Physiology Regarding Apnoeic Oxygenation During Nasal Cannula Therapy at Different Flow Rates

This study compares under controlled conditions if different flow rates affect apnoea time after induction of anaesthesia and how CO2 clearance is influenced. Furthermore, this study enables to quantify the effects of increased pCO2 on vital parameters (e.g. blood pressure, cardiac output, cerebral perfusion, etc.) The investigators will enroll patients undergoing elective surgery at the University Hospital of Bern, Switzerland.

Once anesthesia has been induced, apnoea will set it. During this period, the investigators will compare different methods of apnoeic oxygenation for a maximum of 15 or 30 minutes.

Before discharge, an interview will be conducted, assessing complications and patient satisfaction.

Eligible, consenting adults will be prepared for general anaesthesia in the usual way consisting of ECG, pulse-oximetry, NarcotrendTM, a venous cannula and an arterial line for continuous blood pressure monitoring. They will receive additional monitoring such as transcutaneous measurement of pCO2 and O2, NIRS, and thoracic electrical impedance tomography (EIT, PulmoVista® 500, Draeger, Luebeck, Germany).

Normal pre-oxygenation (until etO2 is \> 90% or time \> 3 minutes) will occur. Anaesthesia will be started (= "induction") using Propofol and Fentanyl, using NarcotrendTM to measure depth of anaesthesia. All patients will receive a standard dose of neuromuscular blockage to facilitate airway management and total intravenous anaesthesia will be installed. Using the train of four measurement (TOF) full neuromuscular blockage with Rocuronium will be confirmed every 30 seconds. If necessary, additional dosage of neuromuscular blockage will be administered.

After administration of Rocuronium, possibility of mask ventilation will be confirmed and the sealed envelope with the randomization will then be opened. As a study intervention, the assigned method (HFNCT 70 l/min with either jaw thrust or laryngoscopy, or 10 l/min or 2l/min with the standard nasal canula, or 0.25l/min delivery of oxgen via a tracheal tube) will be installed and mask ventilation discontinued starting the apnoea period. Nasopharyngoscopy (EF-N slim, Acutronic, Hirzel, Switzerland) will confirm upper airway patency. Blood gas analysis will be conducted: baseline awake, start of apnoea, first minute after apnoea start, and every 2 minutes thereafter with a maximum of 75ml 150 ml in total. Other measurements (ECG, pulse-oximetry, blood pressure, NIRS, thoracic EIT, NarcotrendTM, PtcO2, PtcCO2) will be measured continuously over the study period The study intervention will end when one of the following criteria (study end-points) is met: SpO2 \<92%, PtcCO2 \> 100 mmHg or time \> 30 minutes.

When any of the end points is reached, patient-centred standard anaesthesia care will be continued, as planned for the case.

A post-operative interview will be conducted before discharge to evaluate injuries during airway management (bleeding, sore throat, hoarseness), pain, postoperative nausea and vomiting.

Eligibility Criteria:
Inclusion Criteria:

* \> 18 years
* Written informed consent
* Undergoing elective surgery
* Requiring general anesthesia

Exclusion Criteria:

* Any Indication for fibre optic intubation
* Expected impossible mask ventilation
* Known coronary heart disease
* Known heart failure, NYHA classification ≥ 2
* Therapy including β-receptor antagonists
* Arrhythmias in need of anti-arrhythmic therapy (e.g. implanted cardio defibrillator)
* Peripheral occlusive arterial disease, Fontaine ≥ 2b
* Known stenosis of the (common or internal) carotid or vertebral arteries
* BMI \> 35kg/m2 and BMI \< 16kg/m2
* Hyperkalaemia (K \> 5.5 mmol/l)
* Known COPD Gold classification ≥ 2
* Known pulmonary arterial hypertension, systolic \> 35mmHg
* Known obstructive sleep apnoea syndrome in need of therapy
* High risk of aspiration (requiring rapid sequence induction intubation)
* Increased intracranial pressure
* Intracranial surgery
* Limited knowledge of German language
* Absent power of judgement
* Anaemia, Hb \< 100 g/l
* Pregnancy (pregnancy test in all female patients)
* Neuromuscular disorder
* Known or suspected cervical spine instability
* Nasal obstruction, impossibility of nasal ventilation (both sides patent)
* Allergies or contra-indications to one or more of the used anaesthesia agents