The Effect of High-flow Nasal Oxygen Flow Rate on Gas Exchange During Apnoea (Apox-HFNO)

A Randomised Controlled Trial of Apnoeic Oxygenation With No-flow vs High-flow vs Ultra High-flow Nasal Oxygen

Apnoeic oxygenation refers to oxygenation that occurs through the insufflation of oxygen into the lungs in the absence spontaneous respiration or positive pressure ventilation. It is used to extend the time to desaturation at induction of anaesthesia and as a primary oxygenation technique during airway surgery. The impact of high-flow nasal oxygen flow rate selection on gas exchange is poorly understood. Participants in this study will be randomised to receive a certain nasal oxygen flow rate during apnoea and its effect on gas exchange will be measured by blood gas analysis.

Apnoeic oxygenation with high-flow nasal oxygen has been proposed to result in carbon dioxide clearance. However, this has been poorly quantified. This study will compare use of nasal oxygen at different flow rates during apnoea with that of a control that does not receive nasal oxygen. Participants are anaesthetised after standardised pre-oxygenation with high-flow nasal oxygen, after which they will receive one of three nasal oxygen flow rates (0, 70, 120 L/min). The rate of carbon dioxide elevation will be measured by arterial blood gas analysis after the onset of apnoea and compared between the three groups to discern the relative rates of carbon dioxide clearance after the first minute of apnoea. The effect of nasal oxygen flow rate on oxygenation will also be measured.

100% oxygen is administered via high-flow nasal cannulae at 70L/min from the onset of apnoea until four minutes of apnoea has completed.

100% oxygen is administered via high-flow nasal cannulae at 120L/min from the onset of apnoea until four minutes of apnoea has completed.

After 2mins 45 seconds of pre-oxygenation with high-flow nasal oxygen at Fio2 1.0 at 50L/min, anaesthesia is induced with 1-1.5mcg/kg remifentanil plus 2-3mg/kg propofol. Propofol (10mg/kg/hr) and remifentanil (0.15mcg/kg/min) infusions administered until study conclusion. At the onset of apnoea, 1mg/kg rocuronium administered. Jaw thrust performed. High-flow nasal oxygen adjusted to the randomised flow rate. Videolaryngoscopy after 1 minute of apnoea. Airway patency maintained. Tracheal intubation after 4 minutes of apnoea. Positive pressure ventilation commenced at Spo2 92%. Failure to obtain view of glottis with videolaryngoscope results in withdrawal from the study. Blood samples obtained from an arterial catheter immediately prior to commencing pre-oxygenation, after 90 and 180 seconds of pre-oxygenation, after each minute of apnoea until six minutes of apnoea, and every two minutes of apnoea thereafter, until Spo2 92%.

The rate of rise of the partial pressure of carbon dioxide between 60 seconds and 240 seconds of apnoea as measured by arterial blood gas analysis.

The time period from the onset of apnoea (determined by visual inspection) until an oxygen saturation of 92% is measured by pulse oximetry.

Inclusion Criteria: 18 years of age or older ASA 1 or 2 Receiving a general anaesthetic for non-emergent surgery Exclusion Criteria: ASA score ≥3 BMI ≥ 30 kg/m2 Nasal obstruction Baseline SpO2 ≤95% on room air Anticipated difficult airway management Requirement for awake intubation Pregnancy Positive PCR test for coronavirus in preceding 14 days.

Investigator will consider requests to share patient-specific anonymised data in electronic format for the purpose of meta-analysis