Superior trunk nerve block will be done under ultrasound guidance to patients scheduled for shoulder surgeries. Local anesthetic agent (0.5% levobupivacaine) 8ml will be injected at the superior trunk of the brachial plexus in order to produce surgical anesthesia or analgesia for shoulder surgeries.

This study adopts a "Modified Narayana rule (MNR)", a sequential up-and-down pharmacodynamics method (UDM) to estimate the minimum effective local anesthetic volume of 90% (MELVA90) of cases of the aforementioned local anesthetic (LA), in order to produce the surgical anesthesia of the patients scheduled for shoulder surgeries. Based on the principal investigator's clinical experience, the initial volume of 8ml will be used. Depending on the success or failure of the superior trunk block, which is defined as the readiness for surgery at 30 minutes after the block, the volume for the next patient will be adjusted up or down by 2ml. The upper dose limit of the study to minimize the potential for local anesthetic systemic toxicity will be 16ml and the lower dose limit will be 4ml, which will be of no clinical importance.

The first exact timing (in minutes) [Readiness for surgery] that an Overall sensory score =<30 (NRS: 0-100; 100=normal sensation, 0=no sensation) using ice test and a motor grade =<1 (3 point scale: 2=no block, 1=paresis, 0=paralysis) in the nerves (C5 to C6) tested that are supplied by superior trunk of brachial plexus. Sensation to coldness (ice) will be tested at C5 - Lateral (radial) side of the antecubital fossa (just proximal to elbow crease); C6 - Thumb, dorsal surface, proximal phalanx. Motor block will be graded using a 3-points scale: 2=no block, 1=paresis and 0=paralysis, and will be test at C5 - elbow flexors and C6 - wrist extensors. "Block failure" will be defined as inability to achieve "readiness for surgery" at 30 minutes after the completion of the block.

The lowest volume (in milliliters) of local anesthetic agent that can produce an effective superior trunk block for surgery. Effective superior trunk block is defined as an overall sensory score of NRS =<30 and a motor grade of =<1 during the assessment after the nerve block that can undergo surgery. Sensory assessment will be done by using ice (cold sensation) test in both dermatomes (C5, C6) and a motor grade of =<1 in both myotome (C5, C6) tested. Sensory block will be graded as 0 to 100 where 100 represents normal sensation and 0=no sensation to coldness (ice cube).

Motor blockade will be graded using a 3-point scale: 2=no block, 1=paresis and 0=paralysis. Motor blockade of each individual nerve in the anaesthetized upper extremity will be evaluated by testing C5 - elbow flexors, C6 - Wrist extensors, C7 - Elbow extensors, C8 - Finger flexors to the middle finger, and T1 - small finger abductors. Onset of Motor block for each nerve will be defined as the time it took to achieve motor score =<1.