This study aim to calculate the minimal effective ropivacaine concentration required achieve successful analgesic interscalene block in 90% of patients (MEAC90)

• Before induction of general anesthesia, all patients will receive interscalene block using 15ml of ropivacaine which concentration will be based on the response of the previous patient.
• Based on biased-coin design up-down sequential method: if patient has pain free recovery, a successful block, the next patient will be randomized to receive either the same ropivacaine concentration or a concentration 0.01% w/v less. However, if he has an unsuccessful block, the next patient will receive a concentration 0.01% w/v higher.
• Patients with pain free recovery will be considered to have positive response.
• Patients' responses will be analyzed to calculate the MEAC90.

Inclusion Criteria:

• Patients, who are scheduled for shoulder arthroscopy

Exclusion Criteria:

• - <18 y,

• ASA physical class greater >III

  • BMI >40 has any contraindication for medications or techniques used in the study