Comparison of standard expiratory pressure vs body mass index based expiratory pressure during general anaesthesia

INTERVENTION: Intervention1: BMI based positive end expiratory pressure: Positive end expiratory pressure will be selected according to patients body mass inde Xbased protocol and applied during general anaesthesia after intubation. Ventilatory parameters will be standardized as per the study protocol. Control Intervention1: standard positive end expiratory pressure: a standard positive end exoiratory pressure of 5cm of H2O will be applied after endotracheal intubation and maintained through general anaesthesia CONDITION: Health Condition 1: O‐ Medical and Surgical PRIMARY OUTCOME: intraoperative driving pressureTimepoint: 20 mins after intubation SECONDARY OUTCOME: 1. plateau pressure ; 2. oxygen saturation & oxygen requirement ; 3. lung aeration scoreTimepoint: 1. 20 mins after intubation ; 2. postoperatively after shifting in PACU ; 2 preoperatively 1 hour before intubation & 30 mins after extubation in PACU INCLUSION CRITERIA: ASA physical status I to III surgeries under general anaesthesia with expected duration of more than 2hours