HFNC Versus Conventional Nasal Cannula During EBUS Procedure: a Randomised Controlled Trial

Introduction

1. EBUS - bronchoscopy From a physiological point of view bronchoscopy is an introduction of a foreign body into the airway. Response mechanisms include laryngospasm, bronchospasm, cough, increased secretion of bronchial mucus and reduced depth of breathing. Sympathetic response with increased heart rate and blood pressure is common even before the procedure, while the patient is being prepared on the examination table. The bronchoscope occupies 10 to 15% of the cross-sectional area in the major airways and increases air flow resistance. Suction applied during the procedure causes air stealing through the working channel of the bronchoscope which reduces end inspiratory and expiratory volumes leading to alveolar de-recruitment with increased intrapulmonary shunting consequently.

EBUS bronchoscopy is associated with prolonged procedure time, increased airway contact, a thicker instrument, and multiple needle punctures through the tracheal and/or bronchial wall. All these factors cause additional stress to the patient and require additional local anesthesia and deeper level of sedation. 2. Sedation Although topical anesthesia of the upper airways, vocal cords, trachea, and bronchi enables a tolerable bronchoscopic procedure, a moderate level of sedation during bronchoscopy is currently the golden standard. Use of sedation facilitates the passage through the vocal cords, inhibits airway protective reflexes and increases patient safety (2,3). In combination with physiological responses to bronchoscopy which increase oxygen demand, sedatives used during bronchoscopy reduce respiratory drive and further compromise patients, especially those with chronic diseases. When it occurs, hypoxemia can induce periprocedural arrythmias and ischemia therefore should be generally avoided and, if encountered, must be reverted quickly (4). An oxygenation strategy with appropriate oxygen supplementation is therefore required during most bronchoscopic procedures. 3. Oxygen supplementation Low flow oxygen up to 6 liters per minute applied through a nasal catheter is the most often used method for oxygen supplementation during flexible bronchoscopy. Inspired oxygen fraction (FiO2) can reach up to 45% but cannot be reliably predicted and may not be enough in all cases.

High flow nasal cannula (HFNC), a device first introduced in neonates and pediatric care, is currently used in a wide range of indications in adult respiratory and critical care medicine (5-7). It is a relatively new method in bronchoscopy with several notable theoretical advantages over low flow oxygen via conventional nasal cannula (CNC):

• High flow up to 60 liters per minute ensures a more stable FiO2 and better matches the increased patient's inspiratory flow
• High flow generates a small positive expiratory airway pressure (up to 5 cm H2O) which could stabilize the upper airways during sedation and have a beneficial effect in the lower airways
• High flow reduces dead space in the upper airways and increases alveolar ventilation.

CPAP and NIV can provide similar beneficial effects, but their use is challenging because they require a close-fitting facial mask which limits the use of a bronchoscope, aspiration of secretions from the upper airways, pharynx, or oral cavity and expectoration of sputum. 4. Previous studies on HFNC