“A prospective study to assess the feasibility of High Flow Nasal Oxygen during anaesthesia for Extracorporeal Shock Wave Lithotripsy (ESWL)”

First successive eligible(based on inclusion and exclusion criteria) 20 patients undergoing ESWL procedure at TPCH from the start of study date and giving informed consent will be included in the study. All patients in the study will receive the same intervention. All patients on arrival in theatre will have their baseline vital stats recorded and then subsequently every 5 minutes (or more frequent if required). The SpO2 probe will be attached and baseline reading on room air recorded. The trans-cutaneous CO2 monitor is attached for calibration. The HFNO nasal prongs will be applied at a rate of 70 litres per min for pre-oxygenation. IV access obtained and other monitoring including ECG, Non invasive BP, Bispectral index and peripheral nerve stimulator monitor attached. End tidal CO2 level obtained whenever assisted ventilation using the anaesthetic circuit is required during the anaesthetic will be recorded. Premedication in the form of intravenous midazolam and fentanyl will be given as per choice of the primary anaesthetist in charge of the patient. Intravenous induction of anaesthesia then commenced with propofol target controlled infusion (TCI) based on the Marsh Model to achieve a target BIS between 40-60. When the patient is unconscious and non responsive an appropriate sized Guedels airway will be inserted to ensure upper airway patency throughout the apnoeic period. The patient will be re-positioned as appropriate for the ESWL procedure and then rocuronium @ 0.6 mg/kg will be given for muscle relaxation. The degree of muscle paralysis will be monitored every 5 minutes for further rocuronium top up doses (0.1 mg/Kg) to maintain a paralysis to less than 3 twitches with Train of Four (TOF) count. Additional Fentanyl and Rocuronium doses will be given according to intra operative need as determined by the primary anaesthetist. The aim of the study is to prove that HFNO can be used as the only mode of ventilation (Oxygenation and removal of carbon dioxide) throughout the procedure and thereby avoid tidal volume ventilation induced movement of kidney to optimise ESWL. Usually ESWL procedures last for 20-30 minutes as decided by the surgical team. The HFNO would be ceased and the reminder of the surgery continued using anesthetic circuit and anesthetic ventilator if any of the pre-determined criteria for conversion to default general anesthetic (GA) is reached at any stage (criteria as described below). Rate of oxygenation used during the entire procedure would be the single level of 70 litres per min and is NOT changed during the procedure or any during any adverse events. If any adverse event (defined as significant hypoxia =Spo2<90% and/ or hypercarbia = transcutaneous CO2 monitor CO2>65 mmHg) does occur, the anaesthetists will try initially to give normal tidal volume breaths using the anaesthetic circuit and assess the response. If SpO2/ CO2 levels improve to a satisfactory level as determined by the anaesthetic team, then the ESWL will proceed as per study protocol. . If however, 1)The SpO2/ CO2 levels fails to improve to a satisfactory level as determined by the anaesthetic team 2)Significant hypoxia/hypercarbia (as defined above) recurs frequently (defined as more than once every five minutes) requiring the interruption of the ESWL procedure 3)There is any other associated trends (arrhythmia, hypertension, hypotension) that is concerning for the anaesthetic team 4)The surgeon wants to abort the study for any surgical reason Then the study will be terminated and the anaesthetic converted to the default conventional anaesthetic.{CONVERSION TO GENERAL ANAESTHETIC]. From this point onwards HFNO is ceased and the ventilation continued using standard anesthetic circuit and anesthetic ventilator. Once the surgeon confirms the procedure is over, the patients will be hand ventilated using face mask and anaesthetic circuit. EtCO2 levels will be documented. Propofol TCI will then be ceased and muscle relaxation reversed with appropriate agents (Neostigmine or sugammadex) depending on degree of residual muscle paralysis. All perioperative measurements will be observed and documented in the study data collection form as described below-The aim is to capture as much information as possible regarding the intra operative period. The relevance of various data collected will be discussed at the end of the study and will help formulate a well written study protocol for the future RCT. Demographic data: age, gender, weight, height, BMI. Surgical data details: details of stone (position, number, size, side, type if known) and previous treatment history. Procedure duration details: anaesthetic and surgical start and finish time. (Anaesthetic start time= Patient on operating table and ready to start. Anaesthetic finish time=Patient ready to be transferred the post anaesthesia care unit (PACU) [anaesthetic readiness, irrespective of PACU bed availability) (Surgical start time= time after patient positioning coinciding with the first shock delivered. Surgical finish time = time when last shock is delivered) Procedure details: total number of shocks, total energy (joules),delivered operating theatre time, number of x-rays exposures/screening done, degree of stone movement as measured by fluoroscopy. Anaesthesia details: Baseline and every 5 minute vital statistics in the form of heart rate, systolic and diastolic blood pressure, peripheral pulse saturation level, trans cutaneous CO2 levels, Bispectral index (BIS), train of four counts to assess depth of neuromuscular blockade, propofol TCI levels, HFNO flow level, opioid and other sedation used, drug and dosage of any other drugs used including any vasoactive drugs and details of any other intra or post op adverse events or anaesthetic intervention. [The Primary and secondary end points are the only 3 end points as described under the Endpoints section. The data collection form instructs the investigator to capture as much information as possible regarding the peri operative period of this initial pilot study. The relevance of each individual data collected will be discussed at the end of the study. This additional safety step is included in the study protocol to help formulate a well written study protocol for the future planned RCT. Please note that any relevant additional parameters recorded by the investigator during the study such as incidence of hypertension, arrhythmias or any unanticipated hospital re-admission, etc. are not assessed as secondary outcomes for the study and hence are not included as secondary end points.]