Positive Airway Pressure Under Apnoeic Oxygenation With Different Flow Rates in Nasal Cannula Therapy

The main objective of this explorative observational study is to investigate the pressures in infraglottic airway and the pharynx provided by THRIVE , using flows ranging from 1l/min to 80 l/min, in patients hospitalized for elective surgical procedure; to investigate the correlation between airway pressure and nasal oxygen flow.

Eligible adults with written informed consent will be monitored for general anaesthesia according standard operating procedure of the Bern University Hospital's anaesthesia department. Additional non-invasive monitoring for this study are transcutaneous measurement of carbon dioxide and oxygen (TCM 5®, Radiometer, Krefeld, Germany), NarcotrendTM ( Narcotrend-Group, Hannover, Germany)and thoracic electrical impedance tomography (EIT, PulmoVista® 500, Draeger, Luebeck, Germany).

Normal pre-oxygenation (until end expiratory carbon dioxide is \> 90% or time \> 3 minutes) will occur. Anaesthesia will be started using a target -controlled infusion system for Propofol and Remifentanil, 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. After administration of rocuronium, proper bag-mask ventilation will be confirmed.

Intratracheal pressure will be measured by 11 Fr. catheter (Cook Medical, Bloomington, IN, USA), connected to a CODAN Xtrans®-transducer (CODAN ARGUS Ag, Baar, Switzerland). A 11 Fr. catheter (Cook Medical, Bloomington, IN, USA) will be introduced oral in the trachea facilitated by video-laryngoscopy. To ensure patency of the upper airway jaw thrust via Esmarch manoeuvre will be applied and flexible bronchoscopy will confirm upper airway patency. Initially this 11 Fr. catheter will be positioned in the right main bronchus (2 cm below the carina). The first measurement will be performed with a randomized sequence of flow rates of 80, 60, 40, 20 or 1 l/min with opened and closed mouth. Each pressure measurement will be performed after a stable pressure plateau of 10 seconds will be observed. The catheter will then be retracted and the pressure quantification are performed with the same sequence, defined above, in different positions (50% of trachea length and pharyngeal pressures 10 cm from upper front teeth). The order of flow and mouth opening will be randomized.

The study intervention will end when all measurements are taken or upper airway patency cannot be ensured under direct endoscopy view. If one of the following criteria is met: peripheral oxygen saturation (SpO2) \<92% or transcutaneous carbon dioxide (PtCO2) \>80mmHg, immediately bag mask ventilation will be initiated, until normal values of SpO2 and PtCO2 are reached.

When any of the end points is reached, normal anesthesiologic care will be established as planned.

A safety interview will be conducted on the first post-operative day to evaluate injuries during airway management (e.g. bleeding, sore throat, hoarseness, lip injuries), pain, postoperative nausea and vomiting.

Eligibility Criteria:
Inclusion Criteria:

* \> 18 years
* Written informed consent
* Undergoing elective surgery
* Requiring general anesthesia
* ASA 1-3 (American Society of Anesthesiologists)

Exclusion Criteria:

* Any Indication for fibre optic intubation
* Expected impossible mask ventilation
* Known coronary heart disease
* Known heart failure, NYHA classification( New York Heart Association ) ≥ 2
* Peripheral occlusive arterial disease, Fontaine ≥ 2b
* BMI \> 30kg/m2 and BMI \< 16kg/m2
* Hyperkalaemia (K \> 5.5 mmol/l)
* Known COPD (Chronic obstructive pulmonary disease) Gold classification ≥ 2
* Known pulmonary arterial hypertension, systolic \> 35mmHg
* Known obstructive sleep apnoea syndrome in need of therapy
* High risk of aspiration
* Increased intracranial pressure
* Intracranial surgery
* Limited knowledge of German language
* Absent power of judgement
* Pregnancy (pregnancy test before inclusion)
* 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