Phase 2 The subjects of study are patients undergoing upper extremity operation under supraclavicular brachial plexus block. All patients will be randomly assigned to receive 30ml ropivacaine in concentrations of 0.125%, 0.2%, 0.25%, 0.375%, 0.5%, or 0.75%. Patients are randomised to the four groups by random number(generated by computer) in sealed envelopes. All nerve blocks were performed by a single dedicated anaesthetist who remains blinded to the concentration of ropivacaine.