This is a prospective randomized controlled trial to compare the influence of arm position on ultrasound guided subclavian vein cannulation in mechanically ventilated patients. Clavicular acoustic shadow prevents ultrasonic visualisation of Subclavian vein medially and prevent us to use ultrasound guided cannulation. Recently studies have shown in volunteers that abduction of arm moves the clavicle cephalad and may allow us to scan and puncture more medial part of subclavian vein than adducted arm position. Hence we decided to compare the dynamics and ease of of subclavian vein cannulation under ultrasound guidance in two different arm positions namely arm by the side of the body(adducted arm) and abduction of arm by 90° at shoulder joint and flexion at elbow. After standard induction, intubation and initiation of mechanical ventilation, patients were positioned in supine with arm adducted position and head turned to contralateral side. High frequency linear array transducer (HFL 50) and Sonosite Xporte machine was used. USG probe was aligned in sagittal plane to obtain the short axis view of the SCV in the lateral part of right chest just below the clavicle.Then the subclavian vein was scanned medially till the subclavian artery(SCA) disappear under the clavicular acoustic shadow and the vein was just close to that shadow. This point was marked as Point A in the skin and the sonographic image was saved.Then the arm was positioned in 900 abductions at shoulder and flexion at elbow joint and the probe was placed longitudinally at the previous Point A marked on the skin and the sonographic image was saved here as Point A1. Distance from clavicular acoustic shadow to SCV for cephalic movement at Point A1. Then the probe was moved medially until the disappearance of subclavian artery and the subclavian vein seen as close as to the clavicular acoustic shadow. This point was marked as Point B and the image was saved. The distance between Point A to Point B on the skin was measured using scale. From the saved images, Distance from skin to anterior wall of the Vein, Distance from posterior wall to pleura or rib and  Size of SCV (Maximum vertical diameter, Maximum transverse diameter, Area, Circumference) all were measured by activating the software in the USG machine.After obtaining sonographic measurements, According to the Randomized allocated group, the subclavian cannulation was done using Seldinger’s technique. While inserting the guidewire, IJV was scanned to know the malposition of guidewire to IJV, it was noted and cannulated after redirecting into right atrium. First pass success was defined as the blood aspirate and successful guide wire placement in first attempt. Attempt was defined as change of needle puncture on the skin and maximum of three attempts were allowed in that allocated arm position. .Failure was defined as more than three number of attempts in that allocated group.If venous puncture was unsuccessful for three attempts, the position of arm was changed to other position and if cannulation was still unsuccessful in both the arm positions, Internal Jugular vein was cannulated under USG guidance for central venous access. Complications like arterial puncture, pneumothorax were noted and Chest X-ray was taken postoperatively to confirm the catheter tip position and if any complications, appropriate measures were taken.