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Supraclavicular brachial plexus block is associated with 50% to 67% incidence of hemidiaphragmatic paresis as a result of phrenic nerve block. We examined whether ultrasound-guided compared with nerve stimulation supraclavicular brachial plexus block using 0.75% ropivacaine results in a lower incidence of hemidiaphragmatic paresis.In a prospective randomized observer-blinded controlled trial, 60 patients scheduled for elective elbow, forearm, wrist, or hand surgery under supraclavicular brachial plexus block without sedation were included. Supraclavicular brachial plexus block was performed with 20 mL of 0.75% ropivacaine using either ultrasound or nerve stimulation guidance. Ventilatory function was assessed by ultrasound examination of hemidiaphragmatic movement and spirometry.None of the 30 patients in the ultrasound group showed complete or partial paresis of the hemidiaphragm (95% confidence interval, 0.00-0.14), whereas in the nerve stimulation group, 15 patients showed complete paresis of the hemidiaphragm and 1 patient showed partial paresis of the hemidiaphragm (0% versus 53%, respectively; P < 0.0001). Ventilatory function (forced expiratory volume 1, forced vital capacity, peak expiratory flow) was significantly reduced in the nerve stimulation group compared with the ultrasound-guided group (P < 0.05). Two block failures occurred in the nerve stimulation group compared with none in the ultrasound group (P = 0.49). No adverse effects occurred in either group.Ultrasound-guided supraclavicular brachial plexus block, using 20 mL of 0.75% ropivacaine with the described technique, is not associated with hemidiaphragmatic paresis.