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.