Feasibility and Safety of an Over-pressure wash-in Method Using Fresh Gas Flow 0.5 L and Sevoflurane 8% During Initiation of Low-flow Anaesthesia: a Prospective, Randomised, Description Study

After obtaining the approval from the Institutional Ethics Committee of The Royal Hospital, Muscat, we will register this prospective, randomised, descriptive study in an appropriate online clinical trial registry. A written informed consent will be obtained from all the patients. Forty-eight patients of either gender aged 18-65 years, with American Society of Anesthesiologists (ASA) physical status of 1-2, undergoing elective surgery with the expected duration more than 1 hour under general anaesthesia requiring endotracheal intubation will be included. Pregnant women, smokers, patients with a body mass index (BMI) ≥30 kg/m2, cardiac/pulmonary/renal or liver impairments, upper or lower respiratory infections within the past 6 weeks and anticipated difficult airway will be excluded.

Age, gender, height and weight of the patients will be recorded. All the patients will follow the standard fasting requirements and receive no sedative premedication. They will be randomised before entering the operating room into one of the two groups, 'conventional group' (Group C) and 'over-pressure group' (Group OP), using block randomisation technique with varying block sizes. An appropriate intravenous cannula will be secured in the operating room and Ringer's Lactate 500 ml will be started. The patients will receive standard anaesthetic care and intra-operative monitoring with electrocardiogram, pulse oximeter (SpO2), non-invasive blood pressure (NIBP) and capnography. Anaesthesia workstation (GE Healthcare, Madison, WI, USA) which has an integrated anaesthetic gas analyser, displays age-adjusted MAC according to the internal algorithm and returns sampled gas to the breathing system will be used in this study. The breathing system will consist of a 2 litres reservoir bag, disposable breathing circuit, heat-moisture exchanger and standard circle system with Sodalime as the CO2 absorbent. The workstation will be tested to ensure no leak in the system and vapourisers are calibrated.

Ideal Body Weight (IBW) of the patient will be calculated based on the following formula: IBW (kg)=50 + 0.91 (height in cm - 152.4) for Men and 45.5 + 0.91 (height in cm - 152.4) for Women. The ventilator will be preset in Pressure Control-Volume Guarantee (PCV-VG) mode with a tidal volume of 8 ml/kg of IBW, respiratory rate=14 breaths/min, positive end-expiratory pressure (PEEP)=5 cmH2O and an inspiratory:expiratory ratio of 1:2. The set minute volume will be noted. The alarm for end-tidal Sevoflurane (FES) will be set at 2% to alert the clinician.

Heart rate (HR), blood pressure (BP) and peripheral oxygen saturation will be recorded before induction of anaesthesia, every minute until 5 min after induction and every 5 min intervals thereafter. The FGF will be set at 6 L/min with 100% O2 during pre-oxygenation and manual ventilation. Following pre-oxygenation for 3 min, general anaesthesia will be induced with intravenous Fentanyl 2 μg/kg followed by Propofol 2 mg/kg administered over 30 seconds and Rocuronium 1 mg/kg for neuromuscular blockade. Lungs will be ventilated manually and propofol 20 mg every minute will be administered until tracheal intubation. Tracheal intubation with an appropriate size endotracheal tube (ETT) will be performed 90 seconds later, the cuff inflated to 25-30 cmH2O using a cuff-pressure monitor device and ETT secured with an adhesive tape. The correct ETT position will be confirmed by auscultation and square-wave capnography. FGF will be paused during intubation and restarted after inflating the ETT cuff. The FGF hereafter will consist of O2 and Air with a set FIO2=0.6. FGF and Sevoflurane vapouriser dial (FVS) will be set as per the group allocation and mechanical ventilation will begin. The time of opening of vapouriser will be considered Tzero.

Following will be the steps during wash-in for achieving LFA: