This randomized , double-blind trial was designed to determine whether hourly automated perineural boluses ( 4 mL ) of local anesthesia delivered with patient-controlled pro re nata ( PRN , on demand ) boluses would result in a reduction in total local anesthesia consumption during continuous interscalene blockade after shoulder surgery compared with continuous perineural infusion ( 4 mL/h ) plus patient-controlled PRN boluses .