Following preoxygenation for 3minutes, induction of anaesthesia will be done with injection midazolam (0.02mg/kg), Propofol(2-2.5mg/kg) and Fentanyl(2mcg/kg). After checking the ability to ventilate, Vecuronium (0.1mg/kg) will be administered as a muscle relaxant following which patients will be manually ventilated for 3 minutes. Laryngoscopy will be performed using appropriate size laryngoscopy blade. Endotracheal intubation will be done using appropriate size tube and the airway will be secured. After intubation, general anaesthesia will be maintained with oxygen & air (50:50), isoflurane (Minimum alveolar concentration 

1.0). 

All patients will be ventilated using a closed circuit with the ventilator setting being volumecontrolled mechanical ventilation, with 6-8ml/kg tidal volume, fraction of inspired oxygen 50%, inspiratory: expiratory ratio will be 1:2, and respiratory rate will be adjusted to maintain an end-tidal carbon dioxide of 30-35 mmHg. Normothermia will be maintained throughout the surgery. 

In Group A, a fixed PEEP of 5cm H2O will be delivered throughout the procedure. Static lung compliance will be calculated at three points with 15 minutes interval starting from induction. 

In Group B, incremental PEEP of 5,7 and 9 cm H2O will be delivered at every 15 minutes interval and static lung compliance will be measured at each step keeping Pplat<30 .(16) Static lung compliance (Cs) can be calculated as follows(15): 

Cs=Vt/(Pplat-PEEP)

 Where 

Vt is the Tidal volume. 

Pplat is the plateau pressure. 

Pplat is measured at the end of inspiration by holding the inspiration for few seconds. 

PEEP will be decreased gradually from 9 cm H2O over the next 15 minutes and a PEEP of 5 cm H2O will be maintained throughout the procedure after that. 

Fluids will be given as required and amount and type of fluid will be noted. Any episode of acute hypotension during the study will be treated with Inj. Mephentermine 6mg I/V.  

At the end of the procedure, reversal of neuromuscular blockade will be done using Inj. Neostigmine 0.03-0.07mg/kg with Inj.Glycopyrrolate 0.01mcg/kg. Extubation will be done only after full reversal of neuromuscular blockade with a TOF ratio of more than 0.9 and the patient following verbal commands and ventilating spontaneously. After extubation, the patient will be shifted to postoperative area. Any episode of desaturation (SpO2<92%) will be treated with oxygen supplementation. The patients will be observed for immediate postoperative pulmonary complications using lung ultrasound at two time points:1 hour after extubation(T1) and 24 hours after extubation(T2). 

Low frequency curvilinear (2-5 MHz) probe of the SonoSite Fujifilm Edge II inc.USA 

Ultrasound System will be used for lung ultrasound scanning. Each side of the thorax will be divided into six zones by two vertical (anterior and posterior axillary lines) and one imaginary horizontal line (passing above the nipple), so a total of 12 lung areas will be scanned. Intercostal spaces of each of these areas will be scanned, and an image of the most pathologic area of each quadrant will be saved to digital format for offline analysis and scoring. 

The scoring of lung ultrasound will be done according to a modified lung ultrasound scoring 

(mLUS) system.(9) 

According to the mLUS system, 

Score 0: A-line profile or up to two B-line separated by regular pleural lines. 

Score 1: if there are three or more B-lines with or without small subpleural consolidation and a regular pleural line.  

Score 2: if there are coalescent B-lines and or small subpleural consolidation with an irregular pleural line in between. 

Score 3: if extensive subpleural consolidation is more than 1x2 cm. 

Detailed ultrasound Based upon the mLUS scoring system, each area was scored from 0-3, so a minimum zero or maximum 36 score is possible. Lung ultrasound will be performed at two predefined time points, 1hour after extubation and 24hours after extubation of the trachea.