Airvo/Optiflow High Flow Nasal Oxygenation During Microlaryngeal Surgery

Overview

Optiflow high flow nasal cannula (HFNC) oxygenation is a technique to provide oxygen to patients when they are paralysed under general anaesthesia. Their lungs are not moving, but the high flow allows oxygen to travel into their lungs. This is called 'apnoeic ventilation'. The investigators will be using this for patients undergoing surgery for their throat.

Hypothesis: Optiflow high flow nasal cannula (HFNC) oxygenation is a suitable technique for oxygenation and ventilation (apnoeic ventilation) for patients undergoing microlaryngeal surgery. Aims: 1. Measure continuous pulse oximetry (oxygenation) and intermittent subglottic catheter sampling of end tidal carbon dioxide levels (ventilation) in the distal trachea during HFNC oxygenation 2. Measure intra-tracheal airway pressure during HFNC oxygenation Methodology: As a pilot study, the investigators will recruit up to 20 patients within a 12 month period scheduled for elective microlaryngeal surgery and high frequency jet ventilation. Standard monitoring and standard conduct of anaesthesia will be carried out: induction of general anaesthesia, insertion of a narrow catheter into the trachea and insertion of laryngeal mask airway. In addition, the study device (HFNC) will be placed into the entrance of the patient's nostrils. After removal of the laryngeal mask, HFNC oxygenation will commence. A surgical suspension laryngoscope will be inserted for microlaryngeal surgery (standard technique). The investigators will measure the adequacy of gas exchange, during of HFNC oxygenation and complications (cardiovascular, airway and respiratory). Data will be stored on a secure REDCap (Research Electronic Data Capture) system. Importance: HFNC oxygenation is a novel technique that depends on apnoeic oxygenation. Even though the patient is paralysed and not breathing, oxygenation and carbon dioxide clearance still occurs. This is due to the difference between the alveolar rates of oxygen removal and carbon dioxide excretion which generates a negative pressure gradient. Oxygenation of the patient is therefore due to the high inspiratory fraction of oxygen, apnoeic ventilation and the small continuous positive airway pressure caused by the high flow of gas. This pressure increases lung volume and recruits collapsed alveoli. Carbon dioxide clearance is due to gas mixing and flushing of the airway dead space. The additional humidification of the high flow oxygen prevents mucociliary damage of the airway. HFNC has been shown beneficial in pre-oxygenation, oxygenation after extubation, and in the treatment of respiratory failure and heart failure. New uses in difficult airway management are now emerging. However, the main case series of HFNC use in difficult airways did not measure intra-operative airway pressures end tidal oxygen and carbon dioxide; these gases were only measured at the end of surgery and when a definitive airway was inserted. As far as the investigators are aware, there are no data on intra-operative, intra-tracheal airway pressures or oxygen or carbon dioxide during HFNC oxygenation. Our study aim is to fill this knowledge gap. Potential benefits and risks: The use of HFNC removes obstacles form the surgical field and this may make surgery easier and quicker to perform. Risks include: failed gas exchange, but this is minimized by abandoning HFNC oxygenation and instituting tracheal catheter jet ventilation via the in situ jet catheter (standard practice).

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