Functional Residual Capacity Under Apnoeic Oxygenation with Different Flow Rates in Children

High-flow nasal oxygen (HFNO) is the administration of heated, humidified and blended air/oxygen via nasal cannula at rates ≥ 2 L/kg/min. HFNO is an open system that can be used with nasal prongs of different sizes and was developed in neonatal intensive care unit for preterm babies with apnoea as alternative to continuous positive airway pressure (CPAP). Due to its ease of use and safety to apply to a wide range of indication HFNO is increasingly gaining interest for providing respiratory support in paediatric patients and in adults in ICU with respiratory failure. In adult populations, the use of HFNO permits to prevent desaturation during tracheal intubation of intensive care patients with mild-to-moderate hypoxemia. An application for HFNO in adults and children, is the extension of safe apnoea in patients who were undergoing general anaesthesia for hypopharyngeal or laryngo-tracheal surgery. This method, the so-called safe apnoeic oxygenation, also prevents hypoxemia in children during intubation. By using this technique, Patel et al. demonstrated a significate prolongation of apnoea time and proposed a ventilatory effect, as these studies revealed a slower increase in pCO2 than physiologically was expected. In these studies, researchers compared their data to studies from the 1950-ies, where CO2 increase during apnoea was investigated. In contrast, the investigators' previous research projects with HFNO did not confirm the claimed ventilatory effect in children and adults.

Furthermore studies performed in spontaneously breathing neonates and adults have shown the ability of HFNO to generate some increase in pharyngeal pressure, which could explain the improvement of oxygenation despite prolongation of apnea time. The investigators' previous study on adult patients showed that a relevant increase of pressure was nearly absent while patient's mouth was open. Currently, there is no data on the physiological pressure that is generated in the subglottic airway in apneic children treated with HFNO. The traditional measurement of intratracheal pressure with a catheter in the trachea is considered to pose a risk in small children.

The main objective of this study is thus to investigate the variations of poorly ventilated lung units (i.e., silent spaces) as a surrogate for functional residual capacity measured by electrical impedance tomography to dynamically assess atelectasis formation and regression under apnoeic oxygenation with different flow rates.

Eligible children will receive premedication with Midazolam rectal/oral 0.5 mg/kg or Dexmedetomidine nasal 2 mcg/kg 30 minutes before the beginning of the procedure (local SOPs of the paediatric anaesthesia departments). Mandatory monitoring will consist of non-invasive peripheral oxygen saturation (SpO2), heartrate (HR), and non-invasive blood pressure (NIBP). An intravenous line for drugs injection will be placed.

After start of anaesthesia (="induction"), adequate face-mask ventilation will be established. The sealed envelope for randomisation will then be opened. Standard anaesthesia will be continued using of intravenous propofol. Anaesthetic depth will be assessed using NarcotrendTM (NarcotrendTM, Hannover, Germany), maintaining values between 40 and 60. Additional study related non-invasive monitoring: transcutaneous tcCO2 and O2 (ToscaTM, Radiometer, Neuilly-Plaisance, France) measurement, thoracic electrical impedance tomography (EIT, PulmoVista 500, Draeger, Luebeck, Germany) and NIRS (Niro-200NX (Hamamatsu, Tokyo, Japan). ECG, pulse-oximetry, blood pressure, Narcotrend (NarcotrendTM, Hannover, Germany), thoracic EIT will be measured continuously, starting before induction while spontaneous breathing and ending 1 minute after the recruitment-manoeuvre. All patients will receive neuromuscular blockade medication of 2 x ED95 (standard intubation dose) to facilitate airway management. Neuromuscular block will be assessed using train-of-four (TOF) monitoring (TOF-Watch, Organon Ltd, Dublin, Ireland). A TOF value of zero before apnoea start and throughout the whole procedure will be deemed essential.

After that one minute of pressure support mask ventilation (Pmax 20 cm H20) with a backup respiratory rate of 20/min, normalized at a volume of 6-8 ml.kg-1 with 100% oxygen and will be applied. The ventilation will be discontinued, and the child will be left apnoeic for 5 minutes receiving oxygen according to the randomisation.

Children will be randomized to receive three different flow rates of 100% oxygen, warmed and humidified with the OptiFlow device (Fisher\&PaykelTM, Auckland, New Zealand):

• group 1): 0.2 l/kg/min + continuous jaw thrust
• group 2): 2 l/kg/min + continuous jaw thrust
• group 3): 4 l/kg/min + continuous jaw thrust (control group)
• Group 4): 2 l/kg/min with OptiFlow FiO2 1.0 using OptiFlow-Switch system by Fisher\&Paykel.

The nostrils must not be occluded by the nasal cannula by more than 50%. The time until desaturation from SpO2 100% to SpO2 95% will be measured. A chest ultrasound at end of intervention after definitive airway management will prove that no pneumothorax developed during the procedure.

Break-up criteria during apnoea are: SpO2 below 95%, transcutaneous CO2 above 70 mmHg, or time of apnoea >5 minutes, a decrease of NIRS >30% from baseline.