Futility. Inclusion and logistics of study in a clinical situation proved too difficult to continue.

Apneic oxygenation describes the process of continuous oxygenation of the blood without breathing efforts. This anesthesia technique has been described in the literature for more than 100 years and is sometimes used under general anesthesia, e.g. during surgery of the vocal chords. Although this technique usually provides marked prolongation of the apneic period because of excellent oxygenation, it is limited by the absence of ventilation and the corresponding accumulation of carbon dioxide in the blood. This will lead to worsening respiratory acidosis and associated complications, such as cardiac arrythmias.

In 2015 it was reported that apneic oxygenation with high-flow nasal oxygen delivery systems (HFNO), a device that provides heated humidified oxygen at high flow rates (usually 30-70 L/min), resulted in less carbon dioxide accumulation compared with historical controls. This specific technique of apneic oxygenation was termed transnasal humidified rapid-insufflation ventilation exchange (THRIVE). To date, the impact on different flow rates on blood carbon dioxide accumulation during THRIVE is unknown. Specifically, very high flow rates, exceeding 70 L/min have not been investigated. Therefore, the aim of this trial is therefore to study the rate of accumulation of carbon dioxide during THRIVE at two different flow rates: 40 and 100 L/min.

Eligible participants with signed informed consent presenting for elective surgery will have standard monitoring with ECG, pulse oximetry, blood pressure, train-of-four monitoring and capnography applied at arrival to the operating room. In addition, a circumferential electric impedance tomography thoracic elastic band with equally spaced electrodes will be placed in the 4th to 6th intercostal space. After induction of general anesthesia with a target controlled infusion of Propofol+Remifentanil and muscle relaxation with Rocuronium bromide, adequate face mask ventilation will be confirmed and arterial cannulation will be performed.

Patients will thereafter receive THRIVE at two flow rates for 10 minutes each in a randomized order. During THRIVE, video laryngoscopy will be performed with a grade 2B visualization of the larynx to establish similar airway patency during the interventions.

Before each flow, an end-tidal oxygen value of >0.85 and end-tidal carbon dioxide value of 4.5-5.0 kPa is achieved by controlled pressure regulated volume control ventilation to establish similar arterial partial pressure of oxygen and carbon dioxide at baseline. An arterial blood gas will also be analyzed before the start of each flow rate to ensure that the initial pCO2 is less than 6.0 kPa. Furthermore, a standardized lung recruitment maneuver will be performed to establish similar baseline compliance of the respiratory system.

Termination criteria will be:

• All flow rates have been applied.
• Severe acidemia (pH<7.10), hypercapnia (pCO2>11.0) or desaturation to SpO2 less than 90% for more than 10 seconds. These termination criteria will apply only to the current flow rate being tested. The participant will be mask ventilated until baseline parameters are established and the next THRIVE flow rate according to randomization will be applied.

Inclusion Criteria:

• Patients presenting for elective surgery

Exclusion Criteria:

• Age <18 or >65 years
• Body Mass Index (BMI) >35 kg m-2
• American Society of Anesthesiology classification > 2
• Physical activity level less than 5 metabolic equivalents
• Obstruction of the upper airways
• Known or high clinical suspicion of difficult airway
• Obstructive sleep apnea syndrome
• Increased risk of aspiration
• Pulmonary or cardiac condition resulting in a reduction of physical activity level equivalent to the New York Heart Association class of 2 or higher.
• Any contraindication to high-flow nasal oxygen therapy or hypercapnia
• Pregnancy or breastfeeding
• Allergy to any of the anesthetic agents used in the study
• Inability to comprehend oral or written information.

The participants are aware of the intervention used, but not aware of the order of flow rate tests.