. It should be noted that there is always a gas path to atmosphere to ensure the participant can breathe. Three flow meters (SFM3200-AW, Sensirion, Switzerland) are in the system. These flow meters measure the flow rate: 1. from the flow source, 2. entering or exiting the mouth through the mouthpiece and 3. entering/exiting the mask body through the port through the mask body. The flow meters are sampled, and the results written to a file on the PC. The flow-weighted FiO2 (the primary endpoint) can be calculated from these three flow readings The following flow, cannula and mouth open/closed combinations will be administered for 120s per combination however the first 20s of data will be discarded as this has been set aside to allow the next set up to be established (i.e the 'washout' period of 20s is built into the 120s period and only the last 100s of data will be used for each combination). o 10 L.min-1 through low flow cannula (Hudson standard nasal tip, Teleflex, Morrisville, USA ) o Flows through High Flow Nasal Oxygen (HFNO) cannula (Optiflow, Fisher & Paykel Healthcare, Auckland, New Zealand) of : o 10, 25, 40, 55, 70 L.min-1 with: 1. Mouth open 2. Mouth closed This is a cross over study and each participant will receive all of the combinations (subject to finding these combinations tolerable). We estimate the total time for a participant will be approximately 60 minutes. The concentration of carbon dioxide in the mask body and the participant's oxygen saturation will be monitored for safety.