Comparison of Different Oxygen Flow Rates During Preoxygenation Using High-Flow Nasal Oxygen

High-flow nasal oxygen (HFNO) has long been employed to address respiratory distress in both the intensive care unit and post-anaesthesia unit. Over the past decade, HFNO has emerged as a valuable tool for preventing oxygen desaturation during airway management in the operating theatre. Notably, HFNO demonstrates effectiveness in oxygenating patients during extended periods of apnoea, providing a reliable method for apnoeic oxygenation. This technique serves as an alternative to mechanical ventilation in laryngeal surgical procedures, offering potential advantages such as a clear operating field for surgeons without the interference of a tracheal tube.

More recently, HFNO has found application in preoxygenation before anaesthesia induction. Studies indicate that the preoxygenation efficacy of HFNO is comparable to that of a standard tight-fitting facemask, with added benefits including enhanced patient comfort, improved ease of use as assessed by anaesthetists, and the potential for a seamless transition to apnoeic oxygenation.

One of the suggested mechanisms contributing to the favourable outcomes observed with HFNO in managing patients with respiratory distress is a flow-dependent positive end-expiratory pressure (PEEP) effect. When patients breathe with a closed mouth, HFNO appears to generate a PEEP effect of 1 cmH2O for every 10 l.min-1 of flow. Prior data has demonstrated that an elevated PEEP leads to a greater functional residual capacity (FRC) and improved preoxygenation effectiveness.

Previous studies investigating HFNO for preoxygenation have used flow rates ≤ 60 l.min-1. Consequently, the impact of higher flow rates on preoxygenation effectiveness and the extension of safe apnoea time remains uncertain.

In this randomised prospective study, we aim to investigate the effectiveness of preoxygenation from HFNO using different flow rates. Seventy-five patients (25 per group) scheduled for elective surgery at the Karolinska University Hospital, will be recruited. After a signed consent, the subject will be enrolled and randomised to preoxygenation using HFNO at flow rates of 45 l/min, 70 l/min or 95 l/min.

Routine perioperative monitoring, such as peripheral oxygen saturation (SpO2) and non-invasive blood pressure will be performed. Preoperatively, an arterial catheter will be inserted. An arterial blood gas will be attained before preoxygenation for base line data regarding PaCO2, PaO2 and pH.

To enable lung impedance measurement, all subjects will be applied an appropriately sized circumferential 16-electrode belt around the torso between the fourth and sixth intercostal spaces.

Patients will be positioned supine with the head elevated at 15 degrees. All groups will undergo preoxygenation using HFNO with the flow rate determined by the randomisation. All groups will be preoxygenated for 3 minutes using 100% oxygen and closed-mouth breathing. Immediately prior to anaesthesia induction, patients will evaluate the level of discomfort of preoxygenation. Thereafter, anaesthesia is induced.

Preoxygenation will be administered to all patients until the onset of apnoea, at which point oxygen delivery via HFNO will be immediately discontinued. The patient will then undergo intubation, with apnoea maintained until their oxygen saturation drops to 93%. Once this threshold is reached, mechanical ventilation with 100% oxygen will be initiated.

Inclusion criteria

• Adult, 18-84 years old
• ASA 1-3
• BMI < 35
• Planned for elective surgery

Exclusion criteria

• Cardiac disease (ischemic heart disease, heart failure (NYHA ≥2), ongoing arrhythmias, pulmonary hypertension)
• Severe asthma, moderate to severe COPD
• Pregnancy
• Smokers or former smoker last finished 1 year before inclusion
• Baseline oxygen saturation < 95%
• Nasal obstruction
• Known or anticipated difficult airway
• Patients with electrical active implants where lung impedance analysis is contraindicated
• Not capable of understanding study information and signing a written consent